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• ERI 205513 QS3 Unocal Service Station 5886,Stockton Califorma January 28 1998 Tosca performed a well survey to Identify registered wells within a 114 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 a 1/4-mile of the site, seven of these wells are Cathodic protection wells and three are water supply wells (i e 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, Tosca 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 ONGOING WORK • Based upon previous work completed at the site and in accordance with the correspondence and request from the San Joaquin PHS/EHD, Tosco completed the following • MPDS Services Inc of Concord, California performed groundwater monitoring and sampling at the site on December 1, 1997 The results and data are attached The results of the quarterly monitoring and sampling event indicate the following • Groundwater was measured at approximately 6 7 to 9 9 feet bgs During December 1997, groundwater flow direction was towards the west-southwest at a hydraulic gradient of approximately 0 002 Historic groundwater flow directions and gradients are shown on the Groundwater Flow Direction and Gradient Rose Diagram (Plate 3) • TPPHg, BTEX , and TEPHd were not detected above stated laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells except for 0 51 ug/L benzene detected in the groundwater sample collected from monitoring well 4D, 55 ug/L TEPHd detected in the groundwater sample collected from monitoring well MW5S, and 140 ug/L TEPHd detected in the groundwater sample collected from monitoring well MW6D Sequoia Analytical Laboratories reported dissolved diesel range peaks did not appear to be diesel • MTBE was not detected above stated laboratory method detection limits in monitoring wells MW4S through MW7S, MW4D, MW5D, and MW7D MTBE was detected 1n the remaining groundwater monitoring wells up to 3,600 ug/L Based on the data, 3