Sewage and other objectionable organic matter in water are normally broken down and rendered harmless by beneficial bacteria and other microorganisms living there. When pollutants in enormous amounts are added to a body of water, however, the water's free oxygen supply is depleted, killing off the beneficial organisms. The water is no longer self-cleansing, other forms of life die out, and it eventually becomes biologically dead.

Some pollutants are non-biodegradable—that is, they cannot be broken down by natural biological processes. Examples are certain pesticides, agricultural fertilizers, radioactive material, oil discharges from ships and boats, and various chemicals.

Two types of pollutants fall to earth in a form of precipitation called acid rain. The burning of fossil fuels (coal and oil) by industrial plants releases sulfur dioxide and nitrogen oxide into the air. These oxides then undergo a chemical reaction to become sulfuric acid and nitric acid. These acids are washed out of the air by rain, sleet, or snow and destroy plant and animal life in lakes and ponds. Other pollutants, such as phosphates, provide an excess of plant nutrients, stimulate excessive growth of water plants, and disturb the ecological balance of the waters. Heated water, discharged primarily by the electric power industry, causes thermal pollution. This occurs when water used for industrial cooling is returned to lakes and streams at high temperatures. Not only are most water plants and animals extremely sensitive to even a slight change in temperature, but at high temperatures water cannot hold enough free oxygen.

Efforts to halt water pollution center mainly on the construction of improved sewage treatment plants; the prohibition of the discharge of industrial wastes, municipal wastes, and heated water; and the reduction of the sulfur dioxide content of airborne emissions from industrial plants.