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2 BACKGROUND <br /> The Corral Hollow Sanitary Landfill is a class III landfill located in San Joaquin County <br /> approximately 5 miles south of Tracy, California, along Interstate 580 (see Figure 1). <br /> Monitoring requirements for the landfill are specified in WDR Order No. 94-259, adopted <br /> by the California Regional Water Quality Control Board (RWQCB), Central Valley <br /> Region, on September 29, 1994. Information regarding the site hydrogeology, monitoring <br /> network,and monitoring and response actions is presented below. <br /> 2.1 Regional Hydrology <br /> The site is located on the western boundary of the San Joaquin Valley groundwater basin. <br /> The groundwater reservoir in the Tracy area has been divided into three water-bearing <br /> zones: (1) a lower water-bearing zone which contains confined fresh water below the <br /> Corcoran Clay; (2) an upper water-bearing zone which contains confined, semiconfined, <br /> and unconfined water above the Corcoran Clay and younger deposits; and (3) a shallow <br /> water-bearing zone which contains unconfined water within about 25 feet of the land <br /> surface (Hotchkiss, 1971). In the study area, the upper two zones converge above the <br /> Corcoran Clay,and are considered equivalent to the shallow perched water-bearing zone. <br /> In general, groundwater in the deep regional aquifer in the Tracy area moves toward the <br /> northeast away from the Coast Range and towards the San Joaquin River (Hotchkiss, <br /> 1971). Local cones of depression are located throughout the valley and are due to <br /> groundwater overdrafts caused by extensive irrigation pumping before the introduction of <br /> imported water. Shallow water in the upper parts of the Tulare Formation, alluvium, and <br /> terrace deposits also historically flows from the Coast Range to the northwest toward the <br /> San Joaquin River. <br /> 2.2 Local Hydrology <br /> Two groundwater-bearing zones have been identified on site: (1)a shallowP erched water- <br /> bearing zone within the Corcoran Clay, considered to be unconfined; and (2) a deep <br /> regional water-bearing zone which may be semiconfined to confined (see Drawings 1 <br /> and 2). <br /> SACNN.\PJOM3M390031.9BG-94\sdg:1 Rev.0,420/95 <br /> 0939-003.19 2-3 <br />