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SCIENTIFIC REPORT
SCK•CEN-BLG-990
04/MDC/P-48
Geochemistry of Boom Clay pore
water at the Mol site
Status 2004
M. De Craen, L. Wang, M. Van Geet and H. Moors
September, 2004
SCK•CEN
Boeretang 200
2400 Mol
Belgium
Waste & Disposal Department
© SCK•CEN
Belgian Nuclear Research Centre
Boeretang 200
2400 Mol
Belgium
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2400 Mol, Belgium.
SCIENTIFIC REPORT OF THE BELGIAN NUCLEAR RESEARCH CENTRE
SCK•CEN-BLG-990
04/MDC/P-48
Geochemistry of Boom Clay pore water
at the Mol site
Status 2004
M. De Craen, L. Wang, M. Van Geet and H. Moors
September, 2004
Status: Unclassified
ISSN 1379-2407
SCK•CEN
Boeretang 200
2400 Mol
Belgium
Waste & Disposal Department
3
Executive summary
In Belgium, geological disposal in clay is the primary option for the final disposal of
high-level radioactive waste and spent fuel. The Boom Clay is studied as the reference
host rock for methodological studies on the geological disposal of radioactive waste.
In many of these studies, an in-depth understanding of the Boom Clay pore water
geochemistry is essential.
The objective of this report is to evaluate the most reliable technique(s) to obtain
representative pore water samples, to determine the variation of the pore water
composition in the Boom Clay, to present a coherent geochemical model for
explaining the origin of the Boom Clay pore water composition, and to propose a
reference pore water composition to be used in the laboratory experiments and for
speciation calculations and assessments of perturbation of the Boom Clay. It is
important to mention that this report is not the result of an integrated study on the
Boom Clay pore water composition. In fact, all the available information from
previous studies is put together in this report. It is therefore considered to be a 'state of
the art' report, status 2004.
Pore water sampling is done in situ from various piezometers, or by the mechanical
squeezing or leaching of clay cores in the laboratory. These three pore water sampling
techniques are compared and evaluated. At the present time, piezometer water is
considered to be the most representative for the in situ pore water. This is because
piezometer waters experience minimum laboratory manipulations and therefore suffer
minimum artefacts. Squeezed pore water is comparable to piezometer-derived water
when considering the major ionic composition, but not for trace elements and organic
matter. Squeezed pore water samples can thus be considered as representative for the
in situ conditions, up to a certain degree. Comparing to the piezometer and squeezing
techniques, batch leaching experiments provide comparable results for the major
cation composition if the samples are carefully filtered. Due to the electrostatic
properties of the Boom Clay, i.e., double layer phenomena, the leaching waters reveal
a very different anion composition compared to the waters extracted from compacted
clay using piezometers and squeezing techniques.
A large data-set on pore water composition is available, however, because of the
different sampling techniques, the different design of the piezometers, and the
different filter materials used, it is not always unambiguous to interpret these data.
The reliability and significance of these data have therefore been considered by
statistical analyses and geochemical modelling.
The statistical analysis of the available data at the Mol site (to about 40 m around the
HADES URF) has shown that a vertical spatial variability (perpendicular to the
bedding) is present within the Boom Clay pore water composition. This vertical
variability shows no gradient, and is mostly influenced by the elements Na, Mg, Ca
and Cl. The mechanism behind these variations in major cations is explained by
cation exchange and calcite dissolution/precipitation. The ultimate cause of these
chemical reactions is assumed to be due to the spatial variability in pCO2 and pH,
although the reason of this is not yet understood. Nevertheless, if the assumed pCO2
variation exists, the pore water seemed to respond rapidly to reach a chemical
equilibrium with the clay. Because transport in the Boom Clay is diffusion-controlled,
4
the spatial variability in the pore water composition can still be present, even on small
scales.
Due to this spatial variability, one single mean Boom Clay pore water composition at
the Mol site cannot be given. However, a modelled reference composition, taking into
account the current knowledge of Boom Clay mineralogy and calibrated towards a
dataset including spatial variability, is provided. Boom Clay pore water is basically a
NaHCO3 solution of 15 mM, containing about 115 mg C / l. The observed major
cation concentrations can be explained by cation exchange and mineral
dissolution/precipitation mechanisms. The current model assumes the equilibrium of
calcite, siderite, pyrite, and chalcedony, and the cation exchange between Ca, Na, K,
and Mg. The maximum redox potential Eh is about -270 mV; probably controlled by
the equilibrium of pyrite and siderite under the in situ geochemical conditions. A
lower redox potential is possible as the result of interactions involving natural organic
matter mediated by biochemical processes.
As mentioned before, this report is a 'putting together of all available information' on
the Boom Clay pore water geochemistry. Comparison of the results of various studies
was not always unambiguous, on the one hand because of the different design of the
piezometers and filtermaterial, but on the other hand also because of the lack of
uniformity in sampling conditions and analyses. Therefore, some recommendations
are given for a systematic procedure of pore water sampling and analyses. Finally,
some recommendations for further research on the pore water geochemistry are
formulated.
5
Table of contents
Executive summary
List of abbreviations
Introduction
1
2
Physical and mineralogical characteristics of Boom Clay .......... 13
1.1
Physical characteristics ................................................................................13
1.2
Mineralogical characteristics .......................................................................14
1.2.1
General mineralogical composition of Boom Clay .............................14
1.2.2
Clay-mineralogical composition of Boom Clay ..................................15
Boom Clay pore water sampling and analytical techniques....... 17
2.1
Pore water sampling techniques...................................................................17
2.1.1
Piezometers ..........................................................................................17
2.1.2
Squeezing .............................................................................................19
2.1.3
Leaching...............................................................................................21
2.2
Analytical techniques...................................................................................22
2.2.1
2.2.1.1
pH.....................................................................................................22
2.2.1.2
Eh ......................................................................................................24
2.2.1.3
pCO2 and other dissolved gasses .....................................................25
2.2.1.4
Electrolytic Conductivity (EC) ........................................................27
2.2.2
2.3
pH, Eh, pCO2, electrolytic conductivity and dissolved gases ..............22
Other techniques ..................................................................................28
2.2.2.1
The total organic carbon content (TOC) ..........................................28
2.2.2.2
UV/VIS spectrometry ......................................................................28
2.2.2.3
The cation concentration (Ca, Fe, K, Mg, Na, Si) ...........................28
2.2.2.4
The anion concentration (F-, Cl -, Br -, HPO42-, NO3-, SO4 2-) .........29
2.2.2.5
The trace element concentration ......................................................29
2.2.2.6
The stable isotope composition........................................................29
2.2.2.7
The radiochemical composition .......................................................30
Sample locations ..........................................................................................31
2.3.1
EG/BS piezometer ...............................................................................32
2.3.2
ARCHIMEDE piezometers .................................................................33
6
3
2.3.3
Spring 116............................................................................................34
2.3.4
ORPHEUS piezometer.........................................................................34
2.3.5
MORPHEUS piezometer .....................................................................35
Boom Clay pore water composition and characteristics............. 37
3.1
Chemical composition .................................................................................37
3.1.1
Major elements.....................................................................................37
3.1.2
Trace elements .....................................................................................42
3.1.3
pH, pCO2, and alkalinity of Boom Clay ..............................................44
3.1.3.1
pH of Boom Clay pore water ...........................................................45
3.1.3.2
Partial pressure of CO2 (g) in Boom Clay .......................................49
3.1.3.3
Alkalinity of Boom Clay pore water................................................54
3.1.3.4
Conclusions of pH/ pCO2.................................................................56
3.1.4
Redox processes and redox potential in Boom Clay ...........................56
3.1.4.1
Redox potential in Boom Clay.........................................................57
3.1.4.2
Redox capacity of Boom Clay pore water .......................................59
3.1.4.3
Conclusion of Boom Clay redox conditions....................................59
3.1.5
Electrolytic Conductivity (EC) ............................................................60
3.1.6
Dissolved organic carbon (DOC) and its effect on pore water
composition..........................................................................................60
3.1.6.1
Presence of TOC in Boom Clay pore water.....................................60
3.1.6.2
TOC versus UV measurements........................................................63
3.1.6.3
Characteristics of the mobile organic matter in Boom Clay............64
3.1.7
3.2
Evaluation of extraction techniques and recommendations for water
sampling and storage............................................................................68
Isotope geochemistry ...................................................................................70
3.2.1
Stable isotopes .....................................................................................70
3.2.2
Radioisotopes.......................................................................................72
3.3
3.2.2.1
U-Th isotopes...................................................................................72
3.2.2.2
14
C ....................................................................................................72
3.2.2.3
36
Cl ...................................................................................................72
Spatial variability .........................................................................................73
3.3.1
Vertical variability ...............................................................................73
3.3.2
Lateral variability.................................................................................74
3.4
Data quality ..................................................................................................77
3.4.1
Statistical Analysis...............................................................................77
7
3.4.1.1
Factors that might influence pore water composition......................77
3.4.1.2
Data and statistical techniques .........................................................78
3.4.1.3
Results of the statistical analyses.....................................................79
3.4.1.4
Effect of the filter material on the pore water composition .............82
3.4.1.5
Effect of the spatial variability on the pore water composition .......85
3.4.1.6
Conclusions of the statistical analyses.............................................88
3.4.2
4
Charge balance and equilibrium state of the pore water......................88
Model simulation of pore water chemistry................................... 91
4.1
Equilibrium model and water-rock interaction ............................................91
4.2
Computer code and thermodynamic database .............................................91
4.3
Mineral solubility and ion exchange............................................................92
4.4
Equilibrium model for the simulation of the pore water composition of
Boom Clay...................................................................................................99
4.4.1
Mineral stability constants and ion exchange parameters..................100
4.4.2
Results of model simulations and discussions ...................................103
4.5
Concluding remarks ...................................................................................105
4.6
Future work needed to improve the model ................................................105
5
Reference Boom Clay pore water composition at the Mol site. 107
6
Conclusions .................................................................................... 111
7
Recommendations ......................................................................... 115
8
Acknowledgements ....................................................................... 117
9
References ...................................................................................... 119
10 Annexes .......................................................................................... 127
8
9
List of abbreviations
ANC
Acid Neutralising Capacity
ANOVA
ANalysis Of VAriance
ARCHIMEDE Acquisition et Regulation de la Chimie des Eaux en Milieu Argileux
pour le projet de Stockage de Déchets Radioactifs en Formation
Géologique
BCPW
Boom Clay Pore Water
BDT
Below Drilling Table
BSL
Below Surface Level
CA
Carbo
CEA
Commissariat à l'Energie Atomique
CEC
Cation Exchange Capacity
CERBERUS
Control Experiments with Radiation of the Belgian Repository for
Underground Storage
CDT
Canyon Diablo Troilite
DOC
Dissolved Organic Carbon
EDZ
Excavation Disturbed Zone
EC
Electrolitic Conductivity
EG/BS
Extension Gallery / Bottom Shaft
Extra DOS
depth versus the outside of the lining of the HADES URF
FFFF
Flow Field Flow Fractionation
FT-IR
Fourier Transform Infrared Spectroscopy
HADES
High Activity Disposal Experimental Site
HLW
High Level Waste
HR-ICP-MS
High Resolution - Inductively Coupled Plasma - Mass Spectrometry
IAEA
International Atomic Energy Agency
IC
Ion Chromatography
ICP-AES
Inductively Coupled Plasma - Atomic Emission Spectrometry
ICP-MS
Inductively Coupled Plasma - Mass Spectrometry
Intra DOS
depth versus the inside of the lining of the HADES URF
ISE
Ion Selective Electrode
ISFET
Ion Sensitive Field-Effect Transistor
LLNL
Lawrence Livermore National Library
MANOVA
Multivariate ANalysis Of Variance
10
MORPHEUS
Mobile ORganic matter and Pore water extraction in the Hades
Experimental Underground Site
MWCO
Molecular Weight Cutt-Off
MWL
Meteoric Water Line
NEA
Nuclear Energy Agency
ORPHEUS
Oxidation Reduction Potential and pH Experimental Underground
Station
PE
Poly-Ethylene
PEEK
Poly-Ether-Ether-Keton
PHYMOL
PalaeoHYdrogeological study of the Mol site
SBCPW
Synthetic Boom Clay Pore Water
SCK•CEN
Belgian Nuclear Research Centre
SBCW
Synthetic Boom Clay Water
SCW
Synthetic Clay Water
SG
Sintered Glass
SIC
Synthetic Interstitial Clay water
SICZH
Synthetic Interstitial Clay water without (Zonder) Humus
S/L
Solid/Liquid ratio
SMOW
Standard Mean Ocean Water
SPRING 116
Source Piezonest at RING 116 of the Test Drift gallery
SS
Stainless Steel
ST
SchumaTherm
TAW
Tweede Algemene Waterpassing
TC
Total Carbon
TD
Test Drift
TDS
Total Dissolved Salt
TIC
Total Inorganic Carbon
TOC
Total Organic Carbon
TRANCOM
Migration Case Study: TRANsport of Radionuclides due to
Complexation with Organic Matter in Clay Formations
URF
Underground Research Facility
UV/VIS
Ultra Violet / Visible wavelenghts
XRD
X-Ray Diffraction
YM3
3000 Molecular Weight Cutt-Off
11
Introduction
In Belgium, geological disposal in clay is the primary option for the final disposal of
high-level radioactive waste and spent fuel. The Boom Clay is studied as the reference
host rock for methodological studies on the geological disposal of radioactive waste.
This clay layer is present under the facilities of the SCK•CEN at Mol, at a depth of
190 to 293 m (Figure 0-1).
Mol
Figure 0-1. Location of Mol. Present-day outcrops of the Boom Clay in Belgium are
indicated in black. To the north of the outcrops, the Boom Clay is present in the
subsurface. In Mol, the Boom Clay is present at a depth of 190 to 293 m (Mol-1
borehole).
In 1974, SCK•CEN started with the construction of the HADES underground research
facility (URF), which was build at a depth of 223 m in the Boom Clay. HADES was
designed to carry out experiments related to the disposal of radioactive waste. The
current R&D programme is focussed on the feasibility and safety of HLW disposal in
the Boom Clay. In this framework, a detailed characterisation of the clay is performed
(mechanical, physico-chemical and hydrogeological properties, variability, role of
organic matter, ...). In addition, high priority is given to the understanding of the basic
phenomena which control the retention and/or mobility of radionuclides in the clay.
Therefore, it is very important to characterise the pore water composition in the host
rock.
Previous studies, mainly involved with radionuclide migration studies, already
showed the necessity to understand the pore water composition. Baeyens et al. (1985)
studied the in situ physico-chemical characteristics of Boom Clay. This study mainly
focussed on the pore water composition, the cation exchange capacity and specific
surface of the clay. The study was performed on leached pore water samples. In the
12
frame of radionuclide migration studies, Henrion et al. (1985) sampled Boom Clay
pore water from piezometers under anaerobic conditions. This resulted in the set-up of
an important new data base which gave rise to new questions.
In 1991, a new project was initiated: the ARCHIMEDE-argile project. The main
objective of this project was to better understand the pore water chemistry of Boom
Clay (Griffault et al., 1996; Beaucaire et al., 2000). The project included field work,
field sampling and in situ Eh-pH measurements in the HADES URF, laboratory
investigations and analyses, and modelling.
In 1997, Dierckx published a report on 'Boom Clay in situ pore water chemistry'. In
this report, a mean value for the ionic composition of the EG/BS piezometer was
given and compared with other piezometers. A major conclusion of this study was
that the EG/BS piezometer is not an optimal reference for studying Boom Clay pore
water, and that more measurements are necessary to better understand the Boom Clay
pore water chemistry.
The Boom Clay pore water composition is considered in many studies, such as the
study on the variability of the clay characteristics and its pore water, the study of
dissolved organic matter, Eh and pH studies, migration experiments, geochemical
modelling, ... In each of these studies, only a particular part of the pore water
characteristics is considered. Information was fragmentary.
The objective of this report is to evaluate the most reliable technique(s) to obtain
representative pore water samples, to determine the variation of the pore water
composition in the Boom Clay, to present a coherent geochemical model for
explaining the origin of the Boom Clay pore water composition, and to propose a
reference pore water composition to be used in the laboratory experiments and for
speciation calculations and assessments of perturbation of the Boom Clay. Therefore,
the first step was to put together the information on the Boom Clay pore water
geochemistry from the various studies. This report is thus not the result of an
integrated study on the Boom Clay pore water composition. Consequently, some
important information may still lack if it was not considered in one of the above
mentioned studies.
This report describes the pore water sampling and analytical techniques, the results
and interpretation of a series of studies carried out in situ in the HADES URF and in
the laboratories. Pore water sampling is done in situ from various piezometers, or by
the mechanical squeezing or leaching of clay cores in the laboratory. These three pore
water sampling techniques are compared and evaluated. A large data-set on pore
water composition is available, however, because of the different sampling
techniques, the different design of the piezometers, and the different filter materials
used, it is not always unambiguous to interpret these data. The reliability and
significance of these data have therefore been considered by statistical analyses and
geochemical modelling. This enabled to define a reference Boom Clay pore water
composition at the Mol site. Finally, some conclusions and recommendations for
further research on the pore water geochemistry are given.
13
1
Physical and mineralogical characteristics of Boom Clay
1.1 Physical characteristics
The petrophysical and hydraulic parameters of the Boom Clay are summarised in the
table below (Table 1-1).
Table 1-1: Petrophysical and hydraulic parameters of Boom Clay (compiled from
Baeyens et al., 1985; Henrion et al., 1985; Volckaert et al., 1997; SAFIR 2, 2001)
Parameter
Unity
Value
Bulk density (sat.)
[t/m³]
1.9 - 2.1
Average grain density
[t/m³]
2.65
Water content
[% dry wt]
19 – 24
Total porosity
[vol. %]
36 – 40
Hg injection porosity
Macro-porosity
Micro-porosity
[vol. %]
No reliable data: due to swelling and water
content, outgassing causes serious change
in the Boom Clay fabric
Specific surface
[m²/g]
44
In situ temperature
[°C]
16
Thermal conductivity
[W/mK]
1.68
Specific Heat
[J/kgK]
1400
Heat Capacity
[MJ/m²K]
2.8
Seismic Velocity Vp
[m/s]
1300
1852
1700
2000
Hydraulic conductivity
Lab. experiments
In situ field testing
[m/s]
(from migration experiments)
(laboratory samples)
(DSI sonic-logging) (Vs = 526)
(uphole sonic logging)
(seismic velocity analysis)
Vert. 1.3 - 3.4x10-12; Horiz. 3.5 - 7.9x10-12
Vert. 2.1x10-12; Horiz. 4.5x10-12
14
1.2 Mineralogical characteristics
The Boom Clay is a sedimentary deposit, mainly composed of siliciclastic minerals,
fossils and organic matter.
Identification and semi-quantitative analyses of the minerals is generally performed
by X-ray diffraction, also in the case of the Boom Clay (see references below). As an
alternative, Wouters et al. (1999) used the dual range Fourier Transform Infrared
Spectroscopy method (FT-IR) and demonstrated the accurateness of this technique for
the measurement of most minerals, including clay minerals.
1.2.1 General mineralogical composition of Boom Clay
The mineralogical composition of Boom Clay consists of clay minerals (up to 60
wt%), quartz (~20 wt%), feldspars (~10 wt%), and minor amounts of muscovite,
biotite, and some heavy minerals. The clay mineralogy is dominated by illite,
smectite, illite/smectite interstratifications, and kaolinite. Chlorite, degraded chlorite
and illite/chlorite interstratifications are also present.
The authigenic mineral assemblage in the Boom Clay includes apatite, glauconite,
authigenic quartz, carbonates (calcite and siderite, 1-5 wt%) and pyrite (1-5 wt%).
Gypsum is present as a weathering product.
A summary of the Boom Clay mineralogical composition is given in the NEA 'Clay
Club' Catalogue of the Characteristics of Argillaceous Rocks (Volckaert et al., 1997).
More recent mineralogical analyses (see references in Van Keer and De Craen, 2001;
and De Craen et al., 2004b) enabled to update the summary table mentioned below
(Table 1-2).
Table 1-2: Mineralogical composition of Boom Clay. Values in % total dry wt.
(SAFIR 2, 2001; updated with data from various mineralogical studies, see references
in Van Keer and De Craen, 2001 and De Craen et al., 2004b)
30-60 %
Clay minerals
10-45 %
Illite
10-30 %
Smectite + illite/smectite ML
5-20 %
Kaolinite
0-5 %
Chlorite
0-5 %
Chlorite/smectite ML
Quartz
15-60 %
K-Feldspars
1-10 %
Albite
1-10 %
1-5 %
Carbonates
1-5 %
Calcite
present
Siderite
present
Dolomite
present
Ankerite
Pyrite
1-5 %
Organic Carbon
1-5 %
Others
Glauconite, apatite, rutile, anatase,
present
ilmenite, zircon, monazite, xenotime
present
15
1.2.2 Clay-mineralogical composition of Boom Clay
A lot of effort is done to characterise the clay-mineralogical composition of the Boom
Clay.
Already in the early seventies, an extensive regional sedimentological study of Boom
Clay was performed by Vandenberghe (Vandenberghe, 1974, 1978). In 1976, the clay
mineralogical composition of samples taken from the exploratory borehole at the Mol
site was studied in detail by Thorez (Thorez, 1976). However, some discrepancies
exist between these studies. On the one hand, Thorez (1976) mentioned the presence
of vermiculite and the absence of kaolinite. On the other hand, Vandenberghe (1974;
1978) illustrated the presence of kaolinite, while the occurrence of vermiculite was
not mentioned. To clarify this contradiction, Vandenberghe and Thorez (1985) used
different techniques to identify this mineral. The results obtained do not indicate the
presence of vermiculite.
In the eighties and nineties, additional clay mineralogical studies were carried out on a
limited series of samples (Table 1-3) taken at the Mol site. Samples were taken from
clay cores from exploration boreholes, at the underground research laboratory (Ouvry,
1986; Push et al., 1987; Rousset, 1988; Baldi et al., 1990; Goemaere, 1991; Merceron
et al., 1993; Griffault et al., 1996), and in the second shaft. The corresponding data set
is enlarged by the work of Vandenberghe (1978), Decleer et al. (1983), Laenen (1997)
and De Craen et al. (2000) (Table 1-4), who analysed the mineralogical and
geochemical variations of the Boom Clay in detail.
Table 1-3: Clay mineralogical composition of the Boom Clay near the underground
research facility. Clay mineral amounts are relative percentages of the total clay
content in the <2 µm fraction; n: number of samples. (from Van Keer and De Craen,
2001)
Ouvry
1986
n=2
35
Push et al.
1987
n=?
15
Rousset Vandenberghe Merceron et al. Griffault et al.
1988
1990*
1993
1996
n=?
n=1
n=3
n=3
54
16
Smectite
Mixed layers
10
4
50
38
Illite/smectite
traces
Chlorite/smectite
Illite
25
46
23
41
25
34
Kaolinite
30
39
18
35
20
19
Chlorite + degraded
traces
4
4
traces
9
chlorite
* Since the sample was grinded, only the qualitative characterisation of the <2 µm fraction is
meaningful.
16
Table 1-4: Generalised clay mineralogical composition of the Boom Clay in %. Clay
mineral amounts are relative percentages in the <2 µm fraction; n: number of
samples; min: minimum; max: maximum. (from Van Keer and De Craen, 2001)
Illite
Smectite
Illite/Smectite
Smectite + Illite/Smectite
Chlorite/Smectite
Kaolinite
Chlorite
+
degraded
chlorite
Vandenberghe
1978
n = 30
outcrop
min max mean
37
59
48
9
28
16
0
27
12
Decleer et al.
1983
n = 21
outcrop
mean
24
67
min
6
25
4
Laenen
De Craen et al.
1997
2000
n = 243
n = 40
outcrop
Mol-1 borehole
max mean min max mean
28
18
20
60
40
70
40
18
9
20
60
40
Traces
10
2
31
14
19
6
10
14
1
45
6
30
3
2
2
30
10
It is generally accepted that the clay fraction is dominated by illite, smectite, illitesmectite interstratifications and kaolinite. Chlorite, degraded chlorite and illitechlorite interlayers were only found in small amounts. The reported semi-quantitative
clay percentages differ from one author to the other (see Table 1-3 and Table 1-4).
These differences are mainly due to the application of different quantification
methodologies.
15
5
17
2
Boom Clay pore water sampling and analytical techniques
2.1 Pore water sampling techniques
Pore water extraction from argillaceous rocks is done by either in situ or laboratory
techniques. For Boom Clay, being a relatively soft clay and having a high water
content, almost all standard pore water extraction techniques are feasible (Sacchi and
Michelot, 2000).
In situ pore water extraction from Boom Clay is realised by using piezometers.
Various types of piezometers are placed in different directions and with filters at
different depths (levels) into the clay. Piezometry requires the drilling of boreholes in
which porous filterscreens are placed, that are mounted on supporting tubes. In
general, sealing off of a piezometer in a borehole is necessary to avoid geochemical
alterations of the rock due to the intrusion of atmospheric reactive gasses. However,
the natural convergence of Boom Clay automatically seals off the porous filterscreens
of the piezometers. This property avoids the use of engineered sealing materials, such
as packers or backfill materials.
Laboratory pore water extraction techniques used for Boom Clay are mechanical
squeezing and leaching. Both techniques require the sampling and preservation of
clay cores, in a way that all possible geochemical perturbations are minimised. Since
Boom Clay is sensitive to air-oxidation, the clay cores have been immediately
vacuum-packed in sample-bags made out of aluminium-coated poly-ethylene sheets.
This protects the clay core as much as possible from oxidation and also from drying
out. Samples for pore water characterisation were then stored in a nitrogen-filled
glove box (oxygen level < 10 mg/l), or in PVC tubes filled and flushed with argon to
create the best feasible anaerobic conditions. Whenever possible, long-term storage is
done in dark and cooled facilities where the temperature is around 4 °C. To prevent
geochemical perturbations (in particular oxidation) during sample preparation prior to
analyses, the sample preparation should always be performed in a glove-box (oxygen
level < 10 mg/l).
2.1.1 Piezometers
Besides the initial use of piezometers to determine hydraulic and mechanical
properties of the Boom Clay (pore water pressure, effective stress, hydraulic
conductivity, ...), they are also used to obtain samples of Boom Clay pore water.
These samples are used to determine specific pore water characteristics or to serve as
feed water in all kinds of laboratory experiments.
Piezometers have been designed in different shapes and dimensions containing singleor multiple screens. The piezometer design which is mostly used is the cylindrical
design. Here, a cylindrical porous filter screen is mounted on a supporting tube
equipped with (a) filter chamber(s). Each individual filter chamber (filter screen) is
normally equipped with one or two small diameter water pipes, closed at the gallery
side with valves (see Figure 2-1). The valves allow the sampling of Boom Clay pore
water out of the piezometer whenever needed. Because of the relative high pressures
(lithostatic as well as hydraulic) and the mechanical constructability, the first
piezometers were all made of durable stainless steel alloys. In November 2000, a first
18
non-metallic piezometer, as part from the ORPHEUS-set up, has been constructed and
installed in the HADES-underground laboratory. An overview of the different
piezometers and the filter material used is given in Annex 1 and Annex 2.
Multi-piezometer composed of stainless
steel. Also the filters are composed of
stainless steel.
Reference piezometer R55I,
Connecting Gallery
Close up of the filter (5 cm length).
Reference piezometer R55I,
Connecting Gallery
Piezometer composed of PVC. At the inside,
a filter chamber is present next to the filter
in which the pore water is collected. Two
small-diameter water pipes enable to sample
the pore water in the URF.
MORPHEUS piezometer, Test Drift
Pore water sampling and pressure
measurement.
MORPHEUS piezometer, Test Drift
Figure 2-1: Design of a multi-piezometer for in situ pore water sampling.
19
In situ piezometric water extraction is realised by connecting a recipient (usually
isolated from air to prevent oxidation) to one of the water pipes of the piezometric
filter screen. After opening the water valve, the hydraulic pressure drop will generate
a water flow into the recipient. The flow rate is governed by the hydraulic
conductivity of the Boom Clay in the vicinity of the filter screen. It is important to
note that by applying this in situ pore water extraction technique only the free or
unbound pore water can be obtained and collected. Solutes and colloids (such as
mobile organic material) will simultaneously flow out under the condition that they
are small enough to pass through the hydrodynamic tortuous network of water
conducting pores. Therefore, this in situ pore water extraction technique has to be
considered as a kind of ultra-filtration technique in which the filtration pressure is
equal to the in situ pore water pressure, and, in which the effective pore size of the
Boom Clay, in the vicinity of the piezometer, governs the filtration efficiency. Such
ultra-filtration efficiency is expressed by the "Molecular Weight Cut Off" number
(MWCO). It is questionable whether a fixed piezometric-MWCO number can be
given. The reason for this is that the effective pore size can not always be considered
as constant. Mean pore size around each piezometer can differ from location to
location and is likely to be altered by the Excavation Disturbed Zone (EDZ) of
HADES (caused during the excavation of the underground laboratory), and, the
excavation disturbed zone around each individual piezometer (caused by the borehole
drilling). The piezometer EDZ arises from the necessity to drill an oversized borehole
and the consecutive convergence of the Boom Clay during piezometer installation. A
supplementary pore size reduction might occur during the piezometric pore water
sampling itself: if a piezometer is opened, the water pressure drops to atmospheric
pressure, leaving only the lithostatic pressure to act on the clay skeleton. In other
words, the drop of pore water pressure leads to an increase of the effective stress if
one assumes that the total stress will remain equal. This imposes a squeezing effect
onto the clay fabric that yields to lower pore sizes. Besides these engineered
perturbations on piezometric pore water extraction, the natural spatial variability of
Boom Clay mineralogy might also influence the effective pore size linked to each
specific piezometric filter screen.
2.1.2 Squeezing
The technique applied in this study is mechanical squeezing. Pore fluids are pressed
out of saturated Boom Clay cores by mechanical pressure (Figure 2-2). Squeezing is
analogous to the natural process of consolidation, caused by the deposition of material
during geological times, but at a greatly accelerated rate. Mechanical squeezing is a
widely used technique for the extraction of pore water from low-permeability clays
(Sacchi and Michelot, 2000).
The squeezing technique and its application to clayey sediments were studied amongst
others at the British Geological Survey (Reeder et al., 1998, 1999). Part of the study
was performed on the Boom Clay (Reeder et al., 1992, 1994, see also Sacchi and
Michelot, 2000). The squeezing technique was further studied in the "Natural
analogue study on Boom Clay" (De Craen et al., 2000), and in the study "Natural
evidence on the long-term behaviour of trace elements and radionuclides in the Boom
Clay" (De Craen et al., 2001).
To prevent oxidation of Boom Clay cores by oxygen in air, sample preparation is
always performed in anaerobic conditions, in a nitrogen-filled glove box (oxygen
20
level < 10 mg/l). In the glove-box, the samples are first taken out of the aluminiumcoated poly-ethylene sheets, in which they were packed immediately after drilling.
The outer rim of the clay core, which has been inevitably in contact with air during
the drilling, is then removed to eliminate possible effects of oxidation. Subsequently,
the clay core is transferred to the 'squeezing cell', made of stainless steel type 316
(resistant to corrosion and high tensile strength). The sample chamber has a diameter
of 8 cm and a height of 10 cm. The squeezing cell is then removed from the glove box
and is put under a hydraulic press (COMPAC EMAC HP100).
Important changes in the pore water chemistry occur with increasing pressures
(references in Sacchi and Michelot, 2000). Indeed, squeezing experiments on Boom
Clay have indicated that the pore water chemistry remains more or less the same when
a pressure lower than 30-35 MPa is applied, but important changes in the pore water
chemistry were observed when higher pressures were applied (De Craen et al., 2000).
Therefore, it was decided to squeeze all Boom Clay samples with a relatively low and
constant pressure of 30 MPa during one week (for details and arguments see De
Craen et al., 2000). After the squeezing, the water samples were stored at 4 °C before
chemical or radiochemical analyses.
With this technique and the procedure applied, 40 to 50 millilitres of water is
generally collected out of about 700 g wet clay. This is about 30 to 35 % of the total
water content.
Note: As mentioned above, samples for pore water characterisation were vacuumpacked in aluminium-coated poly-ethylene sheets and stored in a nitrogen-filled glove
box (oxygen level < 10 mg/l), or in PVC tubes filled with argon (to prevent oxidation
as much as possible) at 4 °C. Clay cores which were vacuum-packed in aluminiumcoated poly-ethylene sheets but stored in air and room temperatures, are often
oxidised, resulting in an unrepresentative pore water composition (De Craen, 2001;
De Craen et al., 2002a, De Craen et al., 2003).
Figure 2-2: Set-up for the extraction of pore water by mechanical squeezing of clay
cores.
21
2.1.3 Leaching
A leaching method involves batch experiments and modelling of equilibrium
chemistry of the resulting extracts. Different from a piezometric or a mechanical
squeezing technique in which pore water samples are collected directly, a leaching
method add a leachant to the clay sample, hence leads to a dilution and processes
associated with it. Bradbury and Baeyens (1998) developed a method by which they
determined the soluble salt concentrations of Oplinus clay by the dilution factor using
different solid to liquid ratios (S/L ratio) and the cation concentrations through cation
exchange and mineral solubility equilibriums. In this report, we follow the general
idea of the method but also study the effect of colloids, which is a distinguished
characteristic of Boom Clay. Moreover, the interpretation of our results opens some
new aspects and demonstrates potential differences between Oplinus Clay and Boom
Clay.
The experimental procedure is illustrated in Figure 2-3. Boom Clay samples from the
HADES 2001/4 borehole were grinded, suspended, and agitated in NaHCO3 solution
of 0.01 M. The bicarbonate solution was used as the leachant because Boom Clay
pore water is basically a dilute NaHCO3 solution. Some samples were suspended in
distilled water for comparison. Experiments were conducted in glove boxes to protect
the clay samples from oxidation. The oxygen content of the glove boxes is about 2
ppm but generally below 10 ppm. For practical reasons, some experiments were
carried out in an Ar glove box and some others in an Ar/CO2 (g) glove box. The CO2
content in the glove box was 0.4 percent to mimic the supposed in situ partial pressure
of CO2 (10-2.4 atm). Four different S/L ratio were used: 25, 50, 200, and 800 gram wet
clay per litre of solution. The leaching duration was 2 to 3 months in which a steady
aqueous concentration of the major ions were reached. After the leaching, samples of
the suspension were centrifuged at 21,255 g for 2 hours before the chemical analysis
for major cations and anions. Some samples were further filtered by 0.45 µm filters
and YM3 (3000 MWCO) centriplus ultrafilters to study the possible effect of clay
particulates or colloids on the concentration of clay water components.
solution: NaHCO3 (0.01 M) or water
chemical analysis for
cations and anions
clay
centrifugation
supernatant
filtration
(0.45 µm)
ultrafiltration
(3000 MWCO)
Figure 2-3: Shematic presentation of the experimental procedure of pore water
extraction by leaching.
22
2.2 Analytical techniques
2.2.1 pH, Eh, pCO2, electrolytic conductivity and dissolved gases
2.2.1.1 pH
Three analytical principles exist to determine pH-values: colorimetric,
electrochemical and electronic (Omega Engineering, Inc. 2001, Meier et al., 1989;
Poghossian et al., 2002). The colorimetric principle relies on a detectable colour
change of a dye as function of the varying hydrogen concentration. The
electrochemical principle makes use of a measurable electrical potential change of a
pH sensitive electrode, as function of pH variations. The electronic principle relies on
a signal change in an Ion Sensitive Field-Effect Transistor (ISFET) if the hydrogen
concentration of a sample changes. As the latter principle is a fairly new one that still
needs a lot of scientific development, no analyses based on this principle are
considered in this report.
The simplicity of the colorimetric principle is its main advantage, but colorimetric
techniques suffers from a lack of accuracy: ± 0.1 pH units on the measurement is the
best accuracy that can be achieved (Characteristics of a Fiber Optical pH meter, DBE
Technology GmbH). A second important drawback of colorimetric methods is that
they are often difficult to interpret and this especially in coloured samples (e.g. colour
changes in coloured samples, as in Boom Clay pore water). Therefore, the use of
colorimetric principles can only give a rough indication of the pH-value (Omega
Engineering, Inc. 2001 pH Technical reference guide).
The electrochemical principle, however, is scientifically recognised as being an
accurate, robust and reliable principle, that is applicable under almost every physicochemical measurement condition. To illustrate this, it is worthwhile to mention that
the U.S. Geological Survey only approves pH determinations that are based on this
measurement principle (Radtke et al., 2003). Therefore, the pH of Boom Clay pore
water is generally measured with electrochemical based pH electrodes. A drawback of
the electrochemical principle is the leaching of salt from the pH electrode, more
specifically the reference electrode which is normally build in a pH electrode. For
environmental reasons, the commonly used salt to stabilise the reference signal is
potassium chloride. In aqueous samples this salt is inert and does not influence the pH
of the solution. However, when measuring soil samples, the leached potassium
chloride can disturb the pH measurement through secondary effects like: ionexchange, suspension effect and varying solid–liquid ratios. As these three effects
counteract each other, it is sometimes difficult to quantify the overall effect.
An electrochemical pH measurement system consists of three parts: a pH indicator
electrode, a reference electrode and a mV/pH meter. For each of these three
components, numerous models and types exist. The choice of the complete pH
measuring system depends on the specific measurement requirements: nature of the
samples (aqueous, organic, semi solid, ...), sample conditions (temperature, pressure,
electrolytic conductivity, ...) and system demands (laboratory or in situ measurement,
batch or on line measurement, desired response time, electrode stability, data). From
the measurement requirements point of view, laboratory pH measurements of Boom
Clay pore water samples are not very demanding. Typically the samples are aqueous,
not pressurised, at room temperature and possess sufficient electrolytic conductivity to
be measurable (see Section 2.2.1.4). However, to determine the true in situ pH value
of Boom Clay pore water, laboratory measurements are questionable for two reasons:
23
the low pH buffering capacity and the unavoidable physico-chemical perturbations of
Boom Clay pore water (e.g. loss of carbon dioxide through degassing, perturbation
through oxidation of dissolved organic matter, ...). The physico-chemical
perturbations appear at three stages: during the sample collection, when the samples
are manipulated and during the time of pH-measurements in the laboratory. Because
of the perturbations associated with laboratory pH measurements of Boom Clay pore
water, the pH values reported in most earlier publications or reports were not always
representative of the real in situ pH.
Because laboratory pH measurements fail to render the true in situ pH value of Boom
Clay pore water, pH measurement under undisturbed physico-chemical conditions are
imperative. However, the requirements for such an in situ pH measurement system are
very demanding. The main idea is that the integrity of Boom Clay pore water has to
be maintained during the in situ pH measurements. To realise this, the following
measurement requirements are needed: measuring under in situ hydraulic pressure to
maintain pressure depending dissolution chemistry (e.g. calcite solubility, dissolved
gasses, ...), avoiding contact of the Boom Clay pore water with the atmosphere to
prevent gas exchanges (e.g. CO2 loss, O2 intrusion, ...), and, monitoring of the pH
evolution to determine the moment of geochemical equilibrium and to quantify
reaction kinetics.
The first meaningful attempts to measure in situ pH values were made during the
CERBERUS project (Beaufays et al., 1994). In this project, the pH measurements
were done with electrochemical pH-electrodes screwed in a non-pressurised heated
flow-through cell. Also during the ARCHIMEDE-argile project (Beaucaire et al.,
2000; Griffault et al., 1996) in situ pH measurements have been done using a Fiber
Optical pH measurement system, based on colorimetric principles. As mentioned
above the inaccuracy of such systems is larger then ± 0.1 pH units, and this, even
without taking into account the errors linked to the disturbed geochemistry
(temporarily and locally) of the Boom Clay pore water inside the piezometer, due to
the opening of the piezofilter waterline to introduce the optical pH-fibre. Since 1996,
sound efforts have been made to implement glass electrode technology for in situ
measurements of pH (De Cannière et al., 1997).
