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t <br /> ' grained sediments. No saturated zones were encountered within this unit in the <br /> borings. <br /> The lower zone beneath the Corcoran Clay consists of alluvial sands and gravels with <br /> ' minor fines. The top of this zone was encountered at an elevation of approximately -20 <br /> feet MSL; only the top 20 feet of the unit was penetrated. This unit consists of <br /> ' interbedded sandy-gravels and gravelly sand. The coarse fraction is composed of the <br /> same material found in the upper zone with the exception of the fines. The permeability <br /> ' in this unit appears much higher than the overlying sediments. The borehole was left <br /> open overnight at a depth of approxiamately 5 feet above the lower sand and gravel <br /> zone. No water was observed in the borehole the following morning. Ground water <br /> ' was encountered upon penetrating this zone in borings MW-2 and MW-3 and rose <br /> approximently 10 to 15 feet within an hour in the borehole. <br /> 2.4 HYDROLOGY <br /> ' 2.4.1 Regional Ground-Water Occurrence and Movement <br /> The site is located on the western boundary of the San Joaquin Valley ground-water <br /> basin. The ground-water reservoir in the Tracy area has been divided into three water- <br /> bearing zones: (1) a lower water-bearing zone which contains confined fresh water <br /> below the Corcoran Clay, (2) an upper water-bearing zone which contains confined, <br /> ' semiconfnned, and unconfined water above the Corcoran Clay and younger deposits, <br /> and(3) a shallow water-bearing zone which contains unconfined water within about 25 <br /> ' feet of the land surface (Hotchkiss, 1971). In the study area the upper two zones <br /> converge above the Corcoran Clay and are considered equivalent to the shallow perched <br /> ' aquifer. <br /> ' In general, ground water in the deep regional aquifer in the Tracy area moves toward <br /> the northeast away from the Coast Range and towards the San Joaquin River <br /> (Hotchkiss, 1971). Local cones of depression are located throughout the valley and are <br /> ' due to ground-water overdrafts caused by extensive irrigation pumping before the <br /> introduction of imported water. Shallow water in the upper parts of the Tulare <br /> ' Fonnation, alluvium, and terrace deposits also historically flows from the Coast Range <br /> STL 9390314 9 A'jn OAsg9c13o�� s8 <br />