United States Office of Solid Waste and EPA 542-F-01-004

Environmental Protection Emergency Response April 2001

Agency (5102G) www.epa.gov/superfund/sites

www.cluin.org

A Citizen’s Guide to

Monitored Natural Attenuation

  	
	
 
 
	


Natural attenuation relies on natural processes to clean up or attenuate pollution in soil and

groundwater. Natural attenuation occurs at most polluted sites. However, the right conditions

must exist underground to clean sites properly. If not, cleanup will not be quick enough or

complete enough. Scientists monitor or test these conditions to make sure natural attenuation

is working. This is called monitored natural attenuation or MNA.

	 
	  	

When the environment is polluted with chemicals, nature can work in four ways to clean it up:

1. Tiny bugs or microbes that live in soil and groundwater use some chemicals for food. When they

completely digest the chemicals, they can change them

into water and harmless gases. (A Citizen’s Guide to

Bioremediation [EPA 542-F-01-001] describes how

microbes work.)

2. Chemicals can stick or sorb to soil, which holds them in

place. This does not clean up the chemicals, but it can keep

them from polluting groundwater and leaving the site.

3. As pollution moves through soil and groundwater, it

can mix with clean water. This reduces or dilutes the

pollution.

  EPA uses many methods to clean up pollution at Superfund and other sites. If you live, work, or go to school near

a Superfund site, you may want to learn more about cleanup methods. Perhaps they are being used or are proposed for use at your site. How do they work? Are they safe? This Citizen’s Guide is one in a series to help

answer your questions.

Microbe Microbe chemical

chemical

soil

groundwater

flow

chemical
ground surface gases rise 

through soil

chemical

evaporates

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 write the Technology

Innovation Office at:

U.S. EPA (5102G)

1200 Pennsylvania Ave.,

NW

Washington, DC 20460

or call them at

(703) 603-9910.

Further information also

can be obtained at

www.cluin.org or

www.epa.gov/

superfund/sites.

The time it takes for MNA to clean up a site depends on

several factors:

• type and amounts of chemicals present

• size and depth of the polluted area

• type of soil and conditions present

These factors vary from site to site, but cleanup usually takes years to decades. MNA

is used when other methods will not work or are expected to take almost as long.

Sometimes MNA is used as a final cleanup step after another method cleans up most

of the pollution.

4. Some chemicals, like oil and solvents, can evaporate, which

means they change from liquids to gases within the soil. If these

gases escape to the air at the ground surface, sunlight

may destroy them.

MNA works best where the source of pollution has been removed. For instance, buried waste

must be dug up and disposed of properly. Or it can be removed using other available cleanup

methods. After the source is removed, the natural processes get rid of the small amount of

pollution that remains in the soil and groundwater. The soil and groundwater are monitored

regularly to make sure they are cleaned up.

  

MNA can be a safe process if used properly. No one has to dig up the pollution, and nothing

has to be added to the land or water to clean it up. But MNA is not a “do nothing” way to clean

up sites. Regular monitoring is needed to make sure pollution doesn’t leave the site. This

ensures that people and the environment are protected during cleanup.

  	
	
 
 
	


Depending on the site, MNA may work just as well and almost as fast as other methods.

Because MNA takes place underground, digging and construction are not needed. As a result,

there is no waste to dispose of in landfills. This is less disruptive to the neighborhood and the

environment. Also, it allows cleanup workers to avoid contact with the pollution. MNA requires

less equipment and labor than most methods. Therefore, it can be cheaper. Monitoring for many

years can be costly, but it may cost less than other methods.

MNA is the only cleanup method being used at a few Superfund sites with groundwater pollution. At over 60 other sites with polluted groundwater, MNA is just one of the cleanup methods

being used. MNA also is used for oil and gasoline spills from tanks.

NOTE: This fact sheet is intended solely as general guidance and information to the public. It is not intended, nor can it be relied

upon, to create any rights enforceable by any party in litigation with the United States, or to endorse the use of products or services

provided by specific vendors. The Agency also reserves the right to change this fact sheet at any time without public notice.