EC-G-2002-095
&EPA
United States
Environmental Protection
Agency
Office of
Solid Waste and
Emergency Response
DIRECTIVE NUMBER:
9200.4-17P
TITLE: Use of Monitored Natural Attenuation at Superfund, RCRA
Corrective Action, and Underground Storage Tank Sites
APPROVAL DATE: April 21,1999
EFFECTIVE DATE: April 21,1999
ORIGINATING OFFICE: OSWER
El FINAL
D DRAFT
STATUS:
REFERENCE (other documents):
OSWER OSWER OSWER
DIRECTIVE DIRECTIVE DIRECTIVE
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United States Environmental Protection Agency
MppyV Washington, DC 20460
OSWER Directive Initiation Request
1 . Directive Number
9200.4-17P
2. Originator Information
Name of Contact Person
Hal White
Mail Code
5403G
Office
OUST
Telephone Code
(703) -603-7177
Use of Monitored Natural Attenuation at Superfund, RCRA
Corrective Action, and Underground Storage Tank Sites
4. Summary of Directive (include brief statement of purpose)
The purpose of this Directive is to clarify EPA's policy
regarding the use of monitored natural attenuation (MNA) for
the remediation of contaminated soil and groundwater at sites
administered by EPA's Office of Solid Waste and Emergency
Response (OSWER).
D. Keywords
natural attenuation, remediation, soil, groundwater,
contamination
6a. Does This Directive Supersede Previous Directive(s)?
b. Does It Supplement Previous Directive(s)?
No I—I Yes What Directive (number, title)
No I—I Yes What Directive (number, title)
7. Draft Level
E3 A-SignedbyAA/DAA
B - Signed by Office Director
C -For Review and Comment
ID - In Development
8. Document to be distributed to States by Headquarters? ** YesLJ
No
This Requests Meets OSWER Directives System Format Standards
9. Signature of Lead Office Directives Coordinator
T3iti
, ,
Dtfte /
EPA Form 1315-1(7 (Rev 5-87) PrevloWr editions are obsolete
OSWER OSWER OSWER
DIRECTIVE DIRECTIVE DIRECTIVE
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
APR 21 1999
OFFICE OF
SOLID WASTE AND EMERGENCY
RESPONSE
MEMORANDUM
SUBJECT:
FROM:
TO:
Final OSWER Directive "Use of Monitored Natural Attenuation at Superfund,
RCRA Corrective Action, and Underground Storage Tank Sites" (OSWER
Directive Number 9200.4-17P)
^otsworth, Acting Director
' Office of Solid Waste
Walter W. Kovalick, Jr., Irector
Technology Innovation Office
Stephen D. Luftig, Director
Office of Emergency and Remedial Response
ing Director
Office^ of Underground Stwrfge/ Tanks
Wf^^k
Jam
Federal Facilities Restoration and Reuse Office
Addressees
Purpose
This memorandum accompanies a copy of the Final OSWER Directive regarding the use
of monitored natural attenuation for the remediation of contaminated soil and groundwater at
sites regulated under all Office of Solid Waste and Emergency Response (OSWER) programs. A
draft Interim Final version of this Directive was released on December 1,1997 for use, and for
general public review and comment. In response to comments received on that draft, EPA has
incorporated several changes in this final version dealing with topics such as contaminants of
concern, cross-media transfer, plume migration, and remediation time frame.
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Implementation
This Directive is being issued in Final form and should be used immediately as guidance
for proposing, evaluating, and approving Monitored Natural Attenuation remedies. This Final
Directive will be available from the Superfund, RCRA, and OUST dockets and through the
RCRA, Superfund & EPCRA Hotline (800-424-9346 or 703-412-9810). The directive will also
be available in electronic format from EPA's home page on the Internet (the address is
http://www.epa.gov/swerustl/directiv/d9200417.htm).
Questions/Comments
If you need more information about the Directive please feel free to contact any of the
appropriate EPA staff listed on the attachment.
Addressees: Federal Facility Forum
Federal Facilities Leadership Council
Other Federal Facility Contacts
OSWER Natural Attenuation Workgroup
RCRA Corrective Action EPA Regional and State Program Managers
State LUST Fund Administrators
State LUST Program Managers
UST/LUST Regional Program Managers
UST/LUST Regional Branch Chiefs
State Superfund Program Managers
Superfund Regional Policy Managers
attachment
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Attachment
EPA Contacts
January 1999
If you have any questions regarding this policy, please first call the RCRA/Superfund
Hotline at (800) 424-9346. If you require further assistance, please contact the appropriate staff
from the list below:
Headquarters:
Tim Mott—Federal Facilities (202) 260-2447
Remi Langum—Federal Facilities (202) 260-2457
Ken Lovelace—Superfund (703) 603-8787
Guy Tomassoni—RCRA (703) 308-8622
Hal White—UST (703) 603-7177
Linda Fiedler—Technology Innovation (703) 603-7194
Ron Wilhelm—Radiation & Indoor Air (202) 564-9379
Office of Research and Development:
John Wilson—NRMRL, Ada, OK (580) 436-8532
Fran Kroner—NRMRL, Cincinnati, OH (513) 569-7346
Fred Bishop—NRMRL, Cincinnati, OH (513) 569-7629
Groundwater Forum:
Ruth Izraeli—RCRA, Superfund (212) 637-3784
Region 1
Joan Coyle—UST (617)918-1303
Ernie Waterman—RCRA (617) 918-1369
Richard Willey—Superfund (617) 918-1266
Bill Brandon—Federal Facilities (617) 918-1391
Meghan Cassidy—Federal Facilities (617) 918-13 87
Region 2
Derval Thomas—UST (212) 637-4236
Ruth Izraeli—Superfund (212) 637-3784
Jon Josephs—ORD Technical Liaison (212) 637-4317
Carol Stein—RCRA (212) 63 7-4181
Region 3
Jack Hwang—UST (215)814-3387
Kathy Davies—Superfund (215)814-3315
Deborah Goldblum—RCRA (215) 814-3432
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Region 4
David Ariail—UST (404) 562-9464
Kay Wischkaemper—Technical Support (404) 562-8641
Donna Wilkinson—RCRA (404) 562-8490
Robert Pope—Federal Facilities (404) 562-8506
Region 5
Gilberto Alvarez—UST (312) 886-6143
Tom Matheson—RCRA (312) 886-7569
Luanne Vanderpool—Superfund (312) 353-9296
Craig Thomas—Federal Facilities (312) 886-5907
Region 6
Lynn Dail—UST (214) 665-2234
John Cernero—UST (214) 665-2233
Mike Hebert—RCRA Enforcement (214) 665-8315
Arnold Bierschenk—RCRA permitting (214) 665-7435
Lisa Price—Base Closures (214) 665-6744
Region 7
William F. Lowe—RCRA (913)551-7547
Jeff Johnson—RCRA (913)551-7849
Craig Smith—Superfund (913) 551-7683
Ed Wakeland—UST (913)551-7806
Region 8
Sandra Stavnes—UST (303) 312-6117
Randy Breeden—RCRA (303) 312-6522
Richard Muza—Superfund (303) 312-6595
Region 9
Matt Small—UST (415) 744-2078
Katherine Baylor—RCRA (415) 744-2028
Herb Levine—Superfund (415) 744-2312
Ned Black—Superfund (415) 744-23 54
Mark Filippini—Superfund (415) 744-2395
Region 10
Harold Scott—UST (206) 553-1587
Dave Bartus—RCRA (206) 553-2804
Mary Jane Nearman—Superfund (206) 553-6642
Curt Black — Superfund (206) 553-1262
Nancy Harney—Federal Facilities (206) 553-6635
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USE OF MONITORED NATURAL ATTENUATION
AT SUPERFUND, RCRA CORRECTIVE ACTION,
AND UNDERGROUND STORAGE TANK SITES
U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response
Directive 9200.4-17P
April 1999
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OSWER Directive 9200.4-17P
USE OF MONITORED NATURAL ATTENUATION
AT SUPERFUND, RCRA CORRECTIVE ACTION,
AND UNDERGROUND STORAGE TANK SITES
Contents
PURPOSE AND OVERVIEW 1
BACKGROUND 3
Contaminants of Concern 5
Transformation Products 6
Cross-Media Transfer 6
Petroleum-Related Contaminants 7
Chlorinated Solvents 7
Inorganics 8
Advantages and Disadvantages of Monitored Natural Attenuation 9
IMPLEMENTATION 10
Role of Monitored Natural Attenuation in OSWER Remediation Programs 11
Demonstrating the Efficacy of Natural Attenuation Through Site Characterization .... 13
Sites Where Monitored Natural Attenuation May Be Appropriate 17
Reasonable Timeframe for Remediation 19
Remediation of Sources 21
Performance Monitoring and Evaluation 22
Contingency Remedies 24
SUMMARY 25
REFERENCES CITED 25
ADDITIONAL REFERENCES 28
OTHER SOURCES OF INFORMATION 31
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OSWER Directive 9200.4-17P
NOTICE: This document provides guidance to EPA and state staff. It also
provides guidance to the public and to the regulated community on how EPA
intends to exercise its discretion in implementing its regulations. The guidance is
designed to implement national policy on these issues. The document does not,
however, substitute for EPA's statutes or regulations, nor is it a regulation itself.
Thus, it does not impose legally-binding requirements on EPA, States, or the
regulated community, and may not apply to a particular situation based upon the
circumstances. EPA may change this guidance in the future, as appropriate.
in
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OSWER Directive 9200.4-17P
PURPOSE AND OVERVIEW
The purpose of this Directive is to clarify EPA's policy regarding the use of monitored
natural attenuation (MNA) for the cleanup of contaminated soil and groundwater1 in the
Superfund, RCRA Corrective Action, and Underground Storage Tank programs. These
programs are administered by EPA's Office of Solid Waste and Emergency Response (OSWER)
which include the Office of Emergency and Remedial Response (OERR), Office of Solid Waste
(OSW), Office of Underground Storage Tanks (OUST), and the Federal Facilities Restoration
and Reuse Office (FFRRO). Statutory authority for these remediation programs is provided under
the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and
the Resource Conservation and Recovery Act (RCRA).
