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2012 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California October 2012 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-207 (Graph 18), 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 50), and TW-16 (Graph 52). A depressed ORP is not a requirement for Cr reduction to occur(see Graphs 17, 23, 24, 49, and 51). Experience at this and other sites shows that DO is generally less reliable as an indicator parameter than is ORP. Table 8 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 8, 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 9 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. The estimated lateral extent of the Cr plume in the Shallow Zone has decreased by an estimated 84 percent since initiation of the full-scale in situ remediation in January 2008. Cr concentrations remain above the WQO only in relatively small areas near the source (former (D MWK, 6-3