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u ( ERI 205513.QS&$Tosco 76 Service Station. Stockton. CA _ April 28. 1999 directions and gradients are shown on the Groundwater Flow Direction and Gradient Diagram (Plate 3) TPPHg and benzene were not detected at or above laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells except for 9,300 parts per billion (ppb) TPPHg and 57 ppb benzene detected in the sample collected from well MW2 located northwest and adjacent to the UST cavity Diesel range hydrocarbons were not detected at or above laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells However, detection limits for diesel range hydrocarbons were elevated for samples collected from wells MW through MW4SID and well MWSD MTBE was not detected in samples collected from wells MWSD, MW7S, and MW71) MTBE was detected in the remaining wells at concentrations ranging from 3 9 ppb to 27,000 ppb (2 1 ppb to 31,000 ppb using Environmental Protection Agency (EPA) method 8260) Oxygenated compounds ethanol, tertiary butanol, di-isopropyl ether (DIPE), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME) were not detected at or above laboratory method detection limits • During February 1999, ERI performed a soil and groundwater evaluation downgradient of the site • (ERI, March 2, 1999) The scope of work included drilling three CPT borings (CPTI through CPT3) and three continuous core soil borings (B8 through B10) to depths ranging from 28 to 40 feet bgs The approximate locations of soil borings B8 through B10 are shown on Plate 2 ERI collected depth-specific soil and groundwater samples from these borings for laboratory analyses ERI's observation of lithology and review of laboratory analytical results of the soil and groundwater samples collected during this work indicate the following 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 B8 through B 10 at depths ranging from approximately 8 to 10 feet bgs with increased permeability zones encountered at depths ranging from approximately 13 5 to 18 feet bgs, and approximately 20 to 36 feet bgs Based upon data collected during this investigation and previous investigations, the groundwater beneath and downgradient of the site appears to consist of a single, unconfined aquifer of variable permeability - Laboratory analyses of soil samples collected from borings B8 through B 10 did not detect concentrations of residual hydrocarbons or oxygenated compounds at or above the laboratory method detection limits Residual hydrocarbons to soil are delineated at the site and do not extend off site 4