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.
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.
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.

Community Guide to Monitored Natural Attenuation
EPA-542-F-21-018, 2021

The Community Guide series (formerly Citizen's Guides) is a set of two-page fact sheets describing cleanup methods used at Superfund and other hazardous waste cleanup sites. Each guide answers six questions about the method: 1) What is it? 2) How does it work? 3) How long will it take? 4) Is it safe? 5) How might it affect me? 6) Why use it?

Groundwater Microbiology
Ferris, F., N. Szponar, and B. Edwards. | The Groundwater Project (2021)

This book introduces the principals of groundwater microbiology, from aspects of cell structure and growth, to the bioenergetics and metabolism of subsurface microorganisms, to the geochemical and physical influences of widespread microbial activity on groundwater systems and water quality. The coupling of microorganisms with their geological surroundings – a bond established billions of years ago as life took hold on our planet – is of particular value in bioremediation and microbially induced mineral precipitation applications, such as the clean-up of contaminated groundwater systems or reducing aquifer permeability.

Monitored Natural Attenuation of Chlorinated Solvents: U.S. EPA Remedial Technology Fact Sheet
EPA 600-F-98-022, 1998

This fact sheet explains what monitored natural attenuation means when the term is used to describe a potential strategy to remediate a contaminated site. It also describes the various physical, chemical and biological processes of natural attenuation that may occur at a site. This fact sheet is written for an audience with little or no scientific background and is meant to aid Federal, State, and local regulators in educating the public on complex environmental issues. Other informational materials are in preparation and will provide more specific details and scientific depth for the evaluation of monitored natural attenuation as a remedy at specific sites.

Monitored Natural Attenuation of Petroleum Hydrocarbons: U.S. EPA Remedial Technology Fact Sheet
EPA 600-F-98-021, 1998

This fact sheet explains what monitored natural attenuation means when the term is used to describe a potential strategy to remediate a contaminated site. It also describes the various physical, chemical and biological processes of natural attenuation that may occur at a site. This fact sheet is written for an audience with little or no scientific background and is meant to aid Federal, State, and local regulators in educating the public on complex environmental issues. Other informational materials are in preparation and will provide more specific details and scientific depth for the evaluation of monitored natural attenuation as a remedy at specific sites.

Natural Attenuation of Hazardous Wastes
2004. R. Surampalli, S.K. Ong, E. Seagren, and J. Nuno (eds.). American Society of Civil Engineers, Reston, VA. ISBN: 0784407401, 256 pp.

In addition to a discussion of the regulatory framework, this book covers major pollutants and basic scientific principles on physical, chemical, and biological processes involved in natural attenuation, and contains an extensive review of literature, examples of applications of natural attenuation, and site characterization and monitoring requirements and procedures.