EPA remains fully committed to its goals of protecting human health and the
environment by remediating contaminated soils, restoring contaminated groundwaters to
their beneficial uses, preventing migration of contaminant plumes2, and protecting
groundwaters and other environmental resources3. EPA advocates using the most appropriate
technology for a given site. EPA does not consider MNA to be a "presumptive" or "default"
remedy—it is merely one option that should be evaluated with other applicable remedies. EPA
does not view MNA to be a "no action4" or "walk-away" approach, but rather
Although this Directive does not address remediation of contaminated sediments, many of the same principles
would be applicable. Fundamental issues such as having source control, developing lines of evidence, monitoring and
contingency plans are also appropriate for sediments. However, the Agency is developing the policy and technical
aspects for sediments, specifically.
The outer limits of contaminant plumes are typically defined for each contaminant of concern based on chemical
concentrations above which the overseeing regulatory authority has determined represent an actual or potential threat to
human health or the environment.
Environmental resources to be protected include groundwater, drinking water supplies, surface waters, ecosystems
and other media (air, soil and sediments) that could be impacted by site contamination.
For the Superfund program, Section 300.430(e)(6) of the National Contingency Plan (NCP) directs that a "no
action alternative" (or no further action) "shall be developed" for all feasibility studies (USEPA, 1990a, p. 8849). The
"no action" alternative can include monitoring but generally not other remedial actions, where such actions are defined
in Section 300.5 of the NCP. In general, the "no action" alternative is selected when there is no current or potential
threat to human health or the environment or when CERCLA exclusions preclude taking an action (USEPA, 1991 a). As
explained in this Directive, a remedial alternative that relies on monitored natural attenuation to attain site-specific
remediation objectives is not the same as the "no action" alternative.
1
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OSWER Directive 9200.4-17P
considers it to be an alternative means of achieving remediation objectives5 that may be
appropriate for specific, well-documented site circumstances where its use meets the applicable
statutory and regulatory requirements. As there is often a variety of methods available for
achieving remediation objectives at any given site, MNA may be evaluated and compared to other
viable remediation methods (including innovative technologies) during the study phases leading to
the selection of a remedy. As with any other remedial alternative, MNA should be selected only
where it meets all relevant remedy selection criteria, and where it will meet site remediation
objectives within a timeframe that is reasonable compared to that offered by other methods. In
the majority of cases where MNA is proposed as a remedy, its use may be appropriate as one
component of the total remedy, that is, either in conjunction with active remediation or as a
follow-up measure. MNA should be used very cautiously as the sole remedy at contaminated
sites. Furthermore, the availability of MNA as a potential remediation tool does not imply any
lessening of EPA's longstanding commitment to pollution prevention. Waste minimization,
pollution prevention programs, and minimal technical requirements to prevent and detect releases
remain fundamental parts of EPA waste management and remediation programs.
Use of MNA does not signify a change in OSWER's remediation objectives. These
objectives (discussed in greater detail under the heading "Implementation") include control of
source materials6, prevention of plume migration, and restoration of contaminated groundwaters,
where appropriate. Thus, EPA expects that source control measures (see section on
"Remediation of Sources") will be evaluated for all sites under consideration for any proposed
remedy. As with other remediation methods, selection of MNA as a remediation method should
be supported by detailed site-specific information that demonstrates the efficacy of this
remediation approach. In addition, the progress of MNA toward a site's remediation objectives
should be carefully monitored and compared with expectations. Where MNA's ability to meet
these expectations is uncertain and based predominantly on predictive analyses, decision makers
should incorporate contingency measures into the remedy.
The scientific understanding of natural attenuation processes continues to evolve. EPA
recognizes that significant advances have been made in recent years, but there is still a great deal
to be learned regarding the mechanisms governing natural attenuation processes and their ability
to address different types of contamination problems. Therefore, while EPA believes MNA may
In this Directive, remediation objectives are the overall objectives that remedial actions are intended to accomplish
and are not the same as chemical-specific cleanup levels. Remediation objectives could include preventing exposure to
contaminants, preventing further migration of contaminants from source areas, preventing further migration of the
groundwater contaminant plume, reducing contamination in soil or groundwater to specified cleanup levels appropriate
for current or potential future uses, or other objectives. The term "remediation" as used in this Directive is not limited to
"remedial actions" defined in CERCLA §101(24), and includes CERCLA "removal actions", for example.
"Source material is defined as material that includes or contains hazardous substances, pollutants or
contaminants that act as a reservoir [either stationary or mobile] for migration of contamination to the ground water, to
surface water, to air, [or other environmental media,] or acts as a source for direct exposure. Contaminated ground
water generally is not considered to be a source material although non-aqueous phase liquids (NAPLS [occurring either
as residual- or free-phase]) may be viewed as source materials." (USEPA, 1991b).
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OSWER Directive 9200.4-17P
be used where circumstances are appropriate, it should be used with caution commensurate with
the uncertainties associated with the particular application. Furthermore, largely due to the
uncertainty associated with the potential effectiveness of MNA to meet remediation objectives
that are protective of human health and the environment, EPA expects that source control and
long-term performance monitoring will be fundamental components of any MNA remedy.
This Directive is a policy document and as such is not intended to provide detailed
technical guidance on evaluating MNA remedies. EPA recognizes that at present there are
relatively few EPA guidance documents concerning appropriate implementation of MNA
remedies. Chapter IX of OUST's alternative cleanup technologies manual (USEPA, 1995a)
addresses the use of natural attenuation at leaking UST sites. The Office of Research and
Development (ORD) has recently published a protocol for evaluating MNA at chlorinated solvent
sites (USEPA, 1998a). Additional technical resource documents for evaluating MNA in
groundwater, soils, and sediments are being developed by ORD. Supporting technical
information regarding the evaluation of MNA as a remediation alternative is available from a
variety of other sources, including those listed at the end of this Directive. "References Cited"
lists those EPA documents that were specifically cited within this Directive. The list of
"Additional References" includes documents produced by EPA as well as non-EPA entities.
Finally, "Other Sources of Information" lists sites on the World Wide Web (Internet) where
additional information can be obtained. Non-EPA documents may provide regional and state site
managers, as well as the regulated community, with useful technical information. However, these
non-EPA guidances are not officially endorsed by EPA, EPA does not necessarily agree with all
their conclusions, and all parties involved should clearly understand that such guidances do not in
any way replace current EPA or OSWER guidances or policies addressing the remedy selection
process in the Superfund, RCRA, or UST programs.
BACKGROUND
The term "monitored natural attenuation", as used in this Directive, refers to the reliance
on natural attenuation processes (within the context of a carefully controlled and monitored site
cleanup approach) to achieve site-specific remediation objectives within a time frame that is
reasonable compared to that offered by other more active methods. The "natural attenuation
processes" that are at work in such a remediation approach 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 groundwater.
These in-situ processes include biodegradation; dispersion; dilution; sorption; volatilization;
radioactive decay; and chemical or biological stabilization, transformation, or destruction of
contaminants. When relying on natural attenuation processes for site remediation, EPA prefers
those processes that degrade or destroy contaminants. Also, EPA generally expects that MNA
will only be appropriate for sites that have a low potential for contaminant migration. Additional
discussion of criteria for "Sites Where Monitored Natural Attenuation May Be Appropriate" may
be found later in this Directive. Other terms associated with natural attenuation in the literature
include "intrinsic remediation", "intrinsic bioremediation", "passive bioremediation", "natural
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OSWER Directive 9200.4-17P
recovery", and "natural assimilation". While some of these terms are synonymous with "natural
attenuation," others refer strictly to biological processes, excluding chemical and physical
processes. Therefore, it is recommended that for clarity and consistency, the term "monitored
natural attenuation" be used throughout OSWER remediation programs unless a specific process
(e.g., reductive dehalogenation) is being referenced.
Natural attenuation processes are typically occurring at all sites, but to varying degrees of
effectiveness depending on the types and concentrations of contaminants present and the physical,
chemical, and biological characteristics of the soil and groundwater. Natural attenuation
processes may reduce the potential risk posed by site contaminants in three ways:
(1) Transformation of contaminant(s) to a less toxic form through destructive
processes such as biodegradation or abiotic transformations;
(2) Reduction of contaminant concentrations whereby potential exposure
levels may be reduced; and
(3) Reduction of contaminant mobility and bioavailability through sorption
onto the soil or rock matrix.
Where conditions are favorable, natural attenuation processes may reduce contaminant
mass or concentration at sufficiently rapid rates to be integrated into a site's soil or groundwater
remedy. Following source control measures, natural attenuation may be sufficiently effective to
achieve remediation objectives at some sites without the aid of other (active) remedial measures.
Typically, however, MNA will be used in conjunction with active remediation measures. For
example, active remedial measures could be applied in areas with high concentrations of
contaminants while MNA is used for low concentration areas; or MNA could be used as a follow-
up to active remedial measures. EPA also encourages the consideration of innovative
technologies for source control or "active" components of the remedy, which may offer greater
confidence and reduced remediation time frames at modest additional cost.
While MNA is often dubbed "passive" remediation because natural attenuation processes
occur without human intervention, its use at a site does not preclude the use of "active"
remediation or the application of enhancers of biological activity (e.g., electron acceptors,
nutrients, and electron donors). However, by definition, a remedy that includes the introduction
of an enhancer of any type is no longer considered to be "natural" attenuation. Use of MNA does
not imply that activities (and costs) associated with investigating the site or selecting the remedy
(e.g., site characterization, risk assessment, comparison of remedial alternatives, performance
monitoring, and contingency measures) have been eliminated. These elements of the
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OSWER Directive 9200.4-17P
investigation and cleanup must still be addressed as required under the particular OSWER
program, regardless of the remedial approach selected.
Contaminants of Concern
It is common practice in conducting remedial actions to focus on the most obvious
contaminants of concern, but other contaminants may also be of significant concern in the context
of MNA remedies. In general, since engineering controls are not used to control plume migration
in an MNA remedy, decision makers need to ensure that MNA is appropriate to address all
contaminants that represent an actual or potential threat to human health or the environment.
