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When
dealing with pre-Subtitle D landfills, sometimes it
takes detailed geochemical analyses to resolve ambiguities.
By
Larry Barrows and Brad J. Hunsberger
Livingston
Landfill is located a few miles north of Pontiac, IL,
in the north-central part of the state. Waste disposal
began 25 years ago in a portion of the site now called
the "Old Fill Area." Consistent with the 1978 design
standards, this area had at least 10 ft. of undisturbed,
clay-rich glacial till between the wastes and uppermost
aquifer but did not have an engineered liner or leachate
collection system. A retrofit leachate collection system
that includes vertical wells, horizontal wells, sidewall
slot drains, and peripheral drains is now in place (Friend
and Hock, 1998). The Old Fill Area is surrounded and
overlapped by new waste disposal units with engineered
liners and leachate collection systems (Figure 1).
In response to the Resource Conservation and Recovery
Act Subtitle D regulations, the Illinois Environmental
Protection Agency (EPA) now requires analytic demonstration
of negligible groundwater impact at a landfill compliance
boundary for at least 100 years following closure
of the facility (Title 35, Part 811 of the Illinois
Administrative Code). The compliance boundary is the
property boundary or a margin 100 ft. from the waste,
whichever is closer. This demonstration normally has
the form of solute transport computer simulations
of the landfill and the local geohydrologic system.
A passing model shows groundwater impact less than
the Applicable Groundwater Quality Standards (AGQS)
at the compliance boundary for 100 years following
closure of the facility. AGQS are based on analytical
detection limits and a statistical analysis of background
groundwater chemistry.
Initial models of the Old Fill Area predicted possible
groundwater impact several decades into the future.
A detailed inspection of these models showed the results
were critically dependent on the molecular diffusivity
and mechanical dispersivity assigned to the glacial
till. Site-specific data were not available, so the
parameters were controlled by published values; these
spanned relatively large ranges. In the lower ranges,
the Old Fill Area passed the state regulations, but
in the upper ranges, it failed. This ambiguity successfully
was resolved through geochemical analyses of cores
taken from the glacial till directly beneath the wastes.
Models based on the new site-specific parameters indicated
no adverse groundwater impact. This article describes
the problem, core sampling, chemical analysis, and
results.
Background
Livingston
Landfill currently is the busiest municipal solid waste
landfill in the state of Illinois with an average disposal
rate of 11,000 tpd. It is owned and operated by American
Disposal Services of Illinois Inc., a division of Allied
Waste Industries. The permitted disposal areas cover
254 ac. on a 730-ac. site, with approved proposals to
expand the disposal areas over an additional 207 ac.
The Old Fill Area occupies 16 ac. in a part of the landfill
that is no longer active.
Local geology (Figure 2) includes 40 ft. of Wisconsin-age
glacial drift unconformably overlying Paleozoic shale
and limestone. The drift can be divided into an upper
till, a lacustrine (lakebed) midsection, and a lower
till. The upper till is a massive, gray, silty clay
with suspended granules and pebbles identified as
the Yorkville Member of the Lemont Formation. Twenty-three
permeameter measurements of its vertical hydraulic
conductivity ranged from 1.2 x 10-9 cm/sec.
to 7.8 x 10-8 cm/sec. with a log mean of
7.7 x 10-9 cm/sec. Two discontinuous, silty
sand layers occur in the lacustrine midsection, and
a third groundwater aquifer occurs along the drift
to the bedrock interface. The lacustrine sediments
are assigned to the Equality Formation, and the sand
layers are assigned to the Henry Formation; both formations
are of the Mason Group. The lower till is the Tiskilwa
Formation grading upward into the Batestown Member
of the Lemont Formation.
The upper sand layer within the lacustrine sediments
has been identified as the uppermost aquifer. Monitoring
wells screened across an uppermost aquifer should
provide the earliest and most reliable detection of
a leak from a monitored facility (Sara, 1991). Groundwater
flow in the uppermost aquifer is generally toward
the south or southwest with calculated velocities
of 20 ft./yr. The upper till separates wastes in the
Old Fill Area from the lacustrine midsection and the
uppermost aquifer. Local topography is flat to gently
sloping, with most of the surrounding property being
dedicated to agriculture.