The installation of the ORPHEUS experiment in 2000 (Moors et al., 2002) was the
result of the best available technology and gained expertise to fulfil the requirements
for representative in situ pH measurements. The ORPHEUS set up (see Figure 2-9) is
composed of a flow-through cell equipped with robust and rugged solid-polymer
filled pH electrodes (Xerolyt®, Mettler-Toledo). The flow-through cell is placed in a
closed circuit configuration between the inlet and outlet water pipe of a piezometric
filterscreen. The closed circuit configuration maintains dissolved gas-equilibria. The
polymer filled Xerolyt-electrodes ensure long term stability and combine this quality
with the accuracy and reliability of common electrochemical glass electrodes. A
circulation pump (Milton Roy Solenoid Diaphragm Metering Pump) provides water
circulation inside the closed circuit. This water circulation ensures the contact of the
Boom Clay pore water with the Boom Clay solid phase and the pH electrode. As the
system is closed, geochemical equilibrium of the circulating Boom Clay pore water
with the surrounding solid Boom Clay will be reached after a certain time. A data
acquisition system monitors the evolution of pH measurements and helps to indicate
when geochemical equilibrium is reached. A controlled-atmosphere cabinet (argon
flushed) hosts the pump and flow through cell of the pH measurement system. This
controlled-atmosphere cabinet provides additional protection of the experimental set
24
up against unwanted gas interactions. At the end of a measurement campaign, the
collected measurements are corrected for any electrode drift (see Annex 3 for details
on the electrode drift test). As mentioned above, the inevitable leaching of potassium
chloride when using glass electrodes can cause secondary effects that can cause an
overall effect on the measured pH value. Chemical analyses and geochemical
modelling might aid to correct the measurements, not only for electrode drift but also
for geochemical drift.
2.2.1.2 Eh
To some extent, a similar reasoning as for pH measurements goes for redox
measurements. However, the correct interpretation of measured redox potential values
is far more complicated. Basically any redox reaction can be represented by the
following equilibrium reaction:
i red
Ox + ne − ↔ Red
i ox
Every oxidation and reduction reaction generates a current flowing in opposite
directions (ired and iox). These currents (sum of both is called exchange current) have
to be strong enough so that they are measurable (detectable) with a redox
measurement system (Schüring et al., 2000).
Colorimetric redox indicators are not sensitive enough to determine the redox state of
natural waters. The only accepted measuring principle to measure Eh in natural waters
is the electrochemical principle. And, even with measuring systems based on this
principle, it is questionable whether or not the measured values represent the true
redox state (Christensen et al., 2000).
A classical electrochemical redox measurement consists of three parts: a redox
indicator electrode, a reference electrode and a sensitive millivolt meter. The ideal
requirements for each of these components are the following: a reference electrode
has preferentially following characteristics: its potential does not depend on changing
redox conditions, it has no internal electrical resistance, and, it is in perfect electrical
contact with the media in which it is submersed. The indicator electrode should have
following characteristics: it behaves absolutely inert (i.e. it does not participate to any
of the on going redox reactions inside the medium), on the other hand, it catalyses
every redox reactions at its surface, and, has no double layer or coating that might
limit the passing of the exchange current. The redox meter which is electrically
connected to the electrodes must have an extremely high input resistance, and no
voltage offset. As for pH measurement systems, from each of these three components,
numerous models and types exist. Unlike for pH measurement systems the choice of a
complete Eh measurement system is not solely based on objective criteria. Often, a
compromise between habits and measurement requirements has to be found and is
used. Bearing these facts in mind, it is obvious that analytical redox measurements
will seldom reflect the theoretically calculated values. However, the empirical use of
measured redox values, under the best possible conditions, will always yield valuable
information about the redox-tendency of the measured medium (Christensen et al.,
2000; Nordstrom et al., 2003), in our case the Boom Clay pore water. Combined with
other analytical results and chemical calculations a clear insight of the Boom Clay
pore water redox condition can be obtained.
25
As discussed in Section 2.2.1.1., the reliability of laboratory measurements, in view of
rendering a representative in situ Eh value of Boom Clay pore water, is doubtful.
Again, as for pH measurements, a very low buffering capacity of redox active species
is present and physico-chemical redox perturbations of any sample of Boom Clay
pore water, under laboratory conditions, are unavoidable (possible irreversible redox
reactions, ...). On top of these sample handling problems, the limitations of each
electrochemical redox measurement system complicate the interpretation of
laboratory redox measurements to obtain a representative in situ Eh value for Boom
Clay pore water.
The implementation of redox electrode technology for in situ measurements is, as for
pH-electrodes, far from evident. But the advantage of in situ measurements is to be
able to have measurement conditions which are the closest to the real conditions. This
advantage makes it worthwhile to invest time and work in in situ measurements.
Again, high water pressure and long term electrode stability demand appropriate
precautions and measures to obtain accurate and reliable in situ values of the redox
tendency of Boom Clay pore water. The best solution is the simultaneous use of the
flow-through cell set-up for pH electrodes and redox electrodes. Specifically for redox
measurements, the following extra precautions are foreseen: the anaerobic atmosphere
prevents oxygen perturbation, the exclusive use of polymer or ceramic materials
allows to circulate Boom Clay pore water that never has come in contact with redox
active metal surfaces, and the use of different indicator electrode materials allows
qualitative comparison of the redox measurements.
In contrast with pH and as explained above, just monitoring and mathematically
correcting the measured in situ redox values will seldom yield to representative Eh
values. Nevertheless, in situ redox measurements obtained with an experimental set
up in which all precautions are taken, will render the most representative redox values
of Boom Clay pore water.
To eliminate as much as possible errors linked to the redox measurement system, an
electrode functionality and an electrode drift test (see Annex 3 for details of the tests)
is performed. These tests assure the good functioning of the used redox electrode(s)
during the whole duration of the in situ measurement and, therefore, help with the
correct interpretation of the redox results.
2.2.1.3 pCO2 and other dissolved gasses
The measurement of the partial pressure of carbon dioxide and other dissolved gasses
in argillaceous rocks is a technical challenge. Different methods have been considered
(Henrion et al., 1985). The partial pressure is either calculated based on the relation of
pCO2 with the measured pH and the inorganic carbonate chemistry or the partial
pressure is measured based on the physico-chemical principle on which Henry's law
relies. In its simplified form the law of Henry can be written as follows:
pA = xA K A
With pA the partial pressure of solute A in the gas phase, xA the mole fraction of
solute A in the liquid phase and KA a constant called Henry's law constant with the
dimension of pressure. The law of Henry thus states that there exists a linear
relationship between the concentration of a gaseous solute dissolved in a solution and
26
the concentration of the same gaseous solute in the gas phase, in contact with the
solution (assuming that both phases are equilibrated). Thus, it is sufficient to
determine the concentration of a dissolved gas either in the gas phase or in the
solution to know its concentration in the remaining other phase.
The first attempt to measure the partial pressure of CO2 in Boom Clay was done by
Henrion (Henrion et al., 1985). He assumed, in analogy with common CO2
measurements on frozen pieces of pole ice, that a large piece of frozen Boom Clay
will keep its initial content of CO2. The opportunity to sample a piece of frozen Boom
Clay was given during the construction of the underground laboratory. To be able to
excavate the first access shaft of the HADES-laboratory, the surrounding soil
(including a huge Section of the Boom Clay) had to be frozen. From this frozen Boom
Clay, a large piece was sampled. This piece of frozen Boom Clay was put in a
aluminium-lined sample bag. The sample bag was immediately sealed off to prevent
escape of gasses. After unfreezing, the CO2 content of the gas atmosphere inside the
gastight sample bag was measured. From this measurement the partial pressure of
CO2 was determined assuming the relationship with the dissolved carbonate system
(CO2, CO2(aqua), H2CO3*, HCO3-, CO32-).
Because of the significance of the dissolved carbonate system (e.g. pH controlling
mechanism, ...) in relation to radionuclide migration and speciation, the knowledge of
representative in situ pCO2 values for Boom Clay pore water has become a priority.
This knowledge is also essential for geochemical modelling and to evaluate the
response of Boom Clay towards changing environmental conditions (e.g. alkaline
plume, sodium nitrate perturbation, ...).
The best solution to obtain representative in situ values for pCO2, is to use a set up in
which the experimental boundaries are imposed and controlled by real in situ
conditions. Because, under these conditions no gas phase is present in Boom Clay
pore water and because it is easier to measure CO2 in a gas phase, an artificial gas
phase has to be introduced which can equilibrate with the Boom Clay water. A
schematic presentation of an experimental set up is given in Figure 2-4. In this set up
Henry's law is valid: a gas phase is present by creating a static inert (argon) gas
bubble in the barrel, and equilibrium can be obtained by circulating the Boom Clay
pore water from the barrel towards a piezometric filterscreen and vice versa. In the
pores of the piezometric filterscreen the circulating Boom Clay pore water will
continuously re-equilibrate with the Boom Clay formation and its minerals. Inside the
barrel the circulating Boom Clay pore water will equilibrate with the static gas phase
by Henry's law principle. Periodically, the gas phase will be analysed for CO2
measurements. After a certain time, it is expected to measure stable carbon dioxide
concentrations. From these CO2 concentrations and the total gas pressure, the pCO2
can be determined. Stable isotope composition analysis (δ 13C and/or δ 18O) to try to
determine the origin of the CO2 will be considered after pCO2 quantification.
27
Gas-phase
sampling
Pinlet
Poutlet
level
Water + dissolved gasses
Figure 2-4: Schematic representation of the experimental setup for pCO2
measurement.
The same set up allows the simultaneous determination of all other dissolved gasses,
in Boom Clay pore water, under the condition that they are present in measurable
quantity. It is expected to find gasses such as nitrogen (N2), Hydrogen Sulfide (H2S),
Helium (He), alkanes (Methane (CH4), Ethane (C2H6), ...) and maybe some other
volatile organic components. At first the gas samples will be analysed with a massspectrometer as most of the molecule masses of the expected gasses differ
significantly.
2.2.1.4 Electrolytic Conductivity (EC)
The electrolytic conductivity (EC) of a solution is proportional to the quantity of
dissolved ions (charge carriers) present in the Boom Clay pore water. The obtained
value provides also a very good idea of the Total Dissolved Solids present in Boom
Clay pore water.
A conductivity cell connected with a conductivity meter is the standard analytical
technique which is used to measure Boom Clay pore water conductivity. The
conductivity cell consists of a glass support on which two or more platinum bands are
fixed. These platinum bands are coated with platinum black to limit polarization
effects. Any conductivity cell is characterized by a cell constant to allow
normalization and standardization of different measurement systems. The
conductivity meter is in fact a power supply which generates an alternating current
(AC) potential onto two platinum bands of the conductivity cell. The measured
current is proportional to the conductivity or EC-value of the liquid in which the cell
is submersed.
An EC measurement serves more as a quality indicator rather then as an analytical
parameter. The reason for this is that it is a bulk parameter from which only limited
analytical information can be gathered.
28
2.2.2 Other techniques
2.2.2.1 The total organic carbon content (TOC)
The total organic carbon content in Boom Clay (TOC) is measured with a hightemperature TOC analyser. The total carbon content (TC) is determined by injection
of an aliquot of the sample, without any pre-treatment, in a combustion tube at 680
°C. Catalytic oxidation transforms the TC of the sample completely to CO2 and H2O.
After drying, the CO2 concentration is measured by a non-dispersive infrared detector.
The inorganic carbon content (TIC) is determined by injection of an aliquot of the
sample, without any pre-treatment, in a TIC reactor, which contains acidified water
(20% H3PO4) at room temperature. In this acidic environment, all forms of IC are
purged out of the solution as CO2. After drying, the CO2 concentration is measured by
a non-dispersive infrared detector. The total organic carbon content (TOC) is made by
the difference of TC and TIC.
TOC measurements were performed with a Dohrman DC-190 High Temperature
TOC analyser at the Laboratories of Geological Disposal, Waste and Disposal
Department, and Analyses and Applied Radiochemistry, Nuclear Chemistry and
Services, SCK•CEN, Belgium.
2.2.2.2 UV/VIS spectrometry
A UV/VIS spectrometer is used for the characterisation of Dissolved Organic Carbon
(DOC). The instrument used is a PERKIN ELMER, Lambda 40 at the Laboratory of
Geological Disposal, Waste and Disposal Department, SCK•CEN, Belgium. Two
radiation sources, a deuterium lamp (UV) and a halogen lamp (VIS), cover the
working wavelength range of the spectrometer. The monochromator used is based on
holographic grating, while the exit slit restricts the spectrum segment to a nearmonochromatic radiation beam. The slits provide a spectral band pass of 0.5, 1, 2 or 4
nm. This monochromatic beam is reflected on a beam splitter, allowing 50 % of the
radiation to pass through the sample cell and 50 % of the radiation to pass through the
reference cell (dual beam). The passed beams continue their way onto a photodiode
detector. Absorbance is normally measured at 280 nm for the determination of DOC
content.
2.2.2.3 The cation concentration (Ca, Fe, K, Mg, Na, Si)
The cation concentration (Ca, Fe, K, Mg, Na, Si) in the pore water is measured by
Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). Samples are
diluted ten times and acidified with 1% HNO3 / 5% HCl; higher dilutions may be
necessary when the concentrations are too high. The diluted samples are filtered
through a 0.45 µm filter. Analysis is performed on a simultaneous ICP-AES with
radial plasma view and SCD (Segmented-array Charged-coupled device Detector).
The results obtained with the principal spectral line of each element are confirmed by
measuring an alternative spectral line. Furthermore, spectral interferences are
accounted for by the use of interelement corrections.
Cation concentrations were measured at the Laboratory of Analyses and Applied
Radiochemistry, Nuclear Chemistry and Services, SCK•CEN, Belgium.
29
2.2.2.4 The anion concentration (F-, Cl -, Br -, HPO42-, NO3-, SO4 2-)
Ion chromatography (IC) is used for the analyses of the anions Cl -, Br -, HPO42-, NO3and SO4 2- in the pore water. For the extraction of the anions, a Dionex AG4A-SC
guard column and a Dionex AS4A-SC analytical column are used. To avoid overload
of the separation column, samples are usually diluted ten times. Higher dilutions may
be necessary when the concentrations are too high. The diluted samples are analysed
by the use of a classical ion chromatograph with a 1.8 mM Na2CO3 / 1.7 mM
NaHCO3 solution as eluent and suppressed conductivity detection.
For the measurement of the concentration of F- in the pore water, the ion selective
electrode (ISE) is used. Samples (and standards) are diluted with TISAB (Total Ionic
Strength Adjustment Buffer), which provides a nearly uniform ionic strength
background, adjusts pH and breaks up complexes. The fluoride concentration is
determined by direct potentiometry using a combined fluoride / reference electrode.
Anion concentrations were measured at the Laboratory of Analyses and Applied
Radiochemistry, Nuclear Chemistry and Services, SCK•CEN, Belgium.
2.2.2.5 The trace element concentration
Trace elements are measured by Inductively Coupled Plasma-Mass Spectrometry
(ICP-MS). The pore water samples are first diluted in 2 % nitric acid short before
analysis. An internal standard is added to improve the precision and to correct for
matrixeffects (mainly signal suppression). During analysis, the sample solution is
nebulised into flowing argon gas and passed into an inductively coupled plasma
where the elements are separated according to mass, detected, multiplied and counted.
Trace element concentrations were measured with an Elan 5000 Perkin Elmer ICPMS at the Laboratory of Analyses and Applied Radiochemistry, Nuclear Chemistry
and Services, SCK•CEN, Belgium.
2.2.2.6 The stable isotope composition
The stable isotope composition is measured by a gas source mass spectrometer. The
methodology first involves the conversion of the element of interest into a gas. In
general, hydrogen is analysed as H2, oxygen and carbon are both analysed as CO2, and
sulphur is usually analysed as SO2. The gas is then purified and introduced into the
mass spectrometer for analyses.
Stable oxygen and hydrogen isotope analyses were determined at the Stable Isotope
Laboratory at BGS, UK, using a VG-isogas dual-inlet twin analyser gas source mass
spectrometer. Determination is carried out on carbon dioxide and hydrogen gases, for
O and H respectively, prepared from the original sample according to the procedures
described by Darling et al. (1992). The mass spectrometer has a dual collector
arrangement for each of the two nuclides and a dual inlet system to permit the rapid
switching between the sample gas and a standard gas with calibrated 18O/16O or
2
H/1H. Data are expressed in ‰ relative to the Standard Mean Ocean Water (SMOW).
The overall method precision is estimated to be better than ± 4 ‰ for δ2H and ± 0.2
‰ for δ18O, based on the repeated determination of independent quality control
standards.
30
Oxygen isotope analyses of sulphate were performed at the Isotope Geoscience Unit
of SURRC, Glasgow, UK, following the procedure reported in Hall et al. (1991).
Combustion of an intimate mixture of the sample plus spectrographically pure
graphite took place in a platinum crucible at a temperature around 1200 °C. Evolved
SO2 gas was then analysed on a VG SIRA II mass spectrometer and standard
corrections were applied to the raw data. Reproducibility is about ± 0.2 ‰.
Sulphate isotope compositions were determined at the Isotope Geoscience Unit of
SURRC, Glasgow, UK. Sulphur isotope analyses of pure BaSO4 precipitates were
carried out according to the standard procedures of Coleman and Moore (1978),
involving combustion at 1120 °C of an intimate mixture of the sample, excess Cu2O
and a pure SiO2 catalyst. Product SO2 gas was analysed on a VG SIRA 10 mass
spectrometer and standard corrections were applied to the data. Data are expressed in
‰ relative to the Canyon Diabolo Troilite (CDT). Reproducibility is about ± 0.2 ‰
for δ34S, based on the repeated analyses of internal and international standards.
2.2.2.7 The radiochemical composition
238
U, 234U, 232Th, 230Th and 226Ra isotope concentrations in Boom Clay pore water
were measured at the Laboratory of the Section Mineralogy and Petrography, Royal
Museum for Central Africa, Tervuren, Belgium. Samples were acidified with HNO3
and an internal standard was added. For the accuracy, U is measured in an internal
standard of water (SLRS 4). Analyses were performed with a Finnigan Element 2
high resolution-inductive coupled plasma-mass spectrometer (HR-ICP-MS). HR-ICPMS is similar to the ICP-MS technique previously described with a few
modifications. The mass spectrometer used for detection is a quadripole followed by a
magnetic sector instead of just a quadripole. The main advantages are that most
interferences can be resolved providing essentially interference-free analysis. The
HR-ICP-MS provides detection limits in the ng/l (ppt) to pg/l (ppq) range. Many
isotope ratios can be determined to better than ±0.1%.
The 14C was studied by UPS – GdR Tandétron, Gif-sur-Yvette, France. 14C
measurements were performed on total dissolved inorganic carbon and total dissolved
organic carbon. 14C measurements on total dissolved inorganic carbon were
performed by acidification of the water sample, followed by reduction of the produced
CO2 to graphite by hydrogen, and measurement of the graphite isotope ratios by
accelerator mass spectrometry. 14C measurements on total dissolved organic carbon
were performed by isolation and purification of the fulvic acids by CuO/Cu2O,
reduction of the produced CO2 to graphite by hydrogen, and measurement of the
graphite isotope ratios by accelerator mass spectrometry.
31
2.3 Sample locations
In order to study the Boom Clay pore water composition and the in situ geochemical
conditions, several specific piezometers are considered:
ƒ
EG/BS
Extension Gallery Bottom Shaft
ƒ
ARCHIMEDE
Acquisition et Régulation de la Chimie des Eaux en milieu
argileux.
ƒ
SPRING 116
Source Piezonest at RING 116 of the Test Drift gallery
ƒ
ORPHEUS
Oxidation Reduction Potential and pH Experimental
Underground Station
ƒ
MORPHEUS
Mobile ORganic matter and Pore water extraction in the
Hades Experimental Underground Site
These piezometers are all installed in the HADES URF (Figure 2-5). They were
typically designed either for the sampling of Boom Clay pore water, or for the
measurement of geochemical parameters such as the pH and the Eh. A detailed
description of the various piezometers is given below and summarised in Annex 1.
The many other piezometers in the HADES URF are not described here, since they
are not used for geochemical purposes.
Each pore water sample remains a code in which the name of the piezometer, filter
material, sample number and date of sampling is included (see Annex 1).
Figure 2-5: Location of the piezometers considered in this report, in the HADES
underground facility (Dimensions are not to scale).
In Belgium, the reference level is: "Tweede Algemene Waterpassing" (TAW). It is
important to note that for historical and practical reasons TAW is not always used.
Some depths-values use the earths surface (Below Surface Level, BSL) or drilling
table (Below Drilling Table, BDT) as reference level. For the HADES URL Shaft 1,
the conversion between TAW and BSL is: TAW = BSL + 25.60 [m]. Also for
32
practical reasons, distances relative from gallery lining are used. The indication given
in this case is: distance "Extra DOS", referring to distance relative to the outside
surface of the lining.
2.3.1 EG/BS piezometer
The EG/BS (or EGBS/2; Neerdael, 1984) piezometer was installed in September
1983. It is one of the oldest piezometers still in operation. The initial goal of this
piezometer was to collect large amounts of pore water. The EG/BS piezometer is a
vertically orientated piezometer located at the bottom of the First Shaft. Figure 2-6
shows a schematic view of the complete piezometer-construction at its location.
Coarse sand (0.71 – 1.25 mm) was used to enhance the water-draining capabilities of
this piezometer. The stainless steel cylindrical filter screen of this piezometric set up,
has a length of 60 mm and a diameter of 56 mm. It is made from high porosity
seamless filter tube made by "Krebsöge", quality: SIKA R5, material: 1.4404 (AISI
316 L/B), pore size distribution: 7 to 16 µm. The coarse sand column, in which the
stainless steel filter screen is placed and centralised, is about 13 m long (260 m to 247
m BSL) with a diameter of only 85 mm. This coarse sand column hydraulically
interconnects the pore water from the Boom Clay between septaria-levels S40 and
S60. This large range encompasses also the silty "double band" of which the hydraulic
conductivity is two to three times higher then for the rest of the Boom Clay (Wemaere
et al., 2002). Physically and chemically, the water collected with this EG/BS
piezometer will be proportionally influenced by the characteristics of the "double
band". The reason of including this piezometer into the current report is that it is the
piezometer of which the most diverse analyses are available and this over a period of
more than 20 years. Major disadvantage for geochemistry are the influences of the
coarse sand backfill material, the bentonite top-cover seal and the presence of the
"double band".
COARSE SAND: 0.71 – 1.25 mm
Figure 2-6: Schematic drawing of the complete EG/BS (EGBS/2) piezometerconstruction, shown at its location on the bottom of the First Access Shaft. Depth in
BSL. (from Neerdael, 1984).
33
2.3.2 ARCHIMEDE piezometers
In the frame of the ARCHIMEDE-argile project, in which the better understanding of
the Boom Clay pore water chemistry was the main objective, several piezometers
were installed in the Boom Clay. The ARCHIMEDE #1 piezometer was installed in
March 1992 in the ANDRA gallery of the HADES URF, between sliding ribs 24 and
25. It is a semi-horizontal piezometer (3% inclined upwards) oriented towards the
east. It is 15 m long with a diameter of 60 mm. This piezometer is used for the
sampling and chemical analyses of Boom Clay pore water. The ARCHIMEDE #2
piezometer was installed in April 1992 in the ANDRA gallery of the HADES URF,
between sliding ribs 4 and 5. It is also a 15 m long horizontal piezometer oriented
towards the east, but with a diameter of 140 mm. This piezometer is used for in situ
pH measurements. Both piezometers are entirely composed of stainless steel and
contain five filter screens, also composed of stainless steel. A schematic view of the
location of the ARCHIMEDE piezometers is shown in Figure 2-7.
Figure 2-7: Schematic presentation of the location of the various boreholes and
piezometers in the frame of the ARCHIMEDE-argile project (from Griffault et al.,
1996)
34
2.3.3 Spring 116
Because of the presence of coarse sand around the EG/BS piezometer, the EG/BS
piezometer probably provides Boom Clay pore water which is geochemically
disturbed. Therefore, a new piezometer equipped with large filter screens has been
installed in the HADES laboratory in October 1999. This piezometer is called
SPRING 116. The purpose of this piezometer is to provide sufficient quantities of
representative Boom Clay pore water as feed and reference material for laboratory
experiments. The piezometer is placed horizontally in the Boom Clay, and is located
in the Test Drift part of the HADES URF, at ring 116, pointing towards the east. The
SPRING 116 piezometer is entirely made of stainless steel and contains in total four
large surface filter screens. These are made from high porosity seamless filter tube
made by "Krebsöge", quality: SIKA R5, material: 1.4404 (AISI 316 L/B), pore
size distribution: 7 to 16 µm. A schematic presentation is given in Figure 2-8. The
spring 116 piezometer consists of four filter screens with the following dimensions:
an outside diameter of 149 mm, an inside diameter of 142 mm, and a length of 1500
mm. The first screen starts at about 5 m (Extra Dos) and the last deepest screen ends
at 12 m (Extra Dos). Although the four different filters can be sampled separately, in
total, about six meters of filter screen is placed horizontally in the Boom Clay (see
Figure 2-8. The hydraulic interconnections of this substantial length of Boom Clay
results in a high water flow (approximately 350 ml per day) and, consequently, the
capability of collecting large amounts of Boom Clay pore water.
Figure 2-8: Schematic presentation of the SPRING-116 piezometer, with four large
filter screens of 1500 mm close to each other.
2.3.4 ORPHEUS piezometer
The ORPHEUS piezometer was installed in November 2000 at ring 116 of the Test
Drift gallery. This piezometer is horizontally oriented towards the west (installed just
in front of the SPRING 116 piezometer). Figure 2-9 shows a schematic overview of
the ORPHEUS setup. This figure visualises the goal of the ORPHEUS experiment,
which is to study in situ the geochemistry of undisturbed Boom Clay pore water. For
this purpose, the piezometer of ORPHEUS is the first that is entirely constructed out
of polymer based and metal free materials. With this construction, Boom Clay pore
water has never been in contact with metal surfaces, and, at the same time it is not
oxidised since it can be sampled under protective atmosphere conditions. To evaluate
the influence and constructability of different metal free porous materials, the
35
ORPHEUS piezometer contains four filter screens composed of four different
materials:
ƒ Sintered glass
ƒ Polyethylene
ƒ Carbo
ƒ Schumatherm
Each of these filter screens is 250 mm long with a fixed outside diameter of 120 mm.
The inside diameters vary little from each other due to the four different fabrication
processes. The mean pore sizes of the materials are: for glass between 10 and 16 µm
(porosity-class P4), for Polyethylene 40 µm (Filtroplast 40), for carbon 90 µm (Carbo
40), and, for Schumatherm 60 µm (Schumatherm 30). A data-sheet of the used filter
materials can be found in Annex 2. Every filter screen is equipped with two PEEK
(Poly-Ether-Ether-Keton) water pipes, 1/8" diameter, to conduct the pore water into
the controlled atmosphere cabinet (anaerobic environment). PEEK is chosen as
material for water pipes because it has an extreme low oxygen permeability (oxygen
permeability coefficient = 8.3 x 10-19 [m.s-1.Pa-1] which is at least four orders of
magnitude lower then any other polymer tubing material), which guarantees the best
possible protection of the Boom Clay pore water against oxidation. As extra
precaution, the free space inside the piezometer supporting tube and all voids around
the water pipes have been (back)filled with a hard type protective polymer (Stycast
W19).
pH
Eh
DAQ
P
Cabinet with
controlled
atmosphere +
flow-through cell
with electrodes
Shumatherm
filter
Carbo
filter
Poly
Ethylene
filter
Sintered
Glass
filter
Figure 2-9: Schematic presentation of the ORPHEUS set up and its piezometer. The
four different filter materials are clearly visualised by their specific colour.
2.3.5 MORPHEUS piezometer
The MORPHEUS piezometer, installed in May 2001 in the HADES laboratory (Test
Drift between ring 11 and 12), is a vertically oriented piezometer designed to study
the variability of the Boom Clay pore water composition underneath the HADES
laboratory. In contrast to the other vertically oriented piezometer EG/BS, the
MORPHEUS piezometer allows pore water sampling at 12 distinct stratigraphic
levels of the Boom Clay, including the level of the "double band". Each filter of this
36
multipiezometer is only 10 cm long. In view of studying the geochemistry of Boom
Clay pore water, MORPHEUS is an unique piezometer, that allows the sampling of
pore water from separate Boom Clay layers such as: organic rich layers, carbonate
rich layers, more silty layers, ... . Figure 2-10 gives a schematic view of this
piezometer positioned next to the Boom Clay layering.
All the porous filter screens of this piezometer are made out of "Schumatherm" filters
(see Annex 2 for material specification). Schumatherm is used for its chemically inert
characteristics. The mean pore size of the used Schumatherm is 60 µm. The filter
screens are mounted onto a PVC supporting tube and are all equipped with two water
pipes. For economic reasons, the water pipes are made of nylon and not of PEEK. The
nylon water pipes lead the water into teflon coated stainless steel sample cylinders.
Although the pore water collected from MORPHEUS is assumed to be less protected
against oxidation, the setup still guarantees the sampling of Boom Clay pore water
with only limited geochemical disturbances.
m
Extra DOS
15
Boom Clay
lithostratigraphy
m
TAW
MORPHEUS
piezometer
-212.27
S70
F23
20
-217.27
S61
F20
S60
25
F18
-222.27
F15
S50
30
-227.27
F13
F12
F10
Putte
Member
DB
35
White bands: clayey layers
F6
Grey bands: silty layers
Black bands: septaria layers, indicated by Sxx
-232.27
S40
DB: double band (two very silty layers)
F4
F2
Terhagen
Member
40
F9
F8
Filters in the MORPHEUS piezometer
indicated by Fxx
-237.27
1m
Figure 2-10: View of the MORPHEUS piezometer and its relative positioning towards
the Boom Clay layering.
37
3
Boom Clay pore water composition and characteristics
3.1 Chemical composition
According to Henrion et al. (1985), the Boom Clay pore water in Mol is equivalent to
a 1.25 g/l NaHCO3 solution, rich in humic acids with a large molecular size spectrum,
and a CO2 pressure of 10-2.5 atm. Because of the ionic mobilities in compacted Boom
Clay larger than 10-10 m² sec-1 (Henrion et al., 1985), and the low hydraulic
conductivity in the order of magnitude of 10-12 m sec-1 (De Cannière et al., 1996), it is
assumed that the pore water is in equilibrium with the solid phase and that the pore
water composition is constant throughout the thickness of the formation.
The last few years, several new piezometers were installed in the Boom Clay,
providing pore water at different locations in the clay. A lot of pore water samples
were taken and analysed, resulting in a large dataset. This enabled us to study the
variability of the pore water chemistry, and to define a reference Boom Clay pore
water composition at the Mol site.
3.1.1 Major elements
The ionic composition of Boom Clay pore water, sampled from various piezometers
in the HADES URF, Test Drift, is given in Table 3-1. For each piezometer, the
minimum, maximum, mean, and median values of the ionic concentrations in Boom
Clay pore water are given. The same data is visualised in Figure 3-1.
Note that the various piezometers are designed differently, and that they are not all
composed of the same filter materials (see Section 2.3 Sample location). Moreover,
the filters are often positioned in different layers of the Boom Clay.
•
For the EG/BS piezometer, 74 analyses were considered.
(1 filter)
•
For the ARCHIMEDE piezometer, 14 analyses were considered
(4 filters, 2 to 5 analyses per filter)
•
For the SPRING 116 piezometer, 8 analyses were considered.
(4 filters, 2 analyses per filter)
•
For the ORPHEUS piezometer, 14 analyses were considered.
(4 filters, generally 4 analyses per filter).
•
For the MORPHEUS piezometer, 44 analyses were considered.
(12 filters, generally 4 analyses per filter).
For most of the elements, the variation of concentration within each piezometer is
limited to a few mg/l (Table 3-1). Also, when comparing the five piezometers, some
small variations can often be observed from one piezometer to another.
Up to now, we can conclude from Figure 3-1 that the general Boom Clay pore water
composition can be considered to be comparable in the various piezometers.
However, a detailed discussion on the variability of the Boom Clay pore water
composition will be given further in this report (Section 3.4 statistical analysis).
38
Table 3-1: Chemical composition of Boom Clay pore water sampled from various piezometers. (Min.=Minimum, Max.=Maximum,
Med.=Median, n/a= not analysed)
EG/BS
mg/l
ARCHIMEDE #1
SPRING 116
Min. Max. Mean Med. Min. Max. Mean Med. Min.
ORPHEUS
MORPHEUS
Max. Mean Med. Min. Max. Mean Med. Min. Max. Mean Med.
Ca
3.1
5.4
3.9
3.9
1.6
2.7
1.8
1.8
2.2
4.7
2.8
2.3
2.4
4.6
2.9
2.7
1.3
3.0
1.9
1.8
Fe
0.5
4.6
1.1
1.0
0.1
0.2
0.2
0.2
0.3
1.2
0.6
0.5
0.1
0.3
0.2
0.2
0.1
1.6
0.3
0.2
Mg
1.7
4.0
3.0
3.0
1.0
1.7
1.5
1.5
1.9
4.0
2.5
2.1
1.9
3.7
2.6
2.5
1.1
2.8
1.7
1.6
K
5.2
13.1
9.3
9.4
7.0
10.2
8.1
7.8
8.7
12.0
10.3
10.1
8.7
10.2
9.4
9.0
6.2
9.7
7.5
7.5
Si
1.2
21.2
3.0
2.3
3.4
4.8
4.1
4.2
2.6
4.5
3.5
3.5
5.0
10.3
6.7
5.6
4.1
20.8
5.5
4.6
Na
390
467
410
410
269
290
280
279
298
390
332
320
290
330
309
300
340
440
364
360
F
-
2.7
4.3
3.2
3.1
0.2
3.0
1.6
1.7
2.6
2.9
2.7
2.7
2.8
3.0
2.9
2.9
2.4
3.5
2.9
2.8
-
23.5
34.8
26.0
25.9
17.0
18.4
17.6
17.7
18.5
24.8
20.4
19.3
20.1
25.9
22.6
22.0
22.8
30.1
25.4
25.0
-
Br
0.5
0.8
0.5
0.5
n/a
n/a
n/a
n/a
0.4
0.7
0.5
0.5
0.4
2.0
1.2
1.2
0.4
0.7
0.6
0.6
SO42-
0.3
1.5
0.6
0.4
2.1
4.8
3.4
3.3
<0.25
4.2
1.3
0.6
1.1
7.7
3.7
3.7
0.4
5.6
1.3
0.8
HCO3-
728
4425
1048
871
702
763
731
729
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
835
1092
909
887
TIC
143
871
206
171
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
164
215
179
175
DOC
78
160
96
89
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
97
263
141
124
Cl
39
Figure 3-1: Chemical composition of Boom Clay pore water sampled from various
piezometers, visualised in box plots. The box has lines at the lower quartile, median
and upper quartile values. The whiskers are lines extending from each end of the box
to show the extent of the rest of the data, with a maximum length of 1.5 times the
interquartile range. Outliers are data with values beyond the end of the whiskers and
plotted with a +.
40
The chemical composition of Boom Clay pore water sampled from the MORPHEUS
piezometer has been compared to pore water obtained by the squeezing and by the
leaching of clay cores. Clay cores from the HADES 2001/4 drilling (in which the
MORPHEUS piezometer was installed) were selected at the same stratigraphical
depths as the piezofilters to allow comparison of the pore water composition.
As an example the results from the MORPHEUS filter 18 and the corresponding clay
core are given in Table 3-2 and Figure 3-2. Note that pore water sampled from the
MORPHEUS piezometer is not filtered prior to analyses. This is because earlier
analyses have indicated that filtration of the pore water with a 0.45 µm filter does not
influence the analytical results. In contrast, pore water obtained by the squeezing of
clay cores is always filtered over a 0.45 µm filter before analyses. For F 18, the pore
water obtained by the leaching of clay cores is filtered by both 0.45 µm and YM3
(3000 molecular weight cut off) filters. These results are included in Table 3-2. For
the cations Ca, Fe and Mg, large differences in concentrations were observed after
filtration at different pore size of the filters, suggesting the presence of colloids. For K
and Si, no colloids prevail so species are present as truly soluble forms. For the anion
species, filtration with YM3 did not influence the measured concentrations. The
procedure to derive the leaching concentrations is given in Annex 4 at the end of this
report.
Table 3-2: Comparison of the Boom Clay pore water composition sampled from the
MORPHEUS piezometer filter 18 and from the squeezing and leaching of clay cores
at the corresponding stratigraphical depth (sample H 18, -222.625 m TAW).
HADES 2001/4
Squeezed clay cores
H 18
HADES 2001/4
Leaching experiments
H 18
mg/l
MORPHEUS
Piezometer
Filter 18
0.45 µm filtration
0.45 µm filtrat.
YM3 filtrat.
Ca
Fe
Mg
K
Si
Na
1.3
0.2
1.4
7.2
4.4
340
2.6
0.4
1.5
5.2
7.2
254
1.5 – 7.6
0.35 – 2.8
1.3 – 4.3
7.4
3.4
-
1.3
< 0.05
1.6
7.6
3.8
-
FClBrSO42TIC
DOC
2.8
24.8
0.6
3.6
2.4
26.6
0.7
7.0
23.6
16.8
< 1.9
16.8
170
110
112
74
120 (fixed by 0.01 M NaHCO3)
2708
41
Figure 3-2: Comparison of the Boom Clay pore water composition sampled from the
MORPHEUS piezometer filter 18 and from the squeezing and leaching of clay cores
at the corresponding stratigraphical depth (-222.625 m TAW): MORPHEUS water
(•), squeezed water (), leached water filtrated at 0.45 µm ( | or „), and leached
water filtrated at YM3 (‹).
42
From the above table and figure, it can be concluded that the Boom Clay pore water
composition, at a depth of -222.625 m TAW, is comparable for the major cations,
provided that the leached samples are filtered by a YM3 filter. For the anions,
piezometer waters have a similar composition as the squeezed waters. The leached
waters have a pronounced different composition due to the double layer properties as
discussed in Annex 4. The DOC content is extremely high in the leached waters. This
is probably because both the mobile and (part of the) immobile organic matter are put
in suspension, in contrast to pore water obtained from piezometers or squeezing where
mostly only the mobile fraction of the organic matter is measured.
Similar conclusions can be drawn for all the other pore water samples from which the
three different extraction techniques were compared. These observations indicate that
the measured pore water composition partly depends on the extraction technique.
A few squeezed pore water samples have substantially higher contents of Ca, Mg, Na,
K and SO42-. The pore water composition of these samples is severely modified
because oxidation affected the clay cores (De Craen et al., 2002a, 2004a, 2004b). This
indicates once more that methodology of sampling, sample preservation and sample
treatment is very important. Only well-preserved clay cores are suitable for the
characterisation of the pore water! Note that these data (modified pore water
composition as a result of oxidation) are not included in this report, since they are not
representative of in situ Boom Clay pore water.
3.1.2 Trace elements
The trace element composition of Boom Clay pore water, sampled from piezometers,
is given in Table 3-3. Unfortunately, trace elements were not often analysed.
Therefore, it is not possible to compare the data.
In anaerobic conditions, the presence of nitrate (NO3–) and nitrite (NO2–) is not
expected in the sediment. Nitrate and nitrite are easily detected by ion
chromatography in the mg/l range. For normal pore waters, their concentrations are
usually < 1-2 mg/l, and often below the limit of detection of the instrumental
techniques used. The only time NO3– was measured the last decade in the Boom Clay,
was in squeezed water from a Boom Clay lump sampled in 1999 at the interface with
the concrete blocks of the lining of the small shaft used to host the Reseal-II in situ
experiment. The concentration was anomalously high (90 mg/l) for an unknown
reason.
Ammonium (NH4+) can also indicate chemical or microbial perturbations and was
evidenced for the first time by CEA in the water samples from the piezometer # 2 of
the ARCHIMEDE-argile project (Merceron et al., 1993b). This borehole probably
encountered an oxidative perturbation during the drilling operations, since it remained
open to air 1 week before installation of the piezometer.
