Water pollution: an introduction
by Chris Woodford.
Last updated: January 26, 2022.
Over two thirds of Earth's surface is covered by water; less than a third is taken up by land. As Earth's population continues to grow,
people are putting ever-increasing pressure on the planet's water
resources. In a sense, our oceans, rivers, and other inland waters are
being "squeezed" by human activities—not so they take up less room, but
so their quality is reduced. Poorer water quality means water pollution.
We know that pollution is a human problem because it is a
relatively recent development in the planet's history: before the 19th
century Industrial Revolution, people lived more in harmony with their
immediate environment. As industrialization has spread around the
globe, so the problem of pollution has spread with it. When Earth's
population was much smaller, no one believed pollution would ever
present a serious problem. It was once popularly believed that
the oceans were far too big to pollute. Today, with around 7 billion
people on the planet, it has become apparent that there are limits.
Pollution is one of the signs that humans have exceeded those limits.
How serious is the problem? According to the environmental campaign organization
WWF: "Pollution from toxic chemicals threatens life on this planet. Every ocean and every continent, from the tropics to the once-pristine polar regions, is contaminated."
Photo: Stormwater pollution entering a river from a drain. Photo by Peter C Van Metre courtesy of US Geological Survey.
What is water pollution?
Water pollution can be defined in many ways. Usually, it means one
or more substances have built up in water to such an extent that they
cause problems for animals or people. Oceans, lakes, rivers, and other
inland waters can naturally clean up a certain amount of pollution by
dispersing it harmlessly. If you poured a cup of black ink into a
river, the ink would quickly disappear into the river's much larger volume of clean water. The ink
would still be there in the river, but in such a low concentration that
you would not be able to see it. At such low levels, the chemicals in the ink probably would not present any real problem.
However, if you poured gallons of ink into a river every few seconds through a pipe,
the river would quickly turn black.
The chemicals in the ink could very quickly have an effect on the quality of the water.
This, in turn, could affect the health of all the plants, animals, and humans whose
lives depend on the river.
Photo: Pollution means adding substances to the environment that don't belong there—like the
air pollution from this smokestack. Pollution is not always as obvious as this, however.
Thus, water pollution is all about quantities: how much of a
polluting substance is released and how big a volume of water it is
released into. A small quantity of a toxic chemical may have little
impact if it is spilled into the ocean from a ship. But the same amount
of the same chemical can have a much bigger impact pumped into a lake
or river, where there is less clean water to disperse it.
Water pollution almost always means that some damage has been done to an ocean, river, lake, or other water source.
A 1969 United Nations report defined ocean pollution as:
introduction by man, directly or indirectly, of substances or energy
into the marine environment (including estuaries) resulting in such
deleterious effects as harm to living resources, hazards to human
health, hindrance to marine activities, including fishing,
impairment of quality for use of sea water and reduction of amenities."
Fortunately, Earth is forgiving and damage from water pollution is often reversible.
What are the main types of water pollution?
When we think of Earth's water resources, we think of huge
oceans, lakes, and rivers. Water resources like these are called surface
waters. The most obvious type of water pollution affects surface
waters. For example, a spill from an oil tanker creates an oil slick
that can affect a vast area of the ocean.
Photo: Detergent pollution entering a river—an example of surface water pollution. Photo courtesy of US Fish & Wildlife Service Photo Library.
Not all of Earth's water sits on its surface, however. A great deal
of water is held in underground rock structures known as aquifers,
which we cannot see and seldom think about.
Water stored underground in aquifers is known as groundwater.
Aquifers feed our rivers and supply much of our drinking water. They
too can become polluted, for example, when weed killers used in
people's gardens drain into the ground. Groundwater pollution is much
less obvious than surface-water pollution, but is no less of a problem.
In 1996, a study in Iowa in the United States found that over half the
state's groundwater wells were contaminated with weed killers.
You might think things would have improved since then, but,
two decades on, all that's really changed is the name of the chemicals
we're using. Today, numerous scientific studies are still finding
weed killers in groundwater in worrying quantities:
a 2012 study discovered glyphosate
in 41 percent of 140 groundwater samples from Catalonia, Spain;
scientific opinion differs on whether this is safe or not.
