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2012 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California October 2012 <br /> MW-102: Concentrations of Cr and Cr6+ decreased during the reporting period. It can be seen in <br /> Graph 9 that concentrations at this well have a slightly decreasing trend since fourth quarter <br /> 2010; additionally third quarter 2012 sampling results for MW-102 indicate that Cr and Cr6+ <br /> concentrations decreased further to 503 and 503 gg/L, respectively. <br /> MW-106: Concentrations of Cr and Cr6+ remained relatively stable during the reporting period. <br /> It can be seen in Graph 12 that concentrations at this well have been relatively stable since third <br /> quarter 2010. <br /> MW-207: Concentrations of Cr and Cr6+ increased to slightly above the WQO during this <br /> reporting period, which is consistent with the historical record for this well where fluctuations <br /> slightly above and below the WQO are common (Graph 18). Third quarter 2012 sampling <br /> results for MW-207 indicate that Cr and Cr6+ concentrations decreased to 27 and 25.4 µg/L, <br /> respectively, below the WQO. <br /> MW-213: Cr and Cr6+ were not detected in the first quarter 2012 sample but increased to 5,410 <br /> and 5,120 gg/L, respectively, in second quarter 2012 (Graph 23). Third quarter 2012 sampling <br /> results for MW-213 indicate that Cr and Cr6+ concentrations decreased to 670 and 561 µg/L, <br /> respectively. <br /> MW-301: Concentrations of Cr and Cr6+decreased during the reporting period. It can be seen in <br /> Graph 26 that concentrations at this well have historically fluctuated around 100 gg/L; Third <br /> quarter 2012 sampling results for MW-301 indicate that Cr and Cr6+ concentrations decreased to <br /> 49 and 42.7 µg/L, respectively, below the WQO. <br /> EW-3: Concentrations of Cr and Cr6+ decreased in the first quarter 2012 sample to 4,700 and <br /> 4,650 gg/L, respectively. During second quarter 2012, EW-3 was only sampled for Cr, to be <br /> reported on by Dudek, and the Cr concentration increased to 22,400 gg/L. Third quarter 2012 <br /> sampling results for MW-213 indicate that Cr and Cr6+ concentrations decreased to 12,600 and <br /> 12,500 µg/L, respectively. <br /> I-11: Concentrations of Cr and Cr6+ continued to increase (Graph 47), which is not unexpected <br /> given its location downgradient from monitoring wells with higher concentrations (e.g., MW- <br /> (D <br /> MWH:,: <br /> 4-4 <br />