SITE INFORMATION AND CORRESPONDENCE

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M. Scott Mansholt _� - 2 - 31 October 2008 Chevron Mountain House Site #5 analysis. A soil sample collected at 23.5 feet bgs from one of the borings detected TPHc at a concentration of 1,100 mg/kg and the remaining samples were non detect (ND) or below applicable screening levels. Benzene, toluene, ethylbenzene, and xylenes (BTEX) were ND with the exception of ethylbenzene and xylenes detected at or slightly over their method detection limits (MDLs). Similarly, benzene was detected at concentrations equal to or slightly above the MDL in the five foot-bgs samples from two borings. Low concentrations of polynuclear aromatic hydrocarbons (PAHs) were detected but did not exceed screening levels. SAIC conducted an investigation in latter 2007, primarily to evaluate and define the lateral extent of affected groundwater downgradient (i.e., northeast) of the Site. Three borings were drilled to collect groundwater samples for analysis. Results of the analyses were ND for TPH constituents. Thus, SAIC indicated the downgradient extent of affected groundwater at the Site had been delineated. Depth to groundwater was observed at approximately 13 to 19 feet bgs and based on groun6kiater elevations from nearby Mountain House Development sites, the gradient is northeast. There are no groundwater monitoring wells at the Site. Based on the findings, SAIC concluded the lateral extent of crude oil affected soil was defined with approximate dimensions of 40 feet by 70 feet beneath the highway. The vertical extent was defined at 32 feet bgs by ND results for TPH compounds. The area of crude oil affected groundwater was slightly greater in extent, elongated northeast with approximate dimensions of 75 feet by 125 feet. Affected soil and groundwater is delineated and occurs beneath and within the Byron Road easement and the pipeline corridor. The future Site use and access is not anticipated to change and will likely be limited to construction/maintenance activities associated with the highway and/or possibly the pipeline utilities. Consequently, the ERE was conducted based on the most likely potential exposure for a construction/trench maintenance worker associated with features within the highway and pipeline ROW easements. In conjunction with the ERE, SAIC reviewed the California Department of Fish and Game Natural Diversity Database to research possible threatened, endangered, or sensitive species in the vicinity of the Site. The database query report identified five threatened/sensitive species occurrences from between 0.3 mile to over one mile from the Site. SAIC concluded that the area of affected soil and groundwater is within/beneath the Byron Road ROW, which is not likely to provide adequate habitat for the species noted to occur in the vicinity of Mountain House. According to the ERE report, SAIC concluded that the residual crude oil compounds remaining at the Site do not pose a threat to human health and do not exceed applicable TPH screening values described in - Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater, Interim Final 2008a (ESLs), San Francisco Bay Regional Water Quality Control Board, May for a construction/trench worker exposure scenario. Central Valley Water Board staff concur that based on the investigative findings and the ERE report, affected soil and groundwater have been defined, and the presence of residual petroleum hydrocarbons in soil and groundwater do not pose an unacceptable risk to future