The Factors Affecting the Self-Purification of Streams

An interesting and much-studied topic, stream self-purification describes the ability of streams to gradually clean themselves, improving the quality of water over time if no new pollution is introduced. Given the interest in methods of water purification as fresh water becomes an increasingly scarce resource on Earth, a great deal of research has been devoted to the topic. While self-purification mechanisms in stream ecology are not yet fully understood, several critical factors have been identified.

Biological Factors

• Stream ecosystems have a capacity of biological digestion of pollutants. Microorganisms and larger organisms such as plants and animals, for instance, absorb small portions of contaminants dissolved in water or feed on microbiological contaminants such as sewage bacteria. While such contaminants as mercury cannot be digested by living organisms and thus accumulate in the ecosystem no matter how many times the contaminants are ingested, certain other contaminants can be processed by living organisms. Concentrations of these contaminants in the environment gradually decrease as the ecosystem completes its life cycles.

Chemical Elements

• The chemical composition of water also plays an important role in self-purification. Water flowing through copper-rich areas and thus having a high concentration of dissolved copper, for instance, can gradually remove other suspended metal contaminants as they bond to the copper and fall out as sediment. Concentrations of dissolved oxygen are a particularly critical factor in self-purification, feeding the biological life that digests contaminants and keeping anaerobic bacteria communities from developing. In fact, artificial cascades have become a common tool in stream management, forcing the water to fall through air and increasing the concentration of dissolved oxygen.

Physical Factors

• The physical surroundings of a body of water play a fundamental role in the quality of water. In the case of streams, sands and grasses can act as natural filters for the water, removing large impurities as the water makes its way through the stream or slowly filters through stream sand. Larger topographical features such as long drops or numerous rocks that agitate the water can also contribute to the amount of oxygen dissolved in the water or determine the kinds of organisms that can thrive in the physical conditions present.

Temperature

• As a determining factor, temperature plays a critical role in the rate of stream self-purification. Water temperature can determine the rate at which certain elements dissolve in the water or the amount of sediments that can do so. Relating to other factors of stream self-purification, temperature can also determine the kinds of species present in the water and the rate at which they reproduce. Some studies, for example, show a seasonal change in the rate of stream self-purification as plant species that absorb contaminants grow less during winter months.