Monitored Natural Attenuation
Monitored Natural Attenuation (MNA) is not a "technology," per se. It generally describes a range of physical and biological processes, which, unaided by deliberate human intervention, reduce the concentration, toxicity, or mobility of chemical or radioactive contaminants. These processes take place whether or not other active cleanup measures are in place. Increasingly, parties responsible for cleanup as well as environmental regulators are relying upon natural attenuation as a remediation strategy.
The mechanisms of natural attenuation can be classified as destructive and non-destructive. Destructive processes include biodegradation and hydrolysis. Biodegradation is by far the most prevalent destructive mechanism. Also called bioremediation, it is a process in which naturally occurring microorganisms, such as yeast, fungi, and bacteria, break down target substances, such as fuels and chlorinated solvents, into less toxic or non-toxic substances. Like larger living things, these microbes must eat organic substances to survive. Certain microorganisms digest fuels or chlorinated solvents found in the subsurface environment. Non-destructive attenuation mechanisms include sorption, dispersion, dilution, and volatilization.
Long-term monitoring is necessary to demonstrate that contaminant concentrations continue to decrease at a rate sufficient to ensure that they will not become a health threat or violate regulatory criteria. Monitoring should be designed to verify that potentially toxic transformation products are not created at levels that are a threat to human health; that the plume is not expanding; that there are not releases that could affect the remedy; and, that there are no changes in hydrological, geochemical, or microbiological parameters that might reduce the effectiveness of natural attenuation. Techniques and technologies for predicting and monitoring natural attenuation are still being developed.
Limitations and Concerns
While there is significant debate among technical experts about the application of MNA, a remediation strategy that largely depends on physical mechanisms such as sorption, dilution and dispersion is not attractive to most communities. If MNA is to achieve full community buy-in, it must have a significant amount of contaminant destruction, usually through biodegradation.
In investigating whether MNA is appropriate for chlorinated-solvent contaminated sites, other contaminants likely to be present in plumes should also be included in the investigation and remedy selection.
Some degradation products are more harmful than the original contaminants. For chlorinated solvent plumes, this is especially true. For example, vinyl chloride is more mobile, more toxic, and in certain conditions more persistent than its parent products (e.g., TCE). Project proponents must demonstrate that human or environmental receptors will not be exposed to greater risks during the long natural attenuation process.
TCE, a common contaminant found in groundwater, is sold under about fifty different trade names. Some of these products contain additives used as stabilizers, which make up as much as two percent of the total weight. These stabilizers are numerous and they have not been considered when developing strategies for natural attenuation. One stabilizer, used in both TCE and TCA (trichloroethane), known as 1-4 dioxane, is a problem at many sites. It is a probable carcinogen, is mobile in the environment, and "has not been shown to readily biodegrade in the environment" (USEPA 2009). Additionally, impurities of TCE include vinyl chloride, dichloroethene (DCE), perchloroethylene (PCE), carbon tetrachloride, and acetone. If the line of evidence that is used includes the presence of daughter products of TCE, such as vinyl chloride and DCE, to persuade agencies that natural attenuation is occurring, there is a possibility that this may be misleading.
Monitored MNA is most acceptable to public stakeholders when regarded as just another tool in the remediation toolbox. As suggested in EPAês policy, MNA may complement other remedies.
There is concern that reliance upon MNA may undermine the development and use of innovative alternatives.
There is a need to develop contingency remedies should monitoring demonstrate that MNA is not working as expected. However, because MNA is frequently less costly than other approaches, there is concern that budgets built on the assumption that MNA will do the job may actually lock it in as a remedy, even when it does not work.
Longer time frames may be required to achieve remediation objectives, compared to active remediation. Thus, institutional controls may be required and the site may not be available for reuse, as compared to other strategies. Extended land-use restrictions should be considered in the cost of the remedy. There is concern among communities with closed and closing military bases that MNA, as a slow, uncertain remedy, could delay the transfer and/or reuse of contaminated properties. Any step in the remedial process that delays unrestricted use of property represents a real or potential economic loss to the community or property-owner receiving the property.
Natural attenuation of fuel plumes must consider methyl tertiary butyl ether (MTBE), which is a fuel additive and a suspected carcinogen.
At sites where groundwater flows to surface water, there is concern about the fate of the contaminants in surface water.
Contaminated surface soils, because they are subject to wind and erosion, require long-range and effective containment before being considered for MNA.
MNA is not appropriate where imminent site risks are present.
Where practicable, groundwater should be brought to drinking water or similar standards within a reasonable time frame.
Contaminants potentially addressed by MNA include volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), polychlorinated biphenyls (PCBs), fuel hydrocarbons, metals, and explosives. Fuels and chlorinated VOCs are the substances most commonly evaluated for MNA. MNA may be appropriate for some metals and radionuclides, where natural attenuation processes result in a change in the valence state of the metal that results in immobilization (e.g., changing hexavalent chromium to trivalent chromium), and natural breakdown due to radioactive decay.
Technology Development Status
MNA has been selected at numerous sites. It is now considered a commercially available "technology." Both the U.S. Air Force and the Department of Energy have developed policies that encourage the use of MNA as a first resort.
Other Resources and Demonstrations
See http://www.epa.gov/swerust1/directiv/d9200417.htm, U.S. EPAês Final Directive, Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites, April 21, 1999 (#9200.4-17P).
See http://www.cpeo.org/pubs/narpt.html for "Report on CPEOês Natural Attenuation Conference."
See http://www.itrcweb.org/Documents/EACO-1.pdf for description of Enhanced Attenuation by the ITRC.
See http://www.sandia.gov/eesector/gs/gc/na/mnahome.html#Contaminant for a description of MNA by contaminant.
See Wiedemeier, T.H., D.C. Downey, J.T. Wilson, D.H. Kampbell, R.N. Miller, and J.E. Hansen. 1994. Technical Protocol for Implementing the Intrinsic Remediation (Natural Attenuation) with Long-Term Monitoring Option for Dissolved-Phase Fuel Contamination in Ground Water, Brooks Air Force Base, San Antonio, TX.
See Wiedemeier, J.T. Wilson, Haas et al, 1996. "Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater," Draft—Revision 1, Air Force Center for Environmental Excellence, Brooks Air Force Base, San Antonio, TX.
See http://toxics.usgs.gov/bib/bib-Biodegradation.html for a bibliography on biodegradation and natural attenuation.
See http://www.clu-in.org/characterization/technologies/exp.cfm for a technical description of explosives in different media and the use of some analytical techniques. .