Surface waters and groundwater are the two types of water resources
that pollution affects. There are also two different ways in which
pollution can occur. If pollution comes from a single location, such as
a discharge pipe attached to a factory, it is known as point-source
pollution. Other examples of point source pollution include an oil
spill from a tanker, a discharge from a smoke stack (factory chimney),
or someone pouring oil from their car down a drain. A great deal of
water pollution happens not from one single source but from many
different scattered sources. This is called nonpoint-source
Photo: Above: Point-source pollution comes from a single, well-defined place such as this pipe.
Below: Nonpoint-source pollution comes from many sources. All the industrial plants alongside a river and the
ships that service them may be polluting the river collectively. Both photos courtesy of US Fish & Wildlife Service Photo Library.
When point-source pollution enters the environment, the place most
affected is usually the area immediately around the source. For
example, when a tanker accident occurs, the oil slick is concentrated
around the tanker itself and, in the right ocean conditions, the
pollution disperses the further away from the tanker you go. This is
less likely to happen with nonpoint source pollution which, by
definition, enters the environment from many different places at once.
Sometimes pollution that enters the environment in one place has an
effect hundreds or even thousands of miles away. This is known as transboundary
pollution. One example is the way radioactive waste travels through
the oceans from nuclear reprocessing plants in England and France to
nearby countries such as Ireland and Norway.
How do we know when water is polluted?
Some forms of water pollution are very obvious: everyone has seen TV
news footage of oil slicks filmed from helicopters flying overhead.
Water pollution is usually less obvious and much harder to detect than
this. But how can we measure water pollution when we cannot see it? How
do we even know it's there?
There are two main ways of measuring the quality of water. One is to
take samples of the water and measure the concentrations of different
chemicals that it contains. If the chemicals are dangerous or the
concentrations are too great, we can regard the water as polluted.
Measurements like this are known as chemical indicators of
water quality. Another way to measure water quality involves examining
the fish, insects, and other invertebrates that the water will support.
If many different types of creatures can live in a river, the quality is likely to
be very good; if the river supports no fish life at all, the quality is
obviously much poorer. Measurements like this are called biological
indicators of water quality.
What are the causes of water pollution?
Most water pollution doesn't begin in the water itself.
Take the oceans: around 80 percent of ocean pollution enters our seas from the land.
Virtually any human activity can have an effect on the quality of our
water environment. When farmers fertilize the fields, the chemicals
they use are gradually washed by rain into the groundwater or surface
waters nearby. Sometimes the causes of water pollution are quite
surprising. Chemicals released by smokestacks (chimneys) can enter the
atmosphere and then fall back to earth as rain, entering seas, rivers,
and lakes and causing water pollution. That's called atmospheric deposition.
Water pollution has many different causes and this is one of the reasons why it is such a
difficult problem to solve.
With billions of people on the planet, disposing of sewage waste
is a major problem. According to 2017 figures from the World Health Organization, some
2 billion people (about a quarter of the world's population) don't have access to
safe drinking water
or the most basic sanitation, 3.4 billion (60 people of the population) lack
"safely managed" sanitation
(unshared, with waste properly treated). Although there have been great improvements in securing access to clean water, relatively
little, genuine progress has been made on improving global sanitation in the last decade. 
Sewage disposal affects people's immediate environments and leads to
water-related illnesses such as diarrhea that kills 525,000 children under five each year.
(Back in 2002, the World Health Organization
estimated that water-related diseases could kill as many as 135 million people by 2020;
in 2019, the WHO was still
estimating the annual death toll from poor water and sanitation at over 800,000 people a year.)
In developed countries, most people have flush toilets that take
sewage waste quickly and hygienically away from their homes.
Yet the problem of sewage disposal does not end there. When you
flush the toilet, the waste has to go somewhere and, even after it
leaves the sewage treatment works, there is still waste to dispose of.
Sometimes sewage waste is pumped untreated into the sea.
Until the early 1990s, around 5 million tons of sewage was dumped
by barge from New York City each year. 
