A Citizen’s Guide to

Monitored Natural Attenuation

What Is Monitored Natural 

Attenuation?

Natural attenuation relies on natural processes 

to decrease or “attenuate” concentrations of 

contaminants in soil and groundwater. Scientists 

monitor these conditions to make sure natural 

attenuation is working. Monitoring typically involves 

collecting soil and groundwater samples to analyze 

them for the presence of contaminants and other site 

characteristics. The entire process is called “monitored 

natural attenuation” or “MNA.” Natural attenuation 

occurs at most contaminated sites. However, the 

right conditions must exist underground to clean sites 

properly and quickly enough. Regular monitoring must 

be conducted to ensure that MNA continues to work.

How Does It Work?

When the environment is contaminated with harmful 

chemicals, nature may work in five ways to clean it up:

• Biodegradation occurs when very small

organisms, known as “microbes,” eat contaminants

and change them into small amounts of water

and gases during digestion. Microbes live in

soil and groundwater and some microbes use

contaminants for food and energy. (A Citizen’s

Guide to Bioremediation [EPA 542-F-12-003]

describes how microbes work.)

• Sorption causes contaminants to stick to

soil particles. Sorption does not destroy the

contaminants, but it keeps them from moving

deeper underground or from leaving the site with

groundwater flow.

• Dilution decreases the concentrations of

contaminants as they move through and mix with

clean groundwater.

• Evaporation causes some contaminants, like

gasoline and industrial solvents, to change from

liquids to gases within the soil. If these gases

escape to the air at the ground surface, air will

dilute them and sunlight may destroy them.

• Chemical reactions with natural substances

underground may convert contaminants into

less harmful forms. For example, in low-oxygen

environments underground, the highly toxic

“chromium 6” can be converted to a much less

toxic and mobile form called “chromium 3” when

it reacts with naturally occurring iron and water.

MNA works best where the source of contamination 

has been removed. For instance, any waste buried 

underground must be dug up and disposed of properly, 

or removed using other available cleanup methods. 

When the source is no longer present, natural processes 

may be able to remove the remaining, smaller amount 

of contaminants in the soil or groundwater. The site is 

monitored regularly to make sure that contaminants 

attenuate fast enough to meet site cleanup objectives 

and that contaminants are not spreading.

How Long Will It Take?

MNA may take several years to decades to clean up 

a site. The actual cleanup time will depend on several 

factors. For example, cleanup will take longer when:

• Contaminant concentrations are higher.

• The contaminated area is large.

• Site conditions (such as temperature, groundwater

flow, soil type) provide a less favorable environment

for biodegradation, sorption or dilution.

These factors vary from site to site.
United States Office of Solid Waste and EPA 542-F-12-014

Environmental Protection Emergency Response September 2012

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

www.cluin.org

Example

MNA is being used to complete 

groundwater cleanup at a former 

landfill on the Kings Bay Naval 

Submarine Base, Georgia. From 

1993 to 2001, other cleanup 

methods were used to contain 

and treat the source of solvents 

in the groundwater. The goal was 

to reduce solvent concentrations 

to a level at which MNA would 

ensure safe concentrations at 

the property boundary, and 

unsafe levels of solvents would 

no longer flow beneath nearby 

housing. MNA was considered 

an efficient final treatment 

because of the right conditions 

for bioremediation to occur. 

Monitoring for natural attenuation has been occurring monthly 

since 1998. Groundwater is 

being sampled for solvents 

and other conditions that 

indicate MNA is working. 

The long-term objective is to 

reduce contaminant concentrations across the site to below 

Maximum Contaminant Levels 

(MCLs). Concentrations have 

decreased at most wells, but 

the groundwater in the former 

source area is still expected to 

take decades to reach MCLs. 

For More Information

For more information about 

this and other technologies in 

the Citizen’s Guide Series, 

visit:

www.cluin.org/remediation

www.cluin.org/products/

citguide

www.cluin.org/products/MNA

NOTE: This fact sheet is intended solely as general 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.

Is It Safe?

MNA does not pose a threat to the community or to site workers. MNA 

does not involve excavating soil or pumping groundwater to the surface for 

above ground treatment, so the potential to contact contaminants is limited. 

Long-term, regular monitoring is conducted to make sure contamination does 

not leave the site and that it is being attenuated at a rate that’s consistent with 

cleanup goals for the site. This ensures that people and the environment are 

protected during the cleanup process.

How Might It Affect Me?

Generally, MNA does not cause much disruption to the surrounding community 

since no heavy machinery or other equipment is required during the MNA 

process. Residents and businesses near the site may initially see and hear 

drilling rigs when wells to monitor groundwater quality are installed. Once 

installed, workers will need to visit the site to collect samples of groundwater, 

soil or sediment to ensure MNA is working properly and is protective of human 

health and the environment. At those times, residents may hear the pumps 

and generators often used to collect groundwater samples from the wells.

Why Use Monitored Natural Attenuation? 

MNA is selected when any contaminant source has been removed and only low 

concentrations of contaminants remain in soil or groundwater. The anticipated 

cleanup time for MNA must be reasonable compared to that of other more 

active cleanup methods. MNA requires less equipment and labor than most 

methods, which decreases cleanup costs. However, the cost of many years of 

monitoring can be high. MNA has been selected or is being used at over 100 

Superfund sites across the country.

Monitoring natural attenuation at the site by collecting a groundwater sample.