United States
Environmental Protection
Agency
Environmental
Research Laboratory
Athens, GA 30605-2720
Research and Development
EPA/600/SR-93/125 September 1993
EPA Project Summary
Leachate Generation and
Migration at Subtitle D Facilities:
A Summary and Review of
Processes and Mathematical
Models
Constance L. Travers and Susan Sharp-Hansen
The U.S. Environmental Protection
Agency (EPA) recently promulgated re-
visions to the Solid Waste Disposal Fa-
cility Criteria under Subtitle D of the
Resource Conservation and Recovery
Act (RCRA). These revisions create a
need for tools for predicting the perfor-
mance of facilities regulated under Sub-
title D. Mathematical models, based on
site-specific data, provide a means of
estimating the effects of a variety of
designs and hydrogeologic conditions
on the generation and migration of
leachate at Subtitle D facilities.
Leachate generation and migration
are influenced by numerous interrelated
physical, chemical, and biological pro-
cesses that can occur during the life-
time of a waste disposal facility. Many
of these processes are discussed in
this report. Some processes are already
well understood and easily described
by mathematical models. The math-
ematical representation of other pro-
cesses is the subject of current
research. This report focuses on the
state-of-the-art in modeling leachate
generation and migration. Issues re-
lated to selection of models for appli-
cation to specific scenarios are
discussed.
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 of the same title (see Project
Report ordering information at back).
Overview
Subtitle D of RCRA requires the EPA
to develop criteria for controlling solid
waste management practices. Subtitle D
covers land disposal of all nonhazardous
waste, household hazardous waste, and
hazardous waste from small quantity gen-
erators in landfills, surface impoundments,
land application units, and waste piles.
New criteria for Subtitle D municipal land-
fills provide a need for computer models
in designing the facility and in monitoring
its performance.
The need for computer models appears
at two different stages of landfill develop-
ment and operation. During the design
stage, models will be needed to show that
an alternative design meets the perfor-
mance standards specified in the criteria
and to show that a facility located in a
wetland will not degrade the wetland envi-
ronment. During the monitoring stage, the
use of models will be required of any
operator requesting a suspension of the
groundwater monitoring requirements. In
addition, models can be used to deter-
mine the most effective remediation op-
tions at facilities where contamination of
the environment has occurred.
In anticipation of the need for a suite of
EPA-approved models for use in comply-
Printed on Recycled Paper
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ing with current and future criteria, this
report discusses the potential for contami-
nation of groundwater from the four main
types of Subtitle D facilities and addresses
issues related to modeling contamination
at these sites. Tables characterizing some
currently available models are provided.
Three issues are important in determin-
ing the effect, if any, of land-based Sub-
title D facilities on the subsurface environ-
ment: the quantity of leachate generated,
the composition of the leachate gener-
ated, and the migration of the leachate
from the facility. The report summarizes a
number of the complex, interrelated fac-
tors that affect leachate generation and
migration.
No one model can handle all aspects of
the complex problems that arise at Sub-
title D facilities. For many sites, it will be
necessary to link a series of leachate gen-
eration, geochemical, hydrologic, and pol-
lutant transport models that have been
selected to correspond to specific site con-
ditions. The report discusses the model-
ing of leachate quality and quantity, facil-
ity design, chemical and microbial trans-
port, biodegradation, geochemical reac-
tions, and flow (e.g., fracture, karst,
multiphase). Available models are sum-
marized and the application of specific
models is addressed. Topics include the
selection, verification and validation of
models.
Conclusions
• Many physical, chemical, and biologi-
cal processes can occur during the
lifetime of a waste disposal facility.
These processes influence the gen-
eration and migration of leachate.
Some of the processes are relatively
easy to describe and predict; others
are poorly understood.
Because of the wide variety of wastes
placed in Subtitle D facilities and be-
cause of differences in other factors,
such as the operation, design, and
age of a facility, it is very difficult to
predict the release rate of leachate
generated at a particular disposal fa-
cility. Release rate estimation requires
the determination of both the con-
taminant concentration in the leachate
and the volumetric flux of leachate.
Once released to the subsurface,
leachate migration depends on fac-
tors such as the liquid nature of the
j/y.aste, .the chemical properties of the
leachate constituents, the loading rate,
the climate, and the subsurface hy-
drogeologic properties. Complex in-
teractions among physical, chemical,
and biological processes control mi-
gration.
Revisions to the Solid Waste Disposal
Facility Criteria create a need for mod-
eling tools to predict the performance
of facilities. The models will be used
primarily to demonstrate that alterna-
tive designs will satisfy performance
standards and to show that a particu-
lar design or hydrogeologic setting
ensures that there will be no potential
for migration of contaminants from the
facility to the uppermost aquifer dur-
ing the active life of the unit.
Numerous existing models could be
applied to the simulation of leachate
generation and migration. These mod-
els represent a wide range in com-
plexity, from simple analytical solu-
tions that can be solved with a calcu-
lator to complex numerical models that
require a large computer.
Mathematical models can provide a
useful means of understanding and
predicting the effects of waste dis-
posal facilities on the subsurface en-
vironment. No individual model ex-
ists, however, that can simulate all of
the processes affecting leachate gen-
eration and migration. In some cases,
it is beneficial to link or couple "stand-
alone" codes to produce more accu-
rate representation of a specific facil-
ity.
The use of mathematical models re-
quires simplified representations of re-
ality. Errors are introduced because
of the use of assumptions and simpli-
fications, a lack of data, and a poor
understanding of some processes in-
fluencing the transport and transfor-
mation of contaminants. Normally, it
is difficult to quantify the effects of
these errors.
It is impossible to develop a set of
discrete "scenarios" from which a mod-
eler could confidently select when ap-
plying a model to a specific site. Each
site must be described individually.
Many factors must be considered
when characterizing a site.
Calibration, verification and validation
are critical aspects of the develop-
ment and application of models. They
are particularly important for site-spe-
cific studies.
•6CV.S. GOVERNMENT PRINTING OFFICE: »»3 - 7iMW»7I/g0074
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Constance L. Travers and Susan Sharp-Hansen are with AQUA TERRA Consult-
ants, Mountain View, CA 94043
Gerard F. Laniak is the EPA Project Officer (see below).
The complete report, entitled "Leachate Generation and Migration at Subtitle D
Facilities: A Summary and Review of Processes and Mathematical Models,"
(OrderNo. PB93-217784AS; Cost: $27.00; 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:
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
EPA/600/SR-93/125
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
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