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Natural
attenuation: A potential for environmental clean-up
By: O.P. Shukla
and U.N. Rai
Introduction
The
environmental pollution in India, is rapidly increasing day by day. The
increasing economic development and a rapidly growing population that has taken
the country from 300 million people in 1947 to over one billion people today,
are putting a pressure on the environment, infrastructure, and the country’s
natural resources. Industrial pollution, soil erosion, deforestation, rapid
industrialization, urbanization, and land degradation are all aggravating the
problem.
Natural attenuation relies on
natural processes to clean up or attenuate pollution in soil and groundwater as
a cost-effective technology. Natural attenuation means dilution, dispersion,
bio- degradation, irreversible sorption, and/or radioactive decay of
contaminants in soils and ground waters. It causes a net reduction of
contaminant toxicity and human and ecological risk, but rarely taken advantage
of environmental remediation. Instead, contaminants in the subsurface are
assumed to be 'time bombs' having near eternal lethality. In fact, most
contaminants in soils and ground waters are naturally attenuated rapidly, and
much faster than they can be removed by engineering remediation measures.
However, the
right conditions must exist underground to clean the sites properly. These
various physical, chemical and biological processes of natural attenuation that
may occur at a site. A science-based approach to remediation would be to ignore
those contaminants which are naturally attenuated and remediate those which are
not.
The present
approach requires at least an attempt to remove all contaminants and will
achieve minor decreases in health and environmental risk but has been estimated
to ultimately cost lot of money. Use of unenhanced natural processes as part of
a site remediation strategy is called “natural attenuation.” Some processes that
occur during natural attenuation can transform contaminants to less harmful
forms or immobilize them to reduce risks. Such transformation and immobilization
processes result from biological, chemical, and physical reactions that take
place in the subsurface. These reactions may include biodegradation by
subsurface microbes, reactions with naturally occurring chemicals, and sorption
on the geologic media that store groundwater in the subsurface.
What is Natural
Attenuation?
Natural attenuation is the combination of naturally occurring
processes that act without the need of human intervention or enhancement, and
result in reduced risks posed by contamination in soil and groundwater. A good
working definition is that of the US Environmental Protection Agency (EPA)
Office of Solid Waste and Emergency Response:
“The ‘natural
attenuation processes’ that are at work in such a remediation approach include a
variety of physical, chemical and biological processes that, under favorable
conditions, act without human intervention to reduce the mass, toxicity,
mobility, volume and concentration of contaminants in soil and water. These
in situ processes
include biodegradation, dispersion, dilution, sorption, volatilization,
stabilization, transformation and destruction of contaminants.”
Remediation of
Contaminants
The mechanism 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. Biodegradation 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. Certain microorganisms digest fuels or chlorinated solvents found in
the subsurface environment. Non-destructive attenuation mechanisms include
volatilization, sorption, dispersion and dilution. The last three generally
being the most important.
Groundwater
remediation is a complex process relying on the implementation of a variety of
technologies to effectively degrade or remove the contamination. Remediation
relies on the knowledge of the physical, chemical and hydrogeological properties
of the subsurface. It is acknowledged, however, that it may not be practicable
or feasible to fully restore the contaminated groundwater (US EPA, 1988). Active
remediation may not be effective or feasible at widespread plumes, or because of
hydrogeological constraints (e.g. fractured rock or karst geology), contaminant
properties or the physical/chemical interaction of the contaminants in the
subsurface. In these instances natural attenuation, end-user (wellhead)
treatment with monitoring or institutional controls may be the only feasible
solutions for these sites.
Natural
Attenuation Process
Natural
attenuation is currently the favoured term applied to a multitude of in situ
processes including decay (radioactive), biodegradation, chemical or
biological stabilization, destruction, dilution, dispersion, sorption,
transformation or volatilization. Natural attenuation processes typically occur
at most contaminated sites but its effectiveness is controlled by the
contaminants of concern, and the physical, chemical, biological and
hydrogeological properties of the soil and groundwater. Natural attenuation
processes are largely complex oxidation and reduction processes and under
favourable conditions, reduce the risks to human health and the environment.
Role of Natural Attenuation in
Remediation
The implication of this
study and other similar studies (US EPA, 1999) is that the use of natural
attenuation, as with any other remedial technology, must consider several key
principles to ensure that the risks are managed or that the remedial objectives
are achieved, including:
-
Control of
Source Zones:
Engineering or hydrogeological remedial measures should be used to address
source zones (e.g. phase separated hydrocarbons) wherever practicable. This
may include containment of wastes (or by-products) where treatment is
impracticable;
-
Restoration of
Contaminated Groundwater:
Contaminated groundwater should be remediated to its “beneficial use”
wherever practicable, within a reasonable site-specific timeframe.
-
Mitigating
Media Transfer of Contaminants:
Contaminated soil should be remediated to achieve an acceptable level of
risk to human and environmental receptors, and to prevent any transfer of
contaminants to other media
(e.g..
to surface water, groundwater, air, or sediments) that would present an
unacceptable risk.
-
Risk
Communication/Public Consultation:
Remedial
actions, in general, should include opportunities for public involvement in
the decision making process. This would serve to educate the interested
parties, solicit feedback concerning the remedial process and provide
greater confidence in remedial strategy in the protection of human and
environmental health.
Advantages
Natural attenuation has a
number of advantages as well as disadvantages. Each of these will need to be
considered with respect to impact on various receptors and the sustainability
and efficiency of the remedial efforts. Potential advantages for natural
attenuation (and other
in situ
remediation
alternatives) include:
-
The
generation of lower volumes of waste;
-
Reduced
potential for cross-media transfer or remobilization of contaminants;
-
Reduced risk
of exposure to contaminants, contaminated media (soil, air, liquid) and
other safety hazards;
-
Reduced
disturbance to ecological receptors;
-
In situ
destruction
of some contaminants;
-
Less
intrusive remedial works;
-
Can be used
as a broader application to treat more extensive plumes;
-
Can be used
with other remedial technologies;
-
Monitoring
can allow low cost enhancement of natural remedial processes;
-
Potential
lower remediation costs compared to active remediation, particularly in some
instances where there is little or no energy input.
Future Prospects
While there is
considerable debate among technical experts about the application of NA, for it
to be chosen as a remedy there must be a significant amount of contaminant
destruction. A remediation strategy that largely depends on physical mechanisms
such as sorption, dilution and dispersion, is not attractive to most
communities. Typically, at most sites contaminated with chlorinated solvents,
perchloroethylene (PCE) and trichloroethylene (TCE) are the major threats. It is
assumed that conventional clean-up of these chemicals through pump-and-treat
systems will also remove co-contaminants. This may not be true with NA. In
studying the natural attenuation of chlorinated solvents, other contaminants
likely to be present in plumes, should also be included in the investigation and
remedy selection.
Ecotoxicology
and Bioremediation Group
National
Botanical Research Institute, Lucknow-226 001
E-mail:
[email protected] |
