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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 5.0 GROUNDWATER TREATMENT ACTIVITIES This section presents a summary of the supplemental groundwater treatment completed in 2012. As discussed in the RAIWP (MWH, 2007b), it was anticipated that supplemental treatment (re- dosing) would be necessary after the full-scale in situ injection program in 2008 to address certain locations within the source area or groundwater plume where Cr concentrations remained elevated. Following a review of groundwater data through January 2012, SPX finalized a work plan for supplemental injection to address Cr in the Intermediate Zone in the South Yard area (MWH, 2012). The work plan was updated iteratively prior to finalization to address comments from DTSC. 5.1 SUPPLEMENTAL INJECTION BORINGS The supplemental injection program consisted of 10 vertical boring and injection locations, and was implemented from July 23 through August 24, 2012. Boring locations are shown on Figure 3 and depth intervals and injection volumes for each borehole are presented in Table 8. Eight borings were used for injection into the Intermediate Zone (S-8 through S-17), and two (S-11 and S-15)were used to inject into both the Shallow and Intermediate zones. Soil borings were advanced by sonic drilling technology to the bottom of the shallowest injection interval and the drill stem was pulled back to the top of the injection interval, exposing the target injection zone. Treatment solution, consisting of 2 percent alcohol, 3 percent CaSx solution, and 95 percent potable water, was injected into the open interval until the target volume (Table 8) was achieved. This process was repeated for each subsequent deeper injection interval (i.e., borings and injection proceeded from shallow to deep intervals). The treatment solution was the same mixture that was injected into the deeper soil borings during the full-scale in situ remediation program in 2008. Drilling and injection activities were performed by Cascade Drilling under the supervision of an engineer or geologist from MWH. To produce the reductant solution, ethanol and CaSX were mixed and stored in a series of three 6,500-gallon poly tanks. The ethanol and CaSX solution were blended with hydrant water via dosing meters calibrated to deliver approximately 12.5% by volume. Flow meters located before and after the dosing meters were utilized to check and calibrate the dosing meters to maintain the (D MWH:6: 5-1