SITE INFORMATION AND CORRESPONDENCE

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Mr. Phil Dawson August 26, 2009 Page 3 DLA and its contractor contend that site-specific lithologic data show discontinuity in the sands. The State considered the perceived discontinuities in the sand as a possibility, but point out that the hydraulic communication that DLA observed during the pumping tests show continuity in the sands. For example, the pumping test on EW056A showed that hydraulic communication exists between this well and LM174AU, located greater than 240 feet to the northeast. DLA acknowledged this by making the following statement on page 3-8 of the Dieldrin Mass Removal Pump Test Report, included as Appendix B of the FS Report: "However, the general similarities of the drawdown slopes of observation wells in the Upper zone positioned within 20 feet (LM190A and LM191A) and greater than 240 feet (LM174AU and LM071A) of extraction well EW056A suggest hydraulic communication within the Upper Hydrologic Zone." The State has attached Figure 3-9 of the Pump Test Report to this letter. The drawdown slopes for the observation wells referenced in DLA's statement above are depicted in the figure. The similarity between the slopes of observation wells LM174AU, LM190A, and LM191A are obvious. In addition, all three wells show the rebound effect at the end of the pumping test when EW056A was shutoff in early October. The rapid rebound response of the water level observed in LM174AU, greater than 240 feet away from the pumping well, is strong evidence for the continuity of sand units within the northwest corner dieldrin plume at the Tracy site. The break in the sand units shown on the geologic cross section Figures 3-13a and 3-13b of the Pump Test Report between EW056A and LM174AU is misleading. This break is an artifact caused by under sampling in an aquifer with subunits that are narrower than the average sampling interval. The under sampling is also the result of lack of access to the Union Pacific railroad property that forms a wide swath oblique to the direction of the interconnected stream channels predicted by the State's site conceptual model described to DLA previously. Continuity of formations in the sands is supported by the presence of dieldrin concentrations equal to the injection concentrations over 240 feet from the injection wells. Additional evidence of sufficient continuity of coarse-grained stringers (sand units) is the presence of the dieldrin in randomly placed cone-penetrometer (CPT) sampling locations regardless of the lithology the sample was collected from.