ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec— Cl-Zone and C2-Zone Groundwater Flow Systems The C-zones behave as a semi-confined or leaky aquifer (Weiss, 1995a). Model calibrated hydraulic conductivity values for the C1-zone ranged from 5 ft/d (Geomatrix, 1996) to 125 ft/d (RUST, 1994a). The 5 ft/day value reported in Geomatrix (1996) seems too low, and is based on recovery data from a single pumping test at well M-10C1. Based on the boring logs and reported high water production rates associated with the Turlock Lake Formation, the 125 ft/d K-value is probably more realistic. Assuming K=125 ft/d, and a horizontal gradient and effective porosity of 0.0048 and 0.25, respectively, groundwater velocities as high as 2.4 ft/d or greater can be expected for the C1-zone. Model calibrated hydraulic conductivity values for the C2-zone ranged from 10 ft/d (Geomatrix, 1996) to 50 ft/d (RUST, 1994a). 4.4.3 Corcoran Clay As discussed in Burow et al., (2004), citing Page (1977), the Corcoran Clay is a regional confining unit (aquitard)with vertical hydraulic conductivities ranging from 3 x 10-6 to 9 x 10-6 ft/d, although the calibrated K for a regional model was much higher (1.3 x 10-2 ft/d; Burow et al., 2004, citing unpublished data provided by S. P. Phillips of the United States Geological Survey), presumably because conduit wells transmitting water across the clay resulted in artificially high leakance values. For reference, clay liners used to line municipal solid waste landfills constructed in California must have a hydraulic conductivity value less than approximately 2.8 x 10-3 ft/d (California Code of Regulations, Title 27, Subchapter 2, Article 4). Therefore, the vertical hydraulic conductivity of the Corcoran Clay is very low, such that the migration of water and COCs downwards through the clay is essentially negligible compared to horizontal flow, except where conduit wells penetrate the Corcoran Clay. 4.4.4 Lower Aquifer The Lower Aquifer is a confined aquifer due to the overlying Corcoran Clay, and is composed of alluvial and fluvial sediments of Lower Turlock Lake Formation. High groundwater yields are also associated with this zone. Water levels are several feet lower than those in the Intermediate and Upper Aquifer (Appendix B) and groundwater flow is west and northwest, with no apparent mounding due to recharge at the WWTF or Neenah Paper lagoons (Appendix F). In addition, water levels in D-zone wells generally show a higher amplitude of seasonal variation caused by pumping cycles. The apparent attenuation of surface recharge effects on D-zone water levels and the observed higher response to seasonal pumping is consistent with the confined nature of the Lower Aquifer. A horizontal gradient of 0.001 was measured between M-43D and M-41 D (January 2008; Appendix F). Hydraulic conductivity values used for past numerical groundwater flow models ranged from 150 ft/d (Geomatrix, 1996) to 180 ft/d (RUST, 1994a). Assuming an effective porosity of 0.25, and K=150 ft/d, and a AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 37