SITE INFORMATION AND CORRESPONDENCE_CASE 1

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Lee Higgins -2- 20 April 2011 Chevron US Can-Welty Road, Vernalis groundwater. Following the collection of several years of groundwater data, the field work included destruction of the three remaining monitoring wells in accordance with San Joaquin County well destruction guidelines and oversight. Groundwater at the Site was reported from depths of 32 to 37 feet below ground surface (bgs), consistent with prior investigations. Groundwater samples were collected and submitted for analyses for total petroleum hydrocarbons as diesel (TPHd), benzene, toluene, ethylbenzene, xylene (BTEX), and polynuclear aromatic hydrocarbons (PAHs). Analytical results for the two upgradient samples were non-detect (ND) for TPHd. One of the two downgradient samples reported TPHd at a concentration of 100 micrograms per Liter (pg/L) and the second sample was reported at 3,100 pg/L; BTEX and PAHs were ND and/or were reported at concentrations slightly over respective method detection limits. Concentrations reported did not exceed screening levels. Nine soil samples collected from depths of 5 to 44.5 feet bgs were analyzed for TPH as crude oil (TPHc), BTEX, and PAHs. Analytical results were ND for the petroleum hydrocarbon constituents listed above with the exception of naphthalene at a concentration of 0.00084 milligrams per kilogram (mg/kg) to 0.007 at depths of 34.5 and 38 feet bgs, respectively. The vertical extent of affected soil has been defined by ND results for TPH compounds from other samples collected from depths of between 36 and 44.5 feet bgs. According to the data submitted, the extent of crude oil affected soil and groundwater has been defined. SAIC's report of findings for the investigations summarized the following: • The three remaining monitoring wells were properly destroyed in accordance with San Joaquin County Environmental Health Department guidelines and staff oversight. • The extent of crude oil affected soil and groundwater have been defined. The offsite downgradient extent of affected groundwater to the west-northwest will be addressed in the future as part of the Ahern Road-Vernalis investigations. • Additional soil and groundwater characterization regarding the US Can project Site is not needed and preparation of a human health risk screening assessment is proposed. • Additional research is needed to identify the locations of two private wells noted by a State of California Department of Water Resources (DWR) well search and determine their position relative to current Site investigations. • Samples are needed from water supply wells located near certain areas of the Site which have reported elevated TPH concentrations in groundwater exceeding screening levels. Based on our review, Central Valley Water Board staff has the following comments: • We concur that affected soil and groundwater has been adequately defined and additional characterization is not needed.