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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 Groundwater samples were collected from monitoring wells using a combination of portable and dedicated down-hole pumps. All groundwater samples were collected and field parameters measured by personnel from GeoAnalytical Laboratories, Inc. of Turlock, California. Samples were labeled and stored in ice-chilled coolers for transport to GeoAnalytical Laboratories, Inc., under chain-of-custody protocols for laboratory analyses. Field parameters were measured during sampling, including pH, temperature, electrical conductivity (EC), oxidation-reduction potential (ORP), and dissolved oxygen (DO), and are presented in Table 4. Water-level measurements were collected from each well before sampling and are also presented in Table 4 and Appendix A. Laboratory analytical results for the first- and second-quarter 2013 events are summarized in Table 5. Historical groundwater analytical results and proposed upper background concentration limits for background wells are presented in Table 6. Total dissolved Cr and Cr+6 results are summarized in Table 7, which also includes an evaluation of concentration reductions as a result of the remedial action. 3.1.2 Background and Compliance Wells Background and compliance well sampling results represent conditions outside the established Cr plume boundary and outside areas of estimated in situ treatment. Consistent with MRP No. R5-2012-0814, the following wells are considered to be background or compliance monitoring points and were sampled during the first quarter 2013 monitoring and sampling event: MW-7, MW-201, MW-202, MW-210, MW-211, MW-302, MW-304, MW-310, MW-325, and MW-401. During the second quarter 2013 monitoring and sampling event wells MW-201, MW-304, MW-310, and MW-325 were sampled. Wells MW-7, MW-202, MW-210, MW-211, MW-302, and MW-401 were not sampled based on the rationale provided in the notification letter provided to CVRWQCB and DTSC on June 4, 2013 (MWH, 2013b). Analytical results for the second-quarter 2013 monitoring event, or most recent sample collected, are shown on Figure 4. Note that samples collected from the background and compliance wells were only analyzed for total dissolved Cr per Section II Part A of MRP No. R5-2012-0814. Historical groundwater analytical results and proposed upper background concentration limits for (a MWH 3-2