According to 2002 figures from the UK government's Department for
the Environment, Food, and Rural Affairs (DEFRA), the sewers of Britain collect around 11 billion liters
of waste water every day; there are still 31,000 sewage overflow pipes through which, in certain circumstances,
such as heavy storms, raw sewage is pumped untreated into the sea.
The New River that crosses the border from Mexico
into California once carried with it 20–25 million gallons (76–95 million
liters) of raw sewage each day; a new waste water plant on the US-Mexico border, completed in 2007, substantially
solved that problem.
 Unfortunately, even in some of the richest nations, the
practice of dumping sewage into the sea continues. In early 2012,
it was reported that the tiny island of Guernsey (between Britain and France) has decided to continue dumping 16,000 tons of raw sewage
into the sea each day.
In theory, sewage is a completely natural substance that should be
broken down harmlessly in the environment: 90 percent of sewage is water.
In practice, sewage contains all kinds of other chemicals, from the pharmaceutical drugs people take to
the paper, plastic, and other wastes they flush down their toilets.
When people are sick with viruses, the sewage they produce carries
those viruses into the environment. It is possible to catch illnesses
such as hepatitis, typhoid, and cholera from river and sea water.
Photo: During crop-spraying, some chemicals will drain into the soil. Eventually, they seep into
rivers and other watercourses. Photo courtesy of US Department of Agriculture Agricultural Research Service (ARS).
Suitably treated and used in moderate quantities, sewage can be a
fertilizer: it returns important nutrients to the environment, such as
nitrogen and phosphorus, which plants and animals need for growth. The
trouble is, sewage is often released in much greater quantities than
the natural environment can cope with. Chemical fertilizers used by
farmers also add nutrients to the soil, which drain into rivers and
seas and add to the fertilizing effect of the sewage. Together, sewage
and fertilizers can cause a massive increase in the growth of algae or
plankton that overwhelms huge areas of oceans, lakes, or rivers. This
is known as a harmful algal bloom (also known as an HAB or red
tide, because it can turn the water red). It is harmful because it
removes oxygen from the water that kills other forms of life, leading
to what is known as a dead zone. The Gulf of Mexico has one of
the world's most spectacular dead zones. Each summer, according
to studies by the
NOAA, it typically grows to an area of around 5500–6500 square miles (14,000–16,800 square kilometers), which is
about the same size as the state of Connecticut. 
A few statistics illustrate the scale of the problem that waste
water (chemicals washed down drains and discharged from factories) can
cause. Around half of all ocean pollution is caused by sewage and waste
water. Each year, the world generates perhaps 5–10 billion tons of industrial
waste, much of which is pumped untreated into rivers, oceans, and other
 In the United States alone, around 400,000 factories take
clean water from rivers, and many pump polluted waters back in their
place. However, there have been major improvements in waste water
treatment recently. Since 1970, in the United States, the
Environmental Protection Agency (EPA) has invested about $70 billion in improving water treatment plants that, as of 2021, serve around 90 percent of the US population (compared to just 69 percent in 1972). However, another $271 billion is still needed to update and upgrade the system.
Factories are point sources of water pollution, but quite a lot of
water is polluted by ordinary people from nonpoint sources; this is how
ordinary water becomes waste water in the first place.
Virtually everyone pours chemicals of one sort or another down their
drains or toilets. Even detergents used in
washing machines and
dishwashers eventually end up in our rivers and oceans. So do the
pesticides we use on our gardens. A lot of toxic pollution also enters
waste water from highway runoff. Highways are typically covered
with a cocktail of toxic chemicals—everything from spilled fuel and
brake fluids to bits of worn tires (themselves made from chemical
additives) and exhaust emissions. When it rains, these chemicals wash
into drains and rivers. It is not unusual for heavy summer rainstorms
to wash toxic chemicals into rivers in such concentrations that they
kill large numbers of fish overnight. It has been estimated that, in
one year, the highway runoff from a single large city leaks as much oil
into our water environment as a typical tanker spill.
Some highway runoff runs away into drains; others can pollute groundwater or accumulate in the land next to a road, making it increasingly toxic as the years go by.
