ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec— A-Zone Groundwater Flow System The A-zone behaves as a semi-confined or leaky aquifer (Weiss, 1995a). Model calibrated hydraulic conductivity values for the A-zone ranged from 70ft/d (Geomatrix, 1996) to 125 ft/d (RUST, 1994). Using the same horizontal gradient and porosity as for the B-zone groundwater calculations, but the higher K, groundwater velocities on the order of 1 to 2 ft/d are estimated for the A-zone. Vertical gradients are typically downward from the A-zone to the Intermediate Aquifer zones (Appendix B). 4.4.2 Intermediate Aquifer The Intermediate Aquifer is composed of alluvial and fluvial sediments of Upper Turlock Lake Formation, and therefore high groundwater yields are expected. Water levels are similar to those in the Upper Aquifer, where a mound in the potentiometric surface exists in the vicinity of the WWTF lagoons, with groundwater flow northward, north of the lagoons, westward west of the lagoons towards pumping at Neenah Paper Company wells PW-6, PW-7, and PW-8, and southward, towards the Stanislaus River (Appendix F). The horizontal groundwater flow direction in the Intermediate Aquifer (based on measured water levels in January and July 2008 for wells penetrating both C1- and C2-zones) had a radial pattern, with a similar water level distribution to those observed for the Upper Aquifer Zone in January and July 2008 (Appendix F). This suggests that much of the recharge to the Intermediate Aquifer occurs near the WWTF and Neenah Paper Company lagoons. The steepest horizontal gradients are from the area of recharge (on the Neenah Paper Company property in January 2008) northward along Stockton Avenue, from TH-8 to M-8C1 (approximately 1,250 feet), where the water level difference was 5.94 feet, enhanced by drawdown from pumping at Nestle extraction well E-6. Strong downward gradients develop between the Intermediate Aquifer and Lower Aquifer, and are highest during the summer months. As shown in the hydrographs for the M-23 well cluster (Appendix B), the water level in M-23C was 5.18 feet higher than that measured in M-23D during the February 2007 monitoring event. This difference increased to 13.40 feet by the July event. The higher vertical gradients occur in the summer months presumably because of irrigation pumping and increased municipal pumping. The thickness of the Corcoran Clay at M- 23D is approximately 20 feet, based on the clay interval described as "moderately plastic and sticky" in the boring log, between 197 and 217 feet bgs. If the 13.40 feet head difference is applied across this 20 feet thick layer, the downward gradient is calculated as 13.4 feet/20 feet = 0.67 ft/ft. AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 36