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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 /> Wells with 50-to 79-percent reduction in Cr concentrations <br /> • Shallow Zone wells MW-106, MW-205 (S-7) (Cr+6), MW-207, MW-300, and MW-301 <br /> • 100-Foot Sand Zone well TW-15 <br /> • Intermediate Zone wells MW-1 (Cr+6), MW-2, MW-320, MW-322, MW-323 (Cr), MW- <br /> 324 (1-10) (Cr), MW-447S,MW-448D, EW-3, and EW-5 <br /> Wells with a reduction in Cr concentrations of less than 50 percent <br /> • Shallow Zone wells MW-8, MW-102, and MW-205 (S-7) (Cr), <br /> • Intermediate Zone wells I-11, MW-213, MW-323 (Cr+6), MW-324 (1-10) (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 a few <br /> wells, adjacent to sites of reductant solution injection. The changes in groundwater chemistry <br /> following the full-scale injection have been consistent with expectations and experience at other <br /> sites. Generally, geochemical changes 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 /> Data from MW-101 (Graph 8), MW-206 (Graph 17), MW-214 (1-8) (Graph 24), and TW-13 <br /> (Graph 49) show the rapid effect of the reductant solution on concentrations of Cr(decrease) and <br /> Mn and As (temporary increase). Note that the effect in well TW-13 (located within one of the <br /> 2008 treatment barriers) occurred 2 years after the full-scale in situ treatment phase, which <br /> demonstrates the lasting effect of the reductant solution and injection barrier 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 and dissolution of ferric hydroxide coatings <br /> of aquifer grains and the mobilization of ferrous Fe and reduced Mn originally present as a co- <br /> precipitate in the coatings. Concentrations are expected to attenuate to below respective WQOs <br /> (D MWH. <br /> 6-2 <br />