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Regional Geology and Hydrogeology: The site lies in the Central Valley of California. Geologically, the valley is a large asymmetric trough that is bounded by pre-Tertiary granitic, metamorphic, and marine sedimentary rocks. 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 (USGS 1986). 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 deposits are heterogeneous mixtures of poorly sorted clay, silt, sand, and gravel, and in places some beds of claystone, siltstone, sandstone, and conglomerate. The maximum thickness reported for these formations is 3,000 feet. Continental deposits of Quaternary age crop out primarily along the major rivers and streams of the valley as well as other low-lying areas; the deposits include river and 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 tapped by wells. Flood-basin deposits consist largely of fine-grained beds that restrict the vertical movement of water. Site Geology: A review of boring logs from this and previous studies show that with the exception of the silty, sandy gravel backfilled area, the predominant soil types at the site are clays and silts. Sandy silts and sandy clays are present across the site to approximately 10 to 15 feet below grade (fbg). Underlying these units are fine- to medium-grained sands, sandy silts, and sandy clays, ranging in thickness from less than 1 to approximately 3 feet thick. A medium- stiff to stiff, silty clay underlies the sandy zone, and persists to the total depth of the borings. Saturated soils are generally encountered at a depth of between 10 and 11 fbg. Ground Water Quality and Usage: Ground water in San Joaquin County is designated by the California Regional Water Quality Control Board in the Basin Plan to have beneficial or potentially beneficial use. Currently, however, the uppermost water- bearing zone does not yield water used for domestic, industrial, or municipal purposes; this may be attributed to the degraded nature of this • 2