Ans. Water Pollution
It is the change in the composition of water by the addition of harmful substances. Water pollution severely affects the health of people.
Causes
i) Sewage is one of the major pollutants of water. It contains organic matter and the excreta of human and other animals. Organic 'matter encourages the growth of micro-organisms which spreads diseases.
ii) The wastes of industries (acids, alkalies, dyes and other chemicals) are disposed in nearby water bodies. These wastes change the pH of water and are harmful or even fatal to aquatic organisms.
iii) Certain industries release a lot of hot water from their cooling plants. It results in heating up of water bodies and kills aquatic life.
iv) Fertilizers and pesticides enter into water bodies with the rain water flow and the ground water by seepage. These chemicals remain in water for a long time and can enter food chains. They cause a number of diseases in animals.
v) Oil tankers and offshore petroleum refineries cause oil leakage into water. Oil floats on the water surface and prevents atmospheric oxygen from mixing in water. So, aquatic animals begin to die due to oxygen shortage.
vi) Some heavy metals e.g. lead, mercury, arsenic and cadmium also make the water polluted. Such metals released from industrial and urban areas can be present in the water. If water with such heavy metals is given to plants, the metals enter the vegetables that grow on these plants. Such contaminated vegetables are harmful for human health. Heavy metals reduce growth and development, and cause cancer and nervous system damage. Mercury and lead can cause joint diseases such as rheumatoid arthritis, and diseases of kidneys, circulatory system and nervous system.
Effects of Water Pollution
The following are major effects of water pollution.
(a) Eutrophication: Enrichment of water with inorganic nutrients (nitrates and phosphates) is called eutrophication. The sewage and fertilizers contain large amounts of inorganic material (nutrients). When sewage and fertilizers reach water bodies, the nutrients present in them promote algal blooms (excessive growth) there. Rich algal growth leads to increase in the number of decomposers. Decomposers use the oxygen present in water and it results in the depletion of oxygen. Algal bloom also reduces the light reaching the lower layers in water.
(b) Food chain contamination: The non-biodegradable water pollutants may stay in water for long times. From water, they enter into small organisms, which are fed upon by fish, The fish in turn are consumed by land animals including human.
(c) Epidemics: Organic pollutants in water facilitate the growth of germs. Such polluted water causes epidemics like cholera, gastroenteritis etc.
Control of Water Pollution
i) Public should be made aware of the dangers of water pollution.
ii) Before releasing the sewage into water bodies, it must be purified through sewage treatment techniques. .
iii) Industrial wastes should also be treated before they are released into water bodies.