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June 13, 1997 <br /> Page 2 <br /> gasoline and 12 pounds of benzene have been removed from the soil <br /> by a soil vapor extraction (SVE) system. <br /> • The ,groundwater plume is small and stable in size. No methyl-tert <br /> butyl ether (MtBE) has been detected in any groundwater sample. No <br /> separate-phase hydrocarbon (SPH) has ever been observed in any well. <br /> • The closest public water-supply well is located approximately 800 to <br /> 1,000 feet crossgradient of the site. <br /> • Residual hydrocarbons in the soils at depths of greater than 26 feet <br /> below ground surface (bgs) are unlikely to pose a vapor migration <br /> threat to the surface. <br /> Unocal has submitted several requests for case closure to San Joaquin County <br /> Environmental Health Division (SJCEHD). The latest request from Unocal dated <br /> February 7, 1997 is presented as Attachment B along with SJCEHD's response. In each <br /> case, SJCEHD has requested additional data prior to considering site closure. <br /> BACKGROUND <br /> Site Description (La., 1.b.). The former Unocal Service Station 2859 was located at <br /> 1665 Pacific Avenue, Stockton, California (Attachment C, Figure C-1). The service <br /> station originally included two dispenser islands, two gasoline USTs, and one waste oil <br /> tank (Figure C-2). The site was demolished in February 1988. All product distnbution <br /> piping and USTs were removed. The site now exists as a vacant lot. <br /> Geologic/Hydrogeoiogic Setting (11.). The site is underlain by primarily silt and clay <br /> to the depth explored (60 feet). Geologic cross-sections are presented on Figure C-3. <br /> Data from site monitoring wells have defined a general groundwater flow to the east and <br /> northeast. The latest potentiometric map (February 14, 1997) is presented as <br /> Figure C-4. Depth to groundwater was approximately 30 feet in 1988 when site <br /> assessment was initiated. Groundwater levels subsequently declined reaching depths <br /> greater than 50 feet in 1990 through 1992. Several wells went dry. As of 1996 to <br /> present, groundwater levels have returned to initial depths of approximately 30 to 35 feet <br /> bgs. <br /> Water-Supply Wells (1..1:). A well survey performed by Applied Geosystems (report <br /> dated December 12, 1990) found that four water-supply wells are located within 1 mile <br /> of the site (Attachment D). The following is taken from the Applied Geosystems report: <br /> "Ground-water resources within the Eastern San Joaquin County <br /> Basin are intensively developed for irrigation, domestic, industrial, <br /> municipal, and stock use. The locations of municipal water supply <br /> wells near the site operated by California Water Service of Stockton, <br /> i 100406.AXLSRE.t}PC <br />