Several examples are provided below to illustrate the need to assess both the obvious as well as
the less obvious contaminants of concern when evaluating an MNA remedial option.
• Mixtures of contaminants released into the environment often include some
which may be amenable to MNA, and others which are not addressed
sufficiently by natural attenuation processes to achieve remediation
objectives. For example, Benzene, Toluene, Ethylbenzene and Xylenes
(BTEX) associated with gasoline have been shown in many circumstances
to be effectively remediated by natural attenuation processes. However, a
common additive to gasoline (i.e., methyl tertiary-butyl ether [MTBE]) has
been found to migrate large distances and threaten downgradient water
supplies at the same sites where the BTEX component of a plume has
either stabilized or diminished due to natural attenuation. In general,
compounds that tend not to degrade readily in the subsurface (e.g., MTBE
and 1,4-dioxane) and that represent an actual or potential threat should be
assessed when evaluating the appropriateness of MNA remedies.
• Analyses of contaminated media often report chemicals which are identified
with a high degree of certainty, as well other chemicals labeled as
"tentatively identified compounds" (TICs). It is often assumed that TICs
will be addressed by a remedial action along with the primary contaminants
of concern. This may be a reasonable assumption for an active remediation
system (e.g., pump and treat) which is capturing all contaminated
groundwater, but might not be acceptable for an MNA remedy that is
relying on natural processes to prevent contaminant migration. Where
MNA is being proposed for sites with TICs, it may be prudent to identify
the TICs and evaluate whether they too will be sufficiently mitigated by
MNA.
• At some sites the same geochemical conditions and processes that lead to
biodegradation of chlorinated solvents and petroleum hydrocarbons can
chemically transform naturally occurring minerals (e.g., arsenic and
manganese compounds) in the aquifer matrix to forms that are more mobile
and/or more toxic than the original materials (USEPA, 1998). A
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OSWER Directive 9200.4-17P
comprehensive assessment of an MNA remedial option should include
evaluation of whether naturally occurring metals will become contaminants
of concern.
Addressing the above concerns does not necessarily require sampling and analysis of
extensive lists of parameters at every monitoring location in all situations. The location and
number of samples collected and analyzed for this purpose should be determined on a site-specific
basis to ensure adequate characterization and protection of human health and the environment.
Transformation Products
It also should be noted that some natural attenuation processes may result in the creation
of transformation products7 that are more toxic and/or mobile than the parent contaminant (e.g.,
degradation of trichloroethylene to vinyl chloride). The potential for creation of toxic
transformation products is more likely to occur at non-petroleum release sites (e.g., chlorinated
solvents or other volatile organic spill sites) and should be evaluated to determine if
implementation of a MNA remedy is appropriate and protective in the long term.
Cross-Media Transfer
Natural attenuation processes may often result in transfer of some contaminants from one
medium to another (e.g., from soil to groundwater, from soil to air or surface water, and from
groundwater to surface water). Processes that result in degradation of contaminants are
preferable to those which rely predominantly on the transfer of contamination from one medium
to another. MNA remedies involving cross-media transfer of contamination should include a site-
specific evaluation of the potential risk posed by the contaminant(s) once transferred to a
particular medium. Additionally, long-term monitoring should address the media to which
contaminants are being transferred.
The term "transformation products" in the Directive includes intermediate products resulting from biotic or abiotic
processes (e.g., TCE, DCE, vinyl chloride), decay chain daughter products from radioactive decay, and inorganic
elements that become methylated compounds (e.g., methyl mercury) in soil or sediment. Some transformation products
are quickly transformed to other products while others are longer lived.
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OSWER Directive 9200.4-17P
Petroleum-Related Contaminants
Natural attenuation processes, particularly biological degradation, are currently best
documented at petroleum fuel spill sites. Under appropriate field conditions, the regulated
compounds benzene, toluene, ethylbenzene, and xylene (BTEX) may naturally degrade through
microbial activity and ultimately produce non-toxic end products (e.g., carbon dioxide and water).
Where microbial activity is sufficiently rapid, the dissolved BTEX contaminant plume may
stabilize (i.e., stop expanding), and contaminant concentrations in both groundwater and soil may
eventually decrease to levels below regulatory standards. Following degradation of a dissolved
BTEX plume, a residue consisting of heavier petroleum hydrocarbons of relatively low solubility
and volatility will typically be left behind in the original source (spill) area. Although this residual
contamination may have relatively low potential for further migration, it still may pose a threat to
human health or the environment either from direct contact with soils in the source area or by
continuing to slowly leach contaminants to groundwater. For these reasons, MNA alone is
generally not sufficient to remediate petroleum release sites. Implementation of source control
measures in conjunction with MNA is almost always necessary. Other controls (e.g., institutional
controls8), in accordance with applicable state and federal requirements, may also be necessary to
ensure protection of human health and the environment.
Chlorinated Solvents
Chlorinated solvents9, such as trichloroethylene, represent another class of common
contaminants. These compounds are more dense than water and are referred to as DNAPLs
(dense non-aqueous phase liquids). Recent research has identified some of the mechanisms
potentially responsible for degrading these solvents, furthering the development of methods for
estimating biodegradation rates of these chlorinated compounds. However, the hydrologic and
geochemical conditions favoring significant biodegradation of chlorinated solvents sufficient to
achieve remediation objectives within a reasonable timeframe are anticipated to occur only in
limited circumstances. DNAPLs tend to sink through the groundwater column toward the bottom
of the aquifer. However, they can also occur as mixtures with other less dense contaminants.
Because of the varied nature and distribution of chlorinated compounds, they are typically difficult
to locate, delineate, and remediate even with active measures. In the subsurface, chlorinated
solvents represent source materials that can continue to contaminate groundwater for decades or
longer. Cleanup of solvent spills is also complicated by the fact that a typical spill includes
The term "institutional controls" refers to non-engineering measures—usually, but not always, legal controls—
intended to affect human activities in such a way as to prevent or reduce exposure to hazardous substances. Examples of
institutional controls cited in the National Contingency Plan (USEPA, 1990a, p.8706) include land and resource (e.g.,
water) use and deed restrictions, well-drilling prohibitions, building permits, well use advisories, and deed notices.
Chlorinated solvents are only one type of halogenated compound. Chlorinated solvents are specifically referenced
in this Directive because they are commonly found at contaminated sites. The discussion in this Directive regarding
chlorinated solvents may also apply to other halogenated compounds to be remediated.
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OSWER Directive 9200.4-17P
multiple contaminants, including some that tend not to degrade readily in the subsurface.10
Extremely long dissolved solvent plumes have been documented that may be due to the existence
of subsurface conditions that are not conducive to natural attenuation.
Inorganics
MNA may, under certain conditions (e.g., through sorption or oxidation-reduction
reactions), effectively reduce the dissolved concentrations and/or toxic forms of inorganic
contaminants in groundwater and soil. Both metals and non-metals (including radionuclides) may
be attenuated by sorption11 reactions such as precipitation, adsorption on the surfaces of soil
minerals, absorption into the matrix of soil minerals, or partitioning into organic matter.
Oxidation-reduction (redox) reactions can transform the valence states of some inorganic
contaminants to less soluble and thus less mobile forms (e.g.., hexavalent uranium to tetravalent
uranium) and/or to less toxic forms (e.g., hexavalent chromium to trivalent chromium). Sorption
and redox reactions are the dominant mechanisms responsible for the reduction of mobility,
toxicity, or bioavailability of inorganic contaminants. It is necessary to know what specific
mechanism (type of sorption or redox reaction) is responsible for the attenuation of inorganics so
that the stability of the mechanism can be evaluated. For example, precipitation reactions and
absorption into a soil's solid structure (e.g., cesium into specific clay minerals) are generally
stable, whereas surface adsorption (e.g., uranium on iron-oxide minerals) and organic partitioning
(complexation reactions) are more reversible. Complexation of metals or radionuclides with
carrier (chelating) agents (e.g., trivalent chromium with EDTA) may increase their concentrations
in water and thus enhance their mobility. Changes in a contaminant's concentration, pH, redox
potential, and chemical speciation may reduce a contaminant's stability at a site and release it into
the environment. Determining the existence, and demonstrating the irreversibility, of these
mechanisms is important to show that a MNA remedy is sufficiently protective.
In addition to sorption and redox reactions, radionuclides exhibit radioactive decay and,
for some, a parent-daughter radioactive decay series. For example, the dominant attenuating
mechanism of tritium (a radioactive isotopic form of hydrogen with a short half-life) is radioactive
decay rather than sorption. Although tritium does not generate radioactive daughter products,
those generated by some radionulides (e.g., Am-241 and Np-237 from Pu-241) may be more
toxic, have longer half-lives, and/or be more mobile than the parent in the decay series. Also, it is
For example, 1,4-dioxane, which is used as a stabilizer for some chlorinated solvents, is more highly toxic, less
likely to sorb to aquifer solids, and less biodegradable than some other solvent constituents under the same
environmental conditions.
When a contaminant is associated with a solid phase, it is usually not known if the contaminant is precipitated as a
three-dimensional molecular coating on the surface of the solid, adsorbed onto the surface of the solid, absorbed into the
structure of the solid, or partitioned into organic matter. "Sorption" will be used in this Directive to describe, in a
generic sense (i.e., without regard to the precise mechanism) the partitioning of aqueous phase constituents to a solid
phase.
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OSWER Directive 9200.4-17P
important that the near surface or surface soil pathways be carefully evaluated and eliminated as
potential sources of external direct radiation exposure12.
Inorganic contaminants persist in the subsurface because, except for radioactive decay,
they are not degraded by the other natural attenuation processes. Often, however, they may exist
in forms that have low mobility, toxicity, or bioavailability such that they pose a relatively low
level of risk. Therefore, natural attenuation of inorganic contaminants is most applicable to sites
where immobilization or radioactive decay is demonstrated to be in effect and the
process/mechanism is irreversible.
