Even though three fourth of the earth’s surface is covered with water, the water available for the ordinary use of everyday life is scarce. This limited reserve of fresh water is polluted in several ways.

Biological contamination of water is caused by the presence of disease-causing micro organisms. To remove these micro organ­isms disinfectants such as chlorine are used. Thermal pollution is caused by discharging the coolant water and other industrial waste water into rivers and lakes.

Sedimentary pollution is the result of accumulation of suspended particles. They can be particles of soil washed into the rivers or lakes or particles of insoluble organic and inorganic chemicals car­ried by waste water. These sediments block the sunlight penetrating into the water and affect the photosynthesis of aquatic plants. Sedi­ments also carry absorbed chemicals and biological pollutants with them. Sediments account for the greatest mass of pollutants and generate the greatest amount of water pollution. Sediments are re­moved by filtering, settling and coagulation.

Chemical pollution is caused by the presence of harmful or un­desirable chemicals. Of all the different forms of pollution it is the most subtle and obscure. Thermal pollution generally does not af­fect pot ability of water. Sedimentary pollution is highly visible and can be easily removed. Biological pollution can be overcome by simple methods. Further, their presence is manifest almost immedi­ately or through the outbreak of an epidemic. But chemical pollu­tion is not so simple. The harmful effects of chemical pollution may take time to make they felt.

Some chemicals can be advantageous to one class of living things and at the same time detrimental to another. Agricultural fertilizers, waste of animal farms and sewage are advantageous to plant life. When these chemicals enter the still waters of lakes and slow moving streams, they produce a rapid growth of surface plant life. They form a mat that can cover the surface shielding off the rest of the water from the oxygen of the air. Getting deprived of dis­solved oxygen, fish and other aquatic animals disappear from these waters.

Many of the wastes dumped into the hydrosphere contain or­ganic compounds which nourish the micro organisms in water. The enhanced growth of these micro organisms consumes large amount of dissolved oxygen. This condition results in lack of oxygen for other living organisms. By measuring the biological oxygen demand in a body of water, we can estimate the amount of organic material present and pollute the water.

The ground water stored in aquifers, contain many chemical com­pounds dissolved in it. As the rain water percolates down into the aquifers, pollutants that lie on the ground get dissolved. An aquifer is a large layer of porous rock that holds fresh water.

The only way by which we can safeguard the purity of water is to keep pollutants off the surface of the land as well as water.