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2011 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California January 2012 <br /> surface maps prepared by Dudek are presented in Appendix A, Figures 1 through 4. <br /> Interpretation of the groundwater gradient is complicated by the effects of the groundwater <br /> extraction (for treatment) in an area with an otherwise relatively flat water table surface. <br /> Groundwater elevations, estimated gradients, and linear velocities during the third quarter (July) <br /> and fourth quarter (October) of 2011 are discussed by water-bearing zone in the following <br /> subsections. <br /> 3.2.1 Shallow Zone <br /> Potentiometric surface maps of Shallow Zone groundwater for the third and fourth quarters of <br /> 2011 are presented in Appendix A (Figures 1 and 3). In July 2011, groundwater elevations <br /> ranged from -15.6 feet (ft) relative to mean sea level (msl) at MW-401 to -26.91 ft msl at <br /> MW-205 (S-7), which is an active extraction well. In October 2011, groundwater elevations <br /> ranged from -18.67 ft msl at MW-302 to -24.43 ft msl at MW-304. Groundwater in the Shallow <br /> Zone across the source area moved south in response to a gradient of approximately 0.0029 foot <br /> per foot (ft/ft) in July 2011 and 0.0018 ft/ft in October 2011, as calculated between wells <br /> MW-210 and MW-9. The average linear velocity of groundwater (velocity equals hydraulic <br /> conductivity multiplied by hydraulic gradient divided by effective porosity), in the Shallow Zone <br /> was estimated using: <br /> • A hydraulic conductivity of 25.6 feet per day (ft/day), which was averaged from <br /> hydraulic conductivities estimated from aquifer tests conducted at wells MW-202, <br /> MW-203, MW-206, MW-207, MW-208, and MW-209 (Hargis +Associates, Inc. (H+A), <br /> 1989a and 1989b) <br /> • An effective porosity of 20 percent (H+A, 1990) <br /> • The hydraulic gradient calculated between wells MW-210 and MW-9 <br /> The average linear velocity of groundwater in the Shallow Zone across the source area was <br /> approximately 0.37 ft/day in July 2011 and 0.23 ft/day in October2011. <br /> 3.2.2 100-Foot Sand Zone <br /> In July 2011, groundwater elevations ranged from -15.54 ft msl at TW-16 to -26.37 ft msl at <br /> EW-1 (which is screened across the Shallow and Intermediate zones). In October 2011, <br /> Q) MWH. <br /> 3-4 <br />