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CLEARWATER G R O U P Enyironmen til Serpces Three wells, two generally upgradient of the plume (MW-4 and MW-1), one in the center of the plume (MW-513), and one downgradient of the plume (MW-8) were sampled for concentrations of the following anaerobic parameters nitrate, ortho-phosphate, sulfate, and ferrous iron Analytical Results are summarized on Table 2 and the laboratory report and chain-of-custody form is included in Appendix C Sulfate concentrations are depleted and ferrous iron is elevated within the plume at well MW-5B compared to background well MW-4 (Figure 7) This suggests that denitrification, sulfate and iron reduction are occurring at the center of the contaminant plume, despite the fact that Eh values appear to be slightly high to facilitate the sulfate reduction (Table 2) Microbial Counts and Additional Bioattenuation Parameters Results of microbial enumeration indicate the existence of an indigenous heterotrophic community in the saturated zone beneath the site (Table 2) The total aerobic heterotrophs to hydrocarbon degraders ratios range from 3 1 (MW-5B) to 300 1 (MW-4) with larger populations of aerobic hydrocarbon degraders present in samples from the plume core wells and depleted populations at the plume perimeter This supports the argument that ongoing aerobic bioremediation is occurring at the plume core The total anaerobic heterotrophs to hydrocarbon degraders ratios range from 500 3 (MW-4) to 3 2 (MW-8) Electron receptor concentration analytical data and lowered DO concentrations indicate that anaerobic bioremediation processes are prevalent at the plume's center Concentrations of ammonia, which range from 0 9 to 14 mg/L (Table 2), are sufficient to indicate biodegradation conditions do indeed exist (i e iron reduction, sulfate reduction, and methanogenesis) ZB178C/4Q03 Monitoring Rpt 12 January 29,2004