43
Table 3-3: Trace element composition of Boom Clay pore water sampled from various
piezometers. (Min.=Minimum, Max.=Maximum, n/a = not analysed)
MORPHEUS
Min.
Max.
EG/BS
Min.
ORPHEUS
Max.
Min.
Max.
SPRING 116
Min.
Max.
Li
mg/l
< 0.2
< 0.2
< 0.2
< 0.2
n/a
n/a
< 0.2
< 0.2
B
mg/l
6.7
7.6
6.7
9.7
6.7
11.3
7.4
8.6
Al
mg/l
< 0.2
0.4
< 0.2
0.89
< 0.2
0.25
< 0.2
0.25
Mn
mg/l
0.015
0.036
< 0.05
< 0.05
n/a
n/a
< 0.05
0.14
Ni
mg/l
n/a
n/a
< 0.2
< 0.2
n/a
n/a
< 0.2
< 0.2
Zn
mg/l
n/a
n/a
< 0.2
< 0.2
n/a
n/a
< 0.05
< 0.2
Se
µg/l
0.05 *
5*
n/a
n/a
n/a
n/a
n/a
n/a
Rb
µg/l
1.9
9.2
n/a
n/a
n/a
n/a
n/a
n/a
Sr
µg/l
40
100
90
130
n/a
n/a
60
130
Y
µg/l
< 0.5
0.9
n/a
n/a
n/a
n/a
n/a
n/a
Zr
µg/l
3.1
58.0
n/a
n/a
n/a
n/a
n/a
n/a
I
µg/l
590
840
660
740
n/a
n/a
n/a
n/a
Cs
µg/l
< 0.5
< 0.5
n/a
n/a
n/a
n/a
n/a
n/a
Ba
µg/l
10
60
n/a
n/a
n/a
n/a
13
55
La
µg/l
< 0.5
< 0.5
n/a
n/a
n/a
n/a
n/a
n/a
Ce
µg/l
< 0.5
1.2
n/a
n/a
n/a
n/a
n/a
n/a
Pr
µg/l
< 0.5
< 0.5
n/a
n/a
n/a
n/a
n/a
n/a
Gd
µg/l
< 0.5
0.6
n/a
n/a
n/a
n/a
n/a
n/a
Tb
µg/l
< 0.5
< 0.5
n/a
n/a
n/a
n/a
n/a
n/a
Dy
µg/l
< 0.5
< 0.5
n/a
n/a
n/a
n/a
n/a
n/a
Ho
µg/l
< 0.5
< 0.5
n/a
n/a
n/a
n/a
n/a
n/a
Hf
µg/l
< 0.5
< 0.5
n/a
n/a
n/a
n/a
n/a
n/a
Th
µg/l
0.005 *
< 0.5
n/a
n/a
n/a
n/a
n/a
n/a
U
µg/l
0.200 *
3.5
< 0.5
1.5
n/a
n/a
< 0.25
0.43
* measured with a HR- ICP-MS (better resolution than ICP-MS)
The trace element composition of Boom Clay pore water is slightly different in pore
water sampled from piezometers and pore water obtained by the squeezing of clay
cores (no data are available on pore water from leaching experiments). This can be
illustrated by comparing pore water samples from the MORPHEUS piezometer with
pore water obtained by the squeezing of the HADES 2001/4 clay cores at
corresponding stratigraphical depths (Table 3-4). In a lot of squeezed pore water
samples, the trace elements are more abundant compared to piezometer-derived pore
water. The higher amount of trace elements in squeezed pore water samples is not
fully understood. Possibly it is an artefact of the squeezing technique.
44
Table 3-4: Comparison of the trace element composition of Boom Clay pore water
sampled from the MORPHEUS piezometer and pore water obtained by the squeezing
and leaching of clay cores at corresponding stratigraphical depths (Min.=Minimum,
Max.=Maximum, n/a = not analysed)
MORPHEUS
HADES 2001/4
piezometer
Squeezed clay
cores
Min.
Max.
Min.
Max.
Al
mg/l
< 0.2
0.4
< 0.2
0.4
Mn
µg/l
15
36
60
296
Rb
µg/l
1.9
9.2
2.4
50
Sr
µg/l
40
100
61
425
Y
µg/l
< 0.5
0.9
<1
32
Zr
µg/l
3.1
58.0
<5
71
I
µg/l
590
840
430
610
Cs
µg/l
< 0.5
< 0.5
< 0.5
3.8
Ba
µg/l
10
60
5
51
La
µg/l
< 0.5
< 0.5
<1
63
Ce
µg/l
< 0.5
1.2
<1
180
Pr
µg/l
< 0.5
< 0.5
<1
21
Gd
µg/l
< 0.5
0.6
<1
13
Tb
µg/l
< 0.5
< 0.5
<1
2.2
Dy
µg/l
< 0.5
< 0.5
<1
9.4
Ho
µg/l
< 0.5
< 0.5
<1
1.6
Hf
µg/l
< 0.5
< 0.5
<1
2.8
Th
µg/l
0.005 *
< 0.5
<1
48
U
µg/l
0.200 *
3.5
< 0.5
18
* measured with a HR- ICP-MS (better resolution than ICP-MS)
3.1.3 pH, pCO2, and alkalinity of Boom Clay
The concentration of hydrogen ions (hydrated) is very low for waters that are not
strongly acidic. To quantify the concentration (activity) of hydrogen ions, the more
convenient expression is pH, the negative base-10 logarithm of the hydrogen-ion
activity in moles per litre. The pH of most natural waters is related to reactions
between carbon dioxide (partial pressure, pCO2) and its water soluble species
following the reactions:
CO2(g) + H2O ⇔ H+ + HCO3-
(3.1)
CO2(g) + H2O ⇔ 2 H+ + CO32-
(3.2)
45
In the case of the Boom Clay, since the sediment is water saturated, no free gas phase
is normally present. The pCO2 of the Boom Clay is therefore related only to the
dissolved CO2 as carbonic acid:
CO2 (g) + H2O ⇔ H2CO3 (aq)
(3.3)
Another quantity relating the solution pH and the water soluble carbonate species is
alkalinity. Alkalinity is defined as the capacity of a water to neutralise acid and is
measured by titration with strong acid (HCl or H2SO4) to an end point around pH 4.5.
Different from most quantities determined by chemical analysis, e.g., pH and pCO2
that are intensity functions, alkalinity (also for acidity) is a capacity function so that in
principle all basic species including hydroxide (OH-), silicate, borate, phosphate, and
natural organic ligands should also contribute to the alkalinity. In practice however,
only the dissolved carbonic acid is of quantitative importance as shown in equation
(3.4).
carbonate alkalinity = mHCO3- + 2 mCO32- ≈ total alkalinity
where mHCO3- and mCO32- are the molarity of bicarbonate and carbonate, respectively.
By definition, the carbonate alkalinity is only part of the total alkalinity, but for the
Boom Clay pore water the difference of the two quantities is negligible.
Reactions among the species related to pH, pCO2, and alkalinity are in general fast in
natural waters and can therefore be treated by chemical equilibrium principles. This
should be particularly the case in the Boom Clay because of the extremely long
resident time of the pore water.
3.1.3.1 pH of Boom Clay pore water
The techniques to determine the Boom Clay pH was discussed already in the Section
2.2.1.1 and will not be repeated here.
Until now, the most acknowledged Boom Clay pH value is 8.2 determined in the
project of ARCHIMEDE-argile (Beaucaire et al., 2000; Griffault et al., 1996). This
value was measured in situ by an optode under the Boom Clay hydraulic pressure.
The same value was also determined in the same project by a flow cell with a
combined glass electrode. The recent in situ pH measurement (Moors et al., 2002)
using a closed-circuit technique in the same ARCHIMEDE-argile piezometer revealed
a value of 8.0. The latter measurement was to improve the pH sensing technique, for
example, by replacing the optode sensor with a pressure resisting polymer filled pH
electrode under a 1.8 MPa hydraulic pressure (see section 2.2.1.1). Two problems
were however encountered in this closed-circuit measurement:
1. after 463 days of circulation of clay water, the pH electrode was found having
a drift of 120 mV (about 2 pH unit). Because of the electrode drift, other pH
values before the final reading (pH = 8 after correcting the drift) were not
reliable so we did not know if the pH 8 was the stable value;
2. due to the continuous circulation of the clay water between the measuring cell
(with pH electrode in it) and the filter of the piezometer, a leakage of KCl
from the electrode to the clay water was observed (Table 3-5). The water with
(3.4)
46
a high concentration of K ion was pumped back to the clay and induced cation
exchange reactions as evidenced by the elevated concentration of Na, Ca, and
Mg ions in the pore water.
Table 3-5: Comparison of water compositions collected at the ARCHIMEDE-argile
piezometer (No. 1) for the two pH measurement campaigns. The reference water
composition derived from the current work is also given.
mg/kg water
K
Na
Ca
Moors et al., 2002
50
760
14.3 12.8 0.5 421
Beaucaire et al., 20002
8.2 282.8 1.9
1
This work, table 5-1
1
2
3
7.2 359
2.0
Mg
1.7
1.6
Fe
Cl
SO4
Br
Si
7.8
0.8
5.2
0.2 17.7 3.8
0.6
4.2
0.2 26
2.2
3
0.6
3.4
the water compositions were given in the certificates of chemical analysis MC/PT/C072021/01-032/pt (cations)
and -033/pt (anions)
it is not clear if the water was the same from which the pH was measured
taken from the average of MORPHEUS water compositions
Although it is clear from Table 3-5 that the water from the closed-circuit pH
measurement (Moors et al., 2002) has an unusual high concentration of K, Na, Ca,
Mg, and Cl due to the leak of KCl, it is not clear to what extent the pH value was
affected. One mechanism that might have impacted on the system pH is the cation
exchange, inducing calcite precipitation:
>X2:Ca + 2 K+ + HCO3- ⇔ 2>X:K + Calcite (CaCO3) + H+
The reaction suggests that the process releases acid (H+) and will decrease the pH.
Besides, the high concentration of K was circulated around the filter for more than
one year, so K ions have been exchanged onto the surrounded clay surfaces. The pH
value measured this way is therefore considered as being perturbed to a certain extent.
Apart from the in situ measurements, all other reported pH values for the Boom Clay
were measured in surface laboratories in pore water samples that were unavoidably
contacted with air. The pH measurement in a surface laboratory is normally
performed in the open air by immersing a pH electrode into water samples collected
from the underground gallery. Depending on the in situ pCO2 of the water samples,
i.e., higher or lower than that of the atmosphere, the pH reading may increase
(degassing) or decrease (ingassing) from the initial pH reading, which is mostly close
to the in situ value. To minimise the extent of de- or ingassing of CO2 (g) while
measuring a pH in a surface laboratory, a pH reading should be recorded as soon as
possible to avoid prolonging the contacting time between water samples and the air.
In practice however, a quick pH measurement has not been routinely performed. A pH
reading is normally recorded when the reading stops drifting based on a given
criterion for the pH meter. Because of the problem of de- or ingassing of CO2 (g), a
pH measurement in a surface laboratory is normally considered as unreliable. Dierckx
(1997) reported the pH of EG/BS water as 9.5, which is higher than the available in
situ pH values of 8.0 and 8.2, so it is considered as being caused by artefacts and not
representative for real Boom Clay. The exact cause of the high pH in EG/BS water is
not clear but might relate to the leaching of the filling material between the filter and
the clay formation. Earlier study performed by batch experiments upon leaching clay
47
samples in distilled water resulted in a pH of about 9.3 (Baeyens et al., 1985b). This
value is one pH unit higher than the in situ value of 8.0 and 8.2 because of the low
pCO2 (g) in the N2 (g) or Ar (g) filled glove boxes where pH was measured. The
average pH of 12 MORPHEUS water samples collected in April, 2003 is 8.03 (Figure
3-3). However, the same samples revealed a pH lower than 7.5 after two months of
storage at room temperature instead of 4 °C (routinely applied) suggesting possible
effects of bacterial activity (Figure 3-3). Related to the problem of de- or ingassing of
CO2 (g), Figure 3-3 demonstrates that a laboratory measured pH is influenced by
factors such as the sample storage time and temperature, sample stirring during the
measurement, and the time that the sample was in contact with air. The high pH
approaching to the value of 9.3 is due to the complete degassing of CO2 (g) originally
present in the water samples.
9
at the time of
sampling
1 month storage
pH
8.5
2 month storage
8
45 min stir in air
48 hrs stir in air
7.5
7
-240
-235
-230
-225
-220
-215
depth, m
Figure 3-3: Laboratory pH measurement on MORPHEUS water samples in air and at
room temperature (~23°C). The in situ temperature of pore water is about 16 °C.
Water samples were stored at room temperature in closed vessels under slightly
overpressure before they were measured at open air.
Because of the difficulties encountered in both in situ and laboratory pH
measurements, an alternative way to acquire the pH of the Boom Clay is through
speciation calculation using representative pore water compositions. The basic model
is to assume a chemical equilibrium between calcite and the pore water. The pore
water composition is therefore constrained by the calcite dissolution/precipitation
reaction:
Calcite + H+ ⇔ Ca2+ + HCO3(3.5)
At a chemical equilibrium state, i.e., calcite has a saturation index (SI) of 0, the pH
can then be calculated if Ca2+ and HCO3- are known. However, water analyses in
48
general only provide total calcium (not free Ca2+) and total dissolved inorganic carbon
(not bicarbonate) concentrations and therefore pH cannot be readily estimated using
only the equation (3.5).
A speciation calculation, normally performed with geochemical computer codes, takes
into account all constraints given in equations (3.1) to (3.5), and the pH can be fitted
to the measured calcium and TIC concentrations. Figure 3-4 shows the modelling
results by allowing calcite to be in equilibrium with MORPHEUS waters with
measured Ca and TIC concentrations. Within a varied pH range (imposed), the
saturation index of calcite changes and the pH at which the SI is 0 corresponds to the
pH of the in situ pore water. The results of Figure 3-4 suggest that the pH of
MORPHEUS water is in the range of 8.3 to 8.6.
Calcite saturation index (log Q/K)
0.8
0.6
mor-1
0.4
pH 8.3
0.2
mor-2
mor-3
0
mor-4
-0.2
pH 8.6
mor-5
-0.4
Archimede
-0.6
-0.8
7.75
8
8.25
8.5
8.75
9
pH
Figure 3-4: Saturation indices (SI) calculated at 16 °C using MORPHEUS water
compositions as function of pH. Mor-1 to mor-5 are data from 5 statistic groups of
water samples (see section 3.4.1 latter). Data from the ARCHIMEDE-argile project is
also plotted for comparison. pH at SI = 0 corresponds to in situ pH value.
Data from ARCHIMEDE-argile were fitted in the same way: the pH is about 8.6
which is higher than 8.2, the in situ value measured. In other words, ARCHIMEDEargile water is undersaturated with respect to calcite at pH 8.2. Two possibilities may
have caused the observed undersaturation: (1) the pH of 8.2 measured was too low;
(2) the assumption of calcite equilibrium is not valid. Considering the difficulties
encountered in an in situ pH measurement, the modelling approach for pH estimation
is particularly interesting for the purpose of a long term prediction, for which a
general equilibrium condition should be established.
Table 3-6 summarizes the pH values measured at the underground laboratory, surface
laboratories, and estimated by speciation modelling using measured pore water
compositions. Note that laboratory measurements show the largest extent of variation
suggesting an unfavourable condition for pH determination.
49
Table 3-6: Summary of pH values measured at the underground laboratory, surface
laboratories, and estimated by speciation modelling
In situ
(underground laboratory)
batch
(surface laboratory)
model fit
(speciation modelling)
8.21(optode, ARCHIMEDE)
8.02 (polymer filled electrode)
8.23 (MORPHEUS)
7.54 (MORPHEUS)
9.55 (EG/BS)
9.2-9.36
8.6 (ARCHIMEDE)
8.3 ~ 8.67 (MORPHEUS)
1: ARCHIMEDE piezometer (Beaucaire et al., 2000);
2: ARCHIMEDE piezometer (Moors et al., 2002), disturbed by the leakage of KCl;
3: measured shortly after the sampling;
4: measured after several weeks storage at room temperature (unpublished results);
5: water from the silty layer and the borehole was filled with gravel (Dierckx, 1997);
6: measured in a glove box under inert atmosphere but no CO2 (g). The solid/liquid ratio is 1:1
(Baeyens et al., 1985b);
7: see figure 3-4 for results fitted to measured [Ca] and TIC concentrations for MORPHEUS waters.
3.1.3.2 Partial pressure of CO2 (g) in Boom Clay
The only pCO2(g) measurement on the Boom Clay was performed by Henrion et al.,
(1985) and a value of 10-2.5 atm (25 °C) was reported. The measurement was not in
situ but carried out on preserved clay samples in surface laboratory by an out-gassing
technique. Assuming that the measured pCO2 is equal to its fugacity, Henrion et al.,
(1985) also calculated the Boom Clay pH, using equations (3.1) to (3.3), as being in a
range of 8.5 to 8.8 at varied total inorganic carbon content (TIC, the sum of
bicarbonate and carbonate concentrations).
Different from the approach of Henrion et al., (1985), that is, to estimate pH from the
measured pCO2, Griffault et al., (1996) calculated pCO2 from the measured pH. Using
the in situ measured pH value of 8.2 (see section 3.1.3.1), a pCO2 of 10-2.4 atm was
calculated at TIC of 12 mM based on equation (3.1).
It is well known that pCO2 and pH measurements are technically challenging. This is
because the Boom Clay is susceptible to the loss of CO2 (g) if in contact with air.
Measurement in an absolutely closed system is technically difficult. The so far
measured pCO2 of the Boom Clay is about 10 times higher than the atmospheric pCO2
of 10-3.5 atm. Any contact with air tends to lower the pCO2 of the water sample due to
the loss of dissolved CO2 and many attribute it to a so called degassing or out-gassing
process. The loss of dissolved CO2 can be caused by two different processes:
•
•
degassing due to the difference in CO2 solubility in water;
loss of dissolved CO2 due to the re-equilibrium with the atmospheric CO2
partial pressure.
The first process might occur when a clay or pore water sample was oversaturated in
respect of CO2 (g) under the in situ high pressure. When the sample is brought to low
atmospheric pressure, the CO2 solubility decreases and CO2 (g) will escape from the
50
sample. Such degassing happens when one opens a bottle of sparkling water and CO2
gas bubbles escape from the bottle. Whether or not such a degassing might happen to
Boom Clay or its pore water depends on the in situ saturated of CO2. Whether the
partial pressure of CO2 is equal to the atmospheric pressure can be estimated by
comparing CO2 solubility and the TIC measured in the Boom Clay pore water. The
Henry’s law constant for CO2 (g) is 1.6 × 103 atm at 298 K (Atkins, 1994). The
solubility of CO2 (g) in 1 litre of water at 25 °C (i.e., 55.5 mol) can be approximated
as:
n (CO2) ≈
1atm
× 55.5 = 34.7 mmol
1.6 × 10 3 atm
where n is the number of mols CO2 dissolved when its partial pressure equals to the
atmospheric pressure of 1 atm. The solubility of CO2 at the in situ temperature of 16
°C is even higher and equals to 45 mmol/l. The TIC of a Boom Clay pore water is
about 15 mmol/l which is less than half of the calculated solubility of CO2 . This
suggests that Boom Clay pore water is not oversaturated with respect to CO2 (g) under
the in situ pressure and therefore will unlikely experience degassing due to the
difference in CO2 solubility.
The second process causing the loss of CO2 from samples is due to the re-equilibrium
with the atmospheric CO2 partial pressure. The loss of CO2 will increase pH as shown
in equation (3.1) and (3.2). A Boom Clay pore water containing 15 mmol/l of
inorganic carbon should have a pH of about 9.3 if in equilibrium with air (see Figure
3-5 below for measured value at room temperature). The re-equilibrium process is
however affected by kinetics. Stumm and Morgan (1996) stated that an establishment
of water-atmosphere equilibrium is primarily limited by slow gas transfer reactions.
To a lesser extent, the re-equilibrium can also be slowed down by unfavourable
mixing. Figure 3-5 shows the pH change of a MORPHEUS water when measured in
air under a strong mechanical stirring. The pH approaches 9.3, the value in
equilibrium with air, only after 48 hours. This suggests that a complete degassing of
CO2 even in a surface laboratory needs more than 2 days of time to accomplish. The
initial step of degassing was however quite fast, within one hour, the pH increased
about half a pH unit.
From foregoing discussions plus the results shown in section 3.1.3.1, it can be
concluded that the pH measurements performed in the surface laboratory do not
provide representative values due to possible bacterial effects and the loss of CO2.
Degassing of CO2 in situ due to the solubility difference is unlikely in the Boom Clay.
Since gas transfer reactions are in general slow in kinetics, 'on site' pH measurement
in piezometer water using a carefully designed flow cell might provide relevant pH
values. The difference between an 'on site' measurement and the currently ongoing in
situ closed-circuit measurement is, that an 'on site' measurement does not have to be
performed at the high in situ hydraulic pressure. This will relax most of the constraints
on instrumentation, e.g., to resist to the high in situ hydraulic pressure. A conventional
glass electrode may be used in place of a high pressure resistant electrode, for which a
routine calibration for pressure effects is not straightforward. Another advantage of a
piezometer flow-through cell measurement is that the operation is simple and
therefore can be performed for all piezometers from which pore waters are sampled.
51
9.25
9
calculated pH 9.3 (25 °C)
pH
8.75
8.5
8.25
8
0
10
20
30
40
50
time, hr
Figure 3-5: pH evolution for a MORPHEUS water as function of time with a
mechanical stir. The calculated pH is 9.3 if the water is in equilibrium with air at 25
°C.
The same as in the case of the pH, the partial pressure of CO2 (g) can also be
calculated by assuming calcite equilibrium with water in terms of measured
compositions. The calculation can be done in the same way as shown in Figure 3-4
but the principle variable is pCO2 instead of pH. Since in both cases the TIC
concentration is a measured value (not imposed by the model), the pCO2 (g) can be
derived directly from the calculation shown in Figure 3-4 but plot pCO2 as the x axis
instead of pH (figure not shown). The corresponding results are: pCO2 is in a range of
10-2.4 to 10-2.8 atm while pH changes from 8.3 to 8.6.
Until now the pCO2 (g) has been either measured experimentally or calculated using a
measured pH value without knowing the exact mechanism controlling the parameter.
Coudrain-Ribstein et al., (1998) reviewed literature data of pCO2 in confined aquifers
without a gas phase and concluded that pCO2 is principally constrained only by
mineral assemblage. For sedimentary formations, one example for such CO2 control
mechanism is as:
5 calcite + chlorite + 5 CO2 ⇔ kaolinite + silica + dolomite + 2 H2O
(3.6)
At an equilibrium state, all minerals involved in the reaction have a unity activity so
that the pCO2 can be easily calculated if all stability constants of the involved minerals
needed for balancing the equation (3.6) are known. In addition, the equilibrium is in
principle only dependent on temperature since the system is free of a gas phase and
the equilibrium won’t be affected by the absolute pressure to a big extent. The
problem with this approach is that the uncertainty on the values of the thermodynamic
database is usually large. This is because the stoichiometry and thermodynamic
52
properties of some of the involved minerals are poorly known. This is also the main
drawback of the thermodynamic modelling approach.
The pCO2 controlling mechanism like equation (3.6) should also be applicable to
Boom Clay since all minerals appearing in the equation are present in Boom Clay (see
section 3.1.3.2). The only question is whether or not dolomite exist in Boom Clay.
Van Keer and De Craen (2001) reviewed five representative studies on Boom Clay
mineralogy and only Griffault et al., (1996) reported dolomite in the Boom Clay
based on the XRD analysis performed at ERM (Etudes Recherches Materiaux). The
same institute again identified dolomite in the Boom Clay samples in the recent EC
Eco-clay –II project (Bouchet, 2003). Wouters et al., (1999) determined 1.1 %
magnesite in Boom Clay samples. At the present stage and for the purpose of
geochemical modelling, we consider dolomite as a proxy of magnesium rich
carbonate mineral which likely exists in Boom Clay. Based on the current version of
MOLDATA (Wang, 2003), the Boom Clay pCO2 is calculated to be 10-3.1 atm at 16 °C
following the reaction:
clinochlore-14A + 5 CO2(g) + 5 calcite ⇔
kaolinite + chalcedony + 5 dolomite-dis + 2 H2O
where:
clinochlore-14A: chlorite (Mg5Al2Si3O10(OH)8)
calcite:
CaCO3
kaolinite:
Al2Si2O5(OH)4
chalcedony:
SiO2
dolomite-dis:
CaMg(CO3)2 dis-ordered form of dolomite
According to Coudrain-Ribstein et al. (1998), at 20°C, the pCO2 controlled by the
same mineral assemblage is 10-2.8 bar which is about 1.5 times higher than the value
10-3.0 atm calculated by MOLDATA at the same temperature. The difference is surely
due to the difference between the two thermodynamic databases. Another well known
database Wateq4f (Ball and Nordstrom, 1991) calculates pCO2 at 20 °C in a range of
10-2.8 to 10-3.1 atm using respectively amorphous silica and quartz to represent the
silica mineral. Note that different databases may use different crystallinities to
represent the same mineral identified by, e.g., X-ray diffraction measurement.
Modellers may also make decisions on what minerals to use based on observed pore
water composition, e.g., we use chalcedony in equation (3.7) to replace the silica
mineral, since the silicon concentration measured in the pore water of Boom Clay is
very close to the solubility of chalcedony but much higher than the solubility of
quartz. The same goes for dolomite where a disordered form of dolomite is often used
instead of a well crystalline phase. This is because dolomite has large variations in
solubility depending on its crystallinity. The aqueous concentration of magnesium has
been found mostly governed by the solubility of a disordered form of dolomite rather
than by the well crystallized form (Coudrain-Ribstein et al., 1998). Although the
magnesium concentration in Boom Clay is explained by ion exchange in this report
(see section modelling), the solubility of disordered dolomite calculated on the basis
of MOLDATA is about 2 mg/kg water, which is in good agreement with the
magnesium concentration found in Boom Clay pore water.
(3.7)
53
Besides the reaction (3.7), other possible mineral assemblages were proposed by
Coudrain-Ribstein, et al., (1998). Table 3-7 summarizes some reactions relevant for
Boom Clay, together with the calculated pCO2 values using MOLDATA.
From Table 3-7, it is seen that except the assemblage of illite which resulted in a
much too low pCO2, other groups result in a pCO2 equal to or slightly lower than the
value of 10-2.5 atm (25 °C) measured by Henrion et al. (1985). Note that the current
version of MOLDATA (04-01) contains some questionable data concerning aluminium
species and minerals as pointed out by Nordstrom et al. (1990). Current review, data
selection, and modification of MOLDATA (04-01) is in progress and the pCO2 values
given in Table 3-7 should be recalculated after the MOLDATA is updated.
Another point observed from Table 3-7 is the effect of temperature. The temperature
increase from 16 to 25 °C increases the pCO2 with a factor of 2, as predicted. The
temperature effect has been studied in the Cerberus experiment (Noynaert et al.,
1998) in which Boom Clay was heated up to 80 °C. Although no direct pCO2 was
measured in the experiment, the on site pH measurement evidenced a pH drop to 6~7
from the original value of 8.7. This pH drop has been previously explained as being
due to the pyrite oxidation or bacterial activities. The effect of temperature increase,
which causes an increase in PCO2 so a decrease in pH, should not be overlooked in the
future modelling on the effect of heating on Boom Clay.
Table 3-7: Possible mineral assemblages for controlling pCO2 in sediments and the
calculated pCO2 values at 16 and 25 °C
mechanisms
* pCO2 (atm), 16/25 °C
clinochlore-14A + 5 CO2(g) + 5 calcite ⇔
kaolinite + chalcedony + 5 dolomite-dis + 2 H2O
10-3.1/10-2.8
kaolinite + 3 CO2 + 3 calcite + phlogopite ⇔
muscovite + 2 chalcedony + 2 H2O + 3 dolomite-dis
10-3.0/10-2.7
phlogopite + 3 CO2 + 3 calcite ⇔
K-feldspar + 3 dolomite-dis + H2O
10-2.9/10-2.5
20 illite + 5 CO2 + 2 H2O + 5 calcite ⇔
5 dolomite-dis + 12 muscovite + 5 kaolinite + 24 chalcedony
10-4.9/10-4.6
* calculated using the MOLDATA (04-01) database
clinochlore-14A: Mg5Al2Si3O10(OH)8, chlorite
kaolinite: Al2Si2O5(OH)4
calcite: CaCO3
phlogopite: KAlMg3Si3O10(OH)2, mica group
muscovite: KAl3Si3O10(OH)2, mica group
chalcedony: SiO2
dolomite-dis: CaMg(CO3)2
K-feldspar: KAlSi3O8
illite: K0.6Mg0.25Al1.8Al0.5Si3.5O10(OH)2
54
The change of pCO2 due to the temperature variation has not been realised in a context
of sample storage and experimental protocols. The so far applied procedure is to store
clay samples at 4 °C before use. Depending on the time of storage, a possible decrease
of pCO2 in clay samples may cause a change in pore water chemistry. This is
particularly important for laboratory experiments where squeezing and leaching
techniques are used to extract pore waters from clay samples.
Finally, Table 3-8 summarizes pCO2 values of Boom Clay either measured
experimentally or estimated based on pore water analysis and chemical speciation
calculations.
Table 3-8: Values of pCO2 of Boom Clay obtained by laboratory measurement and
estimations based on model simulation
measurement
10-2.5 (25°C), outgassing clay core1
estimations
10-2.4 (25°C), ARCHIMEDE data2
10-2.8~10-2.4 (16°C), MORPHEUS waters3
10-3.1~10-2.5 (25 to 16 °C), mineral assemblages4
1: outgassing clay cores (Henrion et al., 1985);
2: calculated from the measured pH and alkalinity (Beaucaire et al., 2000; Griffault et al., 1996);
3: calculated based on the assumption of calcite equilibrium and the Ca, TIC concentrations measured in
MORPHEUS waters;
4: calculated using the MOLDATA database (see Table 3-7).
It must be stressed that although attempts have been made to measure or calculate the
pCO2 of Boom Clay, the real mechanism governing the parameters is not understood.
On the one hand, current activities for a more accurate and representative pCO2
measurement should be continued, on the other hand future studies to understand the
mechanism of the CO2 evolution in Boom Clay should be pursued. Last but not least,
the role of natural organic matter and bacterial activity in regulating the pCO2 in Boom
Clay has not been evaluated so far.
3.1.3.3 Alkalinity of Boom Clay pore water
As defined in section 3.1.3, alkalinity is the capacity of a water to neutralise acid.
Rounds and Wilde (2001) also use acid neutralising capacity (ANC) which is
essentially the same but measured in water samples without filtration (0.45 µm). For
Boom Clay pore water collected through piezometers, our titration data (not shown)
illustrated no difference between the alkalinity and the ANC so we do not differentiate
alkalinity from ANC in this report.
The alkalinity of Boom Clay pore water was reported as 12.1 mmol/l in the
ARCHIMEDE-argile project (Griffault et al., 1996). Much lower values, e.g., ~7
55
mmol/l were reported for the pore waters collected by mechanical squeezing,
probably due to the pyrite oxidation (Reeder et al., 1994). Recent measurement on
MORPHEUS waters from different depth resulted in a mean value of 14.9 meq/l
HCO3- or 909 mg/l (see Figure 3-6 below).
It has been unclear as to what extent other solutes than bicarbonate, in particular
natural organic matter, contribute to the total alkalinity of Boom Clay pore water. As
a rough estimate, the contribution of other solutes can be derived by comparing the
total alkalinity to the total inorganic carbon content expressed as bicarbonate. Figure
3-6 shows the comparisons in 12 MORPHEUS water samples.
It is clear from Figure 3-6 that the total alkalinity agrees very well with the TIC
suggesting that other solutes, including natural organic matter, do not contribute
significantly to the total alkalinity. It is notable though that the alkalinity is
systematically higher than TIC for about 2 %, which might be due either to the real
contribution of other solutes or to the underestimation of TIC by assuming only
bicarbonate existing in water samples.
1200
Alkalinity, mg HCO3-/l
1000
800
Titration
TIC
600
400
200
0
-217.1 -220.8 -222.6 -226.0 -227.2 -227.8 -229.1 -229.8 -230.3 -231.8 -233.8 -235.2
depth, m
Figure 3-6: Alkalinity and total inorganic carbon (TIC) content (as HCO3-)
measurements performed in 12 MORPHEUS water samples collected at different
depth. Water samples were collected in April, 2003.
Different from pH and pCO2, alkalinity is a conservative quantity and independent of
pCO2 (Drever, 1997). The alkalinity is therefore not susceptible to the loss of CO2 in
the process of a water sampling. This is because neither the pCO2 nor the dissolved
H2CO3 (aq) will contribute to the charge balance of the system. Although the increase
of pCO2 will increase the TIC through either:
56
or
CO2 + H2O + CO32– ⇔ 2 HCO3-
(3.8)
CO2 + H2O ⇔ H+ + HCO3-
(3.9)
In both reactions, the increased alkalinity due to the increase of [HCO3-] will be
counterbalanced by the disappearance of CO32– and the production of H+,
respectively. The net change of alkalinity will thus be zero. The alkalinity is no longer
conservative if redox reactions between carbon species start to play a role, e.g.,
through biological processes.
One important concern has been to what extent the oxidation of the water sample will
affect the alkalinity. The most probable alkali consuming reaction in Boom Clay pore
water might be the oxidation of Fe(II) and the precipitation of iron hydroxide
following the reaction:
2 Fe2+ + 4 HCO3- + 0.5 O2 + 5 H2O ⇔ 2 Fe(OH)3 + 4 H2CO3
Boom Clay pore water in general contains less than 1 mg/l of Fe, which is 0.018
mmol/l. If such a concentration of Fe2+ precipitates, the water will lose, according to
equation (3.10), twice this molarity of HCO3-, i.e., 0.036 meq/l as alkalinity. This is
about 0.3% of the total 15 meq/l alkalinity measured in MORPHEUS waters, which is
negligible.
3.1.3.4 Conclusions of pH/ pCO2
As the most important parameter, the Boom Clay pH and its controlling mechanisms
are still not conclusive. Apart from the well known in situ ARCHIMEDE-argile
measurement (pH 8.2), the new in situ measurement (pH 8.0) suffered from the leak
of KCl from the electrode and demonstrated the technical difficulties in pursuing a
good quality pH measurement. The model simulation, using water compositions from
a new piezometer (MORPHEUS) suggests a pH range of 8.3 to 8.6. The observed
variation in water compositions suggests that variations in pH or related pCO2 should
exist in the collected water samples. As an absolutely non-disturbed geochemical
condition is not achievable, the variation in pH or pCO2 may well be caused by the
disturbances introduced while installing piezometers and/or collecting waters. The
extent of such disturbances is of course hard to quantify. It is therefore more objective
to use a modelled reference pH instead of measured pH values that are, to certain
degrees, functions of the system disturbances.
3.1.4 Redox processes and redox potential in Boom Clay
Redox is the acronym of 'reduction and oxidation', that is, processes occurring in a
system involving multi-valent elements. These elements are therefore called redox
sensitive elements. A detailed description on redox chemistry is out of the scope of
contamination this report and can be found in standard text books such as Appelo and
Postma (1999), Langmuir (1997), and Hem (1985). We hereafter only give a brief
definition of redox potential and precautions one should bear in mind when applying
the concept to the interpretation of Boom Clay geochemistry.
The redox potential is a numerical index of the intensity of oxidising or reducing
conditions within a system. It is the potential, expressed as Eh in volt or millivolt,
(3.10)
57
developed by a redox reaction involving transfer of electrons. The subscript “h”
implies that the potential is relative to the standard potential of hydrogen (H2/H+)
electrode. Positive values of Eh indicate that the system is relatively oxidising, and
negative values suggest that it is relatively reducing. Another widely used concept
equivalent to the redox potential is pE, defined as the negative logarithm of the
electron concentration:
(3.11)
pE = -log(e-)
The value of pE can be related to Eh at 25 °C and 1 atm pressure as:
pE =
FE h
= 16.9 E h
2.303RT
where F is the Faraday’s constant (96.42 kJ/volt gram equivalent), R the gas constant
(8.314 × 10-3 kJ/deg.mol), and T the absolute temperature.
In the definition, pE is analogous to pH, in reality, there are practical difficulties in
application of pE (or Eh) comparing to pH. Protons exist in water as its hydrated form
(e.g., H3O+), whereas electrons do not exist as free species. Also, most reactions
involving protons are reversible but those involving electrons are not. Most
importantly, electron transfers are often very slow suggesting that redox reactions in
natural system are often kinetically controlled. In general, redox reactions only
proceed at high rate if microbial catalysis is involved. Inorganic reactions involving
electron transfers occur only at an immeasurable rate. Application of the equilibrium
theory to redox processes has been proven invalid for many natural systems (Lindberg
and Runnells, 1984). Common laboratory measurements of Eh may provide a
qualitative indication of the system redox conditions but cannot be used quantitatively
unless the equilibrium state of the system is thoroughly evaluated. Comparing to
normal surface and sub-surface waters where the redox potential is difficult to define,
the Boom Clay redox potential should be thermodynamic indicative for two specific
reasons: (1) water movement in Boom Clay is extremely slow so that the water can be
considered approaching to an equilibrium with the surrounding rocks; (2) in compact
Boom Clay microbial activities are likely very low because of the small pores. The
redox potential is most likely imposed only by chemical interactions involving
inorganic components.
3.1.4.1 Redox potential in Boom Clay
Due to the presence of pyrite and natural organic matter, the Boom Clay is reducing
and therefore has negative redox potentials. The first reported value of the Boom Clay
redox potential was -280 mV measured in a surface laboratory by Baeyens et al.
(1985a) in a Boom Clay slurry. Many subsequent studies have focused on in situ
measurement of Eh and a range of values at -250 to -400 mV has been reported (De
Cannière et al., 1996). In the study of ARCHIMEDE-argile (Beaucaire et al., 2000;
Griffault et al., 1996), although no direct Eh values were reported, the interpretations
on the water composition suggested a similar range of Eh at -240 to -400 mV. Recent
efforts pursuing Eh-pH measurement at the in situ hydraulic pressure (see section
3.1.4.1) report a redox potential Eh of -310 ± 30 mV (Moors et al., 2002).
(3.12)
58
Although the redox potential of Boom Clay has been proven difficult to measure and
interpret, plausible processes controlling the measured Eh can be anticipated using EhpH diagrams in combination with the measured mineralogical and pore water
compositions. Figure 3-7 plots the Eh-pH diagram of the Fe-S-C system at
concentrations representative for Boom Clay pore water. The lines of equilibrium
shown in the figure are drawn from the reaction between the two species which are
separated by the line. For example, the equilibrium between pyrite and siderite is as
follows:
Pyrite + 9H2O + CO2(g) = 18H+ + 2SO42- + Siderite + 14e- logK = -91.2
(3.13)
Eh = 0.38 - 0.076pH + 0.0084log a[SO42-] - 0.0042log f[CO2(g)]
(3.14)
Following the equation (3.14), under the reference Boom Clay condition, e.g., at pH
of 8.2, pCO2 of 10-2.4 atm, and taking a sulphate concentration as 0.1 mg/l (~1 µM),
the redox potential is -283 mV, which is the upper limit of the pyrite stability field.
The sulphate concentration of 0.1 mg/l is about the lowest measured in all piezometer
waters. Except some severely oxidised waters, e.g., as studied by De Craen (2001),
the sulphate concentration in the Boom Clay pore water varies from a few to hundred
mg/l. Since Boom Clay is a marine sediment, the threshold of the present-day sulphate
concentration can be calculated from the seawater ratio between sulphate and
chloride. According to the seawater composition given by Drever (1997), the ratio of
chloride and sulphate in seawater is about 19.3. Taking the chloride concentration in
Boom Clay pore water as 26 mg/l, the sulphate concentration is 3.6 mg/l and the
redox potential calculated by equation (3.14) is -270 mV. This value might be taken
as the maximum redox potential for a non-disturbed Boom Clay.
