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' Saline groundwater also occurs in the western part of Stockton, because of intrusion by poor quality delta water intrusion, induced by intensive groundwater pumping in the Stockton area Groundwater in the area occurs under unconfined water table conditions, but in some areas ' clayey deposits may create a semiconfined saturated zone within the underlying aquifer. These confined zones, however, are hydraulically connected, and over a long period of time, adjustment to equilibrium with the water table takes place in wells completed in different ' zones Recharge to the fresh water-bearing zones in the Stockton area is principally underflow from ' percolation in the outcrop areas to the east, augmented by percolation of rainfall, return irrigation water, and minor contribution from the Calaveras and San Joaquin rivers ' Groundwater flow direction in the Stockton area has been influenced by intensive groundwater pumping in the City of Stockton for public water supply and in areas east- northeast of Stockton for irrigation. As stated in the Department of Public Works - Division of Water Resources report No 7, "Since sprung 1950, and for an undetermined prior period, a groundwater depression has underlain the City of Stockton. The static water levels, as ' determined by the spring 1953 measurements, show elevations 10 feet below sea level near the western boundary of the area, 30 feet below sea level at the low point under the city and 5 feet below sea level at the eastern boundary of the area of investigation " Department of Water Resources Bulletin No. 146, which compiles the data on groundwater conditions in San Joaquin County over the years 1950 to 1964, also indicates that the water ' table under the city was depressed by 40 feet. The difference between current and past groundwater elevation in the area of former Exxon RS 7-3942 and the eastern part of Stockton was about 20 feet in 1950 and about 50 feet in 1964. A continuous westward hydraulic gradient has induced the migration of saline water from the Sacramento-San Joaquin delta area into the aquifer underlying the city. Since 1970, the San Joaquin County Flood Control and Water Conservation District have compiled spring and fall water table elevations and depth to groundwater maps for the area of Stockton. Analysis of these maps indicates that water table elevations in the area of former Exxon RS 7-3942 during the years 1970-1991 were between 15 and 30 feet below sea level, while groundwater elevations east of the site were between 40 and 80 feet below sea level The water table elevations at former Exxon RS 7-3942 and eastern Stockton were 40-60 feet lower than they are now until 1986, when the water table elevations were about 20 feet, due to decreasing groundwater pumping in an effort to slow down or reverse saline water encroachment from the delta area However, between 1990 and 1991 the difference again has increased to 50 feet, as a result of a prolonged period of drought and decreased recharge of the groundwater. 73942{1)/Q49=4 3 I