Advantages and Disadvantages of Monitored Natural Attenuation
MNA has several potential advantages and disadvantages, and the factors listed below
should be carefully considered during site characterization and evaluation of remediation
alternatives before selecting MNA as the remedial alternative. Potential advantages of MNA
include:
• As with any in situ process, generation of lesser volume of remediation
wastes, reduced potential for cross-media transfer of contaminants
commonly associated with ex situ treatment, and reduced risk of human
exposure to contaminants, contaminated media, and other hazards, and
reduced disturbances to ecological receptors;
Some natural attenuation processes may result in in-situ destruction of
contaminants;
• Less intrusion as few surface structures are required;
• Potential for application to all or part of a given site, depending on site
conditions and remediation objectives;
Use in conjunction with, or as a follow-up to, other (active) remedial
measures; and
• Potentially lower overall remediation costs than those associated with
active remediation.
External direct radiation exposure refers to the penetrating radiation (i.e., primarily gamma radiation and x-rays)
that may be an important exposure pathway for certain radionuclides in near surface soils. Unlike chemicals,
radionuclides can have deleterious effects on humans without being taken into or brought in contact with the body due to
high energy particles emitted from near surface soils. Even though the radionuclides that emit penetrating radiation may
be immobilized due to sorption or redox reactions, the resulting contaminated near surface soil may not be a candidate
for a MNA remedy as a result of this exposure risk.
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The potential disadvantages of MNA include:
• Longer time frames may be required to achieve remediation objectives,
compared to active remediation measures at a given site;
Site characterization is expected to be more complex and costly;
Toxicity and/or mobility of transformation products may exceed that of the
parent compound;
• Long-term performance monitoring will generally be more extensive and
for a longer time;
Institutional controls may be necessary to ensure long term protectiveness;
Potential exists for continued contamination migration, and/or cross-media
transfer of contaminants;
• Hydrologic and geochemical conditions amenable to natural attenuation
may change over time and could result in renewed mobility of previously
stabilized contaminants (or naturally occurring metals), adversely impacting
remedial effectiveness; and
More extensive education and outreach efforts may be required in order to
gain public acceptance of MNA.
IMPLEMENTATION
The use of MNA is not new in OSWER programs. For example, in the Superfund
program, use of natural attenuation as an element in a site's groundwater remedy is discussed in
"Guidance on Remedial Actions for Contaminated Groundwater at Superfund Sites" (USEPA,
1988a). Use of MNA in OSWER programs has slowly increased over time with greater program
experience and scientific understanding of the processes involved. Recent advances in the
scientific understanding of the processes contributing to natural attenuation have resulted in a
heightened interest in this approach as a potential means of achieving remediation objectives for
soil and groundwater. However, EPA expects that reliance on MNA as the sole remedy will only
be appropriate at relatively few contaminated sites. This Directive is intended to clarify OSWER
program policies regarding the use of MNA and ensure that MNA remedies are selected and
implemented appropriately. Topics addressed include the role of MNA in OSWER remediation
programs, site characterization, the types of sites where MNA may be appropriate, reasonable
remediation timeframes, source control, performance monitoring, and contingency remedies
where MNA will be employed.
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Role of Monitored Natural Attenuation in OSWER Remediation Programs
Under OSWER programs, remedies selected for contaminated media (such as
contaminated soil and groundwater) must protect human health and the environment. Remedies
may achieve this level of protection using a variety of methods, including treatment, containment,
engineering controls, and other means identified during the remedy selection process.
The regulatory and policy frameworks for corrective actions under the UST, RCRA, and
Superfund programs have been established to implement their respective statutory mandates and
to promote the selection of technically defensible, nationally consistent, and cost effective
solutions for the cleanup of contaminated media. EPA recognizes that MNA may be an
appropriate remediation option for contaminated soil and groundwater under certain
circumstances. However, determining the appropriate mix of remediation methods at a given site,
including when and how to use MNA, can be a complex process. Therefore, MNA should be
carefully evaluated along with other viable remedial approaches or technologies (including
innovative technologies) within the applicable remedy selection framework. MNA should not be
considered a default or presumptive remedy at any contaminated site.
Each OSWER program has developed regulations and policies to address the particular
types of contaminants and facilities within its purview13. Although there are differences among
Existing program guidance and policy regarding MNA can be obtained from the following sources: For
Superfund, see "Guidance on Remedial Actions for Contaminated Groundwater at Superfund Sites," (USEPA, 1988a;
pp. 5-7 and 5-8); the Preamble to the 1990 National Contingency Plan (USEPA, 1990a, pp.8733-34); and "Presumptive
Response Strategy and Ex-Situ Treatment Technologies for Contaminated Ground Water at CERCLA Sites, Final
Guidance" (USEPA, 1996a; p. 18). For the RCRA program, see the Subpart S Proposed Rule (USEPA, 1990b,
pp.30825 and 30829), and the Advance Notice of Proposed Rulemakmg (USEPA, 1996b, pp. 19451-52). For the UST
program, refer to Chapter IX in "How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank
Sites: A Guide for Corrective Action Plan Reviewers;" (USEPA, 1995a).
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OSWER Directive 9200.4-17P
these programs, they share several key principles that should generally be considered during
selection of remedial measures, including:
• Source control measures should use treatment to address "principal threat"
wastes (or products) wherever practicable, and engineering controls such
as containment for waste (or products) that pose a relatively low long-term
threat, or where treatment is impracticable.14
Contaminated groundwaters should be returned to "their beneficial uses15
wherever practicable, within a timeframe that is reasonable given the
particular circumstances of the site." When restoration of groundwater is
not practicable, EPA "expects to prevent further migration of the plume,
prevent exposure to the contaminated groundwater, and evaluate further
risk reduction."16
• Contaminated soil should be remediated to achieve an acceptable level of
risk to human and environmental receptors, and to prevent any transfer of
contaminants to other media (e.g., surface or groundwater, air, sediments)
that would result in an unacceptable risk or exceed required cleanup levels.
Remedial actions in general should include opportunity(ies) for public
involvement that serve to both educate interested parties and to solicit
feedback concerning the decision making process.
Consideration or selection of MNA as a remedy or remedy component does not in any
way change or displace these (or other) remedy selection principles. Nor does use of MNA
Principal threat wastes are those source materials that are "highly toxic or highly mobile that generally cannot be
reliably contained or would present a significant risk to human health or the environment should exposure occur. They
include liquids and other highly mobile materials (e.g., solvents) or materials having high concentrations of toxic
compounds." (USEPA, 1991b). Low level threat wastes are "source materials that generally can be reliably contained
and that would present only a low risk in the event of release." (USEPA, 1991b). Since contaminated groundwater is
not source material, it is neither a principal nor a low-level threat waste.
Beneficial uses of groundwater could include uses for which water quality standards have been promulgated,
(e.g., drinking water supply, discharge to surface water), or where groundwater serves as a source of recharge to either
surface water or adjacent aquifers, or other uses. These or other types of beneficial uses may be identified as part of a
Comprehensive State Groundwater Protection Program (CSGWPP). For more information on CSGWPPs, see USEPA,
1992a and USEPA, 1997b, or contact your state implementing agency.
This is a general expectation for remedy selection in the Superfund program, as stated in §300.430 (a)(l)(iii)(F)
of the National Contingency Plan (USEPA, 1990a, p. 8846). The NCP Preamble also specifies that cleanup levels
appropriate for the expected beneficial use (e.g., MCLs for drinking water) "should generally be attained throughout the
contaminated plume, or at and beyond the edge of the waste management area when waste is left in place" (USEPA,
1990a, p.8713). The RCRA Corrective Action program has similar expectations (see USEPA, 1996b, pp. 19448-
19450).
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diminish EPA's or the regulated party's responsibility to achieve protectiveness or to satisfy long-
term site remediation objectives. EPA expects that MNA will be 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 timeframe that is
reasonable compared to other alternatives. The effectiveness of MNA in both near-term and
long-term timeframes should be demonstrated to EPA (or other overseeing regulatory authority)
through: 1) sound technical analyses which provide confidence in natural attenuation's ability to
achieve remediation objectives; 2) performance monitoring; and 3) contingency (or backup)
remedies where appropriate. In summary, use of MNA does not imply that EPA or the
responsible parties are "walking away" from the cleanup or financial responsibility at a
site.
It also should be emphasized that the selection of MNA as a remedy does not imply that
active remediation measures are infeasible, or are "technically impracticable" from an engineering
perspective. Technical impracticability (TI) determinations are used to justify a departure from
cleanup levels that would otherwise be required at a Superfund site or RCRA facility based on the
inability to achieve such cleanup levels using available remedial technologies (USEPA, 1993a).
Such a TI determination does not imply that there will be no active remediation at the site, nor
that MNA will be used at the site. Rather, such a TI determination simply indicates that the
cleanup levels and objectives which would otherwise be required cannot practicably be attained
using available remediation technologies. In such cases, an alternative cleanup strategy that is
fully protective of human health and the environment must be identified. Such an alternative
strategy may still include engineered remediation components, such as recovery of free phase
NAPLs and containment of residual contaminants, in addition to approaches intended to restore
some portion of the contaminated groundwater to beneficial uses. Several remedial approaches
could be appropriate to address the dissolved plume, one of which could be MNA under suitable
conditions. However, the evaluation of natural attenuation processes and the decision to rely
upon MNA for the dissolved plume should be distinct from the recognition that restoration of a
portion of the plume is technically impracticable (i.e., MNA should not be viewed as a direct or
presumptive outcome of a technical impracticability determination.)
Demonstrating the Efficacy of Natural Attenuation Through Site Characterization
Decisions to employ MNA as a remedy or remedy component should be thoroughly
and adequately supported with site-specific characterization data and analysis. In general,
the level of site characterization necessary to support a comprehensive evaluation of MNA is
more detailed than that needed to support active remediation. Site characterizations for natural
attenuation generally warrant a quantitative understanding of source mass; groundwater flow
(including preferential pathways); contaminant phase distribution and partitioning between soil,
groundwater, and soil gas; rates of biological and non-biological transformation; and an
understanding of how all of these factors are likely to vary with time. This information is generally
necessary since contaminant behavior is governed by dynamic processes which must be well
understood before MNA can be appropriately applied at a site. Demonstrating the efficacy of
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MNA may require analytical or numerical simulation of complex attenuation processes. Such
analyses, which are critical to demonstrate natural attenuation's ability to meet
remediation objectives, generally require a detailed conceptual site model17 as a foundation.
