We can classify the water sources in 4 categories.              

Surface water                                            : – Rivers, lakes, ponds, springs etc.

Underground water                               : – Wells (open and bore wells).

Rain water                                                   : – Water which precipitates as rain snow and sleet.

Sea water                                                     : – Water in all the oceans and sea.

Among the above mentioned categories Rain water is known to be the purest one, as it is formed by the evaporation of water from the surface. But its purity is available only above the atmosphere. When the rain enters atmosphere atmospheric gases such as CO2, NO2 and SO2 gets dissolved along with airborne bacteria and other microorganisms which can be pathogenic (disease causing). Certain amount of organic and inorganic suspended solids also accompanies the rain water while passing through the atmosphere. Hence when the rain water reaches the surface of h earth it is contaminated with various unwanted impurities.

The trusted source of water supply for all our practical usage is surface and underground water. These two sources provide the required water for all kinds of requirements. Normally surface water will be having a greater concentration of organic matter by the decomposition of plant and animal matter. Also it is known to contain minerals from soil. The surface water is fed by springs and rain. The surface water is very often fed by melting of ice to a certain extent. This surface water contains pathogenic microorganisms too. For all practical purposes the surface water can be used with little or no treatment depending upon the source and water quality. Ground water or Underground water is relatively pure. This underground water is collected in a naturally occurring space known as Water Table. This Water Table is related with rain water. We will be discussing the relation in the later part. When the rain water reaches earth’s surface some part of it will percolate in the soil and finally reaches the underground Water Table. During this process it dissolves many minerals from the soil. As the water moves down it gets filtered by the soil and sedimentary rocks. This natural filtration enhances the purity of water by reducing suspended solids. However the dissolved minerals will be present in the water.

Sea water is unusable for all practical purposes without treatment even though it’s available in plenty. Sea water is known as the most impure water. Sea water contains more amounts of dissolved minerals in it. All the rivers join the sea ultimately because of which the amount of minerals in sea water increases. Most of the evaporation takes place from the surface of the sea. During this evaporation only pure water gets evaporated leaving behind the salts or dissolved minerals. This enhances he salt or mineral content of water rendering the water unusable for most of the applications.