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Water pollution is one of the most serious environmental problems. It occurs
when water is contaminated by substances such as human and other animal wastes,
toxic chemicals, metals, and oils. Pollution can affect rain, rivers, lakes,
oceans, and the water beneath the surface of the Earth (groundwater).
Polluted water may appear clean or dirty, but all contains bacteria,
viruses, chemicals, or other materials that can cause illness or death. Impurities
must be removed before the water can be safely used for drinking, cooking, washing,
or laundering.
Major sources of pollution are industrial wastes, sewage, and agricultural
chemicals and wastes. Industrial wastes include the discharge of chemical waste
directly into natural water bodies, and the burning of coal, oil, and other fuels
by power plants, factories, and automobiles.
Industry also uses large water quantities to cool equipment, making the water
hot and causing thermal pollution when the water is discharged into rivers and
lakes; the heat harms plants and animals living near or in the water.
Most sewage goes through treatment plants to remove solids and dissolved
substances such as nitrogen and phosphorus. Some sewage does, however, go
untreated into waterways or the ocean; government regulations usually control the
amount and quality of the discharge.
Agricultural chemicals and wastes come from precipitation that carries chemical
fertilizers and pesticides, along with livestock wastes, into streams or the ground.
Water pollution can be controlled by treating sewage and industrial wastes,
and by following the Environmental Protection Agency's quality standards for
water systems.
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