1.0 Introduction

The Multimedia Environmental Pollutant Assessment System (MEPAS) is a physics-based tool for simulating contaminant source release, environmental transport and transformation, human exposure and dose, and risk in a multimedia environmental system. In the environmental scenarios that MEPAS can address, contamination can move through the environment along several different transport pathways (i.e., vadose zone/groundwater pathway, overland flow/surface water pathway, and atmospheric pathway). MEPAS consists of a collection of linked computer modules (i.e., computer codes for simulating one or more of the aforementioned processes) controlled by a user-friendly interface. The material in this report is documentation for the MEPAS Version 3.2 and the Remedial Action Assessment System (RAAS) Version 1.1.

The MEPAS source-term release module documented in this report includes models for estimating contaminant loss from three different types of source zones: contaminated aquifer, contaminated pond/surface impoundment, and contaminated vadose zone. In all types of contaminant source zones, it is assumed that the contaminants may be present in multiple phases (i.e., in aqueous solution, sorbed to solid particles, in vapor-filled pore space, or in a separate non-aqueous-phase liquid [NAPL] that is immiscible with water and air). The source-term release module contains models to determine which contaminants in the problem scenario will partition into which phases, and to what degree. Depending on the type of source zone, it is assumed that the processes causing contaminant release from the source zone to an environmental transport pathway could be leaching (groundwater pathway), wind suspension (atmospheric pathway), water erosion (overland/surface water pathway), overland flow (overland/surface water pathway), and/or volatilization (atmospheric pathway). In addition, contaminants could be lost from the source zone by radioactive decay or degradation within the zone.

The source-term release module contains models for simulating "baseline" (i.e., unremediated) scenarios for all three types of source zones. These same models can be used to simulate remediated sites (for all three types of source zones) if the remediation methodology can be represented merely by changing the magnitude of certain model parameters from their "baseline" values. In addition, for the vadose zone source zone, the source-term release module contains models that can calculate loss fluxes from vadose zones subjected to two types of remediation methodologies that require special theory: in situ vitrification (ISV) and in situ solidification (ISS).

The ultimate objective of the source-term release component of MEPAS is to calculate a "mass-budgeted" set of contaminant flux versus time functions that describe the simultaneous loss of contaminants from the source zone to a number of different routes. When multiple loss processes are assumed to occur simultaneously, the mathematical models implemented in the source-term release module account for their synergistic or antagonistic interaction. "Mass-budgeted" means that the inventory of each contaminant is apportioned to each loss route (by the implemented physics-based theory) in a manner such that the sum of the calculated contaminant losses to all routes is limited to 100% of the initial inventory.

The source-term release module has been developed to allow the user flexibility in how the contaminant release to different routes is simulated. If the user wishes to explicitly enter the contaminant flux to one or more loss routes (based on known or assumed data, or the predictions of some other model), the source-term release component will use the supplied flux versus time function(s) until mass balance considerations dictate that the flux must go to zero (because cumulative contaminant mass loss to all routes has removed 100% of the initial inventory). If the user cannot or does not wish to supply explicit contaminant fluxes to any or all loss routes, the source-term release component contains models that will calculate the flux verses time functions (also according to mass balance considerations). If the user wants to analyze a scenario that assumes contaminant release to only a subset of the possible loss routes for a given type of source zone, loss to undesired routes can be "turned off" by specifying a known flux of zero to those routes.

Section 2.0 of this report describes some basic attributes of the source-term release module that are applicable to any of the three types of contaminant source zones simulated by it. These include discussions of contaminant phase partitioning theory and the general mathematical approach to describing contaminant mass loss rates.

Sections 3.0, 4.0, and 5.0 give details of the specific approaches for addressing contaminated aquifer, pond/surface impoundment, and vadose zone source zones, respectively. Each of these sections contains a description of: the conceptual model assumed for that source zone, the mathematical models that lead to the contaminant flux expressions (if the user allows MEPAS to compute the release functions), the numerical solution procedure that produces a set of "mass-budgeted" release fluxes for each contaminant, analytical solutions for certain special cases (i.e., simplified scenarios) that could be used to verify the source-term release module outputs, methods already implemented in the module for calculating certain parameters in the contaminant flux expressions, and how to use user-supplied flux versus time functions for one or more loss routes (if the user does not want MEPAS to compute the release functions). The theory presented for the contaminated aquifer and pond/surface impoundment source zones is relatively straightforward because no synergistic or antagonistic interactions between loss processes occur, and because the source zone dimensions remain constant. On the other hand, the theory presented for the contaminated vadose zone source zone is relatively complex because interactions between loss processes do occur, and because the loss processes may cause the dimensions of the source zone to change in several ways. Furthermore, auxiliary theory implemented in the module to calculate selected parameters used in the vadose zone mass loss equations is relatively extensive.

Section 6.0 contains a brief summary of the module and its current limitations, and a discussion of plans for further development. Section 7.0 is a list of all symbols used in this report, along with their definitions and their associated units. Section 8.0 lists all references cited in the report. Finally, a complete listing of all the input parameters required by the source-term release module, along with the necessary units, is given in Appendix A.