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• ERI 205513 QS6 Tosco 76 Service Station 5886, Stockton Californta October 30, 1998 borings Work also included associated well development and sampling, and analyses of soil and groundwater samples The data obtained during the investigation indicated the following • Residual hydrocarbons were not detected at or above the stated laboratory method detection limits in soil samples collected from adjacent to three on-site catch basins located near the pump islands (see Plate 2) except for maximum concentrations of 3 3 milligrams per kilogram (mg/Kg) TPPHg, 8 4 mg/Kg total extractable petroleum hydrocarbons as diesel (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 ft bgs During May 1997, static groundwater elevations in the groundwater monitoring wells ranged from approximately 6 to 9 5 ft 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 TPPHg and benzene, toluene, ethylbenzene and xylene (BTEX) appear to have been delineated beneath the site Concentrations of MTBE were delineated in each direction except towards the south and southwest based on the initial sampling of wells MW4SID through MW7S/D . 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 t«o additional groundwater monitoring and sampling events and Interpreting the data to further eN aluate the extent of dissolved MTBE in groundwater and select additional well locations (if necessary) 3) drilling three soil borings adjacent to former catch basins, 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 County 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 in addition, analyze groundwater samples for the seven oxygenated compounds identified in the California Regional Water Quality Control Board Central Valley Region (Regional Board) document dated July 18, 1997, 2) perform a conduit study and well survey to Identify underground facilities that may provide potential preferential groundwater flow and constituent migration, 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 time frame of MTBE/additives release and history At the request of Tosco, ERI submitted the Work Plan addendum (dated August 8, 1997) The addendum addressed the directive for a full dimensional definition and investigation of soil and groundwater beneath and in the vicinity of the site and submittal of a Soil and Water Investigation Report To date, Tosco has not received a response and/or approval of the Work Plan addendum from the County 2