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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA, Page 21 rte.. 6.0 GEOLOGY AND HYDROGEOLOGY The subject property and the surrounding area are situated on level terrain on the distal, northern slope of an alluvial fan. The shallow underlying alluvial sediments are of ` Quaternary to Recent age. The site and the immediately adjacent property along the south side of West Eleventh Street and the west side of Chrisman Road have been extensively excavated and backfilled during prior filling station construction and remodeling, utility installation, and highway expansion. -� Beneath the paving and fill, the soils are composed of alluvial materials consisting of interbedded clays, silts and sands. These materials have been deposited in a complex lenticular form composed of relatively low-permeability clays and silty clays inter- bedded with permeable,silts and sands. The sizes of individual permeable lenses vary from relatively large features having considerable areal extent to small, localized lenses of limited extent and thickness. In some instances, these lenses merge into each other to form semi-continuous permeable strata within the less permeable clayey material. In the neighborhood of the 7500 West Eleventh Street site, it is estimated that these alluvial materials are some 100 ft. thick. Beneath the alluvial sediments are the poorly sorted clays, silts sands and gravels of the Tulare Formation that were primarily derived from the Inner Coast Range hills that rise along the west side of the San Joaquin Valley. The Tulare Formation is separated into two members, the Upper Tulare Formation and the Lower Tulare Formation. Both members of the Formation are, on the regional scale, moderately to highly permeable and yield moderate to large quantities of water to wells. The Upper Tulare Formation is separated from the Lower Tulare Formation by the low-permeability, lacustrine Corcoran Clay, which acts as a confining bed within the regional groundwater basin. At the subject `i site, the top of the Corcoran Clay is estimated to be at a depth of approximately 230 ft beneath the ground surface and to be some 100 ft. thick. The total thickness of the underlying Lower Tulare Formation is not well documented; however, estimates suggest that it ranges in thickness from 300 ft to greater than 1,400 ft. 6.1 Use of Groundwater in Neighborhood of Site =' Groundwater is the principal source of drinking water in the neighborhood of the 7500 West Eleventh Street site. It serves private residences and commercial and industrial i properties. It also is used for irrigation purposes. Water-supply wells in the neighborhood are shown on Figure 2 and are discussed in Section 8.3. The majority of potable water-supply wells are relatively shallow (some no more than 70- ft. deep) and draw water from the Upper Tulare Formation that lies above the Corcoran Clay. However, the quality of groundwater in the Upper Tulare Formation is of less than desirable quality (particularly in the upper zone, which is usually defined as the upper 30 ft. beneath the ground surface) due to high concentrations of total dissolved solids, boron, sic