| Natural arsenic contamination of drinking water occurs in communities worldwide, presenting a significant threat to public health. The processes that dictate arsenic partitioning between minerals and the aqueous phase are highly variable and intertwined, and require systematic studies to reveal their effects on concentrations of arsenic, and thus on its transport and bioavailability, in increasingly complex systems. In this thesis, we examine chemical and biological factors that impact arsenic partitioning in carbonate-buffered groundwater using model systems.;Various groundwater ions can affect arsenic adsorption to iron oxides, including phosphate, carbonate, and calcium. We observe elevated dissolved arsenate concentrations in systems containing calcium, carbonate, ferrihydrite, and arsenate relative to binary or ternary systems of the same ions. This increase in solubility is not predicted by known thermodynamic relationships, and is particularly dramatic in flowing systems. Given that groundwaters containing these ions are common in many natural systems, this effect likely contributes to arsenic release worldwide, particularly in carbon-rich, biologically active waters.;Sulfate- and iron-reducing bacteria affect arsenic behavior in anoxic systems via dramatic changes in iron and sulfur mineralogy. We present the results of thoroughly characterized incubations of these bacteria with arsenic-bearing ferrihydrite, focusing on the effect of sulfate reduction alone and in combination with iron reduction. Sulfate reduction alone does not release arsenic, despite sulfide-induced dissolution of arsenic-bearing ferrihydrite. Rather, readsorption to iron oxides maintains low dissolved arsenic concentrations in this system. Greater geochemical complexity arises in incubations containing both iron- and sulfate-reducing bacteria, leading to increased variability in arsenic behavior. These cocultures differ dramatically from the sulfate-reducing systems, and are highly dependent on sulfate availability. Contrary to expected relationships, sulfate-limited systems, where precipitation of sulfide minerals is limited, sequester arsenic more effectively than systems with abundant sulfate, by substitution of arsenic into magnetite.;These experiments reveal key aspects of the interrelationships between bulk groundwater geochemistry, bacterial community structure, and arsenic mobility in aquifer systems. The synthesis of our data with detailed geochemical, mineralogical, and biological characterizations of contaminated aquifers should enrich our interpretations of how arsenic becomes dissolved in these systems. |
