HYDROGEOLOGICAL CHARACTERIZATION AND 3D GEOSPATIAL MODELING OF A SUBSURFACE RADIONUCLIDE SOURCE AREA AT A US DOE SITE IN CENTRAL CALIFORNIA

Leaching mixed waste from unlined landfills at LLNL Site 300, a remote high-explosives test facility in the California Coast Range, resulted in release of tritium and uranium to the underlying soil, bedrock, and ground water. To assess the fate of chemicals already released and to properly design remedial alternatives to prevent future releases, we developed geospatial models to examine the distribution of contaminants residing in different geological media and to examine the impact of seasonal water table fluctuations over time. We used geospatial models as a hydrogeological analysis tool, for conducting internal review and design of remedial alternatives, for improving the understanding of the hydrogeological framework, for improved project planning, and to help communicate complex concepts to regulators and the public. We collected surface and subsurface lithology, borehole geophysics, hydraulic testing data, water level data, and chemistry data for tritium and uranium activity, metals, volatile organic compounds, and chlorine-36. Hydraulic parameters vary widely in the surrounding soil and rock due to weathering, fractures and joints, and heterogeneity of the material. The three-dimensional geospatial model incorporated surface and subsurface geologic data coupled with ground water and soil/rock chemistry as well as water level data, resulting in an integrated contaminant hydrogeologic model. We modeled the three-dimensional distribution of contaminants within the landfills, estimated the contaminant mass, and exported geospatial model data to build a numerical flow and transport model. An iterative process between the geospatial model and the numerical model was used to define initial and boundary conditions and to provide quality assurance.