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.
It is possible to broadly subdivide pollutants according to the effects they have on the water system and to the degree of harm they do to the environment: pollutants which are naturally decompose and/or become transformed within natural materials cycles, such as biodegradable organic substances, ammonia, nitrates, phosphate, fluorides; toxic or harmful pollutants which do not accumulate in living organisms, such as slow biodegradable organic substances, cyanides, phenols, mineral oils, aldehydes, surfactants, boron and zinc; pollutants such as mercury, lead, pesticides and solvents, which not only have a pronounced toxic effect, but may also accumulate in organisms and pass from one trophic level to another; and other hazardous pollutants such as pathogens, phenols, viruses, radioactive substances, paints and dyes.
During the 1970s, the pollution that caused most concern was due to sewage, agricultural chemicals, oil, and metals. Metal concentrations were clearly elevated in coastal waters, and in fish and shellfish living there. In some areas, mercury levels in species such as tuna were high enough to make these fish unsuitable as human food. Chemical contamination of the oceans appeared to be localized, with the worst conditions in estuaries and coastal areas in industrial regions, where ecological changes were apparent. Some of this pollution came via rivers: the amount of iron, manganese, copper, zinc, lead, tin and antimony that reached the sea by this route was far greater than would be supplied by natural geological processes. Other contaminants came through atmospheric deposition: the importance of this pathway for metals and synthetic chemicals was increasingly recognized during the decade. Offshore oil and gas exploration and dredging for sand and gravel in coastal areas also increased during the decade. Coastal zone development affected extensive estuarine areas, as well as mangrove swamps and coral reefs. Oil pollution killed sea-birds, fouled beaches and affected tourism. Although tanker accidents were the source of less than 5% of all the oil entering the sea, accidents released large volumes in small areas, and were therefore especially damaging.
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 muncipal and industrial waste pollutes the Sio, Nitra, Morava and Narew rivers of Eastern Europe.
2. If you put a spoonful of water in a barrel full of sewage, you get sewage. If you put a spoonful of sewage in a barrel full of water, you get sewage.