Arsenic (As) is a toxic metalloid that occurs naturally in the Earth’s crust. As is an oxyanion-forming element that exists in several inorganic and organic forms in the environment, which have different toxicities to both humans and plants. Rice paddies and wetlands are of particular interest for As contamination due to their reducing redox conditions that promote As release and mobility. Managing As in contaminated soils as well as its speciation is crucial for protecting human and environmental health. This research seeks to expand on established principles of As mobility, with a focus on the understudied and unique pore water chemical conditions of wetlands. Three research projects were designed to support the goal of exploring organic matter and redox controls on As speciation and bioavailability. The objectives of the first project were to quantify As(III) binding to dissolved organic matter (DOM), evaluate the role of organic sulfur in As(III)-DOM binding, and determine the impacts of As(III)-DOM binding to bioavailability to microorganisms. The results of this work revealed significant binding of As(III) to DOM at environmentally relevant As/dissolved organic carbon ratios, and that the organic sulfur content of DOM was highly correlated with levels of As(III)-DOM complexation. The second project evaluated a two-year field study on the effect of rice cultivars vs irrigation practices on the concentration and speciation of As in addition to the micronutrient content. Linear mixed-effects (LME) models indicated that cultivar selection and alternate wetting and drying (AWD) are important controls on rice grain chemical composition, AWD was found to be most effective at reducing the organic As pool, and interannual variability was significant for trace elements. The final research project investigated the effect of AWD on As demethylation in soil microcosm experiments and benchtop batch reactors. Pore water concentrations of both inorganic and organic As were decreased by AWD, and the addition of nitrate correlated with a decrease in dimethyl arsenic acid (DMA). The results of these three research projects offer new insights into the complex, under-explored biogeochemical reactions controlling As fate in dynamic wetland environments, informing best practices for agriculture and remediation.