When toxic substances enter lakes, streams, rivers, oceans and other water bodies, they get dissolved or lie suspended in water. This causes pollution of water bodies, which degrade the quality of water so that either becomes health hazard or unfit for use and known as water pollution.

It is defined as deterioration of physical, chemical and biological properties of water brought about mainly by human activities and natural activities which causes harmful effects on human, plants and aquatic life.

Example: Domestics sewage, waste water discharge, industrial effluents, surface runoff water into river and sea.