The environmental impact of the explosion at Lac-Mégantic is uncertain as authorities haven't confirmed the type of oil that was spilled.
Quebec estimates that 100,000 litres of oil spilled into the Chaudière River and another 1.5 million litres of oil-and-water mixture have been extracted from the river.
To put the spill estimate in perspective, the rupture of the Enbridge Inc. pipeline in Michigan in 2010 sent 3.3 million litres of heavy crude oil from Alberta's oil sands spewing into the Kalamazoo River.
The impact at Lac-Mégantic is difficult to assess, experts say, in part because authorities have not confirmed what type of oil the 72-car train was carrying when it derailed and exploded early Saturday.
"We're not even sure of the real product inside," said Robert Reiss, a professor of risk management at Sherbrooke University who formerly worked with Environment Canada's emergency management unit. "If it's heavy crude, there will be an effect on the environment, especially the fishes and the vegetation along the river bank."
Lac-Mégantic's sloping topography may prove to have limited the damage on land. Oil that did not burn or evaporate likely would not have had enough time to saturate the ground before flowing into the river.
"We don't think the oil got into the soil," said Mélissa Généreux, a spokeswoman for the area's regional health department. "We think it stayed on the surface."
Lac-Mégantic remained under a boil-water advisory on Monday, as firefighting efforts and a broken downtown water main stressed the system.
Christian Blanchette, the co-ordinator for Environment Quebec, said the town's air quality would be acceptable by late Monday and that the government was planning to allow the 1,500 or so evacuees to re-enter their homes in phases.
Many homes will need a cleaning. The ministry warned that some houses were caked with toxic soot and dripping with yellow oil.
With a report from Kim Mackrael