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a.. Hilvers Residence April 26, 1993 Page 2 ENVIRONMENTAL SETTING Geology The surface geology in the vicinity consists of Recent Alluvial Deposits of gravel, sand and silt associated with the Stanislaus River. Underlying these deposits are primarily older alluvial deposits of Pleistocene and Holocene Ages. The depth of these older alluvium deposits are 250 to 400 feet thick. Underlying the older alluvium are continental deposits of Pliocene and Pleistocene Age and which consist of sand and silt with some gravel and clay units throughout. The Mehrten Formation, a consolidated unit consisting of volcanic clastic sandstone, breccias, +r conglomerate, and tuff, underlies the continental deposits and ranges from 200 to 700 feet in thickness. These deposits are relatively thick towards the western edge of the valley but pinch out towards the eastern edge where the crystalline basement complex of the Sierra Nevada becomes dominant. Hydrology The distribution of groundwater is directly related to the geology and the environments in which they were deposited. There are three groundwater bodies in the Modesto-Merced area: (1) the unconfined water body, (2) the confined body, �.. and (3) the water body in consolidated rocks. The unconfined water body occurs in the unconsolidated deposits above and east of the E-clay, except in the western and southern parts of the area where clay lenses occur and semiconfined conditions exist. The confined water body occurs in the unconsolidated deposits below the E- clay and extends downward to the base of fresh water. The water body in consolidated rocks occurs under both perched and confined conditions (Page, 1973) . The subject site is located within sub-basin C-1 of the San Joaquin River Basin hydrologic study area. This basin is bounded to the north by a topographical high, to the south by the Tuolumne River, and to the west by the San Joaquin River. All major systems in the basin drain toward the west and flow into the San Joaquin River. Groundwater in the vicinity of the subject site has many beneficial uses and is used for municipal, industrial and agricultural uses. The older alluvium of Pleistocene and Holocene age outcrops along most of the eastern edge of the Central Valley and consists of interbedded layers of gravel, sand silt and clay, with some hardpan. In most places, the alluvium underlies slightly dissected hills or nearly flat-lying plains. The depth of this particular unit ranges from 0 to 400 feet. CTE estimates approximately 200 feet of this unit exists below the subject site. This older alluvium is the most extensively developed aquifer in the �. Modesto-Merced area, yielding water to large numbers of domestic, irrigation, industrial and public-supply wells. This extensively utilized aquifer lies beneath the subject site. The subject site lies to the east and upgradient from the E-clay terminus.