Diluted bitumen, the molasses-like product that would be transported by the proposed Northern Gateway pipeline, sinks in salt water when battered by waves and mixed with sediments, according to a new study by the federal government.
However, when free of sediments, the crude floats even after evaporation and exposure to light, the study determined.
The report, conducted by Environment Canada, the Department of Fisheries and Oceans and Natural Resources Canada, also said that the commercial dispersant Corexit 9500 used in cleaning up conventional spills had a limited effect on dispersing diluted bitumen.
Whether the oil sinks or floats in the event of a spill has been a source of debate, pitting environmentalists against supporters of projects such as the Northern Gateway pipeline. Part of the argument has been that if the oil sinks to the ocean floor it's harder to recover.
The study examined two blends of crude, the Access Western Blend and Cold Lake Blend, which represent the highest volume of diluted bitumen, or dilbit, products transported by pipeline in Canada between 2012 and 2013.
"This work demonstrates that, in waters where fine- to moderate-sized sediment is present, these oils are at risk to sink, when there is a high degree of mixing energy available," the report said.
The experiment compared the two dilbit blends to an oil used for marine ship fuel.
The oils were evaporated at what the paper said was a typical marine temperature range. They were also exposed to 120 hours of light under a UV lamp. The dilbit samples did not sink in either cases.
And when mixed in a tube with salt water, the dilbit samples remained afloat. However, when mixed with three types of sediments of various sizes, the results differed.
"In general, mixtures with kaolin [a fine particulate] formed finely divided oil-sediment particles that sank in the salt water, with one exception for the highly weathered fractions for both AWB and CLB," the report said.
"Mixtures with the larger particles of diatomaceous earth likewise dispersed and sank, though a portion of the oil remained unmixed and floating on the surface. Mixtures with the coarser sand, however, formed a single, continuous phase of floating oil-salt water entrained mixtures, and only a small portion sank as an oily sediment mixture."
However, the study acknowledged that the tests were conducted in a synthesized environment, and it's possible that oil products could behave differently in the natural environment.
The dilbit samples were also placed in a wave tank facility in Dartmouth, N.S., to see how they would react to a chemical dispersant that has proven effective with conventional crude spills. In non-breaking waves, the oil remained on the surface.
But under breaking waves, the chemical caused only partial dispersion, the study said.
There is no record of chemical dispersant used on an actual dilbit spill, and since the composition of different dilbit varies, finding an effective dispersant may be challenging, the report concluded.
In 2010, roughly 843,000 gallons of diluted bitumen spilled from an Enbridge pipeline into the Kalamazoo River in Michigan.
The oil was carried downstream during a flood. Even though much of it was sopped up, about 10 to 20 per cent mixed with sediment and sank to the bottom of the river, the report said.
Conducting research on how diluted bitumen would behave in a marine environment was one of the 209 conditions announced by a review panel that approved the proposed Northern Gateway pipeline in December.
The pipeline – if given final approval by the federal government – would carry diluted bitumen from Alberta's oil sands to tankers on the British Columbia coast.