Monitored Natural Attenuation

What is natural attenuation?

Natural attenuation makes use of natural processes to contain the spread of contamination from chemical spills and reduce the concentration and amount of pollutants at contaminated sites. Natural attenuation is also referred to as intrinsic remediation, bioattenuation, or intrinsic bioremediation and is an in situ treatment method. This means that environmental contaminants are left in place while physical, biological and chemical processes degrade the contaminants of concern. Natural attenuation is often used as one part of a site cleanup that also includes the control or removal of the source of the contamination.

How does monitored natural attenuation work?

Monitored natural attenuation refers to the treatment approach of allowing natural processes to reduce contaminant concentrations to acceptable levels. Monitored natural attenuation involves physical, chemical and biological processes, which act to reduce the mass, toxicity and mobility of subsurface contamination. These processes always occur to some extent and, in many cases, may reduce risk to human health and the contamination to acceptable levels.

There are several different physical, chemical and biological processes that comprise monitored natural attenuation. These include:

• Biodegradation: breakdown of contaminants by microorganisms in the environment, the goal being the formation of non-toxic by-products like carbon dioxide and water.
• Chemical degradation or stabilization: reduction in the concentration of the target contaminant as a result of chemical processes.
• Dispersion: the process of mixing that occurs when fluids flow through a porous medium and "clean" groundwater is mixed with contaminated water.
• Sorption: the attachment of compounds to soil or rock by physical or chemical attraction.
• Volatilization: transfer of a chemical from liquid to vapor. Volatilization is analogous to evaporation.

Monitored natural attenuation, by definition, occurs naturally. However, the use of monitored natural attenuation as a specific treatment method is not a "do nothing" approach. It involves modeling, sampling and analysis, active monitoring and evaluation of contaminant reduction rates to determine whether it is a feasible method for contaminant treatment.

Furthermore, to use monitored natural attenuation as a cleanup strategy, sampling must be conducted throughout the time the process is used. This is done to confirm that contaminant attenuation is proceeding at expected rates. The responsibility falls on those required to clean up the site to verify that the contaminants pose no risk to human health or the environment and that natural processes are reducing contaminant levels as predicted and to acceptable levels. Sampling and analysis will determine whether monitored natural attenuation actually is reducing the mass, toxicity and mobility of the contamination. If not, the information can be used to take other appropriate action.

What are the advantages and disadvantages of monitored natural attenuation?

There are several advantages and disadvantages associated with monitored natural attenuation. Each should be carefully considered during site characterization and evaluation of remediation alternatives.

Potential advantages of monitored natural attenuation include:

• Generation of a reduced volume of remediation wastes, as compared to that obtained when the contaminated groundwater is treated "above ground."
• Reduction in potential cross contamination of uncontaminated groundwater because pumping is not required.
• Reduction in the risk of human exposure to contamination, as the extent to which humans contact contaminants is minimized by keeping contaminants below ground.
• Fewer surface structures are required so there is less disruption to the site and less ecological destruction due to construction activities.
• May be applicable to all or part of a site, depending on conditions and objectives.
• May be combined with other active remedial measures.
• Lower overall remediation costs.

The potential disadvantages of monitored natural attenuation include:

• Long-term monitoring is necessary.
• Site characterization may be more complex, costly, and time consuming.
• Possible longer time frames to achieve remediation objectives.
• Intermediate breakdown compounds may be more toxic than initial contaminants.
• Extensive education and outreach efforts are involved in order to inform the public regarding monitored natural attenuation.

What are the design considerations?

Many factors affect whether or not monitored natural attenuation is a feasible treatment alternative at a site. These include:

• Data must be collected to ensure that computer modeling is as accurate as possible.
• An evaluation of intermediate breakdown compounds must be conducted, as these may be more mobile and more toxic than the original contaminant.
• The potential risk level must be evaluated, as monitored natural attenuation should be used only in low risk situations.
• An assessment of the migration rate (of the contaminant and any byproducts) must be made, as contaminants may have to be contained until contaminant levels are reduced.

Why consider natural attenuation?

In certain situations, natural attenuation is an effective, inexpensive cleanup option and the most appropriate way to remediate the site. Natural attenuation is sometimes mislabeled as a "no action" approach. However, natural attenuation is really a proactive approach that focuses on the confirmation and monitoring of natural remediation processes rather that relying totally on "engineered" technologies.

Mobile and toxic fuel hydrocarbons, for example, are good candidates for natural attenuation. Not only are they difficult to trap because of their mobility, but they are also among the contaminants most easily destroyed by biodegradation. Natural attenuation is non-invasive, and, unlike many elaborate mechanical site cleanup techniques, while natural attenuation is working below ground, the land surface above ground may continue to be used. Natural attenuation can be less costly than other active engineered treatment options, especially those available for ground water, and may not require an energy source or special equipment.  

Will natural attenuation work at every site?

To estimate how well natural attenuation will work and how long it will take requires a detailed study of the contaminated site. The community and those conducting the cleanup need to know whether natural attenuation, or any proposed remedy, will reduce the contaminant concentrations in the soil and water to legally acceptable levels within a reasonable time.

Natural attenuation may be an acceptable option for sites that have been through some active remediation that has reduced the concentrations of contaminants. However, natural attenuation is not an appropriate option at all sites. The rates of natural processes are typically slow. Long term monitoring is necessary to demonstrate that contaminant concentrations are continually decreasing at a rate sufficient to ensure that they will not become a health threat. If not, more aggressive remedial alternatives may be considered.

Because the ability of natural attenuation to be an effective cleanup method depends on a variety of conditions, the site needs to be well characterized to determine if natural attenuation is occurring or will occur. Sites where the soil contains high levels of natural organic matter, such as swampy areas or former marshlands often provide successful conditions for natural attenuation. Certain geological formations such as fractured bedrock aquifers or limestone areas are less likely candidates for natural attenuation because these environments often have a wide variety of soil types that cause unpredictable ground water flow and make predicting the movement of contamination difficult.

This fact sheet was written by Dr. Susan Masten and Ashish Chavan, Michigan State University TOSC Program. The information is based in part on "Monitored Natural Attenuation", the Installation Restoration Program for the Massachusetts Military Reservation, and "A Citizen's Guide to Natural Attenuation", the United States Environmental Protection Agency.

The Technical Outreach Services for Communities (TOSC) Program provides independent technical expertise to communities with contaminated sites and promotes community involvement in site-cleanup projects. For more information about TOSC, please contact Kirk Riley, TOSC Program Director, at (800) 490-3890.