Dissolved versus particulate forms of trace elements in the Athabasca River, upstream and downstream of bitumen mines and upgraders

Abstract

Employing trace metal-free analytical protocols developed for polar snow and ice, water samples were collected upstream, midstream and downstream of open pit bitumen mines and upgraders along the Lower Athabasca River (AR, ~125 km). The purpose of this study was to determine whether dissolved (< 0.45 µm) and total (i.e. acid-extractable) trace element (TE) concentrations in the AR have been significantly impacted by industrial operations. Of the TEs known to be enriched in bitumen such as V, Ni, Mo and Re, only total Re was significantly more abundant downstream of industry. Total concentrations of Ag, Bi, Cd, Co, Cu, Sb, Tl and Zn were not significantly more abundant downstream of industry, compared to upstream. Moreover, these elements occur predominantly (63–96%) in the particulate fraction. In contrast to the cationic TEs, elements which occur predominantly as anions in aqueous solution (Mo, Re and Sb) are mainly in the dissolved fraction (89%, 56%, and 73%, respectively). Vanadium, Ni and Pb in the suspended solids revealed similar concentrations downstream and upstream of industry, with concentrations proportional to those of conservative, lithophile elements (Al, La, and Th), but well below their corresponding abundance in crustal rocks. Taken together, these findings suggest that physical weathering and erosion are the primary sources of metals in the AR, both upstream as well as downstream of industry, with silicates the dominant source of most of the TEs of concern.