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CUA"kWA"-T-ER C R i) U I' • ERU"Onmcnlal StrV![rS Anaerobic Electron Receptor Anal ical Results Analytical results from selected wells for anaerobic electron receptors suggest that denitrification, iron, and possibly sulfate reduction are occurring within the contaminant plume Three wells, Two generally upgradient of the plume (MW-4 and MW-1) and one in the center of the plume (MW-5B) were sampled for concentrations of the following anaerobic parameters nitrate, ortho-phosphate, sulfate, and ferrous iron The trends of these analyte concentrations through the plume are shown on Figure 7 Analytical Results are summarized on Table 2 and the laboratory report and chain-of-custody form is included in Appendix C Nitrate and sulfate 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 that 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 hyrocarbon degraders ratios range from 1 1 (MW-4) to 53 1 (MW-5B) with larger populations of • aerobic hydrocarbon degraders present in samples from the plume perimeter wells and depleted populations at the plume core This supports the argument that ongoing aerobic bioremediation is occurring at the plume fringe Populations of total anaerobic heterotrophs versus hydrocarbon degrading heterotrophs are present at a 5 4 ratio in MW-5B This combined with the electron receptor concentration analytical data and depressed DO concentrations indicate that anaerobic bioremediation processes are prevalent at the plume's center Concentrations of total alkalinity, which range from 540 to 670 mg/L (Table 2), are sufficient to buffer organic acids produced during biodegradation of contaminants This conclusion is supported by the neutral pH readings observed using monitoring well purging activities, which ranged from 6 5 to 7 4 Discussion Most NINA parameters and indicators appear favorable for bioremediation However, DO concentrations measured in the field and those measured in the laboratory are somewhat in disagreement While laboratory values are generally different from those measured in the well due to sample disruption resulting in aeration to some extent, the values obtained in the laboratory usually follow the general trend observed in the field, which is the case here The figures prepared for this report reflect field measurement trends It should be noted that field measurements of DO, Eh, pH and conductivity were collected after the wells were purged and sampled so values obtained would reflect aquifer conditions surrounding the sample point While heterotrophic populations indicate that favorable conditions exist for aerobic hydrocarbon ZB178C/1Q03 Monitoring Rpt 9 April 23,2003