Water Pollution and Aquatic Health
Water pollution is the contamination of rivers, lakes, oceans and groundwater by substances that make the water harmful to organisms or unsuitable for use. Common pollutants include sewage, fertilizers, industrial chemicals, heavy metals, oil and plastics. In brief: fertilizer washed into a lake can cause an algal bloom, and when the algae die, bacteria consume the dissolved oxygen so fish suffocate; this process is called eutrophication.
Aquatic ecosystems depend on clean water with enough dissolved oxygen, a suitable temperature and a balanced chemistry. This article describes the main sources and types of water pollution, explains eutrophication and bioaccumulation, shows the indicators used to assess water quality, works through a dissolved oxygen example and outlines prevention methods.
What are the sources of water pollution?
Pollution sources are divided into two groups. Point sources release pollutants from a single identifiable place, such as a factory outlet pipe or a sewage treatment plant. Non-point sources are spread over wide areas, for example fertilizer and pesticide runoff from farmland, oil washed off roads by rain and litter. Non-point pollution is harder to control because it has no single origin.
What types of pollutants are there?
| Pollutant | Typical source | Effect on water and life |
|---|---|---|
| Sewage and animal waste | Households, farms | Disease-causing microbes, oxygen depletion |
| Nitrates and phosphates | Fertilizers, detergents | Algal blooms, eutrophication |
| Heavy metals (mercury, lead, cadmium) | Mining, industry, old pipes | Toxic; accumulate in food chains |
| Pesticides | Agriculture | Harm non-target organisms |
| Oil | Spills, road runoff | Coats animals, blocks oxygen exchange |
| Plastics | Litter, microbeads, fibers | Harm animals that eat or become entangled in them |
| Heat (thermal pollution) | Cooling water from power stations | Warm water holds less dissolved oxygen |
What is eutrophication?
Eutrophication is the overgrowth of algae and plants in water caused by an excess of nutrients, especially nitrogen and phosphorus. The process has several steps.
- Fertilizers and sewage add nitrates and phosphates to the water.
- Algae grow rapidly and form a dense layer called a bloom on the surface.
- The bloom blocks sunlight, so plants below the surface die.
- Dead algae and plants sink and are decomposed by bacteria, which use up dissolved oxygen.
- Fish and other animals die or leave when the oxygen level is too low. Some algae also produce toxins.
What is bioaccumulation?
Some pollutants, such as mercury and certain pesticides, are absorbed by organisms and not easily broken down or excreted. They build up in tissues, which is called bioaccumulation. As predators eat many contaminated prey animals, the concentration rises at each level of the food chain, which is called biomagnification. Large predatory fish such as tuna and swordfish can therefore contain much higher mercury levels than small fish. Health agencies advise limits on how much of such fish certain groups, for example pregnant women, should eat.
How is water quality measured?
| Indicator | What it shows | Healthy value for most freshwater life |
|---|---|---|
| Dissolved oxygen | Oxygen available to animals | Above about 6 mg/L; below 2 mg/L is hypoxic |
| pH | Acidity or alkalinity | About 6.5 to 8.5 |
| Temperature | Heat; affects oxygen and metabolism | Depends on species |
| Turbidity | Cloudiness caused by particles | Low |
| Nitrate and phosphate | Nutrient pollution | Low |
| Coliform bacteria | Contamination by feces | Absent in drinking water |
Worked example: dissolved oxygen
The amount of oxygen that water can hold falls as the temperature rises. At 10 °C, water in equilibrium with air holds about 11.3 mg/L, and at 25 °C it holds about 8.3 mg/L (at sea-level pressure, fresh water).
- Loss of capacity. 11.3 − 8.3 = 3.0 mg/L less in the warmer water.
- Percentage drop. 3.0 ÷ 11.3 × 100 = 26.5%.
- Effect of pollution. If bacteria decomposing sewage use 5.0 mg/L of oxygen, a stream starting at 8.3 mg/L (25 °C, saturated) falls to 8.3 − 5.0 = 3.3 mg/L. This is below the 6 mg/L that is healthy for sensitive fish, such as trout, which means they would be stressed or die.
- Total oxygen in a pond. A pond of 200 m3 (200,000 L) at 3.3 mg/L holds 200,000 × 3.3 = 660,000 mg = 660 g of dissolved oxygen.
Warm water and organic pollution together make a dangerous combination for aquatic life, which is why warm summer months are the time when fish kills occur most often.
How can water pollution be prevented?
- Treat wastewater before it is released into the environment.
- Reduce fertilizer use and plant strips of vegetation along streams to absorb runoff.
- Regulate industry and require treatment of effluent.
- Handle oil and chemicals safely and prepare for spills.
- Reduce plastic use and improve waste collection.
- Protect wetlands, which filter pollutants naturally.
What are some historic water pollution events?
In 1854, John Snow traced a cholera outbreak in London to a contaminated water pump, which helped to establish the link between polluted water and disease. In the 1960s, the Cuyahoga River in Ohio was so polluted by oil and waste that it caught fire several times, which contributed to the passage of the U.S. Clean Water Act in 1972. The Minamata disease in Japan, recognized in the 1950s, was caused by mercury from industrial waste that accumulated in seafood.
Which mistakes are common?
Mistake 1: Thinking that clear water is clean
Water can contain dissolved chemicals or microbes without any visible sign.
Mistake 2: Believing that dilution solves pollution
Some pollutants accumulate in organisms even at low concentrations.
Mistake 3: Blaming only factories
Farms, homes and roads are major sources of water pollution.
Mistake 4: Saying that algae are always harmful
Algae form the base of aquatic food chains. The problem is excessive growth caused by too many nutrients.