A Biogeochemical Study of Groundwater Arsenic Contamination in the Southern Willamette Basin, Oregon, USA
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The mobilization and transformation of arsenic within the critical zone is a major cause of human suffering worldwide. Microorganisms, as they grow and utilize organic matter, accelerate redox processes that can transform and mobilize arsenic within aquifers on a large scale. As such, naturally occurring groundwater arsenic is a particularly hazardous problem that is chronically poisoning over 100 million people annually. Historically, groundwater arsenic research has been focused on the two principal inorganic arsenic species: arsenate [As(V)] and arsenite [As(III)]. Recently, organic arsenic species have garnered more attention due to their mobility, toxicity, and contemporary recognition of the ephemeral yet significant role they have in the global arsenic cycle. Here, I discuss laboratory and in situ experiments focused on exploring how microorganisms transform, mobilize, and sequester arsenic within a biogeochemically complex aquifer system. In my laboratory experiments, I collected aquifer sediments from a naturally contaminated bedrock aquifer and incubated a series of laboratory microcosms. Our results show that simultaneously robust iron and sulfate reduction temporarily mitigated arsenic contamination but then directed arsenic to an unstable adsorbed phase were it was later mobilized. Second, I discuss two aquifer injection experiments designed to examine in situ microbial redox processes and the further explore the potential to stimulate arsenic sequestration through arsenic-sulfide precipitation. Our results show that in situ stimulation of microbial metabolisms accelerated the reduction of arsenic bearing iron (oxy)hydroxides as well as sulfate and arsenic reduction. Within 3 weeks of these contemporaneously occurring redox reactions, 90% of the dissolved inorganic arsenic was removed (~2000 ppb) and an effective long-term, anaerobically stable, sequestration of arsenic was observed by way of a significant increase of arsenic-sulfide precipitate. Finally, using both the laboratory and field experiments, I explore the potential of organic arsenic production rates under stimulated conditions. We report new methylation rates that are consequential to the potential efficacy of enhanced, biologically-driven arsenic remediation and the reconsidered significance of biomethylation pathways in aquifers. These results expand our current understanding of the metabolic reach aquifer microorganisms potentially have over the fate of arsenic.