79.

What are the major causes of water pollution? Explain. 

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Water pollution may be caused in number of ways:

1. Organic pollutants: Organic pollutants such as manures, wastes from food processing, rags, paper discards, decaying plants etc. cause pollution of water. These are decomposed by aerobic bacteria into carbon dioxide, nitrates, sulphates, phosphates etc but they take up dissolved oxygen from water. As a result, the oxygen content in water decreases considerably. This causes the death of aquatic animals such as fish. On the other hand, anaerobic bacteria do not need oxygen for the decomposition of organic matter, however, some toxic gases like hydrogen sulphide, ammonia, phosphine, methane etc. are produced which give an offensive smell. These are quite often noticed in the sewage waste.
The amount of the oxygen consuming waste in water can be determined by biological oxygen demand (BOD).
In addition to the organic waste products, certain other chemical substances react with oxygen dissolved in water and this is known as chemical oxygen demand (C.O.D.). Thus if BOD or COD values of water are large, it indicates that water is heavily polluted.

2. Industrial wastes: Industrial wastes such as compounds of lead, mercury,cadmium, nickel, cobalt, zinc etc. which are the products of chemical reactions, also pollute water to a large extent and are responsible for many diseases, Mercury leads to Minamata disease.

3. Fertilisers: Some fertilisers react with water chemically and pollute the underground water. When this water is used for drinking purposes containing potassium nitrate, in particular, it harms the respiratory system.

4. Pesticides: Pesticides dissolve in water to a small extent and pollute it. Since all the pesticides are toxic in nature, they are injurious to both plants and animals.