EPA recommends the use of conceptual site models to integrate data and guide both
investigative and remedial actions. However, program implementors should be cautious and
collect sufficient field data to test conceptual hypotheses and not "force-fit" site data into a pre-
conceived, and possibly inaccurate, conceptual representation. For example, a common
mechanism for transport of contaminants is advection-dispersion, by which contaminants
dissolved in groundwater migrate away from a source area. An alternative mechanism of
contaminant transport (i.e., NAPL migration) could be associated with a relatively large release of
NAPL into the subsurface such that the NAPL itself has the potential to migrate significant
distances along preferential pathways. Since NAPL migration pathways are often difficult to
locate in the subsurface, one may incorrectly conclude that only the dissolved transport model
applies to a site, when a combined NAPL and dissolved phase migration model would be more
accurate. Applying a wrong conceptual model, in the context of evaluating an MNA (or any
other) remedy, could result in a deficient site characterization (e.g., did not use tools and
approaches designed to find NAPLs or NAPL migration pathways), and inappropriate selection of
an MNA remedy where long-term sources were not identified nor considered during remedy
selection. NAPL present as either free- or residual phase represents a significant mass of
contamination that will serve as a long-term source. Sources of contamination are more
appropriately addressed by engineered removal, treatment or containment technologies, as
discussed later in this Directive. Where the sources of contamination have been controlled,
dissolved plumes may be amenable to MNA because of the relatively small mass of contaminants
present in the plume.
Site characterization should include collecting data to define (in three spatial dimensions
over time) the nature and distribution of contaminants of concern and contaminant sources as well
as potential impacts on receptors (see "Background" section for further discussion pertaining to
"Contaminants of Concern"). However, where MNA will be considered as a remedial approach,
certain aspects of site characterization may require more detail or additional elements. For
A conceptual site model (CSM) is a three-dimensional representation that conveys what is known or suspected
about contamination sources, release mechanisms, and the transport and fate of those contaminants. The conceptual
model provides the basis for assessing potential remedial technologies at the site. "Conceptual site model" is not
synonymous with "computer model"; however, a computer model may be helpful for understanding and visualizing
current site conditions or for predictive simulations of potential future conditions. Computer models, which simulate site
processes mathematically, should in turn be based upon sound conceptual site models to provide meaningful
information. Computer models typically require a lot of data, and the quality of the output from computer models is
directly related to the quality of the input data. Because of the complexity of natural systems, models necessarily rely on
simplifying assumptions that may or may not accurately represent the dynamics of the natural system. Calibration and
sensitivity analyses are important steps in appropriate use of models. Even so, the results of computer models should be
carefully interpreted and continuously verified with adequate field data. Numerous EPA references on models are listed
in the "Additional References" section at the end of this Directive.
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example, to assess the contributions of sorption, dilution, and dispersion to natural attenuation of
contaminated groundwater, a very detailed understanding of aquifer hydraulics, recharge and
discharge areas and volumes, and chemical properties is necessary. Where biodegradation will be
assessed, characterization also should include evaluation of the nutrients and electron donors and
acceptors present in the groundwater, the concentrations of co-metabolites and metabolic by-
products, and perhaps specific analyses to identify the microbial populations present. The findings
of these, and any other analyses pertinent to characterizing natural attenuation processes, should
be incorporated into the conceptual model of contaminant fate and transport developed for the
site.
MNA may not be appropriate as a remedial option at many sites for technological or
economic reasons. For example, in some complex geologic systems, technological limitations
may preclude adequate monitoring of a natural attenuation remedy to ensure with a high degree of
confidence that potential receptors will not be impacted. This situation typically occurs in many
karstic, structured, and/or fractured rock aquifers where groundwater moves preferentially
through discrete pathways (e.g., solution channels, fractures, joints, foliations). The direction of
groundwater flow through such heterogeneous (and often anisotropic) materials can not be
predicted directly from the hydraulic gradient, and existing techniques may not be capable of
identifying the pathway along which contaminated groundwater moves through the subsurface.
MNA will not generally be appropriate where site complexities preclude adequate monitoring. In
some other situations where it may be technically feasible to monitor the progress of natural
attenuation, the cost of site characterization and long-term monitoring required for the
implementation of MNA may be higher than the cost of other remedial alternatives. Under such
circumstances, MNA may not be less costly than other alternatives.
A related consideration for site characterization is how other remedial activities at the site
could affect natural attenuation. For example, the capping of contaminated soil could alter both
the type of contaminants leached to groundwater, as well as their rate of transport and
degradation. Another example could be where there is co-mingled petroleum and chlorinated
solvent contamination. In such cases, degradation of the chlorinated solvents is achieved, in part,
through the action of microbes that derive their energy from the carbon in the petroleum.
Recovery of the petroleum removes some of the source of food for these microbes and the rate of
degradation of the chlorinated solvents is decreased. Therefore, the impacts of any ongoing or
proposed remedial actions should be factored into the analysis of the effectiveness of MNA.
Once site characterization data have been collected and a conceptual model developed, the
next step is to evaluate the potential efficacy of MNA as a remedial alternative. This involves
collection of site-specific data sufficient to estimate with an acceptable level of confidence both
the rate of attenuation processes and the anticipated time required to achieve remediation
objectives. A three-tiered approach to such an evaluation is becoming more widely practiced and
accepted. In this approach, successively more detailed information is collected as necessary to
provide a specified level of confidence on the estimates of attenuation rates and remediation
timeframe. These three tiers of site-specific information, or "lines of evidence", are:
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(1) Historical groundwater and/or soil chemistry data that demonstrate a clear
and meaningful trend18 of decreasing contaminant mass and/or
concentration over time at appropriate monitoring or sampling points. (In
the case of a groundwater plume, decreasing concentrations should not be
solely the result of plume migration. In the case of inorganic contaminants,
the primary attenuating mechanism should also be understood.)
(2) Hydrogeologic and geochemical data that can be used to demonstrate
indirectly the type(s) of natural attenuation processes active at the site,
and the rate at which such processes will reduce contaminant
concentrations to required levels. For example, characterization data may
be used to quantify the rates of contaminant sorption, dilution, or
volatilization, or to demonstrate and quantify the rates of biological
degradation processes occurring at the site.
(3) Data from field or microcosm studies (conducted in or with actual
contaminated site media) which directly demonstrate the occurrence of a
particular natural attenuation process at the site and its ability to degrade
the contaminants of concern (typically used to demonstrate biological
degradation processes only).
Unless EPA or the overseeing regulatory authority determines that historical data
(Number 1 above) are of sufficient quality and duration to support a decision to use MNA,
data characterizing the nature and rates of natural attenuation processes at the site
(Number 2 above) should be provided. Where the latter are also inadequate or
inconclusive, data from microcosm studies (Number 3 above) may also be necessary. In
general, more supporting information may be required to demonstrate the efficacy of MNA at
those sites with contaminants which do not readily degrade through biological processes (e.g.,
most non-petroleum compounds, inorganics), or that transform into more toxic and/or mobile
forms than the parent contaminant, or where monitoring has been performed for a relatively short
period of time. The amount and type of information needed for such a demonstration will depend
upon a number of site-specific factors, such as the size and nature of the contamination problem,
the proximity of receptors and the potential risk to those receptors, and other characteristics of
the environmental setting (e.g., hydrogeology, ground cover, climatic conditions).
Note that those parties responsible for site characterization and remediation should ensure
that all data and analyses needed to demonstrate the efficacy of MNA are collected and evaluated
by capable technical specialists with expertise in the relevant sciences. Furthermore, EPA expects
that documenting the level of confidence on attenuation rates will provide more technically
defensible predictions of remedial timeframes and form the basis for more effective performance
monitoring programs.
18 For guidance on statistical analysis of environmental data, please see USEPA, 1989, USEPA, 1993b, USEPA,
1993d, and Gilbert, 1987, listed in the "References Cited" section at the end of this Directive.
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Sites Where Monitored Natural Attenuation May Be Appropriate
MNA is appropriate as a remedial approach where it can be demonstrated capable of
achieving a site's remediation objectives within a timeframe that is reasonable compared to that
offered by other methods and where it meets the applicable remedy selection criteria (if any) for
the particular OSWER program EPA expects that MNA will be most appropriate when used
in conjunction with other remediation measures (e.g., source control, groundwater
extraction), or as a follow-up to active remediation measures that have already been
implemented.
In determining whether MNA is an appropriate remedy for soil or groundwater at a given
site, EPA or other regulatory authorities should consider the following:
• Whether the contaminants present in soil or groundwater can be effectively
remediated by natural attenuation processes;
Whether or not the contaminant plume is stable and the potential for the
environmental conditions that influence plume stability to change over time;
• Whether human health, drinking water supplies, other groundwaters,
surface waters, ecosystems, sediments, air, or other environmental
resources could be adversely impacted as a consequence of selecting MNA
as the remediation option;
Current and projected demand for the affected resource over the time
period that the remedy will remain in effect;
• Whether the contamination, either by itself or as an accumulation with
other nearby sources (on-site or off-site), will exert a long-term detrimental
impact on available water supplies or other environmental resources;
• Whether the estimated timeframe of remediation is reasonable (see section
on "Reasonable Timeframe for Remediation") compared to timeframes
required for other more active methods (including the anticipated
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effectiveness of various remedial approaches on different portions of the
contaminated soil and/or groundwater);
• The nature and distribution of sources of contamination and whether these
sources have been, or can be, adequately controlled;
Whether the resulting transformation products present a greater risk, due
to increased toxicity and/or mobility, than do the parent contaminants;
• The impact of existing and proposed active remediation measures upon the
MNA component of the remedy, or the impact of remediation measures or
other operations/activities (e.g., pumping wells) in close proximity to the
site; and
• Whether reliable site-specific mechanisms for implementing institutional
controls (e.g., zoning ordinances) are available, and if an institution
responsible for their monitoring and enforcement can be identified.
