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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 /> respectively, at well EW-3 in July 2012. EW-3 is located in the North Yard at the former retort <br /> pit (source area). A summary of the concentration trends for wells where Cr concentrations <br /> exceeded the WQO during the third and fourth quarters of 2012 is presented below. <br /> Wells where Cr concentrations have decreased to below the WOO. Quarterly sampling in <br /> the second half of 2012 showed that concentrations of total dissolved Cr in well MW-207, which <br /> were above the WQO in first and second quarter 2012, decreased below the WQO in third <br /> quarter 2012 (27 gg/L) and then increased to the WQO of 50 gg/L in fourth quarter 2012. <br /> Concentrations of Cr+6 in well MW-207 were also below the WQO in third and fourth quarter <br /> 2012 (25.4 and 48.2 gg/L, respectively). Fluctuations of Cr and Cr+6 around the WQO are typical <br /> for this well (see Graph 18). Concentrations of Cr and Cr+6 decreased at monitoring well <br /> MW-301, located in the South Yard area. The third and fourth quarter 2012 Cr concentration <br /> (49 and 24 µg/L, respectively) and Cr+6 concentration (42.7 and 23.2 gg/L, respectively) were <br /> both all-time lows (Graph 26). <br /> Wells where Cr concentrations are above the WOO. Quarterly sampling in the second half of <br /> 2012 showed that concentrations of total dissolved Cr and Cr+6 is above the WQO in wells MW- <br /> 4, MW-8, MW-102, MW-106, MW-204 (S-8), MW-205 (S-7), MW-213, MW-321, MW-447S, <br /> MW-447D, MW-448S, MW-448D, MW-449S, MW-449D, MW-4505, MW-450D, EW-3, I-11, <br /> and TW-13. Cr and Cr6+ concentrations decreased during the reporting period at wells MW-8, <br /> MW-102, MW-204, and MW-447S. Notable trends in Cr and Cr6+ concentration changes during <br /> the reporting period are summarized below on an individual well basis. <br /> MW4: Concentrations of Cr and Cr6+ increased relative to when the well was last sampled in <br /> third and fourth quarter 2011. Both the third and fourth quarter Cr and Cr+6 concentrations were <br /> all-time highs at the time of collection. Well MW-4 is located in the intermediate zone at the <br /> western edge of the North Yard area. <br /> MW-8: Concentrations of Cr and Cr6+ decreased during the reporting period. As shown on <br /> Graph 6, concentrations at this well fluctuate but had been gradually increasing since mid-2008 <br /> through second quarter 2012. Concentrations declined in third and fourth quarter, and the fourth <br /> (a MWH, <br /> 4-3 <br />