Computer Simulations
There are two parts to the analytic fate and transport
problem. One is determination of groundwater flow.
This is accomplished with MODFLOW, a versatile PC
program developed by the United States Geological
Survey for the general simulation of groundwater systems.
In solute transport applications, MODFLOW is used
to find hydraulic heads, advective (average) fluxes,
and groundwater velocities. MODPATH is a companion
program that operates on the MODFLOW output to find
the advective path lines and travel times.
The second
part of the analytic problem is determination of the
associated solute fate and transport. This is accomplished
with the MT3D program. MT3D calculates time-dependent
evolution of solute concentrations. It can simulate
advective plug flow, molecular diffusion, mechanical
dispersion, sorption and desorption to and from the
soil, and concentration-dependent chemical or biochemical
decay. MODFLOW and MT3D are core programs in the Visual
MODFLOW groundwater modeling package available from
Waterloo Hydrogeologic Inc. The current simulation was
conducted with Visual MODFLOW, Version 3.0.
The solute
transport simulation was a two-dimensional north-south
model through the Old Fill Area and adjoining waste
units to the south. Figure 2 is a geologic cross section
and Figure 3 shows the material configuration and hydraulic
conductivities of the model. The compliance point of
concern is in the uppermost aquifer 100 ft. hydraulically
downgradient of the waste.
Time-dependent
leachate heads on the invert of the Old Fill Area were
based on current observations and projections from a
computer simulation of the retrofit leachate extraction
system. These were +30 ft. for the past 24 years, linearly
decreasing to +4 ft. over the next 20 years, and constant
thereafter. Heads in the newer lined waste units were
+1 ft. above the invert based on performance models
of the landfill design (the Hydraulic Evaluation of
Landfill Performance model). Heads at the boundaries
of the uppermost aquifer were based on measured values
of the groundwater heads. Figure 4 shows the resulting
simulated hydraulic heads and advective groundwater
path lines at 24 years. Tick marks on the path lines
represent 50-year travel-time intervals. The path lines
indicate slow, vertical, downward flow beneath the waste
followed by relatively rapid horizontal flow in the
uppermost aquifer.
Solute transport through a water-saturated porous material
occurs by plug flow, molecular diffusion, and mechanical
dispersion (Zheng and Bennett, 1995). Plug flow
refers to the passive transport of a dissolved solute
by the advective groundwater flux. The velocity of
plug flow equals the advective velocity of the water,
and the solute flux equals the advective flux multiplied
by the solute concentration.
Molecular diffusion results from thermally induced random
motion of the dissolved ions or molecules. According
to Fick's Law, the rate of molecular diffusion is
proportional to the gradient of the solute concentration,
where the proportionality constant is the effective
molecular diffusivity. Reported effective diffusivities
range from 0.0024 to 0.68 ft.2/yr. (Fetter,
1993). Typical values for glacial drift are around
0.2 ft.2/yr. (Rowe et al., 1995). This
value was assumed for all materials in our initial
model.
Mechanical dispersion results from the varied path lengths,
varied velocities, and physical mixing that occur
in a fluid moving through a porous material. The solute
flux due to mechanical dispersion is proportional
to the product of the advective velocity and the gradient
of the solute concentration; the proportionality constant
is the coefficient of mechanical dispersivity. According
to a comprehensive review of experimentally determined
coefficients, a high reliability is assigned to coefficients,
ranging from 0.4 to 4 m (Gelhar et al., 1992). One
meter (3.281 ft.) was assumed for our initial model.
The
leachate was assigned a constant solute concentration
of 1.0, so the calculated concentrations are model
prediction factors. Multiplying the prediction factor
by the leachate solute concentration for a particular
compound yields the predicted concentration at the
time and location of the prediction factor. The time
duration of concern is 124 years. This includes 24
years since the Old Fill Area was opened and 100 years
of postclosure compliance.
