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I SECTIONTWO 31t®laclgr®®nd 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 1,100 gpm, and the maximum expected yield is about 3,200 gpm 2.2.3 Local Geology/Hydrogeology Groundwater at the site is currently encountered at approximately 49 5 feet bgs near the former USTs and at approximately 50 feet bgs south of the former USTs The seasonal groundwater high has been reported at approximately 48 6 feet bgs (Spring 1998) and the seasonal groundwater low has been reported at approximately 67 9 feet bgs (Summer 1993) (Table 2-2) The groundwater flow direction beneath the site is toward the east with an average hydraulic gradient of 0 002 meter per meter(0 002 foot per foot)(PSI, 1998) The California Water Service Company, the primary provider of water to residents in Stockton, was contacted regarding the water supply wells in the area. A water well is located approximately 1,000 feet south of the site that is screened at 500 feet bgs The District Manager of the California Water Service Company, Enc Mar, indicated that he is not aware of any hydraulic communication between the shallow aquifer and the deeper aquifer (Mar E Personal Communication, 12/9/98) Currently 37 percent of the water supply for the California Water Service Company is derived from groundwater 2.3 CLIMATE The elevation at the site is approximately 30 feet above sea level with an annual precipitation average for the Stockton area at 1169 inches of rain with the mal onty(approximately 80 percent) of the annual precipitation occurring from November through March (NOAA, 1991) The average annual temperature is 62 1* F The weather data are from the Stockton Weather Station located at 370 54' latitude and 1210 15W longitude at an elevation of 22 feet above sea level Most of the precipitation that falls on the valley floor evaporates before it can infiltrate downward to become the Much of the moisture that moves inland from the Pacific Ocean is intercepted by the 16 Coast Ranges, so that annual precipitation in the Central Valley is relatively low Annual precipitation is exceeded by potential evapotranspiration throughout the entire valley, which causes a net annual moisture deficit Woodww&oWe ® 711997W73=NA15TOCKTOMREPORT%SHOREP I D0c18-FEB4&a7900WA0NA 2-3