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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 /> • The polysulfide reduces ferric oxyhydroxide coatings on aquifer solids, converting the <br /> ferric ion to the more mobile ferrous iron. <br /> • A portion of the ferrous iron migrates with moving ground water, and reacts with <br /> dissolved hexavalent chromium in the ground water. <br /> • A portion of the ferrous iron forms ferrous minerals which sorb onto the aquifer solids <br /> and thus form a borehole-placed reactive barrier(Rouse et al., 2006), available to react <br /> with and reduce hexavalent chromium dissolved in ground water passing the sorption <br /> site. <br /> During the Phase 1 of the pilot test, it was documented that this sorbed ferrous iron is capable <br /> of exerting reduction for at least two years, in a portion of the aquifer with high dissolved <br /> hexavalent chromium concentration. Thus, while calcium polysulfide is a liquid and able to <br /> migrate through an aquifer, upon entry into an aquifer environment, much of the reductive <br /> capacity becomes fixed in place within the original radius of impact of the pressure injection. <br /> In the case of the horizontal injections, the injections would form a `floor' under the retort <br /> pit, with the capacity to reduce high concentrations of hexavalent chromium being displaced <br /> by shallow input of water, but it might not be able to migrate below the point of injection due <br /> to consumption by oxygen present in the vadose zone pore space. <br /> (B mm" <br /> 2-3 <br />