Monitored
Natural Attenuation
Description
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 EPAÍs 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 "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.
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
Other
Resources and Demonstrations
See http://www.epa.gov/swerust1/directiv/d9200417.htm,
U.S. EPAÍs 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 CPEOÍs 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 also http://www.ert2.org/t2bioportal/.