The widespread practice of releasing raw sewage in shallow soakpits has caused pollution of groundwater in many cities. Pollutants contained in seepage pits, refuse pits, septic tanks and barnyards may percolate through layers of earth and find their way into ground waters. Some times transport accidents may also lead to contamination of underground sources of waters. Some industrial products and wastes may also cause pollution of groundwater. In the industrial areas of Punjab and Haryana, for example, Ambala, Ludhiana, and Sonepat where bicycles and woolen garments are manufactured, significantly high concentration of nickel, iron, copper, chromium and cyanide have been detected in groundwater samples.
Modernisation of agriculture has led to excessive use of nitrogenous fertilizers. The nitrates being soluble in water trickle down through layers of soil into deeper layers of earth and ultimately are added to the underground stores of water. In many villages and townships where groundwater is the only source of drinking water this causes methaemoglobinaemia, particularly in bottlefed infants, because they are very sensitive to this pollutant.
You may wish to know how this disease is caused. What happens is that when water containing nitrates is consumed, it goes to intestines, where intestinal bacteria convert nitrates into nitrites. Nitrite ion combines with haemoglobin to form met haemoglobin, which inhibits the oxygen carrying capacity of the blood, causing a kind of anaemia known as methaemoglobinaemia. The removal of nitrate from water is not accomplished very easily. Nitrate is a soluble toxic waste. Removal of soluble toxic wastes requires elaborate treatment procedures such as chemical coagulation and filtration, carbon absorption chemical oxidation, ion-exchange, electrodialysis and reverse osmosis. Any one or a combination of the methods can be used for removal of nitrates depending on availability of resources.
The indiscriminate release of toxic industrial wastes such as arsenic, lead, cadmiurtl and mercury compounds, and pesticides like polychlorinated biphenyls (PCB) may result in their trickling down to nature's underground water stores. This seriously threatens the quality of groundwater supply, specially in areas where water table is high that is situated near surface of earth. Scientists are concerned that drinking even small quantities over many years may lead to bioaccumulation of these toxins in the body. What happens is that these compounds being difficult to metabolise, are stored in the body, usually in the fatty tissue. The persons dependent on contaminated water supply retain small quantities of these compounds each day. This phenomenon is termed as bio-accumulation.
Human beings consume also the products obtained from various plants which thrive on polluted waters and store these toxic compounds in their biomass. Non-vegetarian diet such as fish, pork and steak is also likely to come from animals which store these pollutants in their biomass. Man here acts as a centre into whose body, pollutants from various kinds of sources pour in. The quantity of toxic substances thus gets magnified. You can see that pollutants are getting concentrated into man through biological sources. And because the amount of toxic substances is increased through biological food chain, this process is also known as Biological magnification. During starvation when the body draws upon reserve food sources, these compounds are released into blood stream producing toxic effects.
Even in regions where water table is low, contamination of groundwater may cause serious problems, as will be evident from the following example.
Case Study :
Water pollution in Pali (Rajasthan) is an excellent example of groundwater pollution. For nearly one lakh population of the city, underground water is the only source of drinking water. There are more than 450 textile units involved in designing, dyeing and bleaching of clothes. All these have been discharging effluents containing sulphuric acid and carcinogenic substances. Coloured effluent containing these toxic substances spread over large areas in the city. A study conducted by Rajasthan State Board for Prevention and Control of Water and Air Pollution concluded that with the onset of monsoon these toxic substances percolate down the earth, mix with underground water and contaminate it. It was reported that appropriate timely measures taken after monitoring samples of ground water at different depths in suspected areas could have avoided the problem, altogether.