Zones of Pollution in a river-stream :
Zone of degradation
Zone of active decomposition
Zone of recovery
4. Zone of clear water
Zone of degradation- This zone is found for a certain length just below the point where sewage is discharged into the river-stream. This zone is characterised by water becoming dark and turbine with formation of sludge deposits at the bottom. D.O is reduced to about 40% of the saturated value. There is an increase in carbon dioxide content. Re-oxygenation occurs but is slower than de-oxygenation. These conditions are unfavourable to the development of aquatic life and such as algae dies out bur fish life may be present feeding on fresh organic matter.
Zone of active decomposition: This zone is marked by heavy pollution. It is becoming greyish and darker than in previous zone. D.O concentration falls to zero and anaerobic condition may set in with evolution of gases like methane, carbon dioxide, hydrogen sulphide etc. The D.O again rises to the original level (i.e about 40%). In this zone bacteria flora will flourish at the upper end. Anaerobic bacteria will replace aerobic bacteria while protozoa and fungi will disappear and then reappear. Fish life will be absent. Algae and tubifex will also mostly be absent.
Zone of recovery: In this zone, the river stream tries to recover from its degraded condition to its former appearance. The water becomes clearer and so the algae reappears. While fungi decreases, BOD falls down and D.O content rises above 40% of the saturation value. Protozoa, rotiters, crustaceans and large plant like sponges, bryozons, etc also reappears.
Zone of clear water: In this zone, the river attains its original conditions with D.O rising up to the saturation value.
