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SITE CONTAMINATION WORKPLAN ARCO Mini-Mart SITE DESCRIPTION The site is located in a flat commercial/residential area in the .� northern part of Stockton (Figure 1 - Location Map, 7 . 5 Minute Stockton West Quadrangle, USGS Topographic Map Series, 1976) . The site is underlain by unconsolidated Quaternary sediment, mostly silt and clay. The depth to groundwater at the site is estimated at `i 80-85 ft (personal communication from Dennis Corcoran, San Joaquin County Public Works Department, Flood Control Section, 1991) . +� PREVIOUS WORK Falcon Energy repaired a leaking product line at this site on 20 February 1991. The leak was located in an unleaded fuel line a few feet south of the northern pump island (Figure 2 - Site Plan) . A trench measuring 20 ft long x 2 ft wide was excavated to expose the product lines, and two soil samples were collected by WaterWork, `i Inc. beneath the line and analyzed for TPH-gasoline and BTE&X. An unauthorized release of unleaded gasoline was indicated by total petroleum hydrocarbon (TPH) concentrations of 100 and 13 , 000 ppm +. and benzene concentrations of 1 and 140 ppm in these samples at depths of 8 and 23 ft below grade (see attached laboratory analysis report from Applied Analytical) . After repair of the leak, the trench was covered with a steel plate and the station was allowed to reopen for business. PROPOSED ASSESSMENT We believe that the contamination is concentrated in the vicinity of the broken coupling in the product line and diminishes rapidly away from leak. The gasoline appears to have drained rapidly in the vertical direction to depths exceeding 25 feet rather than spreading laterally. We propose to assess the vertical extent of the contamination by drilling a soil boring as close to the `~ location of the leak as possible. This boring will be drilled to at least 40 feet; actual depth will be determined during drilling by use of an on-site mobile laboratory. Two additional borings will be +r drilled, one in the down-gradient direction, to assess the lateral extent of contamination. The proposed assessment will be undertaken in the following steps: 1) Submit an Executive Summary of the proposed work to San Joaquin County Environmental Health Division and place copy in Stockton Library (copy transmitted to library on 8/9/91) . 2) Prepare a site health and safety plan in accordance with Occupational Safety and Health Guidance Manual for Hazardous +-. Waste Site Activities - Prepared by National Institute for Occupational Safety and Health, Occupational Safety and Health Administration, U.S. Coast Guard and US Environmental Protection Agency, October 1985.