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I l San Joaquin River Hydrologic Region California's Groundwater San Joaquin Valley Groundwater Basin Bulletin 118 San Joaquin Valley Groundwater Basin Tracy Subbasin • Groundwater Basin Number: 5-22.15 • County: San Joaquin, Contra Costa,Alameda • Surface Area: 345,000 acres (539 square miles) Basin Boundaries and Hydrology The San Joaquin Valley comprises the southernmost portion of the Great Valley Geomorphic Province of California. The Great Valley is a broad structural trough bounded by the tilted block of the Sierra Nevada on the east and the complexly folded and faulted Coast Ranges on the west. The Tracy Subbasin is defined by the areal extent of unconsolidated to semiconsolidated sedimentary deposits that are bounded by the Diablo Range on the west;the Mokelumne and San Joaquin Rivers on the north;the San Joaquin River to the east; and the San Joaquin-Stanislaus County line on the south. The Tracy Subbasin is located adjacent to the Eastern San Joaquin Subbasin on the east and the Delta-Mendota Subbasin on the south. All of the above mentioned subbasins are located within the larger San Joaquin Valley Groundwater Basin. The Tracy Subbasin also lies to the south of the Sacramento Valley Groundwater Basin, Solano Subbasin. The Tracy Subbasin is drained by the San Joaquin River and one of its major westside tributaries; Corral Hollow Creek. The San Joaquin River flows northward into the Sacramento and San Joaquin Delta and discharges into the San Francisco Bay. Annual precipitation within the subbasin ranges from i about 11 inches in the south to about 16 inches in the north. i Hydrogeologic Information Water Bearing Formations The Tracy Subbasin is comprised of continental deposits of Late Tertiary to Quaternary age. These deposits include the Tulare Formation, Older Alluvium,Flood Basin Deposits,and Younger Alluvium. The cumulative thickness of these deposits increases from a few hundred feet near the Coast Range foothills on the west to about 3,000 feet along the eastern margin of the basin. Information regarding the water bearing units and groundwater conditions was taken from(Hotchkiss and Balding 1971), (Bertoldi et al. 1991),and(Davis G.H. et al. 1954). Tulare Formation. The Tulare is exposed in the Coast Range foothills along the western margin of the basin and dips eastward toward the axis of the valley. It consists of semi-consolidated,poorly sorted,discontinuous deposits of clay, silt, and gravel. The Corcoran clay occurs near the top of the Tulare Formation and confines the underlying fresh water deposits. The eastern limit of the Corcoran clay is near the eastern boundary of the basin. + The Tulare formation is moderately permeable,with.most of the larger agricultural,municipal and industrial extractions coming from below the Corcoran clay. Wells completed in this zone produce up to 3,000 gallons per minute. Small domestic wells often obtain their supply from above the Corcoran clay. However,groundwater above the Corcoran clay is often of poor quality. The total thickness of the Tulare Formation is about 1,400 feet. Last update 1/20/06