ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec— Stratigraphic Correlation with Regional Geologic Units The results of the stratigraphic analysis are summarized in Figure 11. The designations of Upper, Intermediate, and Lower Aquifer Zones are retained and their depth intervals and thicknesses have not changed substantially, but have been approximately correlated with regional geologic units, where possible. This analysis suggests that: • The Upper Aquifer Zone (< 110 feet bgs on average) is composed of coarse sands and gravels, with silt and clay interbeds, likely of both the Riverbank and Turlock Lake Formations. • The Intermediate Aquifer Zone is approximately 85 feet thick on average and contains clay zones, of possible lacustrine origin, and coarse sands and gravels of the upper Turlock Lake Formation. • The Corcorcan Clay is a fine-grained laterally continuous lacustrine clay aquitard that forms the base of the upper Turlock Lake Formation and is 15 feet thick on average, encountered at a depth of approximately 196 feet bgs beneath the Site. • The top of the Lower Aquifer zone begins at approximately 207 feet bgs (average depth beneath the Study Area) as a sequence of sands and gravels separated by fine-grained silts and clays of the lower Turlock Lake Formation extending to greater than 300 feet beneath the Study Area. 4.2 HYDROGEOLOGY The hydrogeology of the Study Area is described in greater detail below. Some important points summarizing the hydrogeologic conditions are: • The water table depth below the City of Ripon is typically within 20 —40 feet of ground surface; • Water levels in monitoring wells are influenced by well pumping, precipitation patterns and the stage of the Stanislaus River; • The Upper and Intermediate Aquifers behave as unconfined and semi-confined systems; the Lower Aquifer is confined by the Corcoran Clay; • Lower Aquifer water levels can be greater than 10 feet lower than Intermediate Aquifer levels in the summer months (i.e. strong downward gradients, from 0.1 to 1 are often observed); • Generally, historical and present groundwater flow patterns are from the area near the City of Ripon Wastewater Treatment Facility (WWTF) and Neenah Paper lagoons towards pumping wells to the north, and south, of the Stanislaus River based on monitoring wells screened in the Upper and Intermediate Aquifers; groundwater flow direction has generally been to the west beneath the Corcoran Clay, based on Lower Aquifer monitoring wells; and AMEC Geomatrix, Inc. hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 27