Summary

Groundwater modeling was performed to assess groundwater flow and contaminant migration for a tritium plume at the Savannah River Site (SRS). The study supports the Corrective Measures Study and Interim Action Plan regulatory documents for the Old Radioactive Waste Burial Ground (ORWBG). Modeling scenarios were designed to provide data for an economic analysis of alternatives, and subsequently evaluate the effectiveness of the selected remedial technologies for tritium reduction to surface waters. Scenarios assessed include no action, vertical and surface barriers, pump-treat-reinject, and vertical recirculation wells.

Hydrostratigraphic units in the area consist of fluvial, deltaic, and shallow marine sand, mud, and calcareous sediments that exhibit abrupt facies changes over short distances. The complex heterogeneity of the sediments, along with characterization data, and tritium contaminant source data required a three-dimensional model be developed in order to accurately illustrate the size, shape and orientation of the plume. Results demonstrate that the shallow confining zone in the region controls the migration path of the plume. The size and shape of the plume were modeled in three-dimensions using detailed core, geophysical and cone-penetrometer data, depth-discrete contaminant data, monitoring well data, and seepline/surface water samples. Three-dimensional tritium plume maps were created for the >20,000, >500 and >50 pCi/ml concentration levels. The three-dimensional plume maps and volumetric calculations indicate that 63 percent of the total activity and 12 percent of the volume above 50 pCi/ml resides in a layer less than 6-m thick riding on top of the shallow confining zone.

The unique characteristics of this plume are important remedial design parameters. Computer simulation and analysis were performed on the selected remedial technologies for preliminary design input. Key design parameters were compiled for input to an economic analysis of alternatives. Example parameters include the number, location, and pumping rates of extraction wells. Each design was simulated using the FACT (Flow And Contaminant Transport) finite-element groundwater flow and contaminant transport code. The reduction in tritium discharge to surface waters that would result from each evaluated technology was projected.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah