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Mr. Maurice Benson • - 3 - 4 March 2009 Defense Logistics Agency groundwater extraction and monitoring well data. Spatial and temporal extraction and monitoring well data are much more significant than CPT data collected intermittently from small intervals within the hydrogeologic zones. Furthermore, extraction well data can be correlated directly to concentrations that would reach groundwater users. In any case, the 30-day test showed that dieldrin was mobilized and removed regardless of whether it was suspended or dissolved. The Draft Mass Removal Report compares the proportions of dieldrin in filtered and unfiltered well samples collected during pumping test number 2 (Middle to Upper Zone wells EW056A, LM190A, LM191A, and LM141A) and the proportions of dieldrin in filtered and unfiltered CPT samples collected elsewhere in the NWC. The report concludes that the dieldrin proportions are distinctly different. However, these data sets are not directly comparable. Previous pumping tests performed on the Middle to Upper Zone would be comparable, but have not been performed. Rather than concluding that the test is not valid because the results were different than CPT data collected over the last two years, the data strongly indicate that the act of pumping mobilized the dieldrin. The Draft Mass Removal Report documents one sampling event conducted during the test of the Middle to Upper Zone in which the samples were filtered with a 0.1 µm mesh filter. All the analyses on these samples did not detect dieldrin. Regardless of the presence or absence of dieldrin in filtered samples, the test showed that dieldrin was mobilized. It should also be noted that any conclusions drawn from the results of such filtering would only be of value if repeated during other events. Human Health Risk Comments Appendix C Appendix C of the Draft FS presents the November 2008 Draft Human Health Risk Assessment for the Hypothetical Use of Groundwater from the Northwestern Corner, DDJC- Tracy (Draft HHRA), prepared by URS. The Draft HHRA evaluates the threat to human health from dieldrin in NWC groundwater. The assumptions made in the Draft HHRA rely on the conclusions of the Draft Mass Removal Report and the Draft Sampling Report. The Draft HHRA should be performed again after those conclusions are revised. For example, dieldrin is much more mobile than was concluded in the Draft Mass Removal Report. This would result in increased threat to water supply wells at greater distances than previously estimated. Feasibility Study Comments The Draft FS evaluates three alternatives for response actions to address the presence of dieldrin in groundwater beneath the NWC area of the Tracy site: 1) No action, 2) Land Use Covenants (LUCs) to restrict human exposure to dieldrin in groundwater, and 3) Groundwater pump and treat with discharge to land. Regional Water Board staff concludes that the cost estimates presented in the Draft FS for the LUC alternative and the pump and treat alternative require revisions. 1 . The cost of the LUC alternative presented in the Draft FS is significantly underestimated. Based on the evidence presented in the Draft Mass Removal Report, the area that requires LUCs is much larger than presented in the Draft FS. The dieldrin beneath the NWC, currently not migrating significantly, would likely migrate if groundwater was extracted from current or future water supply wells. Therefore, the