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. ERI 205513 SR21 Tosco Service Station 5886Stockton California Tudw 30 'MWI <br /> o Groundwater was measured at approximately 6 0 to 9 1 feet below ground surface (bgs) in the <br /> groundwater monitoring wells During May 2002, groundwater flow direction remained <br /> towards the southwest with a calculated hydraulic gradient ranging from approximately 0 002 <br /> to 0 007 Historical groundwater flow directions are shown on the Groundwater Plow <br /> Direction Rose Diagram(Plate 4) <br /> o Total petroleum hydrocarbons as gasoline (TPHg), and benzene were not detected at <br /> concentrations at or above laboratory method reporting limits in groundwater samples collected <br /> from the groundwater monitoring wells, however, laboratory reporting limits were elevated for <br /> samples collected from well MW3 <br /> o MTBE was not detected at concentrations at or above laboratory method reporting limits in <br /> groundwater samples collected from wells MW4D/MW4S through MWl0SIMWI0I/MW10D <br /> using Environmental Protection Agency (EPA) Method 8260B MTBE was detected in wells <br /> MW1 through MW3 at concentrations ranging from 28 parts per billion (ppb) to 6,500 ppb <br /> using EPA Method 8260B <br /> o Oxygenated compounds including ethanol, tertiary butyl alcohol (TBA), di-isopropyl ether <br /> (DIPS), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME), and lead <br /> scavengers including 1,2-dichloroethane (1,2-DCA) and ethylene dibromide (EDB) were not <br /> detected at concentrations at or above laboratory method reporting limits in the groundwater <br /> samples collected from the wells <br /> THIRD QUARTER 2002 ACTIVITIES <br /> The following activities are proposed to be completed during third quarter 2002 <br /> • ERI will continue to perform monthly O&M at the site and collect monthly groundwater samples <br /> from wells MW1 through MW3 to evaluate the effectiveness of the microsparge remediation <br /> system and the extent of dissolved TPHg, benzene, toluene, ethyl benzene, total xylenes (BTEX), <br /> and MTBE concentrations in groundwater beneath the site <br /> • Tosca will continue to perform quarterly groundwater monitoring and sampling at the subject site <br /> The water samples will be submitted to a California state-certified laboratory for analysis <br /> • Tosco and ERI will continue to conduct a comprehensive review of the data acquired during <br /> previous work The results will be presented to the County in quarterly reports, which describe the <br /> status of the proposed work by the dates established by the County <br /> 4 <br />