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2.3 SITE GEOLOGY AND HYDROGEOLOGY <br /> 2.3.1 REGIONAL GEOLOGY AND HYDROGEOLOGY <br /> The sediments underlying the site are comprised of the Great Valley sedimentary <br /> sequence. Near-surface sediments are characterized as unconsolidated to <br /> semi-consolidated, mostly nonmarine, alluvium, lake, playa, and terrace deposits that <br /> are Quaternary in age (CDMG,1977). <br /> The San Joaquin River acts as a source of recharge to groundwater beneath the site and <br /> shows an average change in tidal flux of 3 feet (Port of Stockton). A tidal study <br /> conducted at the site in 1991 reported that tidal fluctuations of approximately 4.46 feet <br /> had a very slight effect (0.11 feet) on groundwater levels on site (Harding Lawson <br /> Associates,1991). Saltwater infiltration from the Stockton Deep Water Channel has been <br /> observed in the shallow groundwater beneath the site (URS,2003). <br /> 2.3.2 SOIL TYPES <br /> Subsurface investigation activities characterize soils beneath the site as fine-grained silt <br /> and clay with some coarser-grained (sandy) soils at depths ranging from 5 to 10 feet <br /> below grade (fbg) in the vadose zone. Saturated coarse-grained soils were encountered <br /> in the southern portion of the site. Based on geologic fence prepared by previous <br /> consultants, it does not appear that the saturated coarse-grained layers are horizontally <br /> continuous across the site. Available fence diagrams and exploratory boring logs are <br /> included in Appendix B. The locations of the wells and soil borings are shown on <br /> Figure 2. <br /> 2.3.3 GROUNDWATER DEPTH AND FLOW DIRECTION <br /> Based on previous investigations, groundwater beneath the site appears to be <br /> unconfined with both fine- and coarse-grained layers separating the shallow and deep <br /> monitoring wells. The horizontal groundwater gradient is relatively flat ranging from <br /> 0.002 to 0.04. The groundwater flow direction is generally southwesterly. Historical <br /> flow directions and gradients are shown on Table 2. The depth to groundwater <br /> generally ranges from 10 to 15 fbg (Table 3). Aquifer testing at well MW-20 indicates <br /> 241729(33) 2 CONESTOGA-ROVERS&ASSOCIATES <br />