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2012 Annual Groundwater Monitoring and Evaluation Report Full-Scale In Situ Hexavalent Chromium Reduction Treatment Implementation Former Marley Cooling Tower Site, Stockton,California February 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 third- and fourth-quarter 2012 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 the WDR, the following wells are considered to be background or compliance monitoring points and were sampled during the third and fourth quarter 2012 monitoring and sampling events: MW-7, MW-201, MW-202, MW-210, MW-211, MW-302, MW-304, MW-310, MW-325, and MW-401. Analytical results for the fourth-quarter 2012 monitoring event are shown on Figure 4. Note that samples collected from the background and compliance wells during fourth quarter 2012 were only analyzed for total dissolved Cr per Section II Part A of the revised MRP. Historical groundwater analytical results and proposed upper background concentration limits for background wells are presented in Table 6. Proposed upper background concentration limits are based on historical data available on an individual basis by well. The wide range in concentrations of some historical data supports the proposed changes from the WDR/MRP criteria. (a mm" 3-2