Monitored Natural Attenuation
A fact sheet providing information about one strategy for treating contaminated groundwater at the MMR.

The purpose of this fact sheet is to describe one of the treatment technologies to be used to address the Landfill-1 (LF-1) groundwater plume emanating from the Massachusetts Military Reservation.

Description of the Technology

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 are always occurring 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, sometimes forming non-harmful byproducts like carbon dioxide and water
• chemical stabilization—reduction in contaminant mobility caused by chemical processes
• dispersion—the process of mixing that occurs when fluid flows through a porous medium
• sorption—attachment of compounds to geologic materials by physical or chemical attraction
• volatilization—transfer of a chemical from liquid to vapor; evaporation

Monitored natural attenuation, by definition, occurs naturally. However, 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 plume treatment.

Furthermore, to use monitored natural attenuation as a cleanup strategy, sampling must be conducted throughout the time the process is used 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. 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.

Advantages and Disadvantages

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:

• Generating less volume of remediation wastes, such as treated plume water, reducing potential cross contamination, and reducing the risk of human exposure to contamination.
• Fewer surface structures are required so there is less disruption to the community and less ecological destruction.
• 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 than active remediation methods.

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.

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 make modeling as accurate as possible.
• An evaluation of intermediate breakdown compounds must be conducted, as they may be more mobile and more toxic than the original contaminant.
• Potential risk level must be evaluated, as monitored natural attenuation should be used only in low risk situations.
• An assessment of the migration rate must be made, as contaminants may migrate before they are degraded.
• Restrictions on use of groundwater or surface water downgradient may have to be enforced until contaminant levels are reduced.

Monitored natural attenuation may be appropriate at sites where chlorinated solvents and petroleum compounds are located together, the soil contains high levels of natural organic matter, shallow groundwater is separated from deeper groundwater by a low-permeability layer, or there is little or no source remaining due to active remediation.

Use at Other Locations

Monitored natural attenuation has been used at hazardous waste sites across the country. Although it is a component of remediation activities at many sites, only in the last few years has it been applied specifically as a treatment strategy to be considered along with other engineered solutions. According to A Citizens Guide to Monitored Natural Attenuation (EPA, October 1996) under the Superfund program, monitored natural attenuation has been selected as part of the clean up methods at 73 groundwater contamination sites. The sites include municipal and industrial landfills, refineries and recycling facilities. Within EPA Region I, which covers New England, it is a component of the selected remedy at sites such as Fort Devens in Massachusetts and Savage Municipal Water Supply in New Hampshire.

LF-1 Related Activities

Use of monitored natural attenuation for the western portion of the LF-1 plume meets appropriate criteria for EPA compliance, such as low contaminant concentrations, no immediate threat to human health and the environment, and the source has been remediated using other active technologies.

AFCEE is conducting additional sampling of the LF-1 plume and further modeling of the potential effects of the plume to determine if monitored natural attenuation is an appropriate remedial activity for any portion of the eastern part of the plume. This is being done following guidance issued by EPA and the DoD. Copies of this guidance document can be found in local libraries.

There are opportunities for information exchange and public comment associated with the use of monitored natural attenuation for the portion of the plume east of Route 28. These include fact sheets, neighborhood meetings and a public comment period.

For More Information

For more information, please contact the Community Involvement Office for the Installation Restoration Program by calling (508) 968-4678 or by sending an electronic mail message to: vmusgrave@mafmh.ang.af.mil