Detergents are relatively mild substances. At the opposite end of the
spectrum are highly toxic chemicals such as polychlorinated
biphenyls (PCBs). They were once widely used to manufacture
electronic circuit boards, but their harmful effects have now been recognized and their use is highly restricted in many countries.
Nevertheless, an estimated half million tons of PCBs were discharged
into the environment during the 20th century.
 In a classic
example of transboundary pollution, traces of PCBs have even been found
in birds and fish in the Arctic. They were carried there through the
oceans, thousands of miles from where they originally entered the
environment. Although PCBs are widely banned, their effects will be
felt for many decades because they last a long time in the environment
without breaking down.
Another kind of toxic pollution comes from heavy metals, such as
lead, cadmium, and mercury. Lead was once commonly used in gasoline
(petrol), though its use is now restricted in some countries. Mercury
and cadmium are still used in batteries (though some brands
now use other metals instead). Until recently, a highly
toxic chemical called tributyltin (TBT) was used in paints to
protect boats from the ravaging effects of the oceans. Ironically,
however, TBT was gradually recognized as a pollutant: boats painted
with it were doing as much damage to the oceans as the oceans were
doing to the boats.
The best known example of heavy metal pollution in the oceans took
place in 1938 when a Japanese factory discharged a significant amount
of mercury metal into Minamata Bay, contaminating the fish stocks
there. It took a decade for the problem to come to light. By that time,
many local people had eaten the fish and around 2000 were poisoned.
Hundreds of people were left dead or disabled.
People view radioactive waste with great alarm—and for good reason.
At high enough concentrations it can kill; in lower concentrations it
can cause cancers and other illnesses. The biggest sources of
radioactive pollution in Europe are two factories that reprocess waste
fuel from nuclear power plants:
Sellafield on the north-west coast of Britain and Cap La Hague on the
north coast of France. Both discharge radioactive waste water into the
sea, which ocean currents then carry around the world. Countries such
as Norway, which lie downstream from Britain, receive significant
doses of radioactive pollution from Sellafield. 
The Norwegian government has repeatedly complained that Sellafield has
increased radiation levels along its coast by 6–10 times.
Both the Irish and Norwegian governments continue to press for
the plant's closure.
Photo: Oil-tanker spills are the most spectacular forms of pollution and the ones that catch public attention,
but only a fraction of all water pollution happens this way. Photo courtesy of US Fish & Wildlife Service Photo Library.
When we think of ocean pollution, huge black oil slicks often spring
to mind, yet these spectacular accidents represent only a tiny fraction
of all the pollution entering our oceans.
Even considering oil by
itself, tanker spills are not as significant as they might seem: only
12 percent of the oil that enters the oceans comes from tanker accidents; over
70 percent of oil pollution at sea comes from routine shipping and from the
oil people pour down drains on land.
 However, what makes tanker spills
so destructive is the sheer quantity of oil they release at once — in
other words, the concentration of oil they produce in one very
localized part of the marine environment. The biggest oil spill in
recent years (and the biggest ever spill in US waters)
occurred when the tanker Exxon Valdez broke up in
Prince William Sound in Alaska in 1989. Around 12 million gallons (44
million liters) of oil were released into the pristine wilderness—enough to fill your living room 800 times over!
Estimates of the marine animals killed in the spill
vary from approximately 1000 sea otters and
34,000 birds to as many as 2800 sea otters and 250,000 sea birds.
Several billion salmon and herring eggs are also believed to have been destroyed.
If you've ever taken part in a community beach clean, you'll know
that plastic is far and away the most common substance that washes up
with the waves. There are three reasons for this: plastic is one of
the most common materials, used for making virtually every kind of
manufactured object from clothing to automobile parts; plastic
is light and floats easily so it can travel enormous distances across
the oceans; most plastics are not biodegradable (they do not
break down naturally in the environment), which means that things like
plastic bottle tops can survive in the marine environment for a long time.
(A plastic bottle can survive an estimated 450 years in the ocean
and plastic fishing line can last up to 600 years.)
While plastics are not toxic in quite the same way as poisonous
chemicals, they nevertheless present a major hazard to seabirds, fish,
and other marine creatures. For example, plastic fishing lines and
other debris can strangle or choke fish. (This is sometimes
called ghost fishing.) About half of all
the world's seabird species are known to have eaten plastic residues.
