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2012 Annual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California February 2013 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- precipitate in the coatings. Concentrations are expected to attenuate to below respective WQOs in all the full-scale in situ Cr+6 reduction treatment implementation monitoring wells. This process for Fe and Mn is evident in wells MW-101 (Graph 8), MW-208 (Graph 19), MW-214 (I- 8) (Graph 24), and MW-322 (Graph 33). As expected, As concentrations increased from baseline concentrations in some wells, as presented in Table 5. These results are consistent with experience at this Site (see Graph 10) and other locations; that As increases are localized and temporary because the As III becomes re- oxidized to As V and again becomes insoluble within a relatively short time, and in a spatially restricted area. Injection of the CaSX reductant solution caused the ORP in nearby groundwater to decrease, often resulting in negative ORP readings. The decrease in ORP is temporary, as demonstrated at MW-208 (Graph 19), TW-14 (Graph 54), and TW-16 (Graph 56). A depressed ORP is not a requirement for Cr reduction to occur(see Graphs 17, 23, 24, 53, and 55). Experience at this and other sites shows that DO is generally less reliable as an indicator parameter than is ORP. Table 9 summarizes the analysis of Cr reduction associated with the full-scale in situ remediation program, including trends observed for the various relevant parameters (Cr, Fe, Mn, TOC, and ORP)that are most indicative of treatment conditions and the potential for further reduction. 6.3 ANALYSIS OF IN SITU REMEDIATION EFFECTIVENESS As presented in Tables 5, 7, and 9, significant reductions in Cr have been reported across the site in Shallow and Intermediate zones in direct response to injection of the reductant treatment solution. Table 10 summarizes the change in plume size over time. Note that the addition of monitoring points (new wells) over time provides greater accuracy for Cr concentration contouring; an apparent increase in plume size or concentration over time likely represents an improved understanding of site conditions, not an expansion of the plume. QD MWH 6-3