Municipal waste-water discharges are one of the largest sources of pollution in Canadian waters. In 2006, municipal waste-water effluents produced by households, businesses, and industries generated 84 per cent of the water effluents reported under the National Pollutant Release Inventory.  Municipal waste water is composed of sanitary sewage and storm water, and can contain grit, debris, suspended solids, disease-causing pathogens, decaying organic wastes, nutrients, and about 200 identified chemicals.

Overall, the percentage of Canadians served by waste-water treatment has been increasing. Combined secondary and tertiary waste-water treatment was provided to 78 per cent of the population in 1999, up from 56 per cent in 1983. By 2009, 79 per cent of the population was served by water treatment. Sewage treatment varies greatly across Canada, but it is generally improving as more municipalities upgrade their treatment facilities.

Increases in toxic algal blooms in Canadian lakes and coastal water have increased risks to human health. There has also been an increase in the frequency and spatial extent to which the drinking water guideline for nitrate has been exceeded in groundwater across Canada. Environmental problems caused by excessive nutrients are less severe in Canada, however, than in many countries with a longer history of settlement and agricultural production. Canadian governments have also regulated waste-water discharges from industrial and municipal sewage treatment plants over the last 30 years. And as new information and technologies have become available, measures for addressing excessive nutrient loadings have been refined. Still, environmental and human health problems related to nutrients are evident across Canada.

Overall, Canada’s water quality is better today than it was 30 years ago. But although phosphorous discharge decreased in the 1980’s and 1990’s, there was simultaneous increase in nitrogen discharge. In 1999, 82,750 tonnes of total nitrogen (i.e., nitrogen in all its chemical forms) were released to surface waters in Canada through municipal waste-water discharges—a 24 per cent rise over 1983 levels. Phosphorus, on the other hand, decreased by 44 per cent, with 4,950 tonnes of total phosphorus released to surface waters from municipal waste-water treatment plants in 1999. But phosphorus levels have been on the rise once again. In 2006, phosphorus releases increased to 6,874 tonnes.  Although nitrogen releases have decreased since 1999—down to 52,702 tonnes in 2006—elevated concentrations of these nutrients remain a critical water quality issue in Canada.

Environment Canada puts together data on the quality of freshwater at 173 monitoring stations in rivers in populated areas across the country, using the Canadian Council of Ministers of the Environment’s water quality index. From 2007 to 2009, freshwater quality was rated poor or marginal in 16 per cent of the stations, fair in 41 per cent, good in 33 per cent, and excellent in 10 per cent—slightly better than in the previous three-year period.

The Water Quality Index, developed by the Yale Center for Environmental Law & Policy, is one of the indicators that make up the Environmental Performance Index. It is a proximity-to-target composite of water quality, adjusted for the density of monitoring stations in each country, with a maximum score of 100. The Water Quality Index measures dissolved oxygen, pH, conductivity, total nitrogen, and total phosphorus. It is an indicator of eutrophication, nutrient pollution, acidification, and salinization. More specifically, it measures the following:

• Dissolved oxygen is a measure of free (i.e., not chemically combined) oxygen dissolved in water. It is essential to the metabolism of all aerobic aquatic organisms. Reduced levels have been shown to harm and even kill plants and fish.
• pH is a measure of the acidity or alkalinity of a water body. It can affect aquatic organisms both directly, by impairing respiration, growth, and development of fish, and indirectly, by increasing the bioavailability of certain metals such as aluminum and nickel.
• Conductivity is a measure of the ability of water to carry an electric current, which depends on the presence of ions. Increases in conductivity can lead to changes that reduce biodiversity and alter community composition.
• Nitrogen and phosphorus are naturally occurring elements essential for all living organisms. But they are often found in growth-limiting concentrations in aquatic environments. Increases in nitrogen or phosphorus in natural waters, largely as a result of human activities in the drainage basin (e.g., from agricultural runoff from manure and synthetic fertilizers, and from municipal and industrial waste-water discharge), can overstimulate plant growth and choke off oxygen supplies.