Figure 5
shows contoured prediction factors at 74 years for our
initial model. Figure 6 shows the time-dependent prediction
factors at the compliance point; included are the prediction
factors of the final model described as follows. Multiplying
the initial factors by the leachate solute concentrations
indicated that many compounds would exceed their applicable
groundwater-quality standards within 124 years. Additional
models were run with molecular diffusivities and dispersivity
coefficients for the till that spanned the ranges of
these parameters. The calculated solute concentrations
failed the applicable standards in the higher range
of parameter values but passed the standards in the
lower range.
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| Simulated
time-dependent prediction factors at the compliance
point for the initial and final models |
A literature
review (Rowe et al., 1995) indicated that the uncertainties
might be resolved by direct geochemical analysis of
core samples taken immediately beneath the invert of
the Old Fill Area. Following tentative approval of the
approach by Illinois EPA, the decision was made to proceed
with the investigation.
Field Sampling
Cores of the insitu liner directly beneath 124 ft. of
waste in the Old Fill Area were obtained at two locations
287 ft. apart. Each core was approximately 10 ft.
long. Twenty samples at approximately 1-ft. intervals
were removed for analyses. Additional samples were
obtained: two from the waste, two from clay at the
invert, and two from the background. The background
samples were from cores of the upper till in archival
storage from exploratory borings adjacent to the landfill
waste units. Duplicate cores and duplicate samples
were taken to ensure that the results were not compromised
by natural variability in the measured compound concentrations.
The borings into the insitu liner were advanced with
a CME-75 using 4.25-in. hollow-stem augers. Cores
at the first location were taken with a 5-ft. continuous
split barrel. At the second location, an initial boring
was unsuccessful because waste debris had entangled
the cutting bit. A second boring 13 ft. to the south
was cored with Shelby tube samplers; these were opened
in the landfill maintenance shop to extract the samples
at the earliest possible time. When leachate was encountered,
a low-salinity, organic-polymer drilling fluid (Revert)
was added to the auger columns to maintain a positive
outward head and help prevent leachate contamination
of the cores.
The core samples completely and carefully were shaved
to remove any material that might have come into contact
with the wastes or drilling fluids. The shaved cores
were placed in 4-in. labeled soil sample jars, put
into a cooler, and shipped under chain-of-custody
control to the analytical laboratory. The remaining
cores were wrapped in foil and stored in core boxes.
During project planning, it was recognized that the borings
would penetrate the insitu liner. To prevent creation
of potential contaminant pathways, all holes in the
liner immediately were sealed with a thick bentonite
grout injected through a total-depth tremie tube.
Chemical Analysis
Total chloride
was selected for the indicator parameter. Chloride has
consistently high concentrations in the leachate and
is not subject to chemical or biochemical decay. It
is a conservative parameter normally dissolved in the
leachate or groundwater rather than sorbed by the solid
aquifer matrix. The cores were observed to retain moisture,
so the dissolved solute concentration (mg/l) equals
the mass concentration (mg/kg) divided by the moisture
content (l/kg). The directly measured chloride mass
concentrations were used for data reduction and analysis.
The moisture content of each sample also was measured
to ensure that variations among samples did not significantly
impact the results of the study.
Chloride
measurements were conducted in accordance with EPA Method
300, Revision 2.1, with an estimated accuracy of 1 mg/kg.
Figure 7 is a plot of the measured chloride concentrations
versus depth relative to the landfill invert.
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| Measured
chloride concentrations versus depth relative to
the landfill invert |
Interpretation
The analytic results indicate that the chloride concentrations
in the upper till are less than those of the waste
samples, invert samples, and the higher of the two
background samples (49 mg/kg). The one exception (53
mg/kg) is a sample taken 1 ft. below the invert at
the second boring location. Chloride penetration over
the last 24 years appears limited to 1 or (at most)
2 ft. below the invert. An obvious implication is
that the upper till successfully has isolated the
uppermost aquifer in the lacustrine midsection from
hazardous chemicals in the landfill leachate.
