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Vol. 15 No. 2 - April 2009

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:

  1. 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;

  2. Restoration of Contaminated Groundwater: Contaminated groundwater should be remediated to its “beneficial use” wherever practicable, within a reasonable site-specific timeframe.

  3. 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.

  4. 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: rai_un@rediffmail.com


This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.


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