United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-91/021 July 1991
v?EPA Project Summary
Rate of Flow of Leachate
through Clay Soil Liners
David E. Daniel, Charles D. Shackelford, Wing P. Liao, and Howard M.
Liljestrand
The objective of this research was to
measure the time of travel (TOT) of
inorganic solutes passing through labo-
ratory columns of compacted clay and
to compare measured and predicted
TOT's. Two clay soils were used: ka-
ollnite and Lufkin clay. Anionic tracers
were chloride and bromide; potassium
and zinc were the cationic tracers.
Column tests were used to measure
the TOT of tracers and to determine
the effective porosity ratio, which is
defined as effective porosity divided
by total porosity, of the soils. The
effective porosity is equal to the vol-
ume of the void space that conducts
most of the fluid flow divided by the
total (bulk) volume of the soil. The
effective porosity ratio Increased with
increasing hydraulic gradient in kaolin-
ite from a low of about 0.25 at a gradi-
ent of 1 to a high of 1 at a gradient of
20. With Lufkin clay, the effective po-
rosity ratio was between 0.02 and 0.16.
Breakthrough times were controlled
much more by the low effective porosi-
ties than by molecular diffusion.
The computer program SOILINER,
which was developed by EPA for pre-
dicting TOT's through soil liners, pre-
dicted TOTs that were larger than ac-
tual TOT's by a factor of up to 52. The
failure to account for effective porosity
ratios less than 1 was the cause for the
poor predictions from SOILINER.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the data evaluations
that are fully documented In a separate
report of the same title (see Project
Report ordering information at back).
Objectives
The objectives of this research were to
perform experiments to determine the TOT
of inorganic chemicals through clay soils,
to measure the physical and geochemical
parameters that control the TOT through
specimens of compacted clay soil, and to
compare the measured TOTs with those
predicted from analysis with the computer
program SOILINER. The research was
intended to provide information to help in
answering the following questions:
1. What magnitude of effective poros-
ity should be used in calculating TOT
of constituents through a soil liner?
2. Can retardation factors calculated
from batch adsorption isotherms be
used with confidence to predict TOT
of reactive solutes?
3. How important is molecular diffusion
in determining the TOT of solutes
through soil liners?
4. What methods of calculation should
be used in calculating TOT of con-
stituents through a soil liner?
Methods
Two soils were used: kaolinite, which is
a low-plasticity, commercially-produced
clay, and Lufkin clay, which is a highly
plastic, naturally-occurring clay soil, the
soils were compacted with standard Proc-
tor procedures at a water content just wet
of optimum. The tracers used included
two anions (chloride and bromide) and
two cations (potassium and zinc). Most
tests were performed with the tracers
mixed in water (0.01 N CaSO4) at a con-
centration of 0.01 N, but some tests were
performed using a concentration of tracer
of 0.001 N. After compaction, most soil
Printed on Recycled Paper
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columns were presoaked with water for
about 1 month, and then the tracer solu-
tion was introduced. However, some tests
were performed without presoaking the
soil, i.e., by introducing the tracer solution
immediately after the soils were com-
pacted.
Batch adsorption tests were performed
by adding soil to tracer solutions at differ-
ent concentrations, mixing the soil/liquid
solution, and analyzing the remaining so-
lution to determine the sorbed mass of a
reactive (cationic) tracer. Diffusion coeffi-
cients were measured in fixed-wall diffu-
sional cells in which, after a long soaking
period to destroy any capillary suction in
the soil, the tracer solutions were exposed
to one end of a soil column. Diffusion
coefficients were calculated from the rate
of change of concentration of a tracer in
the sealed reservoir containing the tracer
solution and from profiles of concentration
determined by sectioning the soil after the
diffusion test was complete. Column tests
were performed in fixed-wall cells using
hydraulic gradients of 1, 5, 10, and 20 for
kaolinite and gradients of 20 and 50 for
Lufkin clay. The effluent from column
tests was analyzed for the tracers, and
the data were plotted in the form of rela-
tive effluent concentration versus pore vol-
umes of flow (termed a "breakthrough
curve"). A solution to the advection-dis-
persion equation was fitted by trial to the
experimental data from anionic tracers;
the effective porosity ratio (defined as ef-
fective porosity divided by total porosity
and equal to the ratio of the volume of
pore space conducting most of the flow to
the total volume of void space in the soil)
and hydrodynamic dispersion coefficient
were adjusted until a best fit (by eye) was
obtained. For cationic tracers, the effec-
tive porosity ratio and hydrodynamic dis-
persion coefficient determined for an an-
ionic tracer were assumed to apply to the
cationic tracer as well, and the retardation
factor was determined by trial-and-error
curve fitting.
Results
Batch adsorption tests produced non-
linear isotherms that could be modelled
well with the Freundlich and Langmuir
equations. For kaolinite, zinc and potas-
sium were adsorbed about equally, but for
Lufkin clay, zinc was sorbed much more
strongly than potassium.
Diffusion tests showed that the effective
diffusion coefficient for anions was ap-
proximately 6 - 7 x 10"6 cm2/s for kaolinite
and 1 - 15 x K)-6 cm2/s for Lufkin clay.
The effective diffusion coefficient was
somewhat less for cations, but experimen-
tal problems with precipitation of cations
in the tracer solution and difficulty in
achieving complete extraction of cationic
tracers from soil slices made the data
difficult to interpret. The computed values
of tortuosity factor varied from 0.29 to
0.35 for kaolinite and between 0.05 and
0.71 for Lufkin clay. The tortuosity factors
determined in this study are within the
range of values reported in the literature
for soils.
