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r!1 IL t • ERI 205513 QS2 Unocal Service Statin 5886 Stockton, California October 6, 1997 definition and investigation of soil and groundwater beneath and in the vicinity of the site and submittal of a Soil and Water Investigation report Tosco performed a well survey to identify registered wells within a 1/4 mile radius of the site The purpose of this survey is 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 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 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 September 2, 1997 The results and data are attached The results of the • quarterly monitoring and sampling event indicated the following • Groundwater was measured at approximately 8 2 to 115 feet bgs During September 1997, groundwater flow direction was towards the west-northwest at a hydraulic gradient of approximately 0 003 • TPPHg, BTEX, and TEPHd were not detected above stated laboratory method detection Iimits in groundwater samples collected from the groundwater monitoring wells except for 1 0 ug/L benzene and 410 ug/L TEPHd detected in the groundwater sample collected from monitoring well MW413 and 740 ug/L.TEPHd detected in the groundwater sample collected from monitoring well MW613 Sequoia Analytical Laboratories reported dissolved diesel concentrations were a diesel and discreet peaks mixture • MTBE was not detected above stated laboratory method detection limits in monitoring wells MW4S through MW7S, MW5D, and MW71) MTBE was detected in the remaining groundwater monitoring wells up to 9,100 ug/L Based on the data, MTBE concentrations appear to be decreasing in well MW3 adjacent to the existing tank pit which indicates it is unlikely there is an ongoing source • ERI performed an underground utility search to identify locations and orientations of utility trenches such as sewer lines adjacent to the site to evaluate whether these facilities may provide conduits for the potential lateral off-site migration of dissolved hydrocarbons and oxygenate compounds Based upon data provided by the City of Stockton Public Works 3