SITE INFORMATION AND CORRESPONDENCE

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Soil and Groundwater Investigation Report Mountain House Development Site#6 February 25,2010 determined that all analytical data are usable. The data usability summary is provided as Appendix E.1. Qualified data with reason codes are provided in Tables 1 through 4, and summarized in Appendix E.1; Table E.1. 5. DISCUSSION OF PETROLEUM DISTRIBUTION 5.1 EXTENT OF AFFECTED SOIL Soil analytical results were compared to the lowest, most conservative RSL and ESL values for delineation purposes. Soil analytical results are summarized in Tables 1 and 2. The comparison indicated that soil sample analytical results obtained during this investigation were less than RSLs and ESLs with the following exceptions: • TPHc were detected above the ESLs in two borings. The highest TPHc concentration (6,900 milligrams per kilogram [mg/kg])was detected in SB-39 at 20 feet bgs. • PAHs were detected above the ESLs and/or RSLs in four borings. The highest PAH concentration(2-methylnaphthalene at 4.5 mg/kg) was detected in SB-39 at 20 feet bgs. Two distinct areas of TPH-affected soil, Area 1 and Area 2, were noted based on groups of contiguous borings with crude-oil constituents at levels above RSLs and/or ESLs, as shown on Figure 3. The vertical extent of affected soil in Area 1 was delineated by the sample collected at 39.5 feet bgs from SB-15. The vertical extent of affected soil in Area 2 was delineated by the sample collected at 33.5 feet bgs from SB-16. The naphthalene detected in the soil samples collected from SB-30 and SB-32 exceed, but are very close to, the RSLs; therefore, it is likely that the concentrations decrease to below screening levels in close proximity to the boring locations, and the soil is considered delineated. The extent of crude-oil affected soil is delineated on Figure 3, which was derived from SAIC- Benham's evaluation of the data collected during the previous and current investigations. 5.2 EXTENT OF AFFECTED GROUNDWATER Groundwater analytical results were compared to the lowest, most conservative ESL and WQO values for delineation purposes. Groundwater analytical results are summarized in Tables 3 and 4. The comparison indicated that groundwater sample analytical results obtained during this investigation were less than WQOs and ESLs with the following exceptions: • PAHs were detected above the ESLs and/or WQOs in three borings. The highest PAH concentration(benzo(a)pyrene at 0.004 micrograms per liter [gg/L])was detected in SB-41. Two areas of affected groundwater, Area 1 and Area 2, were noted based on groups of contiguous borings containing SPO or with crude-oil constituents detected at levels above WQOs, as shown on Figure 3. The vertical extent of affected groundwater in Area 1 was delineated by the sample collected at 35 feet bgs from SB-10. 7 Fmin Science to Solutions