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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 • The polysulfide reduces ferric oxyhydroxide coatings on aquifer solids, converting the ferric ion to the more mobile ferrous iron. • A portion of the ferrous iron migrates with moving ground water, and reacts with dissolved hexavalent chromium in the ground water. • A portion of the ferrous iron forms ferrous minerals which sorb onto the aquifer solids and thus form a borehole-placed reactive barrier(Rouse et al., 2006), available to react with and reduce hexavalent chromium dissolved in ground water passing the sorption site. During the Phase 1 of the pilot test, it was documented that this sorbed ferrous iron is capable of exerting reduction for at least two years, in a portion of the aquifer with high dissolved hexavalent chromium concentration. Thus, while calcium polysulfide is a liquid and able to migrate through an aquifer, upon entry into an aquifer environment, much of the reductive capacity becomes fixed in place within the original radius of impact of the pressure injection. In the case of the horizontal injections, the injections would form a `floor' under the retort pit, with the capacity to reduce high concentrations of hexavalent chromium being displaced by shallow input of water, but it might not be able to migrate below the point of injection due to consumption by oxygen present in the vadose zone pore space. In May 2009, the CVRWQCB approved, by electronic mail, a program modification proposal to discontinue nitrate analyses as part of the in situ RAP Amendment activities (CVRWQCB, 2009). This change to the sampling program was incorporated into the third quarter of 2009 and subsequent sampling events. (a MWK, 2-3