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j-HELIMINARY NYDRnGEOLOCEC_AM35M;NT"Pe'"T 3399 East Yossrnits Avenue,Manteca,California Datta project No.40-93-480 Page d 3.2 Sate Sol) and GeoIOY The surface of the site consist of a 2-inch-thick layer of asphalt overlying a 4-inch-thick aggregate base. Subsurface geology+at the site has been inferred from soil horings drilled by EiS and bo-tilips completed by Delta (see Appendix A). The soils encountered during these drilling activities are predominantly silty sant,sand,clayey snnnd,and silty clay. The uppermost layer is a silty sand. Ben"th this is a well-graded, fine- :o coarse-grained sand. Thin clay lenses (less than 5 inches) were uko w, .n 44'lhir. this stratum, Underlying the sand stratum is a silty sand which gradotional:y c4rvnvard hecornes 5 claycf sand. tv'iYuund water was Fwt noticed in this stratum. Underlying these strwa is a saturated silty,clay. UndcYlving the silty clay is a silty said layer which is the dzepest stratum penetrated by bor ngs at the site. Sand was primarily fine-grained,but ranged from tirty fine to corpse grained. Mica is a common mineral component in all soils. A cross-section of the site is presented in figure d. The lii.e of section is presented in T -•re 5. .3 Ret=lonai�}i % ?logy Ground water gists in both confided and unconOned states it the Sscramentn Valley. Confined ground water hinds to be iocated in older and deeper formations. Unconfined water can be found in the alluvial fan, floodplain, and stream channel deports. Ground water is found at a shallower depth in the central part of the Sacramento Vailcy Basin (5 ft:et) and de,:per toward the basin margins,where it may occur at depths greater than 100 feet. 21 Regional ground water flow under natural condition.; is fro3, the valley margins toward the Sacratncato River Delta Development and use:of groun t water has created a major depression ii,the ground water G surface at the site viciai y toward which ground ;nater now flows (DWR Bulletin 146, 1967)- Recharge to the water table occurs through deep percolation of water from streams, rain and snowfAl, and irrigation, Under natural conditions, it is estimated that two-th;rds of recharge to the Sacramento Basin takes place north of the Sutter Buttes. Ground water quality ttiroughout the valley makes it suitable for most purposes. Ground water in the eastern half of the valley is of be--tier quality with respect to mineral content than water from the western side of the valley,where rocks of the Cnast Range contain many soluble minctaLs which promote higher