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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 /> respectively then increased slightly in the second quarter 2013 to 627 gg/L and 508 gg/L, <br /> respectively(Graph 39). <br /> MW-447D: Concentrations of Cr and Cr+6 decrease during the reporting period relative to fourth <br /> quarter 2012. First quarter 2013 sampling results indicate that Cr and Cr+6 concentrations <br /> decreased to 1,630 and 1,620 gg/L, respectively then increased slightly in the second quarter <br /> 2013 to 1,800 gg/L and 1,790 gg/L, respectively(Graph 40). <br /> MW-448S: Concentrations of Cr and Cr6+ remained relatively stable during the reporting period <br /> at 2,780 gg/L and 2,730 gg/L, respectively(Graph 41). <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 gg/L, <br /> respectively then increased to in the second quarter 2013 to 3,900 gg/L and 3,880 gg/L, <br /> respectively. Overall MW-448D Cr and Cr+6 concentrations are still lower than historical values <br /> (Graph 42). <br /> I-11: Concentrations of Cr and Cr+6 declined in second quarter 2013, relative to a high point in <br /> second quarter 2012 (see Graph 51). 1-11 is located in the south yard and is an active <br /> groundwater extraction well. <br /> TW-13: Concentrations had been below the WQO until an increased above the WQO in fourth <br /> quarter 2012 to 262 and 254 gg/L for Cr and Cr+6, respectively. Concentrations in the first <br /> quarter 2013 decreased to below the WQO to 31 gg/L and <0.5 gg/L, respectively (Graph 53). <br /> TW-13 is located in the 100-foot sand zone at the south edge of the North Yard Area. <br /> MW-449S: Concentrations of Cr and Cr6+ increased during the reporting period to 1,940 gg/L <br /> and 1,930 gg/L, respectively. It can be seen in Graph 43 that concentrations at this well have <br /> been on a slight increasing trend since fourth quarter 2012. <br /> © MWH 4-5 <br />