Unit D6 - Water Pollution


Unit D6


Water is essential to all life, and living things are mostly made of water. This is because water is a very good solvent, which means that many kinds of materials can dissolve or wash away in it. Sea water is salty because salts are dissolved in it. The beverages we drink are water with something dissolved in it. When we wash something, the water dissolves and carries away the dirt. Because water is good at dissolving things, it also helps to transport them in dissolved form from one place to another.

These qualities which make water so useful also make it easy for water to be polluted, or made dirty with things that are dangerous or harmful. Often pollution comes from man-made substances or things produced by man's activities that prevent the water from being used naturally. Since nothing can live without water, anything that makes water unusable or dangerous is very serious, and can quickly lead to the death of people, plants or animals if other sources of clean water are not available.

Kinds and sources of water pollution

There are several kinds of water pollution that are important in rural areas. Each one comes from particular sources, and can be harmful in different ways.

Human wastes. Often water is contaminated with human wastes which may carry disease germs and parasites. Wherever people live, they must dispose of their wastes. In urban areas they may use water in toilets to carry these wastes away. Sometimes this water simply drains into a ditch, a stream or into the sea, carrying the pollution with it. More often the water carries the waste into a septic tank where the waste is digested by microbes and much of the pollution removed. Some cities, large hotels and modern buildings have more complicated waste treatment plants to purify their wastes. Unfortunately septic tanks and treatment plants require maintenance to work well, and if this is neglected they also can release polluted water. This happens much too often in villages and other rural areas.

In villages, there may be pit toilets or latrines, or people may simply take care of their needs in the bushes or on the beach. Rain or water runoff may carry these wastes into streams or the lagoon, or wash them down into the ground water where the pollution can get into wells and springs. Animal wastes may also carry disease germs which can get into water and pollute it.

These wastes not only carry diseases and other dangerous things, they also may be high in nitrogen nutrients. Too high a level of nutrients can make algae grow too fast in streams, lakes or lagoons, smothering other kinds of life and building up until they start to rot and kill the fish. If the nitrogen compounds get into drinking water, they can be turned into nitrites which have been linked to cancer in humans.

Washing water. Water is also used by people to bathe or shower in, to wash their clothes and dishes, and to clean their houses and work places. All that water has to go somewhere carrying all the dirt, germs, and soap, detergent or other cleaners. While such pollution is not as much of a health hazard as human wastes, it can still present some dangers. Detergents, for instance, contain phosphorus, which is another nutrient that can cause algae to grow too fast.

Storm water. Whenever there is a heavy rain, it washes the dust and dirt off the roofs and the streets. In towns and cities in particular, that dirt can contain pollutants such as oil spilled from cars and trucks, the poisonous metal lead from lead paints and the lead added to petrol (gasoline), and any other dangerous things that may have been spilled accidentally.

Sediments. When erosion occurs on the land, whether from agriculture, construction, forestry or mining, the fine particles of the soil become pollution in the water. The water turns cloudy or turbid, so fish can no longer see far, and sunlight cannot reach the plants on the bottom that make food for everything else. As the fine particles settle out, they form sediment or mud on the bottom, smothering corals and plants, and covering up the hard surfaces of the reef or rock that such things need to attach to. The ecosystem changes from a stable and productive coral reef or lagoon ecosystem to a much less stable and productive soft bottom community.

Industrial wastes. Factories and industries often use water to wash away their wastes. Fruit and fish processing plants or canneries, sugar refineries, slaughterhouses, coconut oil or oil palm treatment plants, and breweries, among others, produce waste water with lots of organic material from the plants or animals that are processed. As this organic matter is attacked by microbes and decays in the water, it uses up the oxygen; the water and sludge may then turn black and smell bad, and all the fish and other animals will be killed.

The mining and smelting industries sometimes use very poisonous chemicals like mercury or cyanide in their treatment processes. If a metal like copper is mined, it may itself be poisonous. Since it is not possible to stop if from raining at a mine or smelter, or to prevent all accidents, there is always some small but possibly dangerous amount that gets into the water, and big accidental spills can also occur. Since these poisons do not break down, they can go on poisoning the environment for a long time.

Many other industries use chemicals that can be dangerous water pollutants. Not only can chemicals from photographic processors, laundries and dry cleaners, paint factories, metal working or plating companies, etc. be poisonous or harmful in themselves, but if they get into septic tanks, waste treatment plants, streams or lagoons they can also slow or stop the microbes that would otherwise break down and purify organic wastes, making other water pollution problems worse.

Oil pollution. We depend on oil, and oil products such as kerosene and petrol (gasoline), for much of the energy used in modern rural life, as well as to keep motors and machines working properly. Unfortunately we cannot seem to help spilling some of it while we use it, and it can thus get into the water. Since oil products are often transported between countries by ship, the risks of oil spills are greatest in the vulnerable lagoons, harbours and coastal waters, either in accidents while unloading the oil, in spills from storage tanks or facilities, or more rarely in the grounding or sinking of ships carrying oil.

While there are some poisons in oil that can dissolve in the water, the worst effects of oil pollution come from the fact that oil floats on top of the water, sticking to things along the shore and covering animals (like sea birds) that go in and out of the water. Oil can smother animals, mangrove roots, and other seashore life. If waves break the oil up into small globs, it may be eaten by fish and shellfish, giving them an oily taste. Since oil breaks down very slowly, it can take a long time for oil pollution to go away.

Agricultural runoff. Modern agriculture frequently requires chemicals such as fertilizers and pesticides. Unfortunately these do not always stay in the fields where they are needed, but may wash off with the rain into streams or the lagoon. The fertilizers like other nutrients can cause algae to grow too fast, while the pesticides and herbicides are poisons that can kill or weaken sensitive animals or plants even in very tiny amounts. Both can upset the balance of life in productive natural environments.

