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Introduction to water pollution

Last updated: May 10, 2012.

Next time you pour yourself a cool, clear glass of water, hold it up to the light and look very closely. Do you see any pollution? Any spilled oil? Stray bacteria? Toxic chemicals? Bug-eyed microscopic creatures?

Probably not—and nor are you likely to. We assume the water we put inside ourselves is as clean as clean can be, and mostly it is. But think about things logically, and you might start having doubts. We know that water pollution exists (often in quite large quantities) and we also know that the world's water is constantly recycling itself. So how can we be sure that the water we drink is perfectly clean and healthy?

The answer is that water companies go to great lengths to purify drinking water, whether it comes from a bottle or a tap (faucet). That's very reassuring, but what does it tell us about the rest of the water on our planet: the water we swim or bathe in, the water in our rivers, and the water in our seas? Would you be happy to drink water from your local river? Would it bother you if you accidentally swallowed water from the sea? Do you worry about the water that comes out of your tap and drink bottled water instead? The fact that we all do worry about things like this is a sign that water pollution is one of the world's biggest environmental problems. What causes it, what effect does it have, and what can we do to stop it?

What is water pollution?

Close your eyes a moment and say the words "water pollution." What images come into your mind?

Water pollution pipe pouring against background of snow. US Environmental Protection Agency Great Lakes National Program office

Photo: US National Park Service, Indiana Dunes National Lakeshore, courtesy of US EPA Great Lakes National Program office.

Maybe oil tanker spills in the ocean or plastic trash washed up on the beach? Maybe you've seen shopping carts floating in your local river or huge growths of algae turning lakes green? These are all examples of water pollution, but they're by no means the only examples. That's why, if we want to define "water pollution," we have to use quite a wide and general definition. Usually what we mean by pollution is "something that doesn't belong in the natural environment."

An oil spill in the ocean or a sewage spill in a river meet this definition of pollution, but other types of pollution are different. What if you dragged a net through San Francisco bay and hauled up a load of Asian kelp? What if you went fishing in England and caught an American signal crayfish? You might be delighted to have caught something more than an old boot, but both these things are examples of pollution: they are what we call invasive species (or "alien species") that don't belong in the places where we now commonly find them.

So anything that doesn't belong where we find it is pollution, right? Not exactly. Suppose you find one molecule of oil in the Pacific ocean. Would you consider that pollution? How about two molecules? How about a cup of oil... or a gallon.... or 10 barrels... or.... ? At what point does something fairly harmless become a problem? At what point does an ordinary substance become pollution?

Clearly, we have to modify our definition a little bit so it takes account of the factors we've just considered. Let's define water pollution like this:

Water pollution is a chemical or biological substance that builds up in the environment enough to be toxic, harmful, or a nuisance to humans, other animals, or other living things.

What causes water pollution?

That sounds like a simple question, but it's much harder to answer than you might think. To start with, each of the different types of water pollution has a different cause. So, for example, it's relatively common for the world's rivers and seas to be polluted by human sewage and wastewater. But what causes the pollution? It's not simply that humans and animals produce sewage; it's that sewage is often flushed untreated into the water. Even in relatively wealthy countries such as the United States and the United Kingdom, vast amounts of sewage routinely find their way into coastal waters where people bathe and from which fish and shellfish are caught. [1,2]

But why is the sewage flushed into the seas instead of being pumped through a treatment plant to remove toxic chemicals and bacteria? Is it a matter of economics (people don't want to pay more for their water)? Is it a matter of law and regulation (water companies are not fined enough when they pollute)? Is it that people don't care enough to protect the environment we all depend on? Or is it that we don't appreciate the problem of pollution—or perhaps that we have too many other things to worry about? You could argue that sewage and wastewater pollution is caused by any (or all) of these things.

Trash thrown by the side of a highway.

What causes people to throw trash by the side of a highway? Photo courtesy of US Fish & Wildlife Service Digital Library.

The same goes for pretty much all the other types of water pollution. You might believe that careless oil companies are responsible when oil tankers or oil rigs leak oil into the oceans. Then again, you might consider how very upset people become when the price of petrol (gasoline) increases. If we forced all the oil companies to build stronger and more robust tankers, and to adopt much more stringent safety procedures on their oil rigs so accidents and leaks were less likely, how much more would we all have to pay to fill up our cars? How much would the price of everyday goods (which have to factor in a certain amount for transportation) have to rise to pay for it? Maybe a lot, maybe not much at all, but we do at least have to ask the question. As before, you could argue that greedy oil companies cause water pollution—or you could probe further and ask whether society as a whole has come to value cheap gasoline over pristine oceans. Are we all partly to blame?

