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1 3542a/BAV102 5 Clay Member is a major aquitard in the San Joaquin basin (Hotchkiss and Balding 1971). 2.3 Site Hydrogeology The Tracy site is located centrally within the Tracy Groundwater Basin, which is part of the San Joaquin Groundwater Basin. Locally, recharge to the basin occurs from the west, in the foothills of the Diablo Mountain Range, and from surface infiltration. Precipitation in the Tracy area averages about 10 inches per year. Discharge occurs through drainage into the San Joaquin River and its tributaries (locally, the Old River to the north of the site), and by pumping for domestic and industrial uses. Prior to 1951, groundwater was the principal water supply in the Tracy area. In 1951, large scale importation of surface water via the Delta-Mendota Canal was initiated. Since then, surface water has been the principal source of domestic water throughout the area (Hotchkiss and Balding 1971). Regional groundwater is divided into three water-bearing zones: a lower confined zone below the Corcoran Clay Member; a confined zone above the Corcoran Clay Member; and a shallow, unconfined zone within 25 feet of the ground surface. The lower two water-bearing zones are the primary sources of groundwater used in the Tracy area (Hotchkiss and Balding 1971). The groundwater quality in the Tracy area is degraded by the intrusion of saline, connate groundwater as a result of overdraft of deeper aquifers in the Stockton and Tracy areas. Shallow groundwater is generally of poor quality along the western boundary of the San Joaquin Groundwater Basin (DWR, 1975). Water samples collected from monitoring wells screened in the uppermost aquifer installed onsite in February 1988, showed a relatively high conductivity level of 3000 microsiemens per centimeter, indicating a relatively high concentration of dissolved salts exists in the uppermost aquifer. 1 3542a/BAV102 5