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LFR Inc. <br /> 2.0 SITE GEOLOGY AND HYDROGEOLOGY I <br /> 2.1 Site Geology I <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 (ms]). Based on U.S. Geological Survey geological surveys of the <br /> Tracy area (USGS 1971 and 1995), findings from past U.S. Environmental Protection ' <br /> Agency investigations (EEI 2000), and boring logs from the recent installation of the <br /> two new wells at the Site (Appendix A), the following stratigraphic sequence lies <br /> beneath the Site (all depths approximate): ' <br /> • surface to 25 feet below ground surface (bgs): alluvial fan deposits consisting of <br /> silty gravel with some 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 /> • 280 feet to 780 feet bgs: the lower aquifer of the Tulare Formation, consisting of <br /> silt and sand with varying amounts of clay and gravel ' <br /> • below 780 feet bgs: consolidated sedimentary rocks up to 10,000 feet thick <br /> A unit of hard silt with varying amounts of clay, sand, and gravel commonly ' <br /> encountered at approximately 50 feet bgs represents the practical limit to which gravel <br /> can be mined in the area (EEI 2002). <br /> Recent drilling and soil sampling activities conducted by LFR, during September 2005 , <br /> through July 2006, at monitoring wells MW-6 through MW-9 and soil borings SB-1 <br /> and SB-2 indicate that the soil encountered from the surface to approximately 25 feet ' <br /> bgs consisted of silty gravel with minor layers of fines (resembling alluvial fan <br /> deposits). The soils generally consisted of silt and sands with varying amounts of clay <br /> and gravels from approximately 25 feet to 125 feet bgs. A 10-foot-thick clay layer was ' <br /> noted in the cliff face adjacent to the borings from 85-95 feet bgs but was not observed <br /> in the soil samples or cuttings from the borings. <br /> 2.2 Site Hydrogeology/Hydrology ' <br /> Two water-bearing zones exist within the Tulare Formation beneath the Site (EEI ' <br /> 2002). The upper water-bearing zone lies above the Corcoran Clay and is encountered <br /> at approximately 65 feet msl. The lower water-bearing zone lies beneath the Corcoran ' <br /> Clay and is confined. Groundwater monitoring wells installed at the Site intersect the <br /> Page 2 <br /> Final—GW Mon Rpt June 2006.doc:LFR ' <br />