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Modeling of the impact of a hazardous waste storage
centre : sensitivity study
M-A. Aubrya, M-C. Magniéa, A. Budkab, I.Martinc
a
INERTEC,
6 rue de Watford, 92000 Nanterre
b
SITA France
132 rue des trois Fontanot, 92758 Nanterre
c
SITA FD
132 rue des trois Fontanot, 92758 Nanterre
Abstract
The development of the French regulation has led, as early as 1995, to the setting up of
stabilization/solidification industrial units intended to pretreat the hazardous waste
before their storage, so as to reduce the mobility of the pollutants they hold. In addition
to the regulatory compliance targets, many studies have been carried out since more
than ten years, combining laboratories tests, pilot tests and computer modelings, in
order to try to forecast the long term behaviour of stabilized and solidified waste.
Notwithstanding, the question always arises about the reliability of the forecast achieved
in relation to the selection of modeling assumptions (specially uncertainties relating to
the complexity of the waste and the media, and to their variability).
The purpose of the project carried out by SITA, SITA FD and INERTEC, in partnership
with ADEME (Agency for Energy Management), was to gather the available data in
order to asses the overall impact of a hazardous waste storage centre, by completing a
sensitivity study of the source term and transfer term :
 The sensitivity of the source term was studied by considering various waste received
on the CET (Technical Burial Centre) and their variability to assess the respective
impact on various pollutants of parameters significant for the calculation (steadystate concentration, total content, diffusion coefficient…).
 The assessment of the transfer term was performed by the design office, Golder
Associates, by comparing the results of three different models (MISP, MODFLOW
and LANDSIM) for various assumptions of hydrogeological configurations.
Thanks to this study, it became possible to organize into a hierarchy the impact of
various modeling parameters for the forecast of the source term and transfer term. On
the other hand, it allowed the operator to confirm the results of previous studies with a
negligible impact from hazardous waste storage, as described by the regulation.
Keywords: source term, transfer, hazardous waste.
1. Introduction
The study of the environmental impact of a class 1 storage centre called on a coupling
of the modeling of a source term and that of a transfer term.
A development of the source term was considered by taking into account the variability
of waste received on the CET (Technical Burial Centre). The study of the source term
was then performed by INERCTEC, using the digital model SIMPSPEC developed by
INERTEC and Ecole des Mines de Paris and the Pierre et Marie Curie University.
During that assessment of the source term, the impact of the variation of two parameters
(total content and diffusion coefficient) was studied.
The assessment of the transfer term, subcontracted to GOLDER ASSOCIATES, an
English design office, was carried out by using three different digital models : MISP,
MODFLOW and Landsim. A comparison between the results of the modeling was
completed to study their sensitivity according to the digital model used and according to
various hydrogeological configuration assumptions tested.
This document summarizes the main results achieved in the course of this project.
2 Assessment of the source term
The parameters of the source term and their range of variations were selected following
a review paper of the long term behavioural studies performed at SITA’s and
INERTEC’s on various waste, and additional experiments. The results of such forecasts
of the source term were then used as input data for the modeling of the transfer term.
Following the review paper, several wastes were selected according to the data already
available ,to their problems and to their tonnages.
The REFIOMs (of the humid and semi-humid type) were studied more accurately,
because they amount to a significant tonnage of stabilized and solidified waste for
storage in class 1 CET.
The available data were used to simulate the source term, i.e. to simulate the leaching of
stabilized/solidified waste by putting oneself in the case of a failure of the tightness
system of the surface of the class 1 storage centre. The permeability of the compacted
clay layer restricts the seepage stemming from the rainfall : a quantity of water of
around 30 mm/year is added.
Several assumptions were taken to represent the stabilized/solidified waste :
 The treated waste is in solid and monolithic form,
 The main transfer mechanism within the waste is a diffusion mechanism,
 The leaching takes place from the surface of the waste by runoff of the seepage
water.
Thus, for each type of stabilized/solidified REFIOMs, the same approach was taken up,
i.e. :
 creation of an input file describing the total content of the stabilized/solidified waste
(taking into account the test data of basic analysis of the total content, the specific
gravity and the dryness),

modeling of the leaching of a waste pellet subjected to a soxhlet test modified with
the SIMPSPEC software (which enables coupling between a geochemical speciation
module and a transport module),

validation of the simulations by comparison with the experiment results,
 forecasting simulations of a 20 m stabilized waste column, subjected to a seepage
rate of 30 mm/year for 1000 years.