A second observation is that plug flow has not contributed
significantly to the chloride migration. Since plug
flow is the passive transport of a solute by advective
groundwater flow, it should have advanced the leachate
chloride concentration as a dispersed concentration
front at least 2 ft. over the last 24 years. The measured
values show that this has not happened. Two possible
interpretations are as follows:
- Conductivity
of the upper till is less than the log mean of the
permeameter measurements. This could result from
a few very-low-conductivity layers in the sedimentary
section between the wastes and the uppermost aquifer.
- Heads
in the wastes have been significantly lower than
30 ft. above the invert, or flow through the wastes
does not follow Darcy's Law. The observed occurrence
of perched leachate and pressured gas pockets within
leachate-saturated wastes supports complex non-Darcinian
conditions.
To simulate
the indicated reduction in plug flow, the modeled hydraulic
conductivity of the till in the final solute transport
model was reduced from 7.7 x 10-9 cm/sec.
to 7.7 x 10-10 cm/sec.
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| Measured
and simulated chloride concentrations beneath the
landfill invert at 24 years |
Hydrodynamic dispersivity is the combined effect of molecular
diffusivity and mechanical dispersivity. The 24-year
depth-dependent effect of hydrodynamic dispersion
was directly calculated for various dispersivities
and compared with the observed chloride concentrations.
The observed concentrations were comparable with or
less than those due to a dispersivity of 0.01 ft.2/yr.
This value was assumed for the till in the final solute
transport model.
Figure 8 shows the final simulated solute penetration
beneath the Old Fill Area along with the measured
values in the waste, clay at the invert, and upper
till. Figure 6 includes the final time-dependent prediction
factors at the compliance point. The modeled prediction
factor at 124 years was 7.5 x 10-7 cm/sec.,
and all predicted solute concentrations were well
below their applicable groundwater-quality standards.
The measured chloride concentrations resulted from an
inadvertent 24-year-long field experiment of the actual
geochemical system. These are rigorous data that must
be satisfied by any acceptable model; the model parameters
were adjusted within their range of uncertainty to
match these concentrations. This model and analysis
indicated no adverse groundwater impact at the compliance
boundary for 100 years following closure of the facility,
consistent with Section 811 of Title 35 of the Illinois
Administrative Code.
Acknowledgments
The authors thank Allied Waste Industries for supporting
this investigation and encouraging publication of
the results. We particularly thank Mr. Kerry Doetzel
and helpers of AEX Exploration for the determined
and difficult drilling.
References
Fetter,
C.W. Contaminant Hydrogeology. Macmillan Publishing,
New York. 1993.
Friend,
M.C., and J.E. Hock. A Side by Side Evaluation of
Vertical and Horizontal Leachate Extraction Wells in
an MSW Landfill. SWANA, 20th International Madison
Waste Conference, Madison, WI. 1998.
Gelhar,
L.W., C. Welty, and K.R. Rehfeldt. A Critical Review
of Data on Field-Scale Dispersion in Aquifers: Water
Resources Research, Vol. 28, No. 7. 1992.
Rowe,
R.K., R.M. Quigley, and J.R. Booker. Clayey Barrier
Systems for Waste Disposal Facilities. F & FN
Spon, London, England. 1995.
Sara,
M.N. "Ground-Water Monitoring System Design." Practical
Handbook of Ground-Water Monitoring, pp 1768. D.M.
Nielsen, Ed. Lewis Publishing, Chelsea, MN. 1991.
Zheng,
Chunmiao and G.D. Bennett. Applied Contaminant Transport
Modeling. John Wiley & Sons, New York. 1995.
Larry Barrows and Brad J. Hunsberger are geohydrologists
with Andrews Environmental Engineering in Springfield,
IL.
MSW
- March/April 2004
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