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amec�9 <br /> northeast in 2008. Horizontal gradients on the order of 0.0028 to 0.0036 were measured <br /> between pairs of wells (W-7 and M-813 in July and January, 2008, respectively). <br /> B-Zone Groundwater Flow System <br /> The B-zone is the water table aquifer, and behaves as an unconfined aquifer (Weiss, 1995a). <br /> Model calibrated hydraulic conductivity values (K) for the B-zone ranged from 10 ft/d <br /> (Geomatrix, 1996) to 50 ft/d (RUST, 1994a). Three single well pumping tests were performed <br /> in shallow (B-zone) wells near the WWTF to provide information on hydraulic conductivity near <br /> the WWTF (K=0.27 ft/d, 15 ft/d, and 47 ft/d; ECOLOGIC, 2007). The groundwater flow regime <br /> for the B-zone is complex, and influenced by surficial recharge, and discharge to the <br /> Stanislaus River. Based on a horizontal gradient (i) of 0.0036, an assumed effective porosity of <br /> 0.25 (n), and a hydraulic conductivity (K) of 50 ft/d, groundwater velocities (v) on the order of <br /> 0.7 ft/day can be calculated using Darcy's law (v=Ki/n). Geomatrix (2006) estimated a <br /> groundwater velocity of approximately 1 ft/day for the B-zone based on a single well tracer test <br /> performed at Site monitoring well M-1 B. Higher groundwater velocities are possible as the <br /> model calibrated K value is a bulk value, and locally, K can be much higher due to geologic <br /> heterogeneities. <br /> Although water levels in B-zone wells are generally similar to those in co-located A-zone wells, <br /> there are differences, indicating that these intervals can behave as separate hydrostratigraphic <br /> units. Near the WWTF, at well pair M-1 7B and M-1 7A, water levels have generally been lower <br /> in well M-1 7B than in well M-1 7A (during the same monitoring event), indicating that vertical <br /> gradients are upward at the time of monitoring (Figure 15). This observation is unexpected, <br /> because nearby surficial recharge and deeper pumping should impart a downward gradient. It <br /> may be that shallow groundwater drains faster to the Stanislaus River, thereby causing a <br /> transient upward gradient, or that a preferential vertical flow path from a surficial recharge <br /> source to the A-zone exists near this well, or that shallow B-zone pumping is occurring nearby. <br /> For well pairs M-11 B/A and M-1 5B/A, water levels have been higher in the B-zone wells, by <br /> greater than 0.5 feet; indicating stronger downward gradients near these wells, which are <br /> located near areas that are irrigated by Neenah Paper Company. Vertical gradients at wells M- <br /> 10B/A have been variable (both upwards and downwards; Figure 15) in the 1990s, and were <br /> likely responding to pumping cycles at nearby municipal supply well MW-4. <br /> Vertical gradients between M-8B and M-8A (near 4t" Street and Stockton Avenue) have <br /> generally been upward, between 1993 and 2004 (Figure 15); the influence of groundwater <br /> extraction at B-zone well E-2 (beginning in 2002) on vertical gradients is not obvious in the <br /> water level response for M-813 and M-8A. Periods of strong downward gradients at the M- <br /> 1B/M-1A well pair (onsite) are evident in Figure 15; this likely reflects times of increased <br /> groundwater extraction rates from nearby A-zone extraction well EI-1. <br /> AMEC Geomatrix, Inc. <br /> I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 35 <br />