The impact on the stabilized/solidified REFIOMs of two important parameters of the
source term (for a given pollutant total content (C) and diffusion coefficient (D)) was
also studied. In order to get close to a true situation, the ranges of variation of both these
parameters (for an average (M) or extreme (E) situation) have been selected in relation
to the test data of the review paper of the long term behavioural studies carried out at
SITA’s and INERTEC’s on various wastes.
Table 1: description of the REFIOMs studied
Stabilized/solidified waste
Semi-humid Refiom
Parameter studied
CM (mol/l)
CE/CM
Na
0.5000
3
K
0.5000
2
SO4
0.3800
1.3
Cl
2.5500
1.6
Zn
0.0654
1
Pb
0.0090
1
Humid Refiom
Na
1.1920
2.3
K
0.9000
1.1
SO4
0.7400
1.3
Cl
2.100
2.4
Zn
0.1361
1.3
Pb
0.0183
2.4
Parameter
DM (m²/s)
DE/DM
Semi-Humid Refiom
Diffusion coeff.
1.5 10-11
10
Humid Refiom
Diffusion coeff
7.5 10-13
10
CM : average composition in mol/l. CE : extreme composition in mol/l.
DM : average diffusion coefficient (m²/s). DE extreme diffusion coefficient (m²/s).
Considering the results of the forecasting modeling obtained for soluble compounds,
such as chlorides :
 for a given composition, the ratio of released concentrations is logically related to
that of the diffusion coefficients tested : the more important the diffusion coefficient
is, the more significant the released concentrations are.
 For the same diffusion coefficient, the ratio of released concentrations is also related
to the ratio of chloride contents introduced initially :
Considering the results recorded for not much soluble compounds, such as metals (lead
and zinc studied within the framework of this study) :
 as removal is very low, no exhaustion or delay phenomenon (amounting to
conveyance of the metal within the matrix to the leached interface) is recorded : in
view of the simulations performed on the 1000 year period concerned, the total
metal content does not influence the results.

the release of the metal is restricted by the solubility of the mineral phase, in
particular according to the pH of the solution. The variation of the diffusion
coefficient, allowing a more or less speedy salting out, influences indeed the
development of the ionic strength and of the pH of the solution balanced with the
stabilized REFIOM. Such sensitivity of the solubility explains the concentration
differences in released zinc or lead, according to the diffusion coefficient applied.
Considering the important number of results of the source term, a selection was made
before contemplating the simulation of the transfer term, because some data of the
source term tied up. A decrease in the number of parameters monitored for the modeling
of the transfer term was achieved by selecting systematically, for a given component,
the average and maximum concentrations available.
3 Assessment of the transfer term
The assessment of the transfer term, subcontracted to GOLDER ASSOCIATES, a
British design office, was carried out by using three different digital models, MISP,
MODFLOW and Landsim, selected following a paper review of all the transfer models
available on the market. These three models enjoy statutory recognition : nowadays,
Landsim is the model recommended by the British Environment Ministry, and MISP is
the model most used in France, while MODFLOW is today the best known
hydrogeological transfer software worldwide, although it does not allow the operator to
take into account the availability of an unsaturated area.
A study of the sensitivity of the modelings was performed according to the digital
model used and to various assumptions of hydrogeological configurations. The
maximum concentrations of various pollutants downstream of the class 1 CET were
then compared with the quality standards of drinking water as specified by the World
Health Organization (WHO).
3.1 Description of the scenarios :
The storage scenarios suggested within the framework of the drafting of the European
directive on landfilling are shown hereunder :
Unsaturated area :
Thickness :
1 m or 5 m
Permeability :
10-7 m/s
Porosity :
0.3
Longitudinal dispersivity :0.5 m
Specific gravity
1.5 t/m3
Aquifer :
Side flow :4 m/an
Thickness of aquifer :5 m
Porosity :
0.3
Longitudinal dispersivity :1/10 of distance
to checking point
Horizontal transverse dispersivity :1/50 of
distance to checking point
Specific gravity : 1.5 t/m3
Whatever might be the pollutant considered, its concentration is deemed as initially nil
in the aquifer. On the other hand, the authorized limit values shall be the drinkability
standards of WHO. Two checking points are being considered, the first one 20 m from
the site and the other 200 from the site.
The geometry suggested for the site is the following:
Height of waste :
20 m
Dimensions of landfill : 150 m x 150 m
Kind of overburden : Not considered within the framework of the modeling (impact on
underground waters)
Type of bottom tightness : a 50 cm thick drainage blanket with permeability 10 -4 m/s
(silica gravels) was taken into account, then 2 situations with (or without) 2 mm HDPE
geomembrane and 2 thicknesses of the underlying clay layer (1 or 5 m) were tested,
amounting to 4 different containment situations.
