Pollutants flowing into water bodies are broadly classified as domestic sewage and industrial effluents. Domestic sewage contains about 99.9 per cent water and 0.1 per cent impurities such as suspended soil, colloidal and dissolved materials. Industrial effluents contain solids and dissolved salts such as nitrates and phosphates and toxic metals such as lead and mercury, and organic compounds such as dyes and pesticides.
Apart from these pollutants, micro-organisms in sewage decompose the organic matter by utilizing oxygen, which is measured in terms of Biochemical Oxygen Demand (BOD). A high concentration of BOD in sewage water automatically translates into low concentration of dissolved oxygen or DO. High BOD and low DO ultimately cause the death of aquatic organisms. Depletion of oxygen in water bodies is also caused by algal blooms and aquatic weeds, which grow due to large amount of nutrients in water, thereby causing the death of aquatic fauna.
Sewage water and agricultural runoff are rich in nutrients such as nitrogen and phosphorus, which favour the growth of plankton blooms, a process called eutrophication. This leads to the depletion of oxygen in the water bodies and kills the aquatic fauna.
Sewage wastes may also contain pathogenic organisms that are discharged from hospitals. When these wastes are released in water, they r, may cause the outbreak of diseases like dysentery, typhoid, jaundice and cholera. Industries release waste water rich in toxic metals such as lead, mercury and copper along with detergents, alcohol and acids into water bodies. These pollutants are highly toxic to aquatic fauna and also human beings.
Toxic chemicals are not biodegradable and get incorporated with an increase in the concentration of toxins at successive trophic levels, called biomagnification. Even the hot water released from thermal power plants into the water bodies and oil spills in oceans can kill the indigenous flora and fauna.