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SECTION 2.0 GEOLOGY AND HYDROGEOLOGY 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 levels 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 ;n 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 gprn have been recorded The average yield of wells in the San Joaquin Valley is about 1,100 gpm, and the maximum expected yield is about 3,200 gpm (Bailey, 1996) As of September 1999, groundwater at the site was encountered at ganging from 26 92 to 67 W X2,7 48 feet-bgs Groundwater is encountered in a stratigrapluc layer consisting of silts, silty anS ds fA'C r4'.s and clayey sands Silty clay zones have been identified at approximately 45 feet bgs at some of the �r `5 well locations at Stockton Yard (Figures 2-2 through 2-4) The seasonal groundwater high has been reported at approximately 4120 feet bgs (at well MW-19 on June 28, 1999) and the seasonal groundwater low has been reported at approximately 67 94 feet bgs (well MW-19 on October 22, 1993) The groundwater flow direction beneath the site is toward the south-southeast at an average hydraulic gradient of approximately 0 007 foot per foot (PSI, 1999) S IPROJ44iUOl00041SHOPIMSHOPIOSI-DOC 2-2 URS Greiner Woodward Clyde