ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec- • Section 3 summarizes the Site history and conditions, • Section 4 summarizes the history and characteristics of the Study Area, and • Section 5 summarizes key findings and implications from the SCM. 2.0 REGIONAL SETTING The geology and hydrogeology of the Ripon area are described in Sections 2.1 and 2.2. Additional details on the stratigraphy and hydrogeologic conditions beneath the Site and Study Area are described in Sections 3 and 4 of this document. The Study Area is situated within the low-lying eastern flank of the Central Valley of California, at the southern boundary of San Joaquin County. The Stanislaus River crosses the southern portion of the Study Area. Surface elevation is approximately 60 to 70 feet above mean sea level. The incorporated area of Ripon (with a current population of approximately 14,000 residents), is relatively flat and covers approximately 4.3 square miles. The primary industry is agricultural (almonds and grapes); small manufacturing shops supporting this industry have been operating since the 1950's (http://www.cityofripon.org/Community/history.htm). Other important historical industries include Nestle and paper manufacturing (1960 to present— Neenah Paper Company, formerly Fox River and Simpson Paper Company), Guntert Steel, and agricultural and food processing industries (Ripon Canning, Ripon Milling, and Nulaid Foods). 2.1 GEOLOGY Ripon lies within California's Central Valley, a northwest trending asymmetric trough which is bordered on the east by the Sierra Nevada Range and on the west by the Coast Ranges (Bertoldi et al. 1991). The San Joaquin Valley comprises the portion of the Central Valley that lies south of the Sacramento-San Joaquin River Delta, and is underlain by approximately 32,000 ft of sediments of both marine and continental origins from Jurassic to Holocene age (approximately 200 million years ago, to within the past 10,000 years; Bertoldi et al. 1991). A revised interpretation of the geology and hydrogeology of the Modesto area, including Ripon, has been published by Burow et al., 2004. That analysis involved reviewing over 3,500 well logs in the area, mapping the lateral extent of the Corcoran Clay further eastward, and correlating sediment textures with depths, to further the understanding of geologic controls on the groundwater movement and availability. Although the study area referenced in Burow et al., 2004, was centered on the city of Modesto, their study area encompasses Ripon, and therefore their publication is considered to be a primary reference for describing geologic and hydrogeologic conditions for the SCM. Surficial sediments along the Stanislaus River near Ripon are primarily coarse-grained and have been associated with moderately rapid percolation rates (2.5 to 5 inches per hour; Burow et al., 2004). Deeper coarse-grained fluvial deposits are present as disconnected buried AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 3