Occurrence and source of geogenic arsenic in groundwater from the Cold Lake-Beaver River Basin, Alberta

Elevated arsenic concentrations have been observed in shallow groundwater in the Cold Lake-Beaver River Basin (CLBR) of Alberta. The geology of this area includes up to 200 m of unconsolidated glacial deposits, with six regional interglacial sand and gravel aquifers, underlain by marine shale. Arsenic concentrations in unconsolidated sediment samples ranged between 1 to 17 ppm. Mineralogical characterization of the sediment samples revealed the presence of fresh framboidal pyrite in the deeper unweathered sediments with variable As contents of up to 1800 ppm. In contrast, the weathered sediments did not contain framboidal pyrite, but exhibited spheroidal Fe oxyhydroxide grains with elevated As concentrations, interpreted as pseudomorphs after pyrite. X-ray absorption near edge spectroscopy (XANES) indicated that the weathered sediments are dominated by As(V) species having spectral features similar to those of goethite or ferrihydrite with adsorbed As, suggesting that Fe oxyhydroxides are the dominant As carriers. XANES spectra collected from the unweathered sediment samples indicated the presence of a reduced As species characteristic of arsenopyrite and arsenian pyrite. A survey of over 800 water wells, isolated from industrial activity, were sampled for As and found that 50% of the wells contained As concentrations exceeding drinking water guidelines of 10 μg L. Higher As concentrations in groundwater were associated with increasing depth and reducing conditions, circumneutral pH and lower concentrations of SO4. Speciation modelling showed that the majority of groundwater samples were undersaturated with respect to ferrihydrite, suggesting that reductive dissolution of Fe oxyhydroxides, likely formed during glaciation, may be the source of some As in deeper groundwater, whereas sulfide oxidation of pyrite during weathering is the likley source of As released to shallow aquifers. Understanding the distribution and form of As present naturally in the CLBR aquifer basins is important for managing water resources to minimize the potential health risks of As exposure.