Lake Saint-Louis at Risk?
Water Levels and Drinking Water
Christiane Hudon has a PhD in marine biology from Laval University (1982). A specialist in aquatic plant ecology, she has been a research scientist at Environment Canada since 1993.
Dr. Hudon initiated and coordinated the Impacts of Level and Flow Fluctuations in the St. Lawrence River project, which involved about 20 scientists. She was also the Canadian co-chair of the Environmental Technical Working Group (ETWG), established by the Lake Ontario–St. Lawrence River Study Board of the International Joint Commission.
In the Montreal area, 16 municipal water filtration plants produce drinking water for a population of 2.1 million people.
Under low water level conditions, the following situations can develop:
- a change in the relative size of different water masses, resulting in a change in the quality of raw water for treatment;
- a reduction in water depth that may disturb the functioning of certain drinking water intakes.
|1||Saint-Zotique||15||Montreal (borough of Ville-Marie)|
|2||Les Coteaux||16||Laval (3)|
|10||Montreal (borough of Pierrefonds–Senneville) (2)||24||Kahnawake|
|11||Montreal (borough of L'Île-Bizard–Sainte-Geneviève–Sainte-Anne-de-Bellevue)||25||Candiac|
|12||Montreal (borough of Pointe-Claire)||26||La Prairie|
|13||Montreal (borough of Dorval–L'Île-Dorval) (2)||27||Longueuil (2)|
|14||Montreal (borough of Lachine) (2)||28||Varennes|
Water Levels and Problems Related to Drinking Water Supply
A survey has revealed that the functioning of drinking water intakes of eight stations out of 16 are vulnerable to reductions in water levels and are likely to experience problems with the odour and taste of water. Five of these stations are vulnerable to variations in inputs from tributaries, which influence the quality of raw water to be treated.
|Problem||Number of stations||Population||% of population|
|Water quantity||8*||200 000||10|
|Influence of tributaries||5||163 000||7.7|
|Earthy taste and odour||8||495 410||23|
Source: École Polytechnique 2003.
* Excluding Montreal (Charles-J.-Des Baillets and Atwater plants – population 1.5 million), who relocated its intake in 2003.
Improvement in Water Quality
|In 2000, COURD’O, a physico-chemical and bacteriological water quality index, reported a net improvement over the previous 15 years, particularly along the south shore of Montreal Island, where the rating remained between “good” and “very good.” The commissioning of the former Montreal Urban Community’s wastewater treatment plant in 1988 contributed to improving shoreline water quality.|
|This has led, in turn to the recovery of some uses of the water, such as swimming. However, when it rains, water quality is likely to deteriorate because of overflowing wastewater collection systems.|
Improvement in Water Quality
Mouth of the Châteauguay River in Lake Saint-Louis
The quality of raw water varies according to the origin of the water mass. Water from Lake Ontario requires minimal treatment to make it fit to drink, while water from the tributaries is more expensive to treat, particularly during high flow periods, when levels of particulates and dissolved organic substances are higher.
|Nearly 70% of the surface area of the Châteauguay River basin in Quebec is being cultivated. Between 1998 and 2000, the total phosphorus load of the Châteauguay and Ottawa rivers respectively exceeded the threshold for aquatic life in 96% and 23% of the samples.|
High levels of nutrients (nitrogen and phosphorus) favour the proliferation of algae and aquatic plants, particularly under sunny conditions and high water temperatures. The average annual water temperature at the intake to the filtration plants increased by 2°C between 1970 and 2001.
Years of low levels coincide with higher water temperatures.
|The water clarity of the St. Lawrence River and the presence of many water masses of different origins makes it possible for more than 350 species of algae to grow, either freely suspended in the water (plankton) or anchored to sediment and submerged plants.|
When the environmental conditions are right, microscopic algae can proliferate to the point where it blocks the filters of the filtration plants, which occurred in Montreal in November 1955.
Even though suspended algae less abundant under low water level conditions (1955) than under average water level conditions (1994), the proportion of potentially troublesome algal species, such as chlorophyceae and cyanobacteria, increases.
|Source: Hudon et al. 2000.|
Degradation of Algae and Aquatic Plants
The degradation of algae and aquatic plants imparts an earthy, musty taste and odour to drinking water due to the production of volatile organic molecules. Eliminating this problem requires additional water treatment and extra costs.
Zebra Mussel Colonization
Zebra Mussels colonize submerged substrates, including drinking water intakes, sometimes even interfering with their functioning. Under low water-level conditions, there is a higher risk of clogging water intakes, given the higher rate at which Zebra Mussels anchor themselves.
Source: de Lafontaine 2002.
Climate change could have few to many effects on the different aspects of supplying drinking water from the St. Lawrence River.
Anticipated impacts of climate change on drinking water supply
|Problem||Current situation||Anticipated situation (low levels)|
|Water intake location||8 stations affected|
|Variability of water mass (influence of tributaries)||3 stations affected|
|Sewer overflows along the shoreline||General improvement||Stable or declining|
|Toxic algae (cyanobacteria)||Rare||Stable|
|Earthy, musty taste and odour||8 stations affected|
|Zebra Mussel colonization||Local effects||Higher incidence|
École Polytechnique de Montréal. 2003. Impacts of Level Fluctuations in the St. Lawrence River on Water Treatment Plant Operations. Report prepared by Annie Carrière and Benoit Barbeau, submitted to the Water Uses Technical Work Group of the International Joint Commission. 208 pages.
de Lafontaine, Y. 2002. Impacts des fluctuations de débits d'eau douce sur la colonisation annuelle par les Moules zébrées dans le fleuve Saint-Laurent. Technical Report submitted to the International Joint Commission.
Hudon, C., S. Lalonde and P. Gagnon. 2000. Ranking the effects of site exposure, plant growth form, water depth, and transparency on aquatic plant biomass. Canadian Journal of Fisheries and Aquatic Sciences 57: 31–42.
City of Montreal – Réseau de suivi du milieu aquatique (in French only)
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