SITE INFORMATION AND CORRESPONDENCE

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M. Scott Mansholt - 3 - 12 November 2008 ' Chevron Mountain House Development Site #6 ' table. SPO was noted above the water table at approximately three to four feet bgs in borings SB-21 and SB-22. The vertical extent of affected soil was defined at SB-22 where TPHc at a ' concentration of 1,600 mg/kg was detected at 25 feet bgs and ND at 31 feet bgs. BTEX concentrations for some samples were ND to low detections at or slightly above respective method detection limits (MDLs). The vertical extent of petroleum hydrocarbon affected soil ' was defined at the Site at depths of 31 to 39 feet bgs. Consistent with analytical soil results at other OVP/TAOC site investigations, polynuclear ' aromatic hydrocarbons (PAHs) are detected at or above MDLs and/or environmental screening levels when SPO is present, or elevated TPH concentrations are detected in soil samples. In borings SB 10, 21 , and 22, PAHs greater than Screening for Environmental ' Concems at Sites with Contaminated Soil and Groundwater' and/or Preliminary Remedial Goals Z, were detected in soil samples at depths of 4.5 to 18 fleet bgs. Corresponding deeper boring samples at intervals ranging from 17 to 35 feet bgs were ND for PAHs. ' Three soil samples from 14 feet bgs (SB-5), 17 feet bgs (SB-10), and 23 feet bgs (SB-14) were submitted for fuel fingerprint evaluation to verify the source of the TPHc. According to , review of the chromatograms, the TPHc present was moderately degraded crude oil consistent with San Joaquin Valley crude oil. Groundwater The DWR well search identified seven water supply wells within 2,000 feet of the Site. The ' wells identified are either up or cross-gradient of the Site. At least two of the wells were previously sampled for TPH compounds as part of other Mountain House investigation projects. TPH constituents were not detected in samples from those wells. Groundwater samples, including two duplicates, were obtained from 20 borings and submitted for analysis_ TPH as diesel (TPHd) was detected at concentrations ranging from 52 to 94 micrograms per liter (pg/L) in 11 samples. Results for eight other samples were ND, and 130, 150, and 1 ,000 pg/L. The latter results exceeded the ESLs' and Water Quality Objectives (WQOs) for TPHd described in the Central Valley Regional Water Quality Control Board -Water Quality Control Plan for the Sacramento River Basin and the San Joaquin River , Basin, 41h ed., revised August 2006, (Basin Plan). BTEX compounds were ND. Low PAH concentrations slightly over the ESLs and WQOs were detected in two groundwater samples, with the highest being dibenz(a,hlanthracene at a concentration of 0.0064 pg/L (the ESL/WQO ' for this compound, respectively is 0.0048 and 0.0085 pg/L) and 0.004J pg/L for benzo(a)pyrene at 0.004) pg/L was equivalent to the WQO (0.004 pg/L) for this constituent; the ESL for benzo(a)pyrene is 0.014 pg/L_ According to SAICs report, soil and groundwater have only been partially delineated laterally and vertically with additional characterization needed. , 9—ESLs- San Francisco Bay Regional water Quality Control Board, Interim Final. November 2007). ' 2—PRGs - U.S. Environmental Protection Agency (FPA) Region 9, Preliminary Remedial Goals, October 2004.