If water runs downhill to the sea, it carries chemicals (fertilizers and pesticides) which cause water pollution.
Some of the important sources of water pollution are: (i) Domestic effluents and sewage, (ii) Industrial effluents, (iii) Agricultural effluents, (iv) Radioactive wastes, (v) Thermal pollution, and (vi) Oil pollution.
(i) Domestic Effluents and Sewage:
Man, for his various domestic purposes such as drinking, cooking, bathing, cleaning, cooling, etc., uses on an average 135 litres of water per day. About 70 to 80 per cent of this is discharged and drained out, which through municipal drains poured into, in many cases, a river, tank or lake.
This water is known as domestic waste water and, when other waste material such as paper, plastic, detergents, cloth, etc., is mixed in it; it becomes municipal waste or sewage.
The domestic waste water and sewage is the main source of the water pollution. This is the inevitable and unfortunate fallout of urbanisation. This organic waste depletes the oxygen from water and upsets the natural balance of the aquatic ecosystem.
Municipal sewage is considered to be the main pollutant of water. Most of the sewage receives no treatment before discharge, especially in developing countries like India. In Delhi alone, 120 crore litres of water is consumed per day, out of which 96 crore litres is drained into the Yamuna river through 17 big drains. In the same manner, all the 47 towns located on the bank of river Ganga drain their sewage into it.
With the growth of population, the quantity of waste water is also increasing in addition to the production of large quantities of sewage. Sewage contains decomposable organic matter and exert an oxygen demand on the receiving waters.