Temporal and spatial trends in riverine suspended sediment and associated polycyclic aromatic compounds (PAC) within the Athabasca oil sands region

Bitumen-bearing suspended sediment (SS) eroded from the McMurray Formation (MF) are fine grained (silts and clays) and coated with natural hydrophobic oils. This results in poor settling and long range transport of associated contaminants. There was a longitudinal increase in polycyclic aromatic compound (PAC) concentrations for rivers that erode the MF from upstream to downstream regardless of time-of-year, while loads were substantially increased during high flow periods when the erosive forces are the greatest and the overland flow contribution is high. Within the MF, variation in PAC contributions is seen by the Ells River having higher loads than the Steepbank River. Using the Ells and Steepbank as examples, double plot PACs ratios suggest that the PAC concentrations and signatures vary spatially within the MF but that the weathering processes may be the same. Plots of the various homologs of PACs generally illustrated a normal distribution which suggests petrogenic origins. However, several PAC ratios suggested that both the parent material and the SS are pyrogenic in nature. While it is likely that some level of atmospheric deposition (anthropogenic or from forest fire) is incorporated into the SS of the rivers, it is likely to be limited relative to the proportion of naturally eroded MF sediments. Additional analysis will be needed to distinguish the relative risk of both anthropogenic (e.g., industrial operations) and natural sources (bitumen deposits, forest fire) of PACs to the SS and to long-range depositional environments, as they may have potential aquatic ecological effects.