(a) Eutrophication : Eutrophication is the natural aging of a lake by biological environment of its water. Streams draining into the lake introduce nutrients like phosphorus and nitrogen, which encourage the growth of algae that may form a mat on the water surface. The algae deoxygenate the water enough to kill fishes other aquatic organisms. Over the centuries, as silt and organic debris pile up, the lake grows shallower and warmer. Marsh plants take root in the shallows and begin to fill in the original lake basin. Eventually, the lake gives way to large masses of floating, finally coverting into land. Eutrophication may span thousand of years however, effluents from the industries and homes can radically accelerate this process.
(b) Biological Magnification : Biological magnification (Biomagnification) refers to increase in concentration of the toxicant at successive trophic levels. This happens because the toxic substance accumulated by an organism passes to next higher trophic level without being metabolished or excreted. For example, river water may have a very low concentration of DDT, but the carnivorous fish in that river may contain high concentration of DDT and be unfit for eating by man.
(c) Groundwater depletion and ways for its replenishment : The level of groundwater is falling day by day due to high demand in urban areas and in agriculture. As a result water level has falled in several areas.
Ways for its replenishment :
1. Reduction in consumption : Sprinkler and subsurface irrigation techniques reduce the amount of water used in irrigation.
2. Rainwater harvesting :
(i) Rainwater Tanks : Rainwater is stored in tanks. In parts of Rajasthan and Gujarat, roof water is collected in underground storage room.
(ii) Roof top harvesting : Rain water collected over roofs is allowed to pass into ground through deep rain water pipes. It can also be passed directly in old wells and pumps.
