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To make bitumen easier to transport by pipeline, it is diluted with liquid natural gas and industrial thinners, such as benzene, toluene and hexane. The result is diluted bitumen, also known as dilbit.ANDREW CULLEN/Reuters

The B.C. government announced regulations on Tuesday that would block any expansion of heavy-crude shipments off the West Coast while a scientific advisory panel examines the "scientific uncertainties" related to how diluted bitumen interacts with waterways and wildlife. The announcement is seen as an attempt to block Kinder Morgan Canada Ltd.'s proposed expansion of its Trans Mountain pipeline between Alberta's oil sands and the Vancouver region.

Here's what you need to know about diluted bitumen and what experts know – and don't know – about how it would behave in a spill.

What is diluted bitumen?

The sludgy bitumen from Alberta's oil sands is thick like molasses. To make it easier to transport by pipeline, the bitumen is thinned with natural gas condensates, which can include hydrocarbons such as benzene, toluene and hexane. The result is diluted bitumen, also known as dilbit.

What are the concerns?

Regulators, the industry and experts have studied and debated the impact of a diluted-bitumen spill, particularly whether a cleanup would require a different response than a conventional oil spill.

A Royal Society of Canada report published in 2015 highlighted a knowledge deficit about how dilbit oil behaves in different environments.

"The big question people ask is: If it spills and ends up in water, will it sink or will it float?" said Heather Dettman, a Natural Resources Canada scientist in Alberta.

The Royal Society concluded more research was needed.

What have researchers found since?

Dr. Dettman said she and her team have built substantially on the body of dilbit research since the Royal Society report was released three years ago. Their experiments – performed in an open tank filled with fresh North Saskatchewan River water – show that various blends of diluted bitumen won't sink until the sludge has been left alone for at least 21 days, she said.

Even then, she added, only one type of bitumen found its way to the tank floor, even in warm conditions.

She said the data seem to indicate diluted bitumen tends to form a hardy slick on the water's surface – a spill that can be somewhat contained – rather than dissolve into the water and end up coating riverbeds and marine life.

"The misinformation is that diluted bitumen will sink," Dr. Dettman said. "But it's not sinking."

As for how easy, or difficult, it would be to clean up, she said if it doesn't sink, that would make it easier than previously thought.

What are researchers still trying to find out?

Dr. Dettman says one of the major gaps in current understanding is the long-term effect of a bitumen spill – specifically, how long it would take before the spilled oil is no longer a threat to the environment. Her team is also studying how spilled bitumen would be affected by colder water.

Additionally, the toxicity of the solvents used in dilbit remains unclear according to Sachin Goel, a PhD candidate at the University of Toronto who studies the behaviour of dilbit in water. He noted up to 40 per cent of a mixture can be composed of hazardous chemicals such as benzene, toluene and hexane.

"It's toxic to the environment, it's dangerous to the environment," Mr. Goel said. Dilbit and water can also form an emulsion under certain conditions, he added. "You can have tiny drops of oil in the water," which spreads the spill and contaminates the water.

Dr. Dettman's team is working with toxicologists to figure out exactly how those chemicals cause harm.

How does dilbit compare with conventional oil?

Dr. Dettman argues light crude, a low-viscosity oil, may actually be more hazardous. When light crude hits water, it's "like adding cream to coffee. That's it. It's all mixed in, it gets stuck in the sediment."

Dr. Dettman pointed to the 2010 dilbit spill in Michigan's Kalamazoo River.

"It looks ugly and it's not good for the fish. But because it's there you can see it, you can pick it up, and then it's gone," she said. "We get a very high recovery rate."