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07 October 2004 AGE-NC Project No. 01-0887 Page 2 of 6 The thickness of the sedimentary fill ranges from thin veneers along the valley edges to more than 20,000 feet in the south central portion of the valley. Older deposits are primarily marine in origin and younger deposits are primarily continental. Continental-derived sediments, originating in the surrounding mountain ranges, were typically deposited in lacustrine, fluvial, and alluvial environments (Olmsted and Davis, 1961). The Modesto, Riverbank, and Turlock Lake Formations and overlying Recent alluvium are the principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Water Basin (Basin 5-22). This basin is drained primarily by the San Joaquin River. The nearest surface water feature in the vicinity of the property is the Old River,located approximately 2.5 miles west of the property. 2.2. UNDERGROUND STORAGE TANK REMOVAL On 07 December 1992, one 10,000-gallon gasoline UST and approximately 100 feet of associated piping were removed from the site by Western Meter Service, Inc. and transported to Cal Coast Manufacturing Co. for disposal. An Unauthorized Release Report (UAR) was issued by the San Joaquin County Environmental Health Department (EHD) on 31 December 1992. The UAR was issued after laboratory analysis of soil samples collected during the UST and piping removal detected the volatile aromatic compounds benzene,toluene,ethylbenzene,and total xylenes (BTEX) at concentrations as high as 0.026 milligrams per kilogram (mg/kg), 0.007 mg/kg, 0.009 mg/kg, and 0.01 mg/kg, respectively. Total petroleum hydrocarbons quantified as gasoline (TPH-g) and BTEX were detected at concentrations as high as 8.3 mg/kg and 15.9 mg/kg (ethylbenzene), respectively, in soil samples collected from below the former product piping (approximately 4 feet below surface grade: bsg). 2.3 PREVIOUS SITE ASSESSMENT AND SAMPLING On 02 May 2002,three soil probe borings(P-1,P-2,and P-3)were advanced at the site as part of the assessment related to the former UST removal. TPH-g and BTEX compounds were detected in the three analyzed soil samples at concentrations as high as 2,800 mg/kg(TPH-g),24 mg/kg(benzene), 120 mg/kg(toluene), 51 mg/kg(ethylbenzene), and 290 mg/kg (total xylenes) in sample P-2-5.No fuel additives were detected at or above laboratory reporting limits in the three analyzed soil samples (Table 1). TPH-g,BTEX compounds,and MTBE were detected in each of the grab ground water samples.The highest concentrations of target analytes were detected in the grab ground water sample collected Advanced GcoEnvironmental,Inc.