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w ERI 205513.OSR9 Tosco 76 Service Station.Stockton. Califolma _ _ July 22. 1999 • - Sediments encountered beneath and in the vicinity of the site typically consist of heterogeneous mixtures of silt, clay, and fine- to medium-grained sand Groundwater was initially encountered in borings B 11 through B 19 at depths ranging from approximately 9 to 14 feet bgs with static groundwater elevations ranging from approximately 6 0 to 6 5 feet bgs Data collected during this investigation continue to indicate that the groundwater beneath and downgradient of the site appears to consist of a single, unconfined aquifer of variable permeability Dissolved gasoline and diesel hydrocarbons and oxygenated compounds (MTBE, methanol, and ethanol) were not detected at or above laboratory method detection limits in groundwater samples collected from groundwater monitoring well MW9S However, low concentrations of dissolved hydrocarbons and select oxygenated compounds were detected in groundwater samples collected from the remaining wells Laboratory analytical results of the groundwater samples are provided in Attachment B Groundwater samples collected from wells MW8S/I/D through MWIOS/I/D were not subjected to a silica gel cleanup process prior to Environmental Protection Agency (EPA) Method 8015 modified laboratory analysis for TEPHd Laboratory report notes indicate that diesel range hydrocarbons detected in groundwater samples collected from well MW8S through MW8D, MW91, and MW IOD were classified as unknown hydrocarbons or were reported as diesel based upon a single analyte peak because a fuel hydrocarbon pattern was not present • On April 16, 1999, in response to a letter from the County that was not dated but was received (via facsimile) on March 26, 1999, ERI subrrutted a Work Plan to the County for a supplemental evaluation of groundwater downgradient of the site The Work Plan included a recommendation to perform four quarters of groundwater monitoring and sampling of wells MW1 through MWIOS/IID prior to the implementation of the Work Plan • During May 1999, ERI performed an extended vapor extraction test (VET) at the site The results of VET will be reported under a separate cover THIRD QUARTER 1999 ACIVITIES The following activities are proposed to be completed during third quarter 1999 • ERI will submit a report summarizing the results of the VET • Tosco will continue quarterly groundwater monitoring and sampling at the subject site • ERI and Tosco will continue to conduct a comprehensive review of the data acquired during previous work The results will be presented to the County in quarterly reports, which describe the status of the proposed work by the dates established by the County • 5