SITE INFORMATION AND CORRESPONDENCE

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0 0 Mr. Maurice Benson - 2 - 4 October 2010 Defense Logistics Agency Scope of Work The Work Plan proposes to further delineate soil and groundwater organochlorine pesticide contamination beneath the site by collecting soil samples at 3-foot intervals from 18 direct- push soil borings from the surface to the saturated zone, estimated to be 20 feet bgs. Also, hydro-punch groundwater samples would be collected from seven of these borings. Soil samples collected from 1, 3, 6 and 9 feet bgs would be analyzed immediately and the deeper samples would be held until the chemical analyses results are evaluated. In the event that the sample results from both the 6- and 9-foot samples exceed screening levels, the 12-, 15- and 18-foot samples would be analyzed. Comments Central Valley Water Board staff present the following comments on the Work Plan that DLA should address in the final Work Plan document. 1. The Work Plan introduces, but does not define the following terms: residential region screening level (RSL), and preliminary remediation goal (PRG). Section 2.1.4 of the Work Plan text introduces the term residential RSL concentrations for pesticides and compares them to pesticide concentrations detected in soil samples collected in October 2009. In addition, the text of Appendix A introduces the term PRG concentrations for pesticides and also compares them to the October 2009 concentrations. The final Work Plan should define these terms and present the rational for their use as screening tools. 2. The Work Plan introduces, but does not define the following terms: U.S. EPA RSL, industrial RSL, and commercial/industrial RSL. Section 3.1.3 Screening Levels of the Work Plan text introduces the term U.S. EPA RSL concentrations and indicates that concentrations of pesticides in soil samples to be collected from the 18 push-borings will be screened using them. Table 2, also presented in this section of the text, summarizes the U.S. EPA RSL concentrations for pesticides, but names them industrial RSLs. Furthermore, section 3.4.3 of the Work Plan text names the concentrations presented in Table 2 as commercial/industrial RSLs. The final Work Plan should define consistent terminology and present "residential-based" rather than "industrial-based" risk screening tools. 3. The proposed locations and sample depths for direct push borings seem appropriate for the next phase of characterization of the horizontal and vertical extent of pesticide contamination in soil. However, DLA should perform chemical analyses on the deeper soil samples collected from 12, 15, and 18 feet bgs in any boring in which a pesticide is detected in the 9-foot sample regardless of the analysis result on the 6-foot sample. The draft Work Plan indicates that the deeper samples would be analyzed only if pesticides are detected in both the 6- and 9-foot samples. 4. The Work Plan proposes seven locations for groundwater sampling within the area of known pesticide contamination. In order to avoid potential cross-contamination from contaminated soil, DLA should propose locations that are down-gradient and a few feet outside of the contaminated area. 5. The highlighted cells in the table included in Appendix A of the Work Plan are illegible. The cells within the final Work Plan document should be legible. 6. Table 1 of the Work Plan text incorrectly identifies monitoring well LM137A as LM137AU. The monitoring well identifier should be corrected in the final Work Plan.