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ERI 205513 QS6 Tosco 76 Service Station 5886,Stockton California October 30 1998 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 oxygenated compounds and provide information regarding sensitive receptors in the area The data indicates ten registered wells within 114-mile of the site, seven of these wells are cathodic protection wells and three are water supply wells The closest water supply well is approximately 1,000 feet northeast of the site in the generally upgradient direction of interpreted historical 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 to evaluate whether these facilities may provide conduits for the potential lateral off-site migration of dissolved hydrocarbons and oxygenated 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 (approximately 10 5 ft bgs) identified near this site appears to be a 21-inch sewer line located approximately 70 feet south of the southern property boundary of this site Research indicated that this sewer line is a gravity drain system draining towards the west The locations of the 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 (CPT) locations to further evaluate the extent of 0) dissolved MTBE detected in groundwater during ongoing quarterly groundwater monitoring and sampling The proposed CPT locations are shown the Plate 2 Tosco is presently awaiting approval of the proposed locations from the County ONGOING WORK Based upon previous work completed at the site and in accordance with the correspondence and request from the County, Tosco completed the following • Gettler-Ryan, Inc (GRI) of Dublin, California performed groundwater monitoring and sampling at the site on September 1, 1998 The results and data are attached The results of the quarterly monitoring and sampling event indicate the following • Groundwater was measured at approximately 6 6 to 9 8 feet bgs (an elevation decrease of approximately 0 7 to 0 9 feet since the second quarter 1998 groundwater monitoring and sampling event) in the groundwater monitoring wells During September 1998, groundwater flow direction was towards the west at a hydraulic gradient of approximately 0 003 Historic groundwater flow directions and gradients are shown on the Groundwater Flow Direction and Gradient Diagram (Plate 3) • TPPHg, benzene, and TEPHd were not detected above laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells except for 78 micrograms per liter (,ug/L) and 80 pg/L diesel detected in the 3