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URS Ms Lon Duncan Page 2 San Joaquin County Environmental Health Department February 4, 2003 In July 1997, the San Jaoquin County Environmental Health Department (SJCEHD) sent ARCO a letter stating that the former Shell service station approximately 300 feet east of the site had reported increasing concentrations of methyl tertiary-butyl ether (MtBE) in their upgradient wells These wells are to the northeast of the ARCO station, northeast is the historical groundwater flow direction The gasoline stored at the Shell Station never contained MtBE The Shell site was granted closure by the California Regional Water Quality Control Board (RWQCB)and the groundwater monitoring wells at the site were subsequently abandoned On February 10, 11,and 12, 1999,four groundwater monitoring wells(MW-1 through MW-4)were installed at the site by SECOR Soil samples collected from MW-4 during drilling reported total petroleum hydrocarbons as gasoline (TPHg) up to 1 80 mg/kg, benzene up to 0 118 mg/kg, and MtBE up to 15 2 mg/kg Groundwater samples collected from the newly installed wells indicated TPHg was present in two wells at concentrations of 231 parts per billion (ppb) (MW-1) and 15,000 ppb (MW-4) Benzene was reported in two wells at concentrations of 135 ppb (MW-1) and 503 ppb (MW-4) MtBE was reported in monitoring wells MW-3 and MW-4 at concentrations of 7 06 ppb and 9,990 ppb, respectively Concentrations of MTBE have decreased significantly in these wells, suggesting that the source of MTBE contamination has been removed The solvents tetrachloroethene (PCE) and trichloroethene (TCE) have also been detected in groundwater samples collected at the site Historical groundwater analytical data suggest that these solvents are from an off-site source In March 2002, two dual-completion groundwater monitoring wells (MW-9M/D and MW-10M/D) were installed by SECOR on the east side of Center Street(Figure 2) Third quarter 2002 groundwater monitoring data indicated TPHg concentrations up to 3,100 ppb at the top of deeper sand layer (MW-8M) and up to 1,100 ppb at the bottom portion of this sand layer(MW-8D) Groundwater samples collected from both the mid-depth and the deep-screened monitoring wells reported concentrations of benzene up to 86 ppb, and MtBE up to 71 ppb ASSESSMENT ACIVITIES AND FINDINGS A dual-completion groundwater monitoring well was installed on the sidewalk west of El Dorado Street to assess the vertical and lateral extent of MtBE contamination in groundwater associated with the former ARCO station The following paragraphs present a discussion of drilling and monitoring well installation, the subsurface conditions encountered, monitoring well development, sampling and surveying, and the analytical data for groundwater samples collected from the newly installed wells • K V5 BP ARCQ1044931Regulatory Correspondence14493 WIR doc