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Microbial characterization and biogeochemical cycling of iron in a creosote contaminated aquifer along a groundwater flowpath Ross, KellyAnn


Natural attenuation of a contaminant plume in an aquifer is an attractive clean up option that eliminates the expense of engineering processes. A creosote-contaminated aquifer located beneath the Fraser River at Braid Street in New Westminster BC was characterized to deduce the presence of natural attenuation. A geochemical and microbial characterization of the contaminated aquifer was conducted to infer biogeochemical processes. Geochemical analysis of the pore water and the corresponding sediment-bacterial community characterization, via the small subunit 16S rRNA, were implemented to elucidate the microorganisms and biogeochemical processes involved in the degradation of creosote compounds in the plume. From these results it appears that the biogeochemical processes at the Braid Street site were dominated by Fe²⁺ oxidation, which increased along the contaminant flowpath that goes from anaerobic to aerobic in nature. Initially, it was observed that the geochemical and bacterial analysis support the idea of convective flow at the groundwater - river water interface, and spatial variability in hyporheic flowpaths. Secondly, the diversity of the bacterial community increases in the hyporheic zone relative to the anaerobic zone of the plume, where this is attributed to more activity that in tum, is a consequence of steeper gradients in redox solutes found in the hyporheic zone. Thirdly, net Fe²⁺ oxidation is occurring over Fe³⁺ reduction along the contaminant flowpath, where this process was largely attributed to the presence of β-proteobacteria, whose apparent richness increased along the contaminant flowpath in the hyporheic zone. Additionally, the Burkholderia-related phylotype dominant along the flowpath is likely responsible for anaerobic Fe²⁺ oxidation. The presence of Fe²⁺ oxidation at the Braid Street site is attributed to the physicochemical processes associated with a large tidally influenced river such as the Fraser River; where convective flow and spatially variable hyporheic flowpaths increase the extent of the hyporheic zone in the aquifer.

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