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ERI 205513 QSR20 Tosco Service Station 5886,Stockton.Califorma _ April 25,2002 <br /> . Groundwater extraction from well MW3 and the UST cavity well was discontinued in February <br /> 2002 following startup of the AS remediation system <br /> • GRI of Dublin, California, under direct contract to Tosco, collected monthly groundwater samples <br /> from on-site wells MW1 through MW4DIMW4S and MW6D/MW6S for MTBE analysis and <br /> performed quarterly groundwater monitoring and sampling at the site on February 11 and 12, <br /> 2002 The GRI First Quarter 2002 Groundwater Monitoring and Samphng Report is provided as <br /> Attachment A The results of the fourth quarter monitoring and sampling event indicate the <br /> following <br /> o Groundwater was measured at approximately 6 1 to 9 1 feet below ground surface <br /> (bgs) in the groundwater monitoring wells(an elevation increase of approximately 0 1 <br /> to 0 4 foot when compared to the fourth quarter 2001 groundwater monitoring and <br /> sampling event) During February 2002, groundwater flow direction was towards the <br /> southwest with a calculated hydraulic gradient ranging from approximately 0 002 to <br /> 0 004 Historical groundwater flow directions are shown on the Groundwater Flow <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 <br /> collected from the groundwater monitoring wells, however, laboratory reporting limits <br /> were elevated for samples collected from wells MW2 and MW3 <br /> So MTBE was not detected at concentrations at or above laboratory method reporting <br /> limits in groundwater samples collected from wells MW4D/MW4S through <br /> MW7D/MW7S, MW8S/MW81, MW9S/MW91/MW9D, and MWIOSIMWIOD using <br /> Environmental Protection Agency (EPA) Method 8260 MTBE was detected in the <br /> remaining wells at concentrations ranging from 4 8 parts per billion (ppb) to 6,300 ppb <br /> using EPA Method 8260 Detected concentrations of MTBE ranged from 2 6 ppb to <br /> 6,700 ppb using EPA Method 8020, MTBE was detected in well MW4D <br /> o Oxygenated compounds including ethanol, tertiary butyl alcohol (TBA), di-isopropyl <br /> ether (DIPE), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether <br /> (TAME), and lead scavengers including 1,2-dichloroethane (1,2-DCA) and ethylene <br /> dibromide (EDB) were not detected at concentrations at or above laboratory method <br /> reporting limits in the groundwater samples collected from the wells <br /> SECOND QUARTER 2002 ACTIVITIES <br /> The following activities are proposed to be completed during second 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 AS remediation system and the <br /> extent of dissolved TPHg, benzene, toluene, ethyl benzene, total xylenes (BTEX), and MTBE <br /> concentrations in groundwater beneath the site <br /> 0 4 <br />