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SECTIONTWO was classified beneath the fill material as being primarily silty clay, clayey silt and clay, with lenses of sandy silt, silty sand, sand, and sandy clay 2.2.2 Regional Hydrogeology The site is located near the center of the Great Valley of California, a geomorphic province, that is a nearly flat alluvial plain bounded on the east by the Sierra Nevada and on the west by the Coast Ranges The Great Valley is comprised of a thick sequence of sedimentary rocks of Jurassic to Recent age The Great Valley is a elongated, northwest trending, asymmetric trough with a long stable eastern shelf and a short western flank(Bailey, 1996) The Stockton Arch, an area of uplift, extends from the Sierran slope across to the western flank of the trough The uplift is thought to have occurred during Eocene time and may have continued into the Miocene as a result of upward movement on the south side of the Stockton fault The regional hydrogeology of the Central Valley is dominated by a large basin-fill aquifer system comprised of a structural trough that is about 400 miles long and from 20 to 70 miles wide and extends over more than 20,000 square miles The trough is filled to great depths by marine and continental sediments, which are the result of millions of years of inundation by the ocean and erosion of the rocks that form the surrounding mountains Sand and gravel beds in this great thickness of basin-fill material form an important aquifer system From north to south, the aquifer system is divided into the Sacramento Valley, the Sacramento-San Joaquin Delta, and the San Joaquin Valley subregions on the basis of different characteristics of surface-water basins The United States Geological Services (USGS) reports that due to agricultural development, groundwater withdrawals reached a maximum of 15 million acre-feet per year during 1977, a drought year During the 1960's and 1970's, withdrawals greatly exceeded recharge, and water levels declined precipitously, as much as 400 feet in places The declines caused a major reduction in the amount of groundwater in storage and resulted in widespread land subsidence, mainly in the western and southern parts of the San Joaquin Valley Increased rainfall and construction of additional surface-water delivery systems halted most of the serious water-level declines after 1977, and water levels recovered to pre-1960 levels The network of aqueducts in the Central Valley is sufficient to provide one-half or more of the water needed for irrigation in years of average or above-average precipitation (as of 1995) In dry years, however, reliance on groundwater supplies is greater, and aquifers might again be subject to withdrawals in excess of recharge during a severe drought Well depths in the Central Valley aquifer system are determined by the depth of permeable aquifer material and by the quality of the ground water In general, wells are usually less than 500 feet T 11 WM73008NAZTOCKTONIREPORT SHOREM DOM&FEB4M73WONAWNA 2-2