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r,. f."d~Y L `-mss•-. '[a-. ?'s moo.Y h w'� �+ !!: MI the delta. The saline front is moving into the Stockton area at a rate of 150 feet per year. Overdraft of the ground water has also resulted in subsidence of up to 0.1 foot per year in the Stockton area 1 The ARCO service station, located on the northwest side of Stockton. is underlain by alluvial sediments. The shallow ground water of concern in this investigation occurs at depths of 13 to 16 feet in the youngest sediments, the Victor Formation, a floodplain deposit of discontinuous sand lenses interlayered between clay beds. Most water wells in the Stockton area draw ground water from the more continuous sandstone beds at depths greater than 150 feet. At present the ground water at the site has a very flat natural gradient with ground-water elevations varying less than 0.3 foot across the site. Without the imposed stress of the site extraction well the shallow ground-water flows generally to the north to northeast. A northeast ground-water Flow direction is also consistent with the deeper aquifers.2 A water well survey in the area shows that nine water wells exist within a 1-mile radius of the site (Figure 1), none of which are drawing water from the shallow aquifer at the site (see Appendix A for available well logs.) The interconnection between any of these wells and the contami- nation in the shallow aquifer below the site is expected to be moderately low. The combined factors of (1) greater depths of the water supply wells, (2) large thicknesses (25 to 60 feet) of clay deposits between shallow aquifer and deeper aquifers, and. (3) distance (minimum 530 feet) to the supply wells, would act to retard and attenuate the gasoline. However, due to the heterogeneities of the sediments which l.Department of Water Resources, 1967, San Joaquin County Ground Water Inve+-tigation, Bulletin Number 146. 2.San Joaquin County Flood Control and Water Conservation District, 1984, Semi Annual Ground-Water Report, Fall .1984 PJD 438-22.04 3 mon associates e;