DISTRIBUTION AND SOURCE AND OF NATURALLY OCCURRING ARSENIC IN GROUNDWATER FROM THE SOUTHERN ATHABASCA OIL SANDS REGION

Elevated arsenic (As) concentrations (e.g. 360 μg/L) have been measured in shallow groundwater in the Southern Athabasca Oil Sand Region. The geology of this area of Alberta includes a thick cover (up to 200 m) of unconsolidated glacial deposits, with a number of regional interglacial sand and gravel aquifers, underlain by marine shales. As concentrations measured from 201 unconsolidated sediment samples ranged between 1 to 14 ppm. A survey of over 800 water wells sampled in the area found that more than 40% of the wells contained As concentrations exceeding drinking water guidelines of 10 μg/L. Higher As concentrations were associated with reducing conditions. Speciation modeling using the WATEQ4F database showed that the majority of groundwater samples were undersaturated with respect to ferrihydrite, suggesting that reductive dissolution of Fe-oxyhydroxides is the cause of As releases. Mineralogical characterization studies revealed the presence of fresh framboidal pyrite in the deeper unweathered sediments. Electron microprobe analysis employing wavelength dispersive spectrometers indicated that the framboidal pyrite has variable As content with an average As concentration of 530 ppm. In contrast, the weathered sediments did not contain framboidal pyrite, but exhibited spheroidal Fe-oxyhydroxide grains with trace As. The habit and composition suggest that these Fe oxyhydroxide grains in the weathered sediment represent former framboidal pyrite grains. X-ray absorption near edge spectroscopy (XANES) indicated that the weathered sediments are dominated by As⁵⁺ species having spectral features similar to those of goethite or ferrihydrite with adsorbed As, suggesting that Fe-oxyhydroxides are the dominant As carriers. Unweathered sediments, on the other hand, had a shoulder on the lower energy side of the main absorption peak suggesting the presence of As^1- species in addition to As⁵⁺. The derivative value of this shoulder was close to those of arsenopyrite and arsenian pyrite. Least-squares fitting of the XANES spectra with arsenopyrite and goethite model compounds indicate the proportion of As^1- species make up 17 to 26 % of the total arsenic in the unoxidized sediments. These findings indicate that the reductive dissolution of Fe-oxyhydroxides is likely the source of As in the groundwater.