2.1 Contaminants

As is the case for MEPAS in general, the source-term release module can accommodate source zones containing both radioactive and hazardous chemical pollutants.  The hazardous chemicals can be either organic or inorganic compounds.  MEPAS and RAAS, through their user interfaces, have the ability to access databases of physicochemical and exposure-related properties for a wide variety of contaminants.  These databases are augmentations of the database described by Strenge and Peterson (1989), which was originally developed for an earlier version of MEPAS.  The RAAS contaminant database currently contains 409 contaminants categorized into 14 different classes.  Currently, when using RAAS Version 1.1, source-term release calculations can only be performed for contaminants that are in the database.  However, when using MEPAS Version 3.2, source-term release calculations can be performed for contaminants currently in the MEPAS database, plus any contaminants for which the user can supply the required physical and chemical input parameters.

 The source-term release module requires certain contaminant-specific properties (e.g., aqueous solubility, saturated vapor pressure, Henry's Law constant, inorganic or nuclide sorption coefficient, organic carbon partition coefficient, octanol-water partition coefficient, decay/degradation half life, diffusion coefficient in air, and effective diffusion coefficient in grout) as input parameters if loss fluxes are to be calculated by the models implemented in the module.  Exactly which of these properties are required for a given run depends on the contaminant as well as the type of contaminant source zone being analyzed and the types of release pathways being considered.  The source-term release module and the contaminant property databases have been developed in unison so that the mathematical models implemented in the module use the kind of contaminant information available in the databases.  When a contaminant is chosen for inclusion in the analyses, default values for all of the relevant contaminant-specific properties are automatically selected for input parameters.  However, the user also has the option of changing the values of some of these parameters if there are more appropriate data available for the specific scenario under analysis.  (These contaminant property input parameters are included in the Appendix A listing.)

 Half lives for radionuclides are single-valued and well known.  Values of first-order decay half lives and first-order decay rate coefficients for radionuclides are present in the databases.  Many contaminants other than radionuclides can also be transformed into other compounds in real-world environments (e.g., organic chemicals undergoing biodegradation).  However, the degradation process does not necessarily follow first-order kinetics; nor does it necessarily have a unique half life.  Describing the process with a first-order equation involves defining an "effective" first-order decay coefficient or half life that could vary with the microbial population size, the redox potential of the environment, and the concentrations of other substances needed by the microbes (e.g., carbon sources, nutrients, and electron acceptors).  The MEPAS contaminant property database assumes that first-order decay coefficients for nonradionuclides are zero.  On the other hand, the RAAS database contains degradation half life values applicable to biodegradation in a "reference" environment.  However, if users have a specific value for the half life that they wish to use, they can override the default values in the databases.