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• ERI 205513 QS5 Tosco 76 Service Station 5886 Stockton, California July 30, 1998 subnuttal of a Soil and Water Investigation Report To date, Tosco has not received a response from • SJCPHS/EHD Tosco performed a well survey to identify registered wells within a 1/4 mile radius of the site The purpose of this survey was to identify wells that may provide conduits for potential vertical migration of dissolved hydrocarbons and oxygenate compounds and provide information regarding sensitive receptors in the area The data indicates ten documented wells are located within 1/4-mile of the site, seven of these wells are cathodic protection wells and three are water supply wells (i a irrigation) The closest water supply well is approximately 1,000 feet northeast of the site in the generally upgradient direction of interpreted groundwater flow In July 1997, Triangle Environmental, Inc performed a tightness test to evaluate the integrity of the • existing fuel distribution system The results indicate the system passed within allowable tolerances During third quarter 1997, Tosco performed an underground utility search to identify locations and orientations of utility trenches and evaluate whether these facilities may provide conduits for the potential lateral off-site migration of dissolved hydrocarbons and oxygenate compounds The results of this utility search indicated that utilities are primarily located within the March Lane right-of-way and are oriented east to west Lateral utility trench lines servicing specific sites are oriented perpendicular to the main utility lines The deepest utility trench appears to be a 21-inch sewer line located approximately 70 feet south of the site This line is situated approximately 10 5 feet bgs directly south of the site and is inclined towards the west measuring approximately 12 feet bgs 200 feet southwest of the site The locations of underground utilities are shown on Plate 2 On January 28, 1998, Tosco submitted the fourth quarter 1997 Quarterly Summary Report which included proposed off-site cone penetrometer (CP) locations to further evaluate the extent of dissolved MTBE detected in groundwater during ongoing quarterly groundwater monitoring and sampling The proposed CP locations are shown the Plate 2 Tosco is presently awaiting SJCPHS/EHD approval of • the proposed locations ONGOING WORK Based upon previous work completed at the site and in accordance with the correspondence and request from the SJCPHS/EHD, Tosco completed the following • • GRI of Dublin, California performed groundwater monitoring and sampling at the site on June 17, 1998 The results and data are attached The results of the quarterly monitoring and sampling event indicate the following Groundwater was measured at approximately 5 9 to 8 9 feet bgs (an elevation decrease of approximately 1 foot since the first quarter 1998 groundwater monitoring and sampling event) During June 1998, groundwater flow direction was towards the southwest at a hydraulic gradient of approximately 0 003 Historic groundwater flow directions and gradients are shown on the Groundwater Flow Direction and Gradient Rose Diagram (Plate 3) • 3 •