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irk{ Report: Groundwater-quality Monitoring—January 20,2003: 7500 West Eleventh Street, Tracy, CA. Page 3 1.4 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. The stratigraphy described above is typical of the alluvial fan upon which Tracy and the surrounding area arc situated. 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 f '? 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 'l 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 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 d that it ranges in thickness from 300 ft. to greater than 1,400 ft. The depth to groundwater beneath the site varies seasonally between 7 and 11 ft. Regionally, the general direction of groundwater flow is to the north toward the Old River anastomosic branch of the San Joaquin River, the closest tributary of which, the Torn Paine Slough, is one and one-quarter miles north of the 7500 West Eleventh Street site. However, locally, the shallow groundwater gradient tends to follow the topography, which, at the subject property, slopes gently to the north-northeast. The local direction of groundwater flow is also affected by the local sedimentary geology, particularly where continuous or semi-continuous sand strata provide channels for subsurface flow through less penncable facies. sic