Naturally deposited hydrocarbons dominate in Athabasca sediment
By Tyler Irving
Posted November 2012
The load of organic contaminants introduced into the ecosystem via natural erosion of exposed bitumen from the banks of Alberta’s Athabasca River far exceeds that from oil sands development, a new study of sediments suggests.
There have been concerns about polycyclic aromatic compounds (PAC) - some of which are carcinogenic - in the Athabasca River system for some time. A 2009 paper in Proceedings of the National Academy of Sciences (PNAS) by University of Alberta ecologist David Schindler and others examined dissolved PAC concentrations in the waters of the Athabasca near oil sands development. These concentrations showed a slight increase, from around 0.025 μg/L upstream to as high as 0.135 μg/L near industrial operations. But as Roland Hall, University of Waterloo ecologist and author of the recent study, points out, this increase could include contribution from natural processes. “The industry is located in exactly the same place where the natural deposits come out into the riverbank,” says Hall. “You can’t really conclude that the difference is only due to industry.”
Since PACs are hydrophobic, most of them would partition into sediments rather than the water. For this reason, Hall and his colleagues decided to study sediments collected from lakes in the Peace-Athabasca river delta - located 200 kilometres downstream from the oil sands operations - to assess the baseline load of PAC delivered there over the past few centuries. Although such studies had been called for by government-appointed expert panels, NSERC declined funding on the grounds that the research was in industry’s interest and should be funded privately. In the end, Suncor Energy Ltd. stepped forward to provide money for the study. “We knew that with industry funding, the optics would not be in our favour,” says Hall. “But we’ve been up front about this and conducted the research in exactly the same way that we would have if we had been funded by NSERC.”
In a paper recently published in the open-access journal PLoS ONE, Hall and his team reported concentrations of PAC in lake sediments ranging from about 0.1 to 3 milligrams of PAC per kilogram of sediment. Lakes that did not receive floodwaters had levels of PAC three times lower than those that did, indicating that air deposition is small compared to that from waterborne sediment. Significantly, all peaks in PAC concentrations corresponded to known flood events, both before and after 1967. “We could not detect a change since the onset of oil sands development [in 1967] compared to the natural levels that came in previously,” says Hall.
The localized increases in dissolved PAC near oil sands development may well be compatible with undetectable changes in sediments further downstream. “The distance and amount of dilution involved is such that I don’t see a conflict [between our findings and Hall’s],” says Peter Hodson, an ecologist at Queen’s University who was a co-author on the 2009 PNAS paper. However, both Hodson and Hall point out that the overall flow of the Athabasca is decreasing as the glaciers that feed it continue to shrink. This in turn will affect its ability to dilute PAC, regardless of the source.
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