Pesticides and fertilizers are used in crop cultivation. Rain and irrigation water wash these chemicals, especially the soluble nitrate fertilizer into the ground water and they eventually get into rivers, lakes and seas.
Note: Besides nitrates, phosphates are also important plant nutrients that are present in most fertilizers. However, unlike nitrates, phosphates are held strongly by soil particles and do not get into water bodies through agricultural use. The phosphates in polluted water come mainly from detergents used in homes and industries, and from sewage.
Like chemical wastes, many pesticides are synthetic organic compounds that are non biodegradable. Some of them can accumulate in the bodies of plants and animals, and be passed along food chains, becoming concentrated in the process. A well studied example is the build up of DDT in such a manner in large birds like eagles, causing them to lay eggs with thin shells. As a result , the eggs broke easily, causing a fall in the eagle population.
Fertilizers are inorganic salts. Nitrate is the most common salt that gets into ground water and water bodies. In certain regions, the build up of nitrates in drinking water is high enough to become a health risk especially in babied and young children.
Eutrophication :
Plant growth is controlled by the amount if nitrate and phosphate that is present in the environment. When nitrates (from agricultural wastes) and phosphate (from sewage and industrial wastes) get into water bodies, aquatic plant growth, especially algal growth, is greatly increased. This is known as Eutrophication.
Eutrophication upsets the balance of the aquatic ecosystem, as there are not enough primary consumers to eat up the excess producers. When these producers , especially algae, dir at the end of their growing season and start to decay, the level of oxygen in the surrounding water drops. This causes aquatic animals to die. The water becomes foul smelling as decay continues in the absence of oxygen. Large masses of the partly decomposed remains settle to the bottom. Of the water body and eventually fill it up if it is a pond, lake or stream. People in rural area lose their main source of water when this happens to the water bodies that they use for their daily needs.
