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Case Closure Report Former Tosco (Unocal)S5 No 5421, Stockton,California July 8, 1998 • 0 Impacted soil is well defined and residual hydrocarbons are very limited in extent and concentration A maximum concentration of 24 ppm TPHg and 0 77 ppm benzene remain in the former UST excavation at 26 feet bgs, the maximum depth feasible to excavate • Groundwater has been non detected for benzene and MtBE for at least the last four sampling events (24 months/ 2 hydrologic cycles) in all wells at the site In addition, recent analytical data requested by SJCPHS indicate that all oxygenates are non detected in soil and groundwater • There is no potential impact to off-site receptors, nor is there a potential risk to human health or the environment with the remaining residual hydrocarbons on-site The beneficial use of the groundwater beneath the site has not been impacted • Unocal received a UIC Closure Letter from the US EPA on March 9, 1994 regarding the removal and remediation of soils associated with the former septic tank, leach field and seepage pits Recommendations iBased on the analytical results of all of the soil and groundwater samples collected and evaluated to date, and based on the conclusions discussed above, no further delineation or remedial work associated with the petroleum hydrocarbons detected at the former Tosco (Unocal) site is warranted The required RWQCB's "Case Closure Checklist" has been completed and is presented in Appendix F Therefore, GR recommends that Tosco request that"Case Closure" be granted from the SJCPHD and the RWQCB Central Valley Region Once case closure is granted, GR will prepare a Work Plan to destroy the existing four groundwater monitoring wells located at the site Distribution A copy of this report should be sent to Mr Harlin Knoll of the SJCPHD, and to Ms Elizabeth Thayer of the RWQCB Central Valley Region, Sacramento Branch Office 140162 03 14