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LFR Levine-Fricke 2.0 SITE GEOLOGY AND HYDROGEOLOGY 2.1 Site Geology As presented previously, the Site is a former gravel quarry, with associated surface topography consisting of irregular hills and gravel pits. The natural topography surrounding the Site dips gently to the northeast and is approximately 165 feet above mean sea level (msl). Based on U.S. Geological Survey (USGS 1971 and 1995) geological surveys of the Tracy area, findings from past EPA investigations (EEI 2000), and boring logs from the recent installation of the two new wells at the Site (Appendix A), the following stratigraphic sequence lies beneath the Site (all depths approximate): • surface to 25 feet below ground surface (bgs): alluvial fan deposits consisting of silty gravel with some lenses of clayey silt; • 25 feet to 200 feet bgs: the top of the Tulare Formation consisting of silt and sand with varying amounts of clay and gravel, with the upper aquifer encountered at a depth of 100 feet bgs • 200 feet to 280 feet bgs: the Corcoran Clay unit of the Tulare Formation, which acts as an aquitard consisting of reduced gray, sandy to silty clay • 280 feet to 780 feet bgs: the lower aquifer of the Tulare Formation, consisting of silt and sand with varying amounts of clay and gravel • below 780 feet bgs: consolidated sedimentary rocks up to 10,000 feet thick A unit of hard silt with varying amounts of clay, sand, and gravel commonly encountered at approximately 50 feet bgs represents the practical limit to which gravel can be mined in the area (EEI, 2002). 2.2 Site Hydrogeology/Hydrology Two water-bearing zones exist within the Tulare Formation beneath the Site (EEI 2002). The upper water-bearing zone lies above the Corcoran Clay and is encountered at approximately 65 feet msl. The lower water-bearing zone lies beneath the Corcoran Clay and is confined. Groundwater monitoring wells installed at the Site intersect the upper water-bearing zone approximately 100 feet below the natural grade (165 feet msl) surrounding the gravel pit. The groundwater table is located approximately 35 feet below the bottom depth of the gravel pit. Recharge of the upper water-bearing zone is primarily from local irrigation and seepage of surface water through the alluvium. Local groundwater flow was calculated from on-site monitoring well data to be toward the northwest at a shallow gradient of 0.001. Aggregate mining activities to the north and south of the Site may influence the local groundwater flow direction at times of dewatering. Page 2 TTF QMR Q2-03 FINAL(7-2"3):JEE