In September 2005, periodic sampling of groundwater was initiated by the Commodity Credit Corporation of the U.S. Department of Agriculture (CCC/USDA) in the vicinity of a grain storage facility formerly operated by the CCC/USDA at Centralia, Kansas. The sampling at Centralia is being performed on behalf of the CCC/USDA by Argonne National Laboratory, in accord with a monitoring program approved by the Kansas Department of Health and Environment (KDHE). The objective is to monitor levels of carbon tetrachloride contamination identified in the groundwater at Centralia (Argonne 2003, 2004, 2005a). Under the KDHE-approved monitoring plan (Argonne 2005b), the groundwater is being sampled twice yearly (for a recommended period of two years) for analyses for volatile organic compounds (VOCs), as well as measurement of selected geochemical parameters to aid in the evaluation of possible natural contaminant degradation (reductive dechlorination) processes in the subsurface environment. The sampling is presently conducted in a network of 10 monitoring wells and 6 piezometers (Figure 1.1), at locations approved by the KDHE (Argonne 2006a). The results of groundwater sampling and VOCs analyses in September-October 2005, March 2006, September 2006, and March 2007 were documented previously (Argonne 2006a,b, 2007a). The results have demonstrated the presence of carbon tetrachloride …