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amec— <br /> "dark <br /> greenish clay", and formed during the expansion of freshwater lakes during the <br /> Pleistocene epoch (Burow et al., 2004). It is well sorted and predominantly fine-grained, and <br /> has been mapped under much of the western portion of the San Joaquin Valley. Burow et al. <br /> 2004 mapped the extent of the Corcoran Clay. Their map suggests that the eastern boundary <br /> of the clay runs along the axis of the San Joaquin Valley, beneath Ripon in the vicinity of <br /> Highway 99, with a shallow dip (approximately 16 feet per mile) towards the Coast Ranges to <br /> the west (available boring logs for the Study Area indicate that the Corcoran Clay extends east <br /> of Highway 99 and may be continuous beneath most of the Study area). The clay unit can <br /> have higher silt content towards its eastern boundary, making it difficult to recognize along the <br /> edges where it pinches out, becomes oxidized, or grades into coarser materials (Burow et al. <br /> 2004). Burow et al., (2004) indicate that the Corcoran Clay can be up to 100 feet in thickness, <br /> and is encountered at depths of 80 to 210 feet bgs beneath the Modesto area. A similar depth <br /> of the clay is expected for the Ripon area. This unit is important because it acts as a confining <br /> layer that can impede the vertical movement of groundwater. <br /> Deeper unconsolidated sediments beneath the Ripon area include the Laguna Formation, a <br /> Pliocene alluvium that coarsens upward, and the Mehrten Formation, which is identified by <br /> abundant sand and gravels that are black in color, due to their andesitic source rocks (Burow <br /> et al., 2004). The top of the Mehrten Formation may lie within 400 feet of ground surface, <br /> based on the most recent stratigraphic analysis presented in Burow et al. (2004). According to <br /> Burow et al. (2004), the top of the Mehrten Formation can be identified in geophysical logs by <br /> a significant change from coarse-grained dominated units of the overlying younger Pleistocene <br /> sediments, to the relatively fine-grained and more compacted older sediments of the Mehrten <br /> Formation. Light to reddish gravels and sand beds are also a component of the Mehrten <br /> Formation (Burow, et al., 2004). <br /> Earlier groundwater investigations performed on behalf of Nestle resulted in the identification <br /> of four hydrogeologic depth intervals based on observations of sediment characteristics. <br /> These four depth intervals comprise the stratigraphy to a depth of approximately 300 feet bgs: <br /> the Upper Aquifer (approximately 25 - 120 feet bgs), the Intermediate Aquifer (120 - 200 feet <br /> bgs; previously referred to as the Clay Zone from 120 — 220 feet bgs), the Corcoran Clay from <br /> approximately 200 to 220 feet bgs; and the Lower Aquifer Zone underlying the Corcoran Clay <br /> to > 300 feet bgs (Geomatrix, 1996; ECM, 2006). The lithologic packages have further been <br /> subdivided into water-bearing zones (designated shallowest to deepest as the B-Aquifer, A- <br /> Aquifer, C-Aquifer, and D-Aquifer). Further subdivisions among the general depth intervals <br /> have been made recently, with up to 12 layers selected for developing numerical groundwater <br /> flow simulations (ECM, 2006). The hydrostratigraphy has been further revised as part of this <br /> SCM, as discussed in Sections 3.2 and 4.1. <br /> AMEC Geomatrix, Inc. <br /> I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 5 <br />