THREE-DIMENSIONAL MAPPING AND INVENTORY CALCULATIONS FOR CARBON TETRACHLORIDE IN GROUNDWATER AT THE HANFORD SITE, WASHINGTON STATE

Previous work on the carbon tetrachloride (CT) plume in the Hanford Site's 200 West Area suggested that the current location of most of the approximately 750,000 kg of CT that was disposed of during plutonium processing was unknown. Most earlier studies have assumed that the majority of CT in the unconfined aquifer resided within the upper few meters of the aquifer, and that it constituted only 2% of the original mass of CT disposed of at the Site. However, recent drilling data showed that CT exists at high concentrations at significant depths within the aquifer. In order to re-evaluate the inventory of CT in the unconfined aquifer we performed a 3D geostatistical study of the CT and chloroform groundwater concentration data, on the assumption that chloroform in the aquifer resulted from degradation of CT. We used sequential Gaussian simulation to create 1,000 realizations of the 3D distribution of CT and chloroform. Visualization of the median simulated values, and of the probability that the concentration exceeds remediation thresholds, indicate that a substantial plume is present within the lower portion of the unconfined Ringold Formation downgradient of the main source area. A Monte Carlo process was then used to calculate the inventory of CT associated with each set of realizations in a way that attempted to account for uncertainty in the aquifer porosity and in the partition coefficients for CT and chloroform in the aquifer. The inventory results provide improved estimates of the mass of CT present in the groundwater, as well as quantification of the uncertainty in the mass estimates caused by the sparse well control. Taken together, the results indicate that the mass of CT present in the aquifer is at least an order of magnitude larger than previous estimates, and accounts for a much larger proportion of the original CT inventory at the Hanford Site.