Laserfiche WebLink
2012 Annual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California February 2013 <br /> • Intermediate Zone wells MW-214 (I-8), MW-320, MW-321 (I-9), MW-323, MW-4475, <br /> EW-4, , <br /> Wells with 50-to 79-percent reduction in Cr concentrations <br /> • Shallow Zone wells MW-8, MW-102, MW-106, MW-205 (S-7), MW-207, and MW- <br /> 300 <br /> • 100-Foot Sand Zone well TW-15 <br /> • Intermediate Zone wells MW-1, MW-2, MW-213, MW-322, MW-324 (I-10), MW- <br /> 448D, EW-3, and EW-5 <br /> Wells with less than 50 percent reduction in Cr concentrations <br /> • Shallow Zone wells MW-102, and MW-300 <br /> • Intermediate Zone wells I-11, MW-1 (Cr 6+), MW-4, MW-212, MW-323 (Cr+6), MW-324 <br /> (I-10) (Cr+6), MW-447D, 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 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 (I-8) (Graph 24), and TW-13 <br /> (Graph 53) 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. Also note <br /> that Cr and Cr+6 concentrations at TW-13 increased in fourth quarter 2012 and will be closely <br /> monitored during future sampling events to determine if the latest results are anomalous. <br /> (D MWH, <br /> 6-2 <br />