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June 13, 1997 Page 2 gasoline and 12 pounds of benzene have been removed from the soil by a soil vapor extraction (SVE) system The groundwater plume is small and stable in size No methyl-tert butyl ether (MtBE) has been detected in any groundwater sample No separate-phase hydrocarbon (SPH) has ever been observed in any well + The closest public water-supply well is located approximately 800 to ' 1,000 feet crossgradient of the site Residual hydrocarbons in the soils at depths of greater than 26 feetS� � ZG r below ground surface (bgs) are unlikely to pose a vapor migration lef' 1A threat to the surface ' Wj Unocal has submitted several requests for case closure to San Joaquin County Environmental Health Division (SJCEHD) The latest request from Unocal dated February 7, 1997 is presented as Attachment B along with SJCEHD's response In each ' case, SJCEHD has requested additional data prior to considering site closure BACKGROUND 1 Site Description (La., Lb.). The former Unocal Service Station 2859 was located at 1665 Pacific Avenue, Stockton, California (Attachment C, Figure C-1) The service station originally included two dispenser islands, two gasoline USTs, and one waste oil' tank (Figure C-2) The site was demolished in February 1988 All product distribution piping and USTs were removed The site now exists as a vacant lot 1s� ' Geologic/Hydrogeologic Setting (1.f.). The site is underlain by primarily silt and clay to the depth explored (60 feet) Geologic cross-sections are presented on Figure C-3 1 Data from site monitoring wells have defined a general groundwater flow to the east and northeast The latest potentiometric map (February 14, 1997) is presented as ' Figure C-4 Depth to groundwater was apprommately 30 feet in 1988 when site assessment was initiated Groundwater levels subsequently declined reaching depths greater than 50 feet in 1990 through 1992 Several wells went dry As of 1996 to 3 D ' present, groundwater levels have returned to initial depths of approximately 30 to 35 feet bgs ' Water-Supply Wells (11.). A well survey performed by Applied Geosystems (report dated December 12, 1990) found that four water-supply wells are located within I anile of the site (Attachment D) The following is taken from the Applied Geosystems report ' "Ground-water resources within the Eastern San Joaquin County Basin are intensively developed for irrigation, domestic, industrial, municipal, and stock use The locations of municipal water supply wells near the site operated by California Water Service of Stockton, I 100406AACLS RE DOC