United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
National Risk Management
Research Laboratory
Ada, OK 74820
EPA/600/F-98/022
May 1999
SEFft
Monitored Natural Attenuation of
Chlorinated Solvents
U.S. EPA REMEDIAL TECHNOLOGY FACT SHEET
Scope of this fact sheet:
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.
What Is Monitored Natural Attenuation?
The term "monitored natural attenuation," as used by the EPA, refers
to the reliance on natural processes to achieve site-specific remedial
objectives. Where found to be a viable remedy, monitored natural
attenuation may be used within the context of a carefully controlled
and monitored site cleanup approach. To be considered an acceptable
alternative, monitored natural attenuation would be expected to
achieve site remedial objectives within a time frame that is reasonable
compared to that offered by other more active methods. Monitored
natural attenuation is always used in combination with "source
control;" that is, removal of the source of the contamination as far as
practicable.
Natural attenuation processes include a variety of physical, chemical,
or biological processes that, under favorable conditions, act without
human intervention to reduce the mass, toxicity, mobility, volume, or
concentration of contaminants in soil or ground water. These
processes include biodegradation; dispersion; dilution; sorption;
volatilization; and chemical or biological stabilization,
transformation, or destruction of contaminants.
Spills and leaks of chlorinated solvents such as perchlorethylene
(PCE), trichloroethylene (TCE) and trichloroethane (TCA) have
Biodegradation
Sorption
Dispersion,
and Dilution
Processes
of
NATURAL
ATTENUATION
of
Chlorinated
Solvents
Chemical
Reactions
^Volatilization
(Evaporation)
Figure 1. Processes of natural attenuation of chlorinated solvents.
caused widespread contamination in the environment. Generally
these contaminants are present both in NAPL form (non-aqueous
phase liquid; the bulk liquid chlorinated solvent) and also as dissolved
contaminants in the ground water. Cleanup of both the NAPL and
dissolved contamination in soils and ground water using many
common remedial techniques is often expensive and slow. However,
under the proper conditions at some sites, natural attenuation can
contribute significantly to remediation of dissolved chlorinated
solvent contamination and may accomplish site remediation goals
at a lower cost than conventional remediation technologies, within a
similartime frame. Natural attenuation is not expected to remediate
NAPL.
How Does Natural Attenuation Work?
Biodegradation
One of the most important components of natural attenuation is
biodegradation—the change in form of compounds carried out by
living creatures such as microorganisms. Underthe right conditions,
microorganisms can cause or assist chemical reactions that change
the form of the contaminants so that little or no health risk remains.
Biodegradation is important because many chlorinated solvents can
be destroyed by biodegradation, microorganisms that are capable of
biodegrading contaminants are found almost everywhere, and
biodegradation can be very safe and effective. However, most
chlorinated solvents biodegrade only undervery specific conditions,
which are not present at all sites.
Microorganisms are most effective at degrading low to moderate
concentrations of contaminants. High concentrations and very low
concentrations of contaminants may not be biodegradable.
Contaminants in the NAPL phase are not effectively degraded by
microorganisms.
As chlorinated solvents biodegrade, the products of the degrada-
tion process may or may not be less harmful than the original
contaminants. Sometimes chlorinated solvents may degrade to
form more toxic compounds, and these toxic compounds may
accumulate under certain conditions. Also, under some conditions,
the microbial activity involved in degrading the contaminants could
cause mobilization of certain materials such as manganese or
arsenic which could cause environmental problems. Monitoring for
these potential problems is necessary.
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Sorption
Volatilization (Evaporation)
The soil and sediment particles (sand, silt, clay, organic matter)
through which the ground water and dissolved contaminants move
can sorb the contaminant molecules onto the particle surfaces, and
hold bulk liquids in the pores in and between the particles, thereby
slowing or stopping the movement of the contaminants. This
process can reduce the likelihood that the contaminants will reach a
location (such as a drinking water well or stream) where they would
directly affect human or environmental health.
Dispersion and Dilution
As the dissolved contaminants move farther away from the source
area, the contaminants are dispersed and diluted to lower and lower
concentrations overtime. Eventually the contaminant concentrations
may be reduced so low that the risk to human and environmental
health will be minimal.
Chemical Reactions
Some chlorinated solvents such as TCA can undergo significant
degradation by chemical reactions without microbial activity. However,
most chlorinated solvents are not significantly degraded by chemical
reactions in soil or ground water, though exposure to sunlight can
break down many chlorinated solvents. Exposure to sunlight is
significant only for chlorinated solvent vapors in the air, or possibly
dissolved solvents in surface water or on the soil surface.
Chlorinated solvents are volatile and readily evaporate into the
atmosphere, where air currents disperse the contaminants, reducing
the concentration. Also, the solvent vapors may be quickly broken
down by sunlight. Vapors in contact with soil microorganisms may
be biodegraded. Volatilization from NAPL or ground waterto soil gas
may be an important exposure pathway in a risk analysis.
Importance of Natural Attenuation Processes
The processes involved in natural attenuation are operating at all
contaminated sites, but the contribution of natural attenuation to
achieving remediation goals varies in different situations. At some
sites natural attenuation may meet all the remedial goals, and at
other sites natural attenuation may make little or no contribution.
Therefore, before natural attenuation can be selected as a remedial
alternative, it is necessary to study each contaminated site carefully
to determine how effective natural attenuation is for attaining site
remediation goals.
