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LFR Inc. <br /> 50 September 7, 2006 - Groundwater Monitoring Report for Samples Collected <br /> During August 2006, Tracy Tire Fire Site, Former Royster Tire Disposal <br /> Facility, Tracy, California <br /> 51 October 11, 2006 - Human and Ecological Risk Assessment Groundwater, Tracy <br /> Tire Fire Site, Former Royster Tire Disposal Facility, Tracy, California <br /> 52 October 20, 2006 - Groundwater Monitoring Report for Samples Collected <br /> During September 2006, Tracy Tire Fire Site, Former Royster Tire Disposal <br /> Facility, Tracy, California <br /> Electronic copies of the reports are also provided in Appendix A of this report. These <br /> documents are available for public review at the CIWMB document room. Electronic <br /> copies of project-related photographs and recorded videos are provided in Appendix B. <br /> 4.0 SITE GEOLOGY AND HYDROGEOLOGY <br /> This section discusses the site geology and hydrogeology and is based on previous <br /> investigations, U.S. Geological Survey (USGS) reports, and conditions encountered <br /> during site investigation activities. <br /> 4.1 Site Geology <br /> Soils encountered at the Site during previous investigations consist of silty gravel with <br /> interbedded clayey silt lenses to a depth of approximately 25 feet below the average <br /> gravel pit elevation. Silt and sand with varying amounts of gravel and clay occur at a <br /> depth of 25 to 300 feet below ground surface (bgs). The Corcoran Clay is encountered <br /> at depths ranging from 300 to 380 feet bgs (EEI 2002). <br /> As presented previously, the Site is a former gravel quarry, with associated surface <br /> topography consisting of irregular hills and gravel pits. The natural topography <br /> surrounding the Site dips gently to the northeast and is approximately 165 feet above <br /> mean sea level (msl). Based on USGS geological surveys of the Tracy area (USGS <br /> 1971 and 1995), findings from past U.S. EPA investigations (EEI 2002), and boring <br /> logs from the recent installation of groundwater wells at the Site, the following <br /> stratigraphic sequence lies beneath the Site (depths are approximate): <br /> • surface to 25 feet bgs: alluvial fan deposits consisting of silty gravel with some <br /> lenses of clayey silt <br /> • 25 feet to 200 feet bgs: the top of the Tulare Formation consisting of silt and sand <br /> with varying amounts of clay and gravel, with the upper aquifer encountered at a <br /> depth of 100 feet bgs <br /> • 200 feet to 280 feet bgs: the Corcoran Clay unit of the Tulare Formation, which <br /> acts as an aquitard consisting of reduced gray, sandy to silty clay <br /> rpt-RA_summ-Nov06-ftna1-W25 Page 9 <br />