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2013 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site,Stockton,California August 2013 alkaline CaSx solution can cause a rise in pH in influenced wells. This effect has been observed in multiple wells at the Site, but tends to decrease over time due to the buffering capacity of the native soil and groundwater(for example, see Graphs 47, 56, and 57). Groundwater sample temperatures were generally within the range of 15 to 23 degrees Celsius, and generally consistent with data collected since the baseline sampling event. Temperature readings can be variable due to air temperature and the time that elapses before the reading is collected. 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 (e.g., see Graphs 15, 17, 24, and 53). 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 2013, negative ORP readings were not observed at wells sampled. 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, MW-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. © MWH 4-11