Pollution immediately or eventually involves the hydrological cycle of the earth, because even pollutants emitted into the air and those present in the soil are washed out by precipitation. Water is considered polluted when it is altered in composition or condition so that it becomes less suitable for any or all of the functions and purposes for which it would be suitable in its natural state. This definition includes changes in the physical, chemical and biological properties of water, or such discharges of liquid, gaseous or solid substances into water as will or are likely to create nuisances or render such waters harmful to public health, safety or welfare, or to domestic, commercial, industrial, agricultural, fish or other aquatic life. It also includes changes in temperature, due to the discharge of hot water.
Pollution may be accidental (sometimes with grave consequences) but is most often caused by the uncontrolled disposal of sewage and other liquid wastes resulting from domestic uses of water, industrial wastes containing a variety of pollutants, agricultural effluents from animal husbandry and drainage of irrigation water, and urban run-off. The deliberate spreading of chemicals on the land to increase crop yields, or the addition of chemicals to water to control undesirable organisms, is another cause of pollution. Examples are the application of chemical fertilizers, and of pesticides for the control of aquatic weeds, insects and molluscs. Problems are compounded when national boundaries are involved, and cooperation in the management of transboundary waters is becoming essential.
The developing countries are the scene of rapid urbanization and industrial development; and there is a growing demand for water for domestic and industrial purposes, as well as an increase in water pollution, which tends to reduce the available water resources. Water pollution is a particularly acute problem in countries that have scanty water resources, and many of the developing countries are in this situation. Some include arid regions, and in others the rain falls only during a short season and the water cannot be economically stored; much of it runs away to the sea, and much is lost by evaporation. If, in addition to these losses, the natural water resources become increasingly unusable because of pollution, the net reserves of the country are continually reduced. The control of pollution is thus linked closely with the management of water resources. In some areas, ground water has been contaminated by domestic sewage and industrial effluents to such an extent that it has had to be abandoned as a source of supply. If it is not too greatly polluted, water undergoes self-purification, but when pollution is excessive this process is slow and uncertain.
In 1980 four out of five child deaths in the third world resulted from disease from dirty water supplies; 80% of people in developing countries have no sanitation facility. Water shortage and contamination kill 25,000 people a day. More than two thirds of India's water resources are polluted; 98% of China's sewage goes into rivers untreated, and in the Philippines domestic sewage makes up 70% of the Pasig River in Manila. About 2% of the groundwater supplies in the USA are polluted. Japan's Inland Sea has 200 red tides annually. Red tides are caused by the decaying of algae sapping large amounts of oxygen from the water, asphyxiating fish and other marine life. Commercial fishermen dump 22,000 metric tons of plastic packaging into the sea every year, along with 136,000 tons of plastic nets, lines and buoys. Mainly municipal and industrial waste pollutes the Sio, Nitra, Morava and Narew rivers of Eastern Europe.