Underground Storage Tanks

The term "monitored natural attenuation" refers to the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods.

The term "monitored natural attenuation" was first used in the Draft OSWER interim policy directive, which was signed on December 1, 1997. The OSWER Directive (9200.4-17P) was released as Final on April 21,1999. EPA's Office of Emergency Response and Remediation (OERR) requested that the term "monitored natural attenuation (MNA)" be used to emphasize that long-term monitoring is an important component of a remedy where natural processes are to be relied upon to achieve cleanup objectives. Also, the intent was to distinguish between the case where "monitored natural attenuation (MNA)" is used as a remedy, as opposed to the case where "natural attenuation" processes are occurring as part of a no-action remedy and are not being relied upon to attain remedial objectives. Lastly, EPA prefers that the acronym "MNA" be used as opposed to "NA," which is too easily confused with "no action".

More information on Directive 9200.4-17P is available.

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