Center for Exposure Assessment Modeling (CEAM) (EPA)
The Multimedia Exposure Assessment Model (MULTIMED) for exposure assessment simulates the movement of contaminants leaching from a waste disposal facility. The model consists of a number of modules which predict concentrations at a receptor due to transport in the subsurface, surface water, or air. When applying MULTIMED to Subtitle D facilities, the landfill, surface water, and air modules in the model are not accessible by the user; only flow and transport through the unsaturated zone and transport in saturated zone can be considered. A 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 simulates transient, one-dimensional (vertical) transport in the unsaturated zone using either an analytical model that includes the effects of longitudinal dispersion, linear adsorption, and first-order decay or a numerical model that includes the effects of longitudinal dispersion, non-linear adsorption, first-order decay, time variable infiltration rates, and arbitrary initial conditions of chemical concentration in the unsaturated zone. The unsaturated zone transport module calculates steady-state or transient contaminant concentrations. Output from both unsaturated zone modules is used to couple the unsaturated zone transport module with the steady-state or transient, semi-analytical saturated zone transport module. The latter includes one-dimensional uniform flow, three-dimensional dispersion, linear adsorption, first-order decay, and dilution due to direct infiltration into the groundwater plume. The fate of contaminants in the various media depends on the chemical properties of the contaminants as well as a number of media- and environment-specific parameters. The uncertainty in these parameters can be quantified in MULTIMED using the Monte Carlo simulation technique. To enhance the user-friendly nature of MULTIMED, a preprocessor, PREMED, and a postprocessor, POSTMED, have been developed. The preprocessor guides the user in the creation of a correct Subtitle D input file by restricting certain options and parameters and by settingappropriate defaults.
The operation of each module requires specific input, which is organized into data groups. The General Data Group, which is required for all simulations, contains flags and data which describe the scenario being modeled. The input parameters needed for the Saturated Zone Transport Model are arranged in three additional data groups: the Chemical Data Group, the Source Data Group, and the Aquifer Data Group. Use of the Unsaturated Zone Modules requires input found in the above data groups, as well as data from the Unsaturated Zone Flow Data Group and the Unsaturated Zone Transport Data Group. The MULTIMED manual provides help in estimating the model input parameters (Salhotra et al. 1995., Sharp-Hansen et al. 1995).
The MULTIMED manual should be consulted for the output of the different model modules. The POSTMED postprocessor can be used to generate three types of plots; concentration vs. time at a groundwater receptor, cumulative frequency, and frequency or probability density. The cumulative frequency and frequency plots are related to model parameters that are randomly varied within the context of a Monte Carlo simulation.
Assumptions and Limitations
At this time, the air modules of the model are not linked to the other model modules. As a result, the estimated release of contaminants to the air is independent of the estimated contaminant releases to surface and subsurface waters. The simplifying assumptions required to obtain the analytical solutions for the equations used in MULTIMED limit the complexity of the systems that can be modeled. Accordingly, MULTIMED cannot be used to account for site-specific spatial variability or boundary conditions, landfill shape, multiple aquifers and pumping wells, flow in fractures, or chemical reactions between reactants. As a result, MULTIMED should be used only as a screening level tool when applied to complex sites.
MULTIMED was developed primarily for, and has seen extensive application in, predicting leachate movement from a Subtitle D (hazardous waste) landfill. This type of application, however, only utilizes a subset of MULTIMED's full capabilities. When MULTIMED has been used in conjunction with a separate source model, such as HELP (Schroeder, A.C., A.C. Gibson, and M.D. Smolen. 1984), it has been applied to a much larger range of scenarios. Such scenarios may include development and comparison of the effects of different facility designs on ground water quality, prediction of the results of different types of "failure" of a landfill, and to address questions related to appropriate clean-up levels for contaminated soils.
The MULTIMED model, version 2.00 Beta, has under gone a series of tests to verify the correctness of the model. Discussion of these tests and related model application considerations are documented in a report (USEPA 1995) that is included with the distribution release of the MULTIMED model system.
Source of the Abstract: "Readme" directory contained in INSTALMM.EXE
Sharp-Hansen, S., C. Travers, P. Hummel, T. Allison, R. Johns, and W.B. Mills. 1995. A Subtitle D Landfill Application Manual for the Multimedia Exposure Assessment Model (MULTIMED 2.0). U.S. EPA Environmental Protection Agency, Athens, GA. Unpublished Report.
USEPA. 1995. Revised Verification Testing of the Enhancements, MULTIMED Model (2.0). U.S. EPA Environmental Protection Agency, Athens, GA. Unpublished Report.
Schroeder, A.C., A.C. Gibson, and M.D. Smolen. 1984. The Hydrologic Evaluation of Landfill Performance (HELP) Model, Volumes I and II. EPA/530/SW-009 and EPA/530/SW-010, U.S. EPA, Cincinnati, Ohio, 45268.