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III. Environmental Setting, Impacts,and Mitigations K. Hydrology, Geohydrology, and Water Quality Underlying San Joaquin County is a portion of the vast subsurface groundwater aquifer system of the Central Valley of California. Groundwater occurs in unconfined and confined conditions. The upper regional aquifer is typically an unconfined aquifer within the Victor Formation geologic unit. The Victor formation consists of over 100 feet of clay,silt, and fine.to coarse sand. Sedimentary formations underlying the Victor formation include additional,productive confined aquifers. Groundwater within one mile of the site is tapped by irrigation and domestic wells, and most wells in the vicinity are generally drilled several hundred feet deep. Local rainfall and stream infiltration are the main source of recharge to the unconfined aquifer, while the deeper,confined aquifers are recharged by rivers,reservoirs, and surface runoff along the western base of the Sierra Nevada. The project area is not in one of the General Plan designated substantial groundwater recharge areas,although some infiltration and percolation to the groundwater system is expected along Little Johns Creek.I A characteristic of the San Joaquin Valley's groundwater basins has been the tendency for water levels to vary both seasonally and annually,over short periods of time. Seasonal fluctuations mirror the distinct rainy and dry season;rainy season infiltration raises the water table,and dry season pumping lowers the water table. In addition, noticeable decreases in water levels have occurred in years of normal (or low)precipitation(but high pumping). Long-term withdrawal. has resulted in an overdraft conditions(withdrawal rates exceed the recharge rates),and related problems of lowered water table levels,long-term reliability, saltwater intrusion from the Delta, and subsidence(and ground lowering)in the Delta. In the project area,water levels have dropped over the last twenty years by approximately 10 feet due to agricultural pumping demand and drought(EMCON, 1972; Kleinfelder, 1992). Site Groundwater Kleinfelder examined site groundwater conditions based on several sources of information for a 10-year period from Spring 1976 to Fall 1986(Meinfelder, 1988). The groundwater elevation averaged-35 feet MSL(35 feet below mean sea level),with spring levels about-30 feet MSL and fall levels about-41 feet MSL. In dry years the water level dropped to-48 feet MSL. During wet years the groundwater was as high as-12 feet MSL in the spring and-25 feet MSL in the 1 Hydrogeological investigations at the nearby Forward,Inc.Landfill show that recharge along the course of the South Fork of Little Johns Creek,which crosses their property,has a significant influence on water table depth,gradient,and flow direction. III.K.7