A  Citizen's  Guide  to
^nitored   Natural/
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.)
 Biodegradation
 Sorption
    Dilution
Evaporation
                    Chemical
                    Reactions
  •  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 lessfavorable environment
    for biodegradation, sorption or dilution.

These factors vary from site to site.

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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 cc
         1 a groundwater sample.
                                              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 attenua-
                                       tion 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 concentra-
                                       tions 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 on this and
                                                                             other technologies in the Citizen's
                                                                             Guide Series, contact:
                                                                                      U.S. EPA
                                                                                Technology Innovation &
                                                                                Field Services Division
                                                                             Technology Assessment Branch
                                                                                   (703)603-9910
                                                                                       Or visit:
                                                                              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.
United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA 542-F-12-014
September 2012
www.epa.gov/superfund/sites
www.cluin.org

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