Whichever form of water pollution you consider, you'll find the simple causes lead to deeper questions about how our society works. Take the problem of acid rain, for example. That's caused when air pollution from smokestacks blows long distances before meeting up with rain clouds. When the rain falls, it dissolves the pollution and produces acid, which changes the pH (acid-alkali balance) of lakes and rivers, making them too acidic to support fish life. Who or what is responsible for acid rain? Is it the companies who operate the smokestacks? Since many of them are power plants, is it the ordinary people—you and me—who use their electricity? Is it the governments who fail to do anything about the problem?

Once we start to think in this way, and look at the deeper, less obvious causes, it starts to make sense why pollution is such a widespread problem and such a hard thing to solve.

Ten common causes of water pollution

Detergent pollution seeping into a creek.

Photo: Detergent gets your clothes clean, but how dirty does it make the environment? Photo courtesy of US Fish & Wildlife Service Digital Library.

What effect does water pollution have?

Water pollution is often really horrible to look at: no-one likes to see plastic bottles strewn across their favorite beach or plastic bags littering a river. But often the effects are more drastic and much more disturbing: think of dead fish floating on a polluted lake or oil-caked birds flapping hopelessly on a beach inland from a tanker spill.

We've defined water pollution as something "toxic, harmful, or a nuisance," but how bad can its effects be? The answer is all about concentration: how much of a polluting substance or chemical is present in a river, lake, or sea. If there's enough oil to wipe out one or more species, an entire ecosystem may be affected, because each species depends critically on all the others. You might think it doesn't matter if all the insects (say) disappeared from a river corridor. But what feeds on those insects? Bats, birds, and other creatures. And what feeds on those? A relatively small change to one part of an ecosystem can cause knock-on damage felt by many other species. Effects like that can take years to fully manifest themselves, which means they're not always obvious and not always easy to trace back to a single episode of pollution.

Although many people care passionately about the environment, humans are ultimately programmed to be selfish creatures: we care most about our own survival. When we see oil tanker spills on the TV, we get angry and upset—but if we're part of a community that's affected by a spill like that, we care very much more. It's difficult to put a value on a clean river, because it benefits many different communities in many different ways [10]; quantifying the effects of river pollution is therefore very difficult. It's easier to see the effects of localized pollution on something like a coastal community. The 1989 Exxon Valdez oil spill saw at least 42 million liters (11 million gallons) of crude oil discharged into Prince William Sound in Alaska and decimated the local community. [11] Similar devastation was reported in the Gulf of Mexico after a huge oil leak from BP's Deepwater Horizon drilling platform in 2010. [12]

Oil well pollution.

What effect will this spilled oil have when it runs into the ground? Could it damage someone's health years or even decades in the future? Photo courtesy of US Fish & Wildlife Service Digital Library.

Just as we need to think hard about the causes of pollution—looking beyond the immediate reasons for deeper social or economic factors that may have caused them—so we need to take care when we think about pollution's effects. It's relatively easy to consider something like an oil tanker spill and measure the effect on a local fishing community over the coming days, weeks, months, and years. But what about the longer-term effects of pollution happening in many places at once? Although some water pollution can be neatly traced to a single place, time, or event (we call it point-source pollution), much of it builds up from many different sources over a long period of time (we call that nonpoint-source pollution or diffuse pollution). What are the effects of, for example, all the chemical factories in the world that dump toxic liquids into wastewater? How much of that finds its way into rivers, seas, and even groundwater that we might drink? Are the concentrations of toxics too small to worry about? And how do we know? One problem we have is that toxic chemicals may be carcinogenic (cancer-causing), but cancers take years or decades to develop and people get them for many different reasons, so it can be very hard to say, with any certainty, that particular factories have a harmful effect on human health because they contribute to water pollution. [13]

As with many other environmental problems, water pollution is hard to tackle because its effects (which may be long-term and subtle) are often hard to trace to a particular cause. If we know an oil tanker has broken up near the coast, we can instantly identify that as a potential source of water pollution and take immediate action to reduce any effect it may have. But we can look at a dirty river, devoid of fish and other life, running through a city, and find it hard to know who or what to blame. And if we can't easily identify the cause of the problem, coming up with a solution can be almost impossible.

Is an oil tanker spill the worst thing that can hit the ocean?

