Water quality in dynamic redox environments: Coupled hydrologic-biogeochemical controls on metal contaminant mobility

Abstract

Rising population and changing climate threaten to increase the risks posed by anthropogenic and geogenic metal contaminants to our freshwater resources. Increased human demand for freshwater coupled with altered hydrologic cycles will shift (bio)geochemical conditions in soils and sediments, potentially driving transformations of metal contaminants from stationary (solid) phases to mobile forms subject to transport and human consumption. Understanding the fundamental controls on phase transformations of metal contaminants is thus essential for predicting the risk they impose. Often, redox processes control partitioning of metal contaminants in surface and subsurface environments, whether by changing the redox state of the metal itself, or by transforming the metal host(s). In the subsurface, redox processes are commonly driven by hydrologic conditions. My research seeks to understand how redox associated biogeochemical processes arising from and coupled to hydrologic conditions impact metal contaminant partitioning and mobilization. I employ a combination of experimental, spectroscopic, field and modeling approaches to study the partitioning of U, Pb and Cd to solid and dissolved phases in dynamic redox environments. In my first chapter, I find that the calcium-uranyl-carbonato species kinetically limit U(VI) reduction by Fe(II)(aq), thereby impeding transformation of U from a soluble form to an insoluble form. In my second chapter, I show that the affinity of Pb for a range of coordination environments limits dissolved Pb concentration in contaminated floodplain sediments, even during shifts in redox conditions and coupled dissolution of Pb hosts. In my third chapter, I reveal that soil redox environment influences the metal binding properties of DOM, leading to increased complexation of Cd by soft ligands in reduced environments. Finally, in my fourth chapter, I find that the impacts of beaver dams on hyporheic biogeochemical activity dwarfs that of seasonal hydrologic dynamics. Overall, my work both furthers our understanding of the biogeochemical cycles of U, Pb, and Cd and deepens our understanding of how changes in hydrology couple with biogeochemical redox processes to determine water quality. iv Acknowledgements There are many people who made this dissertation possible. I would specifically like to acknowledge my Ph.D. advisor, Scott Fendorf, for his thoughtful mentorship throughout my Ph.D. studies. I would also like to acknowledge Peter Nico and John Bargar, who have served as unofficial advisors over the years, and Kate Maher and Chris Francis, for serving on my committee. v

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