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2011 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California January 2012 <br /> contains DO and that has been impacted by Cr+6, a strong oxidant. The reductant solution, by <br /> contrast, has a negative ORP, commonly in the range of-400 to -500 mV. During the third and <br /> fourth quarters of 2011, wells MW-104 (S-2) and TW-17 indicated that the reductant solution <br /> had affected the surrounding groundwater by exhibiting negative readings. ORP readings at <br /> many wells near where remedial injections occurred have since rebounded to near baseline <br /> conditions while Cr concentrations remain below the WQO (for example, MW-101, WM-105, <br /> MW-303, and TW-16), which indicates the treatment solution has had the desired effect. No <br /> additional action is recommended at this time regarding the ORP readings. <br /> DO measurements indicate the extent to which an environment is either aerobic or anaerobic, <br /> and hence is associated with ORP readings. The biological reduction of Cr+6 and SO4 occurs <br /> under anaerobic conditions; therefore, the extent of the treatment zone would be expected to <br /> have relatively low (less than 0.5 mg/L) DO concentrations. Phase I baseline DO concentrations <br /> at the Site ranged from 4.60 to 10.3 mg/L. It should be noted that field measurement of DO is <br /> subject to variability due to the inevitable introduction of oxygen to the groundwater while it is <br /> pumped to the surface, and historical sampling practices (using submersible pumps) may create a <br /> high bias in DO readings. Experience at this and other sites shows that DO is generally less <br /> reliable as an indicator parameter than is ORP. However, DO data collected during performance <br /> monitoring are of value in the qualitative comparison of DO concentrations since the baseline <br /> sampling event in 2008. Generally the lower DO values are associated with lower ORP and <br /> indicative of reductive conditions (MW-104 and TW-17). No additional action is recommended <br /> at this time regarding the DO readings. <br /> 0) MWH,, <br /> 4-10 <br />