USC Component of PICS Activities:
Modeling and Field Studies.

The Savannah River Site (SRS) has a number of waste sites that will require groundwater modeling to fulfill regulatory requirements. Three particular waste sites, for which a substantial groundwater modeling effort has now been extended, have been selected for use with the PICS GCT code series. These waste sites are the F-Area and H-Area Seepage Basins and the Old Burial Ground, all of which are in the General Seperations Area of the Savannah River Site.

The Seepage Basins received low-level liquid radioactive waste until 1988. The liquid waste discharged to the F\&H Area Basins was acidic (pH 3-4) and the predominant contaminants were tritiated water (tritium), nitrate (from nitric acid), and some heavy metals such as lead and cadmium. The Old Burial Ground (OBG) was used from the middle 1950's until 1972 for the disposal of solid waste. The predominant contaminants that emanate from the OBG are tritium and volatile organic compounds, and to a lesser extent heavy metals. The following image and QuickTime movie are rendered from a simulation of hydraulic pressure with more than one million computational cells using 128 processors on ORNL's xps35.

The pressures are color-coded according to values with blue denoting negative pressures in the vadose zone, and red corresponding to the highest pressures in the saturated zone.

Each site presents its own engineering and scientific, as well as computational challenges. At the Seepage Basin Sites, the pH of the groundwater is becoming more neutral as the system is naturally flushed by rainwater recharge, requiring geochemical reactions to be accurately accomodated for the effectively simulation of the movement of the heavy metals in the subsurface. At the OBG it is important to simulate the mass transfer of tritium from the solid phase to the groundwater in the vadose zone in order to effectively simulate the system. Representative of the complexity of the heterogeneities of the subsurface environment at OBG is the following three-dimensional geophysical interpretation of mud fraction, defined as clay and silt (as opposed to sand and gravel):

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