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 zone’s 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

• simulate contaminant migration for both chemicals and radionuclides through partially saturated porous media (i.e., vadose zones) and provide output consisting of instantaneous, time-varying, contaminant mass fluxes exiting the vadose zone
• have no limits on the number of constituents considered in a scenario
• operate under Windows 95 and have a user-friendly MUI with a standard Windows look and feel
• meet the module specifications for FRAMES.

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 zone’s physical characteristics and the contaminant’s distribution coefficient (Kd) are obtained from the user through the MUI. The contaminant’s chemical properties are obtained from a chemical property database. The boundary conditions are communicatedto the module through the Water Flux File (WFF). The vadose zone’s 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 MUI will operate in Windows 95 and will have a standard Windows look and feel.
• The MUI will have on-line help in an HTML format that provides users with an easy-to-understand description of all input parameters required by the MUI.
• The MUI will provide users with a choice of units for all input parameters having dimensions associated with them.
• The MUI will include a reference feature in which the source of the specified value for each input item can be referenced if the user desires.
• The MUI will show the range of values allowed for each input data item, when the cursor is positioned on that item, as a scrolling message at the bottom of the screen. When an out-of-range value is entered in a field, the MUI will indicate this by a red background in the input field and a scrolling error message in addition to the allowed range message. Data input values within range are shown with a green field background.
• The MUI will display the module version number, obtained from the module description (DES) file, in an About menu.

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:

• time-varying, instantaneous, contaminant mass (or activity) fluxes entering the vadose zone (required)
• width and length of the interface plane through which contaminant enters the vadose zone (required)
• distance from water table
• natural recharge rate
• time-varying water flux entering the vadose zone (required).

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

• vadose zone thickness (i.e., transport distance)
• total porosity
• field capacity
• dry bulk density
• soil type coefficient
• saturated hydraulic conductivity
• longitudinal dispersivity
• constituent distribution coefficient (Kd).

The MEPAS vadose zone transport module obtains the following data from the chemical property portion of the GID file:

• CASID (Chemical Abstracts Service ID Number)
• decay/degradation half-life
• decay chain (for radionuclides)
• solubility limit
• carbon-matter partition coefficient (Koc) (for organics).

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:

• percentage of sand
• percentage of silt
• percentage of clay
• percentage of organic matter
• percentage of iron and aluminum
• pH of the pore water
• carbon-matter partition coefficient (obtained from the chemical database).

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:

• time-varying, instantaneous, contaminant mass (or activity) fluxes exiting the vadose zone
• width and length of the interface plane through which the contaminant exits the vadose zone
• distance from water table
• natural recharge rate
• time-varying water flux exiting the vadose zone.

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

• obey the Law of Mass Conservation
• simulate advection in one dimension
• simulate dispersion along the flow direction
• account for the decay of radionuclides and handle the degradation of chemicals
• account for the ingrowth of progeny resulting from degradation/decay, and handle decay chains with up to 9 members (i.e., one parent and eight progeny)
• compute contaminant mass (or activity) fluxes exiting the vadose zone
• determine a moisture content for the vadose zone that is consistent with the infiltration rate of water entering the vadose zone and the vadose zone’s physical properties - However, if the computed moisture content is less than the field capacity, use the field capacity as the moisture content.
• account for the effect that adsorption of contaminant to soil particles has on travel time through the vadose zone.

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.