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IDENTIFIYING PREFERENTIAL FLOWPATHS IN FRACT
DOMINATED
AQUIFERS:
IDENTIFIYING PREFERENTIAL FLOWPATHS IN FRACTURE
FLOW
THE EASTER
RIVER
PLAIN AQUIFER CASE
DOMINATED AQUIFERS: THE EASTER SNAKE
RIVERSNAKE
PLAIN
AQUIFER
Carlos D. Soto-López ([email protected])
CASE STUDY
November 21 2006
Carlos D. Soto-López ([email protected])
Location:
November
2006
Location:
Eastern Snake River Plain Aquifer (ESRiPA) in Idaho, United States o
113W) (Figure 1)
Eastern Snake River Plain Aquifer (ESRiPA) in
Idaho, United States of America (44°N, 113W)
(Figure 1)
Main problem illustrated:
How to use radiogenic, radioactive and stable
isotope tracers together with major element
concentrations to identify preferential flow pathways
in aquifers?
Summary:
In highly fractured and heterogeneous aquifers (eg:
basalt and karst aquifers) preferential flow is one of
the primary groundwater flow mechanisms because
they act as a major conduit for water, dissolved
matter and contaminants.
Radiogenic isotope tracers can provide a unique
“x-ray” photograph of the physical and chemical
processes occurring in the aquifer that is not
provided by the elemental solute concentration data
alone. Isotopic studies can provide information
about mixing of groundwater, flow patterns,
recharge rates and locations, together with rates
and types of rock-water interaction. In general, the
addition of isotopic measurements will give you a
clearer idea of the chemical and physical processes
than do other chemical parameters alone.
Main problem
How
to
radioactive an
tracers togeth
element
co
identify
pr
pathways in aq
Summary:
In
highly
heterogeneous
basalt and
preferential flo
primary
gro
mechanisms b
a major con
dissolved
contaminants.
Radiogenic is
provide a
photograph of
chemical proc
the aquifer tha
by
the
e
concentration
Isotopic stud
Figure 1. Location of the Eastern Snake River Plain Aquifer in
information a
Idaho, USA.
groundwater, flow patterns, recharge rates and locations, together with
rock-water interaction. In general, the addition of isotopic measureme
Tracers Used:
clearer idea of the chemical and physical processes than do other ch
87
Sr/86Sr, 234U/238U, Tritium (3H), 14C, Stable Isotopes of alone.
Oxygen, Hydrogen, Carbon and Sulfur (Sulfate).
People affected environmental, ecological impacts:
Basalt and Karst dominated aquifers cover large parts of the world and often those aquifers are the primary or sole
provider of potable water for the cities and countries that overlie them. Having an understanding of the location
and extent of preferential flow pathways is of critical importance to the assessment of groundwater resources,
underground disposal sites and prediction of the movement of pollutants in these fractured dominated aquifers.
Basalt and Karst dominated aquifers cover large parts of the world and often those
aquifers are the primary or sole provider of potable water for the cities and countries that
overlie them. Having an understanding of the location and extent of preferential flow
pathways is of critical importance to the assessment of groundwater resources,
underground
disposal
This specific case study is focused
sites and prediction of the
on the Eastern Snake River Plain
movement of pollutants
Aquifer in
in the State
of Idaho,
these
fractured
USA anddominated
more specifically
on
aquifers.
the Idaho National Engineering
and Environmental
Laboratory
This specific
case study is
(INEEL)focused
region (Figure
1 andEastern
on the
Figure 2).
Where in the
1950 Plain
Snake
River
low levelAquifer
radioactiveinandthe
nonState of
radioactive
waste was
disposed
Idaho,
USA
and bymore
means ofspecifically
injections wells
the Idaho
ontothe
National
Engineering
aquifer. These
contaminants
are and
Environmental
moving down
gradient from the
INEEL Laboratory
site taking advantage(INEEL)
of
region
(Figure
1 one
and
preferential
flow. This
aquifer is
Figure
2).
Where
in
of the largest basalt aquifers in the the
low source level
USA
and1950
it
primary
for recharge. Regional ground water flow is from the northeast
about
67is the
percent
of the total
radioactive
and
nondrinking
and
agricultural water
in
to theradioactive
southwest.
