Water Pollution: Sources and Cumulative Effects

Everything ends up downstream

In spring, snowmelt runs off a farmer's field, a shopping mall parking lot and a highway, and it all flows into the same creek. Each source alone might add only a small amount of fertilizer, salt or oil. Together, over years, the amounts add up. This lesson is about where water pollutants come from, why their effects are cumulative, and how chemists use the concentration skills you just learned to measure them.

Major sources of water pollutants

Agricultural runoff (non-point): rain and snowmelt wash fertilizers (nitrates and phosphates), manure (nutrients and bacteria) and pesticides off fields into ditches and streams. Golf courses and lawns can add fertilizer runoff in the same way, especially when they are heavily fertilized and irrigated.

Landfill leachate (point): rain trickling through garbage dissolves substances such as ammonia, metals and organic compounds. Modern Ontario landfills have liners and collection pipes to capture leachate and send it for treatment, but older dumps may not.

Industrial effluent (point): factories, mines, pulp and paper mills and food processors (such as poultry or meat plants) discharge wastewater. Depending on the industry, it may carry metals, acids, nutrients, warm water or organic matter that uses up oxygen as it decays.

Municipal wastewater (point): sewage treatment plants remove most solids, bacteria and nutrients, but traces of medicines, personal-care products and microplastics can pass through.

Road salt (non-point): in Canadian winters, sodium chloride (and some calcium chloride) is spread on roads. The chloride dissolves, does not break down, and can build up in streams, lakes and groundwater.

Cumulative effect 1: eutrophication

Phosphorus is usually the nutrient that limits algae growth in fresh water. When extra phosphorus (and nitrogen) arrives from fertilizer, manure or sewage, algae grow explosively. When they die, bacteria decompose them and use up dissolved oxygen. Fish and other animals can suffocate, and some blooms (blue-green algae, also called cyanobacteria) make toxins.

Lake Erie, the shallowest and warmest Great Lake, has repeated large algal blooms in its western basin, linked mainly to phosphorus from farm runoff. Canada and the United States have agreed on targets to reduce the phosphorus entering the lake.

Cumulative effect 2: bioaccumulation and biomagnification

Some pollutants, such as methylmercury (a form of mercury made by bacteria in water and sediment) and some pesticides, are absorbed by living things but removed very slowly. They bioaccumulate: an older fish has more than a young one. Each predator eats many prey, so the concentration rises at each level of the food chain: biomagnification. Large predatory fish such as walleye and northern pike can carry far higher mercury concentrations than the water around them. That is why Ontario publishes a Guide to Eating Ontario Fish with advice by lake, species and size.

Mercury in the English-Wabigoon River system

In the 1960s, a chemical plant connected to the pulp and paper mill in Dryden, Ontario, discharged mercury into the Wabigoon River, which flows into the English River system. The mercury built up in fish. Downstream, the Anishinaabe communities of Grassy Narrows First Nation (Asubpeeschoseewagong Netum Anishinabek) and Wabaseemoong Independent Nations depend on the river for fishing, food, jobs as fishing guides, and cultural practices. Many community members have experienced health effects linked to mercury, and the contamination has affected their economy and way of life.

The mercury is still in the river sediments today. Community members and leaders continue to lead the push for cleanup of the river, for health care and for compensation, and governments have committed funding for remediation and health supports. This case shows cumulative effects clearly: a pollutant released decades ago, at low concentrations in water, still affects people through the food chain.

When you research cases like this, use reliable sources (A1.9): community statements, government reports and peer-reviewed studies, and notice whose voices each source includes.