Water is said to be polluted when its quality or composition is changed as a result of man's activities.
Water is polluted in various ways by dumping factory,nuclear, household and radioactive wastes into water. It leads to the death of marine organisms. Hot water discharged into rivers leads to algal bloom which depletes the oxygen content in water.
Water can be conserved in the following ways:
i) Do not keep the taps running,  while shaving, washing etc. regularly check the pipes for leakage.
ii) Fill as much water in the washing machine as needed,
iii) Close taps tightly after use,
iv) Water the plants using a bucket rather than a pipe. Water the plants in the evening when evaporation loses are the minimum,
v) Use drip or sprinkle irrigation to improve irrigation efficiency and reduce evaporation,
vi) Build rain water harvesting in the house,
vii) Install water saving toilets.

The preventive measures are:
i) Use of herbicides, insecticides, fungicides and pesticides should be limited.
ii) Proper drainage for industrial for industrial, human and other waste should be provided. Domestic waste water and various industrial effluents should be treated suitably before releasing them into the water bodies. Effluents from industries should not be allowed to flow into agricultural fields.
iii) Excessive chemicals should not be used for water purification. Large scale water purification methods should be adopted.

Some measures undertaken by Government are:
Save Ganga Plan
Removal of industries near to water bodies,
Filtration of wastes expelled into the water bodies.