United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA542-F-96-015
October 1996
&EPA
A Citizen's Guide to
Natural Attenuation
Technology Innovation Office
Technology Fact Sheet
What is natural attenuation?
Natural attenuation makes use of natural processes to
contain the spread of contamination from chemical
spills and reduce the concentration and amount of
pollutants at contaminated sites. Natural attenua-
tion—also referred to as intrinsic remediation,
bioattenuation, or intrinsic bioremediation—is an in
situ treatment method. This means that environmen-
tal contaminants are left in place while natural at-
tenuation works on them. Natural attenuation is
often used as one part of a site cleanup that also
includes the control or removal of the source of
the contamination.
How does natural attenuation work?
The processes contributing to natural attenuation are
typically acting at many sites, but at varying rates
and degrees of effectiveness, depending on the types
of contaminants present, and the physical, chemical
and biological characteristics of the soil and ground
water. Natural attenuation processes are often cat-
egorized as destructive or non-destructive. Destruc-
tive processes destroy the contaminant.
Non-destructive processes do not destroy the con-
taminant but cause a reduction in contaminant
concentrations.
Natural attenuation processes may reduce contami-
nant mass (through destructive processes such as bio-
degradation and chemical transformations); reduce
contaminant concentrations (through simple dilution
or dispersion); or bind contaminants to soil particles
so the contamination does not spread or migrate very
far (adsorption).
Biodegradation, also called bioremediation, is a pro-
cess in which naturally occurring microorganisms
(yeast, fungi, or bacteria) break down, or degrade,
hazardous substances into less toxic or nontoxic sub-
stances. Microorganisms, like humans, eat and digest
organic substances for nutrition and energy. (In
chemical terms, "organic" compounds are those that
contain carbon and hydrogen atoms.) Certain micro-
organisms can digest organic substances such as fuels
or solvents that are hazardous to humans. Biodegra-
dation can occur in the presence of oxygen (aerobic
conditions) or without oxygen (anaerobic condi-
tions). In most subsurface environments, both aerobic
and anaerobic biodegradation of contaminants occur.
The microorganisms break down the organic con-
taminants into harmless products—mainly carbon di-
oxide and water in the case of aerobic biodegradation
(Figure 1). Once the contaminants are degraded, the
A Quick Look at Natural Attenuation
Uses naturally occurring environmental processes to clean up sites.
Is non-invasive and allows the site to be put to productive use while being cleaned up.
Requires careful study of site conditions and monitoring of contaminant levels.
Printed on Recycled Paper
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Figure 1. Schematic Diagram of Aerobic Biodegradation in Soil
Microorganisms eat oil
or other organic
contaminant
Microorganisms digest oil and
convert it to carbon dioxide (CO2)
and water (H2O)
Microoganisms
give off CO2 and
H2O
microorganism populations decline because they
have used their food sources. Dead microorganisms
or small populations in the absence of food pose no
contamination risk. The fact sheet entitled A
Citizen's Guide to Bioremediation describes the
process in detail (see page 4).
Many organic contaminants, like petroleum, can be
biodegraded by microorganisms in the underground
environment. For example, biodegradation processes
can effectively cleanse soil and ground water of hy-
drocarbon fuels such as gasoline and the BTEX com-
pounds—benzene, toluene, ethylbenzene, and
xylenes. Biodegradation also can break down chlor-
inated solvents, like trichloroethylene (TCE), in
ground water but the processes involved are harder
to predict and are effective at a smaller percentage of
sites compared to petroleum-contaminated sites.
Chlorinated solvents, widely used for degreasing air-
craft engines, automobile parts, and electronic com-
ponents, are among the most often-found organic
ground-water contaminants. When chlorinated com-
pounds are biodegraded, it is important that the deg-
radation be complete, because some products of the
breakdown process can be more toxic than the origi-
nal compounds.
The effects of dilution and dispersion appear to re-
duce contaminant concentration but do not destroy
the contaminant. Relatively clean water from the
ground surface can seep underground to mix with
and dilute contaminated ground water. Clean ground
water from an underground location flowing into
contaminated areas, or the dispersion of pollutants as
they spreading out away from the main path of the
contaminated plume also lead to a reduced concen-
tration of the contaminant in a given area.
Adsorption occurs when contaminants attach or
sorb to underground particles. Fuel hydrocarbons
tend to repel water, as most oily substances do.
When they have an opportunity to escape from the
ground water by attaching to organic matter and clay
minerals that also repel water, they do so. This is
beneficial because it may keep the contaminants
from flowing to an area where they might be a health
threat. Sorption, like dilution and dispersion, appears
to reduce the concentration and mass of contamina-
tion in the ground water, but does not destroy the
contaminants.
Why consider natural attenuation?
In certain situations, natural attenuation is an effec-
tive, inexpensive cleanup option and the most appro-
priate way to remediate some contamination
problems. Natural attenuation is sometimes
mislabeled as a "no action" approach. However,
natural attenuation is really a proactive approach that
focuses on the confirmation and monitoring of natu-
ral remediation processes rather than relying totally
on "engineered" technologies. Mobile and toxic fuel
hydrocarbons, for example, are good candidates for
natural attenuation. Not only are they difficult to trap
because of their mobility, but they are also among
the contaminants most easily destroyed by biodegra-
dation. Natural attenuation is non-invasive, and, un-
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like many elaborate mechanical site cleanup tech-
niques, while natural attenuation is working below
ground, the land surface above ground may continue
to be used. Natural attenuation can be less costly
than other active engineered treatment options, espe-
cially those available for ground water, and requires
no energy source or special equipment.
