Officials will be testing the water to make sure there is no dangerous bacteria in the sediment.
City officials have issued a boil water advisory covering all areas east of, and including, LaSalle.
So far, the West Island, Lachine (except for Ville St-Pierre) and parts of Ahuntsic-Cartierville north of the Metropolitan Expressway are the only areas exempt from the boil water advisory.
The head of Montreal's water service, Chantal Morissette, said the advisory is a precautionary measure.
She said the decision comes after test results at the Atwater filtration plant showed water was not up to standard.
Sediment found in water reservoir
According to authorities, workers had lowered the water in the Atwater reservoir for renovations. When the level was raised again, sediment was discovered in the water.
There have been reports of discoloured, brownish water coming from the taps in several boroughs across the city.
At a press conference held shortly before noon on Wednesday, authorities said the advisory will be in effect for 24 hours. Following that, the water will be tested again and the public will be updated on the situation.
Rolling boil for at least 1 minute
Officials are asking the public to bring water to a rolling boil for at least one minute before drinking it.
"We're asking citizens to boil their water … for everything that concerns drinking water, for preparing baby bottles, for brushing teeth, for washing vegetables, and for making ice cubes," said Morissette.
Tap water that has not been boiled is safe to use for washing clothing, bathing and for washing dishes as long as they are well dried afterward.
Advisory a preventive measure
The director of Montreal’s public health department is reminding residents the boil water advisory is a preventive measure.
Dr. Terry-Nan Tannenbaum said the results of the tests on the water in the Atwater reservoir have not come back, so it’s still unknown whether there are bacteria in the water supply.
She advised Montrealers affected by the boil water advisory to simply boil water for one minute before consuming.
She said using a water filter like a Brita would not help if the water was contaminated with bacteria.
If the water is contaminated, she said, people who drink it could experience gastrointestinal problems including nausea, vomiting and diarrhea.
“We have to wait for the results to know really how big a risk this is for the population,” said Tannenbaum.
For more information, residents can contact the City of Montreal by dialing 311.