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W.. =- WESTERN GEa-ENGINEERS Ground water production in this region is predominately from the Tulare Formation a non-marine (continental) sediment deposit of Poo-Pleistocene age. The Corcoran Clay Member of the Tulare Fm., represents a lacustrine (lake bottom) deposit of diatomaceous clay which divides the upper third of the Tulare Formation from the lower two thirds. Overlaying the Tulare Formation are the younger (Pleistocene to Recent) flood basin, alluvial fan and stream channel deposits. Underlaying the Tulare Formation a sequence of older (Miocene thru Cretaceous) marine sedimentary deposits which contain conate water too brackish for any use. To the west, in the Coastal Range foothills, the Tulare Fm. outcrops against the older undifferentiated amarine Cretaceous formations. This unconformity produced by the Coast Range uplift and subsequent subsidence of the Sarin Joaquin Valley created the northwest trending Black Butte Fault, marking the boundary of the Coastal Ranges and the Great Valley provinces. i Ground water recharge to the younger surface to near surface deposits situated above the Corcoran clay is predominately vertical leakage and lateral movement downdip along the less permeable layers. The Corcoran clay forms somewhat of a barrier to the sediment deposits found below it and recharge is presumably slower; by lateral movement downdip from the outcrop areas to the west and southwest along the Coastal Range foothills, by slow leakage from the above clay and by more rapid movement vertically beyond the limits of the, clay. �f The direction of free ground water movement In this region is to the north and M `1 northeast from the Coastal Ranges towards the San Joaquin River to the east and the Sacramento - San Joaquin Delta to the north { see figure 5a ). At the present y. intrusion of waters from the Delta has not occurred in the Tracy area as it has in the Stockton area (see figure 513). SECTION 3.1 LOCAL GEOLOGY AND HYDROGEOLOGY The Tracy area is built upon flood plain/basin deposits of (sown clayey slit. Intermixed with and underlying the flood plain deposits are stream channel deposits consisting of silty sand, sand, and sandy gravels with occasional clay layers and I� clay lenses. The channel deposits do not seem to b.: continuous between wells in the area, indicating a braided and/or meander type deposition. A shallow aquifer exists at approximately 10 to 15 feet below the surface (12 feet below the surface of the site) in the Tracy area. Chloride ion and resistivity measurers ants of this aquifer (from the monitor wells on the site) indicate this water to be slightly brackish; chloride ion ranges between 350 and 500 ppm and resistivity values range from 5.39 to 6.61 ohms/meter 2/meter. Local flow directions and gradients of this aquifer are influenced by recharge and discharge from local irrigation and construction and/or leaks and breakage of the local water and sewer lines. Bul, the general flow direction should be to the north with a relatively flat gradient (see figures 5A and 5B). page 9