SITE INFORMATION AND CORRESPONDENCE

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URS ' Ms.Kathy Amam Page 6 of 9 Regional Water Quality Control Board—Central Valley Region April 30,2014 ' TCE concentrations, in MW-6-BP are primarily due to destructive microbial processes. With regards to DCE concentration reductions, however, partial migration to other site wells, specifically MW-24-BP, cannot be ruled out. Figure C-8 indicates that the total CVOC molar concentration in MW-24-BP started ' increasing roughly at the same time when concentrations in MW-6-BP decreased to trace levels. Additionally, the molar composition in MW-24-BP consists entirely of DCE, indicating a large portion of the DCE may have originated from MW-6-BP. ' Dissolved Metals Figures C-10 and C-11, respectively, depict dissolved arsenic and manganese concentration trends 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 31 µg/l, in 1Q14 (however, this concentration represents little change ' since the previous quarter). ORP decreased only slightly in this well in 1Q14, indicating arsenic concentrations may stabilize or reverse going forward.Arsenic concentrations also remained unchanged in MW-6-BP (19 µg/L) but increased slightly in MW-2-BP (from 14 µg/L to 17 pg/L), even though reducing conditions are still evident in these wells.Manganese concentrations(Figure C-11)also appear to be stabilizing and reversing at 4,300 pg/L in MW-6-BP,MW-24-BP, and MW-2-BP. Fluctuations in metals concentrations may be expected while reducing conditions persist, especially if pH and ORP ' levels continue to decrease in the treatment area. Metal concentrations will continue to be monitored to ensure levels in all wells will return to baseline conditions. Appendix B provides 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 Regula- tions.Appendix D includes GeoTracker upload confirmation sheets. SUMMARY Injection of EOS has 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-2-BP and MW-5-BP; TCE in MW-5-BP; cis-1,2-DCE in MW-6-BP, MW-24-BP, and MW-5-BP; and VC in MW-6-13P. Although additional injection of emulsified oil (or another electron donor substrate) in the areas with residual CVOC ' concentrations is being evaluated, there are indications that the injected oil and its decay products are continuing to promote reducing conditions in several site wells. Therefore, areas with elevated concentrations and no indication of reductive dechlorination should be the primary targets if additional ' injections are considered. Moreover, because metals mobilization (e.g., arsenic and manganese) was observed in the performance well and some other site wells, a bench-scale laboratory test should be performed prior to any additional electron donor injections to evaluate application methods that could ' reduce or eliminate metals mobilization. Such methods may include the addition of a pH-adjusting amendment, such as magnesium hydroxide, or use of other amendments that could have higher buffering characteristics. However, it should be noted that elevated arsenic and manganese H:\Wpro \CDCR\O el Vocetiowl tuutiuu(B Pn)\DVI-BP-IQMR14 Repoadoc