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2013 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site,Stockton,California August 2013 Wells with 50-to 79-percent reduction in Cr concentrations • Shallow Zone wells MW-8, MW-106, and MW-300 • Intermediate Zone wells MW-2, MW-213, MW-322, MW-323, MW-324 (I-10), MW- 448D, and EW-5 Wells with less than 50 percent reduction in Cr concentrations • Shallow Zone wells MW-102, and MW-205 (S-7) • Intermediate Zone wells MW-4, MW-212, MW-320, MW-447D, MW-448S, MW- 449S,MW-449D,MW-450S, MW-450D, and EW-3 5.2 GROUNDWATER GEOCHEMISTRY The full-scale injection of the reductant solution resulted in a decline of oxidized species including Cr+6, and increases in the solubility of reduced species such as Fe, Mn, and As in a few wells, adjacent to sites of reductant solution injection. The changes in groundwater chemistry following the full-scale injection have been consistent with expectations and experience at other sites. Generally, geochemical changes are as expected for key monitoring parameters: • Cr concentrations decrease after injections and remain low • Fe, Mn, and As concentrations temporarily increase after injections, then return to baseline conditions • ORP and DO temporarily decrease after injections, then return to baseline conditions Data from MW-101 (Graph 8), MW-206 (Graph 17), MW-214 (I-8) (Graph 24), and TW-13 (Graph 53) show the rapid effect of the reductant solution on concentrations of Cr (decrease) and Mn and As (temporary increase). Note that the effect in well TW-13 (located within one of the 2008 treatment barriers) occurred 2 years after the full-scale in situ treatment phase, which demonstrates the lasting effect of the reductant solution and injection barrier approach. Also note that the Cr and Cr+6 concentrations at TW-13 that increased in fourth quarter 2012 have decreased during this monitoring period. Increases in Fe and Mn were observed as a direct result of CaSX injection in close proximity to some wells. The increase results from the reduction and dissolution of ferric hydroxide coatings of aquifer grains and the mobilization of ferrous Fe and reduced Mn originally present as a co- MWH 5-2