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2012 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California October 2012 <br /> 3.2.2 100-Foot Sand Zone <br /> In January 2012, groundwater elevations ranged from -13.52 ft msl at TW-16 to -24.37 ft msl at <br /> EW-1 (which is screened across the Shallow and Intermediate zones). In April 2012, <br /> groundwater elevations ranged from -13.14 ft msl at TW-16 to -23.97 ft msl at EW-1. <br /> Groundwater in the 100-Foot Sand Zone across the source area moved south in response to a <br /> gradient of approximately 0.0006 ft/ft in January 2012 and south in response to a gradient of <br /> approximately 0.0004 ft/ft in April 2012, as calculated between wells TW-13 and MW-310. The <br /> average linear velocity of groundwater in the 100-Foot Sand Zone was estimated using: <br /> • A hydraulic conductivity of 269.1 ft/day, which was averaged from hydraulic <br /> conductivities estimated from aquifer tests conducted at wells MW-310, MW-410, <br /> MW-411, and S-3 (H+A, 1989a and 1989b) <br /> • An effective porosity of 20 percent (H+A, 1990) <br /> • The hydraulic gradient calculated between wells TW-13 and MW-310 <br /> The average linear velocity of groundwater in the 100-Foot Sand Zone across the source area <br /> was approximately 0.79 ft/day in January 2012 and 0.58 ft/day in April 2012. <br /> 3.2.3 Intermediate Zone <br /> Potentiometric surface maps of Intermediate Zone groundwater for the first and second quarters <br /> of 2012 are presented in Appendix A (Figures A-2, A-3, A-6 and A-7). In January 2012, <br /> groundwater elevations ranged from—21.94 ft msl at MW-211 to -40.91 ft msl at MW-321 (I-9), <br /> which is an active extraction well. In April 2012, groundwater elevations ranged from -19.84 ft <br /> msl at MW-211 to -40.47 ft msl at MW-321 (I-9). Groundwater in the Intermediate Zone across <br /> the source area moved south to southwest in response to a gradient of approximately 0.0009 ft/ft <br /> in January 2012 and 0.0017 ft/ft in April 2012, as calculated between wells MW-211 and MW- <br /> 322. The average linear velocity of groundwater in the Intermediate Zone was estimated using: <br /> • A hydraulic conductivity of 85.3 ft/day, which was averaged from hydraulic <br /> conductivities estimated from aquifer tests conducted at wells I-7, I-8, MW-211, <br /> MW-320, and MW-212 (H+A, 1989a and 1989b) <br /> • An effective porosity of 7 percent (H+A, 1990) <br /> (D MWH <br /> 3-5 <br />