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3 <br /> r. <br /> ERI 205513 QS4 Unocal Service Station$886, Stockton, California <br /> • April 28, 1998 <br /> submittal of a Soil and Water Investigation Report To date, Tosco has not received a response from <br /> San Joaquin PHS/EHD <br /> Tosco performed a well survey to identify registered wells within a 114 mile radius of the site The <br /> purpose of this survey was to identify wells that may provide conduits for potential vertical migration <br /> of dissolved hydrocarbons and oxygenate compounds and provide information regarding sensitive <br /> receptors in the area The data indicates ten documented wells are located within a 1/4-mile of the <br /> site, seven of these wells are cathodic protection wells and three are water supply wells (1 e <br /> irrigation) The closest water supply well is approximately 1,000 feet northeast of the site 1n the <br /> generally upgradient direction of interpreted groundwater flow <br /> In July 1997, Triangle Environmental, Inc performed a tightness test to evaluate the integrity of the <br /> existing fuel distribution system The results indicate the system passed within allowable tolerances <br /> During third quarter 1997, Tosco performed an underground utility search to identify locations and <br /> orientations of utility trenches and evaluate whether these facilities may provide conduits for the <br /> potential lateral off-site migration of dissolved hydrocarbons and oxygenate compounds The results <br /> of this utility search indicated that utilities are primarily located within the March Lane right-of-way <br /> and are oriented east to west Lateral utility trench Iines servicing specific sites are oriented <br /> perpendicular to the main utility lines The deepest utility trench appears to be a 21-inch sewer line <br /> located approximately 70 feet south of the site This line is situated approximately 10 5 feet bgs <br /> directly south of the site and is inclined towards the west measuring approximately 12 feet bgs 200 <br /> feet southwest of the site The locations of underground utilities are shown on Plate 2 <br /> On January 28, 1998, Tosco submitted the fourth quarter 1997 Quarterly Summary Report which <br /> included proposed off-site cone penetrometer (CP) locations to further evaluate the extent of dissolved <br /> MTBE detected in groundwater during ongoing quarterly groundwater monitoring and sampling The <br /> proposed CP locations are shown the PIate 2 Tosco is presently awaiting San Joaquin County <br /> PHS/EHD approval of the proposed locations <br /> ONGOING WORK <br /> Based upon previous work completed at the site and in accordance with the correspondence and <br /> request from the San Joaquin PHS/EHD, Tosco completed the following <br /> • GRI of Dublin, California performed groundwater monitoring and sampling at the site on <br /> March 25, 1998 The results and data are attached The results of the quarterly monitoring <br /> and sampling event indicate the following <br /> • Groundwater was measured at approximately 5 1 to 7 9 feet bgs (an elevation increase <br /> of 1 6 to 2 0 feet since the fourth quarter 1998 groundwater monitoring and sampling <br /> event) During March 1998, groundwater flow direction was towards the southwest at <br /> a hydraulic gradient of approximately 0 005 Historic groundwater flow directions <br /> and gradients are shown on the Groundwater Flow Direction and Gradient Rose <br /> Diagram (Plate 3) <br /> 3 <br />