Water Pollution: Part 1

We said earlier that the supply of water is constant, and that the system by which it continuously circulates through the biosphere is called the hydrologic cycle. In that cycle, water may change for, and composition, but under natural environmental circumstances, it is purified in the recycling process. When water composition has been so modified that it cannot be used for a specific purpose or is less suitable for that use than it was in its natural state, the water is said to be polluted.

Pollution is caused by discharging into water substances that cause unfavorable changes in its chemical or physical nature or in the quantity and quality of the organisms living in the water. Pollution is a relative term. Water that is not suitable for drinking may be completely satisfactory for cleaning streets.

People are not the only case of water pollution. Decayed leaves, animal wastes, and other natural phenomena may affect water quality. There are natural processes, however, to take care of such pollution. Organisms in water are able to degrade, assimilate, and disperse such substances in the amounts in which they naturally occur.

What is happening now is that the quantities of wastes discharged by people often exceed the ability of a given body of water to purify itself. In addition, we are introducing pollutants, such as metals or inorganic substances, that cannot be broken down at all by natural mechanisms or that take every long time to break down.

The five main contributors to water pollution are agriculture, industry, mining, municipalities, and urban drainage.

Agriculture

The kinds of pollutants associated with agriculture are biocides, fertilizers, and animal wastes. Runoff from farms and feedlots carries these contaminants into underground and surface waters. Fertilizers are responsible for depositing excess nutrients (nitrates and phosphates) in water bodies. There they hasten the process of eutrophication. Algae and other plants are stimulated to grow abundantly. When they die, the level of dissolved oxygen in the water decreases. Fish and plants that cannot tolerate the poorly oxygenated water are eliminated. Symptoms of a eutrophic lake are prolific weed growth, large masses of algae, fish kills, and water that has a foul taste and odor. About one-third of the medium- and large-sized lakes in the U.S. have been affected by accelerated eutrophication.