1998 CPEO Brownfields List Archive

From: Lenny Siegel <lsiegel@igc.apc.org>
Date: Mon, 26 Jan 1998 07:14:38 -0800 (PST)
Reply: cpeo-brownfields
Subject: Natural Attenuation


In November, U.S. EPA issued an Interim Final policy, "Use of Monitored
Natural Attenuation at Superfund, RCRA Corrective Action, and
Underground Storage Tank Sites." (OSWER Directive 9200.4-17) The 20-page
policy provides a valuable framework for evaluating proposals and
overseeing the implementation of natural attenuation as a cleanup
response. The directive is designed to provide "guidance to EPA staff,
to the public, and to the regulated community on how EPA intends to
exercise its discretion in implementing national policy on the use of
Monitored Natural Attenuation." It does not provide all the answers for
determining where and when natural attenuation is a preferable or even
an acceptable cleanup remedy.

Natural attenuation, increasingly being relied upon as a response to
groundwater and soil contamination at federal and private hazardous
waste sites, is defined by EPA to "include a variety of physical,
chemical, or biological processes that, under favorable conditions, act
without human intervention to reduce the mass, toxicity, volume, or
concentration of contaminants in soil or groundwater. These in-situ
processes include biodegradation; dispersion; dilution; sorption;
volatilization; and chemical or biological stabilization,
transformation, or destruction of contaminants."

Natural attenuation, the directive notes, reduces the potential risk
posed by site contaminants 1) by destructive biodegradation and chemical
reaction, 2) by reducing the concentration, or 3) by reducing mobility
or bioavailability.

Natural attenuation processes may be integrated into a remedial strategy
following source control measures. Typically, they will be used in
conjunction with active remediation. However, the transformation of
contamination into more toxic products - such as the degradation of TCE
(trichloroethylene) into vinyl chloride - should be evaluated for its
long-term impact. Similarly, the transfer of contamination from one
medium to another - such as soil to air - is undesirable and acceptable
only in rare cases.

The policy addresses three general categories of common pollutants
susceptible to natural attenuation: petroleum-related products,
chlorinated solvents, and inorganics.

Petroleum-related products:

The policy recognize that the BTEX (benzene, toluene, ethyl benzene, and
xylene) contaminants commonly found in petroleum products break down
predictably and tend not to migrate great distances. However, it notes
that residual contamination may still pose health risks. It therefore
recommends source removal and perhaps institutional controls at
petroleum release sites.

The policy also notes that other chemicals are often found in petroleum
spills and leaks, notably the gasoline additive MTBE (methyl
tertiary-butyl ether) do not break down naturally. It states, "In
general, monitored natural attenuation is not appropriate as a sole
remediation option at sites where non-degradable and nonattenuated
contaminants are present at levels that pose an unacceptable risk to
human health or the environment."

Chlorinated solvents

Though research shows that chlorinated solvents such as TCE biodegrade
under certain conditions, natural attenuation is effective less often as
a remedial option at sites containing those compounds, as opposed to
fuels. Furthermore, such contamination is often mixed with other, less
degradable toxic substances, such as 1,4-dioxane.

Inorganic substances

The most common form of inorganic contamination is metallic. Metals
don't degrade, but they may be transformed into less mobile or toxic
forms through sorption (attachment to soil particles) or chemical
reactions. Radionuclides naturally decay. While some may generate
radioactive daughter products as they decay, tritium, the radioactive
form of water, does not.

The EPA policy identifies numerous advantages and disadvantage of
natural attenuation when, through monitoring, it can be shown to be
occurring. Like active in situ (in place) processes, it generate less
waste, less transfer of contamination to other media, less risk of human
exposure, and less surface disturbance than other remedies. Furthermore,
it usually is less costly than more active responses. On the negative
side, natural attenuation tends to take longer than active methods. Site
characterization - the study phase - may be more complex and costly. And
in certain cases, the transformation products may be more hazardous than
the original contaminants.

In the policy, EPA recognizes that monitored natural attenuation may be
an appropriate remediation option that should be evaluated along with
other alternatives, but "Monitored natural attenuation should not be
considered a default or presumptive remedy at any contaminated site."

EPA identifies three principles underlying all of its cleanup programs.
Nothing in the monitored natural attenuation policy is supposed to
change the application of those principles.

1) Source control actions should treat "principal threat" wastes
(liquids that have not dissolved or soils saturated with contaminants)
where practicable and if not, they should be contained with engineering

2) Where practicable, groundwater should be brought to drinking water or
similar standards. Where that can't be done, the migration of
contamination should be controlled.

