Quantifying the Controls of Manganese Oxides on Geogenic Arsenic Release to Groundwater.

Naturally occurring arsenic (As) is a global contaminant often found in drinking wells. Numerous studies have documented major aquifers with well-water concentrations exceeding the WHO standard of 10 μg L^-1, and chronic consumption above this concentration can lead to adverse health impacts in humans. In areas where As is derived from natural aquifer materials, groundwater could potentially become geogenically contaminated. However, manganese (Mn) oxides can serve as a redox buffer that limit release of geogenic As to groundwater, thus serving as possible predictors for potential future As contamination. The primary objective of this study was to quantify how the abundance and reactivity of Mn oxides influences the release of geogenic As from aquifer solids. Natural sediments were utilized in a laboratory batch experiment from Phnom Penh, Cambodia where groundwater is threatened with geogenic As contamination and concentrations of solid-phase Mn and iron (Fe) vary. Rates of Mn reduction, Fe reduction, and mobilization of geogenic As following the introduction of labile dissolved organic carbon were measured as concentration loss from sediment over time. Total concentrations were measured on days 2, 4, 7, 14, 21, 28 and then every month thereafter. Iron, Mn, and As mineralogical distributions and oxidation and chemical states before and after reduction were determined by μ-X-ray fluorescence (μ-XRF) elemental mapping and μ-X-ray absorption near edge structure (μ-XANES) spectroscopy. Preliminary results indicate that Mn release to solution occurs before Fe and As release, and Fe release occurs sooner in sediments with lower initial Mn concentrations. Additionally, there is a correlation between sediment Mn and As concentrations (with Fe variability), and As exists mostly as As(V) in all samples. This work will determine the extent to which Mn oxides may buffer Fe reduction and concomitant As release and therefore serve as predictors for geogenic As contamination potential of an aquifer.