Of the above factors, the most important considerations regarding the suitability of MNA
as a remedy include: whether the contaminants are likely to be effectively addressed by natural
attenuation processes, the stability of the groundwater contaminant plume and its potential for
migration, and the potential for unacceptable risks to human health or environmental resources by
the contamination. MNA should not be used where such an approach would result in either
plume migration19 or impacts to environmental resources that would be unacceptable to the
overseeing regulatory authority. Therefore, sites where the contaminant plumes are no
longer increasing in extent, or are shrinking, would be the most appropriate candidates for
MNA remedies.
An example of a situation where MNA may be appropriate is a remedy that includes
source control, a pump-and-treat system to mitigate the highly-contaminated plume areas, and
MNA in the lower concentration portions of the plume. In combination, these methods would
maximize groundwater restored to beneficial use in a timeframe consistent with future demand on
the aquifer, while utilizing natural attenuation processes to reduce the reliance on active
remediation methods and reduce remedy cost. If, at such a site, the plume was either expanding
In determining whether a plume is stable or migrating, users of this Directive should consider the uncertainty
associated with defining the limits of contaminant plumes. For example, a plume is typically delineated for each
contaminant of concern as a 2- or 3-dimensional feature. Plumes are commonly drawn by computer contouring
programs which estimate concentrations between actual data points. EPA recognizes that a plume boundary is more
realistically defined by a zone rather than a line. Fluctuations within this zone are likely to occur due to a number of
factors (e.g., analytical, seasonal, spatial, etc.) which may or may not be indicative of a trend in plume migration.
Therefore, site characterization activities and performance monitoring should focus on collection of data of sufficient
quality to enable decisions to be made with a high level of confidence. See USEPA, 1993b, USEPA, 1993c, USEPA,
1994b, and USEPA, 1998b, for additional guidance.
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or threatening downgradient wells or other environmental resources, then MNA would not be an
appropriate remedy.
Reasonable Timeframe for Remediation
EPA recognizes that determination of what timeframe is "reasonable" for attaining
remediation objectives is a site-specific determination. The NCP preamble suggests that a
"reasonable" timeframe for a remedy relying on natural attenuation is generally a "...timeframe
comparable to that which could be achieved through active restoration" (USEPA, 1990a,
p.8734; emphasis added). The NCP preamble further states that "[t]he most appropriate
timeframe must, however, be determined through an analysis of alternatives" (USEPA, 1990a,
p.8732). To ensure that these estimates are comparable, assumptions should be consistently
applied for each alternative considered. Thus, determination of the most appropriate timeframe is
achieved through a comparison of estimates of remediation timeframe for all appropriate remedy
alternatives.
If restoring groundwaters to beneficial uses is a remediation objective, a comparison of
restoration alternatives from most aggressive to passive (i.e., MNA) will provide information
concerning the approximate range of time periods needed to attain groundwater cleanup levels.
An excessively long restoration timeframe, using the most aggressive restoration method, may
indicate that groundwater restoration is technically impracticable from an engineering perspective
(USEPA, 1993a). Where restoration is technically practicable using either aggressive or passive
methods, the longer restoration timeframe required by the passive alternative may be reasonable in
comparison with the timeframe needed for more aggressive restoration alternatives (USEPA,
1996a).
The advantages and disadvantages of each remedy alternative, including the timeframe,
should be evaluated in accordance with the remedy selection criteria used by each OSWER
program. Whether a particular remediation timeframe is appropriate and reasonable for a given
site is determined by balancing tradeoffs among many factors which include:
• Classification of the affected resource (e.g., drinking water source,
agricultural water source) and value of the resource20;
In determining whether an extended remediation timeframe may be appropriate for the site, EPA and other
regulatory authorities should consider state groundwater resource classifications, priorities and/or valuations where
available, in addition to relevant federal guidelines. Individual states may provide information and guidance relevant to
groundwater classifications or use designations as part of a Comprehensive State Groundwater Protection Program
(CSGWPP). (See USEPA, 1992a and USEPA, 1997b).
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• Relative timeframe in which the affected portions of the aquifer might be
needed for future water supply (including the availability of alternate
supplies);
Subsurface conditions and plume stability which can change over an
extended timeframe;
• Whether the contamination, either by itself or as an accumulation with
other nearby sources (on-site or off-site), will exert a long-term detrimental
impact on available water supplies or other environmental resources;
Uncertainties regarding the mass of contaminants in the subsurface and
predictive analyses (e.g., remediation timeframe, timing of future demand,
and travel time for contaminants to reach points of exposure appropriate
for the site);
• Reliability of monitoring and of institutional controls over long time
periods;
Public acceptance of the timeframe required to reach remediation
objectives; and
• Provisions by the responsible party for adequate funding of monitoring and
performance evaluation over the time period required for remediation.
It should be noted that the timeframe required for MNA remedies is often longer than that
required for more active remedies As a consequence, the uncertainty associated with the
above factors increases dramatically. Adequate performance monitoring and contingency
remedies (both discussed in later sections of this Directive) should be utilized because of
this higher level of uncertainty. When determining reasonable timeframes, the uncertainty in
estimated timeframes should be considered, as well as the ability to establish performance
monitoring programs capable of verifying the performance expected from natural attenuation in a
timely manner (e.g., as would be required in a Superfund five-year remedy review).
A decision on whether or not MNA is an appropriate remedy for a given site is usually
based on estimates of the rates of natural attenuation processes. Site characterization (and
monitoring) data are typically used for estimating attenuation rates. These calculated rates may be
expressed with respect to either time or distance from the source. Time-based estimates are
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used to predict the time required for MNA to achieve remediation objectives and distance-based
estimates provide an evaluation of whether a plume will expand, remain stable, or shrink. For
environmental decision-making, EPA requires that the data used be of "adequate quality and
usability for their intended purpose." (USEPA, 1998b). Therefore, where these rates are used to
evaluate MNA, or predict the future behavior of contamination, they must also be of "adequate
quality and usability." Statistical confidence intervals should be estimated for calculated
attenuation rate constants (including those based on methods such as historical trend data
analysis, analysis of attenuation along a flow path in groundwater, and microcosm studies). When
predicting remedial timeframes, sensitivity analyses should also be performed to indicate the
dependence of the calculated remedial timeframes on uncertainties in rate constants and other
factors (McNab and Dooher, 1998). A statistical evaluation of the rate constants estimated from
site characterization studies of natural attenuation of groundwater contamination often reveals
that the estimated rate constants contain considerable uncertainty. For additional guidance on
data quality, see USEPA, 1993c, 1994c, 1995b, and 1995c.
As an example, analysis of natural attenuation rates from many sites indicates that a
measured decrease in contaminant concentrations of at least one order of magnitude is necessary
to determine the appropriate rate law to describe the rate of attenuation, and to demonstrate that
the estimated rate is statistically different from zero at a 95% level of confidence (Wilson, 1998).
Due to variability resulting from sampling and analysis, as well as plume variability over time,
smaller apparent reductions are often insufficient to demonstrate (with 95% level of confidence)
that attenuation has in fact occurred at all.
Thus, EPA or other regulatory authorities should consider a number of factors when
evaluating reasonable timeframes for MNA at a given site. These factors, on the whole, should
allow the overseeing regulatory authority to determine whether a natural attenuation remedy
(including institutional controls where applicable) will fully protect potential human and
environmental receptors, and whether the site remediation objectives and the time needed to meet
them are consistent with the regulatory expectation that contaminated groundwaters will be
restored to beneficial uses within a reasonable timeframe. When these conditions cannot be
met using MNA, a remedial alternative that more likely would meet these expectations
should be selected.
Remediation of Sources
Source control measures should be evaluated as part of the remedy decision process at all
sites, particularly where MNA is under consideration as the remedy or as a remedy component.
Source control measures include removal, treatment, or containment, or a combination of these
approaches. EPA prefers remedial options which remove free-phase NAPLs and treat those
source materials determined to constitute "principal threat wastes" (see Footnote 13).
Contaminant sources that are not adequately addressed complicate the long-term cleanup
effort. For example, following free product recovery, residual contamination from a petroleum
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OSWER Directive 9200.4-17P
fuel release may continue to leach significant quantities of contaminants into the groundwater as
well as itself posing unacceptable risks to humans or environmental resources. Such a lingering
source often unacceptably extends the time necessary to reach remediation objectives. This
leaching can occur even while contaminants are being naturally attenuated in other parts of the
plume. If the rate of attenuation is lower than the rate of replenishment of contaminants to the
groundwater, the plume can continue to expand thus contaminating additional groundwater and
potentially posing a threat to downgradient receptors.
Control of source materials is the most effective means of ensuring the timely attainment
of remediation objectives EPA, therefore, expects that source control measures will be
evaluated for all contaminated sites and that source control measures will be taken at most
sites where practicable. At many sites it will be appropriate to implement source control
measures during the initial stages of site remediation ("phased remedial approach"), while
collecting additional data to determine the most appropriate groundwater remedy.
Performance Monitoring and Evaluation
Performance monitoring to evaluate remedy effectiveness and to ensure protection of
human health and the environment is a critical element of all response actions. Performance
monitoring is of even greater importance for MNA than for other types of remedies due to the
potentially longer remediation timeframes, potential for ongoing contaminant migration, and other
uncertainties associated with using MNA. This emphasis is underscored by EPA's reference to
"monitored natural attenuation".
The monitoring program developed for each site should specify the location, frequency,
and type of samples and measurements necessary to evaluate whether the remedy is performing as
expected and is capable of attaining remediation objectives. In addition, all monitoring programs
should be designed to accomplish the following:
• Demonstrate that natural attenuation is occurring according to
expectations;
Detect changes in environmental conditions (e.g., hydrogeologic,
geochemical, microbiological, or other changes) that may reduce the
efficacy of any of the natural attenuation processes21;
Identify any potentially toxic and/or mobile transformation products;
Verify that the plume(s) is not expanding (either downgradient, laterally or
vertically);
Detection of changes will depend on the proper siting and construction of monitoring wells/points. Although the
siting of monitoring wells is a concern for any remediation technology, it is of even greater concern with MNA because
of the lack of engineering controls to control contaminant migration.
