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2013 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site,Stockton,California August 2013 <br /> MW-449D: Concentrations of Cr and Cr 6+ increased during the reporting period to 8,600 gg/L <br /> and 8,550 gg/L, respectively. It can be seen in Graph 44 that concentrations at this well have <br /> been on an increasing trend since third quarter 2012. <br /> MW-4505: Concentrations of Cr and Cr6+ increased during the reporting period to 5,200 gg/L <br /> and 5,160 gg/L, respectively. It can be seen in Graph 45 that concentrations at this well have <br /> been on an increasing trend since third quarter 2012. <br /> MW-450D: Concentrations of Cr and Cr 6+ increased during the reporting period to 164 gg/L and <br /> 144 gg/L, respectively. It can be seen in Graph 46 that concentrations at this well have been on <br /> an fluctuating trend since third quarter 2012. <br /> EW-3: As outlined in the 2011 2"d Semi-Annual Groundwater Monitoring Report(Dudek, 2012), <br /> the water table elevation at EW-3 in the Shallow Zone, at the retort pit, experienced an <br /> approximate 20+-foot decline from fourth quarter 2006 to fourth quarter 2009. From fourth <br /> quarter 2009 to present, water table elevations have risen approximately 15 feet. The Cr and <br /> Cr 6+ concentration fluctuations observed at EW-3 may be the result of rising ground water <br /> leaching soluble hexavalent chromium from the high concentrations immediately under the retort <br /> pit area. <br /> 4.2.2 Iron <br /> Fe in the soluble ferrous form, which is formed when polysulfide reacts with ferric hydroxide <br /> coatings on aquifer grains, serves as a reductant of Cr+6. As a result, Fe is not generally detected <br /> in samples with elevated Cr+6, and only remains in solution once the Cr+6 has been reduced. <br /> Thus, Fe serves as an effective indicator of the reduction of Cr+6, and its presence serves as a <br /> continued reductant for Cr+6. Analytical data for Fe sampled during the program are summarized <br /> in Table 5 and presented on Graphs 1 through 57. <br /> 4.2.2.1 Background and Compliance Wells. Iron samples were not collected from <br /> background and compliance wells in the first or second quarters of 2013 in accordance with <br /> MRP No. R5-2012-0814. <br /> © M W H 4-6 <br />