The difficulty in defining water pollution lies in the fact that virtually no water is found on the earth in its absolutely purest form.
There are various impurities such as dissolved gases, minerals, suspended matter like clay, silt, sand, etc., besides microbes. But beyond a certain level, these could cause harm to the ecology. Water is said to be polluted when it is turbid, has an unpleasant smell, is not potable, and has the potential to cause diseases such as dysentry, cholera and typhoid.
According to the definition of the World Health Organisation (WHO) (1966), water pollution occurs when “foreign materials either from natural or other sources are contaminated with water supplies and may be harmful to life, because of their toxicity, reduction of normal oxygen level of water, aesthetically unsuitable effects and spread of epidemic diseases”.
The major sources of water pollution are (i) industrial effluents, (ii) industrial wastes derived from chemical industries, thermal power plants and nuclear power stations, (iii) sewage and other waste, and (iv) agricultural discharges.
Types of Water Pollutants:
On the basis of sources of pollution, water pollutants may be considered under the following categories:
(a) Industrial pollutants:
Chlorides, sulphides, carbonates, nitrates, heavy metals, various organic chemical compounds, etc.
(b) Agricultural pollutants:
Chemical fertilisers, insecticides, pesticides, etc.
(c) Urban pollutants:
Sulphates, nitrates and potassium contained in urban sewage water, chemical ions and bicarbonate ions derived from lime and fertilisers used in gardens, and chlorine and’ sodium ions contained in salts used for melting ice which covers roads and buildings in the cities of temperate countries.
On the basis of physical and chemical properties, water pollutants could be (a) physical pollutants like various chemicals, colours, sediments, volcanic dust etc. dissolved or suspended in water, and (b) chemical pollutants like sulphides, chlorides, carbonates, nitrates, etc.
Water pollutants may also be divided into (a) degradable or organic pollutants such as sewage, leaf litters, plants and animals which disintegrate, and (b) non-degradable pollutants, mainly toxic solid substances such as plastics.
1. Surface Water Pollution:
Rivers, lakes and ponds constitute surface water. There are three major sources polluting surface water: (i) pesticides and insecticides, (ii) toxic metals like lead, mercury, calcium, asbestos, zinc etc., and (iii) radioactive wastes from the processing and use of nuclear fuels such as uranium and thorium.
The pollutants may further get mixed with other chemical substances to form secondary pollutants.
River pollution has assumed a serious dimension in the modern world. Urban sewage drains pour pollutants into river water. Non-point pollution occurs if the river water is polluted from toxicants carried by rainwater from agricultural fields.
Soil erosion causes siltation of lakes. Waste sludges released by factories and sewage dumped by tourists also cause lake pollution. The presence of asbestos in the water of Lake Superior (USA) caused a serious disease, later named ‘asbestosis’, when the Reserve Mining Company (USA) started releasing asbestos into it.
In some cases, if organic and inorganic nutrients are concentrated in very high amounts in lake and river water, the aquatic flora and fauna increase beyond sustainable limits. This, too, damages the Eco balance.
2. Sea Water Pollution:
In most cases, sea water pollution occurs near coastal waters due to dumping of pollutants.-Pollution at deep sea is caused by leakage of oil due to shipwreck, deliberate destruction of oil rigs by enemies during war, etc. Oil spills have caused great harm to sea life in the recent past.
3. Groundwater Pollution:
Contamination of groundwater takes place through pollutants like nitrates, phosphorus, potash, insecticides, pesticides, etc. derived from industries, agricultural fields, urban and rural garbage, contaminated ponds, tanks, etc.
Factors such as the lithology of sub-soils, the nature of aquifers, the amount and nature of rainfall, the depth of the water table, the rate of rainwater infiltration and the infiltration of static water of ponds and lakes control the nature and extent of groundwater pollution. Sometimes groundwater pollution-is caused by excessive pumping, which causes the formation of underground cavities through which saline marine water leaks in.
Groundwater pollution causes diseases like cholera, jaundice, typhoid, dysentry, etc. which sometimes assume the proportion of epidemics, if the water is used for drinking, particularly in the developing countries of Asia and Africa. Some of the rare diseases, like ‘minamata’, are caused by mercury contamination.
The presence of toxic chemicals in groundwater causes death of organisms living in lakes, ponds and oceans, if this water finds its way into surface waters. Soil fertility is decreased by contamination of soil if polluted water is used for agricultural purposes.
Controlling Water Pollution:
There should be vigorous efforts to control water pollution with the involvement of individuals, communities, governments and social activist groups. Mass social awareness should be inculcated regarding the nature and effects of water pollution, and remedial measures.
Strict laws should be enforced, and persons violating the provisions of pollution control should be penalised.
Groundwater pollution can be eliminated by- maintaining strict restrictions regarding waste input at source, removal of nutrients, management of fish resources etc. so that bio-diversity in the aquatic ecosystem is adequately maintained.
Industrial effluents should be effectively recycled so that these pollutants are not released into water.
Radioactive, chemical and biological pollutants can be removed from water by absorption, electro-dialysis, ion exchange and reverse-osmosis methods. Scientists of the Council of Scientific and Industrial Research (CSIR), New Delhi, have invented techniques of removing pollutants from water by using ammonia, mercury, phenolics, sodium salts, etc. Scientists in the/USA, have devised a technique of cleaning up polluted water by applying solar power. Nowadays, biotechnology is also being harnessed to clear water pollution.