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sr <br /> V <br /> UP <br /> U& <br /> CHAPTER 3 <br /> HYDROGEOLOGY <br /> To evaluate the potential impact of groundwater quality beneath <br /> ARCO Petroleum Products Company (ARCO) Service Station 2093 on the <br /> City of Tracy ' s groundwater supply, this section presents the <br /> regional and site-specific hydrogeology. Discussions of regional <br /> geology and groundwater occurrence and flow are based on U.S. <br /> LW Geological Survey (USGS) Open-file Report 6433-02. Local geology is <br /> based on borehole lithology from this study. <br /> Regional Geology <br /> The Tracy area is underlain by two major geologic units. In <br /> descending stratigraphic order they are: (1) unconsolidated <br /> sedimentary deposits of Tertiary and Quaternary age and (2) consoli- <br /> dated sedimentary and basement rocks of Jurassic, Cretaceous, and <br /> Tertiary age . <br /> Unconsolidated deposits consist of Quaternary alluvial deposits <br /> and the Tertiary and Quaternary Tulare Formation. Alluvial deposits <br /> LW immediately underlie the Tracy area. These deposits, which may be up <br /> to 350-feet thick, are made up of interbedded clay, silt, sand, and <br /> gravel. <br /> The Tulare Formation, immediately beneath the alluvial deposits, <br /> is made up of beds, lenses, and tongues of clay, sand, and gravel. <br /> The total thickness of the Tulare Formation regionally is less than <br /> 4. 1,350 feet. The relatively impermeable Corcoran Clay Member of this <br /> formation acts as a confining bed separating the Tulare Formation <br /> into an upper and lower section. The Corcoran Clay Member may be up <br /> to 125-feet thick; it occurs at about 150 feet below sea level in the <br /> Tracy area. <br /> The consolidated formation consist of up to 10 , 000 feet of <br /> LO sedimentary rocks underlain by an unknown thickness of crystalline <br /> basement rocks . The sedimentary rocks are mostly marine sandstones <br /> and shales of Cretaceous and Tertiary age. The Jusassic and <br /> �. Cretaceous basement complex is made up of crystalline rocks and <br /> Franciscan formation melange. <br /> The consolidated rocks are at depths of greater than 350 feet, <br /> store and transmit 'Less water than the unconsolidated deposits, and <br /> do not contain fresh water . They will not be discussed further in <br /> this report. <br /> V <br /> BROWN AND CALDWELL e , CONSUL PING ENGfiNEERS <br />