From the mineralogical composition of the Boom Clay given in Table 1-2, it can be
seen that both pyrite and siderite are present in Boom Clay. So, the calculated Eh
based on equation (3.14) should be a reasonable estimate assuming a chemical
equilibrium. Without further information on the redox equilibrium state of the nondisturbed Boom Clay, Figure 3-7 provides the current-state conceptual model for
describing redox processes in Boom Clay. This model suggests that the measured
reducing Eh is controlled by pyrite-siderite equilibrium under the in situ partial
pressure of CO2 (g). If the Boom Clay pH is well buffered, the only factor influencing
the system redox condition is the concentration of sulphate. This suggests that it might
be practically interesting to use the sulphate concentration as an indicator to evaluate
the degree of perturbation on water samples caused by oxidation1. Again, the seawater
ratio of sulphate and chloride can be used to define the sulphate concentration. As
discussed in the previous paragraph, 3.6 mg/l sulphate fixed by seawater SO4/Cl ratio
is a reasonable threshold value. A significant higher concentration of sulphate than 3.6
mg/l should indicate severe oxidation of Boom Clay. A lower sulphate concentration
is the result of biochemical sulphate reduction processes. In argillaceous sediments,
such as the Boom Clay, bacterial sulphate reduction processes occur soon after
deposition, in the shallow burial realm. However, it can also relate to 'recent' renewed
activity of, or contamination with sulphate reducing bacteria.
1
provided that the bacterial activity, e.g., sulphate reducing bacteria, is low.
59
1.2
1
+++
Fe
.8
Eh (volts)
.6
.4
Fe(OH)3
++
Fe
.2
0
›
Pyrite› ›
–.2
›
Troilite
–.4
œ
Siderite
–.6
–.8
16°C
0
2
4
6
8
10
12
14
pH
Figure 3-7: Eh-pH diagram of Fe-S-C system at pCO2 of 10-2.4 atm and sulphate
concentration of 1 µM. The boundary between the siderite and pyrite fields represents
the anticipated Boom Clay conditions. Squares are values measured in the Cerberus
experiment at 80°C (Noynaert et al., 1998); The triangle is measured in the Cerberus
experiment at 23°C; and circle is measured by Moors et al., 2002.
3.1.4.2 Redox capacity of Boom Clay pore water
The redox buffering capacity of Boom Clay pore water isolated from the solid phase
is considered to be very low (De Cannière et al., 1996). Pirlet (2003) reported a
measured value of 0.14 meq/l and interpreted it as being dominated by dissolved
natural organic matter.
3.1.4.3 Conclusion of Boom Clay redox conditions
Both measurements and model interpretation support the conclusion that the reference
redox potential (Eh) of the non-disturbed Boom Clay should be lower than -270 mV.
This reference value is only indicative that Boom Clay is relatively reducing and
implies that laboratory experiments should be performed in an anaerobic environment.
However, because of the slow rate of redox reactions and the extremely low
concentrations of redox species in Boom Clay pore waters, it is not recommended to
use the reference Eh value to constrain the redox condition for laboratory experiments.
Future work should focus on mechanisms controlling the redox potential and
especially the redox equilibrium state of Boom Clay.
60
3.1.5 Electrolytic Conductivity (EC)
The average Electrolytic Conductivity (EC) value for Boom Clay pore water is about
1700 [µS.cm-1]. This value corresponds to a Total Dissolved Salt content (TDS) of
around ≈ 935 [mg.l-1] (for bicarbonate waters TDS, in milligrams per liter, is
calculated by multiplying the EC value, in microSiemens per centimetre, by 0.55).
3.1.6 Dissolved organic carbon (DOC) and its effect on pore water composition
Dissolved Organic Carbon (DOC) is, by definition, the most mobile organic fraction.
Mobile organic matter is commonly divided in three groups (Stevenson, 1982). The
fraction that is not soluble in alkali is the humin fraction, whereas the humic and
fulvic fractions are soluble in alkali. The humic acids precipitate below pH 2, while
the fulvic fraction remains in solution. The most important property of natural organic
matter with respect to environmental chemistry is its polyfunctional structure. The
various functional groups are responsible for the mixed complexation of cations.
Humic and fulvic acids are mostly classified as colloidal matter, with a size variation
arbitrarily defined between 1 nm and 1 µm. Because of their solubility at alkaline pH,
and because of their relatively small molecular size, fulvic and humic acids are mobile
in porous and fractured media. The DOC content is actually a Total Organic Carbon
(TOC) measurement of pore water after a filtration at 0.45 µm. For piezometer water
there is no significant difference between TOC and DOC (Van Geet, 2004). For
squeezing and leaching only TOC values are available.
3.1.6.1 Presence of TOC in Boom Clay pore water
The sampling of mobile organic matter from Boom Clay is performed in three ways
(see Section 2.1). Firstly, pore water is extracted in situ from the clay by means of
piezometers. Secondly, Boom Clay samples are squeezed to extract the water content.
Finally, organic matter is leached from Boom Clay samples at different solid/liquid
ratios. Subsequently, TOC is measured with a high temperature TOC analyser.
Pore waters sampled from piezometers contain variable amounts of TOC in function
of time. This is shown for the MORPHEUS piezometer in Figure 3-8. Two important
observations are made. Firstly, it can be noticed that a very large fluctuation of TOC
is present. The reason for this is not known. Secondly, all filters, except F8 (the filter
within the double band), show a decreasing TOC content during about 2 years of
continuous water sampling. After these two years, a steady state seems to be installed
for the TOC. For F8, in which a much higher flow rate (about 4 times higher) is
present, a steady state was probably reached already when sampling started.
Decreasing TOC contents with time have also been observed in other piezometers
(personal communication, H. Moors). Furthermore, a similar trend has been observed
by sequential leaching of Boom Clay solid phase with synthetic Boom Clay water
(Figure 3-9; Maes et al., 2003). The latter is interpreted by Maes et al. (2003) as a
characteristic dilution pattern showing the presence of an easily soluble organic matter
pool, and an organic matter fraction for which the release is dictated by an
adsorption/distribution mechanism.
61
450
400
350
Filter 2
Filter 4
300
Filter 6
TOC [mgC/l]
Filter 8
Filter 9
250
Filter 10
Filter 12
200
Filter 13
Filter 15
Filter 18
150
Filter 20
Filter 23
100
50
0
0
100
200
300
400
500
600
700
800
900
1000
Time [days]
OM conc. (measured as Abs. at 280 nm)
Figure 3-8: Evolution of the TOC content in the 12 filters of MORPHEUS, since its
installation.
2.0
1.8
1.6
1.4
1.2
S/L 0.02
1.0
S/L 0.06
0.8
S/L 0.15
0.6
S/L 0.25
0.4
0.2
0.0
1
2
3
4
5
extraction step
Figure 3-9: Leaching tests with Synthetic Boom Clay water (absence of OM)
performed at different solid/liquid ratios (1 week intermittent equilibration) (Maes et
al., 2003).
62
The TOC measurements of the MORPHEUS piezometer (mean TOC values at steady
state conditions) were compared with the TOC measurements of pore waters obtained
by the squeezing and the leaching of clay cores at the same stratigraphic depths.
Different amounts of total organic carbon are measured for the three techniques (De
Craen et al., 2002b). The difference in TOC measurements related to the extracting
technique is clearly evidenced for borehole HADES 2001/4, where a core was
retrieved for squeezing and leaching and where the MORPHEUS piezometer was
installed. Table 3-9 summarises the TOC measurements of the pore water for the 3
techniques at all 12 different levels, together with the TOC of the sediment.
Table 3-9: TOC measurements of the solid and the pore water at 12 different levels
within the Boom Clay at the corresponding depths of the MORPHEUS piezometer.
TOC of the pore water is analysed by extracting water in 3 different ways, namely
squeezing of a clay core, leaching of a clay sample, or in situ sampling from the
MORPHEUS piezometer. Pore water samples were not filtered.
F2
F4
F6
F8
F9
F10
F12
F13
F15
F18
F20
F23
Solid (wt%)
1.37
2.7
2.93
2.67
1.82
1.89
2.25
2.79
2.51
3.27
1.26
1.14
Squeezing
(mg C / l)
42.8
50
43.4
81.5
69.9
78.7
105.1
73
41
73.9
54
89
Leaching
(mg C / l)
1356.7
1168.3
3250.4
2883.3
-
3335.0
2686.9
2119.5
1822.2
2806.8
2295.5
2569.8
MORPHEUS
(mg C / l)
101.1
136
98.7
215.7
120.5
118.9
122.8
119.2
196.6
109.1
127.7
264.9
For none of the pore water TOC values, a correlation was found with the TOC present
within the sediment. Moreover, no correlation was found between the pore water TOC
measurements of any of the extracting techniques.
It is clear that squeezing results in very low TOC values compared to the other
techniques, while leaching results in very high measurements. It is believed that the
piezometer gives the most reliable results. For squeezing it is assumed that due to
porosity and pore size decrease the larger molecules cannot be evacuated from the
sample. During leaching, quite the opposite is expected. A slurry is prepared, so that a
filtration through pores is no longer active. A piezometer is assumed to keep the in
situ conditions as close as possible. Moreover, migration experiments of 14C labelled
organic matter through Boom Clay samples illustrated that low molecular size organic
matter is more easily transported through the clay than higher molecular weight
organic matter (Dierckx et al., 2000). Boom Clay, thus, seems to act as a non-perfect
ultra filter.
From the preceding it is clear that it is quite difficult to define the real TOC content of
Boom Clay pore water. If it is agreed that a steady state TOC production of a
piezometer filter is the most representative TOC value, than a mean TOC content of
about 115±15 mg C / l (range between 96 and 146 mg C / l) seems most appropriate.
However, it should be noted that much higher TOC values are recorded within the
double band, namely a mean value of about 220 mg C / l.
63
3.1.6.2 TOC versus UV measurements
Two techniques are frequently used for TOC measurements, namely UV absorbance
and a high temperature TOC analysis. For UV absorbance, two frequencies are mostly
reported in literature, namely 256 and 280 nm. For Boom Clay pore water samples,
the absorption at 280 nm is mostly used. One should be careful, however, in
interpreting results from both techniques. For a long time it was believed that there
was a linear correlation between TOC measurements and UV absorbance
measurements for mobile organic matter from Boom Clay. Henrion et al. (1985)
described a linear correlation (Figure 3-10), i.e. TOC=23.2xAbs+2.5, for a series of
diluted extracts of Boom Clay samples. A similar trend, namely
TOC=22.7xAbs+0.011 was obtained by Maes et al. (2002), once again on diluted
extracts. With the installation of the MORPHEUS piezometer, the correlation between
both techniques could not be established (Figure 3-10). Through time a constant UV
signal was measured (Figure 3-11), although a fluctuating TOC value was measured.
Henrion et al. (1985) also mentioned some measurements on piezometer water that
did not fit the obtained linear regression on diluted extracts. The pore water organic
matter contains a higher TOC content for the same UV280 absorbance as compared to
the linear relationship found by Henrion et al. (1985). This indicates that a part of the
organic carbon in the piezometer waters is UV280 insensitive.
Figure 3-10: UV-VIS absorbance measurements at 280 nm versus Total Organic
Carbon content for the different filters in the MORPHEUS piezometer, measured 800
days after installation. The trend line and data from Henrion et al. (1985) for
piezometer water and organic matter extracts are given for comparison.
64
U.V.-Vis.-measurements at 280 nm y
7
6
Filter 2
5
Filter 4
U.V.-Vis.-absorbance
Filter 6
Filter 8
4
Filter 9
Filter 10
Filter 12
Filter 13
3
Filter 15
Filter 18
Filter 20
2
Filter 23
1
0
0
100
200
300
400
500
600
700
800
900
1000
Time [days]
Figure 3-11: Evolution of the UV absorbance at 280 nm in the 12 filters of
MORPHEUS, since its installation.
3.1.6.3 Characteristics of the mobile organic matter in Boom Clay
An overview of the current knowledge on Boom Clay organic matter characteristics is
given in Van Geet et al. (2003). A short overview will be given here. A first important
characteristic of organic matter is its molecular size distribution. Two techniques have
been used to determine this ditribution, namely ultrafiltration and Flow Field Flow
Fractionation (FFFF).
For ultrafiltration, two experimental set-ups were used. The first uses Amicon filters
at 1, 10, 50 and 100 kDa and a gas pressure was used to force the Boom Clay pore
water through the filters. The second set-up uses centrifugal ultrafiltration units (Pall
Life Sciences: Omega membranes) of 1, 10, 30 and 100 kDa. It should be noted that
the Amicon filters consist of regenerated cellulose, while the Omega membranes
consist of Poly Ether Sulfones. Both types of filters are pre-conditioned by keeping
them in bidistillated water for one week, refreshing the water every day. Blanco
experiments were performed for these pre-conditioning technique and did not show
any measurable TOC content. The first experiment was performed on piezometer
waters at 9 different stratigraphical levels (F23, F20 and F18 were not used) and
sampled 352 days after installation of the MORPHEUS piezometer. The results are
shown in Figure 3-12, illustrating that about 45 % of the mobile organic matter is
smaller than 1000 Da and 45 % is larger than 50,000 Da. This might correspond with
the bimodal structure that is observed in GPC measurements during the TRANCOMClay project, although no calibration has been performed (Dierckx et al., 2000). The
second experiment was performed on piezometer water at 6 different stratigraphical
levels and sampled 853 days after installation of the MORPHEUS piezometer. Results
of the distribution are given in Figure 3-13. Again a bimodal distribution is obtained.
However, most of the mobile organic matter is now smaller than 10 kDa, except for
filter F8 (positioned in the double band, a more silty layer) that contains mostly
mobile organic matter between 30 and 100kDa.
65
DOC molecular size distribution (Amicon UF), incorporating 100k filtration
55
Relative contribution (%)
45
35
F2
F4
25
F6
F8
F9
15
F10
F12
F13
5
F15
50k to 100k
-5
10k to 50k
1k to 10k
<1 000
Molecular weight (Da)
Figure 3-12: Molecular weight distribution of the pore water organic matter found in
the different filters in 9 stratigraphical levels in the MORPHEUS piezometer,
sampled 352 days after installation, by means of Amicon ultrafilters.
80
70
relative contribution (%)
60
50
F6
F8
F12
F18
F20
F23
40
30
20
10
0
-10
100k-.45
30k-100k
10k-30k
1k-10k
<1k
-20
molecular weight (Da)
Figure 3-13: Molecular weight distribution of the pore water organic matter found in
the different filters found at 6 different stratigraphical levels in the MORPHEUS
piezometer, sampled at 853 days after piezometer installation, by means of centrifugal
ultrafiltration units (Omega membranes)
66
The pore water analysed originates from the same filters, but only differs in the timing
of sampling, which also results in different DOC concentrations. A comparison of
both data sets (Figure 3-14) shows that there exists a good correlation between both
experimental set-ups, except for the ultrafitration at 100kDa. The Amicon
ultrafiltration gives an overestimation of the fraction 50 to 100 kDa, compared to the
centrifugal ultrafiltration fraction 30 to 100 kDa. It is unclear whether this difference
is related to an experimental error (poor pre-conditioning of ultrafilter), different
forces with which the organic matter is forced through the filter, or to the difference
observed in DOC between the two moments of sampling.
Thang et al. (2001) performed a flow field flow fractionation (FFFF) on EG/BS
piezometer pore water. A calibration of the technique with polystyrene sulphonate
was used to determine quantitatively the size fraction of the Boom Clay mobile
organic matter. Their study illustrates that the most important fraction of mobile
organic matter has a molecular weight below 4000 Da. It should be noted, however,
that the authors mention possible deviations of the exact values when using other
calibration material.
F6
TOC content after Pall Life
Sciences UF (mg C/l)
135
125
115
105
y = 1.3511x - 33.799
95
2
R = 0.9553
85
75
75
95
115
135
155
175
TOC content after Amicon UF (mg C/l)
195
Figure 3-14: Example of the TOC contents measured after two types of ultrafiltration
(Amicon and centrifugal ultrafiltration units) for filter 6 of the MORPHEUS
piezometer, showing a good correlation between both techniques for most
ultrafilters(◊), except for the 100 kDa UF (■).
A second characteristic of mobile OM is its functional group capacity. The latter has
been determined during the TRANCOM-Clay project (Dierckx et al., 2000). This
functional group capacity was measured on different sources of Boom Clay organic
matter and by means of different techniques. The functional group capacity in the pHrange 7-8 was taken to be representative for the Boom Clay environment and used for
67
comparison (Table 3-10). It was concluded that three types of functional groups are
needed to describe the titration curves within the studied pH-range.
Table 3-10: Comparison of the functional group capacity in the pH-range 7-8 for
different organic matter samples (Dierckx et al., 2000).
Name of
sample
Extraction technique
Measuring technique
Capacity
[Eq/mg]
EG/BS (V103)
Piezometer pore water
Titration
5.95
EG/BS (V3A)
Piezometer pore water
Titration
6.19
EG/BS (3)
Piezometer pore water
Titration
5.10
Total extracted
Boom Clay
Leaching BC with SCW with an L/S ratio of 1/2
Titration
1.80
BCPHA
Concentration of EG/BS and then isolation,
purification and transformation to the proton form
of the HA
Titration
2.85
BCEHA
Successive extraction of HA from Boom Clay
with NaHCO3 0.015 M in air under close to in
situ carbonate concentration conditions
Cobalt hexamine method
2.1
BCEHA (2)
Similar as for BCEHA with an additional
extraction step consisting in bringing the Boom
Clay to pH 3 to break salt bridges
Titration
1.65
A last characteristic of Boom Clay organic matter is its molecular organic chemistry.
The latter has not yet been established due to the experimental difficulty of such a
characterisation. However, a recent technique of electrospray ionisation quadrupole
time-of-flight mass spectrometry has been proposed and tested on pore water from the
Mol aquifer below the Boom Clay. Plancque et al. (2001) used this technique for the
characterisation of the fulvic acids occurring in the aquifer below the Boom Clay.
The fulvic acid solution seems to be a complex mixture of several hundred molecular
structures. Tandem mass spectrometry experiments demonstrated losses of 18 Da
(water molecule) and of 44 Da (CO2 molecule), which indicates the presence of
carboxylic functional groups. A representative molecular structure of the molecules,
which are most likely to be self-assembled in the fulvic acid samples, is given in
Figure 3-15. Consequently, the fulvic acids of the Mol aquifer contain two families
having homologous substructures with alkyl chains terminated by CH3 or NH2. Both
families contain a mixture of species with a range of numbers of carboxyl functions,
alkyl chain lengths and of numbers of substituents on the aromatic ring.
68
Figure 3-15: Representative molecular structure proposed for fulvic acid from the
Mol aquifer (Plancque et al., 2001).
3.1.7 Evaluation of extraction techniques and recommendations for water
sampling and storage
At the present time, pore water samples from piezometers are considered to be the
most representative for the determination of the in situ pore water composition. This
is because piezometer waters experience minimum laboratory manipulations and
therefore suffer minimum artefacts. However, pore waters obtained in the laboratory
from well-preserved clay cores also provide valuable data, which are, in a lot of cases,
not attainable by the piezometer technique. Consequently, the applied pore water
sampling technique should be chosen as a function of the kind of
study/analyses/experiments one wants to perform.
Squeezed pore water samples can be considered as representative for the in situ
conditions, up to a certain degree. These waters generally have a comparable major
element composition, but higher amounts of trace elements and lower amounts of
dissolved organic matter are generally observed compared to piezometer waters (see
Section 3.1). Furthermore, the slightest oxygen perturbation of the clay core strongly
affects the geochemical composition of the pore water
Leached pore water samples have a composition which is strongly different from
piezometer water and squeezed water (see Section 3.1). The leaching technique needs
a lot of laboratory manipulations and is therefore applied under conditions different
from that of in situ. Among many factors, system pH (pCO2), ionic strength, solid to
liquid ratio, and microbial activity are the most probable factors influencing these
water compositions.
Physical changes during pore water extraction (decreasing porosity during squeezing;
increasing porosity during leaching) also influence the composition of the obtained
pore water, in particular the content of natural organic matter. This is because the
natural organic matter in Boom Clay has a wide distribution of molecular weight and
a large fraction is present in colloidal form. The most obvious evidence is that the
squeezed water is colourless comparing to the yellowish to brown piezometer water.
This indicates the removal of the coloured dissolved organic matter by squeezing. In
69
contrast, the leaching technique resulted in deep brownish water containing the
highest amount of natural organic matter (De Craen et al., 2002b).
It is clear that great care should be taken during the drilling of the borehole, the
anaerobic preservation and the handling of the clay cores, to prevent oxygen
perturbation.
The drilling of the borehole should be carefully planned. In the HADES URF,
boreholes are generally air drilled. The main disturbance of air drilling consists of a
desaturation and oxidation of the clay. These processes can be minimalised when the
drilling is performed in a non-oxidising environment, for example when nitrogen or
argon is used.
Pore water sampling from piezometers at a low flow rate probably provides the most
representative pore water, because the system is the least disturbed.
Dehydration and oxidation of clay cores can be prevented with some simple
precautions. The clay cores should be isolated from atmospheric air as soon as
possible. This means that, immediately after drilling (and at the drilling site), the clay
cores should be vacuum-packed in Al-coated poly-ethylene sheets, flushed with
nitrogen or argon, then evacuated and sealed. The clay cores should then be stored at
4°C in anaerobic conditions.
In the laboratory, oxidation of the clay cores can be prevented by working in
anaerobic glove boxes. The outer rim of the clay core, which has been inevitably in
contact with air during the drilling, should be removed to eliminate possible effects of
oxidation.
For the sampling of the pore water, sampling containers, or dark bottles should be
used to avoid organic matter degradation as a result of UV radiation. Finally, the
samples should be stored at low temperatures (at 4 °C) to avoid bacterial activity in
the pore water.
70
3.2 Isotope geochemistry
3.2.1 Stable isotopes
The stable isotope composition of Boom Clay pore water was studied in the frame of
the ARCHIMEDE-argile project (Reeder et al., 1994; Griffault et al., 1996; Beaucaire
et al., 2000). In this study, a profound analysis of the Lower Rupelian aquifer, at a
regional scale, was also performed. This allowed for comparison between the Boom
Clay and its underlying aquifer. Other studies also focussed on the characterisation of
the groundwater in the aquifers above and below the Boom Clay at a regional scale
(Beaufays et al., 1994; Marivoet et al., 2000) with no or few analyses on the Boom
Clay pore water.
During the PHYMOL project, stable isotope analyses were performed on 4 pore water
samples obtained by the distillation and the squeezing of clay cores from the Zoersel
borehole, and on 20 pore water samples obtained by the squeezing of clay cores from
the Weelde borehole (Marivoet et al., 2000; Philoppot et al., 2000). No samples were
taken at the Mol site. δ18O values range from -5.3 to -8.4 ‰. δ2H values range from 43.1 to -58.5 ‰. Pore water samples obtained by distillation always have slightly
more depleted δ18O and δ2H values. No interpretation was however proposed on these
values.
In the ARCHIMEDE-argile project, one pore water sample from the ARCHIMEDEpiezometer #1, and 5 pore water samples obtained by the squeezing of clay cores were
analysed for their stable isotope composition. The results are given in Table 3-11.
Table 3-11: Stable isotope data of Boom Clay pore water, data from the
ARCHIMEDE-argile project (Reeder et al., 1994; Griffault et al., 1996). SMOW =
Standard Mean Ocean Water, CDT = Canyon Diabolo Troilite.
Boom Clay
pore water
Sample reference
δ18O water
δ2H water
δ18O sulph.
δ34S sulph.
‰
vs SMOW
‰
vs SMOW
‰
vs SMOW
‰
vs CDT
-12.5
-10.0
-37.1
-39.1
-6.6
-25.9
ARCHIMEDE
piezo #1
P1,8m
-7.28
-54.9
Squeezed
clay cores
TD R97/98 WH 1,15-1,35 m
TD R97/98 WH 2,35-2,55 m
A07478 7,00-7,10 m
A07479 8,15-8,25 m
A07480 14,20-14,40 m
-7.07
-7.05
-7.10
-7.25
-6.97
-52.3
-54.5
-54.0
-52.3
-53.7
71
The present day isotope composition of ocean water is more or less constant with δvalues very near to zero (Hoefs, 1997). In contrast, the isotope composition of
meteoric waters vary linearly (linear relationship is described as the "meteoric water
line"; IAEA, 1992) on a global scale and are dependent on geographic location. The
isotope composition of the mean worldwide precipitation is estimated to be δ18O = -4
‰ and δ2H = -22 ‰ (Craig and Gordon, 1965).
The isotope composition of Boom Clay pore water is δ18O = -7 ‰ and δ2H = -53 ‰.
These values suggest that the pore water is not of marine origin. A meteoric origin can
explain the δ18O values, but not the δ2H values. In Figure 3-16, the isotope
composition of Boom Clay pore water is compared to the meteoric water line and
groundwater of various Rupelian aquifers (for details see Griffault et al., 1996).
2
H (‰ vs SMOW)
0
M WL
-10
Boom Clay pore water
(ARCHIMEDE piezo #1)
-20
-30
Boom Clay pore water
(squeezed clay cores)
-40
Rupelian aquifers
-50
-60
-10
-8
-6
-4
-2
0
18
δ O (‰ vs SMOW)
Figure 3-16: δ18O versus δ2H of Boom Clay pore water compared to the Meteoric
Water Line (MWL) and groundwater of Lower Rupelian aquifers from various
locations (from Griffault et al., 1996).
Groundwaters from the Lower Rupelian aquifer are in the range of values close to
meteoric water line (Figure 3-16). This indicates that the recharged conditions are
either those of present day or climatically little different from these (Griffault et al.,
1996).
The oxygen and hydrogen isotope composition of Boom Clay pore water fall under
the Meteoric Water Line. This is the case for pore water collected from the piezometer
as well as for pore water derived from the squeezing of clay cores (Figure 3-16). 2Hdepletion is a common observation in water extracted from clayey formations. It is
therefore considered as an artefact of the extraction technique, causing stable isotope
fractionation (Griffault et al., 1996).
According to the chemical analyses performed by Reeder et al. (1994), sulphate is the
dominant anion in most squeezed pore waters (values between 1130 and 11100 mg/l)
and substantial quantities of thiosulphate are often present (200 – 500 mg/l). In order
to characterise the origin of the dissolved sulphate, the δ18O and δ34S isotope
72
composition of the dissolved sulphate is measured. The low δ18O and δ34S values for
dissolved sulphate (see Table 3-11) suggest that sulphide oxidation indeed occurred in
these Boom Clay pore water samples. Also De Craen (2001) and De Craen et al.
(2002a and 2004a) explained these high concentrations of sulphate and thiosulphate as
the result of oxidation.
3.2.2 Radioisotopes
3.2.2.1 U-Th isotopes
In the frame of a regional programme of hydrochemical and isotopic measurements,
uranium series disequilibrium studies were performed on groundwater samples
collected from sand layers above and below the Boom Clay in Mol (Ivanovich and
Wilkins, 1988). Unfortunately, Boom Clay pore water itself was not sampled.
Recently, uranium series disequilibrium studies were performed on pore water derived
from the MORPHEUS piezometer. The uranium content (238U) in the pore water is
generally less than 1 µg/l. Higher uranium contents are present in pore water sampled
from septaria level S50 (1.3 µg/l), and in pore water sampled from an organic-rich
layer at the base of the Putte Member (2.2 µg/l). The 234U/238U activity ratios are
indicative of radioactive disequilibrium, with 234U/238U activity ratios between 1 and
5. An excess of 234U relative to 238U, however, is a common observation in natural
waters (Fleisher, 1988). It is explained as the result of α recoil, either as the direct
recoil ejection of a recoil atom into the pore water, or indirectly by recoil leaching
(the preferential etching of recoil damage and associated preferential solubility of the
products of α decay).
Squeezed pore water samples always contain higher amounts of uranium (generally
about 5 µg/l, but values up to 18 µg/l are also measured) compared to pore water
derived from piezometers. The higher uranium content can be explained by the
presence of colloids in the squeezed pore waters. The 234U/238U activity ratios of
squeezed pore water samples (De Craen et al., 2000; De Craen et al., 2004b) show
radioactive disequilibrium, with ratios between 2 and 6. These results fall in the same
range as the results of the pore water sampled from the MORPHEUS piezometer.
3.2.2.2 14C
In the frame of the PHYMOL project (Marivoet et al., 2000), 14C measurements were
performed on Boom Clay pore water. Unfortunately, no 14C was detected (with a
detection limit of 60 000 years).
3.2.2.3 36Cl
Until now, 36Cl measurements have not been performed on Boom Clay pore water.
73
3.3 Spatial variability
Although the mineralogical and geochemical properties of the Boom Clay are
considered to be rather constant throughout the deposit (Vandenberghe, 1978; De
Craen et al., 2000), Boom Clay is characterised by a banded nature or layering. This
layering is the result of variations in grain-size, organic matter and carbonate content
(Vandenberghe, 1978). Thus a vertical variability exists in the Boom Clay deposit.
Several of these layers were found to have enough specific characteristics to be
recognised whenever they were found in outcrops, clay cores or even in geophysical
logs, the so-called 'marker horizons' (see references in Van Keer and De Craen, 2001).
Examples of marker horizons are: a pinkish layer, calcareous-rich layers containing
septarian carbonate concretions, the double band, the boundary between organic-poor
and organic-rich layers (corresponding to the boundary between the Terhagen
Member and the Putte Member). These layers can be followed on a regional scale. In
general, the composition of Boom Clay doesn't change much laterally, although some
slight variations in the clay mineralogical composition can be recognised (Laenen,
1997; De Craen et al., 2000; Van Keer and De Craen, 2001).
Whether these variations in Boom Clay compositions are also reflected in the pore
water composition is considered in the following paragraphs.
3.3.1 Vertical variability
To study the vertical variability of the Boom Clay pore water composition, the
MORPHEUS piezometer was designed. Pore water is sampled at 12 different
stratigraphic levels. The piezofilters are positioned in silty clay / clayey silt, in
organic-poor / organic-rich clay, in septaria layers, and in the double band.
In Figure 3-17, the concentration of some major elements is plotted against depth. The
chemical composition of the pore water is comparable in most of the filters, except at
a depth of 230.28 m TAW. At this level, a concentration peak can be recognised for
most of the elements. At the same level, much more dissolved organic matter is
measured in the pore water (see Section 3.1.5). This sample corresponds to the double
band.
Thus, apart from the double band, the chemical composition of the pore water seems
to be comparable for most of the filters, suggesting that the variability of the Boom
Clay composition is not reflected in its pore water chemistry. A more detailed study
on the vertical variability of the pore water composition by statistical analyses
(Section 3.4), however, will disprove this conclusion.
74
HADES
borehole 2001/4
-216
-218
-220
dep th (m TA W )
Putte
Member
-222
-224
-226
-228
-230
Terhagen
Member
-232
-234
-236
0
2
4
6
8
10
Ca
Fe
Mg
K
0
10
20
30
concentration (mg/l)
concentration (mg/l)
Si
F-
Cl-
0
200 400 600 800 1000
concentration (mg/l)
Br-
Na
HCO3-
Figure 3-17: Chemical composition of Boom Clay pore water sampled in the
MORPHEUS piezometer.
3.3.2 Lateral variability
The lateral variability of the Boom Clay pore water composition at the Mol site can be
studied by comparing the pore waters sampled from the various piezometers.
Variations in pore water chemical composition at the Mol site are small. Moreover,
these variations might be related to other factors, such as different types of filter
materials, as explained in more detail in Section 3.4.1.
To study the lateral variability of the Boom Clay pore water composition on a
regional scale, squeezed pore water samples from various boreholes were analysed
(Figure 3-18): the Doel-2b borehole, the Zoersel borehole, the Mol-1 borehole, and
the HADES borehole 2001/4 (also at Mol). In these four boreholes, the zone between
S40 and S50 was considered (De Craen et al., 2000; De Craen et al., 2004b).
75
URF Underground Research Facility
Figure 3-18:Location of the boreholes used in the study of the lateral variability of
the Boom Clay: the Doel-2b borehole (in Doel), the Zoersel borehole (in Zoersel),
the Mol-1 borehole (in Mol), and the HADES borehole 2001/4 (also in Mol).
The chemical composition of squeezed Boom Clay pore waters are given in Table
3-12. As mentioned above, the pore water of the double band always has a different
chemical composition, and therefore, the double band is not taken into account for the
calculation of the mean values.
It should be mentioned that the clay cores from the HADES 2001/4 borehole were
well-preserved and squeezing was performed soon after the drilling of the clay cores.
In contrast, clay cores from the Doel-2b, Zoersel and Mol-1 boreholes have been
preserved for a few years in comparable (but not ideal) conditions. These clay cores
might have been oxidised. Oxidation of the clay cores affects the chemical
composition of the pore water (De Craen, 2001; De Craen et al. 2002a; De Craen et
al., 2003). However, some elements, such as chloride, are not influenced by oxidation.
Furthermore, the squeezed pore water samples from the HADES 2001/4 clay cores
were filtered at 0.45 µm before analyses of the cations. Filtration was not done on
pore water samples from the other boreholes. This explains why higher amounts of
Mg and Si are present in pore water samples of the Mol-1 borehole compared to the
HADES 2001/4 borehole. Small clay particles, colloids, were probably measured as
well, resulting in higher amounts of some elements, such as Mg and Si.
76
Table 3-12: Chemical composition of squeezed Boom Clay pore water in various
boreholes.
Doel-2b borehole
Zoersel borehole
HADES 2001/4
borehole
Mol-1 borehole
mg/l
Min.-max.
mean
Min.-max.
mean
Min.-max.
mean
Min.-max.
mean
Ca
8.4 – 339
40.7
3.5 – 10.3
5.6
3.6 – 9.3
5.1
1.2 – 8.4
3.5
Fe
0.12 – 10.1
1.08
0.16 – 0.62
0.37
0.4 – 8.2
2.4
<0.05 – 11.1
0.9
Mg
7.7 – 425
62.8
6.0 – 20.0
11.7
3.4 – 34.2
18.8
0.9 – 10.4
2.8
K
10.1 – 115
27.5
4.9 – 14.4
7.6
4.8 – 29.5
8.5
3.9 – 15.0
6.8
Si
5.1 – 11.5
7.2
5.9 – 10.5
8.3
9.1 – 33.5
15.0
6.4 – 11.3
8.0
Na
381 – 1780
953
365 – 777
502
238 – 1240
378
225 – 730
356
-
F
Cl
0.9 – 1.4
1.1
1.1 – 1.9
1.7
2.2 – 3.0
2.4
1.4 – 3.0
2.2
-
317 – 4000
1230
248 – 347
323
20.4 – 52.2
28.5
20.6 – 39.9
27.3
-
2.1 – 11.2
4.0
1.26 – 1.75
1.45
0.5 – 5.6
1.2
0.43 – 1.43
0.80
53 – 507
215
6 – 618
172
32 – 999
188
3.6 - 700
155
247 - 742
447
448 - 1209
616
422 - 1122
561
422 - 707
575
Br
SO42HCO3
-
The most important lateral variability of the Boom Clay pore water chemistry on a
regional scale is probably the variation of the NaHCO3 -type water in Mol to a
NaCl/NaHCO3 -type water in Doel (Figure 3-19). The mixing of the NaHCO3-type
water with NaCl is clearly increasing from the east (Mol) to the west (Doel).
A gradient from the east to the west is also present for some other elements. These
variations might be linked with the general change of the water-type. However, this
interpretation should be taken with some care, because possible oxidation of the
samples might also have influenced the pore water chemistry.
Doel-2b borehole
DoTP - samples
83
Zoersel borehole
ZTP - samples
mg / l
Depth
(m)
0
1000 2000 3000 4000
vv vvv vvv
150
84
151
85
86
vv vvv vvv
87
vv vvv vvv
153
vvvvv vvv
256
vv vvvvvv
257
vv vvvvvv
258
vv vvv vvv
156
90
91
vv vvv vvv
259
vvvvv vvv
vv vvv vvv
vvvvv vvv
158
vv vvvvvv
260
261
vv vvv vvv
vv vvvvvv
263
160
94
161
95
Na
Cl
HCO3-
162
Na
Cl
HCO3-
258
259
vvvvvvvv
vvvvvvvv
vvvvvvvv
260
261
262
262
159
93
vvvvvvvv
vvvvvvvv
92
257
vvvvvvvv
vvvvvvvv
157
255
256
vvvvvvvv
vv vvvvvv
vv vvv vvv
1000 2000 3000 4000
255
vvvvv vvv
155
0
254
Depth
(m)
vvvvv vvv
vvvvvvvv
vv vvv vvv
mg / l
Mol-1 borehole
MTP - samples
vvvvv vvv
152
154
89
1000 2000 3000 4000
vv vvv vvv
vv vvv vvv
88
mg / l
0
Depth
(m)
vvvvvvvv
263
264
264
265
265
266
Na
Cl
HCO3-
Figure 3-19: Lateral variability on a regional scale of the NaHCO3 and NaCl
concentration in the Boom Clay.
77
3.4 Data quality
3.4.1 Statistical Analysis
Several piezometers starting from the URF are available for the characterisation of the
Boom Clay pore water in Mol. However, the question raises whether we can provide
one mean composition from the waters sampled and analysed from those different
piezometers. It is clear that several factors may influence the measured composition,
so that a statistical analysis is appropriate. In this Section we will discuss all factors
that might have an influence on the measured pore water composition, the factors that
will be analysed statistically, the data used for the statistical treatment, and the results.
It should be noted that this is only a statistical treatment of data. When statistically
significant differences are found, one still needs to look for the possible geochemical
causes for these differences.
3.4.1.1 Factors that might influence pore water composition
Pore water sampling from piezometers is a quite simple procedure. However, many
processes are involved so that many factors might influence the final pore water
composition. The following factors are assumed to be the most important.
•
Spatial variability
Boom Clay is sometimes treated as a homogeneous material. However, it is
known that natural layering does occur and variances in mineralogy, grain
size, etc. are present (see Section 1 and Section 3.3). Consequently, it should
be considered that the 3D position of the filter within the Boom Clay might
have an effect on the pore water composition.
•
Evolution in time
Placing a piezometer causes a disturbance of the host rock (see Section 2). It is
known that this disturbance will be restored. However, it should be tested if an
evolution in time of the pore water composition is noticed.
•
Effect of filter material
As described in Section 2, different filter materials can be used in the design of
a piezometer. The effect of this filter material should be tested as well.
•
Effect of sampling conditions
The final pore water sampling can be performed in aerobic or anaerobic
conditions. This also might influence the measured composition.
It is thus clear that many factors might influence the pore water composition. In this
study, the effect of only some of the factors is studied. The factor of spatial variation
is only partly studied. It is known that the natural layering within the Boom Clay is
very continuous parallel to the bedding. Moreover, only the Boom Clay pore water
from the Mol site is used. The used piezometers are laterally only separated by a
maximum of 67 m of clay. Consequently, the effect of spatial variation is studied in
one dimension, namely perpendicular to the bedding. The effect of time evolution is
also only partly considered. Actually, samples that might have an influence of the
disturbance due to the emplacement of the piezometer are excluded. Since it is known
that SO42- will be produced during installation due to the oxidation of the surrounding
clay, a criterion based on the sulphate content is defined. It is stated that the sulphate
78
content should be constant and low (<6 mg/l). The effect of filter material is taken
into consideration in this study. However, it should be noted that only a very limited
amount of data is available to check this influence. The effect of sampling in
anaerobic or aerobic conditions is not incorporated in this statistical evaluation. It is
assumed that the latter has no important influence on the results.
Furthermore, it should be noted that different analysis techniques might influence the
measured composition. However, this influence is not considered because of the lack
of sufficient data. Moreover, the oldest results, date from 1996 and we have checked
that since then, no new techniques became in operation at the Nuclear Chemistry and
Services, SCK•CEN.
