Monitored natural attenuation (intrinsic bioremediation) is one method
of applying in situ bioremediation. In addition to abiotic mechanisms, natural
attenuation includes the use of indigenous microorganisms to degrade the contaminants of concern,
all without human intervention (such as supplementing the available nutrients). Site
characterization, reactive flow and transport modeling, and long term monitoring
comprise the activities required to implement natural attenuation. The site characterization
determines the extent of contamination and the properties of the aquifer. This characterization
information can then be used in a reactive transport model to predict the fate of the contaminants and
whether the contaminants will affect the receptors of concern. Long-term monitoring
is used to assess the fate and transport of the contaminants compared against the predictions. The
reactive transport model can then be refined to obtain better predictions.
Quoting from the U.S. EPA OSWER Directive [U.S. EPA, 1997], the following is a definition
of monitored natural attenuation:
The term "monitored natural attenuation", as used in this Directive, refers to the reliance
on natural attenuation processes (within the context of a carefully controlled and monitored site
cleanup approach) to achieve site-specific remedial objectives within a time frame that is
reasonable compared to that offered by other more active methods. The "natural attenuation
processes" that are at work in such a remediation approach include a variety of physical, chemical,
or biological processes that, under favorable conditions, act without human intervention to reduce
the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater.
These in situ processes include biodegradation, dispersion, dilution, sorption, volatilization, and
chemical or biological stabilization, transformation, or destruction of contaminants.
Other terms associated with natural attenuation in the literature include
"intrinsic remediation", "intrinsic bioremediation", "passive bioremediation", "natural recovery",
and "natural assimilation". While some of these terms are synonymous with "natural attenuation,"
others refer strictly to biological processes, excluding chemical and physical processes.
Natural attenuation processes are typically occurring at all sites, but to varying degrees of
effectiveness depending on the types and concentrations of contaminants present and the physical,
chemical, and biological characteristics of the soil and groundwater. Natural attenuation
processes may reduce the potential risk posed by site contaminants in three ways:
- The contaminant may be converted to a less toxic form through destructive
processes such as biodegradation or abiotic transformations
- Potential exposure levels may be reduced by lowering of concentration
levels (through destructive processes, or by dilution or dispersion)
- Contaminant mobility and bioavailability may be reduced by sorption to the soil or rock matrix
When we implement Battelle's Chlorinated Solvent Bioremediation Design Service,
we first consider if natural
attenuation is an appropriate technology to remediate a site. If our evaluation shows that
natural attenuation is infeasible, we then proceed to investigate an
accelerated in situ bioremediation system. For example,
our remediation alternative evaluation of a site at
Naval Air Station Point Mugu showed that at one portion of the site natural attenuation was the
best in situ bioremediation alternative, while at another portion, natural attenuation was
infeasible.
If natural attenuation is the most feasible alternative, we evaluate the movement and destruction
of contaminant with respect to the point(s) of compliance and/or sensitive receptors. Over
time, the site will be monitored and the effectiveness of natural attenuation re-evaluated.
Predictive modeling will be repeated as the site data and model are refined. Two examples of sites
where we have applied a natural attenuation analysis are Dover
Air Force Base and the Petro-Processor's Brooklawn site.
Advantages of Monitored Natural Attenuation
- As with any in situ process, generation of lesser volume of remediation
wastes, reduced potential for cross-media transfer of contaminants
commonly associated with ex situ treatment, and reduced risk of human
exposure to contaminated media
- Less intrusion because few surface structures are required
- Potential for application to all or part of a given site, depending on site
conditions and cleanup objectives
- Can be used in conjunction with, or as a follow-up to, other (active) remedial measures
- Lower overall remediation costs than those associated with active remediation
Disadvantages of Monitored Natural Attenuation
- Longer time frames may be required to achieve remediation objectives,
compared to active remediation
- Site characterization may be more complex and costly
- Toxicity of transformation products may exceed that of the parent compound
- Long term monitoring will generally be necessary
- Institutional controls may be necessary to ensure long term protectiveness
- Potential exists for continued contamination migration, and/or cross-media
transfer of contaminants
- Hydrologic and geochemical conditions amenable to natural attenuation are
likely to change over time and could result in renewed mobility of
previously stabilized contaminants, adversely impacting remedial
effectiveness
- More extensive education and outreach efforts may be required in order to
gain public acceptance of monitored natural attenuation
References/Further Reading
- Borden, R.C. 1994. "Natural Bioremediation of Hydrocarbon-Contaminated Ground Water."
In: Handbook of Bioremediation, R.D. Norris, R.E. Hinchee, R. Brown,
P.L. McCarty, L. Semprini, J.T. Wilson, D.H. Kampbell, M. Reinhard, E.J. Bouwer,
R.C. Borden, T.M. Vogel, J.M. Thomas, and C.H. Ward eds. Lewis Publishers,
Boca Raton, FL. pp. 177-199.
- U.S. EPA. 1997. Use of Monitored Natural Attenuation at Superfund,
RCRA Corrective Action, and Underground Storage Tank Sites. United States
Environmental Protection Agency, Office of Solid Waste and Emergency Response,
Washington D.C. OSWER Directive 9200.4-17.
- U.S. EPA. 1997. Proceedings of the Symposium on Natural Attenuation of
Chlorinated Organics in Ground Water. United States Environmental Protection
Agency, Office of Research and Development, Washington D.C. EPA/540/R-97/504.
- Vogel, T.M. 1994. "Natual Bioremediation of Chlorinated Solvents."
In: Handbook of Bioremediation, R.D. Norris, R.E. Hinchee, R. Brown,
P.L. McCarty, L. Semprini, J.T. Wilson, D.H. Kampbell, M. Reinhard, E.J. Bouwer,
R.C. Borden, T.M. Vogel, J.M. Thomas, and C.H. Ward eds. Lewis Publishers,
Boca Raton, FL. pp. 201-225.
- Wiedemeier, T.H., M.A. Swanson, D.E. Moutoux, E.K. Gordon, J.T. Wilson, B.H. Wilson, D.H.
Kampbell, P.E. Haas, R.N. Miller, J.E. Hansen, and F.H. Chapelle. 1998.
Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in
Ground Water. United States Environmental Protection Agency, Office of
Research and Development, Washington D.C. EPA/600/R-98/128.
Last Revised: 2001-Jan-16