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08 December 2005 AGE-NC Project No. 01-0828 Page 2 of 9 y continental. Continental-derived sediments were typically deposited in lacustrine, fluvial, and alluvial environments with sediment sources being the mountain ranges surrounding the valley (Olmsted and Davis, 1961); the site itself is located on unconsolidated and semi-consolidated alluvium, lake, playa, and terrace deposits of Quaternary age (California Division of Mines and Geology, 1977).Rocks composing the basement complex of the Province have not been completely defined, but are believed to be metamorphic and igneous in origin. Based on the General Soil Map from the San Joaquin County Soil Survey,published by the United States Department of Agriculture Soil Conservation Service in 1992, the site area is within the Jackson-Hallenbeck-Stockton (JHS) association. The JHS soils are located within basins, and generally consist of moderately-to poorly-drained,fine-textured soils.The soils are generally derived from both marine and non-marine sediment, and are generally formed in alluvium derived from mixed rock sources. v 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).Based on review of the Lines of Equal Depth to Groundwater, Fall 1999 map published by the San Joaquin County Flood Control District and Water Conservation District (FCD&WCD),the estimated depth to ground water at the site is between 40 feet and 50 feet below surface grade (bsg). However, historical ground water depth may have been between 70 feet and 80 feet bsg. During the October 2001 investigation conducted by AGE, depth to first encountered ground water was approximately 55 feet bsg. Review of the map Lines of Equal Elevation of Groundwater, Fall 1999(FCD&WCD)depicts the regional ground water flow direction toward the northeast, however, this may vary due to local recharge and discharge patterns. 2.2. UNDERGROUND STORAGE TANK REMOVAL An Unauthorized Release Report(UAR)was submitted to the San Joaquin County Environmental Health Department(EHD) in 1986 for two 6,000-gallon diesel USTs(Tank No. 4 and Tank No. 5) which were removed from the site sometime prior to 1986. Laboratory analysis of soil samples collected from beneath the north and south ends of the tank pit detected total extractable hydrocarbons at concentrations as high as 17,600 milligrams per kilogram (mg/kg). In June 1987, three additional soil samples were collected from the former Tank No.4 and Tank No. 5 excavation. Laboratory analysis of the three samples detected concentrations of total volatile petroleum hydrocarbons at concentrations ranging from 4.2 mg/kg to 130 mg/kg. In 1992, a UAR was filed with the EHD for one 2,000-gallon and one 500-gallon(Tank No. 1 and Tank No. 2,respectively) former gasoline USTs. Soil samples collected from beneath Tank Nos. 1 and 2 did not contain detectable concentrations of petroleum hydrocarbons. On 17 May 1990, one Advanced GeoEnvironmental,Inc. r