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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-102: Concentrations of Cr and Cr 6+ increased slightly during the reporting period. It can be <br /> seen in Graph 9 that concentrations at this well had a slightly decreasing trend since fourth <br /> quarter 2010 however, increased during this reporting period. MW-102 is located in the shallow <br /> zone west of the Former Retort Pit area. <br /> MW-106: Concentrations of Cr and Cr 6+ increased slightly during the reporting period. It can be <br /> seen in Graph 12 that concentrations at this well are still relatively stable since third quarter <br /> 2010. MW-106 is located southeast of the Former Retort Pit area. <br /> MW-204: Concentrations of Cr and Cr 6+ decreased below the WQO during second quarter 2013 <br /> sampling. Concentration graphs are presented in in Graph 15. MW-204 is located in the shallow <br /> zone in the South Yard Area. <br /> MW-213: Monitoring well MW-213 is located in the intermediate zone in the southwest <br /> quadrant of the North Yard area. Since 2008, when concentrations of Cr and Cr+6 initially <br /> decreased as a result of nearby supplemental injection, concentrations have fluctuated over a <br /> wide range. Cr and Cr 6+were not detected in the first quarter 2012 sample but increased to 5,410 <br /> and 5,120 µg/L, respectively, in second quarter 2012 (Graph 23). Concentrations decreased in <br /> third quarter 2012 to 670 and 561 µg/L, respectively, but increased in fourth quarter 2012 to <br /> 2,880 and 2,710 gg/L, respectively. Concentrations have decreased slightly in the current <br /> reporting period to 2,460 µg/L and 2,450 µg/L, respectively. <br /> MW-321: Concentrations of Cr and Cr+6 decreased below the WQO for the first time in second <br /> quarter 2012. However, in third quarter 2012 concentrations of Cr and Cr+6 increased to 150 and <br /> 149 gg/L, respectively. In fourth quarter 2012, concentrations decreased to 57 and 49.5 gg/L, <br /> respectively. Concentrations decreased during the current reporting period to <10 µg/L and <0.5 <br /> gg/L, respectively. MW-321 is located in the intermediate zone in the southern South Yard <br /> Area. <br /> MW-447S: Concentrations of Cr and Cr+6decreased from 2,060 and 1,950 gg/L, respectively, in <br /> third quarter 2012 to 716 and 700 µg/L, respectively, in fourth quarter 2012. First quarter 2013 <br /> sampling results indicate that Cr and Cr+6 concentrations decreased to 590 and 590 µg/L, <br /> © MWH 4-4 <br />