Biogeochemical Controls on Arsenic Mobility within Hyporheic Zone Sediments

Arsenic contaminated drinking water is a global concern, specifically in the Bengal basin where millions rely on arsenic contaminated groundwater for drinking purposes. Currently, the primary process believed to be responsible for the high dissolved arsenic is the microbially-mediated reductive dissolution of arsenic-bearing iron-oxides. Recent studies suggest that the interactions between oxygen-rich surface water and iron-rich groundwater in the hyporheic zone precipitates abundant iron-oxide minerals which sequester arsenic. The objective of this dissertation is to investigate the comprehensive role of hyporheic zone processes on the cycling of arsenic in sediment along the Meghna River, Bangladesh, and the Hooghly and Beas Rivers in India. The inorganic and organic chemical properties of the riverbank sediments were evaluated and the resulting biogeochemical processes influencing arsenic mobility within the hyporheic zone were determined. The findings show three distinct hyporheic zone scenarios which impact the fate of arsenic through differing biogeochemical processes. Along the Meghna River, a shallow silt layer, rich in labile organic matter, promotes arsenic mobility in the riverbank by fueling the microbially-mediated reductive dissolution of arsenic-bearing iron-oxides. Along the Hooghly River, surficial sands and minimal organic matter permits the precipitation of arsenic attenuating iron-oxides, maintaining low arsenic concentrations in the riverbank. Along the high-energy Beas River, a low residence time and oxic conditions prevents the microbial reduction of oxides, allowing for efficient transportation of As-bearing minerals to the underlying deltas. Together, this research provides a comprehensive analysis on the solid-phase properties of hyporheic zone sediments influencing the fate and transport of arsenic.