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ERi 205513 QS1 Unocal Service Station 5886 Stockton,California August 21 1997 Sduring December 1996 Concentrations of MTBE have been detected in each on-site well up to 20,000 ppb (MPDS Services, Inc [MPDS], January 21, 1997) In April 1997, ERI performed an Investigation to evaluate the source and extent of MTBE (ERI, June 3, 1997) The investigation included hand-auguring three soil borings adjacent to select drains, drilling four soil borings and constructing dual completion groundwater monitoring wells (MW4S/D through MW7S/D) at discreet depths in the borings, associated well development and sampling, and analyses of soil and water samples The data obtained during the investigation indicated the following 0 Residual hydrocarbons were not detected at or above the stated laboratory method detection limits in soil samples collected from adjacent to three on-site drains (see Plate 2) except for maximum concentrations of 3 3 mg/Kg TPHg, 8 4 mg/Kg TEPHd, and 12 mg/Kg MTBE • Residual hydrocarbons in soil appear to have been delineated beneath the site • Groundwater was encountered at approximately 9 to 14 feet bgs During May 1997 static groundwater elevations ranged from approximately 6 to 9 5 feet bgs and the groundwater gradient was towards the southwest at a hydraulic gradient of 0 002 Groundwater elevations were essentially the same in the shallow and deep wells, indicating hydraulic communication between the two depths • The lateral and vertical extent of TPHg and BTEX appear to be delineated beneath the site • Concentrations of MTBE have been delineated in each direction except towards the south and southwest based on the initial sampling of wells MW4SID through MW7SID Based on the results of the investigation, ERI submitted a Work Plan which included 1) obtaining and reviewing available MTBE data from previous investigations, 2) performing two additional groundwater monitoring and sampling events and interpreting the data to further evaluate the extent of dissolved MTBE in groundwater and select additional well Iocations (if necessary), 3) drilling three soil borings adjacent to former drains, 4) obtaining appropriate permit(s) and access agreement(s), drilling off-site soil boring(s), collecting soil samples, constructing groundwater monitoring well(s), developing and purging the new well(s), collecting groundwater samples, analyzing selected soil and groundwater samples, and 5) preparing a report The San Joaquin County PHS/EHD response to the work plan requested Tosco complete the following, 1) perform a detailed site assessment to evaluate the lateral and vertical extent of MTBE in groundwater and whether these compounds have migrated off-site and additionally analyze groundwater samples for the seven oxygenate compounds Identified In the California Regional Water Quality Control Board Central Valley Region (CRWQCB) document dated July 18, 1997, 2) perform a conduit study and well survey to identify underground facilities that may provide potential preferential groundwater flow conduits, 3) characterize hydrogeology beneath and in the vicinity of the site, and 4) identify the source(s) of MTBE detected in groundwater beneath the site and the approximate timeframe of MTBE/additives release and history . 2