3.4.1.2 Data and statistical techniques
To evaluate the effect of the above described factors, different piezometers are
available. Within this statistical evaluation, 5 piezometers are considered, namely
EG/BS, MORPHEUS, ORPHEUS, SPRING 116 and ARCHIMEDE #1 (see Annex
5). EG/BS is used already a long time for many laboratory experiments and
consequently provides a large amount of data (70 analyses of major elements are
available). MORPHEUS contains 12 filters at different layers. For 11 of these filters 4
analyses are available. ORPHEUS is a horizontal piezometer with 4 filters of different
materials. For each filter material, a different amount of data is available, with a
maximum of 5 analyses (n ≤ 5). For SPRING 116, a horizontal piezometer with 4
stainless steel filters, only two analyses for each filter are available (n=2).
ARCHIMEDE #1 is a horizontal piezometer with 5 filters; 2 to 5 analyses are
available for the 4 deepest filters. Although pore water sampling is taking place
already a long time, the amount of useful data is limited. Especially the amount of
data to test the effects of filter material and the effects of spatial variability is low.
This results from the fact that the piezometers that might give an answer to these
questions were recently installed in the URF. Most other piezometers are in the
horizontal plane, or were used for pore water pressure measurements. For the
statistical evaluation, only the major elements occurring in the pore water are
included: B, Br, Ca, Cl, F, Fe, K, Mg, Na, Si. TIC is not available for all the
measurements and is only used if available.
For easy recognition, each pore water sample got a code in which the name of the
piezometer, filter material, sample number and date of sampling is included (see
Annex 1).
For the statistical evaluation it is first tested whether the pore water sampled from
different filters of one single piezometer can be grouped to one single population.
This is done element per element using an analysis of variance (ANOVA1). This
analysis provides a probability value indicating whether all filters can be grouped to
one single population (H0: µ1=µ2=...=µm). This is the case when the probability
p>0.05 (95% confidence interval). The basic assumptions when using the ANOVA
test are:
•
•
All data are normally distributed.
The variance within each of the populations is equal.
79
A limited amount of data is available, but the assumptions can be supposed to be true.
A description of the ANOVA technique and the check of the assumptions is given in
Annex 6.
Next, several elements will be considered together to answer the following questions:
•
•
Is there an effect of the filter material on the measured pore water
composition?
Is there an effect of the spatial variability on the measured pore water
composition?
To this end, a multivariate analysis of variance (MANOVA) is used. This MANOVA
allows to determine whether the entire set of means of all variables (elements) is
different from one group to the next. In MANOVA several linear combinations of the
original variables (elements) are defined, so that the largest separations between
groups occur. From these linear combinations an estimate of dimension (d) of the
groups means is defined. If the means were all the same, the dimension would be 0. If
the means differed, but fell along a line, the dimension would be 1, and so on. For
each dimension a probability (p) can be calculated. A scatter plot of the linear
combinations that result in the largest separation between groups will thus show more
separation between groups than a scatter plot of any pair of original variables. If the
estimated dimension is 2, than this scatter plot most probably shows all variation
between the groups. The basic assumptions when using the MANOVA test are
•
•
•
•
Data are independent.
Data are normally distributed.
No big difference of the variances between groups of the same variables.
MANOVA is sensitive to outliers.
A description of the MANOVA technique and an evaluation of the assumptions is
given in Annex 6.
3.4.1.3 Results of the statistical analyses
First of all the ANOVA analysis of each piezometer will be discussed. Later on, the
MANOVA analysis related to the questions whether filter material and spatial
variability influence the pore water composition will be given.
EG/BS
EG/BS is a vertical piezometer surrounded with coarse sand, so that a large vertical
segment of the Boom Clay is draining into the filter. EG/BS is in use already for a
long time, so that measurements over a longer period are available. The first results
date from 1996. For all measurements available, the sulphate value is very low and
can be assumed as being constant. The first 3 measurements have an unknown starting
date of sampling and were excluded from the statistical evaluation. For all elements a
visual inspection of the analytical results did not define any systematic time trend.
However, it was noticed that two groups can be distinguished in time. The first group
consists of the measurements from 1996-03-13 till 1999-04-07. The second group
contains data from 2000-08-08 till 2003-02-17. In between the two groups, a serious
time lack of data is present, and based on the visual inspection a shift in the mean
value of both groups is possible. It is unknown whether anything happened during this
80
time gap that might have affected the composition of the pore water. For all major
elements listed above, 69 or 70 data points are available, except for Br for which only
26 data points are available. For TIC, 52 data points are available. For most of the
elements, group 1 contains 45 (or 44) data points and group 2 contains 25 data points.
Table 3-13 lists the p values obtained with the ANOVA test. For the elements Na, K,
TIC, Cl and F, there is significant evidence that these elements belong to one single
group. However, for the other half of elements considered (Ca, Fe, Mg, B and Si) this
is not the case. Consequently, the most conservative option would be to take into
account that both groups may be different. However, it should be noted that the most
abundant elements (Na, TIC) can be considered to belong to one single group.
Table 3-13: ANOVA probability values for each element testing the hypothesis that
the two groups distinguished in time in the EG/BS piezometer belong to the same
group. P-values above 0.05 (in bold) are accepted as significant.
K
Mg
Na
Si
B
Ca
Cl
F
Fe
HCO3
8.8·10-8
0.017
0.090
0.2353
0.010
0.826
0.810
0.002
0.250
0.016
ARCHIMEDE #1
The ARCHIMEDE #1 piezometer is a semi-horizontal piezometer (3% inclined
upwards) with 5 filters at respectively 15, 14, 8, 7 and 3 m from the gallery. Pore
water analyses are only performed for the 4 deepest filters. From the deepest to the
closest filter, respectively, 2, 5, 5 and 2 analyses are available. The sulphate criteria
are met in all the analyses. However, not all the major elements were always
measured. Therefore, the ANOVA 1 test is limited to Cl, F, Na, K, Ca and Mg. The
results of the ANOVA 1 test suggest that all the analyses can be considered as
belonging to one single group (see Table 3-15).
Table 3-14: ANOVA probability values for several element testing the hypothesis that
the filters of the ARCHIMEDE #1 piezometer belong to the same group. P-values
above 0.05 (in bold) are accepted as significant.
Cl
F
Na
K
Ca
Mg
0.59
0.78
0.21
0.85
0.21
0.26
SPRING 116
The SPRING 116 piezometer is a horizontal piezometer containing 4 large stainless
steel filters. Filter 1 is the deepest filter, while filter 4 is the nearest one. For each of
these filters only 2 measurements are available. This is too few for statistical analyses.
However, if the groups do not seem to differ, all measurements can be combined to
increase the number of data points in a further statistical evaluation. In all these
measurements the sulphate content is below 6 mg/l. For filters 1, 2 and 3 the sulphate
content seems to be more or less constant, but for filter 4 the sulphate content still
81
shows a decreasing trend (more than 2 sulphate measurements are available for
deduction of these conclusions). However, all data samples are used. It is assumed
that for each filter the data of each element follows a normal distribution (see Annex
6). Table 3-15 shows the probability values that all filters belong to 1 group. For most
of the elements (Na, Ca, Mg, Fe, Cl and Si) this probability is very low and thus it can
be concluded that not all filters belong to the same group. From a multicomparison of
the ANOVA results it can be concluded that filter 1 has the most deviating values
compared to the other filters. Therefore, an ANOVA test was also performed on filters
2, 3 and 4, only. Table 3-15 gives the p-values, showing that, apart from Mg, all
elements of these 3 filters seem to belong to one group. Consequently, filters 2, 3 and
4 are considered as one group with 6 data points, while filter 1 is considered as a
different group with only 2 data points. Up to now, it is unknown what caused the
difference between the filters of this piezometer.
Table 3-15: ANOVA probability values for each element testing the hypothesis that
the filters of the SPRING 116 piezometer belong to the same group. P-values above
0.05 (in bold) are accepted as significant.
B
Br
Ca
Cl
F
Fe
K
Mg
Na
Si
All
filters
0.074 0.661 0.003 0.002 0.232 0.011 0.062 0.003
Filters
2, 3
and 4
0.303 0.977 0.268 0.282 0.211 0.258 0.555 0.0149 0.175 0.173
0.001 0.043
ORPHEUS
The ORPHEUS piezometer is a horizontal piezometer containing 4 different filters,
each of another material, namely: PolyEthylene (PE), Glass, Carbo, and a sintered
aluminium silicate (Schuma). For the PE, glass and Schuma filters respectively 5, 4
and 3 measurements are available for which a constant and low (<6 mg/l) sulphate
concentration can be deduced. For the Carbo filter only 1 measurement is available
that fits the sulphate criteria stated above. However, for an ANOVA test, at least two
measurements are needed. Therefore the measurement with the lowest but one
sulphate content, i.e. 7.7 mg/l, is included as well. Consequently, one should be very
cautious in making conclusions on this filter material. For all the major elements
listed above data are available. For TIC, however, no data are available. Table 3-16
lists the probability values that all filters belong to one group for each element. It is
clear that only for the elements Ca and Fe no distinction can be made on the filter
material. For all other elements there is a significant difference.
Table 3-16: ANOVA probability values for each element ANOVA testing the
hypothesis that the 4 different filter materials in the ORPHEUS piezometer belong to
the same group. P-values above 0.05 (in bold) are accepted as significant.
B
Br
Ca
Cl
F
Fe
K
Mg Na
Si
9.5·10-7
0.035
0.364
0.001
0.029
0.253
0.001
0
0
1.43·10-11
82
MORPHEUS
The MORPHEUS piezometer is a vertical piezometer with 12 different filters (each
10 cm high) at several distinguished layers. Due to some technical errors the top filter
did not provide water during a long time and consequently only one analysis of the
pore water composition of this filter is available. Therefore, this filter was not
included in the statistical evaluation. For each of the other filters, 4 measurements are
available. In all 4 of them the sulphate content is very low and can be assumed to be
constant. In all measurements, data for all the major elements listed above and for the
TIC are available, except B for which only 2 data points are available. As only 4
measurements are available, a histogram cannot be provided, and thus it is assumed
that the measurements of each filter are normally distributed (see Annex 6). Table
3-17 gives the p-values that are obtained for each element, considering all 11 filters. It
can be concluded that, except for B and Br, none of the elemental analysis of all filters
belongs to 1 group. Box plots of each element and a multicomparison of the ANOVA
analysis illustrates that filter F8, located within the double band, seems to differ
extremely from all other filters for most of the elements. Consequently, it was tested if
all filters except F8 of the double band, belong to one group considering each element
apart. From Table 3-17, it can be concluded that excluding F8 allows to consider the
elements B, Br, Fe, TIC and K as originating from one group. However, for the
elements Na, Ca, Mg, Cl, F and Si there is no significant evidence that they belong to
the same group, even when excluding F8.
Table 3-17: ANOVA probability values for each element testing the hypothesis that
the measured element concentrations in the MORPHEUS filters belong to one single
group. P-values above 0.05 (in bold) are accepted as significant.
B
Br
Ca
Cl
F
Fe
HCO3
K
Mg
Na
Si
All filters
0.970
0.078
2.11·10-7
0
4.3·10-8
0.033
3.6·10-5
0.006
4.9·10-9
9.9·10-16
0.028
All filters
except F8
0.964
0.380
0.005
1.9·10-14
0.003
0.307
0.211
0.087
0.021
1.1·10-6
0.035
3.4.1.4 Effect of the filter material on the pore water composition
Is there any effect of the filter material on the pore water composition? Two
piezometers are used to test the effect of the filter material. The ORPHEUS
piezometer is a horizontal piezometer with 4 different filter materials. SPRING 116 is
also a horizontal piezometer with 4 stainless steel filters. For the SPRING 116
piezometer, only the filters 2, 3 and 4 are used, as filter 1 seems to be significantly
different and only contains 2 data points (see ANOVA above). From the assumption
that the spatial variability can be reduced to a one-dimensional vertical problem, it
can be assumed that possible differences between the pore water composition of
ORPHEUS and SPRING 116 are only due to the filter material. A MANOVA test is
now used to compare these different filter materials. However, it should be noted once
more that only a limited amount of data is available and results should be treated with
care. The obtained results might indicate whether the raised question should be re-
83
analysed in future when more data points are available. All major elements listed
above are taken into consideration together. From a grouped scatter plot of all
elements and for all filter materials, it is clear that B and Si have extremely high
values in the sintered glass filter of ORPHEUS compared to the other filter materials
(Figure 3-20). This clearly indicates a leaching of both elements from the sintered
glass filter material.
Figure 3-20: Cross plot of the concentration of all elements (see diagonal) in function
of the filter material. It is noticed that B and Si (outer columns and rows) of the
sintered glass filter (open circles) are extremely different from the values for the other
filter materials.
Therefore these two elements were removed from the MANOVA analysis, as to be
able to compare all filter materials. The MANOVA test shows a significant
probability to result in a 2D space (p=0.070), so that a 2D scatter plot illustrates the
maximum difference between all groups (Figure 3-21). From this graph it can be
concluded that the water provided by the poly-ethylene (PE) and sintered glass (SG)
filter materials is very similar. The first 3 stainless steel (SS) filters of SPRING 116
(SP-SS group 2) show a small difference with the PE and SG group. A larger
difference is noticed for the Schumatherm (ST) filter and the Carbo (CA) filter.
However, it should be noted that for the latter two filters only a very limited amount
of data is available. The elements mostly influencing the separation are F, Cl and Na.
84
Figure 3-21: MANOVA result for all filter materials, illustrating the maximum
separation between groups. Pore water collected with a PE or SG filter seems to have
a very comparable composition. SS gives a slight deviation, while ST and CA create
the largest deviations.
With the limited amount of data available, it can be concluded that the filter material
has an effect on the measured pore water composition. Table 3-18 gives an overview
of the filter materials tested and the elements contributing to differences. From this
table it can be concluded that the sintered glass (SG) filter causes an increase in B and
Si content. Apart from these elements there is no difference between the SG and PE
filter materials. The CA and ST filter materials only differ in their Mg content. As this
is a rather minor element, it can be concluded that CA and ST hardly differ from each
other. A small difference (except for B and Si) between the PE–SG and CA–ST pore
water composition is observed. However, additional data points are certainly
necessary to confirm this observation. The stainless steel (SS) filters cause an increase
in Fe content and depleted concentrations for several other elements. Finally, it can be
concluded that Na, Cl, Mg and F are the most important elements influencing the
separation between all groups. Once again it should be noted that additional data
points in the future are needed to confirm or disapprove the observed trends.
85
Table 3-18: Overview of the elements influencing the difference in pore water
composition for each filter material studied. For each comparison the elements are
ordered from left to right with decreasing contribution to difference. The elements
marked in grey are more abundant in the material mentioned at the top of the column
for that comparison.
PE
PE
SG
CA
SG
CA
ST
SS2
Si, B
Mg, Cl, Na
Cl
Fe, Si, Cl, F
Si, Mg, B, Cl,
Na
Si, B, Mg, Cl
Si, Fe, B, Cl, F
Mg
Br, Fe, Mg, Si,
Cl
ST
Br, Si, Mg, Cl,
F
SS1
Fe, Ca, Mg, Cl,
Na, K
SS2
3.4.1.5 Effect of the spatial variability on the pore water composition
Is there any effect of the spatial variability on the pore water composition? To
evaluate this question, it is necessary to compare pore water of piezometers with the
same filter material. As the question of spatial variability around the URF is reduced
to a vertical 1D problem, we only need a vertical piezometer with different filters of
the same material, like MORPHEUS (all Schumatherm filters). However, as one filter
in ORPHEUS also contains a Schumatherm filter in a layer that is not sampled in
MORPHEUS, it can be included in the MANOVA test. Consequently, 12 different
locations are taken into account. For this test all major elements are taken into
account, except B, as only a limited amount of data are available for this element. A
first MANOVA test results in a 4D separation of several groups. A scatter plot of the
two most important separations illustrates that the pore water sampled in the
ORPHEUS piezometer and the pore water provided by F8 of MORPHEUS is clearly
different from all other filters (Figure 3-22). The elements causing this separation are
Na and Cl (and to a lesser extent F, Ca and Mg). A boxplot for Na and Cl for all filters
at different depths is given in Figure 3-23. Filter F8 of MORPHEUS is providing
water from the "double band", a more silty layer within the sampled Boom Clay
interval. The presence of a geologically particular layer close to the gallery, that might
influence the pore water composition of the ORPHEUS filter is not known up to now.
To decrease the dimensionality of the separation between several groups (and to
optimise the visualisation), those filters (F8 and the ORPHEUS filter) are excluded
from a new MANOVA test. This new test results in a 3D separation (p=0.177). From
this MANOVA test, it can be concluded that filters F18 and F20 are very similar
(results not shown).
Distinguishing other groupings, however, is very difficult. The elements mostly
influencing this 3D separation are once again Na and Cl.
86
Figure 3-22: MANOVA result for all Schumatherm filters at different depths,
illustrating the two largest linear combinations of elements to optimise the separation
between groups. Pore water sampled at the gallery (gal) and at the double band (F8)
seem to have a different composition from all other sampling filters.
a
b
Figure 3-23: Boxplot for Na (a) and Cl (b) for all depths studied, showing the major
difference of the pore water sampled at the double band and in the ORPHEUS filter.
87
It is clear that only a limited amount of data are available to answer the proposed
question. However, it might be justified to try to group the filters of MORPHEUS into
geologically justified sets. Therefore, a new MANOVA was performed on the
remaining filters by excluding not only B, but also Na and Cl. The new separation is
2D (p=0.433), so that a maximum separation is visualised in Figure 3-24. Now,
several groupings can be made. A first group encompasses F20, F18 and F15, which
are all located above S50, a septarian horizon within the Putte member. A second
group contains filters F6, F9, F12 and F13, which are all located between the base of
the Putte member and S50. One filter located in the same area seems to have a
different composition of pore water, namely F10. For F10 there is no geological
peculiarity known. A last group is formed by filters F2 and F4 that are located within
the Terhagen member. It should be stressed once more that the grouping made here is
probably very conservative as the amount of data is very limited and there is not really
a reason to reject the Na and Cl elements from the analysis. The most important
elements contributing to these groupings (causing the largest "difference") are Mg, Ca
and F, which are present in only relatively low amounts in the Boom Clay pore water.
Hence, the fact that Na and Cl play a major role in the Boom Clay pore water
composition is probably a more important conclusion from this analysis.
Figure 3-24: MANOVA result for filters of MORPHEUS, except the one in the double
band (F8), illustrating the maximum separation between groups, based on all major
elements except B, Na and Cl. Pore water sampled at Terhagen (F2 and F4) or
between base Putte and S50 (F6, F9, F12, F13) or above S50 (F15, F18, F20) seem to
be distinguishable.
88
3.4.1.6 Conclusions of the statistical analyses
The amount of data presently available has certainly revealed some statistical
differences of the measured pore water composition. First of all it is clear that there is
an effect of the filter material on the measured pore water composition. The use of a
glass filter will increase the amount of B and Si within the pore water. Stainless steel
filters increases the Fe content. Apart from B and Si, no difference is observed
between the SG filter and a PE filter. The CA and ST filter only differ in Mg content.
However, additional data points are needed to increase our confidence in these results.
Apart from the filter material, there also seems to be an influence of the vertical
position of the filter. Without any doubt the pore water composition of the double
band is different from the rest of the Boom Clay. With the limited amount of data and
the exclusion of Na and Cl from the analysis, a distinction can be made between the
pore water composition of the Terhagen member, the base of the Putte member up to
S50, and the Putte member above S50. However, it should be noted that the latter
needs confirmation or rejection when more data points are available.
Within the SPRING 116 piezometer having 4 different stainless steel filters and
positioned in the same layer, a distinction between the deepest filter and the three
filters closer to the gallery was observed. Up to now it is not known what causes this
difference. Some suggestions might be a relict disturbance caused by the excavation
of the gallery, the effect of oxidation caused by coring not completely removed yet in
the first three filters, or a lateral variation even on short distance in the same layer.
Continued follow-up of this piezometer might clarify this.
The EG/BS filter provided many data through time. There is no continuous or
systematic evolution found in time. However, two groups of results separated by a
time gap can be distinguished. It is unknown what has caused this difference.
Finally, it can also be concluded that Na and Cl play a major role in discriminating
several groups of pore water composition.
3.4.2 Charge balance and equilibrium state of the pore water
To evaluate the quality of a chemical analysis, one may look at the accuracy of the
analysis and the equilibrium state of the measured water composition. The precision
of the analysis is determined by the sensitivity of the analytical technique itself and
will not be discussed here.
The accuracy of a water analysis is normally assessed by the degree of charge
imbalance calculated from the given water composition. The principle of
electroneutrality requires that the ionic species in any water sample must remain
charge balanced. The degree of charge imbalance is thus a measure for the error of a
water analysis:
Charge Imbalance (%) =
( sum cations − sum of anions )
× 100
( sum cations + sum anions )
where cations and anions are expressed in meq/l. It is obvious that a charge imbalance
is inevitable in a chemical analysis but an acceptable charge imbalance should be less
than 5 % for a good water analysis (Appelo and Postma, 1999).
(3.1)
89
For the Boom Clay pore water, the major contributions of the cationic charge are from
Na+, K+, Ca2+, and Mg2+. Negative charges are from the total alkalinity and other
conservative anions Cl-, F-, and SO42-. The total alkalinity is the total titratable bases
and is equal to the sum of equivalents of HCO3-, CO32–, hydroxide (OH–), silicate,
borate, phosphate, and natural organic ligands. The charge balance equation for Boom
Clay pore water can therefore be simplified to:
mNa+ + mK+ + 2 mCa2+ + 2 mMg2+ = alkalinity + mCl- + mF- + 2 mSO42where m refers to the molarity of the different species.
Figure 3-25 plots the charge imbalance for the MORPHEUS water collected in April,
2003. It is seen that the charge imbalance is well below 2 % suggesting that the
analysis is quite accurate. Also shown in Figure 3-25 is the charge imbalance
calculated considering the TIC as the total alkalinity. In the latter case, the charge
imbalance is higher than using the alkalinity but still below 5 %.
The use of TIC to replace the alkalinity in equation (3.2) has been practiced for
calculating the charge imbalance of some previously studied Boom Clay water
samples (Noynaert et al., 1998; De Cannière et al., 1994a; De Cannière et al., 1994b)
and some high values of charge imbalance were found specifically for waters of high
pH (9 and above). That was probably because the use of TIC to represent the
alkalinity underestimated the total negative charges of the water by neglecting the
contribution of CO32–. The approximation that the equivalent of TIC is equal to the
total alkalinity is only valid at a pH below 8. At a higher pH range, the use of the TIC
to calculate the charge imbalance needs a representative pH measurement which
generally has not been available (see section pH and pCO2). A precise charge
imbalance should be calculated using the total alkalinity and not the TIC.
The charge imbalance was more pronounced in a lot of squeezed water samples as
evidenced by Reeder, et al. (1992) and De Craen (2001). These waters were mostly
severely oxidised so the possible reason for the observed charge imbalance is the lack
of data for thiosulphate. Anionic deficit of 10 to 20 % has been observed in those
waters.
Another useful indication for evaluating the quality of the water sampling and
analysis is the saturation state of the water in terms of minerals that are in contact with
the water. As will be discussed in section 4 on modelling, the calcite saturation index
is used widely to evaluate the equilibrium state of a given water composition.
Nordstrom and Munoz (1994) pointed out that the calcite saturation index should
approach to zero in almost all types of water but not surpassing it. A water analysis
showing significant oversaturation of calcite normally suggests that either the
sampling procedure is inadequate or the chemical analysis is not accurate. For the
Boom Clay pore water compositions discussed in this report, a direct calculation of
the saturation state is not possible because of the lack of relevant pH measurements. A
reverse calculation has been performed therefore assuming a priori that calcite is in
equilibrium with the MORPHEUS water compositions and that the resulted pH values
are in a range of 8.3 to 8.6 (see section 3.1.3). This pH range is higher than the in situ
pH measurements of 8 to 8.2, suggesting that the MORPHEUS waters are slightly
undersaturated with calcite at the in situ measured pH values. No indication of
oversaturation of calcite is observed, so the quality of water analysis should be
acceptable in terms of the calcite saturation state.
(3.2)
90
4.5
3.5
charge imbalance, %
2.5
1.5
0.5
-0.5
-217.1 -220.8 -222.6 -226.0 -227.2 -227.8 -229.1 -229.8 -230.3 -231.8 -233.8 -235.2
alkalinity
TIC
-1.5
-2.5
-3.5
-4.5
depth, m
Figure 3-25: Charge imbalance calculated for MORPHEUS water compositions
(collected in April, 2003).
91
4
Model simulation of pore water chemistry
The present-day chemical composition of the Boom Clay pore water is the result of
the early diagenesis and the subsequent water-rock interactions, mass transfers, and
the mixing of groundwaters from the surroundings with the Boom Clay pore fluid.
Broad interrelationships among these processes can be discerned by application of
chemical thermodynamics. In addition, because of the low hydraulic conductivity, the
Boom Clay pore water is practically immobile so that a chemical equilibrium is likely
established between the pore water and the minerals. The system can therefore be
evaluated by principles of chemical equilibrium. We present in this section
equilibrium model simulations, calibrated with the detailed analysis of Boom Clay
pore water as discussed in previous Sections, to produce a probable interpretation of
the measured water compositions.
4.1 Equilibrium model and water-rock interaction
An equilibrium model describes the distribution of chemical components in a system
containing fluid, minerals, and gases at a chemical equilibrium state. The system is
constrained at the initial state by known temperature, pressure, and composition. At
the equilibrium, the redistribution of the involved chemical components among
species in fluid, minerals, and gases is so that the value for the total Gibbs free energy
of the system is minimised, that is, the lowest potential for chemical reactions. The
chemical equilibrium model consists of two parts: the constraint of chemical
equilibrium, and the constraint of conservation of mass. The constraint of chemical
equilibrium is a set of mass action expressions in terms of equilibrium constants. The
constraint of conservation of mass is a set of mass balance equations, one for each
component. The combination of the mass action and the mass balance equations
results in a set of nonlinear equations to which the mathematical solutions describe the
final distribution of the chemical components. For the simulation on water-rock
interactions of the Boom Clay, the pore water composition is calculated at the
equilibrium state as the result of interactions between the pore fluid and minerals.
4.2 Computer code and thermodynamic database
Most of calculations were performed using the computer code The Geochemist’s
Workbench 3.2.2 (Bethke, 2001). Some calculations were done with the new GUI
(graphic user interface) version of the code 4.0.2 (Bethke, 2002). In calculations
where temperature corrections were needed, the latest working version of 4.0.3 was
used (only available to SCK•CEN at the present time). The version 4.0.3 allows to fix
the values for the ion exchange selectivity coefficients at 25°C while the solubilities
of the minerals are calculated at 16°C, the temperature of in situ Boom Clay.
The thermodynamic database used is the MOLDATA (04-01) (Wang, 2004), a
database maintained at SCK•CEN for the study of geochemistry and migration of
radionuclides in Boom Clay. MOLDATA is a database derived from the EQ3/6
database version 8 release 6 which has its roots in the SUPCRT database (Johnson et
al., 1991) developed at Lawrence Livermore National Laboratories (LLNL). The
database supports activity coefficients calculated by an extended form of the Debye-
92
Hückel equation (the B-dot equation). Concerning aqueous species and mineral
stability constants, MOLDATA is basically identical to the LLNL database except
some corrections of known errors. The only difference, relevant to this report, is that
MOLDATA contains a sub-dataset for ion exchange selectivity coefficients. This ion
exchange dataset is calibrated with the pore water composition of the Boom Clay and
literature data on ion exchange so that the data are considered to be specific for the
Boom Clay conditions. An example of input and output files from geochemical
modelling simulations is given in Annex 7.
4.3 Mineral solubility and ion exchange
Solubility is an important mechanism controlling the compositions of natural waters
since many common minerals dissolve and precipitate readily and reach solubility
equilibrium. However, it is also known that many other minerals do not readily reach
solubility equilibrium in the temperature range of 0-100 °C. In a clay rich system such
as the Boom Clay, which contains 60 wt% of clay minerals and which is practically
free of metal oxides, ion exchange is as important as solubility in regulating the pore
water composition. In the framework of the EC ARCHIMEDE-argile project
(Griffault et al., 1996), two distinguished approaches have been applied for the
interpretation of measured Boom Clay pore water composition. The first approach
(Beaucaire et al., 2000) stressed the difficulties in defining the ion exchange
complexes and used only solubility constraints to explain measured concentrations of
major ions present in the Boom Clay pore water. The second approach (Sanjuan et al.,
1994) applied a combined solubility and ion exchange model and claimed a better fit
of the simulation to the measured data. In comparing the two basic approaches,
Pearson (2001) noticed that the addition of ion exchange to the model at least
reproduced equally well the observed water composition in Mont Terri project. Our
scoping calculations revealed that neither pure solubility, nor ion exchange models
explain the observed Boom Clay pore water composition satisfactorily. We therefore
in this report explore a combined solubility and ion exchange model.
Concerning solubility mechanisms, a common difficulty is to decide priorily what are
the reactive minerals and if they are in chemical equilibrium with the pore water.
Nordstrom and Munoz (1994) summarised a guide to which minerals are likely to
reach solubility equilibrium (given sufficient time) and which ones are not likely
based on mineral reactivity, kinetics, and degree of complexity of mineral
stoichiometry. Following this guide we can classify the Boom Clay minerals (Table
1-2) according to their reactivity and their plausibility to reach an equilibrium with the
pore fluid:
•
•
•
calcite, siderite, and pyrite are likely in chemical equilibrium with the pore
water. These secondary minerals are reactive and have a simple stoichiometry.
The simple stoichiometry facilitates geochemical modelling since the mineral
formulas and the mass action laws are easily defined;
illite, smectite, feldspars, and chlorites are reactive but unlikely reach chemical
equilibrium with the Boom Clay pore water due to either the slow kinetics at
low temperatures, incongruent dissolutions, or complex stoichiometries. These
minerals are generally not considered suitable to be included in equilibriumbased model simulations;
kaolinite might reach an equilibrium but has a complex stoichiometry;
93
•
quartz is not reactive.
A common mineral that is considered as being in equilibrium for almost all types of
water-rock interactions is calcite (Langmuir, 1997; Nordstrom and Munoz, 1994;
Pearson et al., 1978). This has also been recognised and applied in previous studies of
Boom Clay (Beaucaire et al., 2000; Griffault et al., 1996). Calcite is therefore taken as
the control mineral for the calcium concentration in the Boom Clay pore water
through the following dissolution reaction:
Calcite + H+ ⇔ Ca2+ + HCO3-
(4.1)
Iron has never been taken into account in previous studies of the Boom Clay, mainly
because its concentration and behaviour are not supposed to influence the
geochemistry of the Boom Clay to a noticeable extent. Another reason might have
been that iron is a redox sensitive element and the water sampling and preservation
procedure will influence the analytical results. In this report, siderite is used to
constrain the iron concentration under an undisturbed in situ condition. We are aware
of, however, the sensitivity of the iron concentration to the redox condition, evidenced
in a batch experiment (Wang et al., 2002). Siderite dissolution likely follows two
reactions depending on pH:
Siderite + H+ ⇔ FeHCO3+
(4.2)
Siderite ⇔ FeCO3 (aq)
(4.3)
where FeHCO3+ species dominate at pH less than 8.4 and FeCO3 (aq) is important at
higher pH values. If only Fe(II) prevails, the concentration of the free Fe2+ in the
Boom Clay pore water is about an order of magnitude lower than the two ion pair
species.
Pyrite is an important mineral since it is sensitive to oxidation and contributes largely
to the redox buffering capacity of the Boom Clay. It is not uncommon in literature
that pyrite is used to control the aqueous concentration of sulphate in a pore water.
However, an accurate quantification of pyrite dissolution through reaction modelling
needs an accurate redox potential measurement. As discussed in the Section of redox
(Section 3.1.4), Boom Clay redox measurements have been proven elusive and
technically challenging. On the other hand, due to the marine origin of Boom Clay,
there is no evidence that the sulphate content of the present-day pore water is
exclusively constrained by the dissolution of pyrite. We therefore use pyrite to buffer
the system redox but use a sulphate concentration measured from piezometer waters
as the initial condition. The pyrite-sulphate couple, i.e., FeS2/SO42- will fix the redox
potential. This is more workable and sound since the sulphate analysis is more reliable
than the redox measurement. The redox potential of the Boom Clay can be calculated
as demonstrated in Section 3.1.4 if the system pH, alkalinity, sulphate and total iron
concentrations are known:
FeS2 + 8 H2O + HCO3- ⇔ 16 H+ + 2 SO42- + FeHCO3+ + 14 e-
(4.4)
In the Boom Clay, illite, smectites, feldspars, and chlorites are important minerals in
terms of weight percentage (see Table 1-2). Although they belong to groups of
reactive minerals, they unlikely reach chemical equilibrium with the pore water at low
94
temperatures so that they are not taken in this report as controlling minerals for
simulating equilibrium solubilities. However, it is important to note that although the
pore water of the Boom Clay may not achieve stoichiometrical solubility equilibrium
with these minerals, they do exist in the Boom Clay and they do react with the pore
water and affect the overall water-rock mass balances. Beaucaire, et al., (2000)
considered albite and microcline (K-feldspar) as the minerals controlling Na and K
concentrations and found comparable results to the measured water compositions
(ARCHIMEDE-argile project). Typical dissolution reactions of these minerals as
written in the LLNL database are as follows:
Illite ⇔ 1.2 H+ + 0.25 Mg2+ + 0.6 K+ + 2.3 AlO2- + 3.5 SiO2(aq) + 0.4 H2O
(4.5)
Montmor-Na (Smectite) + 6 H+ ⇔
0.33 Mg2+ + 0.33 Na+ + 1.67 Al3+ + 4 H2O + 4 SiO2(aq)
(4.6)
Albite ⇔ AlO2- + Na+ + 3 SiO2(aq)
(4.7)
K-Feldspar ⇔ AlO2- + K+ + 3 SiO2(aq)
(4.8)
Clinochlore-14A (chlorite) + 8 H+ ⇔ 2 AlO2- + 3 SiO2(aq) + 5 Mg2+ + 8 H2O
(4.9)
Kaolinite is also reactive but complex in stoichiometry. The relatively faster kinetics
comparing to illite and feldspars makes kaolinite-pore water equilibrium possible:
Kaolinite ⇔ 2 H+ + 2 AlO2- + H2O + 2 SiO2(aq)
Kaolinite is commonly used as the controlling mineral for the Al concentration in
natural waters. The thermodynamic stability constant of kaolinite has been a subject
of lively discussions (Nordstrom and Munoz, 1994). Based on a self-consistent
solubility dataset (Nordstrom et al., 1990), at 16 °C and pH of 8 to 8.5, the Al
concentration controlled by the dissolution of kaolinite at the Si concentration of
Boom Clay pore water (~5 mg/l) is in a range of 2 to 8 µg/kg water which is about an
order of magnitude lower than the Al concentration normally measured in unfiltered
Boom Clay piezometer waters. This may be explained either by the possible presence
of Al colloids/particulates in water samples or the choice of thermodynamic data for
kaolinite in terms of crystallinity. Aluminium is known to form colloids in natural
waters. To remove colloids, a careful ultrafiltration of water samples is necessary
using filters of pore size in a nanometer range. Beaucaire et al., (2000) found that the
measured Al concentration in the Boom Clay pore water was generally overestimated
if the water sample was not carefully filtered. They found that the ultrafiltration of a
water sample at 10 nanometer (about 200,000 MWCO) resulted in a representative
and truly dissolved Al concentration which was in a solubility equilibrium with
kaolinite. The second possible explanation might be that the kaolinite presence in
Boom Clay is less stable than the assumed crystalline kaolinite. Coudrain-Ribstein
and Gouze (1993) studied the Dogger aquifer (Paris Basin) and concluded that
kaolinite in equilibrium is not a well-crystallised mineral but a kind of disordered
form, that is less stable and has a higher solubility. Ultrafiltration has not been
generally practiced in the procedure of sampling and analysis of Boom Clay waters.
Pre-filtration on water samples should be performed in future analyses to better define
the Al concentration. Aluminium is not a routinely measured element in the Boom
Clay water characterisation campaign. As aluminosilicates are not considered as being
(4.10)
95
in equilibrium with pore water for the current simulation, the Al concentration will
not influence the other major ions concentration. For the purpose of modelling when
Al is to be treated, we currently use kaolinite as the solubility controlling mineral. We
recommend to carry out more measurements on the Al concentration in Boom Clay
pore waters in the future.
Quartz consists of 40 wt% of the Boom Clay. As the second most abundant element in
the Earth’s crust, Si in quartz is however rarely found in equilibrium with natural
waters. This is because most of the dissolved silica observed in natural waters results
originally from the chemical breakdown of silicate minerals in processes of
weathering. These processes are irreversible, and the silica concentration in water is
likely controlled either by kinetics of dissolution processes, surface processes like
sorption, or by precipitation of secondary minerals of less organised structures. Boom
Clay pore waters are generally oversaturated in terms of quartz. The Si concentration
in Boom Clay pore waters is close to the solubility of chalcedony, a more soluble
phase but having the same stoichiometry as quartz. We therefore use chalcedony to
model the concentration of Si. Although the use of chalcedony in place of quartz is a
widely applied approach in water rock interaction modelling, e.g., Beaucaire et al.,
(2000) and Bradbury and Baeyens (1998), the existence of chalcedony in the Boom
Clay is still to be demonstrated.
Besides the solubility approach, ion exchange is a well known process regulating the
water composition in clays. Ion exchange is an adsorption process which removes one
solute from the aqueous phase and releases another from the clay surface. Exchange
reactions occur at the surfaces of clay particles where the isomorphic substitution
results in a permanent surface charge. Ion exchange reactions follow a mass action
law such as, for example, for a potassium-sodium exchange:
>X:Na + K+ ⇔ >X:K + Na+
(4.11)
with a mass action coefficient expressed as:
KC (K-Na)=
[> X : K ]{Na + } = N K {Na + }
[> X : Na ]{K + } N Na {K + }
In this equation, KC is the selectivity coefficient, the “>” sign means exchange
complexes on the clay surface, that is, >X:Na and >X:K express the sodium and
potassium exchange complexes on the surface of the clay. {Na+} and {K+} are the
activities in the aqueous phase. Depending on molal or equivalent fractions being
adopted for expressing the quantity of the exchange complexes within [ ], KC takes
different forms, namely, the Gaines-Thomas, Vanselow, or Gapon convention. In this
report, we use the Gaines-Thomas convention, i.e. the surface exchange complex is
expressed as an equivalent fraction of the total ion exchange capacity of the clay. The
activity of the exchange complex is therefore written as equivalent fractions NNa and
NK, respectively, for Na and K occupancies. Limited only to metal cations, the
capacity of the Boom Clay to exchange cations is the cation exchange capacity
(CEC), which is an important characteristic of Boom Clay and represents the major
buffering sink for the cation composition in the pore water.
According to the well established ion exchange selectivity behaviour in common soils
and sediments, for example, the data given by Appelo and Postma (1999), sodium
(Na), potassium (K), calcium (Ca), and magnesium (Mg) are dominant cations
96
contributing to the major part of exchange complex on clays. In addition to the K-Na
exchange reaction given in reaction (4.11), Ca-Na and Mg-Na exchange reactions are
as follows:
2 >X:Na + Ca2+ ⇔ >X2:Ca + 2 Na+
2+
KC (Ca-Na) =
+
2 >X:Na + Mg ⇔ >X2:Mg + 2 Na KC(Mg-Na) =
{ }
{ }
N Ca Na +
2
N Na
Ca 2+
{ }
{Mg }
N Mg Na +
2
N Na
2
(4.12)
2
2+
(4.13)
If other cations occupancies are negligible, Na, K, Ca, and Mg should take all
exchange sites so that:
NNa + NK + NCa + NMg = 1
(4.14)
Ion exchange is an important mechanism regulating the water composition of the
present-day marine sediments, analogues to the Boom Clay. Sea water is Na+ and Cldominant and sediment in contact with sea water will have a large Na+ occupation on
clay surfaces. Fresh water, commonly rich in Ca2+ and HCO3-, infiltrates into the
sediment resulting in an exchange of Ca2+ onto clay surfaces and Na+ in return to the
water. The water therefore changes from the Ca-HCO3 type to the present-day NaHCO3 type.
Ion exchange in the Boom Clay has been studied by Baeyens (1982), Baeyens et al.