Plastics. Another kind of danger comes from the plastics, bottles, plastic bags, pieces of fishnet, etc., which get thrown in the sea. Such plastics break down very slowly or not at all, and meanwhile they drift around causing trouble. Fishnets may go on catching and killing fish long after their owners have lost them. Small bits of plastic may be eaten by fish which think they are food. Some turtles have been known to die from eating too many plastic bags thinking they are jellyfish. All the plastic things on the beaches look ugly and can hurt tourism.

Fresh water pollution

Some rural areas have fresh water running down from the mountains or collecting on the land surface in streams, rivers and lakes. Since streams collect much of the rain that falls on the land, they receive the pollution that the rain picks up as it runs off the land and drains into them. This water is often used for drinking as well as for washing, bathing and swimming, so the pollution can hurt anyone who uses it. However, since this surface water is usually quickly replaced by new rainfall, stopping the sources of pollution can clean up the problem quickly.

The same is not true for ground water, which is the water that collects under the surface of the ground in spaces in the sand and rock. This fresh water comes from rainfall that seeps down into the ground. On small islands and some coastal areas on porous rock or sand, the ground water may float as a layer on top of the heavier sea water. As the rain soaks into the ground, it may take with it any pollution on the surface or in the soil. Once this pollution gets into the ground water, it may stay there for years and years, since ground water may run off into the sea and be replaced only very slowly. Ground water is the most important fresh water resource in many rural areas and islands, supplying wells, springs and water galleries. Pollution of this water can have a serious long-term effect on rural development, and can be expensive or impossible to correct.

Marine pollution

Pollution can reach the coastal waters of the marine environment either directly or by being carried down rivers or streams. Even though no one drinks sea water, it is used for fishing and recreation, and can thus be a direct threat to people who come in contact with it. Particular problems can come from big pollution accidents like shipping accidents or oil spills, which are rare but have occurred on a number of rural coasts and islands. Sometimes the pollution is intentional, as when someone goes fishing with poisons (usually illegally).

There have been cases where marine pollution has had a serious effect on a whole lagoon or coastal area. In Kaneohe Bay, Hawaii, sewage from housing developments drained into a bay with some of the best coral reefs in Hawaii. The nutrients in the wastes so fertilized a kind of green bubble algae that it grew all over everything and smothered and killed the corals. The bay was an ugly mess. However, when a new sewage treatment system was put in to keep the pollution from going into the bay, most of the algae died off and the corals recovered. Another case was the cholera epidemic on Tarawa in Kiribati. The microbes that cause cholera were brought to the island by one sick person, who began to spread the disease to others. Their wastes got into the ground water and the lagoon, where the germs were collected by shellfish that were a popular local food. Many people got sick very quickly and a few died. The people had to stop eating food from the lagoon or using untreated well water, and an expensive waste treatment system had to be installed. Other rural or coastal areas have also had cholera epidemics spread by water pollution.

Dangers of water pollution

As shown in the case of Tarawa Lagoon, water pollution can kill people by spreading diseases. It can also cause poisonings, and make food like fish taste bad, or even be dangerous to eat. Some kinds of pollution in water may not be strong enough to hurt us immediately, but they may slowly collect in our bodies until there is enough poison to make us sick. Since it happens slowly, we may not even know why we are sick. Water pollution is also ugly: oil and tar on the beaches; water a dirty brown or green colour; plastic litter. Such water pollution can be a threat to local people and damaging to the tourist industry.

Pollution also damages natural systems. It may hurt the productivity of mangroves, lagoons and coral reefs that are important coastal resources. It can upset the delicate balance between different kinds of life, as when excess nutrients encourage algae to grow.

Cleaning up water pollution

All wastes put in water that are not broken down along the way eventually find their way into the ocean, where they accumulate like the salt that makes sea water salty. Already some man-made pollutants from industrialized countries have contaminated all the world's oceans. Once a persistent pollutant is in the sea, it is almost impossible to remove, but because the oceans are so large, most such pollutants are diluted to the point that they are not a danger, at least at present. No one knows what effect they may have in the long term. Since most of the pollutants produced in rural coastal areas will eventually break down, disposal in the oceans away from the coast may be a reasonable solution if only small quantities are generated.

Fortunately, when water evaporates from the ocean to become the water vapour that makes clouds and rain, the pollutants stay behind. Unless the rain encounters air pollution (as in the case of the acid rain of Europe, North America and Asia) rain water is clean and pure, and it renews the rural fresh water supplies.

The best way to stop water pollution is to keep potential pollutants from getting into the water, or if the waste is the result of washing or some other use of water, to treat the water to remove the pollutants before it is released back into the natural environment. There are many ways to clean water, including filters of sand, or biological filters where microbes remove pollutants, or chemical additives to kill dangerous germs or to make chemical wastes turn solid and settle out of the water. Sand filters on water supplies, and septic tanks, waste digesters, oxidation ponds or reed beds for human and animal wastes are relatively simple techniques that can even be used in village areas where water pollution is a problem.


Why is water so easily polluted?

How do human wastes become a source of water pollution?

How do nutrients in water become pollutants? Where do they come from?

What are some of the industrial wastes in your country that might pollute water?

What are the risks of oil pollution where you are?

What agricultural chemicals are used in your country that might become pollutants?

If there are streams in your area, are they safe to drink from?

Do many people suffer from diarrhoea in your village or town?

If ground water is used in your area, what are the risks that it could be polluted? Are there other water supplies you could use?

Do you see signs of pollution on your shorelines and beaches?

What steps are taken in your area to prevent water pollution?

What more needs to be done?

Instructions for trainers in the use of this unit

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Last updated 1 January 2008