ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec�9 stream channels north of the Stanislaus River (Burow et al., 2004). According to Page (1986), Ripon is underlain by river deposits, younger alluvium of Holocene age (i.e. deposited within the last 10,000 years), older alluvium (Pleistocene age; approximately 1.8 million to 10,000 years old) and continental rocks and deposits of the Pliocene and Miocene Epochs (1.8 to 23 million years ago). Three formations of Pleistocene age are the Modesto, Riverbank, and Turlock Lake Formations (Page, 1986; Burow et al., 2004). The characteristics of these formations are described in order of increasing depth: Modesto Formation: The Modesto Formation is encountered at the surface in the Study Area, and is composed of late Pleistocene deposits of arkosic sand, gravel and silt composed primarily of quartz and feldspar, with lesser biotite and minor amounts of magnetite and hornblende (Burow et al., 2004). This formation is mapped in Burow et al. (2004) beneath the Ripon area and is described as a thin veneer overlying the Riverbank Formation. Riverbank Formation: This formation is composed of Pleistocene arkosic sands, gravels, and silts, with similar lithology and mineralogy to the overlying Modesto Formation (Burow et al., 2004). Clay-rich hard pan tops the Riverbank Formation and has been mapped at depths of a few feet bgs in the Ripon area (Burow et al., 2004). The Riverbank Formation contact with the Upper Turlock Lake formation can be difficult to distinguish beneath the Study Area because of similar lithology, although it is characterized as finer-grained and containing more magnetite. The Riverbank Formation is characterized as having moderate groundwater yields. Turlock Lake Formation: This formation has been described as a coarsening upward sequence of Pleistocene arkosic sands, gravels and silts, and relatively thick clay zones (Burow et al., 2004). Coarser materials include andesite, rhyolite, quartz, greenstone, schist, and granodiorite; clays and silts are gray to grayish pink; sands are reddish in color (Burow et al., 2004). The lower portion of the Upper Turlock Lake Formation is finer-grained and may represent lacustrine deposition. The Corcoran Clay forms the base of the Upper Turlock Lake Formation (Burow et al., 2004). Sands, gravels, and silts underlie the Corcoran Clay and form the Lower Turlock Lake Formation. High groundwater yields (exceeding 2,000 gallons per minute; gpm) are common for wells screened in sands and gravels of the Turlock Lake Formation. The top of the Turlock Lake Formation generally ranges from 50 to 100 feet deep in the Modesto Area (according to Burrow et al., 2004); similar depths are likely for the Ripon area, as Ripon is approximately 7 miles northwest of Modesto, parallel to the axis of the San Joaquin Valley. Corcoran Clay: This regionally extensive lacustrine clay has formerly been described as part of the Tulare Formation, but Burow et al., (2004) refer to the Corcoran Clay as part of the Turlock Lake formation. The Corcoran Clay is readily identified in well logs as "blue clay" or AMEC Geomatrix, Inc. hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 4