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h <br /> i <br /> REGIONAL AND SITE GEOLOGY AND HYDROGEOLOGY <br /> The Tracy service center is situated within the northwestern part of the San <br /> Joaquin Valley between the San Joaquin River and the Coast Range, and <br /> L bounded on the north b the Old <br /> y e River (a channel of the San Joaquin River) . <br /> The San Joaquin Valley is a topographic and structural basin bounded on the <br /> �* east by the Sierra Nevada fault block and on the west by the Coast Range. <br /> The service center is situated on a veneer of recent flood plain deposits <br /> that overlies older Recent deposits. This veneer consist mainly of clays <br /> w <br /> and silts. The Recent deposits consist of unconsolidated clays, silts, <br /> sands, and gravels to a depth of over 100 feet. This sequence overlies the <br /> Corcoran Clay Member, a major aquitard in the San Joaquin basin (Hotchkiss <br /> and Balding, 1971) . <br /> Shallow stratigraphy specific to the site can also be described from boring <br /> logs obtained during the 1986 field investigation (see Appendix A) and this <br /> investigation (see Appendix B) . The surficial material typically consists <br /> of 10 to 15 feet of silty clay or clayey silt overlying fine to medium silty <br /> sand. Sieve analyses performed on selected soil samples (see Appendix B) <br /> support the soil texture descriptions recorded by the project geologist in <br /> the drilling logs. <br /> The Tracy service center is located centrally within the Tracy Groundwater <br /> Basin, which is part of the San Joaquin Groundwater Basin. Locally, <br /> recharge to the basin occurs to the west of the site in the foothills of the <br /> io Diablo Mountain Range, and from surface infiltration. Precipitation for the <br /> Tracy area averages about 10 inches per year. Discharge occurs through <br /> fim drainage into the San Joaquin River and its tributaries (locally, the Old <br /> River to the north of the site) , and by pumping for domestic and industrial <br /> uses. <br /> 60. <br /> Prior to 1951, groundwater was the principal water supply in the Tracy area. <br /> In 1951, large scale importation of surface water via the Delta-Mendota <br /> / Canal was initiated. Since then, surface water has been the principal <br /> 3286a/BAV100 9 <br />