SITE INFORMATION AND CORRESPONDENCE

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June 13, 1997 Page 3 / California are shown on Plate P-I [Attachment D]. One municipal water supply well operated by California Water Service is within 1,000 feet of the site. A municipal well operated by California Water Service is approximately 800 feet northeast and down gradient of the site. According to California Water Service personnel, the well's submersible pump is 160 feet below ground surface (pp. 9 and 10)." Previous Assessments (Le., 1.d.). Site investigations began in January 1988 with the drilling of five borings designated KB 1 through KB5 (Figure C-2). Elevated concentra- tions of total petroleum hydrocarbons (TPH) were detected in soil samples at 20.5 feet at the bottom of Borings K133 and KB5 (239 and 370 parts per million [ppm]), respec- tively. Soil analytical data are presented as Attachment E. In March I988 site UST and associated piping was removed. One soil sample from beneath'the tank had elevated concentration of total purgeable petroleum hydrocarbons calculated as gasoline (TPPH-g) (582 ppm). Analytical data from tank removal samples are presented as Attachment E. A quarterly groundwater monitoring and sampling program was initiated in February 1988 with the installation of Wells MW-I through MW-3. Initial monitoring data found a small dissolved hydrocarbon plume to be centered in the eastern portion of the site near Well MW-2. Subsequently, six additional wells (MW-4 through MW-9) were installed to define the lateral extent of the plume. Groundwater analytical data from the new wells confirmed that the hydrocarbon plume is localized in the area of Well MW-2 (Figure C-5). No SPH has been found in any site well_ MtBE has been below detection in all samples since it was included in the monitoring program in mid-1996. Historic groundwater analytical data are presented as Attachment F. The most recent quarterly groundwater monitoring report prepared by MPDS Services and dated June 13, 1997 is enclosed with this report. SITE REMEDCATI<ON Soil Excavation (Le.). In March 1988, all USTs and associated piping were removed from the site. Six soil samples were collected from native silty clay at approximately 16 feet bgs in the gasoline storage tank cavity. Obvious hydrocarbon odor was present in three soil samples. Some evidence of soil contamination was noted in the native material below the tanks and in the walls of the excavation. Additional excavation was performed in an attempt to remove all obviously impacted soil. The tank cavity was enlarged to approximately 25 by 35 feet. The excavation was extended to a total depth of 26 feet. According to Unocal, approximately 1,100 cubic yards of hydrocarbon- impacted soil were removed from the excavation. Soil Vapor Extraction (SVE) (Le.). A SVE was operated at the site between January and November 1996. Hydrocarbons were extracted from the subsurface using three 3100406A\CLS RE.DOC