SITE INFORMATION AND CORRESPONDENCE

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Lee Higgins - 3 - 2 December 2010 Former Vernalis Pump Station Vernalis, San Joaquin and Stanislaus Counties • The additive oral, dermal, and inhalation total potential onsite residential cancer risk of 1 E-06 is equivalent to the de minimus level of 1 E-06. A conservative approach for a residential scenario was utilized in the Screening Evaluation. The most likely future onsite use will be for commercial/industrial purposes and/or maintenance trench worker(s). • BTEX and PAHs were detected in shallow groundwater samples collected during characterization activities. Benzene was detected in only one of 43 samples analyzed, and the concentration reported was below the maximum contaminant level (MCL). Toluene and some PAHs were detected at very low concentrations indicating the potential risks associated with this pathway are low. • Shallow groundwater exceeds numerous California and Federal total dissolved solids standards for drinking water and agricultural purposes. Depth to onsite groundwater is approximately 36 to 40 feet bgs. Also, it is unlikely shallow onsite groundwater would be considered for a drinking water source and therefore the direct exposure route is incomplete. Therefore, health risks and hazards for potential residential exposure to onsite groundwater were not evaluated quantitatively. • The California Natural Diversity Database, 2009, maintained by California Department of Fish and Game, was reviewed to search for threatened, endangered, or sensitive species in the vicinity of the Site. The assessment indicates that a threat to sensitive ecological receptors does not appear to exist and current and future land use is unlikely to provide suitable habitat. In summary, SAIC's report determined that the potential cancer risk to a potential future resident was 1 E-06, equivalent to the de minimus level of 1 E-06. The non-cancer health hazard, at 3E-03 is well below the de minimus level of 1 which is also a conservative estimate of risk. This level of cancer risk and non-cancer health hazard exposure to onsite soils therefore indicates an acceptable level of exposure for a hypothetical future resident. An ecological screening evaluation,indicates that threats to sensitive ecological receptors do not appear to exist at the Site and no occurrences were identified within one-mile. SAIC concluded that no remedial action is necessary to protect human health or ecological receptors, and that a decision for no further action determination could be made. Based on my review, I have the following comments: • Consistent with other TAOC sites, a Soil and Groundwater Management Plan (SGMP) needs to be submitted. The SGMP designates Chevron as the responsible party and serves as a guideline for Chevron to implement for handling soil affected by the degraded crude oil that may be encountered during future Site activities. • Central Valley Water Board staff concurs with the findings of the Screening Evaluation and related supporting documentation, and that the Site is a candidate for closure. A request for a No Further Action Required (NFAR) determination may be prepared.