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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 EC is a direct measure of the TDS concentration of a water sample; hence, the behavior is consistent with the observed changes in TDS. In general, EC measurements closely match the trends described for TDS in the preceding section. The most dramatic changes in EC measurements during quarterly sampling events appear to correlate with periods of treatment solution injection. ORP is a measure of the general oxidizing or reducing conditions in the saturated zone. Generation of a strongly reducing environment enhances the reduction of Cr+6 to the trivalent state. Negative ORP readings represent reducing conditions, which are generally conducive to removing Cr from groundwater. Of the field parameters measured, ORP is generally the most indicative of remediation potential. However, site data show that a negative ORP is not necessary for Cr+6 reduction to occur (see Graphs 17, 23, 24, 49, and 51). Baseline sampling indicated that the saturated zone was at a positive ORP, ranging from 31 to 480 millivolts (mV) in the groundwater of the Shallow and Intermediate zones. This is typical of groundwater that contains DO and that has been impacted by Cr+6, a strong oxidant. The reductant solution, by contrast, has a negative ORP, commonly in the range of-400 to -500 mV. During the first and second quarters of 2012, negative ORP readings were observed at wells MW-105 and TW-17, indicating that reductant solution had affected the surrounding groundwater. ORP readings at many wells near where remedial injections occurred have since rebounded to near baseline conditions while Cr concentrations remain below the WQO (e.g., MW-101, WM-105, MW-303, and TW-16), which indicates the treatment solution has had the desired effect. No additional action is recommended at this time regarding the ORP readings. DO measurements indicate the extent to which an environment is either aerobic or anaerobic, and hence is associated with ORP readings. Biological reduction of Cr+6 and SO4 occurs under anaerobic conditions; therefore, the extent of the treatment zone would be expected to have relatively low (less than 0.5 mg/L) DO concentrations. Phase I baseline DO concentrations at the Site ranged from 4.60 to 10.3 mg/L. It should be noted that field measurement of DO is subject to variability due to the inevitable introduction of oxygen to the groundwater while it is pumped to the surface, and historical sampling practices (using submersible pumps) may create a high bias in DO readings. Experience at this and other sites shows that DO is generally less (a mmi 4-11