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ERI 205513 QSR16 Tosco Service Station 5886 Stockton,Califorma Ann'26,_2001 • Groundwater was measured at approximately 6 0 to 9 0 feet below ground surface (bgs) in . the groundwater monitoring wells (an elevation variation of approximately 0 0 to 0 2 feet when compared to the fourth quarter 2000 groundwater monitoring and sampling event) Duffing February 2001; groundwater flow direction was towards the southwest at a- - calculated hydraulic gradient of approximately 0 005 Historical groundwater flow directions are shown on the Groundwater Flow Direction Rose Diagram (Plate 3) • Total purgeable petroleum hydrocarbons as gasoline (TPPHg) and benzene were not detected at or above laboratory method detection limits in groundwater samples collected -=from the groundwater monitoring wells—However,—laboratory detection limits were - elevated for groundwater samples collected from wells MW1 through MW3 • Diesel-range hydrocarbons were not detected at or above laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells with the exception of well MW3 Diesel-range hydrocarbons were detected in the groundwater sample collected from well MW3 at 52 4 parts per billion (ppb) However, diesel-range hydrocarbons were not detected at or above laboratory method detection limits in this sample after the silica clean-up was performed • MTBE was not detected at or above laboratory method detection limits in groundwater samples collected from wells MWSD, MWSS and MW7D/MW7S through MW IOS/MW l0I/MW 10D MTBE was detected in the remaining wells ranging from 3 7 ppb to 18,000 ppb using EPA Method 8260, (ranging from 5 4 ppb to 16,400 ppb using EPA Method 8020) MTBE was not detected in well MW4S using EPA Method 8020 . • Oxygenated compounds ethanol, tertiary butyl alcohol (TBA), di-isopropyl ether (DIPE), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME) and lead scavengers 1,2-dichloroethane (1,2-DCA) and ethylene dibromide (EDB) were not detected at or above laboratory method detection limits in the groundwater samples collected from the wells SECOND QUARTER 2001 ACTIVITIES The tollowmg activities are proposed to be completed during second quarter 2001 • Tosco will continue weekly purging activities at the site utilizing the UST cavity well and groundwater monitoring well MW3 as extraction wells The extracted groundwater will be transported to the Tosco refinery for disposal • Tosco will collect monthly groundwater samples from wells MW1 through MW4S/D and wells MW6S/D for a period of 12 consecutive months to evaluate dissolved MTBE concentrations in groundwater beneath the site • Tosco will seal surface cracks on the site to reduce the possibility of surface hydrocarbon spills impacting soil and conduct a tracer test of the underground storage systems to evaluate the system integrity 4 •