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09/12/97 16:42 GR/GSI—SACTO - 12094640138 P07 Work Plea For Soil Boring and Monitoring Well Liewlation-UaQcal Station No. 6981 September 12, 1997 - SITE DESCRIPTION The subject site is an operating Unocal Service Station situated on the northwest corner of the intersection of Pacific Avenue and March Lane in Stockton, California, as shown on the vicinity Map (Figure I). The site is bounded to the north and west by an asphalt parking lot, to the east Pacific Avenue, and to the south by March Lane. Properties in the general site vicinity are used for commercial purposes. Locat topography is flat-lying, with an elevation of approximately 13 feet above mean seal level (MSL). Site improvements consist of three product dispenser islands and two 12,000-gallon underground storage tanks (USTs) in a common excavation east of the building. The building is not utilized by Unocal. The former UST's were replaced with new double-walled fiberglass USB in May and June, 1988. Locations of the pertinent site features are shown on the Site Plan (]Figure 2). PREVIOUS ENVIRONMENTAL WORK In May and June 1988, RESNA Industries collected soil samples during replacement of the UST§ and from teat pits excavated adjacent to the product dispenser islands. Petroleum hydrocarbons were detected in soil samples collected from the UST pit, product lines, and test pits. Approximately 1,600 cubic yards (ydt or hydrocarbon-impacted soils were excavated and treated at the site prior to disposal, Due to the proximity of roads and structures, overexcavation was terminated before all hydrocarbon-impacted soils could be removed. A,soil vapor survey was performed to assess the extent of residual hydrocarbons in the soil beneath the site. Thirteen groundwater monitoring wells, four vapor extraction wells, and one groundwater extraction well have been installed at or near the subject site. These wells were installed between January 1990 and August 1992. A vapor extraction test (V)3T) was performed in March 1992 to evaluate the feasibility of vapor extraction as a remedial option for soil beneath the site. An aquifer pumping test was performed in August 1992 to evaluate the feasibility of utilizing groundwater attraction to remediate hydrocarbon-impacted groundwater. A vapor extraction system has operated at the site since September 1994. To date, approximately 12,000 pounds of gasoline hydrocarbons and 169 pounds of benzene have been extracted and treated. The VES continues to operate_ zassara 2