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Mr. Maurice Benson • - 2 - 4 March 2009 Defense Logistics Agency The Draft Sampling Report's conclusion that the dieldrin beneath the NWC remains bound in the fine-grained layers is not supported by the monitoring well data. Monitoring wells LM190A, LM191A, and LM141A are screened within course-grained materials and dieldrin concentrations are currently detected at the upper range of historic concentrations. The Draft Sampling Report also concludes that dieldrin is not dissolved in the groundwater and that most of it is sorbed to particle colloids at sizes greater than 0.45 um. However, the data collected from extraction well EW056A and monitoring wells LM190A, LM191A, and LM141A do not support this conclusion (see attached dieldrin concentration trend plots for samples collected during the pumping tests in September and October 2008 from these wells). On each sampling event, two laboratory chemical analyses were performed on every groundwater sample from every well: the samples were split and one split filtered through a 0.45 micrometer (µm) mesh filter before analysis and the other split not filtered before analysis. The plots clearly show that significant concentrations of both filtered and unfiltered dieldrin concentrations occurred in all of the wells. Dieldrin Mass Removal Pumping Test Comments Appendix B of the Draft FS presents the November 2008 Draft Dieldrin Mass Removal Pump Test Report(Draft Mass Removal Report), prepared by URS. This report presents the results of two pumping tests performed in the NWC during September and October 2008. The purpose of the work was to complete characterization of dieldrin extent in the Middle and Lower Hydrologic Zones and to evaluate the feasibility of dieldrin removal by pumping of these zones and the Upper Zone. The Draft Mass Removal Report concludes that the dieldrin is for the most part present in the Middle to Upper Zone within silt and clay materials and that dieldrin removal by pumping is not effective. Regional Water Board staff concurs with the observation that the dieldrin appears to be present primarily in the Middle to Upper Zone. However, we disagree with the conclusion that pumping is not effective. The Draft Mass Removal Report presents evidence to the contrary. The data collected from groundwater extraction well EW056A and monitoring wells LM190A, LM191A, and LM141A show that dieldrin mobilized and was removed throughout the 30-day test period. The attached plots, with the possible exception of LM191A, clearly show that both filtered and unfiltered dieldrin concentrations increased over the test period. The Draft Mass Removal Report indicates that the dieldrin data collected during the pumping tests were not representative of conditions in the NWC, arguing that CPT data collected previously are representative. However, Regional Water Board staff concludes that dieldrin data collected from pumping and monitoring wells over a 30-day period are more representative of dieldrin concentrations over time than are data from discrete CPT samples. The Draft Mass Removal Report concludes that total dieldrin cannot be mobilized from more than 6 feet from an extraction well. This conclusion was based on pre- and post-test CPT groundwater sampling events performed approximately 6 feet and 12 feet from the extraction well for the Middle to Upper Zone pumping test. Laboratory analyses were presented to show that dieldrin concentrations in groundwater within silt or clay materials collected more than 6 feet from an extraction well did not change. This is week evidence when compared to well data. The dieldrin concentrations in CPT-collected groundwater samples from fine-grained materials before and after the pumping test are not compelling when compared with