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Jonas & Associates Inc ' The Tracy Service Center is located centrally within the Tracy Groundwater Basin, which is part of the San Joaquin Groundwater Basin Locally, recharge to the basin ' occurs to the west of the Site in the foothills of the Diablo Mountain Range, and from surface infiltration Precipitation for 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 (PG&E, 1988) ' 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 the water throughout the area (Hotchkiss and Balding, 197 1) Regional groundwater is divided into three water-bearing zones 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 use in the Tracy area (Hotchkiss and Balding, 197 1) 4 2 Beneficial Uses of Groundwater �Iewt 111K1 The groundwater quality in the Tracy area is degraded by the intrus= of saline, connatYeas oundwater as a result of overdraft of deeper aquifers in the Stockton and Shallow groundwater is generally of poor quality along the western ' boundary of the San Joaquin Groundwater Basin (DWR,_ 1975) Water sarriples� collected from wells installed on-site showed typical conductivity levels of 3,000 Dmicrosiemens per centimeter, indicating a relatively highconcentration of dissolved_ j • saltsin the uppermost aquifer (PG&E, 1988) (,-'( ,7 -2 x000 ( �� According to Mr Karl Borgman (telephone correspondence, 7/30/96) of the San ' Joaquin County Environmental Health Division, groundwater in the Tracy area is r used for drinking water, irrigation and industrial supply ' 5.0 REMEDIATION ACTIVITIES With the exception of the UST removal and minimal soil excavatio];