SITE INFORMATION AND CORRESPONDENCE

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Lee Higgins - 3 - 9 July 2010 Chevron Mountain House Site#3 • SAIC reviewed Department of Water Resources files to identify water supply wells within 2,000 feet of the Site. Three wells identified as lateral-cross gradient, are greater than one-quarter mile from the Site with well screens below the maximum depth of affected groundwater. Considering the distance from the Site and the well screen depths, it is not likely these wells would be affected by the TPH compounds in groundwater. • Site characterization information describes exposure point concentrations (EPCs) that were used to calculate the upper confidence limit (UCLs) with a statistical software program to analyze the analytical datasets. Since soil and groundwater were determined to be delineated, and the characterization dataset adequately describes Site conditions, the EPCs were calculated as the UCL using the mean soil concentration for each constituent of potential concern (COPC). • In accordance with the National Contingency Pian, an incremental lifetime cancer risk of 1 E--06 (i. e., 1 in 1 million), was used as a point of departure consideration for the Screening Evaluation. For non-cancer hazard indices, a hazard index of 1 or less is considered an acceptable health hazard level. The Screening Evaluation was prepared to estimate future onsite residential risk and hazards under current site conditions. In addition, the evaluation was developed using health protective, conservative risk and hazard algorithms developed by DTSC and provided within the PEA Guidance Manual. • Future oral and direct contact exposures to onsite soils and cancer risks were estimated and the risk for individual COPCs and summation of risks for all COPCS for these exposure routes were determined. Potential future indirect inhalation exposures to onsite soils and cancer risk were also estimated, The combined risks for oral, dermal, and inhalation cancer risks were completed. The total potential cancer risk to a hypothetical future resident from for oral, dermal, and inhalation exposures was estimated to be 5E-07 which is below the threshold lifetime cancer risk of 1 E-06, and thus indicates an acceptable cancer risk for exposure to onsite soils for hypothetical future residents. • The total potential non-cancer hazard from oral, dermal, and inhalation exposures to onsite soils was determined to be 0.002 which is well below a hazard index of 1, and indicates an acceptable non-cancer health hazard exposure to onsite soils for hypothetical future residents. • Since affected onsite groundwater exceeds numerous California and Federal total dissolved solids standards for drinking water and some agricultural purposes, the health risks and hazards were not evaluated quantitatively for potential residential exposures to onsite, shallow groundwater. • Results of the Screening Evaluation indicate that exposure to chemicals present at the Site should not result in an unacceptable health risk for an onsite resident. Therefore, SAIC concluded that no remedial action is necessary for protection of human health, and that a risk management decision for no further action could be made for the Site. • As part of the Screening Evaluation, a feasibility evaluation of potential technology options previously conducted was reviewed for addressing weathered crude oil found at