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S E C O R ' Conceptual Site Model Chevron Service Station #9-6171 March 25, 2005 Page 34 MtBE has only been detected in one soil sample (MW-8 at 38 5 feet) collected beneath the Chevron service station MtBE was detected in this soil sample at a concentration of 0 065 mg/kg in a saturated soil sample collected from beneath the static groundwater surface Additionally, a soil sample collected at 30 feet bgs from unsaturated soil in the same boring contained no detectable concentrations of MtBE This implies that some, or a majority of the petroleum hydrocarbons detected in groundwater beneath the Chevron site may not be the result of past activities at the Chevron site This assumption is supported by the complex history of groundwater gradient fluctuations produced by pumping and extraction of groundwater beneath the site by multiple extraction wells in the site vicinity Prior to 1992, MtBE comprised only 1-2% of the total gasoline components From approximately 1992 to the present, reformulated gasoline in California has contained approximately 15% MtBE The Chevron service station was remodeled in 1986, which was before MtBE was a significant constituent in gasoline In 1986, a new UST pit was dug and new double walled tanks, associated product piping, and dispenser islands were Installed The old USTs, product piping, and dispenser islands were removed and the former UST pit backfilled with uncontaminated material MtBE has also been detected in soil and groundwater beneath the former Regal/Exxon service stations, the Shell service station, and Canepa's Carwash The soil sample from the Chevron station that contained MtBE was collected at a depth of 38 5 feet bgs and was neither from the former UST pit nor near the current UST pit The soil and groundwater samples collected from beneath the former Regal/EXXON stations, the Shell station, and Canepa's Carwash includes soil that was possibly contaminated in the late 1980's Therefore, the detection of MtBE in groundwater beneath the Chevron service station Implies that the source is from underneath one of the cross-gradient sites the former Regal/Exxon service stations, the Shell service station, and Canepa's Carwash Considering the present and former groundwater flow directions to the east/southeast for the Chevron station and to the north/northeast for the remaining service stations, the detection of MtBE in soil and groundwater beneath the Chevron service station Implies, in part, that any one of the former and active service stations southeast of the Chevron station may also be responsible for hydrocarbons detected beneath the Chevron service station In addition, the abandoned oil UST at the former Norge Cleaning Village is located upgradient from the Chevron station Any leaks in the tank would have gone unnoticed and been naturally attenuated by the time the abandoned UST was discovered It is possible that this UST also contributed, In part, to the presence of hydrocarbons detected beneath the Chevron service station in the early 1990's Subsurface investigation completed at the Chevron service station suggests that two distinct horizons of groundwater impact may exists beneath the station Shallow groundwater contained in the A-Zone Aquifer is contaminated, and Is separated from the Sdeep B-Zone Aquifer by a laterally discontinuous low permeability aquitard The deeper N IFEIes\Projects\Chevron\RetarR#9-617115ite Conceptual ModeIIFtNAL_CSM doc