United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens, GA 30605-2720
Research and Development
EPA/600/SR-93/081 June 1993
Project Summary
MULTIMED, the Multimedia
Exposure Assessment Model for
Evaluating the Land Disposal of
Wastes-Model Theory
Atul M. Salhotra, Philip Mineart, Susan Sharp-Hansen, and Terry L. Allison
The MULTIMED computer model
simulates the transport and transfor-
mation of contaminants released from
a hazardous waste disposal facility into
the multimedia environment. Release
to air and soil, including the unsatur-
ated and saturated zones, and possible
interception of the subsurface contami-
nant plume by a surface stream is rep-
resented by the model. The model fur-
ther simulates contaminant movement
through the air, soil, ground water, and
surface water media to humans and
other potentially affected species.
MULTIMED is intended for general ex-
posure and risk assessments of waste
facilities and for analyses of the im-
pacts of engineering and management
controls. This report provides the con-
ceptual and theoretical details of the
various modules and the Monte Carlo
simulation technique.
This Project Summary was devel-
oped 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
The Multimedia Exposure Assessment
Model (MULTIMED) simulates the move-
ment of contaminants leaching from a
waste disposal facility. The model includes
two options for simulating leachate flux.
Either the infiltration rate to the unsatur-
ated or saturated zone can be specified
directly or a landfill module can be used
to estimate the infiltration rate. The landfill
module is one-dimensional and steady-
state, and simulates the effect of precipi-
tation, runoff, infiltration, evapotranspira-
tion, barrier layers (which can include flex-
ible membrane liners), and lateral drain-
age.
A steady-state, one-dimensional, semi-
analytical module simulates flow in the
unsaturated zone. The output from this
module, water saturation as a function of
depth, is used as input to the unsaturated
zone transport module. The latter simu-
lates transient, one-dimensional (vertical)
transport in the unsaturated zone and in-
cludes the effects of longitudinal disper-
sion, linear adsorption, and first-order de-
cay.
Output from the unsaturated zone mod-
ules—that is, steady-state or time series
contaminant concentrations at the water
table—is used to couple the unsaturated
zone module with the steady-state or tran-
sient, semi-analytical saturated zone trans-
port module. The latter includes one-di-
mensional uniform flow, three-dimensional
dispersion, linear adsorption, first-order de-
cay, and dilution due to direct infiltration
into the ground-water plume.
Contamination of a surface stream due
to the complete interception of a steady-
state saturated zone plume is simulated
by the surface water module. Finally, the
air emissions and the atmosphere disper-
sion modules simulate the movement of
chemicals into the atmosphere.
At this time, the air modules of the
model are not linked to the other model
modules. As a result, the estimated re-
lease of contaminants to the air is inde-
pendent of the estimated contaminant re-
lease to the subsurface and surface wa-
ter.
^,A) Printed on Recycled Paper
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The fate of contaminants in the various
media depends on the chemical proper-
ties of the contaminants as well as a num-
ber of media- and environment-specific
parameters. The uncertainty in these pa-
rameters is quantified using the Monte
Carlo simulation technique.
To enhance the user-friendly nature of
the model, separate interactive pre- and
postprocessing software have been de-
veloped for use in creating and editing
input and n plotting model output.
MULTIMED uses analytical and semi-
analytical solution techniques to solve the
mathematical equations describing flow
and transport. The simplifying assump-
tions required to obtain the analytical so-
lutions limit the complexity of the systems
that can be represented by MULTIMED.
The model does not account for site-
specific spatial variability, the shape of
the land disposal facility, site-specific
boundary conditions, multiple aquifers or
pumping wells. Nor can MULTIMED simu-
late some processes, such as flow in frac-
tures and chemical reactions between con-
taminants, that can have a significant ef-
fect on the concentration of contaminants
at a site. In more complex systems, it may
be beneficial to use MULTIMED as a
"screening level" model, which would al-
low a user to obtain an understanding of a
system of interest. A numerical model
could then be used if sufficient input data
are available and if decision makers re-
quire the detailed information available
from a more' complex model.