3.2 Modelings performed by GOLDERS ASSOCIATES
The flow speed of the selected aquifer is very low and represents therefore a pessimistic
assumption in relation to the transfer term.
The quantity of leachate out of the CET was calculated with Landsim taking into
account the thickness of the clay layer (1 or 5 m) and used as an input parameter for the
three digital models.
Only the pessimistic concentrations of the source term were used to simulate the
transfer term with the MISP and MODFLOW models, according to the various
containment configurations studied. The variability of the waste received on the CET
was considered by performing, for the 4 containment configurations (cf. 3.1), two
simulations with the Landsim digital model : the first one with the concentrations
matching the average situation and the second one with those matching the pessimistic
situation.
3.4 Results achieved in term of environmental impact
The transfer modelings, from the average assumptions of the source term, have been
performed only with the Landsim model. Considering the results achieved with the
steady (scenario 1) or variable according to time (scenario 2) source term, whatever the
pollutants, the scenarios and the containment configurations considered might be, the
concentrations remain below the limits of the quality of drinking waters specified by
WHO. By way of illustration, the developments of zinc and chlorides release are given
hereunder.
Figure 1: Forecasting simulations of zinc release – Average assumptions on the source
term, variable concentrations according to time – 5 m clay layer and unsaturated
underlying area
Figure 2: Forecasting simulations of chlorides release – Average assumptions on the
source term, variable concentrations according to time – 5 m clay layer and unsaturated
underlying area
3.5 Comparison of results achieved with the three software tested
A systematic selection of the most pessimistic assumptions on the source term (in term
of diffusion coefficient and total content) and on the transfer term amounting to an
extreme situation (very unlikely relative to the one described by the average
assumptions on the source term) was used to study the impact of the choice of the
digital transfer model on the results of the coupled modeling source term/transfer.
Considering the results achieved, for the common situations tested, the modeling results
obtained with the three software used are of the same order of magnitude at the
observation point located 200 m downstream of the class 1 CET.
The trends of the concentration curves according to time are similar, whatever the
model might be, with the occurrence and increase of concentrations happening at about
the same dates.
In all the examples submitted, the conveyance time through the unsaturated area is
short, by comparison with the total transport time to the receiver.
Notwithstanding, the results achieved should be interpreted cautiously, because the
conceptual modelings of the leakages and of the area where leachates mix with aquifer
waters are not insignificant and explain, for example, the differences recorded between
the results of Modflow and those of the other 2 software at the point of impact 20 m
downstream of the CET.
In the three models, the thickness of the clay layer has only a very limited impact on the
maximum concentrations calculated at the receiver, because the chemical interactions of
the leachate with the clay are not taken into account.
This study underlines the benefits and drawbacks of each model. As LandSim applies
directly to the cases studied, the conceptual model suggested could be shown without
need for any adjustment. The results of LandSim are similar to those of
MODFLOW/MT3DMS at the receiver located within 200 m, which strengthens the
validity of the software.
MISP can forecast, for pollutants in the underground water downstream of the landfill,
concentrations consistent with those achieved with the other models, while MISP
requires a specific format of data input files and seems unable to execute itself for some
combinations of parameters.
MODFLOW and MT3DMS have the advantage of being digital models, whereby more
complex hydrogeological conditions can be simulated. However, such programs are not
explicitly designed to represent landfills and they restrict themselves to modeling
conveyance in the saturated area.
4. Conclusion
Owing to the important number of results of the source term, a selection was made
before considering the simulation of the transfer term, as some data of the source term
tied up. After highlighting the benefits and the drawbacks of each of the transfer model
selected (MISP, MODFLOW and Landsim), the environmental impact of a class 1
storage centre was contemplated with the Landsim digital model, by considering the
coupling of the source term and transfer term from the forecasts of the source term,
calculated within the framework of the average situations and according to various
containment configurations.
Considering the results achieved, it seems that :
 the modeling of the impact of a storage centre with Landsim is not much significant:
all the concentrations of monitored elements remain below the limits of drinking
water specified by WHO, and this for a study or variable source term and for all the
containment configurations tested.

the 1 to 5 m thickness of the clay layer does not seem to influence the concentrations
of leachates originating from the landfill. These findings, which are a priori
amazing, can be explained by the fact that the chemical reactions within this clay
layer have not been taken into account by the transfer models tested.
In this program, we have chosen to favour the comparison of the results of the
modelings of the transfer term achieved according to three different software, more than
a detailed understanding of the source term/transfer term coupling. Considering the
results of the program, an interesting deepening could lie in the study of the chemical
reactions within the layer clay.
Acknowlegements
Special thanks to ADEME for their help and support provided on this study.