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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 /> downgradient of the former retort pit (source area). A summary of the concentration trends for
<br /> wells where Cr concentrations exceeded the WQO during the first and second quarters of 2012 is
<br /> presented below.
<br /> Wells where Cr concentrations have decreased to below the WOO. Quarterly sampling in
<br /> the first half of 2012 showed that concentrations of total dissolved Cr or Cr+6 in well MW-321 (I-
<br /> 9), which were above the WQO through January 2012, have now decreased to below the WQO
<br /> (see Graph 32).
<br /> Wells where Cr concentrations are above the WOO. Quarterly sampling in the first half of
<br /> 2012 showed that concentrations of total dissolved Cr and Cr+6 is above the WQO in wells MW-
<br /> 8, MW-102, MW-106, MW-204 (S-8), MW-205 (S-7), MW-207, MW-213, MW-301, MW-
<br /> 447S, MW-447D, MW-448S, MW-448D, EW-3, and I-11 (see Graphs 6, 9, 12, 15, 16, 18, 23,
<br /> 26, 39, 40, 41, 42, 44, and 47).
<br /> Cr and Cr6+ concentration changes during the reporting period are summarized below. In some
<br /> cases third quarter 2012 data is presented, on a preliminary basis, to provide additional
<br /> understanding of the concentration trend at a particular well. All third quarter 2012 data will be
<br /> documented in the 2012 Annual Report, which is due by February 2013.
<br /> Wells MW-204 (S-8), MW-205 (S-7), MW-447S, MW-447D, MW-448S, and MW-448D are
<br /> located in the South Yard and are in the area were reductant injections were implemented during
<br /> third quarter 2012 in accordance with the Final Work Plan for Focused Supplemental
<br /> Hexavalent Chromium Reduction as Part of the In Situ Remedial Action Plan Amendment
<br /> Activities (MWH, 2012), and will be documented in the 2012 Annual Report, which is due by
<br /> February 2013. Cr and Cr6+ concentrations decreased during the reporting period at wells MW-
<br /> 204 and MW-448D. Cr and Cr6+ concentrations remained stable during the reporting period at
<br /> wells MW-205, MW-447S, MW-447D, and MW-4485.
<br /> MW-8: Concentrations of Cr and Cr6+ increased during the reporting period. It can be seen in
<br /> Graph 6 that concentrations at this well do fluctuate; additionally, third quarter 2012 sampling
<br /> results for MW-8 indicate that Cr and Cr6+ concentrations decreased to 400 and 388 gg/L,
<br /> respectively, as compared to the second quarter 2012 results.
<br /> (a MWH,
<br /> 4-3
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