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OSWER Directive 9200.4-17P
Verify no unacceptable impact to downgradient receptors;
Detect new releases of contaminants to the environment that could impact
the effectiveness of the natural attenuation remedy;
• Demonstrate the efficacy of institutional controls that were put in place to
protect potential receptors; and
Verify attainment of remediation objectives.
The frequency of monitoring should be adequate to detect, in a timely manner, the
potential changes in site conditions listed above. At a minimum, the monitoring program should
be sufficient to enable a determination of the rate(s) of attenuation and how that rate is changing
with time. When determining attenuation rates, the uncertainty in these estimates and the
associated implications should be evaluated (see McNab and Dooher, 1998). Flexibility for
adjusting the monitoring frequency over the life of the remedy should also be included in the
monitoring plan. For example, it may be appropriate to decrease the monitoring frequency at
some point in time, once it has been determined that natural attenuation is progressing as expected
and very little change is observed from one sampling round to the next. In contrast, the
monitoring frequency may need to be increased if unexpected conditions (e.g., plume migration)
are observed.
Performance monitoring should continue until remediation objectives have been
achieved, and longer if necessary to verify that the site no longer poses a threat to human
health or the environment. Typically, monitoring is continued for a specified period (e.g., one
to three years) after remediation objectives have been achieved to ensure that concentration levels
are stable and remain below target levels. The institutional and financial mechanisms for
maintaining the monitoring program should be clearly established in the remedy decision or other
site documents, as appropriate.
Details of the monitoring program should be provided to EPA or the overseeing
regulatory authority as part of any proposed MNA remedy. Further information on the types of
data useful for monitoring natural attenuation performance can be found in the ORD publications
(e.g., USEPA, 1997a, USEPA, 1994a) listed in the "References Cited" section of this Directive.
Also, USEPA (1994b) published a detailed document on collection and evaluation of performance
monitoring data for pump-and-treat remediation systems.
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OSWER Directive 9200.4-17P
Contingency Remedies
A contingency remedy is a cleanup technology or approach specified in the site remedy
decision document that functions as a "backup" remedy in the event that the "selected" remedy
fails to perform as anticipated. A contingency remedy may specify a technology (or technologies)
that is (are) different from the selected remedy, or it may simply call for modification of the
selected technology, if needed. Contingency remedies should generally be flexible—allowing for
the incorporation of new information about site risks and technologies.
Contingency remedies are not new to OSWER programs. Contingency remedies should
be included in the decision document where the selected technology is not proven for the specific
site application, where there is significant uncertainty regarding the nature and extent of
contamination at the time the remedy is selected, or where there is uncertainly regarding whether
a proven technology will perform as anticipated under the particular circumstances of the site
(USEPA, 1990c).
It is also recommended that one or more criteria ("triggers") be established, as
appropriate, in the remedy decision document that will signal unacceptable performance of the
selected remedy and indicate when to implement contingency remedies. Such criteria should
generally include, but not be limited to, the following:
• Contaminant concentrations in soil or groundwater at specified locations
exhibit an increasing trend not originally predicted during remedy selection;
Near-source wells exhibit large concentration increases indicative of a new
or renewed release;
• Contaminants are identified in monitoring wells located outside of the
original plume boundary;
Contaminant concentrations are not decreasing at a sufficiently rapid rate
to meet the remediation objectives; and
• Changes in land and/or groundwater use will adversely affect the
protectiveness of the MNA remedy.
In establishing triggers or contingency remedies, however, care is needed to ensure that
sampling variability or seasonal fluctuations do not unnecessarily trigger a contingency. For
example, an anomalous spike in dissolved concentration(s) at a well(s) might not be a true
indication of a change in trend.
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OSWER Directive 9200.4-17P
EPA recommends that remedies employing MNA be evaluated to determine the need for
including one or more contingency measures that would be capable of achieving remediation
objectives. EPA believes that contingency remedies should generally be included as part of a
MNA remedy which has been selected based primarily on predictive analyses rather than
documented trends of decreasing contaminant concentrations.
SUMMARY
EPA remains fully committed to its goals of protecting human health and the
environment by remediating contaminated soils, restoring contaminated groundwaters to
their beneficial uses, preventing migration of contaminant plumes, and protecting
groundwaters and other environmental resources. EPA does not view MNA to be a "no
action" remedy, but rather considers it to be a means of addressing contamination under a limited
set of site circumstances where its use meets the applicable statutory and regulatory requirements.
MNA is not a "presumptive" or "default" remediation alternative, but rather should be evaluated
and compared to other viable remediation methods (including innovative technologies) during the
study phases leading to the selection of a remedy. The decision to implement MNA should
include a comprehensive site characterization, risk assessment where appropriate, and measures to
control sources. In addition, the progress of natural attenuation towards a site's remediation
objectives should be carefully monitored and compared with expectations to ensure that it will
meet site remediation objectives within a timeframe that is reasonable compared to timeframes
associated with other methods. Where MNA's ability to meet these expectations is uncertain and
based predominantly on predictive analyses, decision-makers should incorporate contingency
measures into the remedy.
EPA is confident that MNA will be, at many sites, a reasonable and protective component
of a broader remediation strategy. However, EPA believes that there will be many other sites
where either the uncertainties are too great or there is a need for a more rapid remediation that
will preclude the use of MNA as a stand-alone remedy. This Directive should help promote
consistency in how MNA remedies are proposed, evaluated, and approved.
REFERENCES CITED
Gilbert, R.O. 1987. Statistical Methods for Environmental Pollution Monitoring. Van Nostrand
Reinhold Co., New York, NY, 320p.
McNab, W.W., Jr. and B.P. Dooher. 1998. A critique of a steady-state analytical method for
estimating contaminant degradation rates. Ground Water 36, no.6:983-87.
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United States Environmental Protection Agency (USEPA). 1988a. Section 5.3.3.1. Natural
attenuation with monitoring. Guidance on remedial actions for contaminated groundwater at
Superfundsites, OSWER Directive 9283.1-2, EPA/540/G-88/003, Office of Solid Waste and
Emergency Response. Washington, D.C.
United States Environmental Protection Agency. 1989. Methods for evaluation attainment of
cleanup standards, Vol. 1: Soils and solid media, EPA/230/02-89-042, Office of Solid Waste.
Washington, D.C.
United States Environmental Protection Agency. 1990a. National oil and hazardous substances
pollution contingency plan (NCP); final rule, Federal Register 55, no. 46:8706 and 8733-34.
Washington, D.C.
United States Environmental Protection Agency. 1990b. Corrective action for releases from
solid waste management units at hazardous waste management facilities; proposed rule, Federal
Register 55, no. 145:30825 and 30829. Washington, D.C.
United States Environmental Protection Agency. 1990c. Suggested ROD language for various
groundwater remediation options, OSWER Directive 9283.1-03, Office of Solid Waste and
Emergency Response. Washington, D.C.
United States Environmental Protection Agency. 199la. Guide to developing Superfundno
action, interim action, and contingency remedy RODs, Superfund Publication 9355.3-02FS (Fact
Sheet), Office of Emergency Remedial Response. Washington, D.C.
United States Environmental Protection Agency. 1991b. A guide to principal threat and low
level threat wastes, Superfund Publication 9380.3-06FS (Fact Sheet, November version), Office
of Emergency Remedial Response. Washington, D.C.
United States Environmental Protection Agency. 1992a. Final comprehensive state ground
water protection program guidance, EPA 100-R-93-001, Office of the Administrator.
Washington, D.C.
United States Environmental Protection Agency. 1993 a. Guidance for evaluating the technical
impracticability of ground-water restoration, OSWER Directive 9234.2-25, EPA/540-R-93-080,
Office of Solid Waste and Emergency Response. Washington, D.C.
United States Environmental Protection Agency. 1993b. Guidance document on the statistical
analysis of ground-water monitoring data at RCRA facilities, Office of Solid Waste.
Washington, D.C.
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OSWER Directive 9200.4-17P
United States Environmental Protection Agency. 1993c. Data quality objectives process for
Superfund: Interim final guidance, EPA/540-R-93-071, Office of Solid Waste and Emergency
Response. Washington, D.C.
United States Environmental Protection Agency. 1993d. Methods for evaluation attainment of
cleanup standards, Vol. 2: Ground Water, EPA/230/R-92/014, Office of Policy, Planning, and
Evaluation. Washington, D.C.
United States Environmental Protection Agency. 1994a. Proceedings of symposium on natural
attenuation ofgroundwater, EPA/600/R-94/162, Office of Research and Development.
Washington, D.C.
United States Environmental Protection Agency. 1994b. Methods for monitoring pump-and-
treatperformance, EPA/600/R-94/123, Office of Research and Development. Washington, D.C.
United States Environmental Protection Agency. 1994c. EPA requirements for quality
assurance project plans (QAPP) for environmental data operations, EPA/QA/R-5, Office of
Water. Washington, D.C.
United States Environmental Protection Agency. 1995a. Chapter IX: Natural attenuation. How
to evaluate alternative cleanup technologies for underground storage tank sites: A guide for
corrective action plan reviewers, EPA 510-B-95-007, Office of Underground Storage Tanks.
Washington, D.C.
United States Environmental Protection Agency. 1995b. Guidance for data quality assessment,
EPA/QA/G-9, Office of Research and Development. Washington, D.C.
United States Environmental Protection Agency. 1995c. Guidance for the preparation of
standard operating procedures (SOPs) for quality-related documents, EPA/QA/G-6, Quality
Assurance Division. Washington, D.C.
United States Environmental Protection Agency. 1996a. Presumptive response strategy and
ex-situ treatment technologies for contaminated ground water at CERCLA sites, Final Guidance,
OSWER Directive 9283.1-12, EPA 540-R-96-023, Office of Solid Waste and Emergency
Response. Washington, D.C.