Bulk chlorinated solvents-in the NAPL form, ratherthan dissolved
in water or sorbed on soil particles - are not readily degraded by
microorganisms. Also, dispersion, dilution and sorption of the
NAPL is slow. Therefore, it is important to determine where this
NAPL may be at a polluted site, in orderto remove or contain as much
of it as possible, because the processes of natural attenuation would
not effectively remediate most of this material in a reasonable time
Expanded View of Residual NAPL
Trapped in Pores Between Soil &
Sediment Particles
Chlorinated
Solvents
Storage Tank
LOWER
AQUIFER
Figure 2. As the bulk chlorinated solvent moves through the subsurface, some of the liquid may be trapped in the soil or sediment pores (residual
saturation); some may evaporate (volatilization); some may become sorbed to the surface of the soil particles (sorption) and some may
dissolve in the ground water (dissolved plume). Since bulk chlorinated solvents are more dense than water, the liquid tends to move
down below the water table. As the dissolved plume moves, the concentration of the dissolved solvents is lowered by dispersion and
dilution effects. Microorganisms may degrade hydrocarbons that are dissolved, volatilized or sorbed.
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frame. Natural attenuation processes are usually of most signifi-
cance forthe remediation of those contaminants dissolved in water,
sorbed on soil particles, or in the vapor form.
How Is Natural Attenuation Evaluated?
In order to decide what contribution natural attenuation can make to
meeting site remediation goals, very detailed site investigations
must be carried out. Generally, the investment in site characterization
for determining the applicability of natural attenuation is at least as
expensive and time consuming, if not more so, than for any other site
remediation technology. However, the long-term costs for natural
attenuation (if natural attenuation is able to achieve most of the site
remediation goals) may be less than for other remedial technologies.
In order to properly evaluate natural attenuation at a site, it is
necessaryto knowthe location and concentration ofthe contaminants,
and how the contaminants move in the environment. Since
contaminants commonly move dissolved in water, the movement of
ground water at the site must be carefully investigated to determine
how the water moves, when it moves and where it moves. The
subsurface is often very complex in terms of water movement
pathways, and determination of these pathways can be expensive.
Also, evaluation of natural attenuation processes may require a
detailed understanding ofthe site geochemistry, especially where
biodegradation processes are involved. The compounds that may
be associated with microbial activity, such as oxygen, carbon dioxide,
nitrate, sulfate, iron, etc., should be measured in order to gain
understanding of what processes the microorganisms are using,
how fast these processes are occurring, and what the results of these
processes are likely to be. Biodegradation of many chlorinated
solvents takes place under very specific conditions, which may not
be present at many sites.
Evaluation of natural attenuation usually involves not only the
determination of what processes of natural attenuation are occurring,
but also the estimation of what the results of these processes will be
in the future. Therefore, use of natural attenuation as part ofthe site
remedial plan will necessarily require that a long-term monitoring
plan be instituted. The monitoring plan should provide information to
allow regulators to decide if natural attenuation is meeting site
objectives, and to verify that there are no changes in conditions
affecting natural attenuation. Adetection system for early warning of
impacts on sensitive receptors, such as drinking waterwells, streams
and wetlands should be provided. Also, plans must be developed for
contingency remedial efforts that can be implemented if natural
attenuation processes do not fulfill expectations.
Summary
Natural attenuation processes occur to varying degrees in all
chlorinated solvent contamination sites, and may contribute
significantly to site remedial goals. Biodegradation processes can
be particularly important for natural attenuation of some chlorinated
solvents, under specific environmental conditions. Chlorinated
solvents dissolved in water, sorbed on soil particles or in vapor form
are the most readily subject to natural attenuation processes, but
bulk chlorinated solvents (NAPLs) are not readily subject to natural
attenuation in the short term. The significance of natural attenuation
processes at a given site for achieving site remedial goals must be
carefully evaluated, and extensive site characterization and monitoring
is usually necessary.
Additional Information:
U.S. EPA. A Citizen's Guide to Natural Attenuation. EPA 542-F-
96-015. October 1996. http://www.epa.gov/swertio1/download/
remed/citguide/natural.html.
U.S. EPA. Commonly Asked Questions Regarding the Use of
Natural Attenuation for Chlorinated Solvent Spills at Federal
Facilities, http://www.epa.gov/swerffrr/chlorine.htm(20 May 1999).
U.S. EPA. Proceedings ofthe Symposium on Natural Attenuation of
Chlorinated Organics in Ground Water. EPA/540/R-97/504. May
1997. http://www.epa. gov:80/ordntrnt/ORD/WebPubs/natural/
natural.pdf.
U.S. EPA. Technical Protocol for Evaluating Natural Attenuation of
Chlorinated Solvents in Ground Water. EPA/600/R-98/128.
September 1998. http://www.epa.gov/ada/reports.html.
U.S. EPA. Use of Monitored Natural Attenuation at Superfund,
RCRA Corrective Action, and Underground Storage Tank Sites.
OSWER Directive 9200.4-17P, April 21, 1999. http://
www.epa.gov:80/ordntrnt/ORD/WebPubs/biorem/D9200417.pdf.
For more information, contact:
Jerry N. Jones or John T. Wilson
R.S. Kerr Environmental Research Center
Subsurface Protection and Remediation Division
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
P.O. Box 1198
Ada, OK 74820
Authors:
Daniel F. Pope, Dynamac Corporation
Jerry N. Jones, RSKERC/SPRD/NRMRL/ORD
Notice: This document is being distributed solely for informational
purposes. The information presented in this document is not
intended, nor can it be relied upon, to create any rights enforceable
by any party in litigation with the United States. EPA officials may
decide to follow steps in concert with this information, or to act in
variance with the enclosed information based on an analysis of
specific site circumstances. The Agency also reserves the right to
change the information provided in this document at any time
without public notice.
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