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Stephen Donell - 6 - 31 January 2014 concur. The Basin Plan designates all groundwater within the San Joaquin basin as drinking water. Furthermore, nitrate contamination is widespread and a continuing threat to water supplies within the San Joaquin basin. Based on site data presented in the Report, MW-8, in the northwestern corner of the Site, should be representative of Site background groundwater conditions. Monitoring well MW-8 contained 24.75 mg/L nitrate as NO3. Downgradient wells MW-2, MW- 7, MW-6, and TW-1 contained 1,186 mg/L, 934 mg/L, 819 mg/L, and 797 mg/L of nitrate, respectively. The data shows that the site is a significant source of nitrate pollution that needs to be addressed. A similar pattern is found for chloride, TDS, and sulfate therefore, the Site appears to be the source of groundwater pollution. 7. In Table 3-Soil Sample Results SVOCs, TOC, Arsenic, Mercury; analytical results for total organic carbon (TOC) were reported as a percent. Please report TOC in units of milligrams per liter (mg/L) or micrograms per liter(ug/L). 8. Figure 3 Sampling Locations depicts a stormwater drainage running from north to south along the eastern portion of the Site. This drainage needs to be sampled for Site contaminants of concern. 9. TPH results for groundwater needs to be included in a Report summary table. 10. All Tables in Appendix A need to include sampling dates. QA/QC Comments 1. The laboratory reports in Appendix F indicate that Trip Blank#1 and Trip Blank #2 contained bromomethane at 5.52 ug/L and 5.85 ug/L, respectively. Please discuss possible sources of bromomethane contamination in the sampling event trip blanks. Staff note that bromomethane was formerly used as a soil fumigant and may originate from Site contamination. 2. Appendix F indicates that sample MW-6-25 duplicate (lab sample ID 130531-86) experienced a toluene-d8 surrogate failure of 67% recovery during EPA Method 82608 QA/QC due to matrix interference. Sample MW-6-25 (lab sample ID 130531-76) did not experience the same matrix interference with a reported recovery of 99%. The Regional Water Board is concerned that the matrix interference did not occur in both sample analyses for MW-6-25, as would be the case if the source of matrix interference was the sample. However, the analysis was considered under control by the certified lab because other control parameters were within acceptable limits. Please discuss if the laboratory verified that matrix interference was from the sample through empirical testing. 3. Sample analyses for MW-6-5 (lab sample ID 130531-72) indicated dibromofluoromethane matrix interference with 132% recovery. Sample TW-4-25 (lab sample ID 130531-158) also experienced a dibromofluoromethane surrogate failure of 169% recovery due to matrix interference. Both interferences occurred during EPA Method 8260B QA/QC. The analyses were considered under control by the lab because other control parameters were within acceptable limits. Please discuss if the laboratory verified the matrix interference source was the sample by re-testing or LCS analysis for dibromofluoromethane for both samples.