EPA's Monitored Natural Attenuation Policy

[Lenny Siegel <lsiegel@cpeo.org>]

EPA'S "MONITORED NATURAL ATTENUATION" POLICY

In November, U.S. EPA issues 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 controls.

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 understood.

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, including:

* Do transformation products present a greater risk than the
original contaminants?

* Have sources been controlled?

* Is the plume stable?

* Are the conditions that make natural attenuation possible
likely to change?

* Is the time frame reasonable compared to other methods?

* Are there adequate vehicles to enforce institutional controls
at the site?

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 affected.

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 environment."

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
lsiegel@cpeo.org