Finally
waste
wasgroundwater discharges in a series of large springs between
southern
Idaho.
Twin Falls and King Hill (Figure 3).
disposed by means of Figure 2. Snake River Plain Aquifer (ESRiPA) and watersheds that recharge it.
regional groundwater flow direction is given by arrows. Also the boundaries
injections wells to the Approximate
of the Idaho National Engineering and Enviromental Laboratory (INEEL) are shown.
Hydrogeological
setting:
aquifer. These contaminants are moving down gradient from the INEEL site taking
The ESRiPA
covers an
of
advantage
of area
preferential
flow. This aquifer is one of the largest basalt aquifers in the
2
approximately
25,000
kmthe
and
and itconsists
is
primary
The USA
ESRiPA
of source for drinking and agricultural water in southern Idaho.
3
stores
1.2 to 2.5thin
x 1012 m
(1 to 2of
multiple
flows
billion
acre-feet) and
of water.younger
Recharge
Pliocene
Hydrogeological
setting:
occurs
along
the
Snake
River
and
basaltic rock interbedded
its tributaries on the northeastern
with
unconsolidated
2
12
3
Theedges
ESRiPA
and eastern
of the covers
aquifer. an area of approximately 25,000 km and stores 1.2 to 2.5 x 10 m
alluvial
deposits
(1 toaccounts
2 billion
Precipitation
for 10acre-feet) of water. Recharge occurs along the Snake River and its
underlain
by
Miocene
the from
northeastern and eastern edges of the aquifer. Precipitation accounts for
percent, tributaries
while surfaceon
water
basaltic-rock
aquifers
and
10
percent,
while
surface
water from streams, diversion and irrigation of land comprises
streams, diversion and irrigation
of
in
some
places
underlain
land comprises about 67 percent of
silicic
volcanic
theby
total
recharge.
Regional rocks.
ground
In flow
general
aquifer tois
water
is fromthe
the northeast
unconfined
but
thean
southwest.
Finallysystem
groundwater
in
discharges
insome
a series of largeplaces
springs
Figure 3. Large springs that issue from the north wall of the Snake River Canyon
unfractured
basalt
and
between
Twin Falls and
King Hill
near Twin Falls, Idaho, discharge thousands of gallons of water per minute.
interbedded
clay layers
(Figure
3).
cause
confining
The ESRiPA consists of multiple
conditions.
Permeability
thin
flows of Pliocene
and younger basaltic rock interbedded with unconsolidated alluvial deposits underlain by
on
the
younger
of the
aquifer
is by
highly
variable
dueIntogeneral
the different
Miocene basaltic-rock upper
aquifers layers
and in some
places
underlain
silicic volcanic
rocks.
the aquifer kinds
is an of
fracturessystem
and but
features
pillow
columnarclay
basalt
lava flows)
unconfined
in some(lava
places tubes,
unfractured
basaltlava,
and interbedded
layers and
causeindividual
confining conditions.
that compose
it (Figure
highland
mountainous
at depth
Permeability
on the younger
upper4).
layersTherefore,
of the aquiferinis the
highly
variable due
to the differentregion
kinds of and
fractures
and
rainfall
snowmelt
infiltrate
features
(lavaand
tubes,
pillow lava,can
columnar
basalt quickly.
and individual lava flows) that compose it (Figure 4). Therefore, in
the highland mountainous region and at depth rainfall and snowmelt can infiltrate quickly.
Water sampling and analysis summary:
Sixty-four water samples were taken from several purged groundwater wells near the
INEEL screened in the upper part of the ESRiPA aquifer. All samples were filtered to
0.45 ȝm on site, preserved with Nitric Acid (HNO3) and stored in acid washed HDPE
high isotopic ratio zone. The inverse is also true for the low isotopic ratio zones (Figure
5 B-D).
Water-rock interactions affect groundwater elemental concentrations through different
processes, including mineral dissolution, ion exchange, desorption and adsorption. Unlike
Water
sampling and
analysis summary:
elemental
concentrations,
isotopic
Sixty-four
water
samples
were
taken
from
ratios are unaffected by several
those
purged
groundwater
wells
near
the
INEEL
processes, because the Uranium
screened
the upper parttransferred
of the ESRiPAto the
and inStrontium
aquifer.
