Water pollution is caused by the discharge of harmful substances into rivers, lakes and seas.
The increasing human population has led to an increase in pollution. Some of this is due to:
Nitrate fertilisers are very soluble in water. They are easily washed off fields by rain, and then into rivers and lakes. Because nitrates are soluble they cannot easily be removed from the water. Pesticides used by farmers include herbicides (to kill weeds) and insecticides (to kill insects). These substances may be washed or blown into rivers and lakes. Sulfur dioxide in the air can dissolve in water to form an acidic solution.
Human sewage is another source of water pollution. If untreated sewage gets into rivers, microorganisms decompose it. They use oxygen from the water for aerobic respiration. As a result there is less oxygen dissolved in water, so aquatic organisms such as fish and insects may be unable to survive.
In sewage treatment works, lots of oxygen is provided by stirring the waste or by injecting jets of compressed air. This allows the microorganisms to break down the waste completely before the treated water is discharged into a river or the sea.
Many aquatic invertebrate animals cannot survive in polluted water, so their presence or absence indicates the extent to which a body of water is polluted.