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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 /> surface maps prepared by Dudek are presented in Appendix A, Figures A-1 through A-8. <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 first quarter <br /> (January) and second quarter (April) of 2012 are discussed by water-bearing zone in the <br /> following subsections. Figures A-4 and A-8 in Appendix A present the groundwater elevations <br /> and chromium concentrations in the deep zone in the first and second quarters of 2012, <br /> respectively, and are provided for informational purposes. <br /> 3.2.1 Shallow Zone <br /> Potentiometric surface maps of Shallow Zone groundwater for the first and second quarters of <br /> 2012 are presented in Appendix A (Figures A-1 and A-5). In January 2012, groundwater <br /> elevations ranged from -21.06 feet (ft) relative to mean sea level (msl) at MW-401 to -25.13 ft <br /> msl at MW-302. In April 2012, groundwater elevations ranged from -14.33 ft msl at MW-302 to <br /> -23.37 ft msl at MW-9. Groundwater in the Shallow Zone across the source area moved south in <br /> response to a gradient of approximately 0.0008 foot per foot (ft/ft) in January 2012 and 0.0016 <br /> ft/ft in April 2012, as calculated between wells MW-210 and MW-9. The average linear velocity <br /> of groundwater (velocity equals hydraulic conductivity multiplied by hydraulic gradient divided <br /> by effective porosity), in the Shallow Zone 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 0.11 ft/day in January 2012 and 0.21 ft/day in April 2012. <br /> (a MWH <br /> 3-4 <br />