All samples
were filtered toidentical
0.45 μm to
water
is isotopically
on site,
preserved
with
Nitric
Acid
(HNO
)
3
the host rock. With this important
and stored in acid washed HDPE bottles.
fact in mind both isotopic ratios
The samples were analyzed for uranium and
(Uranium and Strontium) suggest a
strontium isotopic composition as well as major
channeling
of
radiogenic
elemental chemistry, using different analytical
groundwater from the north,
methods such as Thermal Ionization Mass
through (Uranium,
zonesStrontium
of isotopes),
relatively
Spectrometry
unradiogenic
and(anions).
around
ICPMS
(cations), Ionmaterials
chromatography
the western and central zones that
have low isotopic ratios. These
Results of tracer studies:
same preferential flow paths
Figure 5 shows the result of the sample analysis
appear to cause some sort of
as a contour plot interpolated using the
stagnation
of method.
watersThewith
KRIGIN
interpolation
contourlow
isotopes
ratios
(CLZ,
WLZ)
plots of the Uranium and Strontium isotopic
(Figure
5D).
ratios
show water
masses with high isotopic
ratios emanating from Little Lost River and
One
key
about
Birch
Creek
thatfact
penetrate
waterthese
massesaquifer
of
is that
the host
basalt
rock
lowsystems
isotopic ratios
originating
elsewhere
in the
aquifer.
distinct water
massesaquifer
continue are
andThese
sediments
in the
for characterized
tens of kilometers away
from
their
by low apparent
isotopic
sources.
Also
it
is
important
to
note
the
low and
ratios while mountain ranges
isotope
ratio
zones
in
the
Southwest
and
their sediments (Paleozoic and
Central
regions, near the
INEEL Rocks)
site (Figure are
Precambrian
Clastic
5 A and E). The elemental concentrations
characterized by high isotopic
data also reflect some spatial similarities to
ratios. Therefore we can use the
the isotopic ratio data. For example high Mg
isotopic ratios as an indicator of
concentration and low Na and Si concentration
water-rock contact time when
slightly resembles the high isotopic ratio zone.
totrue
elemental
Thecompared
inverse is also
for the lowchemistry.
isotopic
ratio zones (Figure 5 B-D).
Figure 4. Surficial features show the characteristic type of openings in the
Pliocene and younger basaltic-rock aquifers. A and B show, undulating
surface of a basalt flow with collapsed and partly collapsed lava tubes. C and
D shows pillow basalt raging from large boulders to tand grain sand that
formed when lava entered water and cooled quickly. E shows side view of
columnar basalt showing typical cooling joints. These openings provide easy
access for water from recharge sources to enter the underlying aquifer.
The results then suggest that the
Water-rock interactions affect groundwater elemental concentrations through different processes, including mineral
higher the isotopic value the
dissolution, ion exchange, desorption and adsorption. Unlike elemental concentrations, isotopic ratios are unaffected
shorter
the residence time of
by those processes, because the Uranium and Strontium transferred to the water is isotopically identical to the
ground
waters.
Nevertheless, the
host rock. With this important fact in mind both isotopic ratios (Uranium and Strontium) suggest a channeling of
data suggest that preferential flow
radiogenic groundwater from the north, through zones of relatively unradiogenic materials and around the western
and central zones that have low isotopic ratios. These same preferential flow paths appear to cause some sort of
stagnation of waters with low isotopes ratios (CLZ, WLZ) (Figure 5D).
One key fact about these aquifer systems is that the host basalt rock and sediments in the aquifer are characterized
by low isotopic ratios while mountain ranges and their sediments (Paleozoic and Precambrian Clastic Rocks) are
characterized by high isotopic ratios. Therefore we can use the isotopic ratios as an indicator of water-rock contact
time when compared to elemental chemistry.
is not the only mechanisms acting in the aquifer. As the elemental chemistry suggests,
mixing of ground waters also plays an important role. Other alternate interpretations of
the data were explored but failed to adequately explain the observed phenomena.
Figure 5. Contour plots of (A) Strontium Isotopic Ratios, (B) concentration of (B) Si, (C) Mg, (D) Na and (E) Uranium Isotopic
Ratios. (E) also shows location of river channels. Arrows indicate direction of groundwater influxes.
