During recent years, there has been an increasing awareness of, and concern about, water pollution all over the world, and new approaches towards achieving sustainable exploitation of water resources have been developed internationally.
It is widely agreed that a properly developed policy framework is a key element in the sound management of water resources. With the rapid growth of population, industrialization and urbanisation, environmental pollution has greatly increased. The major sources of water pollution arise from many activities.
They include domestic, industrial and agricultural wastes. Domestic waste contains pathogenic organisms which are responsible for the spread of communicable diseases such as cholera, typhoid, dysentery and other gastrointestinal diseases.
These wastes also contain materials which are responsible for obnoxious conditions and irreparable damage to aesthetics of land and water environment. Industrial wastes contain a wide variety of organic substances and minerals including cyanides, arsenic, mercury, and cadmium, carcinogens which are toxic to human and plant life. Agricultural drainage carries dangerous pesticide residues and unused fertilizer ingredients.
The term 'water pollution' can be briefly defined as any alteration in physical, chemical or biological properties of water, rendering the water harmful to public health and safety. For convenience, the sources of contamination of water can be classified as natural and anthropogenic (man-made).
Important natural sources are surface run-off, seepage from ground water and swamp drainage. In urban areas, rain water is reported to be acidic. This is due to reaction between water droplets and atmospheric oxides of sulphur and nitrogen. The atmospheric sulphur dioxide (S02) is always accompanied by a little amount of sulphur tri-oxide (S03) which, under humid condition, reacts with water vapour to form sulphuric acid thus causing acid rain. The chemical reaction involved in this process is represented as follows:
Leachates from animal excreta, decaying bodies of animals and plants, solid waste landfill sites and the decay of large quantities of organic matter in swamps or deep ponds also introduce appreciable amounts of soluble organics and microorganisms which in turn contaminate the adjacent ground water.
Anthropogenic sources are the result of industrial, domestic, agricultural and mining activities of man.
Nowadays, industries are the major contributors of water pollution. Water is an essential raw material in almost all manufacturing plants. In India, industries such as tanneries, sugar mills, pulp and paper mills, distilleries, oil refineries, etc. generate a large quantity of wastewater which is discharged into natural waterways either without treatment or after partial treatment. The characteristics of industrial wastewater depend primarily on the type of industry and the chemicals used in various processes.
In urban areas, municipal sewage is discharged into the nearby canal, thus polluting the canal and also deteriorating the ground water. Municipal sewage includes wastewater from houses, commercial buildings and institutions. The important pollutants present are biodegradable organic matter, coliforms and pathogens.
Pollutants discharged into water courses due to agricultural activities include:
1. Soil and silt removed by erosion
2. Agricultural run-off
3. Synthetic fertilizers, herbicides and insecticides
4. Plant residue.
Receiving water bodies get fertilised with nutrients, thus resulting in Eutrophication. Some common insecticides in use are chlorinated hydrocarbons such as DDT (dichloro diphenyl trichloroethane), aldrin, heptachlor, PCBs (polychlorinated biphenyl) etc. Most of the chlorinated hydrocarbons are persistent to degradation and hence remain in the environment for a very long time. Indiscriminate use of insecticides could make them an integral part of the biological, geological and chemical cycles of the earth. Measurable quantities of DDT residues may be found in air, soil and water several thousand kilometres away from the point where it originally entered the ecosystem.
Natural or man-made geochemical alterations are also sources of wastewater pollution. Fines from ore washings disposed off in water suspension may be transferred to the natural water bodies to pollute them in due course. Mining operation also produces soluble toxic materials depending on the geological formation. Acid drainage from coal mines and arsenic residue from gold mines are some of the burning problems of environmental concern.