SITE INFORMATION AND CORRESPONDENCE

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Mr. Maurice Benson • - 3 - 0 3 November 2008 Defense Logistics Agency exception, have always been below the cleanup concentration after the remediation work. Attachment D presents a table of the last three years of TCE concentrations in groundwater samples collected from LM093AU before remediation; the average concentration was 21.0 pg/L. Attachment D also presents the data collected from LM175AU after remediation; the average has been 1.8 pg/L. May 2008 TCE Concentration Data. Regional Water Board staff prepared a concentration contour map for TCE in soil samples collected by DESJC beneath and adjacent to the northeast corner of Warehouse 10 in 2008 (Attachment E). The base map for Attachment E is Figure 2-1 of the Draft Report; the concentration data are from Figure 3-1 of the Draft Report. Also, Regional Water Board staff prepared a concentration contour map for TCE in groundwater beneath the same area (Attachment F). The base map for Attachment F is Figure 3-4 of the Draft Report; the concentration data are from the same figure. The extents of contamination shown on the Attachments are very similar to each other, and to the extent of TCE in soil vapor shown on Figure 3-2 of the Draft Report (Attachment G). During April and May 2008 seven of the groundwater samples DESJC collected from CPT borings within the northeast quadrant of Warehouse 10 had TCE concentrations greater than the ROD-specified cleanup standard of 5 pg/L (Attachment F). The concentrations tabulated in Attachment H averaged 14.1 pg/L. Data Comparisons. The 1999 soil remediation work performed adjacent to the northeast corner of Warehouse 10 reduced concentrations of TCE in groundwater from an average of 21 pg/L to an average of 1.8 pg/L, as documented in the Attachment D tables. It is likely that if soil remediation work were conducted beneath the northeast quadrant of Warehouse 10, after the planned 2009 building demolition, TCE concentrations in groundwater shown in the Attachment H table would also decrease. 2. SVE should be reevaluated as an alternative remedial action. Removal of diesel- range TPH would not be a limiting factor in the cleanup requirements of SWMU 20, as was suggested in the 2004 ESD. For the most part, concentrations of TPH in soil and groundwater samples collected in April and May 2008 were below the ROD- specified cleanup levels. However, TCE concentrations in soil continue to be present above the ROD-specified cleanup values in soil and groundwater. As discussed above, excavation of TCE-contaminated soil causes a concomitant reduction in TCE impacts to groundwater. Furthermore, removal of TCE contaminated soil by SVE should also be as effective. Also, SVE should be significantly less expensive than excavation. After Warehouse 10 is demolished, source area TCE contaminated soil beneath the warehouse will be accessible for remediation. Please submit a plan that presents the remedial response DESJC will implement to reduce CDCs in soil, soil vapor, and groundwater.