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a^ I e r :.n 2.3 Regional Geol The following description of regional geology with respect to the site was based on United States Geological Survey Professional Paper 1401-C (1986) . The site lies in the Central Valley of California, which is about 400 males long and averages about 50 miles wide, encompassing about 20,000 square miles. Geologically, the valley is a large asymmetric trough that is bounded by granitic, metamorphic, and marine sedimentary rocks of pre-Tertiary age. The trough has been filled with as much as 30,000 feet of sediment in the San Joaquin Valley to the south and as much as 60,000 feet of sediment in the Sacramento Valley to the north. These sediments range in age from Jurassic to Holocene and include both marine and continental rocks and sedimentary deposits. The scope of this investigation includes only the uppermost deposits within the Central Valley. Continental rocks and deposits of Tertiary and Quaternary age compose a number of formations; in total they constitute the major aquifer of the Central Valley and yield large quantities of water to wells. Lithologically these deposi-'s are heterogeneous mixtures of poorly sorted clay, silt, sane, and gravel with local beds of claystone, siltstone, sandstone, and conglomerate. The maximum thickness reported in these formations is 3,000 feet. Continental deposits of Quaternary age outcrop primarily along the major rivers and streams of the valley as well as other low-lying areas; the deposits include river deposits, flood-basin deposits, and sand dunes. River deposits, including channel and flood-plain deposits, are considered to be the most permeable deposits in the valley; however, they are generally not used for water supply. Flood--basin deposits in the ;area consist largely of fine-grained beds that restrict the vertical movement of water. 3.0 FI£LD METHODS AND xROCEDURE.`A To assess the nature and extent of hydrocarbon constituents in the soil and/or ground water and the quality of the shallow ground water both onsite and offsite, Alton Geoscience performed a qualitative shallow ground water survey, installed monitoring wells, and collected soil and ground water samples for laboratory analysis. The procedures and methods used during field activities were in accordance with the requirements of the City of Tracy, the SJCEHS, and the RWQCB. 4