The contamination of water by undesirable substances is termed as water pollution. The agents which bring about the change in the water quality are called pollutants of water.
Water pollution can be broadly classified into two types.
I. Chemical Pollution
Water pollution caused due to the discharge of harmful chemical substances in the water bodies is termed as chemical pollution.
The chemical wastes that cause water pollution can be put into the following categories:
1. Industrial Wastes
The throwing of industrial wastes into rivers and lakes is the most important source of water pollution.
The industrial wastes of industries like paper , paint, pesticide, metal etc. contain toxic chemicals like mercury, lead, copper, cadmium etc. which can enter the human body through food chains and affect their health. The accumulation of toxic harmful pesticides (bio concentration) in the human body can even affect the nervous system.
The mineral acids from different industrial plants , coal mines and acid rain affect the aquatic lives on the earth. The acids and alkalis present in industrial wastes kill those microorganisms which are the natural cleansing agents of water.
The disposal of these chemical wastes in the water bodies causes a great increase in the water pollution.
2. Domestic Wastes
The dumping of sewage into rivers and lakes is a major cause of water pollution in big cities. This includes all kinds of wastes like human excreta, food waste, soaps and detergents, garbage and other materials. The oxygen – demanding wastes also include wastes from food-processing industries, wastes from hospitals, dead bodies thrown into water etc. The organic substances from these wastes absorb dissolved oxygen in the water in the presence of some bacteria. This leads to depletion of oxygen in water bodies which badly affects the aquatic lives.
3. Agricultural Wastes
Fertilisers, insecticides, pesticides and are essential for food production and protection. However, their excessive use is not desirable as they contain harmful substances. They seep through the ground and pollute the underground water sources. They also get washed away by rains into rivers and lakes.
The sewage and fertilizers present in polluted water are rich in nutrients. This leads to an excessive growth of weeds like algae which cover the whole surface of a lake. This process is called eutrophication. When the algae die, the decomposers use up all the dissolved oxygen for decomposing the vast amount of dead algae. Since no dissolved oxygen is left in the water of the lake, all aquatic animals like fish die.
4. Leakage of Oil and Grease in Water body
The leakage of petroleum oil into the sea, during drilling and shipping operations pollutes the sea water.
The film of oil that floats over the water body affects the process of photosynthesis in the aquatic plants. It is because oil film affects the transmission of light through the water body. Moreover, petroleum or grease spilled over water also produces chemicals that are extremely harmful for marine animals.
II. Thermal Pollution
Industrial plants use water as a coolant. They draw water from the nearby water bodies like a river or a lake. The used-up water is returned to the water bodies. The temperature of discharged water is higher than the water in the water bodies. This difference in temperature is harmful for aquatic lives as they are not adapted to this temperature change. The pollution thus caused is called thermal pollution. The thermal pollution also causes decrease in the level of dissolved oxygen present in the water which affects the aquatic lives on the earth.