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L <br /> ERI 205513 QS2 Unocal Service Station 5886,Stockton,California October 6, 1997 <br /> In April 1997, ERI performed an investigation to evaluate the source and extent of MTBE (ERI, June <br /> 3, 1997) The investigation included hand-auguring three soil borings adjacent to select drains, drilling <br /> four soil borings and constructing dual completion groundwater monitoring wells (MW4S/D through <br /> MW7S/D) at discreet depths In the borings, associated well development and sampling, and analyses of <br /> soil and water samples The data obtained during the investigation indicated the following <br /> • Residual hydrocarbons were not detected at or above the stated laboratory method detection <br /> limits in soli samples collected from adjacent to three on-site drains (see Plate 2) except for <br /> maximum concentrations of 3 3 mg/Kg TPHg, 8 4 mg/Kg TEPHd, and 12 mg/Kg MTBE <br /> • Residual hydrocarbons in soil appear to have been delineated beneath the site <br /> Groundwater was encountered at approximately 9 to 14 feet bgs During May 1997 static <br /> groundwater elevations ranged from approximately 6 to 9 5 feet bgs and the groundwater <br /> gradient was towards the southwest at a hydraulic gradient of 0 002 Groundwater elevations <br /> were essentially the same in the shallow and deep wells, indicating hydraulic communication <br /> between the two depths <br /> • The lateral and vertical extent of TPHg and BTEX appeared to have been delineated beneath <br /> the site Concentrations of MTBE were delineated in each direction except towards the south <br /> • and southwest based on the initial sampling of wells MW4S/D through MW7S/D <br /> Based on the results of the investigation, ERI submitted a Work Plan which included 1) obtaining and <br /> reviewing available MTBE data from previous investigations, 2) performing two additional <br /> groundwater monitoring and sampling events and interpreting the data to further evaluate the extent of <br /> dissolved MTBE in groundwater and select additional well locations (if necessary), 3) drilling three soil <br /> borings adjacent to former drains, 4) obtaining appropriate permit(s) and access agreement(s), drilling <br /> off-site soil boring(s), collecting soil samples, constructing groundwater monitoring well(s), developing <br /> and purging the new well(s), collecting groundwater samples, analyzing selected soil and groundwater <br /> samples, and 5) preparing a report <br /> The San Joaquin County PHS/EHD response to the work plan requested Tosco complete the following, <br /> 1) perform a detailed site assessment to evaluate the lateral and vertical extent of MTBE in groundwater <br /> and whether these compounds have migrated off-site and additionally analyze groundwater samples for <br /> the seven oxygenate compounds identified in the California Regional Water Quality Control Board <br /> Central Valley Region(CRWQCB) document dated July 18, 1997, 2) perform a conduit study and well <br /> survey to identify underground facilities that may provide potential preferential groundwater flow <br /> conduits, 3) characterize hydrogeology beneath and in the vicinity of the site, and 4) identify the <br /> source(s) of MTBE detected in groundwater beneath the site and the approximate time frame of <br /> MTBE/additives release and history <br /> At the request of Tosco, ERI submitted the addendum to our work plan (dated July 14, 1997) for the <br /> subject site by the requested date The addendum addressed the directive for a full dimensional <br /> • <br /> 2 <br />