Will natural attenuation work at every
site?
To estimate how well natural attenuation will work
and how long it will take requires a detailed study of
the contaminated site. The community and those con-
ducting the cleanup need to know whether natural at-
tenuation, or any proposed remedy, will reduce the
contaminant concentrations in the soil and water to
legally acceptable levels within a reasonable time.
Natural attenuation may be an acceptable option for
sites that have been through some active remediation
which has reduced the concentrations of contami-
nants. However, natural attenuation is not an appro-
priate option at all sites. The rates of natural
processes are typically slow. Long-term monitoring
is necessary to demonstrate that contaminant concen-
trations are continually decreasing at a rate sufficient
to ensure that they will not become a health threat. If
not, more aggressive remedial alternatives should be
considered.
What Is An Innovative
Treatment Technology?
Treatment technologies are
processes applied to the treatment of
hazardous waste or contaminated
materials to permanently alter their
condition through chemical,
biological, or physical means.
Innovative treatment technologies are
those that have been tested, selected
or used for treatment of hazardous
waste or contaminated materials but
lack well-documented cost and
performance data under a variety of
operating conditions.
Because the ability of natural attenuation to be an ef-
fective cleanup method depends on a variety of con-
ditions, the site needs to be well-characterized to
determine if natural attenuation is occurring or will
occur. Sites where the soil contains high levels of
natural organic matter, such as swampy areas or
former marshlands often provide successful condi-
tions for natural attenuation. Certain geological for-
mations such as fractured bedrock aquifers or
limestone areas are less likely candidates for natural
attenuation because these environments often have a
wide variety of soil types that cause unpredictable
ground water flow and make predicting the move-
ment of contamination difficult.
Where is natural attenuation being used?
Natural attenuation is being used to clean up petro-
leum contamination from leaking underground stor-
age tanks across the country.
Within the Superfund program, natural attenuation
has been selected as one of the cleanup methods at
73 ground-water-contaminated sites—but is the sole
treatment option at only six of these sites. Some of
these sites include municipal and industrial land fills,
refineries, and recyclers.
At the Allied Signal Brake Systems Superfund site in
St. Joseph, Michigan, microorganisms are effectively
removing TCE and other chlorinated solvents from
ground water. Scientists studied the underground
movement of TCE-contaminated ground water from
its origin at the Superfund site to where it entered
Lake Michigan about half a mile away. At the site it-
self, they measured TCE concentrations greater than
200,000 micrograms per liter (Jlg/L), but by the time
the plume reached the shore of Lake Michigan, the
TCE was one thousand times less—only 200 |lg/L.
About 300 feet offshore in Lake Michigan, the con-
centrations were below EPA's allowable levels. EPA
estimated the plume took about 20 years to move
from the source of contamination to Lake Michi-
gan—plenty of time for the microorganisms natu-
rally present in the ground water to destroy the TCE
without any outside intervention. In fact, microor-
ganisms were destroying about 600 pounds of TCE a
year at no cost to taxpayers. EPA determined that na-
ture adequately remediated the TCE plume in St.
Joseph.
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For More Information
The publications listed below can be ordered free of charge by faxing your request to NCEPI at 513-489-8695. If
NCEPI is out of stock of a document, you may be directed to other sources. Some of the documents listed also can
be downloaded free of charge from EPA's Cleanup Information (CLU-IN) World Wide Web site (http://clu-in.com) or
electronic bulletin board (301-589-8366). The CLU-IN help line number is 301-589-8368.
You may write to NCEPI at:
National Center for Environmental Publications and Information (NCEPI)
P.O. Box 42419
Cincinnati, OH 45242
• A Citizen's Guide to Bioremediation, April 1996, EPA 542-F-96-007.
• Symposium on Intrinsic Bioremediation of Ground Water, August 1994, EPA 540-R-94-515.
• Bioremediation Research: Producing Low-Cost Tools to Reclaim Environments, September 1995, EPA 540-R-95-
523a.
• "Natural Bioremediation of TCE," Ground Water Currents (newsletter), September 1993, EPA 542-N-93-008.
• "Innovative Measures Distinguish Natural Bioattenuation from Dilution/Sorption," Ground Water Currents
(newsletter), December 1992, EPA 542-N-92-006.
• How to Evaluate Alternative Cleanup Technologies for UST Sites, (Chapter on Natural Attenuation), May 1995,
EPA510-B-95-007.
Bioremediation Resource Guide, September 1993, EPA 542-B-93-004. A bibliography of publications and
other sources of information about bioremediation technologies.
• Engineering Bulletin: In Situ Biodegradation Treatment, April 1994, EPA 540-S-94-502.
• Selected Alternative and Innovative Treatment Technologies for Corrective Action and Site Remediation: A
Bibliography of EPA Information Sources, January 1995, EPA 542-B-95-001. A bibliography of EPA
publications about innovative treatment technologies.
• WASTECH® Monograph on Bioremediation, ISBN #1-883767-01-6. Available for $49.95 from the American
Academy of Environmental Engineers, 130 Holiday Court, Annapolis, MD 21401. Telephone 410-266-3311.
NOTICE: This fact sheet is intended solely as general guidance and information. It is not intended, nor can it be relied upon, to create any rights enforceable by any
party in litigation with the United States. The Agency also reserves the right to change this guidance at any time without public notice.
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