"Natural attenuation" refers to the ability of a ground water system to rid itself of contamination resulting from a spill or improper disposal of wastes. It is emerging as a viable and in some cases, the preferred remedy for contaminated ground water. The basic concept of natural attenuation is not new. We depend on it, for example, in municipal wastewater permits that allow treatment plants to discharge a certain amount of wastes without unduly depleting oxygen in surface water. Another example is the use of drainfields in septic systems. Bacteria that naturally inhabit many ground water environments are able to break down chemicals once thought to be virtually non-biodegradable. For example, components of gasoline such as benzene, toluene, xylene, and ethylbenzene are now known to biodegrade in ground water to carbon dioxide and water. Other contaminants, including chlorinated solvents (e.g., dry-cleaning solvents) can also biodegrade under particular conditions. In some cases, natural biodegradation may break down contaminants in ground water faster than they can be removed by engineered systems. Natural attenuation is sometimes a preferred remedy because it does not transfer pollutants from one location to another. Rather they are broken down in place, converted usually to non-toxic end products. However, natural attenuation is not always a completely effective remedy by itself. In cases where the contamination is spreading more quickly than it can break down, where drinking-water wells are threatened by contamination, or when toxic breakdown products occur, engineered systems are needed so that exposure to contaminants is eliminated.
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