A great deal of water pollution happens not from one single source but from many different scattered sources. This is called nonpoint-source pollution. 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. 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. 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. 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. 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 figures from the World Health Organization the most recent available at the time this article was updated in , some 2. Sewage disposal affects people's immediate environments and leads to water-related illnesses such as diarrhea that kills , children under five each 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 s, around 5 million tons of sewage was dumped by barge from New York City each year. In early , it was reported that the tiny island of Guernsey between Britain and France has decided to continue dumping 16, 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.
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. 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 grows to an area of around — square miles 14,—15, 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 waterways. However, there have been major improvements in waste water treatment recently.
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. 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 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 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. 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. 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 Around 12 million gallons 44 million liters of oil were released into the pristine wilderness—enough to fill your living room times over!
Estimates of the marine animals killed in the spill vary from approximately sea otters and 34, birds to as many as sea otters and , sea birds.