Findings and conclusions:
The results then suggest that the higher the isotopic value the shorter the residence time of ground waters.
Nevertheless, the data suggest that preferential flow is not the only mechanisms acting in the aquifer. As the
elemental
chemistry
suggests,
mixing
ground
also plays
an importantflowpaths
role. Other in
alternate
interpretations
Sr and
U isotope
ratios
wereofused
towaters
distinguish
preferential
aquifer
systems
of the
data wereofexplored
but failed topredictable
adequately explain
the observed
phenomena.
because
their relatively
reactions.
Therefore
in the ESRiPA, in the vicinity
of the INEEL it was determined that the mixing of groundwaters from the mounts of
Birchand
Creek
and Little Lost River with waters from the east and south seems to be an
Findings
conclusions:
important process. More important was the fact that two preferential flow pathways
Sr and U isotope ratios were used to distinguish preferential flowpaths in aquifer systems because of their relatively
extending southeast from these two rivers dominate the entire flow system in this area.
predictable reactions. Therefore in the ESRiPA, in the vicinity of the INEEL it was determined that the mixing of
The findings of these studies illustrate the importance of understanding fractured flow to
groundwaters from the mounts of Birch Creek and Little Lost River with waters from the east and south seems to
the management of groundwater resources and contaminant transport.
be an important process. More important was the fact that two preferential flow pathways extending southeast from
these two rivers dominate the entire flow system in this area. The findings of these studies illustrate the importance
of understanding fractured flow to the management of groundwater resources and contaminant transport.
Take home message:
Take home message:
Using
radiogenic
isotope ratios
together
with
elementalwith
chemistry
in the ESRiPA
madeinpossible
the identification
Using
radiogenic
isotope
ratios
together
elemental
chemistry
the ESRiPA
made
of ground
waterthe
sources,
preferential of
flow
paths, together
with a better
understanding
of paths,
the chemical
and physical
possible
identification
ground
water sources,
preferential
flow
together
with
processes in a highly heterogeneous aquifer system. This case study shows that a similar approach could potentially
be used in other heterogeneous aquifer systems.
Credits:
Case study was written by Carlos D. Soto, Department of Hydrology and Water Resources, University of Arizona,
Tucson. Original work by Johnson et al. (2000) and Roback et al. (2001)
Further reading:
Friedman, L. (2006) Hydrology of Fractured Rocks. Accessed 11/21 2006. http://water.usgs.gov/nrp/proj.bib/hsieh.html
Geller, D. (2006) Eastern Snake River Plain Aquifer. Accessed 11/21 2006. http://academic.emporia.edu/schulmem/hydro/
TERM%20PROJECTS/Geller/Eastern%20Snake%20River%20Plain%20Aquifer.html
Johnson, T. M., Roback, R. C., McLing, T. L., Bullen, T. D., DePaolo, D. J., Doughty, C., Hunt, R. J., Smith, R. W., Cecil, L. D., Murrell, M. T.
(2000) Groundwater “fast paths” in the Snake River Plain aquifer: Radiogenic isotope ratios as natural groundwater tracers. Geology 28(10),
871-874.
Roback Robert C, Johnson Thomas M, McLing Travis L, Murrell Michael T, Luo Shangde, Ku, T. L. (2001) Uranium isotopic evidence for
groundwater chemical evolution and flow patterns in the eastern Snake River Plain Aquifer, Idaho. Geological Society of America Bulletin
113(9), 1133-1141.
Schramke Janet A, Murphy Ellyn M, Wood, B. D. (1996) The use of geochemical mass-balance and mixing models to determine groundwater
sources. Appl. Geochem. 11(4), 523-539.
Whitehead, R. L. GROUND WATER ATLAS of the UNITED STATES: Idaho, Oregon, Washington. Accessed 11/21 2006. http://capp.water.
usgs.gov/gwa/ch_h/H-text8.html
Wood, W. W. and Low, W. H. (1987) Solute Geochemistry of the Snake River Plain Regional Aquifer System, Idaho and Eastern Oregon.
Available from Books and Open File Report Section CO 80225. USGS Open-File Report 86-247, 132 ref.