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2012 Semiannual Groundwater Monitoring and Evaluation Report <br /> Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation <br /> Former Marley Cooling Tower Site, Stockton,California October 2012 <br /> consumes the carbon and removes Cr+6 from solution. No additional action is recommended at <br /> this time regarding the observed TOC concentrations. <br /> During the first and second quarters of 2012, TDS concentrations in the injection area wells <br /> ranged from 81 mg/L (MW-101 in January 2012) to 2,540 mg/L (TW-17 in April 2012). <br /> Concentrations in all but ten wells exceeded the WQO of 450 mg/L. TDS concentrations have <br /> been relatively stable at most wells since the baseline sampling event. However, several wells <br /> show relatively high increases and decreases in TDS concentrations which appear to correlate <br /> with periods of treatment solution injection or impact (for example, reductions in Cr <br /> concentrations) (see Graphs 1 through 53). TDS concentrations are expected to decrease over <br /> time. No additional action is recommended at this time regarding the observed TDS <br /> concentrations. <br /> 4.3 FIELD PARAMETERS <br /> Field parameters were measured at each well before sample collection, and results are <br /> summarized in Table 4. The pH, temperature, EC, ORP, and DO measurements are discussed <br /> below. Field sampling logs are provided in Appendix B. Field parameter measurements for pH, <br /> EC, and ORP are also presented on Graphs 1 through 53. <br /> Field measurements during the first and second quarters of 2012 showed the pH in the wells was <br /> generally within the range of 6.5 to 8 standard units. Results from the current period are <br /> generally consistent with trends since the baseline sampling event in 2008. Injection of the <br /> alkaline CaSX solution can cause a rise in pH in influenced wells. This effect has been observed <br /> in multiple wells at the Site, but tends to decrease over time due to the buffering capacity of the <br /> native soil and groundwater(for example, see Graphs 43, 52, and 53). <br /> Groundwater sample temperatures were generally within the range of 17 to 23 degrees Celsius, <br /> and generally consistent with data collected since the baseline sampling event. Temperature <br /> readings can be variable due to air temperature and the time that elapses before the reading is <br /> collected. <br /> (a MWH,, <br /> 4-10 <br />