Trace elements in the Athabasca Bituminous Sands: A geochemical explanation for the paucity of environmental contamination by chalcophile elements

The Athabasca Bituminous Sands (ABS) in northern Alberta, Canada, represent one of the largest reserves of hydrocarbons on the planet, yet there is remarkably little published data on the abundance of potentially toxic trace elements (TEs) in this resource. Here, we present the concentrations and review the relevance of 30 TEs in bulk samples of ABS as well as the organic and mineral fraction of representative samples. The distribution of TEs is dichotomous: they occur primarily in the organic fraction (Mo, Ni, Re, V and Se) or almost exclusively in the mineral fraction (virtually all of the other TEs). Except for Mo and Re, TEs in the ABS are depleted relative to the composition of the Upper Continental Crust (UCC), a reference level commonly used in quantifying the extent of contamination by TEs in the environment. Based on the published data available for comparison, TE concentrations in ABS are similar to those of sandstones, well below the average value for shale, and far below the values reported for organic-rich, black shales. The data presented here explains why recent studies of contamination of air, water, soil, plants in this region of northern Alberta, when viewed critically, reveal limited enrichments of chalcophile TEs, relative to crustal abundance. The abundance and distribution of TEs in the ABS also explains why atmospheric transport of TEs is largely restricted to the immediate region (< 50 km) surrounding the open pit bitumen mines, and dispersion patterns resemble those of dust deposition. Based on these findings, most of the chalcophile TEs mobilized by mining are expected to have limited bioaccessibility and bioavailability, simply because they are mainly hosted by silicate minerals with limited solubility at ambient pH.

Automated summary

The study found that in the Athabasca Bituminous Sands in northern Alberta, Canada, most potentially toxic trace elements are primarily distributed in the organic fraction, and their concentrations are relatively low compared to the Upper Continental Crust. In addition, the atmospheric transport of these elements is restricted to a specific area around mining activities.