Monitored Natural Attenuation
Description
Monitored
Natural Attenuation (MNA) is not a Òtechnology,Ó 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 Òhas not been shown to
readily biodegrade in the environmentÓ (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 EPAÕs 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.
Applicability
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 Òtechnology.Ó Both the U.S. Air Force and the Department of Energy
have developed policies that encourage the use of MNA as a first resort.
Web
Links
http://www.frtr.gov/matrix2/section4/4-32.html
http://clu-in.org/download/citizens/mna.pdf
http://www.clu-in.org/download/techdrct/tdeec01004.pdf
http://www.epa.gov/ada/download/reports/600R07139/600R07139.pdf
http://www.epa.gov/ada/download/reports/600R07140/600R07140.pdf
http://www.pnl.gov/main/publications/external/technical_reports/PNNL-13569.pdf
Other
Resources and Demonstrations
See
http://www.epa.gov/swerust1/directiv/d9200417.htm,
U.S. EPAÕs 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 ÒReport on CPEOÕs Natural Attenuation Conference.Ó
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. ÒTechnical Protocol for Evaluating
Natural Attenuation of Chlorinated Solvents in Groundwater,Ó
Draft—Revision 1, Air Force Center for Environmental Excellence, Brooks
Air Force Base, San Antonio, TX.
See
http://www.epa.gov/ada/download/reports/epa_600_r01_025.pdf
for EPAÕs Technical Protocol for Evaluating Natural Attenuation of
Chlorinated Solvents in Ground Water (EPA/600/R-98/128).
See
http://toxics.usgs.gov/bib/bib-Biodegradation.html
for a bibliography on biodegradation and natural attenuation.