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2 The Great Valley is an asymmetrical synclinal trough with its axis off center to the west. The San Joaquin Valley is an asymmetric structural trough with abroad,gently inclined,and little deformed east flank and a relatively narrow west flank. Two major faults, the Stockton and White Wolf faults, cut perpendicular to the axis of the valley. ' The Stockton fault is located approximately one mile north of the Site and the White Wolf fault is located at the southern end of the valley. The Stockton Arch, a broad structure that is bound on the north by the Stockton fault but has a poorly defined southern limit, separates the San Joaquin and 1 Sacramento sedimentary basins. The Stockton Arch does not have appreciable structural relief,but it did have an influence on sedimentation. Cenozoic deposits in the San Joaquin Valley thicken southeastward from 800 meters over the western part of the Stockton Arch to over 9,000 meters in ' the southern part of the Valley(USGS, 1991). The valley fill consists of an up to a 9 kilometer thick sequence of Jurassic to Holocene aged marine, ' lacustrine, fluvial, and eolian sediments. The valley fill rests on a westward-tilted block of crystalline basement composed of Sierra Nevada plutonic and metamorphic rocks under the eastern part of the valley and mafic and ultramafic rocks under the central and western parts of the valley (USGS, 1991). 2.3 Regional Hydrogeology ' The Great Valley aquifer system is formed primarily of sand and gravel with significant amounts of silt and clay, all of which have been eroded mainly from older rocks at the boundaries of the valley. Little groundwater flows through the extensive deposits of consolidated marine and mixed marine and continental sediments that overlie the crystalline basement rocks. The marine sediments usually contain saltwater or brine,but near the northwestern,western, and southeastern margins of the San Joaquin Valley, some fresh water is withdrawn from these deposits (USGS, 2002). The environments in which the continental sediments were deposited varied, but most were deposited in a fluvial environment; however, the deposits contain some lacustrine beds. Locally, volcanic rocks and dune deposits are part of the aquifer system (USGS, 2002). jThe Site lies in the Eastern San Joaquin Subbasin of the San Joaquin Valley Groundwater Basin (Groundwater Basin Number 5-22.01). The Eastern San Joaquin Subbasin is defined by the areal extent of unconsolidated to semiconsolidated sedimentary deposits that are bounded by the Mokelumne River on the north and northwest, the San Joaquin River on the west, the Stanislaus River on the south, and consolidated bedrock on the east(DWR, 2003). ' FREY