United States Environmental Protection Agency. 1996b. Corrective action for releases from
solid waste management units at hazardous waste management facilities; advance notice of
proposed rulemaking, Federal Register 61, no. 85:19451-52.
United States Environmental Protection Agency. 1997'a. Proceedings of the symposium on
natural attenuation of chlorinated organics ingroundwater; Dallas, Texas, September 11-13,
EPA/540/R-97/504, Office of Research and Development. Washington, D.C.
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United States Environmental Protection Agency. 1997b. The role ofCSGWPPs in EPA
remediation programs, OSWER Directive 9283.1-09, EPA F-95-084, Office of Solid Waste and
Emergency Response. Washington, D.C.
United States Environmental Protection Agency. 1998a. Technical protocol for evaluating
natural attenuation of chlorinated solvents in ground water, EPA/600/R-98/128, National Risk
Management Research Laboratory, Ada, Oklahoma.
United States Environmental Protection Agency. 1998b. Policy and program requirements for
the mandatory agency-wide quality system, EPA Order 5360.1 CHG 1, Office of Research and
Development, Washington, D.C.
Wilson, John T. 1998. Personal communication, U.S. EPA, NRMRL, Ada, Oklahoma.
ADDITIONAL REFERENCES
American Academy of Environmental Engineers. 1995. Innovative site remediation technology,
Vol. 1: Bioremediation, ed. W.C. Anderson. Annapolis, Maryland.
American Society for Testing and Materials. 1998. Standard guide for accelerated site
characterization for confirmed or suspected petroleum releases, ASTM E 1912-98.
Conshohocken, Pennsylvania.
American Society for Testing and Materials. 1998. Standard guide for remediation of ground
water by natural attenuation at petroleum release sites, ASTM E 1943-98. Conshohocken,
Pennsylvania.
Black, H. 1995. Wisconsin gathers evidence to support intrinsic bioremediation. The
bioremediation report, August:6-7.
Borden, R.C., C.A. Gomez, and M.T. Becker. 1995. Geochemical indicators of intrinsic
bioremediation. Ground Water 33, no.2:180-89.
Hinchee, R.E., J.T. Wilson, and D.C. Downey. 1995. Intrinsic bioremediation. Columbus,
Ohio: Battelle Press.
Klecka, G.M., J.T. Wilson, E. Lutz, N. Klier, R. West, J. Davis, J. Weaver, D. Kampbell, and B.
Wilson. 1996. Intrinsic remediation of chlorinated solvents in groundwater. Proceedings of
intrinsic bioremediation conference, London Wl, United Kingdom, March 18-19.
McAllister, P.M., and C.Y. Chiang. 1993. A practical approach to evaluating natural attenuation
of contaminants in groundwater. Groundwater Monitoring & Remediation 14, no.2:161-73.
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New Jersey Department of Environmental Protection. 1996. Site remediation program,
technical requirements for site remediation, proposed readoption with amendments: N.J.A.C.
7:26E, authorized by Robert J. Shinn, Jr., Commissioner.
Norris, R.D., R.E. Hinchee, R.A. Brown, P.L. McCarty, L. Semprini, J.T. Wilson, D.H.
Kampbell, M. Reinhard, E.J. Bouwer, R.C. Borden, T.M. Vogel, J.M. Thomas, and C.H. Ward.
1994. Handbook of bioremediation. Boca Raton, Florida: Lewis Publishers.
Salanitro, J.P. 1993. The role of bioattenuation in the management of aromatic hydrocarbon
plumes in aquifers. Groundwater Monitoring & Remediation 13, no. 4:150-61.
United States Department of the Army. 1995. Interim Army policy on natural attenuation for
environmental restoration, (12 September) Memorandum from the Assistant Chief of Staff for
Installation Management. Washington, D.C.: the Pentagon.
United States Environmental Protection Agency. 1978. Radionuclide interactions with soil and
rock media, Vol. 1: Element chemistry and geochemistry., EPA 520/6-78-007, Office of Research
and Development. Washington, D.C.
United States Environmental Protection Agency. 1988b. Groundwater modeling: an overview
and status report, EPA/600/2-89/028, Office of Research and Development. Washington, D.C.
United States Environmental Protection Agency. 1992c. Quality assurance and control in the
development and application of ground-water models, EPA/600/R-93/011, Office of Research
and Development. Washington, D.C.
United States Environmental Protection Agency. 1993e. Compilation of ground-water models,
EPA/600/R-93/118, Office of Research and Development. Washington, D.C.
United States Environmental Protection Agency. 1994d. The hydrocarbon spill screening model
(HSSM), Vol. 1: User's guide, EPA/600/R-94-039a, Office of Research and Development.
Washington, D.C.
United States Environmental Protection Agency. 1994e. Assessment framework for ground-
water model applications, OSWER Directive 9029.00, EPA 500-B-94-003, Office of Solid Waste
and Emergency Response. Washington, D.C.
United States Environmental Protection Agency. 1994f. Ground-water modeling compendium,
EPA 500-B-94-004, Office of Solid Waste and Emergency Response. Washington, D.C.
United States Environmental Protection Agency. 1994g. A technical guide to ground-water
model selection at sites contaminated with radioactive substances, EPA 402-R-94-012, Office of
Air and Radiation. Washington, D.C.
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United States Environmental Protection Agency. 1994h. Guidance for conducting external peer
review of environmental models, EPA 100-B-94-001, Office of Air and Radiation. Washington,
D.C.
United States Environmental Protection Agency. 19941. Report of the agency task force on
environmental regulatory modeling, EPA 500-R-94-001, Office of Air and Radiation.
Washington, D.C.
United States Environmental Protection Agency. 1995a. The hydrocarbon spill screening model
(HSSM), Vol. 2: Theoretical background and source codes, EPA/600/R-94-039b, Office of
Research and Development. Washington, D.C.
United States Environmental Protection Agency. 1996c. Documenting ground-water modeling
at sites contaminated with radioactive substances, EPA 540-R-96-003, Office of Air and
Radiation. Washington, D.C.
United States Environmental Protection Agency. 1996d. Three multimedia models used at
hazardous and radioactive waste sites, EPA 540-R-96-004, Office of Air and Radiation.
Washington, D.C.
United States Environmental Protection Agency. 1996e. Notes of Seminar—Bioremediation of
hazardous waste sites: Practical approaches to implementation, EPA 510-B-95-007, Office of
Research and Development. Washington, D.C.
United States Environmental Protection Agency. 1997c. (Draft) Geochemicalprocesses
affecting sorption of selected contaminants, Office of Radiation and Indoor Air. Washington,
D.C.
United States Environmental Protection Agency. 1997d. (Draft) The KD model and its use in
contaminant transport modeling, Office of Radiation and Indoor Air. Washington, D.C.
United States Environmental Protection Agency, Air Force, Army, Navy, and Coast Guard.
1996a. Commonly asked questions regarding the use of natural attenuation for chlorinated
solvent spills at federal facilities, Fact Sheet, Federal Facilities Restoration and Re-Use Office.
Washington, D.C.
United States Environmental Protection Agency, Air Force, Army, Navy, and Coast Guard.
1996b. Commonly asked questions regarding the use of natural attenuation for petroleum
contaminated sites at federal facilities, Fact Sheet, Federal Facilities Restoration and Re-Use
Office. Washington, D.C.
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Wiedemeier, T.H., J.T. Wilson, D.H. Kampbell, R.N. Miller, and I.E. Hansen. 1995. Technical
protocol for implementing intrinsic remediation with long-term monitoring for natural
attenuation of fuel contamination dissolved in groundwater. United States Air Force Center for
Environmental Excellence, Technology Transfer Division, Brooks Air Force Base, San Antonio,
Texas.
Wiedemeier, T.H., J.T. Wilson, D.H. Kampbell, I.E. Hansen, and P. Haas. 1996. Technical
protocol for evaluating the natural attenuation of chlorinated ethenes in groundwater.
Proceedings of the petroleum hydrocarbons and organic chemicals in groundwater: Prevention,
detection, and remediation conference, Houston, Texas, November 13-15.
Wilson, J.T., D.H. Kampbell, and J. Armstrong. 1993. Natural bioreclamation of alkylbenzenes
(BTEX) from a gasoline spill in methanogenic groundwater. Proceedings of the second
international symposium on in situ and on site bioremediation, San Diego, California, April 5-8.
Wisconsin Department of Natural Resources. 1993. ERRP issues guidance on natural
biodegradation. Release News, Emergency and Remedial Response Section, February, vol. 3, no.
1.
OTHER SOURCES OF INFORMATION
http ://www. epa.gov/ORDAVebPubs/biorem/
EPA Office of Research and Development, information on passive and active bioremediation
http://www.epa.gov/ada/kerrlab.html
EPA Office of Research and Development, R.S. Kerr Environmental Research Laboratory
http://www.epa.gov/OUST/cat/natatt.htm
EPA Office of Underground Storage Tanks, information on natural attenuation
http://www.epa.gov/swerffrr/chlorine.htm
EPA Federal Facilities Restoration and Reuse Office, fact sheet on natural attenuation of
chlorinated solvents
http ://www. epa.gov/swerffrr/petrol .htm
EPA Federal Facilities Restoration and Reuse Office, Fact sheet on natural attenuation of
petroleum contaminated sites
http://www.epa.gov/hazwaste/ca/subparts.htm
EPA Office of Solid Waste, information on RCRA Subpart S
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http://www.epa.gov/swerosps/bf/
EPA Office of Outreach Programs, Special Projects, and Initiatives, information on Brownfields
http://www.epa.gov/oerrpage/superfnd/web/programs/clp/quality.htm
EPA Office of Emergency and Remedial Response, information on data quality
http ://es. epa. gov/ncerqa/qa/qatool s. html
EPA Office of Research and Development, National Center for Environmental Research and
Quality Assurance, information on data quality, quality assurance and quality control
http://clu-in.com
EPA Technology Innovation Office, information on hazardous site cleanups
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