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LFR Inc. • 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). Recent drilling and soil sampling activities conducted by LFR along the north- northwestern extent of the Site during September through November 2005, at monitoring wells MW-6 and MW-7 and soil borings SB-1 and SB-2 (Figure 3), indicate that the soil encountered from the surface to approximately 25 feet bgs consisted of silty gravel with minor layers of fines (resembling alluvial fan deposits). The soils generally consisted of silt and sands with varying amounts of clay and gravels from approximately 25 feet to 125 feet bgs. A 10-foot-thick clay layer was noted in the cliff face adjacent to the borings from 85-95 feet bgs, but was not observed in the soil samples or cuttings from the borings. 4.2 Site Hydrogeology/Hydrology Groundwater occurs in two water-bearing zones at the Site: an upper water-bearing zone and a lower water-bearing zone. The upper water-bearing zone (i.e., the water table) is encountered at an approximate depth of 75 feet bgs. The lower water-bearing unit is encountered at an approximate depth of 380 feet bgs, and is separated from the upper zone by the Corcoran Clay (EEl 2002). Regionally, recharge of the upper water- bearing unit is primarily from irrigation and seepage of surface water through the alluvium with little contribution from rainwater (USGS 1971). The project site receives an approximate average of 10.5 inches of precipitation per year and has a total estimated evapotranspiration rate of 61 inches per year as measured 3 miles south of the Site in Tracy, California. Based on depth to water measurements recorded during ground sampling events from 2003 through 2006, groundwater within the upper water- bearing unit generally flows toward the north-northwest with a calculated gradient ranging between approximately 0.001 to 0.002 feet per foot (ft/ft). Figure 4 shows groundwater elevation contours for the measurements recorded during September 2006. The Site is a closed basin with no paths for surface-water outfall and there are no significant surface-water sources (e.g., streams, ponds) located at the Site. The Banta Carbons Irrigation District Canal bounds the gravel pit to the east, and Corral Hollow Creek, an intermittent stream, is located 500 feet to the south. Other surface water in the vicinity of the Site consists of the Delta Mendota Canal (located approximately 0.5 mile to the southwest) and the California Aqueduct (located 1 mile to the southwest). The topographic nature of the Site precludes it as a potential source of runoff to any of these water features. Because the Site is a former gravel mine, it is essentially a closed basin with no outfall. Page 10 rpt-RA_sunun-Nov06-final-09025:FNC