Bar chart of five big ocean oil spills

Wikipedia has a useful list of the biggest oil spills in history. Here are five of the best known, with approximate figures for the oil spilled into the ocean in each case.

Now we often assume that oil tanker spills are the worst things that ever hit the ocean, but this chart shows otherwise: the Gulf War and Deepwater Horizon spills dwarf even the giant Exxon Valdez tanker accident, and the other two tanker spills I've included here barely even show up on this scale.

What's the solution to water pollution?

You might think water pollution would be an easy problem to solve, but it's not. I've taken pains to point out that the causes and effects of water pollution are more subtle than they might, at first, appear. By recognizing the complexity of the problem, we stand a better chance of solving it.

The first thing we need to note is that water pollution is not a single problem but many different ones. Although lots of different things can lead to a river or sea being heavily polluted, it can be very helpful to see them as separate problems and try to find specific solutions in each case.

Suppose we consider the hypothetical city of Nowheresville, with the appallingly polluted Nowhere River running through it. The pollution could be a combination of sewage, oil, acid rain, highway runoff, and all kinds of other things. What can we do about it?

We can let water pollution defeat and deflate us, or we can break the seemingly huge problem into a number of smaller, more manageable problems and go after smaller, highly targeted solutions; eventually, all these things will make a noticeable difference to the quality of our water. It may take years or decades, but we can and will turn things around. We can make a difference; we can beat water pollution!

Ten pollution solutions

Volunteers clean trash from the side of a harbor as part of a community program to clean up their local wetland.

We're all part of the pollution problem—and we all need to be part of the solution. Community cleanups like this directly improve the environment, but they also remind people that pollution is a problem we have to keep under control. Photo by John Wallace Ciccarelli Jr courtesy of US Navy.

Written by Chris Woodford for the UK Rivers Network. © Copyright Chris Woodford 2012.

Find out more

On the web



Detailed general books on water pollution

Chemical and biological effects of water pollution

Political and economic analysis



  1. According to NRDC, over 700 US communities have CSOs (combined sewer overflows) that can discharge raw sewage and runoff into waterways during high rainfall. See NRDC: Testing the Waters, 2011.
  2. In the UK, there are an estimated 20,000 CSOs, of which 500 flow directly onto or near beaches. See Britain's Dirty Beaches, BBC News, 7 September 2009.
  3. British environmental group Surfers Against Sewage was set up in the early 1990s to fight this problem.
  4. A 1994 leaflet by the US EPA reported: "200 million gallons of used motor oil are improperly disposed of... one gallon of used oil [can]... contaminate up to one million gallons of drinking water." See EPA Collecting used Oil for Recycling/Reuse [PDF].
  5. A 1999 report by David Pimentel et al estimated the cost of invasive species to the US at $138 billion. See Environmental and economic costs associated with non-indigenous species in the United States, Cornell University Press Release, June 12, 1999.
  6. According to NRDC: "In 2010, polluted runoff and stormwater caused or contributed to 8,712 closing/advisory days at coastal and Great Lakes beaches, making it the largest known source of contamination problems." See NRDC: Testing the Waters, 2011.
  7. See Marine Conservation Society: The Problem with Plastics, 2011.
  8. For more about how oil gets into the oceans, see Accidental discharges of oil. For an example of how oil affects sensitive marine areas, see Shipping and Oil Spills, Australian Government: Great Barrier Reef Marine Park Authority, March 2006 [PDF].
  9. Notably International Convention for the Prevention of Pollution from Ships (MARPOL), originally adopted in 1973 and revised several times since then. On the effectiveness of MARPOL, see The Marpol Convention (Good Planet, 200) and Has MARPOL reduced intentional oil discharges by Ronald B. Mitchell (1994).
  10. In May 2012, US community alliance Protect the Flows estimated that the Colorado River brought recreational benefits of $17 billion. See Study: Colorado River generating money and jobs
  11. For lasting ecosystem damage, see What we learned from the Exxon Valdez by Stephen Dowling, BBC News, 26 March 2009. For the effect on the local community, see Exxon Valdez oil disaster still affects communities, wildlife Sydney Morning Herald, May 4, 2010.
  12. Gulf spill's effects 'may not be seen for a decade' By Jason Palmer, BBC News, 21 February 2011.
  13. Sandra Steingraber's Living Downstream: An Ecologist's Personal Investigation of Cancer and the Environment is a classic account of attempts to trace human health problems to environmental pollution.
  14. The "drittsekk" incident is briefly recounted in Gummer goes on getting greener by Geoffrey Lean, Telegraph, 31 December 2009.

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