Selenium in surface waters of the lower Athabasca River watershed: Chemical speciation and implications for aquatic life.

Selenium in the lower Athabasca River (Alberta, Canada) is of concern due to potential inputs from the weathering of shallow bitumen deposits and emissions from nearby surface mines and upgraders. Understanding the source of this Se, however, is complicated by contributions from naturally saline groundwater and organic matter-rich tributaries. As part of a two-year multi-disciplinary study to assess natural and anthropogenic inputs, Se and its chemical speciation were determined in water samples collected along a ∼125 km transect of the Athabasca River and associated tributaries. Selenium was also determined in the muscle of Trout-perch (Percopsis omiscomaycus), a non-migratory fish species, that were sampled from selected locations. Dissolved (<0.45 μm) Se in the Athabasca River was consistently low in 2014 (0.11 ± 0.02 μg L; n = 14) and 2015 (0.16 ± 0.02 μg L; n = 21), with no observable increase from upstream to downstream. Selenate was the predominant inorganic form (∼60 ng L) and selenite was below detection limits at most locations. The average concentration of Se in Trout-perch muscle was 2.2 ± 0.4 mg kg (n = 34), and no significant difference (p > 0.05) was observed between upstream and midstream (industrial) or downstream reaches. Tributary waters contained very low concentrations of Se (typically < 0.1 μg L), which was most likely present in the form of dissolved organic colloids.