Physical Scenario
The physical scenario simulated by the
model is a land disposal facility that re-
leases pollutants into the air, soil, and/or
ground water. In response to a number of
complex physical, chemical, and biologi-
cal processes, pollutants move in the mul-
timedia environment, resulting in potential
toxic exposure to humans and other re-
ceptors.
In the model, the processes affecting
air emissions are not linked to the pro-
cesses affecting subsurface transport In
other words, the concentration calculated
m the one medium is not affected by the
release of the contaminant to the other
medium.
The sources of pollutants considered in
the initial version of the model are either
leachate from a waste disposal facility or
air emissions during the post-closure pe-
riod. Inadequate long-term functioning or
failure of the facility's engineering controls
(i.e., caps and liners) are assumed to oc-
cur after closure and to result in the re-
lease of leachate to soil or ground water
beneath the facility and emission of vapor
to the atmosphere. Note that the use of
the air emission module is most appropri-
ate for high concentrations of waste in the
facility. Also, the model does not include
fate processes that affect metals, such as
complexation and solids precipitation.
Model Capabilities
During the course of model develop-
ment, emphasis was placed on the cre-
ation of a unified, user-friendly software
framework, with the capability of perform-
ing uncertainty analysis, that can easily
be enhanced by adding modules or modi-
fying existing modules.
The transport and transformation of con-
taminants critically depend on a number
of media-specific parameters. Typically
many of these parameters exhibit spatial
and temporal variability as well as vari-
ability due to measurement errors
MULTIMED can analyze the impact of un-
certainty and variability in the model in-
puts on the model outputs (concentrations
at specified points in the multimedia envi-
ronment), using the Monte Carlo simula-
tion technique.
To enhance the user-friendly nature of
the model, separate interactive preprocess-
ing and postprocessing software have
been developed, using the ANNIE Inter-
action Development Environment (AIDE)
to create and edit input and to plot model
output. The pre- and postprocessors have
not been integrated with MULTIMED be-
cause of the size limitations of desktop
computers. Therefore, after using the pre-
processor to create or modify input the
model is run in batch mode. Afterwards
the postprocessor can be used to pro-
duce plots of the Monte Carlo output or
concentration versus time.
Finally, model results can be used
manually to "back-calculate" the maximum
source concentration (for a steady-state
infinite contaminant source) of a chemicai
that would ensure protection of human
health or the environment at a down-gra-
dient point of exposure.
Obtaining MULTIMED Software
The MULTIMED computer code may
be obtained by sending a request to Model
Distribution Coordinator, Center for Expo"-
sure Assessment Modeling, Environmen-
tal Research Laboratory, U.S. Environmen-
tal Protection Agency, Athens GA 30605-
2720. Please include either five 5 25-in
(double-sided, double density, DS/DD
360KB) diskettes or two 3.5-in (double-
sided, high-density, DS/HD 1.44MB error
free) diskettes. The MULTIMED code will
be copied to your diskettes and returned
to you. You also may obtain model theory
documentation on diskette by sending two
3.5-in (DS/HD) diskettes to the modeling
center; hard copies may be obtained from
the National Technical Information Ser-
vice (see ordering information on the next
page).
•U.S. Government Printing Office: 1993 - 750-071/60256
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Atul M. Salhotra and Philip Mineari are with Woodward-Clyde Consultants, Oak-
land, CA; Susan Sharp-Hansen is with AQUA TERRA Consultants, Mountain
View, CA; and Terry L. Allison is with Computer Sciences Corp., Athens, GA.
Gerard F. Laniak is the EPA Project Officer (see below)
The complete report, entitled "MULTIMED, the Multimedia Exposure Assessment
Model for Evaluating the Land Disposal of Wastes-Model Theory," (Order No.
PB93-186 252/AS; 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
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use
$300
EPA/600/SR-93/081
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