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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALITY r • Hydrologic study completed on the property adjacent to the southern boundary of the proposed project site(James M.Montgomery Consulting Engineers (JMM) 1993). The Sacramento/San Joaquin Valley is underlain by a large groundwater aquifer known as the Central Valley Regional Aquifer System. The southern two-thirds of this aquifer system,known as the San Joaquin Valley Basin(DWR, 1975),covers 13,500 square miles from the Delta to r Bakersfield. The California Department of Water Resources (DWR) further divides this basin into numerous smaller units called sub-basins. Sub-basins in the northern half of the San Joaquin Valley Basin include the Tracy,Eastern San Joaquin County, Modesto,Turlock, Merced,"Chowchilla, Madera,and Delta-Mendota sub-basins (DWR, 1994). The Tracy and Delta-Mendota Sub-basins lie beneath the proposed project site with the boundaries defined by the Coast Range and Diablo Range on the west; the Mokelumne and San Joaquin Rivers on the north;and the San Joaquin River,Chowchilla Bypass,Fresno Slough, and the Tranquility Irrigation District boundary to the south. The San Joaquin and Stanislaus County line roughly divides the Tracy sub basin from the Delta-Mendota sub basin. Groundwater Characteristics Groundwater occurs in void spaces below the ground surface. The physical properties of underlying geologic units control the occurrence and ability to store and transmit water. The key factors in defining basin hydrologic properties are permeability(the ability of a material to convey water)and porosity (the percentage of void space available for water storage). The geologic units that contain groundwater are aquifers, and the non-water bearing,semi- impermeable layers that divide them are aquitards. Typically, an unconfined aquifer,or what is ` also considered the water table,is closer to the surface and bound at the top by unsaturated and permeable alluvium and bound at the bottom by an aquitard. A confined aquifer is typically much deeper than the water table and is bound on the top and bottom by impermeable to semi- impermeable materials. In perched aquifers,unconfined and confined water occurs as saturated lenses in the soil near the surface. r The groundwater aquifers and aquitards in the Tracy and Delta-Mendota sub-basins have formed in geologic deposits consisting of discrete lenses of clay, sand, and gravel. These water-bearing ` geologic units include alluvial fan deposits,buried stream channel deposits, and ancient terrace deposits all formed by Hospital Creek and other local streams located in the Coast Range foothills. The geologic deposits are a few hundred feet thick near the Coast Range foothills and ., increase to about 3,000 feet thick along the eastern margin of the basin. One major geologic unit,referred to as the Tulare Formation, contains the regionally extensive, r semi-impermeable Corcoran Clay, which is a buried fine-grained deposit formed from ancient lake flooding events. The Corcoran Clay divides upper and lower water-bearing sand and gravel units and is the aquitard between the lower confined zone and the upper zones that underlie �• portions of the San Joaquin Valley(JMM, 1990;DWR, 1981 in DWR 2003). Near the proposed project site,the upper Tulare Formation is approximately 100 to 200 feet thick and overlies the 60-foot thick Corcoran Clay. The lower Tulare Formation is beneath the Corcoran Clay and r r RMC Pacific Vernalis Quarry Mining and Reclamation Pmjmt 4.8-5 ESA 1203015 Adtninist an,e Draft EIR Minch 2004 1.n