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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 /> were labeled and stored in ice-chilled coolers for transport to GeoAnalytical Laboratories, Inc., <br /> under chain-of-custody protocols for laboratory analyses. <br /> Field parameters were measured during sampling, including pH, temperature, electrical <br /> conductivity(EC), oxidation-reduction potential (ORP), and dissolved oxygen (DO). Results are <br /> presented in Table 4. Water-level measurements were collected from each well before sampling <br /> and are also presented in Table 4 and Appendix A. <br /> Laboratory analytical results for the first- and second-quarter 2012 events are summarized in <br /> Table 5. Historical groundwater analytical results and proposed upper background concentration <br /> limits for background wells are presented in Table 6. Total dissolved Cr and Cr+6 results are <br /> summarized in Table 7, which also includes an evaluation of concentration reductions as a result <br /> of the remedial action. <br /> 3.1.2 Background and Compliance Wells <br /> Background and compliance sampling results represent conditions outside the established Cr <br /> plume boundary, and areas of estimated in situ treatment. <br /> Consistent with the WDR, the following wells are considered to be background or compliance <br /> monitoring points and were sampled during the first two 2012 quarterly monitoring and sampling <br /> events: MW-7, MW-201, MW-202, MW-210, MW-211, MW-302, MW-304, MW-310, <br /> MW-325, and MW-401. Analytical results for the second-quarter 2012 monitoring event are <br /> shown on Figure 4. Additionally, historical groundwater analytical results and proposed upper <br /> background concentration limits for background wells are presented in Table 6. Figure 4 also <br /> presents the proposed upper background concentration limits for background and compliance <br /> wells. The proposed changes to upper background concentration limits are based on historical <br /> data available on an individual basis by well. The wide range in concentrations of some <br /> historical data supports the proposed changes from the WDR/MRP criteria. <br /> (D MWH, <br /> 3-2 <br />