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2012 Annual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California February 2013 <br /> MW-448D: Concentrations of Cr and Cr+6 had been declining steadily at MW-448D since June <br /> 2011, but increased slightly to 2,930 and 2,860 gg/L, respectively, in fourth quarter 2012. The <br /> fourth quarter data are within the range of historical results. However, first quarter 2013 <br /> sampling results indicate that Cr and Cr+6 concentrations decreased to 1,470 and 1,460 µg/L, <br /> respectively. <br /> I-11: Concentrations of Cr and Cr+6 declined in third and fourth quarter 2012, relative to a high <br /> point in second quarter 2012 (see Graph 51). I-11 is located in the south yard and is an active <br /> groundwater extraction well. <br /> TW-13: Concentrations of Cr and Cr+6 increased above the WQO in fourth quarter 2012. <br /> Concentrations had been below the WQO since July 2010, with concentrations ranging from <br /> non-detect to 14 µg/L, but increased to 262 and 254 µg/L for Cr and Cr+6, respectively, in fourth <br /> quarter 2012. TW-13 is located in the 100-foot sand zone at the south edge of the North Yard <br /> Area. <br /> New Wells: Two new dual-zone well clusters, MW-449S/MW-449D and MW-450S/MW-450D, <br /> were installed in 2012 and samples from all wells collected in third and fourth quarter 2012 <br /> exceeded the WQO for Cr and Cr+6. At the MW-449 well cluster, concentrations of Cr and Cr+6 <br /> are higher in the deeper well (see Graphs 43 and 44) while concentrations are higher in the <br /> shallower well at the MW-450 cluster(Graphs 45 and 46). <br /> EW-3: As outlined in the 2011 2nd 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 /> Cr6+ 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 /> (D MWK, <br /> 4-5 <br />