Natural Attenuation
The term natural attenuation refers to the reliance on natural processes to achieve site-specific remedial objectives. Where found to be a viable remedy, monitored natural attenuation may be used within the context of a carefully controlled and monitored site cleanup approach. To be considered an acceptable alternative, monitored natural attenuation would be expected to achieve site remedial objectives within a timeframe that is reasonable compared to that offered by other more active methods. Monitored natural attenuation is always used in combination with source control.
Natural attenuation processes include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or ground water. These processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization, transformation, or destruction of contaminants.
Spills and leaks of petroleum fuels, and chlorinated solvents such as PCE, TCE and TCA have caused widespread contamination in the environment. Generally these contaminants are present both in NAPL form and as dissolved contaminants in groundwater. Cleanup of both the NAPL and sorbed or dissolved contaminants in soils and groundwater using many common remedial techniques often is expensive and slow. However, under the proper conditions at some sites, natural attenuation can contribute significantly to remediation of dissolved fuel hydrocarbons and chlorinated solvents and may accomplish site remediation goals at a lower cost than conventional remediation technologies, within a similar timeframe.
One of the most important components of natural attenuation is biodegradation -- the change in form of compounds carried out by living creatures such as microorganisms. Under the right conditions, microorganisms can cause or assist chemical reactions that change the form of the contaminants so that little or no health risk remains. Biodegradation is important because many fuel constituents and chlorinated solvents can be destroyed by biodegradation, microorganisms that are capable of biodegrading contaminants are found almost everywhere, and biodegradation can be very safe and effective. However, most organic chemicals, including fuels and chlorinated solvents, biodegrade only under very specific conditions, which are not present at all sites. Microorganisms are most effective at degrading low to moderate concentrations of contaminants. High concentrations and very low concentrations of contaminants may not be biodegradable.
As chlorinated solvents biodegrade, the products of the degradation process may or may not be less harmful than the original contaminants. Sometimes chlorinated solvents may degrade to form more toxic compounds, and these toxic compounds may accumulate under certain conditions. Also, under some conditions, the microbial activity involved in degrading the contaminants could cause mobilization of certain materials such as manganese or arsenic which could cause environmental problems. Monitoring for these potential problems is necessary.
Links
|
|
