What Is Water Pollution?

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Oil spills are a very common form of water pollution, and has detrimental effects on aquatic life. David Wall Photo/Lonely Planet Images/Getty

Water pollution is when water contains contaminants. In the context of environmental science, a contaminant is usually a substance which can be harmful to living things like plants or animals. Environmental contaminants can be the result of human activity, for example a by-product of manufacturing. However, they can also occur naturally, like radioactive isotopes, sediment, or animal waste.

Because of how general the concept of pollution is, we can assume that polluted waters have been around even before humans were here.

For example, a spring might have high sulfur levels, or a stream with a carcass in it would have been unfit for other animals to drink from. However, the number of polluted streams, rivers, and lakes multiplied rapidly as the human population increased, agricultural practices intensified, and industrial development spread.

Important Sources of Pollution

A number of human activities lead to water pollution harmful to aquatic life, aesthetics, recreation, and human health. The main sources of pollution can be organized in a few categories: 

  • Land use. We have a heavy impact on the land: we cut forests, plow grasslands, build homes, pave roads. Land use activities intercept the water cycle during precipitation events and snowmelt. As water flows over the land and into streams, it picks up anything small enough to be carried away. Vegetation does an important job of holding back organic and mineral components of the soil, but clearing that vegetation means a lot of substances make it into streams, rivers, wetlands, and lakes, where they become contaminants.
  • Impervious surfaces. Most man-made surfaces cannot absorb water like soil and roots would. Rooftops, parking lots, and paved roads allow rain and snowmelt runoff to flow with great speed and volume, picking up along the way heavy metals, oils, road salt, and other contaminants. The pollutants would otherwise have been absorbed by the soil and vegetation, where they would have been naturally broken down. Instead, they concentrate in runoff water, overwhelming the streams’ capacity to process them.
  • Agriculture. Common agricultural practices, like exposing soils to the elements, using fertilizers and pesticides, and concentrating livestock, routinely contribute to water pollution. Nutrient runoff, mostly phosphorus and nitrates, leads to algae blooms and other problems. Mismanagement of farm soils and livestock can also lead to significant soil erosion. Soil picked up by rain makes its way into streams where it becomes sediment pollution, with harmful consequences on aquatic life.
  • Mining. Mine tailings are the piles of rock discarded after the valuable portion of the ore has been removed. Tailings can leach to surface and ground waters large amounts of contaminants, some occurring naturally in the waste rocks, others a product of the ore processing methods. Mining by-products are sometimes stored in impoundments as a slurry or sludge (for example, coal ash), and failure of the dams holding back these artificial ponds can lead to environmental disaster. Abandoned coal mines are a notorious source of acid mine drainage: water in flooded mines and in contact with mine tailings sometimes oxidizes sulfur-bearing rocks, and turns extremely acidic.
  • Manufacturing. Industrial activities are a major source of water pollution. In the past, liquid waste was dumped directly into rivers, or put into toxic waste barrels which were then buried somewhere. Those barrels then deteriorated and leaked, resulting in heavily contaminated sites we are still dealing with today. In the United States, regulations now severely limit these practices, notably the 1972 Clean Water Act, the Resource Conservation Recovery Act of 1976, and the Superfund Act of 1980. The release of toxic materials at industrial sites continues, either at levels below regulatory thresholds, or simply illegally. In addition, accidental spills occur all too frequently – for example with the recent West Virginia MCHM spill. In developing countries, pollution from industrial sources is still widespread and dangerous to human and ecosystem health.
  • Energy sector. The extraction and transportation of fossil fuels, notably oil, is prone to spills that can have long lasting effects on aquatic systems. In addition, coal-fired power plants release large amounts of sulfur dioxide and nitrogen oxides in the air. When those contaminants dissolved in rain water and enter waterways, they significantly acidify rivers and lakes. Coal plants also emit mercury, a very toxic heavy metal, polluting lakes throughout the world and making fish unsafe to eat. The production of electricity through hydropower produces much less pollution, but still has some deleterious effects on aquatic ecosystems.
  • Household practices. There are numerous actions we can take every day to prevent water pollution: avoid lawn pesticides, slow rainwater runoff, collect pet waste, properly dispose of household chemicals and medicine, avoid products with microbeads, attend to oil leaks on the mower or car, have the septic tank maintained and inspected.
  • Thrash. A lot of trash persists in the environment, and plastic matter breaks down into harmful microplastics.

Are Contaminants Always a Substance?

Not always. For example, nuclear power plants use vast amounts of water to cool down the steam generator by the reactor and used to spin the turbines. The warm water is then released back into the river it was pumped from, creating a warm plume that affects downstream aquatic life.