(1985), and Griffault et al. (1996). The first two papers were actually based on the
same experiment, so we consider them as one study. Ion exchange properties of the
Boom Clay derived from these studies are summarised in Table 4-1. Parameters
needed for modelling the ion exchange in the Boom Clay will be taken from Table 4-1
with some re-evaluation as given below.
Table 4-1: Summary of ion exchange data determined from previous studiesa
Ion exchange properties
Baeyens (1982) and
Griffault et al. (1996)
Baeyens et al. (1985)
CEC, meq/100 g clay
24
24
NNa
0.365
0.36
NK
0.16
0.096
NCa
excluded
0.159
NMg
0.475
0.154
NH
Not considered
0.229b
KC (K-Na)
10
15
KC (Mg-Na)
3.8
Not consideredc
KC (Ca-Na)
Not applicable
2.1
a
KC were expressed following the Gaines-Thomas convention in Baeyens (1982) and the Vanselow
convention in Griffault et al. (1996)
b
the proton occupancy was reported by Sanjuan et al. (1994)
c
magnesium concentration was considered being controlled by the dissolution of dolomite, not by
ion exchange
97
Both studies resulted in exactly the same CEC value of Boom Clay, i.e., 24 meq/100 g
clay. Griffault et al. (1996) tried to divide experimentally the total CEC into two
fractions, namely the exchange capacity from clay and from natural organic matter.
Baeyens (1982) and Baeyens et al. (1985) estimated that the contribution of natural
organic matter to the total CEC is about 5-10 meq/100 g clay. They did not separate
the two fractions and the CEC was determined as a total value. We in this study treat
the CEC as the total without making difference between inorganic and organic
fractions. Separation between clay and the natural organic matter makes sense only if
the two phases can be described by two distinct mechanisms attributed to two sets of
model parameters for the clay and the natural organic matter. As seen from Table 4-1,
both studies defined only one set of parameters for ion exchange in terms of
selectivity coefficients and exchange occupancies, therefore it makes no sense to split
the total CEC into two fractions.
The studies of Baeyens (1982) and Baeyens et al. (1985) were characterised by the
exclusion of calcium exchange, that is, no exchangeable calcium occupancy on Boom
Clay surface. It was considered that calcium in the Boom Clay exists exclusively as
calcite. This was explained by the fact that the carbonate contents measured in the
Boom Clay matched stoichiometrically the total amount of calcium in the clay
samples. To our opinion, this way of reasoning depends on the extent of errors
associated with the quantification of calcite and the total calcium concentration on
clay. Considering the CEC of 24 meq/100 g clay and a 0.2 equivalent fraction
occupancy of calcium on the clay, the amount of exchangeable calcium expressed by
Baeyens et al. (1985) as calcite is about 8 % of the total amount of the calcite mineral.
This percentage is within the error range normally found in the X-ray diffraction
method used to determine the quantity of calcite. Since no error range on the calcite
and the total calcium determinations was given in the work of Baeyens et al. (1985), it
is difficult to judge if the calcium exchange occupancy exists or not. In any case,
whether or not calcium is part of the exchange complex is very important for choosing
an ion exchange model for Boom Clay. Some further discussion will follow in the
Section 4.4.2 of modelling results.
The pH value of the water derived from the batch leaching experiment of Baeyens and
co-workers is 9.2 to 9.3 (Baeyens et al., 1985, Table II), which is higher than the pH
of the Boom Clay as estimated in Section 3.1.3. This is probably because the
experiments were performed in a glove box under inert atmosphere to prevent the clay
from oxidation. Cautions were apparently taken to maintain the in situ redox condition
of the Boom Clay but the importance of keeping a correct CO2 (g) partial pressure
was not realised. Glove boxes filled with (an) inert gas(es) contain practically no
CO2(g), and the clay suspension in equilibrium with (or partly in equilibrium with) the
gas phase likely experienced severe degassing in terms of CO2(g). The pH will
increase when the system was degassed, i.e., decrease in partial pressure of CO2(g),
following the reaction:
CO2 (g) + H2O ⇔ H+ + HCO3-
(4.15)
An increase in pH will change the system chemistry in terms of calcite dissolution,
carbonate to bicarbonate ratio, and ion exchange reactions. A higher pH due to the
degassing of CO2(g) leads to a decrease in calcite solubility, i.e., lower calcium
concentration than it would be under the in situ partial pressure of CO2(g). If cation
exchange between calcium and sodium occurred, i.e., not excluding Ca from the
98
exchange complex as concluded by Baeyens et al. (1985), the concentrations of other
exchangeable cations in the aqueous phase, i.e., Na, K, and Mg, will also decrease
accordingly. Under the conditions of the Baeyens experiment, i.e., pH of 9.2 to 9.3,
the calcium concentration controlled by the calcite dissolution is 3 × 10-5 molal, which
is identical to the value measured by Baeyens et al. (1985). This is an indication that
the batch water composition reported by Baeyens et al. (1985) was likely extracted at
a relative higher pH than the expected value for in situ Boom Clay. It must be noted
however that the effect of dilution must also play a role since the water was extracted
at a solid to liquid ratio of 1:1 g/ml. However, as demonstrated by Henrion et al.
(1985) in the same project, the influence of CO2(g) degassing dominated over the
dilution effect.
In the ARCHIMEDE-argile project (Griffault et al., 1996; Sanjuan et al., 1994) the
ion exchange composition was interpreted differently comparing to that of Baeyens
(Table 4-1). The fact that about 0.2 (NH) equivalent fraction of cation occupancy was
attributed to protons was remarkable. Since no explanations were given in neither of
the two papers regarding why such high proton occupancy was derived, we assume
that the NH value was probably resulting from the difference between the total CEC
and the sum of individual cation occupancies measured by selective extraction
experiments. It has been noticed from the Table 18 in Griffault et al. (1996) that: (1)
the sum of the cation exchange occupancies of Na, K, Ca, and Mg is about 0.8, i.e.,
0.2 in deficit; and (2) no proton occupancy was reported.
It is not uncommon that the sum of extracted cations turns out being less than the total
CEC in ion exchange experiments. Bradbury and Baeyens (1998) found the deficit of
about 25% in Opalinus clay, which is comparable to the 23 % being noticed in the
ARCHIMEDE-argile project for Boom Clay. However, Bradbury and Baeyens (1998)
explained that the difference is most likely due to the uptake of the index cation by the
amphoteric functional groups existing at the edge sites of the clay, in other words, the
total CEC measurements tend to overestimate the true exchange capacity. Amphoteric
functional groups on clay behave similarly to the hydroxyl groups on oxi- hydroxides
and react with solutes in aqueous phase through surface complexation reactions
(Dzombak and Morel, 1990). These reactions occur on the variable charge sites and
are different from ion exchange reactions taking place on the permanent charge sites
and must be therefore treated differently. We follow the notion of Bradbury and
Baeyens (1998) that the sum of the extracted cations is a better measure for the total
CEC than the global CEC measurement using an index cation. The CEC measured in
the ARCHIMEDE-argile project should therefore be 18.5 meq/100 g clay instead of
24 meq/100 g clay.
In addition to the above given argument, a high proton exchange occupancy is
unlikely due to the low selectivity of proton to clay surfaces. Bradbury and Baeyens
(1998) pointed out that the Na-H selectivity coefficient is about unity on clays. At a
pH value of 8-9 for the Boom Clay, the proton occupancy is not expected to be
significant. In this report, we do not consider proton exchange. We do not, at this
stage, consider amphoteric groups on Boom Clay since no data available for
characterising the edge sites. As a result, we consider the CEC derived from the
ARCHIMEDE-argile project as being 18.5 meq/100 g clay, and comprising of Na, K,
Ca, and Mg as exchangeable cations.
Another concern about the model used in the ARCHIMEDE-argile project is the use
of dolomite to control the magnesium concentration in the pore water. First of all, the
99
question is still open if dolomite exist or not in Boom Clay (see Section 3.1.3).
Secondly, since the measured Ca/Mg ratio in Boom Clay pore waters is relatively
constant, ion exchange approach will also explain the measured Ca and Mg
concentrations. In this report, we use ion exchange to estimate the magnesium
concentration, assuming that dolomite does not exist. It is important to remember that
mineral dissolution mechanisms, including solubility of dolomite (either as pure
dolomite phase or as a proxy of magnesium rich calcite), illite, and chlorite, may also
contribute to the total concentration of magnesium measured in the pore water
especially under severely perturbed conditions.
4.4 Equilibrium model for the simulation of the pore water
composition of Boom Clay
The equilibrium model is based on a system containing 1 liter of water in contact with
5 kg dry clay, that is, a water content of about 17 wt%, as found in clay samples
collected in the MORPHEUS drilling (calculated as weight of the water / weight of
the wet clay; De Craen et al., 2004b). The amount of minerals are roughly taken as the
weight percentage of minerals given in Table 1-2. Except in the cases of severe
perturbations, for example, pyrite oxidation or alkaline plume - in which large amount
of minerals will dissolve - the amount of minerals present in an equilibrium model
simulation is not important since the concentration of dissolved species is much
smaller than the total quantity of minerals present. The individual equivalent fraction
of adsorbed cations will be discussed and derived from the following Section.
According to the arguments given in the previous Section, we can summarise a
general model to be used for simulations (Table 4-2). The model is a set of
constraints, each for one element of interest in Boom Clay pore water, that regulate
the water composition.
Table 4-2: Models constraints applied in the simulation of the Boom Clay pore water
composition
Element or variables
Constraints
Na
ion exchange
K
ion exchange
Ca
Calcite
Mg
ion exchange
Al
Kaolinite
Fe
Siderite
Si
Chalcedony
SO42Fixed initial con. (imposed)1
2
C
pCO2 (imposed)
pH
balancing charge
Eh
Pyrite
1
2
initial value 2.31 mg/l, the average value found in one of the statistic groups from
MORPHEUS water compositions (Section 3.4.1);
in the range of 10-2.8 to 10-2.2.
100
4.4.1 Mineral stability constants and ion exchange parameters
The stability constants used in solubility calculations are listed in Table 4-3. All
values are given for a temperature of 16°C, which is the in situ temperature of the
Boom Clay. Stability constants at a specific temperature are interpolated using a
polynomial fit at a temperature span of 0-300°C.
Table 4-3: Minerals considered in the model simulation and their dissolution
constants
minerals
Chalcedony
Kaolinite
Calcite
reactions
Chalcedony = SiO2 (aq)
-3.9397
+
3+
Kaolinite + 6 H = 2 SiO2(aq) + 2 Al + 5 H2O
+
2+
-
8.2205
-
1.9825
2+
-0.0224
Calcite + H = Ca + HCO3
+
logK (16°C)
Siderite
Siderite + H = HCO3 + Fe
Pyrite
Pyrite + H2O + 3.5 O2(aq) = 2 H+ + Fe2+ + 2 SO42-
225.0954
As discussed in Section 3 and shown in Table 4-1, both existing ion exchange models
have some limitations. Because we have not performed ion exchange experiments for
the purpose of this report, to apply ion exchange mechanisms in modelling, we need
to derive ion exchange parameters based on the two existing studies with some
modifications.
The model of Baeyens et al. (1985) does not involve calcium exchange. Our scoping
calculations revealed that, without the calcium exchange mechanism, the generally
observed variation of major cations cannot be explained. This can be elucidated as
follows:
•
The calcium concentration is controlled by the solubility of calcite;
•
The pH/pCO2 variation is the only system variable imposed by still unknown
mechanisms (see discussion in the Section 3.1.3);
•
Variations in concentration of sodium, potassium, and magnesium are
regulated by ion exchange and directly linked with the change in calcium
concentration, which in turn, is controlled by the pH/pCO2 dependent calcite
dissolution.
Based on the above reasoning, the dissolved calcium ion must be exchanged with
other cations present on the Boom Clay surface to have any impact on variations of
major cation concentrations (unless ion exchange does not play any role, which is
highly unlikely). We therefore in this report do not use ion exchange data from
Baeyens et al. (1985) and do consider the calcium ion exchange reaction. As one of
the most important model assumptions, the existence or the non-existence of calcium
exchange complex on Boom Clay should be demonstrated by future experiments.
101
The ion exchange data collected in the ARCHIMEDE-argile project (Griffault et al.,
1996) demonstrated the presence of calcium exchange complex on the Boom Clay
(Table 4-1). Except the questionable high proton occupancy claimed in the model
interpretation (Sanjuan et al., 1994), we will use the ion exchange data derived from
the ARCHIMEDE-argile project as follows:
•
We consider the CEC value is equal to the sum of the extracted cation
concentrations, i.e., CEC = [Na] + [K] + [Ca] + [Mg], where [] is the
concentration in equivalents used for expressing cation occupancies;
•
According to the data given in Table 18 in Griffault et al. (1996), CEC = 8.7 +
2.3 + 3.8 + 3.7 = 18.5 meq/100 g clay;
The individual cation occupancies are:
NNa = 8.7/18.5 = 0.47;
NK = 2.3/18.5 = 0.12
NCa = 3.8/18.5 = 0.2
NMg = 3.7/18.5 = 0.2
With known cation occupancies and using the mass action equations (4.11) to (4.13),
ion exchange selectivity coefficients can be calculated if the cation activity ratios in
the aqueous phase are known. To calibrate the ion exchange model, we will use the
real pore water composition derived from the collected piezometers to calculate the
selectivity coefficients.
Boom Clay pore water is basically a low ionic strength NaHCO3 water of 15 mmolal.
The total pool of cations in the aqueous phase represents therefore only 1 % of the
total CEC. This means that the ion exchange complex on the Boom Clay is large
enough so that the composition of the exchanging surface remains invariant. In other
words, the equivalent fractions (N terms) in equations (4.11) to (4.13) are constant
values. Taking KC, i.e., the selectivity coefficient as a constant under certain chemical
conditions, the activity ratio of exchangeable cations in the aqueous phase should be
invariant. This implies that, if ion exchange is the dominant mechanism, the activity
ratio of exchangeable cations in the aqueous phase should be a constant value.
Figure 4-1 plots the measured concentration ratios and the calculated activity ratios of
major cations from 5 statistic groups of 44 MORPHEUS water samples. Although the
measured concentration ratio is different from the free cation activity ratio expressed
in the ion exchange reactions (equation 4.11 to 4.13), it gives a good indication as if
ion exchange mechanism is important or not.
Figure 4-1 indicates that the K/Na activity ratio is constant suggesting that it is fixed
by ion exchange between the two cations. The Mg/Na2 activity ratios are more
scattered but have a mean value around 0.19. The Ca/Na2 activity ratio varies in a
range of 0.1 to 0.14. This variation, although small, cannot be explained at the present
time. One aspect can be considered in the future is to carry out more careful sample
preservation and handling. Calcium is susceptible to precipitation when water samples
were brought to a surface laboratory because of CO2 degassing and the consequent pH
rise. On site filtration and immediate acidification of water samples to pH < 2 can
overcome the precipitation of calcite.
102
cation concentration/activity ratio
0.35
0.3
0.25
con. Mg/Na^2
act. Mg/Na^2
con. Ca/Na^2
act. Ca/Na^2
con. K/Na
act. K/Na
0.2
0.15
0.1
0.05
0
1
2
3
4
5
statistic groups
Figure 4-1: Measured concentration ratios (closed symbols) and calculated activity
ratios (open symbols) of major cations in 5 statistic groups of 44 MORPHEUS water
samples.
For a modelling purpose at the present stage, the activity ratio of Ca/Na2 is taken as
the average value of the 5 statistic groups, i.e., around 0.13. Activity ratios derived
from the MORPHEUS water compositions for the purpose of ion exchange model
calibration are:
K+/Na+ = 0.012; Mg2+/(Na+)2 = 0.19; Ca2+/(Na+)2 = 0.13.
With the known cation occupancies and free cation ratios, selectivity coefficients for
the ion exchange on Boom Clay can be readily calculated using equations (4.11) to
(4.13). The results are as follows:
KC (K-Na) = 21.28; KC (Mg-Na) = 4.76; KC (Ca-Na) = 6.96
The value of selectivity coefficient for potassium KC (K-Na) is higher than literature
value generally reported for common soils and sediments, e.g., KC (K-Na) = 4-7
(Appelo and Postma, 1999). The higher KC (K-Na) calibrated by MORPHEUS water
compositions suggests that Boom Clay has a higher selectivity for potassium. This
may be due to the high content of illite clay which is known for its high potassium
selectivity. Thellier and Sposito (1988) reported a KC (K-Na) value of 15 for Silver
Hill illite, the value was taken by the ARCHIMEDE-argile project to explain the
composition of Boom Clay pore water.
Selectivity coefficients for magnesium and calcium calibrated for MORPHEUS water
compositions are in good agreement with literature values compiled by Appelo and
Postma (1999), e.g., KC (Mg-Na) = 4 and KC (Ca-Na) = 6.
103
It is important to note that when the ion exchange pool in the pore water is small
compared to the ion exchange complex on the clay surface, one can readily calculate
major cation concentrations by fixing the cation ratios without knowing the exact
composition of the ion exchange complex on the clay surface. This means that we do
not really need the values of cation occupancies and selectivity coefficients to
calculate the major cation concentrations. Those open questions concerning the cation
exchange compositions as discussed in previous Sections therefore do not influence
our present calculations as far as non-disturbed pore water composition is to be
calculated. To make the model intrinsic and especially to apply the model to
conditions of severe perturbations where the dissolved cation concentration is not
small comparing to the total CEC, we do need the knowledge about the exchange
complex and selectivity coefficients of Boom Clay. The cation exchange composition
and the selectivity coefficients derived in this Section are reference model parameters
calibrated on the most extensive set of water compositions, and are therefore
considered as plausible interpretations for Boom Clay pore water chemistry.
4.4.2 Results of model simulations and discussions
With the model parameters derived from the previous Section, the Boom Clay pore
water compositions can be calculated using the constraints given in Table 4-2.
Figure 4-2 shows the variations in concentration of the major cations that are
controlled by calcite dissolution (Ca) and ion exchange reactions (Mg, K, Na).
Major cation concentration, mM
0.25
0.2
0.15
K
Mg
Ca
0.1
0.05
0
320
340
360
380
400
420
440
[Na], mg/kg H2O
Figure 4-2: Variations in concentration of the major cations in 44 water samples
sampled from the MORPHEUS piezometer. Dots are measured data and lines are
calculated values. The system pH varies in the range of 8.3 to 8.6 in equilibrium with
calcite (16°C).
104
From the modelling result of Figure 4-2, it is seen that the combined calcite
dissolution and ion exchange model represents the measured data well.
Although plotted as the concentration of major cations versus sodium concentration,
the system variable in the calculation was pCO2. As shown in Figure 3-4, the
equilibrium pH of MORPHEUS waters was estimated in the range of 8.3 to 8.6,
which corresponds to pCO2 of 10-2.3 to 10-2.8 atm. This variation in pCO2 resulted in a
variation in calcium concentration controlled by calcite dissolution. The calcium in
turn exchanges with Boom Clay to release Na, K, and Mg into aqueous phase, hence
regulating the major cation concentrations in the Boom Clay pore water.
Other cation concentrations, i.e., Fe, Si, and Al, that are governed by the solubility of
minerals, are plotted in Figure 4-3. The iron concentration calculated based on the
siderite solubility agrees generally with the measured data, except that few data points
at the high end of the sodium concentration. These points are data collected at the
double band from where higher concentrations of ions are in general observed (F8,
MORPHEUS piezometer). Some batch experiments have suggested the presence of
Fe-containing colloids. Although colloids may or may not be present in the
piezometer waters, future experiments with the determination of the Fe concentration
using careful filtration is essential. Silicon concentrations are invariant and the
measured concentrations are in agreement with the calculated solubility of
chalcedony.
1.0E+00
cation concentration, mM
1.0E-01
1.0E-02
Si
Fe
Al
Al model
1.0E-03
1.0E-04
1.0E-05
1.0E-06
320
340
360
380
400
420
440
460
[Na], mg/kg H2O
Figure 4-3: Variations in concentration of Fe, Si, Al in 44 water samples collected
from the MORPHEUS piezometer. Dots are measured data and lines are calculated
values. The system pH varies in the range of 8.3 to 8.6 in equilibrium with calcite
(16°C).
105
4.5 Concluding remarks
•
Although statistically very different, the observed variations found in the
MORPHEUS waters can be generally explained by principles of chemical
equilibrium.
•
pH and pCO2 values are critical in choosing the model. More field
measurements of pH and pCO2 are needed to better constrain the model.
•
A combined ion exchange and solubility model explains well the observed
water compositions.
•
The calcium concentration is controlled by the calcite solubility, provided that
the system pH or pCO2 is imposed.
•
The concentrations of Na, K, and Mg can be satisfactorily simulated by ion
exchange reactions.
•
The measured iron concentrations agree, for most of data points, with the
calculated values based on the solubility of siderite. However, the iron
concentration determined piezometer waters without filtration may involve
colloids as suggested by the batch leaching experiments (see Annex 4). Future
iron determination with careful filtration is needed to test the siderite control
mechanism.
•
The silicon concentration can be explained by the solubility of chalcedony.
•
A slight variation in composition of the Boom Clay pore water is generally
observed and attributed to the change of pCO2 (g). However, the cause of the
pCO2 (g) variation is not clear at the moment. It is therefore difficult to assign
a single composition representing the Boom Clay pore water. At present, the
pore water composition is therefore best represented by a reference model
which can be used to calculate the specific water composition under the given
chemical condition.
4.6 Future work needed to improve the model
The model presented in this report is based on pH and pCO2 values in equilibrium
with calcite. As the most important model assumption, the calcite equilibrium state
should be experimentally demonstrated. Although a lot of high quality water samples
became available recent years, the lack of field pH and/or pCO2 measurements
hampered the evaluation of the equilibrium state of these waters. It is desirable to
have more accurate and an increased amount of field pH/ pCO2 measurements. It is
also recommended that pH, pCO2, and the water composition should be determined at
the same time for the same sample to better define the system and to evaluate the
system equilibrium state unambiguously.
The need of well-thought procedures for the sampling, handling and preservation of
the water samples should be emphasised. On site acidification of water is necessary
before analysis of metal ion concentrations. The aluminium concentration in the pore
water should be re-evaluated with the support of careful pre-filtration or ultrafiltration
procedures.
106
The ion exchange complex should be re-measured to confront with the existing data.
The most important issue is whether or not the calcium exchange complex exists in
the Boom Clay.
Finally, the current model uses an imposed pH and pCO2 as the system variable
without knowing the mechanisms controlling the observed variation of pH or pCO2.
First of all, more efforts should be made to identify possible artefacts associated with
the piezometer water sampling. It is desirable to be able to quantify the extent and the
rate of degassing, the degree of bacterial activities in water samples, and the extent of
Boom Clay oxidation due directly to the installation of a piezometer. Secondly, the
intrinsic cause of the pCO2 variation can be studied through the planned heater test,
i.e., the effect of a temperature increase on the variation of the pCO2 and the water
composition. The effect of temperature on the Boom Clay pCO2 can also be studied by
a surface laboratory experiment. A pCO2 measurement can be performed on a well
preserved clay core with the ‘out-gassing’ technique but under different temperatures.
107
5
Reference Boom Clay pore water composition at the Mol site
As described in the statistical analysis of available data (Section 3.4.1), many factors
can influence the measured Boom Clay pore water composition. A major factor is the
spatial variability. In that sense, it is not possible to define one single mean of the
Boom Clay pore water composition at the Mol site, as:
•
•
For some filter materials a straightforward chemical perturbation is noticed
(e.g. B and Si increase with glass filter), but other elementary differences
between filter materials are unexplained.
A natural spatial variation is present, and including all available data does not
provide a Gaussian distribution of measured concentrations for all elements
(for some elements a bimodal distribution is noticed, while for others a normal
distribution is present).
However, as a basic input for geochemical modelling and performance assessment, a
reference Boom Clay pore water composition is needed. As a mean composition
cannot be derived statistically from the measurements, we propose to use a reference
Boom Clay pore water composition as given in Table 5-1. This reference composition
is modelled taking into account the current knowledge of Boom Clay mineralogy and
calibrated towards the MORPHEUS water samples available. From the statistical
analysis performed, it can be concluded that the MORPHEUS piezometer is a good
choice to perform this calibration. First of all, a quite extensive and complete dataset
is available. Secondly, no major disturbance of the borehole or surrounding host rock
is observed (in contrast with EG/BS). Thirdly, the Schumatherm filters do not have a
major effect on the analysed pore water composition. Only the measured Mg content
can differ somewhat from the PE, glass and Carbo filters. A comparison with stainless
steel filters is less straightforward due to the statistically differentiated groups in the
SPRING 116 piezometer. Fourthly, the extent of the influenced zone due to pore
water sampling is smallest for the MORPHEUS piezometer, compared to the EG/BS
and SPRING 116 piezometers. The ORPHEUS piezometer has an even smaller
influenced zone, but due to in-situ pH-Eh measurements, the system might be more
disturbed (leak of KCl, and possibly also glycerine and acrylate from electrodes, pers.
comm. from N. Bogaerts, Elscolab). Finally, the MORPHEUS piezometer comprises
at least some of the spatial variation noticed around the HADES URF as indicated by
the variation of measured concentrations (also shown in Table 5-1).
For laboratory experiments, a synthetic Boom Clay pore water composition is also
needed to mimic the real Boom Clay water. Within the framework of Belgian R&D
for radioactive waste disposal in Boom Clay, two names of synthetic water have
appeared in literatures, namely SIC and SCW, standing for synthetic interstitial clay
water and synthetic clay water, respectively. A subdivision of SIC was also made
between the waters with and without humic acid (Lemmens, 2001), i.e., SIC (with
humic acid) and SICZH (without humic acid). Maes (2000) also reported a modified
SCW in comparison to the water used by Dierckx et al., (2000). Although rather
confusing in terminology, these synthetic waters have basically two compositions as
given in Table 5-2.
108
Table 5-1: The reference Boom Clay pore water composition and the measured
MORPHEUS water composition. The major ion concentrations of the reference water
are calculated by cation exchange and mineral dissolution reactions that are
calibrated against the measured MORPHEUS water compositions.
composition
reference
MORPHEUS water*
water
-217~ -235 m TAW
mg/l
mmol/l
mg/l
mmol/l
Na
K
Ca
Mg
Fe
Si
Al
359
7.2
2.0
1.6
0.2
3.4
0.6E-3
15.6
0.2
0.05
0.06
0.003
0.1
2.4E-5
348-431
6.7-8.3
1.5-2.9
1.3-2.6
0.10-0.68
4.2-5.5
0.03-0.06
15.1-18.7
0.17-0.21
0.04-0.07
0.05-0.11
0.002-0.012
0.1-0.2
1.1-2.2E-3
HCO3TIC (mg C/l)
alkalinity (meq/l)
Cl
total S
SO42HPO42NO3F
Br
B
878.9
181.3
15.12
26
0.77
2.2
14.4
15.1
173-206
14.9
24-30
na
0.63-2.31
na§
~0.4
2.6-3.3
~0.6
~7
14.4-17.2
DOC (mg C/l)
Cs
Sr
U
pH
pCO2 (atm)
Eh (mV)
temperature (°C)
conductivity (µS.cm-1)
ionic strength
*
0.7
0.02
0.02
0.7-0.8
6.5E-3-0.02
6.4E-3
0.13-0.17
7.5E-3
0.6
120-200
8.5
10-2.62
-274
16
0.016#
<0.5 (µg/l)
46-90 (µg/l)
0.3-1.2 (µg/l)
na
na
na
~16
1700
<4E-9 (M)
5-10 E-7 (M)
1-5 E-9 (M)
the range observed in 5 statistic groups of 44 measurements.
na: not analysed.
#
calculated by the B-dot method (Helgeson and Kirkham, 1974; Helgeson, 1969).
§
not measured in MORPHEUS waters but in general below the detection limit of 0.5 mg/l.
109
It is apparent from Table 5-2 that SIC and SCW are similar in major components and
the difference between them will unlikely have any impact on experimental results.
From Table 5-1 and the detailed interpretation given in this report, an undisturbed
Boom Clay pore water is basically a solution of 15 mM NaHCO3, rich in organic
matter. It is therefore our opinion that a synthetic Boom Clay pore water made of 15
mM NaHCO3 will suffice to perform most of the laboratory experiments. It will be the
decision of experimentalists if additional components should be added according to
the objective of the experiment. In case a more detailed water composition is needed,
Table 5-1 provides the reference.
Table 5-2: Compositions of synthetic interstitial clay water (SIC) and synthetic clay
water (SCW) used in the laboratory experiments*.
composition
elemental
reference
composition
composition
mg/l
SIC
SCW
mg/l
NaHCO3
NaCl
MgSO4
Na2SO4
KCl
NaF
MgCl2•6H20
H3BO3
FeCl2
1250
44
12
1.5
20
8
1170
10
Na
K
Ca§
Mg
Fe
B
humic acid
150
*
0.3
25
11
22
43
3
TIC (mg HCO3-)
Cl
SO42F
SIC
SCW
363
330
10.5
13
saturated saturated
2.4
2.6
1.3
7.5
909
33.7
10.6
3.6
848
27.3
0.2
4.9
359
7.2
2.0
1.6
0.2
878.9
26
2.2
-
detailed procedures for preparation of SIC and SCW are given in Lemmens (2001) and Maes (2000)
respectively. See also Annex 8.
§
saturated with calcite by equilibrating synthetic water with calcite.
110
111
6
Conclusions
In the frame of the methodological studies on the Boom Clay as the reference host
rock for the geological disposal of radioactive waste in Belgium, a good
understanding of the Boom Clay pore water composition is necessary. Indeed, in the
laboratory, representative Boom Clay pore water is used in the experiments.
Furthermore, the Boom Clay pore water composition is a basic input for geochemical
modelling and performance assessment calculations. It is therefore important to
carefully know the Boom Clay pore water composition.
In this report, different technique(s) to obtain representative pore water samples were
evaluated. The Boom Clay pore water was sampled, chemical analyses were
performed and geochemical parameters such as the pH, Eh, pCO2, and the alkalinity
were studied. This was done in order to determine the variation of the pore water
composition in the Boom Clay, to present a coherent geochemical model for
explaining the origin of the Boom Clay pore water composition, and to propose a
reference pore water composition to be used in the laboratory experiments and for
speciation calculations and assessments of perturbation of the Boom Clay.
Evaluation of the different pore water extraction techniques
Pore water extraction from the Boom Clay is done by either in situ or laboratory
techniques, all carried out at SCK•CEN. In situ pore water extraction is realised by
using various types of piezometers, which are placed in different directions and at
different depths into the clay. In the laboratory, the pore water is extracted from wellpreserved clay cores either by mechanical squeezing or leaching. Great care should be
taken during the drilling of the borehole, and the anaerobic preservation and handling
of the clay cores, to prevent oxygen perturbation.
At the present time, piezometer water is considered to be the most representative for
the in situ pore water. This is because piezometer waters experience minimum
laboratory manipulations and therefore suffer minimum artefacts. Squeezed pore
water is comparable to piezometer-derived water when considering the major ionic
composition, but not for trace elements and organic matter. Squeezed pore water
samples can thus be considered as representative for the in situ conditions, up to a
certain degree. Comparing to the piezometer and squeezing techniques, batch leaching
experiments provide comparable results for the major cation composition if the
samples are carefully filtered. Due to the electrostatic properties of the Boom Clay,
i.e., double layer phenomena, the leaching waters reveal a very different anion
composition compared to the waters extracted from compacted clay using piezometers
and squeezing techniques. This aspect deserves a more detailed study in the future.
Chemical composition of the Boom Clay pore water
The Boom Clay pore water is basically a NaHCO3 solution of 15 mM, containing an
important amount of dissolved organic matter (about 115±15 mg C / l). The observed
major cation concentrations can be explained by cation exchange and mineral
dissolution/precipitation mechanisms. The current geochemical model assumes the
equilibrium of calcite, siderite, pyrite, and chalcedony, and the cation exchange
between Ca, Na, K, and Mg.
112
pH, pCO2, and alkalinity of the Boom Clay
As the most important parameter, the Boom Clay pH and its controlling mechanisms
are still not conclusive. Apart from the well known in situ ARCHIMEDE-argile
measurement (pH 8.2; Beaucaire et al., 2000; Griffault et al., 1996), the new in situ
measurement (pH 8.0; Moors et al., 2002) suffered from the leak of KCl from the
electrode and demonstrated the technical difficulties in pursuing a good quality pH
measurement. An alternative way to acquire the pH of the Boom Clay is through
speciation calculations. This model simulation, using water compositions from the
MORPHEUS piezometer, suggests a pH range of 8.3 to 8.6.
Both pH and pCO2 measurements are technically challenging. This is because the
Boom Clay is susceptible to the loss of CO2 (g) if in contact with air. Although
attempts have been made to measure or calculate the pCO2 of Boom Clay, the real
mechanism governing the parameters is not understood. On the one hand, current
activities for a more accurate and representative pCO2 measurement should be
continued, on the other hand future studies to understand the mechanism of the CO2
evolution in Boom Clay should be pursued. Until now, the calculated pCO2 value in the
range of 10-2.4 to 10-2.8 atm is used.
The total alkalinity of Boom Clay pore water is 12.1 mmol/l and agrees very well
with the total inorganic carbon content suggesting that other solutes, including natural
organic matter, do not contribute significantly to the total alkalinity.
Redox processes and redox potential of the Boom Clay
The maximum redox potential Eh is about -270 mV; probably controlled by the
equilibrium of pyrite and siderite under the in situ geochemical conditions. A lower
redox potential is possible as the result of interactions involving natural organic
matter mediated by biochemical processes.
Spatial variability
The statistical analysis of the available data at the Mol site (to about 40 m around the
HADES URF) has shown that a vertical spatial variability (perpendicular to the
bedding) is present within the Boom Clay pore water composition. This vertical
variability shows no gradient, and is mostly influenced by the elements Na, Mg, Ca
and Cl. The mechanism behind these variations in major cations is explained by
cation exchange and calcite dissolution/precipitation. The ultimate cause of these
chemical reactions is assumed to be due to the spatial variability in pCO2 and pH,
although the reason of this is not yet understood. Nevertheless, if the assumed pCO2
variation exists, the pore water seemed to respond rapidly to reach a chemical
equilibrium with the clay. Because transport in the Boom Clay is diffusion-controlled,
the spatial variability in the pore water composition can still be present, even on small
scales.
An important horizontal variability in the pore water composition is present at a
regional scale, where a seawater contribution is obvious towards the west (NaHCO3
dominated versus NaHCO3-NaCl mixed waters).
113
Model Simulation of the Boom Clay pore water chemistry
Although statistically very different, the observed spatial variations in pore water
composition can be explained by principles of chemical equilibrium and cation
exchange. The current geochemical model assumes the equilibrium of calcite, siderite,
pyrite, and chalcedony, and the cation exchange between Ca, Na, K, and Mg.
The model presented in this report is based on pH and pCO2 values in equilibrium
with calcite. The lack of accurate field pH and/or pCO2 measurements hampered the
evaluation of the equilibrium state of these waters. Furthermore, as the most important
model assumption, the calcite equilibrium state should be experimentally
demonstrated. In order to test the calibrated geochemical model in a wider range of
conditions, some recommendations for further study are formulated (see Section 7).
Reference Boom Clay pore water composition at the Mol site
Due to the spatial variability, one single mean Boom Clay pore water composition at
the Mol site cannot be given. However, a modelled reference composition, taking into
account the current knowledge of Boom Clay mineralogy and calibrated towards a
dataset including spatial variability, is provided.
114
115
7
Recommendations
A systematic follow-up of the pore water composition, with a systematic set of
analyses and a centralised data-base is necessary in order to:
•
•
•
•
study the variability of the pore water composition (spatial variability);
follow the evolution of the perturbations (variability with time);
improve the statistical data set;
calibrate the geochemical model in a wider range of conditions.
Well-though procedures for the sampling, handling and preservation of the pore water
samples should be emphasised.
Regarding to the chemical analyses, appropriate on site filtration should be practiced
to remove possible particulates and colloids. Pre-acidification of sample to a pH < 2 is
required to preserve the concentrations of the major cations. This is especially
important for cations that precipitate easily such as Ca and Fe. Sample preservation at
low temperature and/or in the presence of bactericide like thymol is needed to restrict
bacterial activity. Aluminium and total sulphide (HS- and H2S) should be analysed.
More accurate in situ pH measurements are needed. Hence, the technique for in situ
pH measurement should be evaluated. The currently used closed-circuit technique
encountered several important problems, such as the electrode drift and the leakage of
KCl of the electrode into the pore water. In the future, the electrode drift should be
calibrated. Also the influence of the KCl leakage on the pore water chemistry and the
possibly associated pH drop should be studied in more detail.
In order to test the calibrated geochemical model in a wider range of conditions, the
following aspects should be further studied:
•
Although a lot of high quality water samples became available recent years,
the lack of field pH and/or pCO2 measurements hampered the evaluation of the
equilibrium state of these waters. It is necessary to have more accurate (see
above) and an increased amount of field pH/ pCO2 measurements. It is also
recommended that pH, pCO2, and the water composition should be determined
at the same time for the same sample to better define the system and to
evaluate the system equilibrium state unambiguously;
•
In addition to the pCO2 measurements, the CO2 sources and controlling
mechanism should be studied;
•
Further mineralogical analysis is needed to reveal if dolomite exists or not.
This is directly related to the way how magnesium containing minerals should
be modelled;
•
Cation exchange experiments should be performed to indicate if a calcium
exchange complex exists or not on the surface of the Boom Clay;
•
The nature and the extent of biochemical processes (as an artefact in the
surroundings of the piezometer) should be evaluated to better scope the effect
of sulphate reduction, variation on pCO2 and pH, organic matter oxidation,
methane formation…
116
117
8
Acknowledgements
This work would not have been possible without the technical support of Marc Van
Gompel, Louis Van Ravestyn, Frank Vandervoort and Tom Maes. We are very
greatful to them.
We also thank Norbert Maes, Isabelle Wemaere and Christelle Cachoir for reviewing
particular sections of this report. Geert Volckaert, Ann Dierckx, Elie Valcke and
Pierre De Cannière are thanked for the review of the entire document and the many
fruitful discussions concerning the Boom Clay pore water chemistry.
Finally, NIRAS/ONDRAF is thanked for the financial support.
118
119
9
References
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A. Balkema, Rotterdam.
Atkins, P. W. (1994) Physical chemistry, Oxford University Press, Oxford,
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Baeyens, B. (1982) Strontium, cesium and europium retention in Boom Clay: A
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10 Annexes
Annex 1:
Overview of the general characteristics of the various piezometres
considered in this report (EG/BS, ARCHIMEDE, SPRING 116,
ORPHEUS, MORPHEUS).
Annex 2:
Materials used as piezometric filterscreen
Annex 3:
Assessing the performance of pH and Eh electrodes used for in situ
measurement campaigns
Annex 4:
The procedure, results, and interpretations for batch leaching
experiments to determine the concentration of Boom Clay pore water
components
Annex 5:
Boom Clay pore water geochemistry: analytical data used in the
statistical analyses
Annex 6:
Statistics: methodology
Annex 7:
The input and output files from geochemical modelling simulations
Annex 8:
Prescriptions for the preparation of synthetic Boom Clay water
129
Annex 1: Overview of the general characteristics of the various piezometres considered in this report
(EG/BS, ARCHIMEDE, SPRING 116, ORPHEUS, MORPHEUS).
piezometer name
piezometer
emplacement
piezometer location
piezometer
material
piezometer length
piezometer
diameter
filter
amount
filter material
filter size
vertical depth (m
TAW)
EGBS/2
September 1983
vertical piezometer, downward from
HADES, located at the bottom of the first
schaft
stainless steel
~13 m
stainl. steel 6 cm,
coarse sand 8.5 cm
1
stainless steel
stainl. steel 9 cm,
coarse sand 120 cm
-227.6
ARCHIMEDE #1
March 1992
semi-horizontal piezometer (3 % inclined
upwards) towards the east, HADES URL,
ANDRA Gallery, between sliding ribbs 24
and 25
stainless steel
15 m
60 mm
5
?
