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2, FIELD INVESTIGATIONS <br /> 2.1 PREVIOUS INVESTIGATIONS <br /> 2.1.1 Hydrogeology <br /> The Stockton area is underlain by unconsolidated Quaternary <br /> deposits of the Victor formation, the Laguna formation, which <br /> includes the Arroyo Seco gravel, and related continental depos- <br /> its. The Laguna formation may be partly of late Pliocene age. <br /> Beneath these deposits lie consolidated and semi-consolidated <br /> Tertiary deposits of the Mehrten and Valley Springs formations <br /> and undifferentiated older marine sediments. Beneath these <br /> sedimentary formations lies a basement complex of pre-Cretaceous <br /> metamorphic and igneous rocks (Figure 3) . The Victor and Laguna <br /> formations contain the principal fresh-water aquifers in this <br /> region. <br /> The alluvium and Victor formation together are roughly 150 feet <br /> thick, and the Laguna formation is about 1, 000 feet thick, in the <br /> Stockton area. These were deposited in flood plains and alluvial <br /> fans. Locally, these anits make up the fresh--water-bearing <br /> series, with a total thickness of some 1,200 feet. Lithological- <br /> ly, these materials are very heterogeneous. They are made up by <br /> discontinuous sand and gravel beds and lenses, separated by sandy <br /> silt, silt, and clay. On the east side of Stockton, sand and <br /> gravel constitute some 32 percent of the total fresh-water- <br /> bearing series (CDWR 1955) . <br /> t <br /> The Victor formation and alluvium have moderate permeabili.ties <br /> and contain unconfined ground water, which stands at a depth of <br /> 50 feet in the vicinity of the Chevron site (SJCFC & WCD 1986) <br /> The Laguna formation has moderate permeabilities and contains <br /> partially confined aquifers, although there is no evidence for <br /> continuous-pressure aquifers. Recharge to these aquifers occurs <br /> B <br />