Early Wednesday morning, 28,000 liters of diesel from a generator at a Longueuil waste water treatment plant started leaking into the St. Lawrence River.
The provincial environment ministry was notified just before 9:30 a.m. ET.
Urgence-Environnement officials arrived on the scene about six hours after the after the initial leak.
The city issued a statement saying the water was safe to drink Wednesday evening. On Thursday morning, the city issued another statement telling residents in some areas of Longueuil and surrounding communities not to drink the tap water.
The city is setting up four sites to distribute water to residents. The locations will be announced soon.
Who is affected?
Residents in the areas of Saint-Hubert, Vieux-Longueuil, Boucherville and Saint-Bruno-de-Montarville should not drink water from the municipal supply until further notice.
The Marie-Victorin and Riverside school boards have also issued preventative bans on drinking municipal water in their institutions.
Drinking water in Brossard and St-Lambert, as well as Lemoyne and Greenfield Park, is not affected, according to the City of Longueuil.
What will happen if you drink the water?
The smell and taste of the water is likely to cause the most immediate physical reaction, according to water experts.
However, the average healthy adult, or even an elderly person, should not suffer ill effects, even if the water smells or tastes like diesel.
McGill University professor Ronald Gehr says the average person would have to drink a significant amount of straight diesel to have serious health consequences.
Most people would likely throw up before they got to that point, he said.
On mobile? Click here for live updates.