Microbial Iron Geochemistry of a Pristine Glaciofluvial Groundwater System

The biogeochemical cycling of iron by microorganisms has a strong impact on groundwater resources due to their influence on aqueous geochemistry, including contaminant mobility. This thesis expands on knowledge of groundwater-surface transitions through the characterization of an iron cycling biofilm dominated groundwater spring, and presents the first in situ investigation of microbial iron cycling in a pristine, glaciofluvial aquifer that has not been used as a groundwater resource, anthropogenically contaminated, or stimulated to enhance microbial activity. Metagenomic evidence from the groundwater spring with a sharp reduction-oxidation gradient demonstrated the presence of iron biofilm forming microbes in the groundwater system being studied, as well as showing a diversity of nitrogen and sulfur cycling microorganisms. Changes in redox potential along the length of the stream corresponded with pronounced shifts in the relative abundance of bacteria known to mediate the biogeochemical cycling of iron, sulfur, and nitrogen. Five years of geochemical surveying showed a spatiotemporally persistent zone of elevated iron concentrations in an aquifer which had previously been shown to have potential for microbial iron cycling through genomic characterization. Based on the analysis of terminal electron acceptor concentrations in the groundwater, the aquifer was found to be graded into zones of O2 reduction, NO3- reduction, Fe3+ reduction, oxygen-dependent Fe2+ oxidation, and nitrate-dependent Fe2+ oxidation. Hydrogeological and geophysical evidence suggested that groundwater flow conditions differed in the vicinity of the elevated iron zone, which may have contributed to conditions conducive to the growth of a subsurface microbial community. Environmental electron scanning microscopy showed bacterial morphologies similar to known iron cycling bacteria were present in the elevated iron zone, but not outside of it. The findings of this thesis are broadly applicable to other aquifers of glacial and glaciofluvial origin, where reducing conditions will arise through non-aerobic forms of microbial respiration, and observed patterns of redox zonation will be affected by the mixing of groundwater along three dimensional flow paths. Taken together, this thesis underscores the importance of iterative interdisciplinary collaboration, including hydrogeology, geochemistry, near-surface geophysics, and microbiology, as a means of building the most complete possible representations of groundwater resources.Ph.D.2019-11-10 00:00:0