Monitored Natural Attenuation (MNA)
Monitored natural attenuation (MNA) is the sum of natural processes that leads to the decreasing of contaminant concentrations in groundwater over time.
The primary objective of MNA is to demonstrate that natural processes will reduce contaminant concentrations in groundwater to levels below regulatory standards before a point of compliance is reached. MNA as a remedial alternative is highly dependent on a good understanding of localized hydrogeologic conditions and may require considerable information and monitoring over an extended period of time.
An initial period of monitoring of an approved monitoring well network is needed to establish the effectiveness of MNA as a remedial option. The monitoring well network should include contaminated wells and appropriate sentinel wells. An approved monitoring well network should be sampled for all MNA parameters on a semiannual basis for at least two years to establish baseline trends. Please contact the North Carolina Solid Waste Section for a list of the MNA parameters required, and please refer to the Environmental Protection Agency (EPA) MNA guidance documents on the EPA’s website: www.epa.gov.
Any approved MNA program should include a contingency plan with a list of triggering events and established responses to those triggering events. If contaminants migrate off site, active remediation should be initiated and adjacent property owners notified.
Uncertainty associated with estimated rates of attenuation over extended periods of time is a major consideration with MNA. Hydrologic and geochemical conditions amenable to MNA can change due to (1) natural or anthropogenic causes, and (2) changes in the mobility of a plume over time. MNA should not be considered a presumptive remedy, but should be evaluated along with active remediation options to restore groundwater to its designated beneficial use considering cost, technical practicability, remedial objectives, and protection of human health and the environment.
Suggested readings:
- EPA OSWER Directive 9200.4-17P Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites, dated April 21, 1999.
- Monitored Natural Attenuation of Petroleum Hydrocarbons, U.S. EPA Remedial Technology Fact Sheet dated May 1999.
- Monitored Natural Attenuation of Chlorinated Solvents, U.S. EPA Remedial Technology Fact Sheet dated May 1999.
- Handbook of Groundwater Protection and Cleanup Policies for RCRA Corrective Action, Section 11, Monitored Natural Attenuation, U.S. EPA, September 2001.
- Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water, EPA/600/R-98/128, September 1998.