Column tests for presoaked soils
showed that the effective porosity ratio for
kaolinite varied from a low of 0.25 - 0.3 at
a hydraulic gradient of 1 to a high of 1 at
a hydraulic gradient of 10 and 20. It was
postulated that at low hydraulic gradient,
viscous water adsorbed to the surfaces of
kaolinite particles blocked flow paths and
produced a relatively bw effective poros-
ity. At higher hydraulic gradient, the vis-
cous water was mobilized and contributed
to, rather than blocked, flow paths. With
Lufkin clay, effective porosity ratios were
very low — only 0.02 to 0.16. Adsorbed
water layers or preferential flow paths (or
both) evidently produced the low effective
porosities. With lufkin clay, the effective
porosity ratio was approximately the same
on soil columns permeated at hydraulic
gradients of 20 and 50. There was no
evidence of a threshold hydraulic gradient
(below which no flow occurs) in the col-
umn tests.
Hydrodynamic dispersion coefficients in
the kaolinite columns varied with hydraulic
gradient. At low gradient (1 and 5), the
hydrodynamic dispersion coefficient was
on the same order as the effective diffu-
sion coefficient, indicating that mechani-
cal mixing was unimportant compared to
molecular diffusion. At the highest hy-
draulic gradient used for testing kaolinite
(20), the hydrodynamic dispersion coeffi-
cient was 3 to 4 times larger than the
effective diffusion coefficient, indicating (as
expected) that mechanical mixing was
much more important at high hydraulic
gradient. With Lufkin clay columns, the
hydrodynamic dispersion coefficient was
on the same order as the effective diffu-
sion coefficient.
Retardation factors for potassium and
zinc calculated from batch adsorption tests
compared favorably with values deter-
mined from column tests, although there
were discrepancies involving up to 100%
difference in retardation factors. Several
geochemical differences existed between
the column and batch tests and probably
combined to cause the differences.
An analysis of the relative importance
of molecular diffusion in determining the
TOT of nonreactive (anionic) tracers
through the soil columns revealed that
diffusion was of little significance in these
particular experiments; the relatively fast
breakthrough times (less than 1 month) in
the column tests were controlled more by
low effective porosity than by molecular
diffusion.
Soil columns that were not presoaked
were analyzed with the computer program
SOILINER to determine breakthrough
times. SOILINER is a computer program
that predicts TOT of nonreactive solutes
through a compacted soil liner. SOILINER
consistently predicted longer TOT's
through the soil columns than were mea-
sured. With kaolinite, the actual TOT was
2 to 5 times less than predicted, and with
Lufkin clay, nonreactive tracers broke
through the column more than 50 times
faster than predicted (the predicted break-
through time was > 2 years, but the actual
breakthrough time was about 2 weeks).
The cause for the poor results with
SOILINER was the assumption in
SOILINER that the effective porosity
equals the total porosity; the soil columns
used in this study had effective porosities
that were generally much less than the
total porosity.
Conclusions
The main conclusions from this study
are:
1. The effective porosity of compacted
kaolinite was a function of hydraulic
gradient; lower values of effective
porosity were measured at low hy-
draulic gradient than at high gradi-
ent in kaolinite.
2. The magnitude of effective porosity
ratio (defined as effective porosity
divided by total porosity) was 0.25
to 0.3 for kaolinite tested at a hy-
draulic gradient of 1, and was 0.02
to 0.16 for Lufkin clay tested at a
hydraulic gradient of 20 and 50.
These effective porosity ratios, it is
noted, are significantly less than
unity.
3. Breakthrough of nonreactive tracers
occurred much faster than predicted
by the computer program SOILINER.
The problem with results from
SOILINER appeared to be that the
effective and total porosity are as-
sumed to be equal in SOILINER.
4. Molecular diffusion played a relatively
unimportant role in determining the
TOT of constituents through the soil
columns used in these experiments;
low effective porosity had a much
more significant effect.
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Recommendations
The main recommendations that are
made as a result of this research are:
1. Effective porosity should not be as-
sumed to equal total porosity unless
actual experimental data confirm that
the two are equal.
2. Column tests similar to those uti-
lized for this research are recom-
mended for the determination of ef-
fective porosity; the tests should be
performed at a hydraulic gradient that
is as close to the anticipated field
value as possible and not at a grossly
elevated gradient.
3. SOILINER should not be used to
predict TOT of constituents through
a soil liner, unless the total porosity
is approximately equal to the effec-
tive porosity. A simple, direct calcu-
lation of TOT, using the velocity of
seepage equal to the hydraulic con-
ductivity times hydraulic gradient di-
vided by effective porosity, is rec-
ommended instead.
4. The computer program SOILINER
should be refined to account for ef-
fective porosity less than total po-
rosity.
The full report was submitted in fulfill-
ment of Cooperative Agreement No. CR-
812630 by the University of Texas at Aus-
tin under the sponsorship of the U.S. En-
vironmental Protection Agency.
6 U.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/400Z3
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David E. Daniel, Charles D. Shackelford, Wing P. Liao, and Howard M. LJIjestrand
are with The University of Texas, Department of Civil Engineering, Austin, TX
78712.
Walter E. Grube, Jr. is the EPA Project Officer (see below).
The complete report, entitled "Rate of Flow of Leachate through Clay Soil Liners,"
(OrderNo. PB91-196 691/AS; Cost:$23.00 subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/021
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