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United States Office of Solid Waste and EPA 542-F-96-015

Environmental Protection Emergency Response October 1996

Agency (5102G)

A Citizen’s Guide to

Natural Attenuation

Technology Innovation Office Technology Fact Sheet

Printed on Recycled Paper

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.

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 attenuation—also referred to as intrinsic remediation,

bioattenuation, or intrinsic bioremediation—is an in

situ treatment method. This means that environmental contaminants are left in place while natural attenuation 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 categorized as destructive or non-destructive. Destructive processes destroy the contaminant.

Non-destructive processes do not destroy the contaminant but cause a reduction in contaminant

concentrations.

Natural attenuation processes may reduce contaminant mass (through destructive processes such as biodegradation 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 process in which naturally occurring microorganisms

(yeast, fungi, or bacteria) break down, or degrade,

hazardous substances into less toxic or nontoxic substances. 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 microorganisms can digest organic substances such as fuels

or solvents that are hazardous to humans. Biodegradation can occur in the presence of oxygen (aerobic

conditions) or without oxygen (anaerobic conditions). In most subsurface environments, both aerobic

and anaerobic biodegradation of contaminants occur.

The microorganisms break down the organic contaminants into harmless products—mainly carbon dioxide and water in the case of aerobic biodegradation

(Figure 1). Once the contaminants are degraded, the
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Figure 1. Schematic Diagram of Aerobic Biodegradation in Soil

Microorganism

Oil

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

Oil

CO2 + H2O

CO2 + H2O

CO2 + 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 hydrocarbon fuels such as gasoline and the BTEX compounds—benzene, toluene, ethylbenzene, and

xylenes. Biodegradation also can break down chlorinated 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 aircraft engines, automobile parts, and electronic components, are among the most often-found organic

ground-water contaminants. When chlorinated compounds are biodegraded, it is important that the degradation be complete, because some products of the

breakdown process can be more toxic than the original compounds.

The effects of dilution and dispersion appear to reduce 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 concentration 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 contamination in the ground water, but does not destroy the

contaminants.

Why consider natural attenuation?

In certain situations, natural attenuation is an effective, inexpensive cleanup option and the most appropriate 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 natural 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 biodegradation. Natural attenuation is non-invasive, and, un-
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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.

like many elaborate mechanical site cleanup techniques, 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, especially 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 conducting the cleanup need to know whether natural attenuation, 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 contaminants. However, natural attenuation is not an appropriate option at all sites. The rates of natural

processes are typically slow. Long-term monitoring

is necessary to demonstrate that contaminant concentrations 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.

Because the ability of natural attenuation to be an effective cleanup method depends on a variety of conditions, 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 conditions for natural attenuation. Certain geological formations 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 movement of contamination difficult.

Where is natural attenuation being used?

Natural attenuation is being used to clean up petroleum contamination from leaking underground storage 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 itself, they measured TCE concentrations greater than

200,000 micrograms per liter (μg/L), but by the time

the plume reached the shore of Lake Michigan, the

TCE was one thousand times less—only 200 μg/L.

About 300 feet offshore in Lake Michigan, the concentrations were below EPA’s allowable levels. EPA

estimated the plume took about 20 years to move

from the source of contamination to Lake Michigan—plenty of time for the microorganisms naturally present in the ground water to destroy the TCE

without any outside intervention. In fact, microorganisms were destroying about 600 pounds of TCE a

year at no cost to taxpayers. EPA determined that nature adequately remediated the TCE plume in St.

Joseph.
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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.

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,

EPA 510-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.