La Fondation Rivières -logo blanc
Close this search box.

E. coli in water: how do you know if swimming in rivers is safe?

Des gens sautent à l'eau au Vieux-Port de Montréal

As heatwaves become more and more frequent, the desire to swim in one of Quebec's many lakes or rivers grows. But is the water quality suitable for safe swimming? How to know? 

Contrary to what many might think, the temperature, color and turbidity of fresh water are not necessarily indicators of its quality. Besides the cyanobacteria (blue-green algae), you have to trust the fecal coliform count, more preciselyEscherichia coli, frequently called E. coli. And to know their quantity, you need the right tools… when they are available!

E.coli, it's what?

E.coli is a bacteria of fecal origin found in the digestive tracts of mammals, including humans and birds. L'E.coli represents 90% from fecal coliforms, also called thermotolerant coliforms.

On the other hand, certain types of thermotolerant coliforms are pathogenic. Although the majority of strains ofE.coli are not pathogenic, the bacteria remains an excellent indicator of fecal contamination in waterways. A high concentration then represents a high risk of the presence of pathogenic bacteria.

Why are there fecal coliforms in the water?

The wastewater treatment systems are one important source of contamination of watercourses. When the flows circulating in the sewer pipes or in a sewage treatment plant are too large, it becomes impossible to treat wastewater effectively. Result: untreated overflows in overflow structures, or diversions of partially treated wastewater into the plant. These events are more frequent during snowmelt and heavy rains. In theory, they are prohibited in dry weather. 

Rivière des Mille Îles
Credit: André Chevrier

That said, some stations do not have the necessary infrastructure allowing disinfection. That is to say that the tributaries do not undergo tertiary treatment, only physicochemical and biological treatments. Therefore, discharges contain fecal coliforms of human origin. Other sources of contamination? Illegal dumping, runoff from agricultural environments where manure spreading and livestock farming are practiced, and contamination of avian origin. And yes: certain beaches heavily frequented by seagulls, ducks and geese show contamination linked to their droppings! 

How to check the rate'E. coli in water? 

In Quebec, standards regarding water quality for swimming and practicing water activities are measured in Colony Forming Unit per 100 mL (CFU/100mL). Traditional laboratory analyzes count bacteria inE.coli cultivable. There standard for bathing is set at 200 CFU/100mL while that for secondary contacts (canoe, kayak, etc.) is 1,000 CFU/100 mL. 

Water Atlas

TheWater Atlas allows you to evaluate the water quality at a certain location in a preliminary way since the data is not provided in real time and there are not always numerous, that is to say that alone, this tool requires others. This interactive map developed by the Government of Quebec allows you to identify overflow structures, wastewater treatment plants, industrial discharges, agricultural plots and certain sampling sites in Quebec as well as their histories. By selecting the blue colored icon named Water quality then ticking Bacteriological monitoring – Fecal coliforms and Trends: Fecal coliforms, we have a good overview of their presence in the past, which can give a good idea for the future! It is also possible to select the green icon titled Pollution Sources and check the contaminants that interest us to know what, in addition to the E.coli, could affect water quality.

Des gens sautent à l'eau au Vieux-Port de Montréal
In anticipation of La semaine du Grand Splash, a mobilization event to demand better access to swimming, Fondation Rivières makes sure to carry out an analysis of water quality in the form of a vulnerability notice. Photo : Cristian Mijea /


For citizens of Montreal, it is also possible to visit the website of the Aquatic environment monitoring network (RSMA) to identify sites in the St. Lawrence River or the Rivière des Prairies generally having good water quality. Please note that sampling results are not instantaneous. Consequently, following rain, it is preferable to wait 48 hours before being in contact with water in the event of overflowing of overflow structures.

A reliable tool? 

In reality, it proves difficult to monitor the microbiological quality of water effectively. The traditional analyzes on which the standards are based are slow since an incubation period of 24 hours is necessary. This is why it is impossible to obtain a faithful portrait of the real-time reality of water quality in a given location with the tools available to the general public. 

On the other hand, new technologies are emerging, such as ColiMinder and Fluidion, which produce analyzes in 15 minutes and a few hours, respectively. Although they are not yet approved at the regulatory level, they have great potential to improve the management of recreational water quality, by detecting contamination peaks quickly and on site. After all, it's much better to know the water quality while you're swimming in it! 

Coralie, intern at Fondation Rivières, samples river water, at Verdun beach, in order to know the count of E. coli.

Until the use of effective technologies is widespread by the authorities concerned, a good assessment of water quality requires a thorough understanding of historical and potential upstream sources of contamination, as well as characterization of the site. swimming. Fondation Rivières is also undertaking several projects this summer in the hope of being able to inaugurate safe swimming sites, particularly in the St. Lawrence River as well as the Richelieu and Châteauguay rivers. Campaigns to detect sources of pollution and monitor water quality will be carried out, based on the counting of'E. coli using traditional analyzes combined with those using ColiMinder. 

Fondation Rivières takes concrete actions to ensure water quality and promote access to waterways.

Main photo: Cristian Mijea /

About the Author

Marisol is a water quality field technician trainee at Fondation Rivières. She is currently completing a bachelor's degree in geological engineering, environmental profile, at Polytechnique Montréal.


Our recent news

Contribute to the protection of Quebec’s rivers

In addition to supporting our team in protecting rivers, your donation entitles you to a charitable tax receipt.

Make a difference for the environment

Become a member of the Foundation by donating. Your contribution allows our team to continue to mobilize to protect Quebec's rivers and waterways for future generations. 

Do you have a question?

Follow us

Our charity number : 14505 9911 RR0002

Stay informed of our actions to protect Quebec's rivers