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MNA Frequent Question 3. What is meant by the term "natural attenuation?"
"Natural attenuation" is the effect of natural processes (i.e., those which do not require human intervention such as engineered enhancements or controls) which reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. When relying on natural attenuation processes for site remediation, EPA prefers those processes that degrade or destroy contaminants. Also, EPA generally expects that MNA will only be appropriate for sites that have a low potential for contaminant migration.
"Natural attenuation" is passive in the sense that natural attenuation processes occur without human intervention, however, the use of "natural attenuation" at a site does not preclude the use of "active" remediation or the application of enhancers of biological activity (e.g., electron acceptors, nutrients, and electron donors). By definition, a remedy that includes the introduction of an enhancer of any type is no longer considered to be "natural" attenuation. EPA encourages the consideration of innovative technologies for source control or "active" components of the remedy, which may offer greater confidence and reduced remediation time frames at modest additional cost.
Other terms associated with "natural attenuation" in the literature include "intrinsic remediation", "intrinsic bioremediation", "passive bioremediation", "natural recovery", and "natural assimilation". While some of these terms are synonymous with "natural attenuation", others refer strictly to biological processes, excluding chemical and physical processes. Therefore, EPA recommends that for clarity and consistency, the term "monitored natural attenuation" should be used unless a specific process (e.g., reductive dehalogenation) is being referenced.
There is still a great deal to be learned regarding the mechanisms governing natural attenuation processes and their ability to address different types of contamination problems. Therefore, while EPA believes that natural attenuation processes may be capable of achieving remedial objectives at a particular site, EPA expects that source control and long-term performance monitoring will be fundamental components of any remedy that relies primarily on natural attenuation processes.