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4.8 Hydrology and Water Quality 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 _ subbasin from the Delta-Mendota subbasin. 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 referred to 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.6 In perched aquifers,unconfined and confined water occurs as saturated lenses in the soil near the surface. 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, semi-impermeable Corcoran Clay,which is a buried deposit of fine-grained sediments formed from ancient lake flooding events. The Corcoran Clay divides upper and lower water-bearing sand and gravel units of the Tulare Formation 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 100-to 200-foot-thick upper Tulare _ Formation underlies the geologically younger alluvial deposits and overlies the 60-foot-thick Corcoran Clay. The lower Tulare Formation is beneath the Corcoran Clay and ranges in thickness from 200 to 500 feet. Figure 4.8-2 presents a conceptual cross-section of the geologic formations in the proposed project vicinity. All three types of groundwater aquifers(confined,unconfined, and perched)are present in the vicinity of the proposed project site(JMM, 1993). Groundwater is generally present beneath the proposed mine site in the upper alluvium,in the discrete confined, semi-confined,and unconfined zones in the upper Tulare Formation and in the confined fresh water zone in the lower Tulare Formation (Davis 1959 in DWR,2003b).Most of the larger agricultural,municipal, and 6 Water levels in unconfined wells represent the actual elevation of the water table.In confined aquifers,the groundwater is under pressure and the groundwater levels represent the level to which the water would rise in a well tapping the aquifer. r. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-5 ESA/203015 Draft Environmental Impact Report May 2006