Pure uncontaminated water dose not occur in nature. The natural water contain impurities both natural and man made. The natural impurities are not dangerous like man made impurities. The natural impurities are dissolved gas (nitrogen, carbon dioxide, hydrogen sulphide which are picked up during rain), and dissolved minerals (salts of calcium, magnesium and sodium). The minerals are due to contact of water with soil. Natural water also contain suspended impurities (clay, sand, mud, silt etc.) and microscopic organisms (bacteria, virus, planktons, fungi etc). All the natural impurities are derived from the soil, catchments area and atmosphere.
Water pollution by man made containments are more serious which is generally due to human actives mainly urbanization and industrialization. The sources of man made water pollution are
(1) Sewage: This contain pathogenic organism and organic (decomposable) matter. Main constituents of sewage is human excreta with water.
(2)Industrial and trade wastes: These contain toxic agents from synthetic, organic compounds to metals and metal salts.
(3) Agriculture pollutants: These are mainly chemical fertilizers and pesticides. In many parts of the world like India and Kazakhstan due to uncontrolled use of pesticide and fertilizers(chemical) their content in soil and water has crossed normal acceptable limits, which is not harmful to health of general population.
(4) Physical pollutants: Heat (thermal pollution) and radioactive substance are present in water.
Indicators of water pollution:
The following are the indicators of water pollution, the total amount of suspended solids, concentration of chlorides, BOD (Biological Oxygen Demand) at 20 degree centigrade, nitrogen, Phosphorus and absence of dissolved Oxygen.
In presence of high quality treatment (purification) of water, pollution of water may still be there due to corrosion of pipelines, leaky joints and due to cross connection of water supply pipe with that of sewage drain. That is why surveillance is required at every point in the water supply system to ensure safe drinking water to the consumer.