In recent years, The U.S. Environmental Protection Agency (EPA)
has employed groundwater contaminant fate and transport models to
set chemical-specific concentration limits that dictate how solid
wastes are managed. One such model, the EPA Composite Model for
Landfills (EPACML) (Woodward-Clyde) was first published in 1990 and
enhanced in 1992. EPA used EPACML to set Toxicity Characteristic
(TC) concentration limits which determine whether a waste is
hazardous under the Resource Conservation and Recovery Act.
A key component of the proposed 1992 Hazardous Waste
Identification Rule (since withdrawn, but expected to be
re-proposed) was to establish regulatory levels (i.e.,
constituent-specific health-based levels multiplied by a
dilution/attenuation factor (DAF)) to be used as a threshold for
excluding wastes from hazardous waste regulation. EPACML was used
to determine appropriate DAFs under an infinite waste source
EPA recognizes that the infinite source scenario may not be
appropriate for groups of chemicals which are either disposed of in
small quantities or have high leaching potentials. As a result, EPA
modified the source term in the original EPACML model with a
finite, constant concentration source.
API performed an initial review (API, 1992) of this enhanced
EPACML model that indicated flaws in the finite source methodology.
This initial review also suggested that the model had potential
problems with conserving mass.
To confirm the results of the initial review, API undertook a
more rigorous analysis of EPACML. The objective of the analysis,
documented in this report, is to provide technical information on
the model's problems, and where possible, to propose some
alternative solutions to EPA and the regulated community affected
by the use of the model. Specifically, this report documents a
technical evaluation of the finite source methodology and the
analytical solutions that comprise the enhanced EPACML model. The
report contains an overview of the following model components:
• Unsaturated zone transport module
• Finite source methodology
• Saturated zone transport module
• Coupling of the unsaturated zone and saturated zone transport
• Estimation of retardation factors and decay coefficients
Each overview is immediately followed by a critique of the
component. Flaws in the model are demonstrated with model runs and
simple calculations where possible.
GENERAL DESCRIPTION OF THE EPACML MODEL
The model consists of two major parts: (1) unsaturated transport
simulator, and (2) saturated transport simulator. Operated in the
deterministic mode, the model calculates a constituent
concentration at a designated drinking water (receptor) location
for a given set of input parameters. To obtain a statistical
distribution of possible receptor well concentrations for ranges of
waste, soil, aquifer properties, the model utilizes a Monte Carlo
method, which randomly generates a large number of possible
combinations of these parameters. For each set of these properties,
the model calculates the ratio of the source concentration to the
receptor concentration (dilution/attenuation factor - DAF).
Finally, a cumulative probability distribution for DAF is estimated
based on the results from the Monte Carlo simulations. This
cumulative distribution is subsequently used to estimate an
allowable leachate concentration. In order to obtain a realistic
cumulative probability distribution for DAF, it is important to
ascertain whether the deterministic part of the model, i.e. the
part that calculates the receptor concentration for each Monte
Carlo-generated set of input data, is correct.
This report focuses on the deterministic part of EPACML. Each
major component of the EPACML model is described in the report,
followed immediately by a critical analysis of the component.