At least 15 people were reported dead and dozens remain missing following the weekend derailment of a train carrying crude oil from North Dakota across the border to a Canadian refinery.
Operator Montreal, Maine & Atlantic said the release of the air brakes holding the locomotive in place may have led to the derailment. Canadian authorities have launched a sweeping investigation into the disaster.
Quebec Minister for the Environment Yves Francois Blanchet said some oil spilled from the accident was still visible in area rivers. He said during a Tuesday news conference water quality was the ministry's biggest concern because of the presence of oil sheen on the water.
He said steps were taken to prevent oil from seeping into the sewer system in Lac-Megantic, a town near the U.S. border. The city sewer system was closed as a precaution.
"Regarding the quality of the air, the latest analysis ... confirmed that the level of air contamination is now within the limits acceptable," he said in a statement.
Regarding drinking water, he said adequate precautions were taken to avoid the disruption of water supply to area municipalities. "Priority is given to drinking water and the work of containment and recovery of spilled oil to minimize the impact on people, wildlife and the environment," he said.