Characterizing sediment sources and natural hydrocarbon inputs in the lower Athabasca River, Canada

Abstract

The Athabasca River drains an area of 160 000 km² in northern Alberta, Canada, with much of the lower basin underlain by oil-sand deposits. The oil sands occur primarily in the McMurray Formation of the Cretaceous Period, with outcrops evident along the banks of the Athabasca River, as well as the lower portions of several tributaries. Since the oil sands represent a natural diffuse source of hydrocarbons to the aquatic environment, understanding the nature and extent of sediment-bound hydrocarbon contaminants in the context of the sediment regime of the Athabasca River is important. Described are fluvial geomorphic characteristics of the lower Athabasca River, which provide a basis for assessing sediment-bound hydrocarbon contaminants. Suspended sediment derived from main stem and tributary sources between Fort McMurray and Embarras account for 1.2 Mt, or 18%, of the mean annual load of the Athabasca River. Of this load, approximately 53% of the sediment input originated from tributaries, the remainder from main-stem sources. The majority of sediment contributed along the main stem occurs in the vicinity of Embarras, well downstream of oil-sand sources. Natural oil-sand sediment contributions are likely much less than 3% of the annual load downstream of Fort McMurray. Key words: sediment, fluvial geomorphology, oil sands, hydrocarbons, polycyclic aromatic hydrocarbon (PAH), contaminants, environmental monitoring, Athabasca River.