Multimedia Exposure Assessment Model
MULTIMED, Multimedia Assessment Model, is a user-friendly model which simulates the fate and transport of contaminants leaching from a waste disposal facility into the multimedia environment. Release to either air or soil, including the unsaturated and saturated zone, and possible interception of the subsurface contaminant plume by a surface stream are included in the model. The model includes two options for simulating leachate flux. Either the infiltration rate to the unsaturated 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 precipitation, runoff, infiltration, evapotranspiration, barrier layers (which can include flexible membrane liners), and lateral drainage.
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 simulates transient, one-dimensional (vertical) transport in the unsaturated zone and includes the effects of longitudinal dispersion, linear adsorption, and first-order decay. Output from the 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. Contaminant of a surface stream due to the complete interception of a steady-state saturated zone plume is simulated by the surface water module. The air emissions and the atmosphere dispersion modules simulate the movement of chemicals into the atmosphere.
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 of these parameters is quantified using the Monte Carlo simulation technique.
Interactive pre and postprocessing modules are included for creating and editing input data and in plotting model output.
The major functions performed by the model include:
Depending on user-selected options:
- Allocation of default values to input parameters and variables.
- Reading of the input data files.
- Echo of input data to output files.
- Derivation of some user specified parameters.
Generation of random numbers for Monte Carlo simulations.
- Simulation of leachate flux emanating from the source.
- Simulation of unsaturated zone flow and transport.
- Simulation of saturated zone transport only.
- Computation of in-stream concentrations due to contaminant loading assuming complete interception of a plume in the saturated zone.
- Computation of the rate of contaminant emission from the waste disposal unit into the atmosphere.
- Simulation of dispersion of the contaminants into the atmosphere.
Performance of statistical analyses of Monte Carlo simulations.
Writing the concentrations at specified receptors to output for deterministic runs. In the Monte Carlo mode, writing the cumulative frequency distribution and selected percentiles of concentrations at receptors to output.
Printing the values of randomly generated input parameters and the computed concentration values for each Monte Carlo run.
Model results can be used to manually "back-calculate" the maximum source concentration (for a steady-state, infinite contaminant source) of a chemical which ensures protection of human health and/or the environment at a down-gradient point of exposure.
IBM-PC or compatible with 640K RAM, math coprocessor, and hard disk.
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Τελευταία Ενημέρωση 27 Ιουλίου 2004 - Last Revised on July 27th 20048
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