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2011 Semiannual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California January 2012 4.3 FIELD PARAMETERS Field parameters were measured at each well before sample collection, and results are summarized in Table 4. The pH, temperature, EC, ORP, and DO measurements are discussed below. Field sampling logs are provided in Appendix B. Field parameter measurements for pH, EC, and ORP are also presented on Graphs 1 through 53. Field measurements during the third and fourth quarters of 2011 showed the pH in the wells was generally within the range of 6.5 to 8 standard units. Results from the current period were generally consistent with trends since the baseline sampling event in 2008. Injection of the 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 43, 52, and 53). Groundwater sample temperatures were generally within the range of 19 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 (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 (a mWH 4-9