Three days after a freight train derailed and exploded in Lac-Megantic, Que., there are still important, pressing questions about the environmental toll of the disaster.

MMA Railway has said little about the kind of oil the train was hauling. On Monday, the company confirmed it was shale crude oil extracted through fracking from North Dakota's Bakken shale development, and that the oil was being transported to Irving Oil's plant in Saint John, New Brunswick.

But Steven Guilbault, the co-founder and senior director of the not-for-profit group Equiterre says there are many kinds of shale oil, and a lot remains unknown.

Shale oil can contain a number of different toxic compounds including heavy metals and other natural by-products, he says, so the effects the spill will have on the environment can’t yet be assessed.

Shale oil can contain a number of and other substances that can disperse in the environment and won’t break down naturally.

“We don’t have the full story on what kind of oil is,” Guillbault told CTV’s Canada AM from Montreal.

“It could be light crude or it could be heavy. And depending on whether it’s one of the other, the cleanup methods, the environmental impact, the safety measures for workers there, these things are still unknown.”

One bit of good news is that it appears that air emissions from the explosion and the fire were contained to the town and have now dispersed.

“That shouldn’t be too much of a concern. So right now, the real issue is water,” he said.

Quebec estimates 100,000 litres of oil spilled into the nearby Chaudière River and Lac Megantic. If the oil was light crude, it would mostly float, so it would be easier to remove from waterways, although it could still contaminate the water for drinking.

If it was heavy oil, the bitumen would sink down to the riverbed, making the cleanup more difficult. Heavy oil also raises the chance of severe fish and vegetation damage.

Quebec’s environment minister has said the spill has been contained to a 100-kilometre radius in the Chaudiere. While it has been contained, there is still concern for the many villages and towns nearby that get their drinkable water from the river.

Guilbault says he’s “quite troubled” that the public hasn’t been given more information about the spilled oil, saying it’s just one of many unanswered questions he has.

He also wants to know why the 72-car train was allowed to run with only one engineer, and why the rail company was allowed to have so many trains and so many cars.

“There are so many things we don’t know and it’s already been three days since the accident,” he said.