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2011 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California January 2012 <br /> Wells with 50-to 79-percent reduction in Cr concentrations <br /> • Shallow Zone wells MW-8, MW-106, MW-204 (S-8) (Cr), and MW-300, <br /> • Intermediate Zone wells MW-2, MW-324 (I-10) (Cr), MW-447S, MW-448D, EW-3, <br /> and EW-5 <br /> Wells with a reduction in Cr concentrations of less than 50 percent <br /> • Shallow Zone wells MW-102, MW-204 (S-8) (Cr 16), MW-205 (S-7), and MW-301 <br /> • Intermediate Zone wells I-11, MW-1 (Cr+6), MW-4, MW-212, MW-324 (I-10) (Cr+6), <br /> MW-447D, and MW-448S <br /> 6.2 GROUNDWATER GEOCHEMISTRY <br /> The full-scale injection of the reductant solution resulted in a decline of oxidized species <br /> including Cr+6, and increases in the solubility of reduced species such as Fe, Mn, and As in <br /> selected wells. The changes in groundwater chemistry following the full-scale injection have <br /> been consistent with expectations and experience at other sites. Generally, geochemical changes <br /> are just as expected for key monitoring parameters: <br /> • Cr concentrations decrease after injections and remain low <br /> • Fe, Mn, and As concentrations temporarily increase after injections, then return to <br /> baseline conditions <br /> • ORP and DO temporarily decrease after injections, then return to baseline conditions <br /> Recent data from MW-206 (Graph 17), MW-213 (I-7) (Graph 23), MW-214 (I-8) (Graph 24), <br /> and TW-13 (Graph 49) show the rapid effect of the reductant solution on concentrations of Cr <br /> (decrease) and Mn and As (temporary increase). Note that the effect in well TW-13 (located <br /> within one of the 2008 treatment barriers) occurred 2 years after the full-scale in situ treatment <br /> phase, which demonstrates the lasting effect of the reductant solution and injection barrier <br /> approach. <br /> Increases in Fe and Mn were observed as a direct result of CaSx injection in close proximity to <br /> some wells. The increase results from the reduction of ferric hydroxide coatings of aquifer <br /> grains and the mobilization of ferrous Fe and reduced Mn originally present as a co-precipitate in <br /> the coatings. Concentrations are expected to attenuate to below respective WQOs in all the full- <br /> MWH, <br /> 6-2 <br />