-196.5
ARCHIMEDE #2
April 1992
horizontal piezometer towards the east,
HADES URL, ANDRA Gallery, between
sliding ribbs 4 and 5
stainless steal
15 m
140 mm
5
?
-196.5
SPRING 116
October 1999
horizontal piezometer towards the east, Test
Drift, ring 116
stainless steel
12 m
15 cm
4
November 2000
horizontal piezometer towards the west,
Test Drift, ring 116
PVC
8m
12 cm
4
MORPHEUS
May 2001
vertical piezometer, downward from
HADES, Test Drift, between ring 11 and 12
PVC
40 m
15 cm
12
152 cm
152 cm
152 cm
152 cm
25 cm
25 cm
25 cm
25 cm
10 cm
10 cm
10 cm
10 cm
10 cm
10 cm
10 cm
10 cm
10 cm
10 cm
10 cm
10 cm
-196.5
ORPHEUS
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
sintered glass
poly ethylene
carbo
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
schumatherm
-196.5
-217.07 to -217.17
-220.77 to -220.87
-222.57 to -222.67
-225.98 to -226.08
-227.18 to -227.28
-227.78 to -227.88
-229.08 to -229.18
-229.73 to -229.83
-230.23 to -230.33
-231.78 to -231.88
-233.79 to -233.89
-235.19 to -235.29
distance from the code piezometer- code stalen
gallery (in m, filter
extra dos Test
Drift)
stainl.steel 30 m, EGBS-SS
coarse sand 22-34 m
3m
7m
8m
14 m
15 m
3m
7m
8m
14 m
15 m
5.44 - 6.94
7.16 - 8.66
8.88 - 10.38
10.60 - 12.10
5.25 - 5.50
5.90 - 6.15
6.45 - 6.70
7.10 - 7.35
18.12 - 18.22
21.92 - 22.02
23.72 - 23.82
27.13 - 27.23
28.33 - 28.43
28.93 - 39.03
30.23 - 30.33
30.88 -30.98
31.38 - 31.48
32.93 - 33. 03
34.94 - 35.04
36.34 - 36.44
AR1-SS-03
AR1-SS-07
AR1-SS-08
AR1-SS-14
AR1-SS-15
AR2-SS-03
AR2-SS-07
AR2-SS-08
AR2-SS-14
AR2-SS-15
SP-SS-4
SP-SS-3
SP-SS-2
SP-SS-1
OR-ST
OR-CA
OR-PE
OR-SG
MO-ST-23
MO-ST-20
MO-ST-18
MO-ST-15
MO-ST-13
MO-ST-12
MO-ST-10
MO-ST-09
MO-ST-08
MO-ST-06
MO-ST-04
MO-ST-02
EGBS-SS/yyyymmdd
AR1-SS-03/yyymmdd
AR1-SS-07/yyymmdd
AR1-SS-08/yyymmdd
AR1-SS-14/yyymmdd
AR1-SS-15/yyymmdd
AR2-SS-03/yyymmdd
AR2-SS-07/yyymmdd
AR2-SS-08/yyymmdd
AR2-SS-14/yyymmdd
AR2-SS-15/yyymmdd
SP-SS-4/yyyymmdd
SP-SS-3/yyyymmdd
SP-SS-2/yyyymmdd
SP-SS-1/yyyymmdd
OR-ST/yyyymmdd
OR-CA/yyyymmdd
OR-PE/yyyymmdd
OR-SG/yyyymmdd
MO-ST23/yyyymmdd
MO-ST-20/yyyymmdd
MO-ST-18/yyyymmdd
MO-ST-15/yyyymmdd
MO-ST-13/yyyymmdd
MO-ST-12/yyyymmdd
MO-ST-10/yyyymmdd
MO-ST-09/yyyymmdd
MO-ST-08/yyyymmdd
MO-ST-06/yyyymmdd
MO-ST-04/yyyymmdd
MO-ST-02/yyyymmdd
130
Annex 2: Materials used as piezometric filterscreen
For more then 20 years multipiezometers were all made of stainless steel (sometimes in
combination with classic sand-filter systems). The main raisons for this choice were the
machinability, and the pressure and corrosion resistance of stainless steel.
As new materials became available, the applicability of these materials as piezometric
filterscreen was investigated. Nowadays, the choice of piezometric filterscreen is a strategic
item in the construction phase of a multi-piezometer, that can be adapted in function of the
intended uses of multi-piezometers.
Coarse sand
The filterscreen of the EG/BS piezometer consists of a column of purified coarse sand with a
granulometry of 0.71 to 1.25 mm.
Stainless steel (SS)
The stainless steel filterscreens of SPRING 116 and the collecting chamber of the EG/BS
piezometer are manufactured by GKN Sinter Metals (Germany). The filter elements are
produced by cold isostatic pressing of metal powder followed by a sintering process that
bonds and fuses the powder particles to each other.
The filter elements have the following characteristics:
•
•
•
•
material: stainless steel, 1.4404 (AISI 316 L/B); filter type: SIKA-R5 IS which stands
for self-supporting structural element
porosity: 30 % (DIN ISO 30911-3)
average CCE pore diameter: 9 µm (ASTM F 902)
permeability coefficients: α = 0.8 x 10-12 m2; β = 0.9 x 10-7 m (DIN ISO 4022)
Glass (SG)
The glass filterscreen which is part of the ORPHEUS multi-piezometer is manufactured by
ROBU GLASFILTER-GERÄTE GMBH (Germany). It has the trade-name VitraPOR, which
stands for a complete series of glass filter-products.
The filter element has the following characteristics:
•
•
•
material: Borosilicate glass 3.3 (DIN-ISO 3585)
porosity: porosity grade 4, ISO 4793 designation P16
nominal pore size: 10-16 µm
Filtroplast (PE)
The plastic filterscreen which is part of the ORPHEUS multi-piezometer is manufactured by
Schumacher Umwelt- und Trenntechnik GmbH (Germany). The filterelements are know as
FILTROPLAST. FILTROPLAST is a porous sintered plastic product of pure Polyethylene
(PE-HD). Rigid elements of different dimensions and shapes are manufactured from special
PE granules of defined size and particle distribution in a controlled sintering process. The
filtration and aeration properties of the product depend on the pore size, which is a function of
both the raw material and the production parameters.
132
The filter element has the following characteristics:
•
•
•
material: High Density Polyethylene, grade 40
porosity: 45 %
mean pore size: 40 µm
Carbo (CA)
The carbon filterscreen which is part of the ORPHEUS multi-piezometer is manufactured by
Schumacher Umwelt- und Trenntechnik GmbH (Germany). The filterelements are known as
CARBO. CARBO consists of pure carbon (98 % C). The various types of CARBO are based
on different grain size fractions that are linked by carbon bridges which were built from tar
during a sintering process. The material is very resistant against chemical reactions due to its
binder-free structure.
The filter element has the following characteristics:
•
•
•
material: Carbon (98 % C), grade 40
porosity: 35 %
mean pore size: 90 µm
Shumatherm (ST)
The Schumatherm filterscreens are used as filterscreens for the MORPHEUS multi
piezometer and are part of the ORPHEUS multi-piezometer. Schumatherm is manufactured
by Schumacher Umwelt- und Trenntechnik GmbH (Germany). Schumatherm is a high-quality
fireclay rich in mulite(a), obtained by sintering of refractory clay and subsequent crushing.
This raw material is sintered with an alumo-siliceous bond. The binding phase imparts good
mechanical properties and is mostly of amorphous structure.
Schumatherm filter elements have the following properties:
•
•
•
material: fireclay rich in mulite, grade 30.
Mullite is a nonstoichiometric compound of approximate composition
Al6Si3O15. It is rare as mineral but commonly used in artificial Al2O3–SiO2
systems at high temperature.
porosity: 37 %
mean pore size: 60 µm
Annex 3: Assessing the performance of pH and Eh electrodes used for in situ
measurement campaigns.
Introduction
Electrodes used for in situ measurements are generally submitted to harsh conditions (e.g.
high pressure, long contact time between electrode and the measuring medium, influence of
the ligand and complex-forming foreign species, ageing, ...). To demonstrate the performance
of such electrodes over the whole period of in situ measurement an adapted testing procedure
is required. This adapted testing procedure is composed out of two successive tests:
• a first test that allows to interpret the offset, the deviation with respect to the zero point and
the slope or linearity of the electrode at moment of testing, and,
• a second test that provides evidence on the origin of the deviation and the quantification of
the electrode drift.
Background
Ideal pH-electrodes display 0 mV when immersed in a solution of pH = 7 and respond linear,
with a slope of 59.16 mV per pH unit (@ 25 °C) when immersed in solutions having other pH
values. Similarly, ideal Eh-electrodes display 0 mV when immersed in a pH buffer solution of
pH = 8.5 saturated with quinhydrone and respond linear with an identical slope of 59.16 mV
per pH unit (@ 25 °C) when immersed in quinhydrone saturated solutions buffered at other
pH-values. The differences between the theoretical values and the measured ones, determines
the offset values of the tested pH or Eh-electrode. Linearity is evaluated by comparing the
calculated electrode slopes, based on the electrode measurements in different pH buffered
solutions, with the theoretical slope.
Offset values within ±10 mV and slopes deviating ±5 % from the theoretical values are
allowed for most laboratory applications. This kind of testing, especially for pH electrodes, is
used in many standard calibration protocols and electrode functionality checks. However, for
electrodes used in long term in situ measurements, the deviation of the offset can become
quite large. The logic source for this deviation is a change of the chloride concentration inside
the in situ reference electrode, and, consequently, a change of the reference potential. To
check and quantify this change of reference potential, an additional test is used. For this test, a
separate reference electrode, of the same type and filling as the in situ reference electrode, is
required and connected to a pH-meter together with the reference electrode of the in situ
electrode. If the offset of the in situ electrode is solely due to a drift of the electrodes reference
potential than, immersing the in situ reference electrode in conjunction with a separate
reference electrode in an aqueous solutions, either pH, Eh or salt buffered, should always
display the same value. A value equal to the previously determined offset potential. In this
case, the data set collected during the in situ measurement campaign can be appropriately
corrected. The correction is done assuming a linear drift of the reference potential over the
measurement period. All data points are pro rata corrected with the calculated drift-slope.
Procedure
The first test: Determination of the electrode offset and linearity
pH-electrodes
Three certified buffer solutions are used: 4.00, 7.00 and 9.00. The pH-electrode, connected to
its pH-meter, is successively immersed in each of the three buffer solutions. When the meter
134
reading has stabilised the displayed value (preferably in mV) is recorded and compared with
the theoretical value. This test is repeated at the beginning and at the end of an in situ
measuring campaign.
An electrode is considered as functioning properly if the absolute– and relative differences of
the measured values are smaller than ± 0.15 pH units. If the absolute differences are larger
than 0.15 pH-units, but the relative differences are smaller than 0.15 pH-units, then only the
linearity of the pH-electrode is acceptable. In this case a supplementary test is needed to
determine the origin and to verify the offset of the electrode.
Eh-electrodes
As for pH-electrodes, three certified pH buffer solutions are used. To these solutions and just
before the test, an excess of Quinhydrone is added. The Eh-electrode, connected to its Ehmilli-voltmeter, is successively immersed in each of the three solutions. When the meter
reading has stabilised the displayed value is recorded and compared with the theoretical value.
This test is repeated at the beginning and at the end of an in situ measuring campaign.
An electrode is considered as functioning properly if the absolute– and relative differences of
the measured values are smaller than respectively ± 20 and ± 10 mV. If the absolute
differences are larger than 20 mV, but the relative differences are smaller than 10 mV, then
only the linearity of the Eh-electrode is acceptable. In this case a supplementary test is needed
to determine the origin and to verify the offset of the electrode.
The second test: determination of the value and the origin the electrode drift
This test is similar for pH and Eh-electrodes. It is used to check the signal deviation of the
reference electrode, that is "build in" in pH and Eh-electrodes. For this test a separate properly
functioning reference electrode is required, preferably of the same type as the in situ reference
electrode. The one lead of this separate reference electrode is connected to one of the two
signal inputs of a pH-milli-voltmeter. The lead of the reference electrode of the in situ pHelectrode is connected to the remaining electrical signal input of the milli-voltmeter. The
indicator electrode lead, or glass membrane signal lead, is left unconnected. Both reference
electrodes are successively immersed in the three certified buffer solutions of the first test,
and, if possible in a non buffered electrolyte solution. Typically, in a solution containing 3
moles per litre of potassium chloride. When the meter reading, which indicates during these
measurements the deviation between two reference electrodes, has stabilised the value is
recorded and evaluated.
Evaluation
From the results of the first test we learn whether or not the electrode functions properly
(absolute differences between measured values and theoretical values), and if not, whether the
response behaviour is linear or not (relative differences = comparison of the differences
between the measurements). If linearity is considered to be good, the second test allows to
interpret the origin and to verify the electrode drift.
To conclude that the in situ electrode offset is solely due to a drift of the reference electrode,
the relative differences between the measured values of the second test, and in all buffer
solutions, may not exceed ± 10 mV. Also, the recorded values of this second test have to be
equal (within a range of ± 10 mV) to the offset values determined in first test. An example of
an evaluation report is given.
The change of the potassium chloride concentration inside the reference electrode is
controlled by diffusion. The larger the concentration difference between reference electrode
electrolyte and measuring solution, the stronger the rate of out-diffusion. For standard
laboratory application this out-diffusion can easily be neglected. However, for long lasting in
situ measurement campaigns this out-diffusion cannot be neglected. Due to this potassium
chloride leaching problem and the non uniform construction of electrochemical electrodes, the
true evolution of the reference electrode drift is unknown. The easiest approach is to consider
a linear process, and, proportionally correct all measured data with the appropriate slope or
drift speed of the in situ electrode. This approach is used until experimental evidence will
come up with the correct interpretation of the evolution of the drift speed.
136
Example of an "evaluation report" of in situ used pH and Eh electrodes:
Archiméde 2 - assessment of the electrode performance
First measurement period: 1-March-2000 to 3-July-2001
pH-electrode
Electrode data
Electrode type:
Serial number:
Laboratory tested on:
Method:
Test result:
Ingold, Xerolyt, glass-pH electrode
8238228
29-February-2000
Standard "Metrohm" electrode test procedure
good electrode
Field data
Electrode placed in flow-through cell on 1-March-2000
Chronological event history (see logbook ARCHIM2.xls)
Electrode taken out of flow-through cell on 3-July-2001
Covering a period of 463 days
Test methods
1) Potential measurement in three different pH-buffer solutions
Table 1: Results of the potential measurement in pH-buffer solutions
Time/Evaluation
[dd-mm-yyyy]
Theoretical
29-February-2000
Difference
3-July-2001
Difference
Buffer 4
[mV]
177
181
+4
56
-125
Buffer 7
[mV]
0
4
+4
-124
-124
Buffer 9
[mV]
-118
-110
+8
-239
-121
2) Reference-Reference measurement
Table 2: Results of the Reference-Reference measurement
Time/Evaluation
[dd-mm-yyyy]
Theoretical
29-February-2000
Difference
3-July-2001
Difference
Buffer 4
[mV]
0
+3
+3
-119
-119
Buffer 7
[mV]
0
+3
+3
-120
-120
Buffer 9
[mV]
0
+4
+4
-121
-121
3M KCl
[mV]
0
n.a.
n.a.
-
Conclusions for the pH-electrode performance
From the potential measurements in pH-buffer solutions we can calculate an average
difference of 123.3 (± 2.1) mV over the pH-region 4 to 9. From the Reference-Reference
measurement we calculate an average difference of 120.0 (± 1.0) mV over the pH-region 4 to
9. We therefore conclude that:
• the measured difference is solely due to a drift of the reference electrode
• the drift is in a logical direction (depletion of KCl results in higher pH values)
• the end value of the drift can be taken as: 120 mV
If one considers a linear drift, measurement correction should be done with a drift-speed or
slope = 0.25918 mV per day or
-0.00438 pH units per day
138
Example of an "evaluation report" of in situ used pH and Eh electrodes:
Archiméde 2 - assessment of the electrode performance
First measurement period: 1-March-2000 to 3-July-2001
Pt-Eh-electrode
Electrode data
electrode type:
serial number:
Tested on:
Method:
Test result:
Ingold, Xerolyt, Platinum redox-electrode
8484707
8-June-1999, 29-February-2000
SCK•CEN redox-electrode test procedure
Good Electrode
Field data
Electrode placed in flow-through cell on 1-March-2000
Chronological event history (see logbook ARCHIM2.xls)
Electrode taken out of flow-through cell on 3-July-2001
Covering a period of 463 days
Test methods
1) Potential measurement in three different Eh-buffer solutions
Table 1: Results of the potential measurement in pH-buffer solutions
Time/Evaluation
[dd-mm-yyyy]
Theoretical
8-June-1999
Difference
29-February-2000
Difference
3-July-2001
Difference
Buffer4 +
Quinhydrone
[mV]
266
264
-2
263
-3
142
-124
Buffer 7 +
Quinhydrone
[mV]
89
88
-1
89
0
-36
-125
Buffer 9 +
Quinhydrone
[mV]
-30
-29
+1
-29
+1
-143
-113
2) Reference-Reference measurement
Table 2: Results of the Reference-Reference measurement
Time/Evaluation
[dd-mm-yyyy]
Theoretical
29-February-2000
Difference
Buffer 4 +
Quinhydrone
[mV]
0
n.a.
-
Buffer 7 +
Quinhydrone
[mV]
0
n.a
-
Buffer 9 +
Quinhydrone
[mV]
0
n.a
-
3M KCl
[mV]
0
n.a
-
Conclusions for the Pt-Eh-electrode performance
From the potential measurements in Eh-buffer solutions we can calculate an average
difference of 121 (± 7) mV over the Eh-region +266 to -30 mV. The Reference-Reference
measurements have not been performed. Therefore, we can only base the conclusions on the
Quinhydrone measurements and evaluate the observed differences at the start and the end of
the measurement period. We can assume that:
• the measured difference is probably due to a drift of the reference electrode
• the observed drift is logical (depletion of KCl results in apparently lower Eh values)
• the final value of the reference drift can be taken as: 121 mV
If one considers a linear drift, measurement correction should be done with a drift-speed or
slope = -0.26 mV per day
140
Example of an "evaluation report" of in situ used pH and Eh electrodes:
Archiméde 2 - assessment of the electrode performance
First measurement period: 1-March-2000 to 3-July-2001
Au-Eh-electrode
Electrode data
electrode type:
serial number:
Tested on:
Method:
Test result:
Ingold, Xerolyt, gold redox-electrode
9084656
8-June-1999, 29-February-2000
SCK•CEN redox-electrode test procedure
Good Electrode, slightly over the linearity limit
Field data
Electrode placed in flow-through cell on 1-March-2000
Chronological event history (see logbook ARCHIM2.xls)
Electrode taken out of flow-through cell on 3-July-2001
Covering a period of 463 days
Test methods
1) Potential measurement in three different Eh-buffer solutions
Table 1: Results of the potential measurement in pH-buffer solutions
Time/Evaluation
[dd-mm-yyyy]
Theoretical
8-June-1999
Difference
29-February-2000
Difference
3-July-2001
Difference
Buffer4 +
Quinhydrone
[mV]
266
269
+3
254
-12
144
-122
Buffer 7 +
Quinhydrone
[mV]
89
87
-2
89
0
-31
-120
Buffer 9 +
Quinhydrone
[mV]
-30
-28
+2
-28
+2
-134
-104
2) Reference-Reference measurement
Table 2: Results of the Reference-Reference measurement
Time/Evaluation
[dd-mm-yyyy]
Theoretical
29-February-2000
Difference
Buffer 4 +
Quinhydrone
[mV]
0
n.a.
-
Buffer 7 +
Quinhydrone
[mV]
0
n.a
-
Buffer 9 +
Quinhydrone
[mV]
0
n.a
-
3M KCl
[mV]
0
n.a
-
Conclusions of the Au-Eh-electrode performance
From the potential measurements in Eh-buffer solutions we can calculate an average
difference of 115 (± 10) mV over the Eh-region +266 to -30 mV. The Reference-Reference
measurements have not been performed. Therefore, we can only base the conclusions on the
Quinhydrone measurements and evaluate the observed differences at the start and the end of
the measurement period. We can assume that:
• the measured difference is probably due to a drift of the reference electrode
• the observed drift is logical (depletion of KCl results in apparently lower Eh values)
• the final value of the reference drift can be taken as: 115 mV
If one considers a linear drift, measurement correction should be done with a drift-speed or
slope = -0.25 mV per day
142
Annex 4: The procedure, results, and interpretations for batch leaching
experiments to determine the concentration of Boom Clay pore water
components
Background
In parallel with using the piezometric and mechanical squeezing techniques, the batch
leaching experiment was performed for a purpose of comparison. Summaries of the
techniques and the results are given in Section 2.1 and 3.1 of this report. Different from the
piezometer and squeezing techniques, the batch leaching method does not sample pore water
directly without diluting the system, i.e., the addition of water to clay samples is necessary in
batch experiments. Due to the addition of water to the system, the resulting solution
composition cannot directly be used as the real pore water composition of the Boom Clay
without interpretations or modelling. This annex documents the details of the procedure,
results, and interpretations for the batch leaching technique.
The experiment was designed in the way that the approach of Bradbury and Baeyens (1998),
referred as the B&B approach hereafter, can be used for the data interpretations. The essential
feature of the B&B experiment is to measure the water composition change as a result of
leaching as a function of varied clay to the solution ratio (solid to liquid ratio, S/L). The
chemical principles of the B&B approach are based on the dilution effect for soluble salts
(NaCl, NaHCO3 or Na2SO4), the cation exchange equilibrium for major cations, and the
solubility equilibrium for solubility controlled phases like calcite and dolomite. Because of
the general similarities between the Opalinus Clay and the Boom Clay, we expected that the
B&B approach, at the stage of experiment, should work for the Boom Clay as well. The major
difference between our experiments and that of B&B is that we did look at the effect of
colloids by filtration techniques and B&B did only centrifugation. In addition, we mostly used
a NaHCO3 solution as the leachant while B&B used only distilled water. We didn’t perform
the exhaustive cation exchange extraction, e.g., using nickelethylenediamine as B&B did.
Different from the Opalinus Clay for which the data of ion exchange complex was not
available (to our knowledge), Boom Clay ion exchange has been well studied (Baeyens et al.,
1985).
The experimental procedure
The experimental procedure is illustrated in Figure A4-1. Boom Clay samples from the
HADES 2001/4 borehole were grinded, suspended, and agitated in a NaHCO3 solution of
0.01 M. Bicarbonate solution was used as the leachant because Boom Clay pore water is
basically a dilute NaHCO3 solution. Some samples were suspended in distilled water for
comparison. Experiments were conducted in glove boxes to protect the clay samples from
oxidation. The oxygen content of the glove boxes is about 2 ppm but generally below 10 ppm.
For practical reasons, some experiments were carried out in an Ar glove box and some others
in an Ar/CO2 (g) glove box. The CO2 content in the glove box was fixed at 0.4 percent to
mimic the supposed in situ partial pressure of CO2 (10-2.4 atm). Four different S/L ratio were
used: 25, 50, 200, and 800 gram wet clay per litre of solution. The leaching duration was 2 to
3 months in which a steady aqueous concentration of the major ions were reached. After the
leaching, samples of suspension were centrifuged at 21,255 g for 2 hours before the chemical
analysis for major cations and anions. Some samples were further filtered by 0.45 µm filters
and YM3 (3000 MWCO) centriplus ultrafilters to study the possible effect of clay particulates
or colloids on the concentration of clay water components.
144
solution: NaHCO3 (0.01 M) or water
chemical analysis for
cations and anions
clay
centrifugation
supernatant
filtration
(0.45 µm)
ultrafiltration
(3000 MWCO)
Figure A4-1: Schematic illustration of the batch leaching experiment to determine the
concentration of Boom Clay pore water components
Results and discussion
General
The detailed experimental results are given in the end of this annex from Table A4-1 to A4-5.
The general trend of the observation is similar to that of the B&B experiments, i.e., the
concentration of cations, except Si and K, and all anions increases linearly with the S/L. Our
data show however important differences comparing to the data of B&B and are explained in
detail hereafter.
The pH value of the leaching suspension
The pH was measured at the end of the leaching period inside the glove boxes. The pH
electrode was immersed in suspension before centrifugation, so the pH was recorded in
suspension, not in solution. Figure A4-2 shows the pH values of all samples as a function of
the S/L ratio, the type of infill gas in the glove box, and the type of leachant used.
The pH in the Ar box is higher than the N2/CO2 box as expected because of the lower pCO2 in
the Ar box. Also the pH in the Ar box is around 9 and is higher than the anticipated pH of
Boom Clay at pH around 8.5 due to degassing of CO2(g). In the N2/CO2 box, the pH is around
8.2 at low S/L and increases as a function of the S/L. This contradicts the findings of B&B in
which they observed that the lower pH are associated with higher S/L. B&B explained that
the pH decrease as a function of S/L was the result of calcite/dolomite solubility equilibrium
using the Ca/Mg activity ratio of 1.35 according to their solubility data. As will be seen from
the later Sections on Ca and Mg concentrations, the Ca/Mg concentration ratio in our batch
extracts is about 0.5 (after ultrafiltration) which has been explained as being controlled by
cation exchange reactions (see modelling Section 4).
The increase of pH found in our batch experiments as a function of S/L is likely due to the
increase of the concentration of bicarbonate released into the solution following the reaction:
CO2(g) + H2O = H+ + HCO3-
(A4-1)
HCO3-
As the S/L ratio increases, more
will be released from the clay to the liquid phase and
so the concentration of HCO3 will rise. Following the reaction A4-1, at a constant pCO2, the
H+ activity should decrease, i.e., increasing the pH.
9.2
9
8.8
pH
8.6
8.4
8.2
Ar box
8
N2/CO2 box
7.8
distilled water N2/CO2 box
7.6
0
100
200
300
400
500
600
700
800
900
S/L, g/L (wet clay/NaHCO3 solution)
Figure A4-2: pH values in suspensions of leaching experiments.
The role of S/L on the apparent leached concentrations
Following the approach of B&B, the S/L ratio is the key system variable to derive
concentrations of leached components from clay. For non-filtered samples, our data agrees
with the data of B&B in the sense that the concentration generally increases as S/L. Figure
A4-3 and -4 show the cases of Ca and Cl from one experiment as an example for cations and
anions. Other data show similar behaviours and are therefore not plotted here. All data are
listed in Table A4-1 and A4-2 at the end of this annex.
8
7
[Ca], mg/L
6
5
centrifuge
0.45 µm
YM3
4
3
2
1
0
0
100
200
300
400
500
600
700
800
900
Solid/liquid, g/L (wet clay/NaHCO3 solution)
Figure A4-3: Calcium concentrations in the aqueous extracts as function of S/L.
146
3
2.5
[Cl], mg/L
2
centrifuge
0.45 µm
YM3
1.5
1
0.5
0
0
100
200
300
400
500
600
700
800
900
Solid/liquid, g/L (wet clay/NaHCO3 solution)
Figure A4-4: Chlorine concentrations in the aqueous extracts as function of S/L.
In the case of Ca, concentrations in the centrifuged extracts and in the 0.45 µm filtered
extracts are identical suggesting that no coarse clay particulates containing Ca are present in
the extracts. The important finding is that the Ca concentration is independent of S/L after the
extracts were ultrafiltered by YM3 filters. This evidences the possible existence of colloids in
the extracts. B&B didn’t use ultrafiltration to remove colloids, so we can’t compare our data
with theirs. The detailed discussion about the filtration will be given in the next Section.
In the case of Cl, concentration increases in all three samples indicating no colloids involving
Cl are present in the extracts, as expected.
The effect of filtration
From the example of Ca as shown in Figure A4-3, the YM3 ultrafiltration effectively removed
colloids, so the ‘real’ dissolved Ca concentration is independent of S/L. From this result, we
conclude that the extracts obtained only by the centrifugation and the 0.45 µm filtration
contain colloids and cannot be used for the determination of soluble concentration of Boom
Clay components. The concentration of extracts should be obtained from the samples
ultrafiltered by filters of nanometre scale.
Because of the colloids, the observed S/L effect as given in the above Section is actually an
effect of colloids. When the extracts were not ultrafiltered, more clay was added to the
system, more colloids would release into the suspension as the result of agitation. This
behaviour has been observed in our earlier study (Wang et al., 2002) and also found by
Cremers and Maes, (1986) but interpreted as being due to the effects of natural organic
matter. The real nature of colloids, i.e., if they are real clay colloids or natural organic matter
colloids, is still not clear.
As discussed in the Section of background, the S/L is the key system variable in the work of
B&B and the effects of S/L are the central information for data interpretation. Our data
however suggest no S/L effect for cations if ultrafiltration is applied, so our data cannot be
interpreted in the same way as that of B&B. Also for the anion extraction data, our results
indicate strong effects of clay compaction, so the anion concentration cannot be determined
only by dilution, as done by B&B. The detailed procedure to derive the extracted cation and
anion concentrations will be further elaborated when discussing the result of each element in
later Sections in this annex. It is not clear from the work of B&B if colloids were also present
in their systems since their samples were not ultrafiltered (or not reported). It is important to
note however that the system of Opalinus Clay can be quite different from Boom Clay in
terms of colloids stability. Opalinus Clay has a much higher ionic strength, up to 0.5 molal
comparing to the low ionic strength of Boom Clay of 0.02 molal. It is possible that colloids
are much less stable in Opalinus Clay because of the high ionic strength induced peptisation.
Also, the Opalinus Clay may contain less natural organic matter especially the immobile
fraction at the surfaces of clay. The presence of natural organic matter may enhance the
stability of colloids.
The effect of glove box atmosphere
Because of the restricted availability of glove boxes, some experiments were performed in the
Ar filled glove box and others were done in the N2/CO2 box. Figure A4-5 and -6 show the Ca
and Cl concentrations measured under the two atmospheres.
12
10
[Ca], mg/L
8
Ar box
N2/CO2 box
6
4
2
0
0
100
200
300
400
500
600
700
800
900
Solid/liquid, g/L (wet clay/NaHCO3 solution)
Figure A4-5: Calcium concentrations in the aqueous extracts as function of S/L under the Ar
and the N2/CO2 atmospheres.
8
Ar box
N2/CO2 box
N2/CO2 box sample 2
7
[Cl], mg/L
6
5
4
3
2
1
0
0
100
200
300
400
500
600
700
800
900
Solid/liquid, g/L (wet clay/NaHCO3 solution)
Figure A4-6: Chlorine concentrations in the aqueous extracts as function of S/L under the
Ar and the N2/CO2 atmospheres.
148
Figure A4-5 indicates that the evolutions of the Ca concentration are similar under the two
atmospheres. The Ca concentration is slightly higher in the N2/CO2 box than in the Ar box.
This is due to slightly higher solubility of calcite when CO2 is present, i.e., lower pH as
indicated by Figure A4-2.
The Cl concentrations are scattered. One sample (cross) shows a higher Cl concentration and
is less independent on S/L in the N2/CO2 box. The other sample (triangle, sample 2) shows a
similar behaviour to the case of the Ar box except one point at the S/L of 50 g/L. Both data
points at 50 g/L in the N2/CO2 box are considerably higher than other data points suggesting
experimental artefacts at this specific S/L. In general, there is no distinguished effects of the
glove box atmosphere on the measured Cl concentration.
The effect of leachant type
Different from the work of B&B, we use in most of the experiments a NaHCO3 (0.01 M)
solution as the leachant instead of distilled water. The use of NaHCO3 is to minimise the
disturbances to the original Boom Clay geochemistry because Boom Clay pore water is a 15
mM NaHCO3 solution. The approach of B&B is indirect, i.e., the measurements must be
interpreted by a kind of model before being able to calculate the real pore water compositions.
Therefore, the type of leachant is not important, to our opinion, as long as the composition of
the leachant is well defined. According to experiences, even a slight disturbance on Boom
Clay geochemistry may cause severe changes of the pore water composition. We therefore
decided to use NaHCO3 as the leachant. We also used distilled water for one sample in
parallel to check the influence of the type of leachant (Figure A4-7) on the concentration of
the extracts.
9
8
concentration, mg/L
7
6
Ca, NaHCO3
Ca, distilled water
Cl, NaHCO3
Cl, distilled water
5
4
3
2
1
0
0
100
200
300
400
500
600
700
800
900
Solid/liquid, g/L (wet clay/NaHCO3 solution)
Figure A4-7: The effect of leachant type on the extracted concentrations of Ca and Cl as a
function of S/L.
For both the Ca and Cl cases, distilled water seems to extract more Ca and Cl into the aqueous
phase comparing to the NaHCO3. The measurements in the distilled water samples show also
less dependency on S/L. However, it is difficult to observe a general difference between the
case of distilled water and the case of NaHCO3.
Chlorine concentration in the extracts
For easily soluble Cl, B&B assume that the source of Cl in the extracts can only be the
original pore water in the Opalinus Clay. They found exactly the 1:1 correspondence between
the Cl concentration and the dilution at the different S/L. To their data, the original Cl
concentration in the pore water can be easily calculated by:
CCl , pore =
CCl ,ext
S
L
where CCl,pore is the Cl concentration in the original pore water, CCl,ext is the Cl concentration
in the extracts of batch leaching experiment, and S/L is the solid to liquid ratio used.
B&B derived from 15 extracts a mean value of 12.3 mmol/kg Opalinus Clay. Apparently, the
calculated CCl,pore in the case of Opalinus Clay is independent of S/L although the CCl,ext is
dependent on S/L due to the dilution effect.
Following the same approach, we also calculated the CCl,pore based on our leaching data but
found out that the CCl,pore for Boom Clay is dependent on the S/L as shown in Figure A4-8.
100
converged at high S/L
[Cl], mg/L
10
C(Cl, ext)
C(Cl, pore)
1
0.1
0
200
400
600
800
1000
Solid/liquid, g/L (wet clay/NaHCO3 solution)
Figure A4-8: Chlorine concentrations in the aqueous extracts as function of S/L. See text
for the definitions of CCl,pore and CCl,ext.
Although the CCl,ext (triangles in Figure A4-8) increases linearly as a function of S/L, the same
as the case of B&B for the Opalinus Clay, the CCl,pore (cross symbols in Figure A4-8)
calculated for the Boom Clay is not a constant value but decreases as a function of S/L. Also
interesting is that the CCl,pore and CCl,ext are converging at the high end of the S/L.
The observation of Figure A4-8 is not fully understood at the present time. A likely
explanation is related to the colloids feature of the Boom Clay in terms of the double layer
structure, hyper-filtration phenomena, and anion exclusion mechanism. The Boom Clay is
known to have very small pores and the double layer is overlapped. Under such condition,
anions are not free to move so under the confined conditions, i.e., when collecting pore water
via piezometers or mechanical squeezing techniques, only part of the total Cl is extractable, so
the Cl concentration is smaller than the inventory Cl concentration. In the batch experiment
where the confined clay core is suspended, Cl ions are freed from the pores and the apparent
concentration increases. This is probably why the Cl concentration is higher in a more diluted
system. As the S/L increases, a negative potential increases (so as the stability of colloids) so
150
the Cl ions are trapped between pores in a similar way as under the in situ or squeezing
conditions. This is at the moment a statement without being tested by rigorous calculations.
Theories exist, for example, Donnan equilibrium in suspensions (Appelo, 1977), and should
be applied to our experimental systems in the near future.
Following this reasoning, the Cl concentration determined at the highest S/L should be close
to the concentration measured in piezometer and squeezing waters. The Cl concentration
measured at the lowest S/L should approach the Cl inventory in Boom Clay.
With the known Cl inventory CCl,pore, i.e., the number of moles of Cl per kilogram of wet
Boom Clay, we can calculate the Cl concentration in Boom Clay ‘total water’, the amount of
water measured by the method of evaporation, if the water content in the sample is known:
CCl =
CCl , pore
WH 2 O
where CCl is the Cl concentration in Boom Clay total water, WH2O is the total water content in
the sample from which the extraction was performed.
Using all the data presented in Figure A4-8 and the average water content of 16.5 wt%
(determined separately), CCl is calculated as being in the range of 15 to 425 mg Cl/kg water.
As the highest S/L is favourable for determination of in situ Boom Clay pore water
concentration, we calculated the average value from 11 samples of the HADES 2001/4
borehole and the value is 18 mg/kg water, which is lower than 26 mg/kg water as the
reference Cl concentration.
It is important to note that the value of 26 mg/kg water is the Cl concentration under the
confined condition and can be higher than the real pore water concentration of Cl, due to the
anion exclusion effect.
The batch experiments show that the Cl inventory concentration can be as high as 425 mg/kg
water. If the hyper-filtration and anion exclusion effects exist as expected, the Cl inventory
can only be determined with batch experiments.
Fluorine concentrations in the extracts
If the Cl concentration in the Boom Clay is controlled by the double layer properties, the same
is expected for F. A similar behaviour is indeed observed in the extracts for F, i.e., the
extracted concentration increases linearly as a function of S/L but the inventory concentration
decreases with S/L. The range of F concentration in the batch accessible water is: 21 to 104
mg/kg water. The average of 11 measurements on the highest S/L is 26 mg/kg water which is
about 10 times higher than the value found in MORPHEUS waters as 2.6 to 3.3 mg/kg water.
Sulphate and thiosulphate concentrations in the extracts
The evolution of SO42- concentration in the extracts are similar comparing to that of Cl and F.
The inventory concentration measured is in the range of 16 to 2542 mg/kg of batch accessible
water. The high concentration is due to the oxidation of some samples. The average of 11
samples is 31 mg/kg water which is about 10 times higher than the reference water
composition of 2 mg/kg.
Thiosulphate ion was only measured in some samples suggesting the oxidation of these
samples. Figure A4-9 shows the concentration of SO42- vs that of S2O32- and the linear
relationship indicates the oxidation of the samples.
40
[S2O32-], ppm
30
20
10
0
0
40
80
120
160
200
2-
[SO4 ], ppm
Figure A4-9: Concentration of S2O32- as a function of SO42- in the extracts.
Calcium concentration in the extracts
As shown in the Figure A4-3, the Ca concentration is only independent to S/L if the extracted
water was filtered by YM3 filters. The centrifuged and the 0.45 µm filtered samples contain
colloids and hence cannot be used for the determination of the dissolved Ca concentration.
We therefore only use the data from 8 determinations performed on two clay samples (Table
A4-1). The average Ca concentration found is 1.5 mg/kg water which is close to the reference
value of 2 mg/kg water.
Iron concentration in the extracts
Similar to Ca, iron was also found in colloidal fraction. Ultrafiltration data show all 8
determinations (Table A4-1) are under the detection limit of 0.05 mg/kg. This is an important
finding suggesting that the Fe concentration measured in piezometer waters may contain
colloids. Further tests are needed to clarify: (1) if the low Fe concentration found in batch
system is due to the oxidation of the samples, i.e., a precipitation of Fe(III) oxi- and
hydroxides; or (2) the high concentration of Fe, as we always assumed controlled by siderite
solubility, is the result of the involvement of colloids.
Magnesium concentration in the extracts
Colloids containing Mg are also observed in the extracts. The average value of 8
measurements (Table A4-1) from the ultrafiltered samples is 1.8 mg/kg which is in good
agreement with the 1.6 mg/kg derived from the reference water composition.
Potassium concentration in the extracts
There is no evidence that colloids involving K ions are present in the extracts. All data are
quite consistent and the average value from the 8 ultrafiltration determinations is 8.9 mg/kg
water which agrees with the value of reference water composition. Different from the case of
Ca, Fe, and Mg, where non-filtered samples give higher concentrations due to colloids, the
average of all the non-filtered K determinations is identical to the measurements in the
ultrafiltered samples and is also 8.9 mg/kg.
152
Silicon concentration in the extracts
There are no Si colloids observed in the extracts. Ultrafiltered samples result in a value of 3.8
mg/kg while the average of all the non-filtered samples is 2.8 mg/kg. The ultrafiltered value
agrees better with the value 3.4 mg/kg derived from the reference water composition but we
take the latter value since it is an average from many more determinations.
