United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
National Risk Management
Research Laboratory
Ada, OK 74820
EPA/600/F-98/021
May 1999
SEBV
Monitored Natural Attenuation of
Petroleum Hydrocarbons
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 petroleum hydrocarbons such as gasoline, diesel,
motor oils, and similar materials have caused widespread
Biodegradation
Sorption
Dispersion,
and Dilution
Processes
of
NATURAL
ATTENUATION
of
Petroleum
Hydrocarbons
Chemical
Reactions
| Volatilization
(Evaporation)
Figure 1. Processes of natural attenuation of petroleum hydrocarbons.
contamination in the environment. Generally these contaminants
are present both in NAPL form (non-aqueous phase liquid; the bulk
liquid petroleum hydrocarbon) 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 petroleum hydrocarbon
contamination and may accomplish site remediation goals at a lower
cost than conventional remediation technologies, within a similar
time 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 important components
of petroleum hydrocarbon contamination can be destroyed by
biodegradation, biodegrading microorganisms are found almost
everywhere, and biodegradation can be very safe and effective.
Many of the most environmentally significant components of petroleum
hydrocarbons such as BTEX (benzene, toluene, ethyl benzene, and
the xylenes) and some PAHs (polynuclear aromatic hydrocarbons)
can be biodegraded under the proper environmental conditions.
However, some PAHs, MTBE (methyl tertiary butyl ether, a gasoline
additive) and other components of petroleum hydrocarbons may not
readily biodegrade. Generally speaking, the petroleum hydrocarbons
that are most mobile in the environment (except for MTBE) are also
readily biodegraded. Once the more mobile and easily degradable
petroleum hydrocarbons are removed, the remaining hydrocarbons,
which are not readily degraded, can still pose a high risk in the
immediate vicinity of the area in which they remain.
Microorganisms are most effective at degrading low to moderate
concentrations of contaminants. High concentrations and very low
concentrations of contaminants may not be biodegradable.
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Contaminants in the NAPL phase are not effectively degraded by
microorganisms.
As contaminants biodegrade, the products ofthe degradation process
may or may not be less harmful than the original contaminants.
Therefore, it is important to investigate the site processes carefully
to be sure that biodegradation is making the site safer. Fortunately,
petroleum hydrocarbons appearto degrade to less harmful products
in almost all cases. 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.
Sorption
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. Eventuallythe contaminant concentrations
may be reduced so low that the risk to human and environmental
health will be minimal.
Chemical Reactions
Some contaminants degrade by chemical reactions (that are not
facilitated by microorganisms). However, most petroleum hydro-
carbons are not significantly degraded by chemical reactions in soil
or ground water.
Volatilization (Evaporation)
Many petroleum hydrocarbons evaporate readily into the atmosphere,
where air currents disperse the contaminants, reducing the
concentration. In some cases, this means of natural attenuation may
be useful, since the hydrocarbons can be broken down by sunlight.
Vapors in contact with soil microorganisms may be biodegraded.
Volatilization from NAPL or ground water into 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
Expanded View of Residual NAPL
Trapped in Pores Between Soil &
Sediment Particles
GROUND
SURFACE
Petroleum
Hydrocarbon
Storaae
Tan
Volatilization
Sorbed
Contaminants
Petroleum
Hydrocarbon
Leak
LNAPL
(Bulk
Fluid)
Biodegradation /
Dissolved
Hydrocarbons
WATER
TABLE
Dissolved
Hydrocarbon
Plume
Figure 2. As the bulk hydrocarbon 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 ofthe soil particles (sorption) and some
may dissolve in the ground water (dissolved plume). Since bulk petroleum hydrocarbon liquids are less dense than water, the liquid
may float on top ofthe ground water table, rising and falling as the water table rises and falls through the seasons. This process can
create a smear zone of residual saturation. As the dissolved plume moves, the concentration ofthe dissolved hydrocarbons is lowered
by dispersion and dilution effects. Microorganisms may degrade hydrocarbons that are dissolved, volatilized or sorbed.
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alternative, it is necessary to study each contaminated site carefully
to determine how effective natural attenuation is for attaining site
remediation goals.
Bulk petroleum hydrocarbons-gasoline, diesel, motor oil, etc., in the
NAPLform, ratherthan dissolved in waterorsorbed 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 order to
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 frame. Natural attenuation processes
are usually of most significance for the 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
necessary to know the location and concentration of the contami-
nants, 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 of the 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.
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 of the 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. A detection system for early warning
of impacts on sensitive receptors, such as drinking water wells,
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.
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 Petroleum-Contaminated Sites at Federal
Facilities, http://www.epa.gov/swerffrr/petrol.htm. (20 May 1999).
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.
Summary
Natural attenuation processes occur to varying degrees in all
petroleum hydrocarbon contamination sites, and may contribute
significantly to site remedial goals. Biodegradation processes can be
particularly important for natural attenuation of petroleum
hydrocarbons, because microorganisms can degrade most
environmentally significant petroleum hydrocarbons. Hydrocarbons
dissolved in water or sorbed on soil particles are the most readily
subject to natural attenuation processes, but bulk hydrocarbons
(NAPLs) are not readily subject to natural attenuation in the short
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