Distribution and Range of Radionuclide Sorption Coefficients in a Savannah River Site Subsurface: Stochastic Modeling Considerations - 10259

The uncertainty associated with the sorption coefficient, or Kd value, is one of the key uncertainties in estimating risk associated with burying low-level nuclear waste in the subsurface. The objective of this study was to measure >648 Kd values and provide a measure of the range and distribution (normal or log-normal) of radionuclide Kd values appropriate for the E-Area disposal site, within the Savannah River Site, near Aiken South Carolina. The 95% confidence level for the mean Kd was twice the mean in the Aquifer Zone (18–30.5 m depth), equal to the mean for the Upper Vadose Zone (3.3 – 10 m depth), and half the mean for the Lower Vadose Zone (3.10 – 18 m depth). The distribution of Kd values was log normal in the Upper Vadose Zone and Aquifer Zone, and normal in the Lower Vadose Zone. To our knowledge, this is the first report of natural radionuclide Kd variability in the literature. Using ranges and distribution coefficients that are specific to the hydrostratigraphic unit improved model accuracy and reduced model uncertainty. Unfortunately, extension of these conclusions to other sites is likely not appropriate given that each site has its own sources of hydrogeological variability. However, this study provides one of the first examples of the development stochastic ranges and distributions of Kd values for a hydrological unit for stochastic modeling.