Aluminium concentrations in the extracts
The contents of Al from all samples were under the detection limit given as 0.2 mg/kg. The Al
concentration of Boom Clay pore water should be much lower than that value. Water analysis
in the future should determine Al with an improved detection limit.
Natural organic matter concentration in the extracts
Natural organic matter is present in Boom Clay pore water as colloids. The centrifugation and
the 0.45 µm filtration do not result in different concentration of TOC (total organic carbon)
suggesting no coarse particulate materials containing natural organic matter are present in the
extracts. Our experiments with YM3 ultrafiltration failed due to the leaching of organic matter
from the filter material. The concentration of TOC is therefore derived from non-filtered
samples.
From 11 measurements, the TOC ranges from 1284 to 5857 mg C/kg water. Following the
reasoning given in the case of chlorine, if the value at the highest S/L represents most closely
the value of the pore water, the average TOC from 11 samples is 2289 mg C/kg. This value is
somewhat 10 times higher than the value determined in MORPHEUS water.
If the electro-double layer properties such as hyper-filtration and anion exclusion are effective
in the Boom Clay, they will affect the migration of natural organic matter as well. There has
been a commonly observed feature in Boom Clay when collecting pore water from
piezometers: the TOC at the beginning stage of water collection is always high and gradually
decrease till a certain plateau value (Van Geet, 2004). This is perhaps related to the anion
exclusion phenomena as explained by Appelo (1999) so that the natural organic matter
molecules are hesitant to enter the pores with negative potential and will show an earlier
breakthrough than water.
Concluding remarks
•
the B&B approach for batch experiments cannot be directly applied to Boom Clay
suspensions mostly because of the involvements of colloids (for cations) and the
electro double layer related behaviours (for anions) of the Boom Clay. The colloids
and the double layer properties may well be the reasons for the observed differences
between Opalinus Clay and Boom Clay;
•
for the major cations, the batch leaching method provides comparative results to those
determined by piezometric and squeezing techniques if ultrafiltration is applied to
remove the colloids;
•
for the anions, the batch leaching method in general results in higher concentrations
comparing to piezometric and squeezing techniques suggesting that an interpretation
using double layer theory is needed to correct the in situ compaction of the Boom
Clay;
•
colloids containing Ca, Mg, and Fe are evidenced by ultrafiltration experiments. This
finding may suggest that colloids are of the secondary mineral nature. No colloids of
K and Si are found indicating that no clay colloids are present. Future experiments
should focus on the determination of Al concentration and the possible involvement of
Al colloids to test the above statements;
•
finally, the batch leaching experiment provides a way to determine the inventory
concentration of Cl and other anions in Boom Clay. The inventory concentration of,
e.g., Cl is important for the corrosion study of overpack materials. Due to the in situ
compaction of the Boom Clay and the related double layer properties, piezometric and
squeezing techniques tend to underestimate the total inventory concentration of
anions.
154
Table A4-1: Cation concentrations measured in batch leaching extracts. Glove box atmosphere Ar.
S/L
25
50
200
800
25
50
200
800
25
50
200
800
25
50
200
800
25
50
200
800
25
50
200
800
25
50
No.
(1)-0.5
(1)-1
(1)-4
(1)-16
(2)-0.5
(2)-1
(2)-4
(2)-16
(3)-0.5
(3)-1
(3)-4
(3)-16
(4)-0.5
(4)-1
(4)-4
(4)-16
(5)-0.5
(5)-1
(5)-4
(5)-16
(6)-0.5
(6)-1
(6)-4
(6)-16
(7)-0.5
(7)-1
Al
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
Ca
1.26
1.47
2.5
6.9
1.51
1.79
3.0
7.4
1.49
1.61
2.9
7.4
1.67
1.85
2.6
6.1
1.56
1.60
2.6
6.4
1.63
1.60
2.7
6.8
1.68
1.93
0.45µm YM3 Fe
0.12
0.10
0.20
0.83
0.06
0.13
0.29
0.95
1.50
1.48 0.08
1.64
1.30 0.11
2.8
1.2
0.37
7.6
1.3
1.15
1.64
1.40 0.08
1.86
1.57 0.08
2.5
1.6
0.20
6.0
2.2
0.55
0.05
0.09
0.22
0.86
0.06
0.08
0.24
1.03
0.06
0.13
0.45µm YM3
0.35
0.39
1.06
2.80
0.29
0.26
0.67
1.60
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
Mg
1.17
1.22
1.82
4.2
1.28
1.40
2.05
4.4
1.17
1.36
2.03
4.6
1.24
1.53
1.84
4.1
1.19
1.16
1.68
3.7
1.12
1.22
1.74
3.9
1.29
1.44
0.45µm YM3 K
6.9
8.1
9.3
10.6
7.1
8.0
9.6
11.2
1.30
1.77 6.7
1.50
1.72 8.1
2.09
1.57 9.0
4.3
1.4
9.8
1.32
1.76 8.4
1.58
1.92 10.8
1.89
1.88 11.0
3.8
2.6
14.3
7.5
8.0
9.2
10.0
7.4
7.9
9.0
9.7
7.5
8.4
0.45µm YM3 Si
2.5
2.4
3.2
3.3
2.5
2.4
2.7
3.3
6.4
6.3
2.4
7.4
7.3
2.4
8.5
8.4
2.9
10.0
8.6
3.8
7.8
7.6
2.7
10.2
10.2 2.6
10.0
9.8
2.8
13.0
13.0 3.0
2.3
2.7
3.0
3.8
2.2
2.3
3.3
3.5
2.7
2.7
0.45µm YM3 Na
242
241
250
283
239
244
254
289
3.5
3.3
241
3.1
3.2
245
3.7
3.8
257
10.0
4.8
278
3.4
3.6
249
3.5
3.4
254
3.9
4.1
272
6.0
4.0
354
243
246
259
285
245
242
256
278
249
251
200
800
25
50
200
200
200
200
200
200
800
25
50
200
800
25
50
200
800
25
50
200
800
(7)-4
(7)-16
(8)-0.5
(8)-1
(8)-4
(8)-4-1
(8)-4-2
(8)-4-3
(8)-4-4
(8)-4-5
(8)-16
(9)-0.5
(9)-1
(9)-4
(9)-16
(10)0.5
(10)-1
(10)-4
(10)16
(11)0.5
(11)-1
(11)-4
(11)16
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
3.5
9.9
1.37
1.54
2.7
2.3
2.22
2.7
2.4
2.6
7.2
1.63
1.93
3.1
8.7
0.34
1.27
0.09
0.11
0.29
0.18
0.16
0.25
0.21
0.24
0.99
<0.05
0.12
0.32
1.20
2.27
5.7
1.23
1.22
1.86
2.01
2.15
2.28
1.93
1.83
4.4
1.18
1.31
1.96
5.2
9.1
9.7
6.7
8.2
9.6
9.1
9.2
9.2
9.3
9.0
11.2
6.7
7.6
8.6
11.0
3.1
3.3
2.0
2.4
2.8
3.4
3.3
3.5
3.2
2.9
3.1
2.6
2.8
3.0
3.5
259
289
244
257
290
260
268
269
266
269
313
253
259
271
337
<0.2 1.29
<0.2 1.63
<0.2 2.5
<0.05
0.10
0.27
1.28
1.18
1.69
7.8
8.6
10.0
2.4
2.4
2.5
249
253
297
<0.2 3.9
0.35
2.55
11.6
2.5
318
<0.2 1.12
<0.2 1.41
<0.2 2.15
<0.05
0.09
0.30
0.93
1.22
1.50
6.5
7.6
8.6
2.1
2.2
2.6
244
247
258
<0.2 4.5
0.43
2.65
9.6
2.9
294
156
Table A4-2: Anion concentrations measured in batch leaching extracts. Glove box
atmosphere Ar.
S/L
25
50
200
800
25
50
200
800
25
50
200
800
25
50
200
800
25
50
200
800
25
50
200
800
25
50
200
800
25
50
200
200
200
200
200
200
800
25
50
200
800
No.
(1)-0.5
(1)-1
(1)-4
(1)-16
(2)-0.5
(2)-1
(2)-4
(2)-16
(3)-0.5
(3)-1
(3)-4
(3)-16
(4)-0.5
(4)-1
(4)-4
(4)-16
(5)-0.5
(5)-1
(5)-4
(5)-16
(6)-0.5
(6)-1
(6)-4
(6)-16
(7)-0.5
(7)-1
(7)-4
(7)-16
(8)-0.5
(8)-1
(8)-4
(8)-4-1
(8)-4-2
(8)-4-3
(8)-4-4
(8)-4-5
(8)-16
(9)-0.5
(9)-1
(9)-4
(9)-16
F0.26
0.42
1.07
3.27
0.35
0.53
1.46
3.7
0.30
0.44
1.27
3.12
0.43
0.52
1.26
2.82
0.32
0.50
1.37
3.48
0.31
0.44
1.35
3.26
0.32
0.53
1.50
3.8
0.32
0.46
1.40
1.20
1.21
1.24
1.22
1.27
3.10
0.37
0.56
1.58
3.7
Cl0.55
0.61
0.92
2.30
0.66
0.64
0.81
2.35
0.56
0.74
0.86
2.22
1.00
8.1
1.31
2.15
0.47
0.75
0.83
2.18
0.67
0.61
0.77
2.69
1.44
0.66
0.77
2.36
1.09
1.20
2.05
0.83
1.01
0.92
0.93
0.86
2.82
0.91
0.48
1.12
2.50
0.45
µm
0.85
1.02
1.22
2.77
1.77
8.2
1.62
2.87
YM3 Br<0.25
<0.25
<0.25
<0.25
0.27
<0.25
<0.25
<0.25
1.12 <0.25
1.11 0.27
1.22 <0.25
2.49 <0.25
1.38 <0.25
8.3
0.37
1.79 <0.25
2.89 <0.25
<0.25
<0.25
<0.25
0.34
<0.25
0.28
<0.25
0.28
0.43
0.32
0.34
0.30
0.30
<0.25
<0.25
1.06
0.66
0.52
0.54
0.34
0.25
0.32
0.26
0.35
0.28
HPO42<0.5
<0.5
0.73
1.60
<0.5
<0.5
0.71
1.47
<0.5
<0.5
0.66
1.58
<0.5
<0.5
<0.5
0.50
<0.5
<0.5
0.80
1.70
<0.5
<0.5
0.75
1.52
<0.5
<0.5
0.77
1.58
<0.5
<0.5
17.1
0.70
0.65
1.53
0.55
0.61
1.05
<0.5
<0.5
0.74
1.30
SO420.79
1.03
1.19
4.40
1.03
1.40
2.20
6.4
0.96
0.62
1.01
2.22
10.5
16.7
35.0
166
0.97
0.62
1.28
3.78
1.18
0.74
1.10
2.91
0.64
0.69
1.07
2.96
9.6
6.6
18.1
24.5
27.8
18.7
19.6
22.5
55.1
4.35
5.55
15.4
58.5
0.45
µm
1.18
1.17
1.60
3.85
10.4
16.7
35.8
157
YM3
2.08
1.35
1.63
2.85
10.7
17.2
34.8
165
S2O32(1)
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
2.82
32.6
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
4.06
<1
<1
5.23
3.89
4.31
10.1
<1
<1
2.73
11.2
25
50
200
800
25
50
200
800
(10)-0.5
(10)-1
(10)-4
(10)-16
(11)-0.5
(11)-1
(11)-4
(11)-16
0.38
0.62
1.49
3.9
0.33
0.57
1.41
3.9
1.75
0.55
2.23
2.04
0.73
2.03
1.18
1.95
0.35
<0.25
0.26
0.29
<0.25
0.46
0.25
0.34
<0.5
<0.5
15.6
0.66
<0.5
1.15
0.60
1.09
3.98
7.4
19.3
71
1.74
3.23
7.1
23.2
<1
<1
1.76
5.05
<1
<1
<1
2.96
158
Table A4-3: Major cation and anion concentrations measured in the N2/CO2 box and in
distilled water leachant for the purpose of comparison.
S/L
25
50
200
800
(3) - II - 0,5g
(3) - II - 1g
(3) - II - 4g
(3) - II - 16g
Ca
[ppm]
2.07
2.08
3.1
9.6
Fe
[ppm]
0.05
0.09
0.32
1.39
Mg
[ppm]
1.89
1.88
2.24
5.7
K
[ppm]
9.2
13.1
10.7
11.7
Si
[ppm]
2
2.1
2.8
2.8
Cl
[ppm]
2.55
7.2
2.83
2.96
SO4
[ppm]
1.29
1.2
1.76
4.76
25
50
200
800
(4) - II - 0,5g
(4) - II - 1g
(4) - II - 4g
(4) - II - 16g
3.5
2.79
2.9
6.7
<0,05
<0,05
0.22
0.51
2.24
2.07
2.2
4.4
9.3
11.8
11.9
14.1
2.1
2.2
3
2.4
1.12
3.07
1.19
2.77
8.1
11.9
37.1
168
25
50
200
800
(4) - H2O - 0,5g
(4) - H2O - 1g
(4) - H2O - 4g
(4) - H2O - 16g
4.6
3.1
3.1
7.5
0.17
0.09
0.35
0.78
3.1
1.82
1.55
3.8
9.6
9.8
8.2
10.7
2.4
1.8
1.9
1.8
2.16
2.46
6.7
2.97
7.2
13.5
38.1
166
Table A4-4: Concentrations of natural organic matter in batch leaching extracts. Glove box atmosphere Ar. Samples centrifuged at 21,255 g for 2 hrs.
S/L
25
50
200
800
1
TOC
ext
[ppm]
19
28
80
319
pore
2
TOC
ext
4688
3407
2438
2417
[ppm]
21
38
118
370
pore
3
TOC
ext
4972
4621
3573
2799
[ppm]
19
32
118
395
pore
4
TOC
ext
4640
3899
3588
2994
[ppm]
21
35
73
245
pore
5
TOC
ext
5152
4296
2217
1852
[ppm]
16
25
75
290
pore
6
TOC
ext
3884
2972
2276
2199
[ppm]
20
26
84
319
pore
7
TOC
ext
4812
3155
2551
2418
[ppm]
24
42
126
488
pore
8
TOC
ext
5857
5075
3824
3696
[ppm]
24
32
91
366
pore
9
TOC
ext
5833
3853
2748
2773
[ppm]
19
36
123
450
pore
10
TOC
ext
4507
4417
3718
3407
[ppm]
16
32
73
170
pore
11
TOC
ext
pore
3976
3856
2198
1285
[ppm]
15
28
63
190
3583
3347
1894
1441
Table A4-5: Comparisons of the TOC content as the result of centrifugation, filtration, and ultrafiltration of the leaching extracts. Glove box
atmosphere N2/CO2
TC
IC
TOC
St.
Dev.
TC
IC
TOC
St.
Dev.
(3) - 0,5
not
filtered
125.6
108.5
17.11
0,45µm
127.7
110.5
17.18
0.77
0.836
YM3
5,103
217.7
4885
(3) - 1
not
filtered
137.5
110.6
26.85
0,45µm
157.7
111.6
46.12
47.40
1.778
4.963
(4) - 0,5
not
filtered
127.5
109.8
17.73
0,45µm
127.8
110.3
17.48
YM3
3,873
163.1
3710
(4) - 1
not
filtered
148.7
113.8
34.93
0.672
1.436
36.53
2.807
YM3
3,974
205.8
3769
(3) - 4
not
filtered
215.2
113.1
102
YM3
4,157
185.5
3971
(3) - 16
not
filtered
476.5
119
357.5
0,45µm
217.3
114.1
103.2
0,45µm
479.7
119
360.7
YM3
4,312
185.6
4127
29.42
1.244
0.706
25.47
1.798
1.677
21.25
0,45µm
144.2
113.7
30.5
YM3
4,562
170.9
4392
(4) - 4
not
filtered
180.5
113.7
66.77
0,45µm
178.7
113.6
65.1
YM3
4,221
126.9
4094
(4) - 16
not
filtered
333.7
115
218.7
0,45µm
332
113.8
218.2
YM3
4,938
153.5
4784
1.595
0.00
0.825
0.841
21.39
3.106
1.452
42.33
160
Annex 5: Boom Clay pore water geochemistry: analytical data used in the statistical analyses
EG/BS (statistical group 1)
162
EG/BS (statistical group 2)
ARCHIMEDE #1
reference
Mol(8)
Mol(8)
Mol(8)
Mol(8)
Mol(8)
Mol(7)
Mol(7)
Mol(14)
Mol(14)
Mol(14)
Mol(14)
Mol(14)
Mol(15)
Mol(15)
N°piezo
A50001
A50001
A50001
A50001
A50001
A50001
A50001
A50001
A50001
A50001
A50001
A50001
A50001
A50001
N°Echant
A07502
A07505
A07509
A07510
A07513
A07501
A07514
A07503
A07506
A07507
A07508
A07512
A07504
A07511
Prof
8
8
8
8
8
7
7
14
14
14
14
14
15
15
date perlvt pH
09.07.1992 8,5
28.10.1992 8,86
06.11.1992
17.11.1992
24.03.1993
09.07.1992 8,5
24.03.1993
09.07.1992 8,7
28.10.1992 8,8
17.11.1992 8,7
30.11.1992 8,8
24.03.1993
09.07.1992 8,7
24.03.1993
only 1 group based on ANOVA1 analysis of Cl, F, Na, K, Ca, Mg
mean
8,695
SPRING 116
ppm
alcalini
732,24
762,75
762,75
713,934
732,24
713,934
701,73
726,138
ppm
SO4
4,803
4,51482
3,3621
3,65028
3,16998
4,3227
2,30544
4,3227
3,74634
2,97786
3,07392
2,78574
2,8818
2,11332
ppm
NO3
31
0,806
0,62
1,116
0,31
0
0,248
0,62
0,434
<0.372
1,24
0,248
0
0,31
ppm
Cl
17,725
18,434
17,016
17,016
17,725
17,725
17,725
17,725
17,725
18,0795
18,0795
17,3705
17,725
17,016
ppm
Br
7,8302
730,7145 3,430714 2,842462 17,64904 7,8302
ppm
F
0,152
1,9
1,672
1,71
3,04
0,152
2,85
0,152
1,729
2,09
1,691
2,85
0,152
2,85
ppm
S 2O 3
n.d.
n.d.
n.d.
n.d.
n.d.
0,67278
n.d.
n.d.
n.d.
n.d.
1,00917
n.d.
ppm
PO4
0,66479
0,66479
0,66479
0,66479
0,56982
0,28491
0,66479
0,66479
0,56982
0,47485
0,75976
0,56982
0,37988
0,66479
ppm
SIO2
9,012
10,2136
9,6128
10,2136
ppm
Na
285,076
275,88
268,983
273,581
280,478
ppm
K
7,82
7,82
7,429
7,429
10,166
ppm
Ca
2,004
2,004
1,6032
1,6032
1,92384
ppm
Mg
1,2155
1,7017
1,4586
1,4586
1,7017
287,375
10,166
2,72544
1,67739
275,88
278,179
278,179
280,478
289,674
280,478
7,429
7,038
7,038
9,384
7,82
8,211
1,76352
1,6032
1,6032
1,92384
1,6032
1,8036
1,55584
1,4586
1,33705
1,7017
0,9724
1,4586
ppm
Li
ppm
NH4
0,0902
0,029842 0,5412
0,029842 0,43296
0,029842 0,46904
3,7884
ppm
Fe
ppm
Mn
ppm
Al
0,072605 0,170314
0,17872 0,488966
0,161965 0,071422 0,035883
n.d.
7,2096
8,4112
9,012
8,4112
7,2096
1,642143 0,840975 0,590171 8,811733 279,5201 8,145833 1,84702
5,0512
0,029842 0,39688
0,029842 <0.054
0,006038 <0.054
3,4276
1,9844
4,1492
0,17872
0,032964 0,028059
0,23457
0,065928 0,028059
0,161965 0,076916 0,028059
1,474807 0,025875 2,033108 0,164758 0,151085 0,030015
164
ORPHEUS
MORPHEUS (major elements)
166
MORPHEUS (trace elements)
Annex 6: Statistics: methodology
ANOVA1
First it is checked whether data provided by the different filters of one single
piezometer all belong to the same population. In case of the EG/BS, consisting of 1
major filter, two different groups in time are compared. This is done element per
element using an analysis of variance (ANOVA1). For this ANOVA test, the mean
( xi ), standard deviation (si) and amount of samples (ni) of each group (k groups) is
calculated. The grand mean ( x ) is calculated as the mean of all groups together. Next
the between group variability (SSB) is calculated as :
k
(
SS B = ∑ ni ⋅ xi − x
i =1
)
2
and the degrees of freedom for the between group variability is
dfB=k-1. The variance between groups is then defined as s B2 =
SS B
.
df B
The within groups variability is defined as SSW = ∑ (ni − 1) ⋅ si2 . The degrees of
freedom are defined as dfW=N-k, with N the number of samples of all groups together
SS
and the variance within groups is defined as sW2 = W . The ratio of the variance
df W
s B2
between groups and the variance within groups is the F statistic: Fstat = 2 . Under the
sW
null hypothesis, namely that the mean value of each group is not differing, this test
statistic has an F sampling distribution with dfB and dfW degrees of freedom. The
probability (p) value can then be calculated. The null hypothesis is accepted if
p>0.05. Based on this information it will be possible to consider all filters of one
piezometer as one single population or to assume several groups within one
piezometer.
The basic assumptions when using the ANOVA 1 test are
All data are normally distributed
The variance within each of the populations is equal.
As only a limited amount of data are available, both assumptions cannot be tested on
all data sets.
The EG/BS data are the largest and thus these are used to test the first assumption of
normallity of the data. Figure A6-1 illustrates the normal-quantile-quantile plots for
all elements of both data sets discriminated in the EG/BS data. From these plots it is
concluded that all elements are normally distributed, although some tailings are
sometimes observed. For the other filters, not enough data points are available to
check the normallity. However, it is assumed that the assumption of normally
distributed data can be transposed to all filters of all piezometers.
168
170
Figure A6-1: Normal – quantile-quantile plots of all elements of the two groups of
data discriminated in the EG/BS data set (left: EG/BS1, referring to the data from
1996-03-13 till 1999-04-07, and right EG/BS2, refering to the data after from 200008-08 till 2003-02-17).
The second assumption of homogeneity of variance is especially important when the
number of measurments in each group are not equal. Therefore, this should be
checked for the EG/BS and ORPHEUS data sets. The Brown-Forsythe test can be
used to check for homogeneity of variance in the data considered. This test is simple
to execute and is robust to nonnormality. The steps are:
•
Calculate the median score, Mdi, for each of the i groups;
•
Replace each observed score, Yij, with a new variable, Zij, which is equal to
the absolute difference of the observed score and group median. That is:
Zij=|Yij-Mdi|
•
Run an ANOVA on the new scores, Zij. The overall F is a test of the
hypothesis that all groups are drawn form populations having the same
variance.
The results of this Brown-Forsythe test is given in Table A6-1. It can be concluded
that for EG/BS and ORPHEUS data sets, there is no need to worry about a violation
of the homogeneity of variance assumption.
Table A6-1: P-values of the Brown-Forsythe test to check the hypothesis that there is
a homogeneity of variance. P-values above 0.05 (in bold) are accepted as significant.
Ca
Cl
F
Fe
HCO3 K
Mg
Na
Si
EG/BS
0.92
0.38
0.15
0.07
ORPHEUS 0.16
0.19
0.19
0.72
0.90
0.61
0.18
0.40
0.32
0.52
0.30
0.38
0.66
MANOVA
Multivariate analysis of variance (MANOVA) is simply an ANOVA with several
dependent variables. Instead of a univariate F value, we would obtain a multivariate F
value (Wilks' lambda) based on a comparison of the error variance/covariance matrix
and the effect variance/covariance matrix. Although Wilks' lambda has been used
here, there are other statistics that may be used, including Hotelling's trace and Pillai's
criterion. Testing the multiple dependent variables is accomplished by creating new
dependent variables that maximise group differences. These artificial dependent
variables are linear combinations of the measured dependent variables.
The basic assumptions when using the MANOVA test are:
•
Data are independent
•
Data are normally distributed
•
No big difference between the variances between the groups of the same
variables
•
MANOVA is sensitive to outliers
First of all it is assumed that the data are independent.
Moreover, the normality of data is assumed as mentioned before in the ANOVA test.
The assumption of homogeneity of variance is especially important when the group
samples are not equal. Therefore, it should only be considered for the question on the
effect of filter material on pore water composition. A possible test of the homogeneity
of variance is Box's M test. This test could, however, not be applied on the question of
the effect of filter material on pore water composition, since the matrix is singular. It
is clear that additional data are needed to confirm the obtained results so far.
To avoid any problems with outliers, these have been removed before performing the
MANOVA test. Data are considered outliers when they fall outside the range of 1.5
times the interquartile range away from the 25th or 75th percentile of the sample.
172
Annex 7: The input and output files from geochemical modelling
simulations
Input file
This input calculates the variation of the Boom Clay pore water composition while the
system pCO2 is varying between 10-2.8 to 10-2.2 atm at 16°C in equilibrium with the
minerals calcite, pyrite, siderite, chalcedony, and kaolinite. An ion exchange complex
of 0.925 eq per 5 kg of clay, i.e., 18.5 meq/100 g clay is also present.
# React script, saved Tue Dec 09 2003 by lwang
data = "C:\Program Files\Gwb\Gtdata\MOLDATA.dat" verify
surface_data = "C:\Program Files\Gwb\Gtdata\MOLDATA ION EX.dat"
exchange_capacity IonEx = 0.925 eq
work_dir = "N:\USERS\Lwang\Projects\Topical reports\Pore water chemistry\GWB"
temperature = 16
swap Calcite for Ca++
swap Pyrite for O2(aq)
swap Siderite for Fe++
swap CO2(g) for HCO3swap Chalcedony for SiO2(aq)
swap Kaolinite for Al+++
1 kg free H2O
free kg Calcite = .15
free gram Pyrite = 100
free gram Siderite = 5
fugacity CO2(g) = .00158489319
free gram Chalcedony = 2000
free gram Kaolinite = 1000
total mol Na+ = .01417
total mol Mg++ = 5.41e-5
total mol K+ = .000168
total mg/kg SO4-- = 2.31
balance on H+
total mg/kg Cl- = 26
slide log fugacity of CO2(g) to -2.2
suppress >X2:Fe >X3:Al
alter >X:K -1.328 -1.328 -1.328 -1.328 -1.328 -1.328 -1.328 -1.328
alter >X2:Ca -.843 -.843 -.843 -.843 -.843 -.843 -.843 -.843
alter >X2:Mg -.678 -.678 -.678 -.678 -.678 -.678 -.678 -.678
extrapolate
precip = off
174
Output file
This output only lists the results at the step 30 of the simulation which has been used
as the reference Boom Clay pore water composition as given in Table 5-1 of this
report. The full output file is too lengthy to be included in this annex and can be
obtained if desirable.
Step #
30
Xi = 0.3000
Temperature =
16.0 C
pH =
8.487
Eh =
-0.2744 volts
Pressure =
log fO2 =
pe =
1.013 bars
-71.013
-4.7833
Ionic strength
=
0.016057
Activity of water
=
0.999974
Solvent mass
=
0.999986 kg
Solution mass
=
1.001316 kg
Solution density
=
1.019
Chlorinity
=
0.000733 molal
Dissolved solids
=
Rock mass
=
Carbonate alkalinity=
g/cm3
1329 mg/kg sol'n
3.254925 kg
756.71 mg/kg as CaCO3
IonEx sorbing surface:
Exchange capacity =
Reactants
0.925 eq
moles
moles
grams
cm3
remaining
reacted
reacted
reacted
---------------------------------------------------------------------------CO2(g)
-- sliding fugacity buffer --
Minerals in system
moles
log moles
grams
volume (cm3)
---------------------------------------------------------------------------Calcite
1.498
0.175
149.9
55.32
Chalcedony
33.29
1.522
2000.
755.2
Kaolinite
3.874
0.588
1000.
385.5
0.8335
-0.079
100.0
19.95
0.04316
-1.365
Pyrite
Siderite
5.000
_____________
(total)
Aqueous species
3255.
molality
mg/kg sol'n
act. coef.
1.268
_____________
1217.
log act.
--------------------------------------------------------------------------Na+
HCO3-
0.01531
351.6
0.8829
-1.8690
0.01442
878.9
0.8829
-1.8950
Cl-
0.0007320
25.92
0.8789
-3.1916
NaHCO3(aq)
0.0002941
24.67
1.0000
-3.5315
CO3--
0.0002409
14.43
0.6104
-3.8326
K+
0.0001855
7.244
0.8789
-3.7877
SiO2(aq)
0.0001149
6.894
1.0000
-3.9397
CO2(aq)
0.0001084
4.766
1.0000
-3.9648
Mg++
5.535e-005
1.344
0.6443
-4.4478
Ca++
3.928e-005
1.572
0.6257
-4.6094
SO4--
2.251e-005
2.159
0.6051
-4.8659
NaCO3-
9.503e-006
0.7877
0.8829
-5.0762
CaCO3(aq)
6.437e-006
0.6434
1.0000
-5.1913
MgHCO3+
5.709e-006
0.4865
0.8829
-5.2975
MgCO3(aq)
4.585e-006
0.3861
1.0000
-5.3387
CaHCO3+
4.042e-006
0.4081
0.8829
-5.4475
HSiO3-
3.394e-006
0.2613
0.8829
-5.5234
NaHSiO3(aq)
2.207e-006
0.2206
1.0000
-5.6563
OH-
1.686e-006
0.02864
0.8809
-5.8281
FeCO3(aq)
1.619e-006
0.1873
1.0000
-5.7908
NaCl(aq)
1.385e-006
0.08086
1.0000
-5.8584
NaSO4-
1.378e-006
0.1638
0.8829
-5.9149
FeHCO3+
1.245e-006
0.1453
0.8829
-5.9589
Fe++
3.884e-007
0.02166
0.6257
-6.6143
MgSO4(aq)
9.967e-008
0.01198
1.0000
-7.0014
CaSO4(aq)
4.140e-008
0.005629
1.0000
-7.3830
AlO2-
2.401e-008
0.001414
0.8829
-7.6736
FeOH+
2.036e-008
0.001481
0.8829
-7.7453
MgCl+
2.007e-008
0.001198
0.8829
-7.7515
KSO4-
1.897e-008
0.002561
0.8829
-7.7760
(only species > 1e-8 molal listed)
Exchanging species
molality
moles
act. coef.
activity log activity
------------------------------------------------------------------------------------>X:Na
0.4339
0.4339
1.0811
0.4690
-0.3288
>X:K
0.1114
0.1113
1.0811
0.1204
-0.9195
>X2:Ca
0.09532
0.09531
2.1621
0.2061
-0.6860
>X2:Mg
0.09458
0.09458
2.1621
0.2045
-0.6893
Mineral saturation states
log Q/K
log Q/K
---------------------------------------------------------------Nontronite-Na
3.6185s/sat
Cristobalite(alp
-0.2915
Nontronite-Mg
3.5426s/sat
Greenalite
-0.3026
Nontronite-Ca
3.4947s/sat
Dolomite-dis
-0.3125
Nontronite-K
3.3517s/sat
Magnesite
-0.3858
Nontronite-H
2.4546s/sat
Sanidine_high
-0.3942
Stilbite
2.2431s/sat
Albite
-0.4100
Mesolite
1.6165s/sat
Boehmite
-0.4626
Celadonite
1.4773s/sat
Coesite
-0.5542
Dolomite-ord
1.2994s/sat
Gibbsite
-0.5955
Dolomite
1.2994s/sat
Saponite-H
-0.6214
Muscovite
1.2476s/sat
Beidellite-Na
-0.6399
Cronstedtite-7A
1.0652s/sat
Beidellite-Mg
-0.7159
Hematite
1.0306s/sat
Analcime
-0.7471
Minnesotaite
0.9181s/sat
Cristobalite(bet
-0.7609
Annite
0.9048s/sat
Beidellite-Ca
-0.7637
Maximum_Microcli
0.8662s/sat
Dawsonite
-0.8067
K-Feldspar
0.8647s/sat
Monohydrocalcite
-0.8106
Montmor-Na
0.6158s/sat
Smectite-high-Fe
-0.8356
176
Magnetite
0.5481s/sat
Phlogopite
-0.8453
Montmor-Mg
0.5439s/sat
Scolecite
-0.8973
Saponite-Na
0.5424s/sat
Beidellite-K
-0.9066
Saponite-Mg
0.4675s/sat
Paragonite
-0.9483
Montmor-Ca
0.4232s/sat
SiO2(am)
-1.0811
Saponite-Ca
0.4186s/sat
Pyrophyllite
-1.1561
Daphnite-14A
0.4157s/sat
Mordenite
-1.2239
Montmor-K
0.3533s/sat
Ferrosilite
-1.3049
Quartz
0.2797s/sat
Albite_high
-1.7893
Saponite-K
0.2756s/sat
Beidellite-H
-1.8036
Tridymite
0.0982s/sat
Clinoptilolite-h
-1.9538
Illite
0.0934s/sat
Clinoptilolite-C
-2.0052
Clinoptilolite-N
0.0540s/sat
Ripidolite-14A
-2.0697
Clinoptilolite-h
0.0537s/sat
Jadeite
-2.3127
Goethite
0.0489s/sat
Clinoptilolite-K
-2.4232
Talc
0.0206s/sat
Laumontite
-2.4750
Calcite
0.0000 sat
Chrysotile
-2.5484
Chalcedony
0.0000 sat
Natrolite
-2.5708
Kaolinite
0.0000 sat
Chamosite-7A
-2.5782
Siderite
0.0000 sat
Clinoptilolite-h
-2.6250
0.0000 sat
Pyrite
Lansfordite
-2.7484
Diaspore
-0.0432
Huntite
-2.7944
Ice
-0.1042
C
-2.8648
Aragonite
-0.1448
Kalsilite
-2.9715
Smectite-low-Fe-
-0.2040
(only minerals with log Q/K > -3 listed)
Gases
fugacity
log fug.
----------------------------------------------H2O(g)
CO2(g)
0.01481
-1.829
0.002399
-2.620
H2(g)
2.383e-008
-7.623
CH4(g)
1.536e-009
-8.814
H2S(g)
5.785e-010
-9.238
CO(g)
1.859e-014
-13.731
HCl(g)
4.093e-019
-18.388
SO2(g)
4.046e-025
-24.393
S2(g)
1.529e-030
-29.815
C2H4(g)
6.335e-034
-33.198
Na(g)
3.505e-059
-58.455
K(g)
8.817e-062
-61.055
Cl2(g)
2.766e-064
-63.558
O2(g)
9.705e-072
-71.013
Mg(g)
2.052e-098
-97.688
Ca(g)
5.390e-122
-121.268
C(g)
4.188e-125
-124.378
Al(g)
2.671e-145
-144.573
Si(g)
6.140e-158
-157.212
In fluid
Original basis total moles
moles
Sorbed
mg/kg
moles
Kd
mg/kg
L/kg
------------------------------------------------------------------------------>X:Na
0.925
Al+++
7.75
2.44e-008
0.000657
Ca++
1.59
4.98e-005
1.99
Cl-
0.000733
0.000733
26.0
Fe++
0.877
3.27e-006
0.183
H+
-23.1
-0.000162
-0.163
0.0953 3.81e+003
H2O
74.0
HCO3-
1.56
0.0151
920.
0.112
0.000186
7.24
0.111 4.35e+003
Mg++
0.0946
6.58e-005
1.60
0.0946 2.30e+003
Na+
0.449
0.0156
359.
0.434 9.96e+003
O2(aq)
-2.92 -4.49e-009 -0.000143
K+
55.5 9.99e+005
SO4--
1.67
2.40e-005
2.31
SiO2(aq)
41.0
0.000120
7.23
Sorbed
fraction
log fraction
-----------------------------------------------Ca++
0.9995
-0.000
K+
0.9983
-0.001
Mg++
0.9993
-0.000
Na+
0.9652
-0.015
Elemental composition
In fluid
total moles
moles
Sorbed
mg/kg
moles
mg/kg
------------------------------------------------------------------------------Aluminum
7.747
2.438e-008
0.0006569
Calcium
1.593
4.981e-005
1.994
181.1
Carbon
1.556
0.01510
Chlorine
0.0007334
0.0007334
25.97
Hydrogen
126.5
111.0
1.118e+005
0.1825
Iron
0.8766
3.273e-006
0.09465
6.577e-005
1.596
161.6
55.55
8.877e+005
0.1115
0.0001855
7.245
Silicon
41.03
0.0001205
3.379
Sodium
0.4495
0.01562
358.6
Sulfur
1.667
2.405e-005
0.7701
Magnesium
Oxygen
Potassium
0.09531
3815.
0.09458
2296.
0.1113
4348.
0.4339
9961.
178
Annex 8: Prescriptions for the preparation of synthetic Boom Clay
water (SBCW)
General
Boom Clay pore water is basically a NaHCO3 solution of 15 mM. A 15mM NaHCO3
solution can thus be used in the experiments.
SBCW as used in the section 'Geological Disposal'
The directives for the preparation of Synthetic Boom Clay Water which is used in the
section 'Geological Disposal' is recently published by Maes et al. (2000 and 2004).
The directives are the following:
If you are planning to work in anaerobic conditions, make sure your water is
degassed! Always leave the bottle open in the glovebox, to allow the last ppm O2 to
diffuse out. Be aware of the oxygen level while you do that. Then, close the bottle.
To prepare 1 L of synthetic clay water, add the following products to 1 L of high
quality water (demineralised, bidistilled, milli-Q,...) :
Table 1: Weight of salts needed for the preparation of 1 L of SBCW
Salt
Quantity (mg) sorted by descending order
NaHCO3
1 170 mg
H3BO3
43 mg
KCl
25 mg
MgCl2 · 6 H2O
22 mg (hygroscopic salt !)
NaF
11 mg
NaCl
10 mg
FeCl2
3 mg (very sensitive to oxidation !)
Na2SO4
0.3 mg (10 ml of 3 mg dissolved in 100 ml)
TDS
1 284.3 mg (-12 mg H2O) = 1 272.3 mg (anhydr.)
TDS: Total Dissolved Salts (control of mass balance in Table 2).
•
•
•
•
MW
83.996
61.834
74.555
203.218
41.988
58.443
126.751
142.041
(Mole·L-1)
1.39 E-02
6.95 E-04
3.35 E-04
1.08 E-04
2.62 E-04
1.71 E-04
2.37 E-05
2.11 E-06
—
1.55 E-02
Supersaturation with 1000 mg CaCO3 to adjust Ca2+, followed by one week of
stirring.
Bubbling with 0.4 CO2 % gas mixture until constant pH (carrier gas Argon or N2)
Always filter through a 0.22 µm filter before use. In case of solubility
experiments, perform an ultrafiltration through a 30 000 MWCO filter.
Bubbling with 0.4 CO2 % gas mixture until constant pH.
If this SBCW is to be used in the presence of organic matter, organic matter is added
AFTER filtration.
We advise to analyse the solution before using it, the minimum analysis required is
the Fe, Ca and inorganic carbon (IC) content and pH.
Table 2:Reference to check the analyses of the SBCW
Cations
Na+
K+
Mg2+
Ca2+
Fe2+
Sum cations (eq.g.)
(mole · L-1)
1.44 E-02
3.35 E-04
1.08 E-04
—
2.37 E-05
1.50 E-02
MW
22.990
39.098
24.305
40.078
55.845
(mg · L-1)
330.3
13.1
2.6
—
1.3
347.3
Anions
HCO3ClFSO42Sum anions (eq.g.)
(mole · L-1)
1.39 E-02
7.70 E-04
2.62 E-04
2.11 E-06
1.50 E-02
MW
61.017
35.453
18.998
96.064
(mg · L-1)
849.9
27.3
5.0
0.2
882.4
Neutral Species
B(OH)3
(mole · L-1)
6.95 E-04
MW
61.834
(mg · L-1)
43.0
TDS (mg · L-1)
1 272.8
SBCW as used in the section 'Waste Characterisation'
For the composition of Synthetic Boom Clay Water as used in the section 'Waste
Characterisation', we refer to the SCK•CEN document WI.WD.062 (Lemmens, 2001).