United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30605-2720
Research and Development
EPA/600/SR-92/233 January 1993
EPA Project Summary
Evaluation of Source Term Initial
Conditions for Modeling
Leachate Migration from Landfills
Lee A. Mulkey, Anthony S. Donigian, Jr., Terry L. Allison, and C.S. Raju
Leachate migration modeling is a
necessary part of conducting exposure
assessments in the development of
land disposal regulations for solid
wastes. Development of toxicity char-
acteristics to define leachate concen-
tration levels that pose unacceptable
risks to humans and the environment
requires modeling studies for a wide
range of leachate generation-migration
scenarios. This study investigated the
influence of five alternative modeling
initial conditions on down-gradient
ground water concentrations predicted
by two models (MULTIMED and
AT123D) and for four typical hazardous
waste constituents. Differences be-
tween steady-state and transient con-
ditions also were investigated.
The alternate initial conditions stud-
ied were shown to be representative of
typical leachate patterns reported from
laboratory and field studies. The square
wave pulse initial condition was found
to be the most conservative represen-
tation of leachate generation for both
models. Sorptive and degradation prop-
erties of chemicals strongly influence
predictions and, for some chemicals,
steady-state modeling and transient
modeling of the same scenario pro-
duced significantly different results.
This Project Summary was developed
by EPA's Environmental Research
Laboratory, Athens, GA, to announce
key findings of the research project
that Is fully documented In a separate
report (see Project Report ordering In-
formation at back).
Overview
The study described in this report is
part of an ongoing investigation of the
behavior of land disposal systems for solid
and hazardous wastes. Major program ob-
jectives are development, testing, and
implementation of predictive tools for use
in land disposal regulations that protect
human health and the environment. The
U.S. Environmental Protection Agency's
Office of Solid Waste and Emergency Re-
sponse is a major collaborator with the
Office of Research and Development in
the overall research program and has par-
ticipated in the development and execu-
tion of this study. The modeling scenario
and the modeling details under investiga-
tion are part of the Office of Solid Waste's
current approach to modeling leachate mi-
gration from land disposal units. In particu-
lar, the development of solid waste
characteristics that render such wastes haz-
ardous under expected future management
conditions has evolved to include the use
of models and their boundary conditions
as described in this report.
The work summarized in this report
advances knowledge of how modeling as-
sumptions influence predicted exposures,
how boundary conditions compare to labo-
ratory and field observations, how different
dominant chemical properties influence pre-
dicted exposures, and how currently avail-
able models can be used to refine exposure
estimates.
The appropriate selection and use of
mathematical models in conducting expo-
sure estimates within a regulatory frame-
work depends on many different factors.
Paramount among them are the system
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being modeled and the data available to
describe its features. In cases where site-
specific decisions are under study—for ex-
ample, the siting, design, or permitting of a
disposal facility—site characterization data
are particularly important.
Once a site has been completely char-
acterized and the study objectives clearly
defined, then choices among individual
models to use, boundary conditions to ap-
ply, and validation studies to complete can
be made. Often the model calibration pro-
cess is used to refine the approach and
test the model to insure that the specific
circumstances understudy are represented
in the most nearly accurate manner. For
example, measured site data can be used
to define the most appropriate model, in-
cluding the model boundary conditions,
and the best choice of parameter values.
Boundary conditions and parameter
values become much more problematic,
however, when scenarios or hypothetical
case studies are modeled as a means of
developing more generic regulations and
standards or when only screening level
analyses are desired. The most appropri-
ate set of assumptions is not always ap-
parent, and often the major issue is not
only how representative but also how "safe"
the assumptions are given the wide variety
of possible circumstances.
The main emphasis of the report is an
analysis of two major assumptions used in
the OSW modeling. The first assumption is
that the contaminant source has a suffi-
ciently large mass to enable an assump-
tion of an infinite source. That is, the down
gradient contamination, once reached, will
be maintained. The second assumption is
that transient behavior is unimportant, per-
mitting the overall response to be repre-
sented by steady-state estimates.
These two assumptions are convenient
as an interface between the modeling and
the physical tests proposed as a means to
measure the teachability of a given waste.
The assumptions also dominate the choice
of modeling boundary conditions, espe-
cially the initial conditions. The form or
shape of the initial conditions is deter-
mined by the steady-state assumptions;
magnitudes are determined by reference
to the leaching test values.
In the study, literature searches were
performed to identify alternate leachate
models, leachate characteristics, and model
initial conditions. Based on available
leachate data, the range of initial condi-
tions that are most consistent with ob-
served data was determined. Modeling
simulations were conducted with different
models and initial conditions to character-
ize groundwater contaminant sensitivities
for a range of scenarios and chemical
properties.
Findings
Four key findings were apparent with
respect to leachate quantity and quality.
First, leachates vary widely in quantity and
quality among different sites and change
with time at a given site. Second, leachate
quantity responds to precipitation infiltra-
tion and the subsequent water balance.
Several models exist with which to predict
quantities, and many are variants on the
USEPA HELP model. Third, time histories
of leachates rarely are measured, but ex-
isting data suggest a range of patterns
from relatively constant concentrations to
declining concentrations with time. Fourth,
for regulatory modeling purposes, leachate
time patterns can be represented by a
limited number of simple curves including
square wave pulses, exponentially decay-
ing pulses, and a series of pulses emulat-
ing highly variable concentrations.
For some chemicals, steady-state and
transient modeling of leachate migration
give remarkably different results. In such
cases, steady-state modeling is inappro-
priate. Sorption and transformation rates
are most important in this regard.
Transient modeling results are influ-
enced strongly by the duration of inputs
corresponding to available source mass
within the facility. Departure from steady-
state results for any given duration de-
pends on chemical properties.
The square wave pulse load pattern
consistently produced the highest concen-
trations for all chemicals and all locations.
Differences in concentrations among the
five pulse types were insignificant for some
combinations of chemicals and locations
forthe scenario investigated.
For highly sorptive chemicals, time vari-
ability of the leachate inputs is relatively
unimportant when predicting maximum
down gradient concentrations. This applies
as long as the total mass and duration of
leaching is the same.
For less sorptive, more mobile chemi-
cals, the duration of the leaching period is
more important in influencing down gradi-
ent concentrations than the leachate vari-
ability during the period. This is true under
conditions where the total mass loading is
the same.
Transient modeling should be imple-
mented in regulatory modeling analyses of
leachate migration from landfills.
Forthe OSW modeling scenario, square
wave pulse inputs are the most conserva-
tive option in selection of modeling initial
conditions to represent leachate genera-
tion.
Additional field studies to characterize
detailed patterns of solid waste leachates
are needed to better define modeling as-
sumptions.
•U.S. Government Printing Office: 1993 — 750-071/60189
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LOB A. Mulkey (also the EPA Project Officer, see below) is with the Environmental
Research Laboratory, Athens, GA; Anthony S. Donigian, Jr., and C.S. Raju are
with AQUA TERRA Consultants, Mountain View, CA; Terry L Allison is with
Computer Sciences Corporation, Athens, GA.
The complete report, entitled "Evaluation of Source Term Initial Conditions for
Modeling Leachate Migration from Landfills, "(Order No. PB93-131464/AS; Cost:
$19.50; subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telaphone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Athens GA 30605-2720
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
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EPA
PERMIT No. G-35
EPA/600/SR-92/233
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