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Pale 2 Tracy Tire Fire Celeste Temple, Project Officer • Boring Logs prepared by a company named RESNA for four monitor wells installed at 29099 South MacArthur Road in Tracy, California(provided by the San Joaquin County, Public Health Services, Environmental Health Division). • "Traina\Costello Test Hole Summaries,"November 1991, including prior testing in 1972 (provided by the California Regional Water Quality Control Board, Central Valley Region). • Harding Lawson Associates(HLA),Phase 1 Preliminary Site Assessment and Limited Phase 11 Site Investigation, Traina Property, APN 253-210-15, San Joaquin County, California, November 18, 1996. • William R. Hotchkiss and Gary O. Balding, 1971, Geology, Hydrology and Water Quality of the Tracy-Dos Palos Area, San Joaquin Valley, California, U.S. Geological Survey, Waier Resources Division, Open File Report, August 6;"1971. ._ • State of California. Department of-Water-Resources, Water Xell Standards: San -- Joaquin County (Preliminary Edition). Bulletin 74-5, March 1965. REGIONAL GEOLOGY/HYDROGEOLOGY The geology found in the area consists of a series of coalescing alluvial fans and plains. Topographic relief created by headward erosion of the fan surfaces by active stream channels reaches about 10 feet. The alluvial fans contain about 100 feet of Pleistocene to Holocene age unconsolidated clay, silt, sand and gravel derived from several seasonal east-flowing streams draining the Diablo Range to the west. Tile alluvial fans are underlain by approximately 800 feet of Pliocene to Pleistocene age unconsolidated alluvium of the Tulare Formation. The Tulare consists of upper and lower members of mainly coarse- ;rained lenticular alluvium. The Tulare Formation conformably overlies the San Pablo Group marine sediments of Miocene age. Near or at the top of the Tulare Formation, at a depth of approximately 100 feet, an extensive clay bed called the Corcoran Clay separates alluvial sediments into upper and lower aquifers. The lower aquifer is confined by the Corcoran Clay. The upper aquifer is predominantly unconfined; however it is semi- confined and confined in some areas. The Corcoran Clay ranges in thickness from 10 to 160 feet. Recharge to the upper aquifer is by infiltration of water from rainfall, streams, and excess irrigation. The infiltrated water cannot permeate the Corcoran Clay and is recovered by wells drawing from the upper water-bearing zone. The recharge areas for the confined aquifers is along the foot hills where the Tulare Formation outcrops, and the Corcoran Clay is absent. Movement of groundwater is in a general northeasterly direction. The principal fresh water-bearing aquifer, located at a depth of approximately 400-500 feet, is overlain by the Corcoran Clay. The Corcoran Clay separates the fresh water-bearing deposits of the Lower Tulare iTTzur30.'99 wpd