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2012 Annual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California February 2013 <br /> DO measurements indicate the extent to which an environment is either aerobic or anaerobic, <br /> and hence is associated with ORP readings. Biological reduction of Cr+6 and SO4 occurs under <br /> anaerobic conditions; therefore, the extent of the treatment zone would be expected to have <br /> relatively low (i.e., 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 /> (a MWH <br /> 4-12 <br />