ARCHIVED REPORTS_THIRD QUARTER 2014 GROUNDWATER MONITORING REPORT

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URS Mr. Durin Linderholm Page 7 of 10 Regional Water Quality Control Board—Central Valley Region ' October 30, 2014 Dissolved Metals Figures C-10 and C-1 ldepict dissolved arsenic and manganese concentration trends, respectively, for all site wells. Figure C-10 indicates that due to the persistence of reducing conditions in the treatment area, there was a significant increase in dissolved arsenic concentrations, especially in MW-24-BP, where arsenic was detected at the maximum concentration of 31 gg/L in 1Q14. However, by 2Q14, dissolved arsenic had decreased in MW-24-BP to 19 pg/L, but has since rebounded to 33 Vg/L. This rebound is probably caused by the unexpected strengthening of reducing conditions in 3Q14. Dissolved arsenic concentrations also rebounded in MW-6-BP (from 12 gg/L to 27 gg/L)and in MW-2-BP (from 11 gg/L to 17 pg/L), due to the persistent reducing conditions in these wells. Dissolved arsenic also increased ' slightly in MW-25-BP (to 5 Ag/L). Contrary to arsenic, there was no observed rebound in dissolved manganese concentrations (Figure C-11). Only upgradient well MW-7-BP and transition well MW-25- BP showed an increase in manganese concentrations. Fluctuations in metals concentrations are to be ' expected while reducing conditions linger, especially if pH and ORP levels continue to support metals mobilization in the treatment area. Metal concentrations will continue to be monitored to ensure levels in all wells will return to baseline conditions. ' Figure 2 shows the groundwater analytical summary. Appendix B includes the laboratory analytical data report and chain-of-custody documentation. Groundwater analytical data were submitted electronically to the State Water Resources Control Board GeoTracker System, per Article 12, Chapter 17, Division 3, Title 23, of the California Code of Regulations. Appendix D includes GeoTracker upload confirmation sheets. SUMMARY EOS injections have been effective in reducing CVOC concentrations in the target remediation area. ' CVOC concentrations in most site wells are at historically low levels. Reductions in CVOC concentrations likely have resulted from the dual effect of reductive dechlorination and a reduced mass flux from upgradient areas. However, residual contamination at concentrations equal to or greater than ' MCLs persists in some site wells, specifically, PCE in MW-5-BP; cis-1,2-DCE in MW-6-BP, MW-24-BP, MW-2-BP, and MW-5-BP; and VC in MW-6-BP (Figure 4). Additional injection of emulsified oil (or another electron donor substrate) in the areas with residual CVOC concentrations is being evaluated, but with concentrations decreasing to levels just above MCLs, a more focused injection area will be targeted following additional CVOC delineation. Based on site soil properties, further characterization of areas downgradient of MW-24-BP is recommended. If such characterization indicates areas with elevated CVOC concentrations, then additional injections should be considered. A rebound in arsenic concentrations was observed in the performance well and in other site wells, likely as a result of unexpectedly resilient reducing conditions. A reversal to baseline levels is expected once organic carbon and biological activity reverses to baseline levels. RECOMMENDATIONS 1 Based on 3Q14 groundwater monitoring results and historical data, URS recommends the following: \\ACDATA01\Ducipliu6\Wp.ccs CDCRDadVocations bsumtiov(Bum PiQOVI-BP-3QA 14 Repoli doc