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Case Closure Report Former Tosco(Unocal)SS No 5421, Stockton,California July 8, 1998 . Analytical results of the groundwater samples collected from monitoring wells MWA, MW-2, and MW-4 indicated non-detectable levels of TPH as gasoline and BTEX In well MW-3, TPH as gasoline and benzene were detected at levels of 220 ppb and 7 6 ppb, respectively In well MW-1, TPH as diesel was detected at a level of 170 ppb, TOG was non-detectable, and all EPA Method 8010 constituents were non-detectable, except for chlorobenzene and 1,2-dichlorobenzene at concentrations of 2 3 ppb and 7 4 ppb, respectively The results of the soil and groundwater analyses, and sample locations are presented in Appendix A Tables 2B and 3B, Figure IB Based on the analytical results, KEI recommended the implementation of a monthly monitoring and quarterly sampling program for all of the monitoring wells Documentation of the well installation protocol, sample collection techniques, and sample results are presented in KEI's report (KEI-P89-1104 R7) dated December 10, 1990 UIC Closure Activities As requested by the US EPA UIC Division, Unocal proceeded with the closure of the septic system on-site On June 5, 1992, one 1,000-gallon septic tank (partially filled with sludge and liquids) was exposed during initial excavation Samples of the sludge and liquid were collected to determine proper disposal prior to tank removal On July 14, 1992, KEI returned to the site in order to observe the removal of the former leach lines Six soil samples, labeled P-I through P-6, were collected from beneath the piping located along the leach line at depths of about 7 feet below grade, except for sample P-1, which was collected from beneath the former leach field at a depth of approximately 10 feet below grade The undisturbed samples were collected from bulk materials excavated by backhoe The sludge and liquid were removed on July 15, 1992 KEI returned to the site on October 20, 1992, when the concrete septic tank was destroyed and removed in conjunction with the leach field This work was performed in accordance with an agreement between Unocal and the U S Environmental Protection Agency The sludge and liquid in the tank had previously been properly disposed of The septic tank was made of reinforced concrete and no apparent holes or cracks were observed in the tank prior to removal One soil sample, labeled S(9), was collected from beneath the septic tank at a depth of about 9 feet below grade The former leach field (located at the southwest corner of the referenced site) had been partially excavated and one soil sample, labeled SW-1, was collected from the east sidewall of the leach field excavation at a depth of about 15 feet below grade The undisturbed samples were collected, handled, and stored as previously described The sample point Iocations are shown on the attached Site Plan, Figure IC All excavated soil was stockpiled on-site and sampled prior to disposal Mr Harlin Knoll (SJCPHS) was present on-site during excavation activities 140162 03 5