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SECTION 2. GEOLOGY i 0 AND HYDROGEQLOGY basin form the 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 Survey (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 Ievels declined as much as 400 feet in places The declines caused a mayor 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 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 deep in the Sacramento Valley but are as deep as 3,500 feet in the San Joaquin Valley The greater depth of wells is a result of the low permeability of the sands in the unconfined aquifer in the western and southern San Joaquin Valley and of highly mineralized water and water high in selenium in the upper parts of the aquifer system in the western San Joaquin Valley Well yields of more than 1,000 gallons per minute (gpm) are commonly obtainable throughout the aquifer system The average yield of wells in the Sacramento Valley is approximately 800 gpm, but yields as large as 4,000 gpm have been recorded The average yield of wells in the San Joaquin Valley is about 1,I00 gpm, and the maximum expected yield is about 3,200 gpm (Bailey, 1996) During the discrete vertical sampling in April 1999, groundwater at the site was encountered at approximately 44 90 feet bgs northeast of the former USTs (at well MW-9) and at approximately ' 46 60 feet bgs south of the former USTs (at well MW-3) Groundwater is encountered in a stratigraphic layer consisting of silts, silty sands and clayey sands Silty clay zones have been identified at approximately 45 feet bgs at some of the well locations at Stockton Yard (Figures 2-2 through 2-4) The seasonal groundwater high has been reported at approximately 49 feet bgs (at well MW-33on June 26, 1998) and the seasonal groundwater low has been reported at approximately 66 feet bgs (well MW-3 on April 2, 1992) The groundwater flow direction beneath the site was estimated by Professional Services Industries (PSI) to be toward the northeast at an . average hydraulic gradient of approximately 0 007 foot per foot (PSI, 1999) S IPROJI410401Q0D41SYYARDINIAIRTYAROSADOC 2-2 URS Gruner Woodward Clyde