PNNL-SA-32286

Written by: John P. McDonald

Prepared for the U.S. Department of Energy

Pacific Northwest National Laboratory
Operated by the U.S. Department of Energy
By Battelle




Requirements for the MEPAS Vadose Zone Transport Module

1.0 Introduction

This document describes the requirements for the Multimedia Environmental Pollutant Assessment System (MEPAS) vadose zone transport module. The module is designed specifically as an object for inclusion into the Framework for Risk Analysis in Multimedia Environmental Systems (FRAMES), which is a platform to link various multimedia modules into complete 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 Vadose Zone Transport Module

The purpose of the MEPAS vadose zone transport module is to simulate the migration and fate of chemical and radionuclide constituents through partially saturated porous media (i.e., vadose zones). Input to the module consists of time-varying, contaminant mass fluxes entering the vadose zone, the physical characteristics of the vadose zone itself, and the constituent properties.  Output consists of time-varying, contaminant mass fluxes exiting the vadose zone. Currently, mass fluxes entering the vadose zone can originate from a source term or from a previous vadose zone. The vadose zoneís physical characteristics are entered through a module user interface (MUI), and constituent properties are obtained from a constituent properties database. The contaminant mass fluxes exiting the vadose zone can be used as input to another vadose zone or to an aquifer module, or can also serve as the endpoint of the simulation. Contaminant concentrations in the vadose zone can be obtained by dividing the output contaminant mass fluxes by the water flux through the vadose zone.

3.0 Summary of Requirements for the MEPAS Vadose Zone Transport Module

This section provides an overall summary of the requirements for the MEPAS vadose zone transport module. Detailed input, output, and scientific requirements are described in Sections 4, 5 and 6 respectively.

The MEPAS vadose zone transport module will

4.0 Input Requirements for the MEPAS Vadose Zone Transport Module

Data needed to simulate contaminant migration through a vadose zone vadose zone is obtained from three sources. The boundary conditions (i.e., time-varying contaminant mass fluxes entering the vadose zone, infiltration rate, and source dimensions) are obtained from the previous module (i.e., source term or vadose zone), the vadose zoneís physical characteristics and the contaminantís distribution coefficient (Kd) are obtained from the user through the MUI. The contaminantís chemical properties are obtained from a chemical property database. The boundary conditions are communicatedto the module through the Water Flux File (WFF). The vadose zoneís physical characteristics are communicated from the MUI to the model through the Global Input Data (GID) file and the model pre-processor. The GID file also is used to store the contaminant chemical property data.

There are some general requirements associated with the MUI, which are

The following data are obtained from the WFF by the MEPAS vadose zone transportmodule. 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:

The following data are obtained from the MUI and are needed by the MEPAS vadose zone transport module to perform its computations:

The MEPAS vadose zone transport module obtains the following data from the chemical property portion 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 provides the user with options to automatically use the estimated Kd values for single constituents or automatically use estimates for all constituents.

5.0 Output Requirements for the MEPAS Vadose Zone Transport Module

The MEPAS vadose zone transport module is required to output its results to a WFF. The module is also required to produce a list 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 or decay constant) and the simulation results (peak flux and time of peak).

The following data are output to the WFF:

6.0 Scientific Requirements for the MEPAS Vadose Zone Transport Module

This section describes the scientific requirements for the MEPAS vadose 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).

7.0 References

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.