3) Soil should be remediated "to achieve an acceptable level of risk to
human and environmental receptors, and to prevent any transfer" to water
or other media.

In the directive, EPA makes it clear that neither the agency nor
responsible parties can use the presence of natural attenuation to walk
away from cleanup or financial responsibility. It states, "Monitored
natural attenuation is an appropriate remediation method only where its
use will be protective of human health and the environment and it will
be capable of achieving site-specific remediation objectives within a
time frame that is reasonable compared to other alternatives."

The agency makes it clear that the selection of monitored natural
attenuation as a remedy is different than a determination of "technical
impracticability." On the one hand, active measures may be employed at
sites where complete cleanup is technical impracticable. On the other
hand, monitored natural attenuation may be selected over other
practicable, but less preferable remedies.

The central tenet of the monitored natural attenuation policy is that
the both the short-term and long-term sufficiency of natural attenuation
processes must be demonstrated and backed up. Most important, "Decisions
to employ natural attenuation as a remedy or remedy component should be
thoroughly and adequately supported with site-specific characterization
data and analysis." That is, the evaluation of natural attenuation
requires more study than the evaluation of active remediation
alternatives. Those responsible for cleanup must collect data on the
nature and distribution of contaminants. In the case of groundwater, the
hydrogeology of the site must be well understood. For biodegradation and
other reactions, the availability of nutrients and reactants must be
documented. It generally will necessary to analyze the data using a
conceptual site model, such as a computer simulation. If all these steps
cannot be carried out in a way that demonstrates the likely
effectiveness of natural attenuation, then it should not be accepted as
a remedy.

Monitoring and modeling should demonstrate that contaminant mass or
concentration is decreasing. Notably, in the case of a groundwater
plume, it must be demonstrated that decreasing concentrations are not
simply the result of plume migration - in lay terms, dilution. In the
case of inorganics, the actual mechanism causing attenuation must be

The decision to rely upon monitored natural attenuation should be made
through the remedy selection critieria that normally govern cleanup
decisions under the EPA program through which the site is being
addressed. Rather than function as a stand alone remedy, natural
attenuation is most likely to be selected as a remedy in conjunction
with active remediation or as a follow-up to active measures.

In evaluating monitored natural attenuation as a potential remedy, EPA
or other regulatory authorities should consider numerous factors,

* Do transformation products present a greater risk than the original

* Have sources been controlled?

* Is the plume stable?

* Are the conditions that make natural attenuation possible likely to

* Is the time frame reasonable compared to other methods?

* Are there adequate vehicles to enforce institutional controls at the

That is, there is no cookie-cutter set of criteria to be used to
evaluate the acceptability of monitored natural attenuation. However,
the policy suggests, "monitored natural attenuation would more likely be
appropriate if the plume is not expanding, nor threatening downgradient
wells or surface water bodies ... Therefore, sites where the contaminant
plumes are no longer increasing in size, or are shrinking in size, would
be the most appropriate candidates" - only if there are no unacceptable
impacts on human or environmental receptors.

Monitoring does not stop, however, with the acceptance of monitored
natural attenuation as a remedy. "Performance monitoring should continue
as long as contamination remains above required cleanup levels." It's
even more important at natural attenuation sites than at other sites.
Through ongoing monitoring, the responsible parties and EPA should
demonstrate that natural attenuation is indeed occurring as expected,
and that the conditions remain in place for natural attenuation to
continue. They should identify transformation products. They should
determine if a plume is expanding in any direction. And they should make
sure that no people or ecological receptors are being adversely

The flip side of performance monitoring is the establishment, up front,
of contingency, or "backup" remedies. EPA recommends that they be
considered as part of the original remedy selection decision,
particularly if predictive modeling, rather than historical data, has
been used as evidence of natural attenuation. While contingency remedies
should be flexible enough to take into account new information, they
should be based upon pre-established trigger criteria, such as increased
contaminant concentrations at specific sampling points (over a
sufficient duration to cancel seasonal variations). 

In conclusion, EPA stresses that monitored natural attenuation is not a
"no action" remedy. The directive does not signify a changes in the
agency's remediation goals: "Monitored natural attenuation should be
selected only where it will be fully protective of human health and the

This is merely an introductory summary of the EPA policy, which with
references runs 25 pages. The complete directive may be found on the
World Wide Web at http://www.epa.gov/OUST/directiv/d9200417.htm.


Lenny Siegel
Director, SFSU CAREER/PRO (and Pacific Studies Center)
c/o PSC, 222B View St., Mountain View, CA 94041
Voice: 650/961-8918 or 650/969-1545
Fax: 650/968-1126

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