File: ABSTRACT.TXT MULTIMDP Model System Abstract Multimedia Exposure Assessment Model Fate of Transformation (Daughter) Products Center for Exposure Assessment Modeling (CEAM) National Exposure Research Laboratory - Ecosystems Research Division Office of Research and Development (ORD) U.S. Environmental Protection Agency (U.S. EPA) 960 College Station Road Athens, Georgia 30605-2700 706/355-8400 ----------------------------------------------------------------------------- Summary 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. 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. Modification The MULTIMED model has been modified to simulate the transport and fate of first- and second-generation transformation (daughter) products that migrate from a waste source through the unsaturated and saturated zones to a downgradient receptor well (5). For the unsaturated zone, solutions to the transformation product equations are obtained using Stehfest and De Hoog methods to numerically invert transformed solutions, convolution, and analytes solutions expressed in terms of exponential and complementary error functions. The analytical solution approach has been extended from the approach in CML to handle complex-valued arguments. Solutions expressed in terms of the convolution integral, while stable for parent compound equations, have exhibited unstable behavior for certain parameter ranges when the approach was applied to the transformation product equations. Guidelines are provided in the user manual to avoid this range of parameters. For the saturated zone, both steady-state and dynamic solutions have been developed, and presented in terms of infinite series. Asymptotic expansions are used to solve the dynamic equations in the Laplace domain. Numerical inversion is accomplished by Stehfest and De Hoog methods. The capability to Monte Carlo solutions has been extended to the unsaturated zone, as well as to the saturated zone. An algorithm has been included in the model to calculate, within the Monte Carlo framework, maximum concentrations to reach a receptor location. The algorithm has been modified to produce a better initial guess of the time to attain the maximum concentration. Data Requirements 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 (1,2). Output 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. Application History 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 (4), 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. Testing 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 (3) that is included with the distribution release of the MULTIMED model system. References 1. Salhotra, A.M., P. Mineart, S. Sharp-Hansen, T. Allison, R. Johns, and W.B. Mills. 1995. Multimedia Exposure Assessment Model (MULTIMED 2.0) for Evaluating the Land Disposal of Wastes--Model Theory. U.S. EPA Environmental Protection Agency, Athens, GA. Unpublished Report. 2. 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. 3. USEPA. 1995. Revised Verification Testing of the Enhancements, MULTIMED Model (2.0). U.S. EPA Environmental Protection Agency, Athens, GA. Unpublished Report. 4. 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. 5. Liu, S., W.B. Mills, and B. Baena. 1995. Multimedia Exposure Assessment Modeling Including Fate of Transformation Products (MULTIMED-DP 1.0): Implementation, Tests and Users' Manual. U.S. Environmental Protection Agency, Athens GA. Unpublished Report. -----------------------------------------------------------------------------