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amec— <br /> Two <br /> apparently laterally continuous geologic features were identified and form the basis for <br /> the revised stratigraphy. These geologic features are: <br /> • The base of Upper Aquifer Zone (or top of the Intermediate Aquifer Zone) <br /> • The top of the Corcoran Clay. <br /> A description of these features is provided below. <br /> Base of the Upper Aquifer Zone <br /> Geophysical logs can provide a "signature" for geologic formations or stratigraphic units, and <br /> can be used to identify textural changes in sediment characteristics with depth (e.g. <br /> coarsening and fining upward sequences, erosional surfaces, buried stream channels and clay <br /> layers (Burow, et al., 2004). The geophysical logs reviewed for this SCM were derived from <br /> test holes for municipal supply wells (MW-7, MW-8, MW-9, MW-10, MW-11 and MW-12), and <br /> Nestle wells (N-2, M-24C1, M-25C2, M-26C2, M-27D, M-28C1, M-29C2). Based on the <br /> geophysical logs and Site boring logs, soils to 110 feet bgs contain an abundance of sands <br /> and gravels that do not appear to correlate far across borings. In general, the base of the <br /> Upper Aquifer Zone is encountered at elevations above -40 feet msl beneath the northeast <br /> and east portion of the Study Area (MW-7, MW-8, MW-9, M-5C2, and M-26C2) and probably <br /> slopes to the west and southwest. The Upper Aquifer Zone base generally is described as <br /> brown stiff silty clay in boring logs, but has also been referred to as white or gray in color <br /> (boring logs M-81D, M-13, M-15A, ED-1, EI-1) and red-brown silty clay (ED-2). <br /> Lateral Extent and Slope of the Corcoran Clay <br /> The Corcoran Clay is more easily recognized by its color in boring logs than by it geophysical <br /> signature, as fine-grained sediments deposited on top of the Clay Layer have a different color <br /> (predominantly brown) and are not widely correlated among borings. The clay unit has been <br /> identified in borings beneath the Study Area as blue, gray or green clay, with an average <br /> thickness of approximately 15 feet and generally moderate to high plasticity. The top of the <br /> Corcoran Clay is generally encountered at variable depths beneath the Study Area; between <br /> 178 and 210 feet bgs, where it is encountered at shallower elevations beneath the northeast <br /> portion of the Study Area (approximately -120 to -125 feet msl at M-61D, M-25C2, M-5C1 and <br /> M-1 6C1) to less than -135 feet msl towards the southwest, south of the Stanislaus River (M-34 <br /> and M-35). <br /> AMEC Geomatrix, Inc. <br /> hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 26 <br />