This document describes the requirements for the Multimedia Environmental Pollutant Assessment System (MEPAS) saturated zone transport module. The module is specifically designed as an object for inclusion in the Framework for Risk Analysis in Multimedia Environmental Systems (FRAMES), which is a platform that allows linking of various multimedia modules into complete source/transport/exposure assessment systems (Whelan et al., 1997). These requirements can be used by software engineers and testers to ensure this module meets the needs of the clients and users.
2.0 Purpose of the MEPAS Saturated Zone Transport Module
The purpose of the MEPAS saturated zone transport module is to simulate the migration and fate of chemical and radionuclide constituents through saturated porous media (e.g., aquifers). Input to the module consists of time-varying contaminant mass (or activity) fluxes entering the aquifer, the physical characteristics of the aquifer itself, and the constituent properties. Output consists of time-varying contaminant mass fluxes exiting the aquifer or time-varying contaminant aqueous concentrations at a point within the aquifer. Currently, contaminant mass fluxes entering the aquifer can originate from a source term within the aquifer or from a vadose zone. The aquifer’s physical characteristics are entered through a module user interface (MUI), and constituent properties are obtained from a constituent property database. The output contaminant mass fluxes can be used as input to a river module, and contaminant aqueous concentrations can be used as input to an exposure module. In addition, the output contaminant mass fluxes or aqueous concentrations also can serve as the endpoint of the simulation.
3.0 Summary of Requirements for the MEPAS Saturated Zone Transport Module
This section provides an overall summary of the requirements for the MEPAS saturated zone transport module. Detailed input, output, and scientific requirements are described in Sections 4, 5 and 6.
The MEPAS saturated zone transport module will
4.0 Input Requirements for the MEPAS Saturated Zone Transport Module
Data needed to simulate contaminant migration through a saturated zone is obtained from three sources. The boundary conditions (i.e., time-varying, contaminant mass fluxes entering the aquifer, water flux from the previous medium or source term, and source dimensions) are obtained from the previous module (i.e., source term or vadose zone). The aquifer’s physical characteristics and the constituent’s distribution coefficient (Kd) are obtained from the user through the MUI. Constituent chemical properties are obtained from a chemical property database. The boundary conditions are communicated to the module through the Water Flux File (WFF). The aquifer’s physical characteristics are communicated from the MUI to the model through the Global Input Data file (GID) and the model’s pre-processor. The GID file is also used to store the chemical property data. The specifications for the WFF and GID file are described in Whelan et al. (1997).
There are some general requirements associated with the MUI, which are
The following data are obtained from the WFF by the MEPAS saturated zone transport module. Those items listed as required are needed by the module to perform its computations. Other items are read and simply reported to the output WFF, in order to meet the WFF specifications:
The following data are obtained from the MUI and are needed by the MEPAS saturated zone transport module to perform its computations:
The MEPAS saturated zone transport module obtains the following data from the chemical property section of the GID file:
The MUI provides users with an estimate of the Kd for each contaminant based on soil property data entered by the user. The Kd values are computed according to Strenge and Peterson (1989). The following soil property data are needed for this estimate:
In addition to allowing the user to enter their own Kd values, the MUI will provide the user with options to automatically use the estimated Kd value for single constituents or automatically use estimates for all constituents.
5.0 Output Requirements for the MEPAS Saturated Zone Transport Module
The MEPAS saturated zone transport module is required to output its results to a WFF (for contaminant mass flux results) or Water Concentration File (WCF) (for contaminant aqueous concentration results). The module is also required to produce a listing file (*.WLS file) that documents the data actually read in by the model and provides a summary of intermediate calculation results (e.g., retardation factor and decay constant) and the simulation results (peak flux or concentration and time of peak).
The following data are output to the WFF for contaminant mass flux results:
Data output to the WCF for contaminant aqueous concentration results includes instantaneous, time-varying, contaminant aqueous concentrations for each receptor locations in the aquifer.
6.0 Scientific Requirements for the MEPAS Saturated Zone Transport Module
This section describes the scientific requirements for the MEPAS saturated zone transport module. The primary scientific requirements are
The implementation of these requirements, in the form of mathematical formulations, are documented in Whelan et al. (1996). However, their document is out-of-date with respect to the effect of inflowing water from the vadose zone. The current technique to handle both near-field and far-field cases is described only in project quality assurance documentation.
Strenge, D. L., and S. R. Peterson. 1989. Chemical Data Bases for the Multimedia Environmental Pollutant Assessment System (MEPAS): Version 1. PNL-7145, Pacific Northwest Laboratory, Richland, Washington.
Whelan, G., J. P. McDonald, and C. Sato. 1996. Multimedia Environmental Pollutant Assessment System (MEPAS): Groundwater Pathway Formulations. PNNL-10907, Pacific Northwest National Laboratory, Richland, Washington.
Whelan G., K. J. Castleton, J. W. Buck, G. M. Gelston, B. L. Hoopes, M. A. Pelton, D. L. Strenge, and R. N. Kickert. 1997. Concepts of a Framework for Risk Analysis in Multimedia Environmental Systems (FRAMES). PNNL-11748, Pacific Northwest National Laboratory, Richland, Washington.