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Report:Groundwater-qualityMonitoring—July 27,2004: 7500 West Eleventh street,Tracy, CA. Page 4 f=� characterization program in April 2004, five floating product monitoring wells were installed at the locations shown on Figure 2 to investigate the down-gradient extent to which LNAPL may have migrated in the interior of the plume of affected groundwater emanating from the 7500 West Eleventh Street property. The thickness of floating product in those five wells was checked at that time and tests were also made to detect the presence of LNAPL on the water table in groundwater-quality monitoring wells MW- 7, MW-13 and MW-14. The same array of wells was again checked for the presence of r ; floating product on July 26, 2004. The historical record of LNAPL thicknesses measured in the array of floating product monitoring wells is presented in Table 3. At the direction of the SJCEHD, the quality of water in six potable water supply wells in the neighborhood of the Navarra Site was analyzed in April 2004. A second round of sampling and analysis of the water produced from those wells was conducted on July 27, 2004. The results of those two sampling rounds are recorded in Table 4. 1.4 Geology and Hydrogeology i 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 i ` 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 f. 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 are 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 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 site, the top of the Corcoran Clay is estimated to be at a depth of approximately 230 ft. sic