In one study of 450 shearwaters in the North Pacific,
over 80 percent of the birds were found to contain plastic residues in their stomachs.
In the early 1990s, marine scientist Tim Benton collected
debris from a 2km (1.5 mile) length of beach in the remote Pitcairn islands
in the South Pacific. His study recorded approximately a thousand
pieces of garbage including 268 pieces of plastic, 71 plastic bottles,
and two dolls heads.
Today, much media attention focuses on the
Great Pacific Garbage Patch,
a floating, oceanic graveyard of plastic junk roughly three times the size of France,
discovered by sailor Charles J. Moore in 1997.
But, as you'll know well enough if you've ever taken part in a community beach
clean, persistent plastic litters every ocean on the planet: some
8 million tons of new plastic are dumped in the sea every single year.
Most people's idea of water pollution involves things like sewage,
toxic metals, or oil slicks, but pollution can be biological as well as
chemical. In some parts of the world, alien species are a major
problem. Alien species (sometimes known as invasive species)
are animals or plants from one region that have been introduced into a
different ecosystem where they do not belong. Outside their normal
environment, they have no natural predators, so they rapidly run wild,
crowding out the usual animals or plants that thrive there. Common
examples of alien species include zebra mussels in the Great Lakes of
the USA, which were carried there from Europe by ballast water (waste
water flushed from ships). The Mediterranean Sea has been invaded by a
kind of alien algae
called Caulerpa taxifolia. In the Black Sea, an alien jellyfish
called Mnemiopsis leidyi reduced fish stocks by 90 percent after
arriving in ballast water. In San Francisco Bay, Asian clams called Potamocorbula
amurensis, also introduced by ballast water, have dramatically
altered the ecosystem. In 1999, Cornell University's David Pimentel estimated that alien invaders like this cost the US economy $123 billion a year;
in 2014, the European Commission put the cost to Europe at €12 billion a year and "growing
all the time.
Photo: Invasive species: Above: Water hyacinth crowding out a waterway around an old fence post. Photo by Steve Hillebrand. Below: Non-native zebra mussels clumped on a native mussel.
Both photos courtesy of US Fish & Wildlife Service Photo Library.
Other forms of pollution
These are the most common forms of pollution—but by no means the
only ones. Heat or thermal pollution from factories and power
plants also causes problems in rivers. By raising the temperature, it
reduces the amount of oxygen dissolved in the water, thus also reducing
the level of aquatic life that the river can support.
Another type of pollution involves the disruption of sediments
that flow from rivers into the sea. Dams built for hydroelectric power
or water reservoirs can reduce the sediment flow.
This reduces the formation of beaches, increases coastal erosion (the
natural destruction of cliffs by the sea), and reduces the flow of
nutrients from rivers into seas (potentially reducing coastal fish
Increased sediments can also present a problem. During construction work, soil, rock, and other fine powders sometimes enters nearby rivers in large quantities, causing it to become turbid (muddy or silted). The extra sediment can block the gills of fish, effectively suffocating them.
Construction firms often now take precautions to prevent this kind of pollution from happening.
What are the effects of water pollution?
Some people believe pollution is an inescapable result of human
activity: they argue that if we want to have factories, cities, ships,
cars, oil, and coastal resorts, some degree of pollution is almost
certain to result. In other words, pollution is a necessary evil that people
must put up with if they want to make progress. Fortunately, not
everyone agrees with this view. One reason people have woken up to the
problem of pollution is that it brings costs of its own that
undermine any economic benefits that come about by polluting.
Take oil spills, for example. They can happen if tankers are too
poorly built to survive accidents at sea. But the economic benefit of
compromising on tanker quality brings an economic cost when an oil
spill occurs. The oil can wash up on nearby beaches, devastate the ecosystem,
and severely affect tourism. The main problem is that the people who
bear the cost of the spill (typically a small coastal community) are
not the people who caused the problem in the first place (the people
who operate the tanker). Yet, arguably, everyone who puts gasoline
(petrol) into their car—or uses almost any kind of petroleum-fueled
transport—contributes to the problem in some way. So oil spills are a
problem for everyone, not just people who live by the coast and tanker
Sewage is another good example of how pollution can affect us all.
