Requirements of 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 the sections that follow. 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). The MEPAS Vadose Zone Transport Module will

• G1 - 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
• G2 - have no limits on the number of constituents considered in a scenario
• G3 - operate under Windows 95 and have a user-friendly MUI with a standard Windows look and feel
• G4 - meet the module specifications for the Framework for Risk Analysis in Multimedia Environmental Systems (FRAMES).

Input Requirements of the MEPAS Vadose Zone Transport Module

Data needed to simulate contaminant migration through a vadose zone is obtained from three sources. The previous module (i.e., source term or vadose zone) provides boundary conditions (i.e., time-varying contaminant mass fluxes entering the vadose zone, infiltration rate, and source dimensions) through the Water Flux File (WFF). The user provides the vadose zone's physical characteristics and the contaminant's distribution coefficient (Kd) through the Module User Interface (MUI) via the Global Input Data (GID) file and model pre-processor. The chemical property database provides the contaminant's chemical properties via the GID file.

General requirements for the MUI are as follows:

• M1 - The MUI will operate in Windows 95, 98, 2000, NT, ME, and XP and will have a standard Windows look and feel.
• M2 - The MUI will have online help in an HTML format that provides users with an easy-to-understand description of all input parameters required.
• M3 - The MUI will provide users with a choice of units for all input parameters having dimensions associated with them.
• M4 - The MUI will include a reference feature in which the source of the specified value for each input item can be referenced if desired.
• M5 - 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 indicated by a green field background.
• M6 - 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 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:

• 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 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. (PNL-7145). 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 (M7) to automatically use the estimated Kd values for single constituents or automatically use estimates for all constituents.

Output Requirements of the MEPAS Vadose Zone Transport Module

The MEPAS Vadose zone transport module is required to provide the following output:

• O1 - results as a WFF
• O2 - list file (*.WLS file) that documents the data actually read in by the model and summarizes 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.

Scientific Requirements of the MEPAS Vadose Zone Transport Module

The primary scientific requirements for the MEPAS vadose zone transport module are as follows:

• S1 - obey the Law of Mass Conservation
• S2 - simulate advection in one dimension
• S3 - simulate dispersion along the flow direction
• S4 - account for the decay of radionuclides and handle the degradation of chemicals
• S5 - account for the ingrowth of progeny resulting from degradation/decay and handle decay chains with up to nine members (i.e., one parent and eight progeny)
• S6 - compute contaminant mass (or activity) fluxes exiting the vadose zone
• S7 - 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.
• S8 - account for the effect of adsorption of contaminant to soil particles on travel time through the vadose zone.

Other References

Whelan et al., 1997 (PNNL-11748)

Home | Security and Privacy | Contact Us