Sewage discharged into coastal waters can wash up on beaches and cause
a health hazard. People who bathe or surf in the water can fall ill if
they swallow polluted water—yet sewage can have other harmful effects
too: it can poison shellfish (such as cockles and mussels) that grow
near the shore. People who eat poisoned shellfish risk suffering from
an acute—and sometimes fatal—illness called paralytic shellfish
poisoning. Shellfish is no longer caught along many shores because
it is simply too polluted with sewage or toxic chemical wastes that
have discharged from the land nearby.
Pollution matters because it harms the environment on which people
depend. The environment is not something distant and separate from our
lives. It's not a pretty shoreline hundreds of miles from our homes or
a wilderness landscape that we see only on TV. The environment is
everything that surrounds us that gives us life and health. Destroying
the environment ultimately reduces the quality of our own lives—and
that, most selfishly, is why pollution should matter to all of us.
How can we stop water pollution?
There is no easy way to solve water pollution; if there were, it
wouldn't be so much of a problem. Broadly speaking, there are three
different things that can help to tackle the problem—education, laws,
and economics—and they work together as a team.
Making people aware of the problem is the first step to solving it.
In the early 1990s, when surfers in Britain grew tired of catching illnesses
from water polluted with sewage, they formed a group called Surfers
Against Sewage to force governments and water companies to clean up
their act. People who've grown tired of walking the world's polluted
beaches often band together to organize community beach-cleaning
sessions. Anglers who no longer catch so many fish have campaigned for
tougher penalties against factories that pour pollution into our
rivers. Greater public awareness can make a positive difference.
One of the biggest problems with water pollution is its
transboundary nature. Many rivers cross countries, while seas span
whole continents. Pollution discharged by factories in one country with
poor environmental standards can cause problems in neighboring
nations, even when they have tougher laws and higher standards.
Environmental laws can make it tougher for people to pollute, but to be
really effective they have to operate across national and international
borders. This is why we have international laws governing the oceans,
such as the 1982 UN Convention on the Law of the Sea (signed by over
120 nations), the 1972
London (Dumping) Convention, the 1978
MARPOL International Convention for the Prevention of Pollution from Ships,
and the 1998 OSPAR Convention for the Protection of the Marine
Environment of the North East Atlantic. The European Union has
water-protection laws (known as directives) that apply to all of its
member states. They include the 1976 Bathing Water Directive (updated 2006), which seeks to ensure the quality of the waters that people use for
recreation. Most countries also have their own water pollution laws. In
the United States, for example, there is the 1972
Clean Water Act
and the 1974 Safe Drinking Water Act.
Most environmental experts agree that the best way to tackle
pollution is through something called the polluter pays principle.
This means that whoever causes pollution should have to pay to clean it
up, one way or another. Polluter pays can operate in all kinds of ways.
It could mean that tanker owners should have to take out insurance that
covers the cost of oil spill cleanups, for example. It could also mean
that shoppers should have to pay for their plastic grocery bags, as is
now common in Ireland, to encourage recycling and minimize waste. Or it
could mean that factories that use rivers must have their water inlet
pipes downstream of their effluent outflow pipes, so if they cause
pollution they themselves are the first people to suffer. Ultimately,
the polluter pays principle is designed to deter people from polluting
by making it less expensive for them to behave in an environmentally
Our clean future
Life is ultimately about choices—and so is pollution. We can live
with sewage-strewn beaches, dead rivers, and fish that are too
poisonous to eat. Or we can work together to keep the environment clean
so the plants, animals, and people who depend on it remain healthy. We
can take individual action to help reduce water pollution, for example,
by using environmentally friendly detergents, not pouring oil down
drains, reducing pesticides, and so on. We can take community action
too, by helping out on beach cleans or litter picks to keep our rivers
and seas that little bit cleaner. And we can take action as countries
and continents to pass laws that will make pollution harder and the
world less polluted. Working together, we can make pollution less of a
problem—and the world a better place.