United States
          Environmental Protection
          Agency
                 Office of
                 Solid Waste and
                 Emergency Response
&EPA
DIRECTIVE NUMBER:
                               9200.4-17
          TITLE:    Use of Monitored Natural Attenuation at Superfund, RCRA
                   Corrective Action, and Underground Storage Tank Sites

          APPROVAL DATE: December 1, 1997

          EFFECTIVE DATE: December 1, 1997

          ORIGINATING OFFICE:   OSWER

          D   FINAL

          0   DRAFT Interim Final

          STATUS:
          REFERENCE (other documents):
  OSWER       OSWER       OSWER
   DIRECTIVE     DIRECTIVE    DIRECTIVE

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           united btates environmental Protection Agency
                  Washington, DC 20460
OSWER Directive  Initiation Request
                                                                     1. Directive Number

                                                                     	9200.4-17
                                   2. Originator Information
      Mame of Contact Person
        Hal White
                   Mail Code
                    5403G
Office
 OSWER/OUST
Telephone Code
 703-603-7177
      3'Title  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  for  the remediation of contaminated soil and groundwater
        at sites regulated under Office of Solid Waste and Emergency Response (OSWER) programs
      5. Keywords
        natural  attenuation, remediation, soil, groundwater,  contamination
      6a. Does This Directive Supersede Previous Directive(s)?
       b. Does It Supplement Previous Directive(s)?
                                              No
                                            X No
                                     Yes   What directive (number, title)


                                     Yes   What directive (number, title)
      7. Draft Level
        X  A - Signed by AA/DAA
              B - Signed by Office Director
       C - For Review & Comment
          D - In Development
8.
Document
to
be
distributed
to
States
by Headquarters?
X

Yes


No
      This Request Meats OSWER Directives System Format Standards.
      9. Signature/Of Lead Office Directives Coordinator
                                                   Date
                                                                       //
      10. Name
      EPA Fonrr1315-17 (Rev/f-87) Previous editi
                   obsolete.
   OSWER           OSWER                OSWER                O
VE     DIRECTIVE          DIRECTIVE         DIRECTIVE

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                    UNITED  STATES ENVIRONMENTAL PROTECTION AGENCY
                                  WASHINGTON, D.C.  20460
                                  NOV  I 8
                            OFFICE OF
                     SOLID WASTE AND EMERGENCY
                            RESPONSE
MEMORANDUM

SUBJECT:   Draft Interim Final OSWER Monitored Natural Attenuation Policy
             (OSWER^Directive 9200.4-17)
FROM:
            'Office^ Solid Waste

             Walter W. Kovalick, Jr., Director
             Technology Innovation Office
             Stephen D. Luftig, Director
             Office of Emergency and Remedial Response
Cxtx/TuLj
             Anna Hopkins Virbick, Director8
             Office of Underground Storage Tanks
             James E. Woolford, Director
             Federal Facilities Restoration and Reuse Office

TO:         Addressees

Purpose

      This memorandum accompanies a draft Interim Final Policy (OSWER Directive
9200.4-17) regarding the use of monitored natural attenuation for the remediation of
contaminated soil and groundwater at sites regulated under all programs administered by EPA's
Office of Solid Waste and Emergency Response (OSWER),  including Superfund, RCRA
Corrective Action, and Underground Storage Tanks. The Directive incorporates extensive
comments received from EPA Regional and Headquarters reviewers (including the Office of
General Counsel), as well as state agencies and federal facility representatives.

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Summary of the Directive

       This Directive clarifies the U.S. Environmental Protection Agency's (EPA) policy
regarding the use of Monitored Natural Attenuation for the remediation of contaminated soil and
groundwater at sites regulated under Office of Solid Waste and Emergency Response (OSWER)
programs.  These include programs administered under the Comprehensive Environmental
Response, Compensation, and Liability Act (CERCLA or "Superfund"), the Resource
Conservation and Recovery Act (RCRA), the Office of Underground Storage Tanks (OUST), and
the Federal Facilities Restoration and Reuse Office (FFRRO).  The Directive is intended to
promote consistency in how monitored natural attenuation remedies are proposed, evaluated, and
approved.  As a policy document, it does not provide technical guidance on evaluating Monitored
Natural Attenuation remedies. This Directive is being issued as Interim Final and may be used
immediately.  It provides guidance to EPA staff, to the public, and to the regulated community on
how EPA intends to exercise its discretion in implementing national policy on the use of
Monitored Natural Attenuation.  The document does not, however, substitute for EPA's statutes
or regulations, nor is it a regulation itself and, 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.

Implementation

       This Directive is being issued in Interim Final form and should be used immediately as
guidance for proposing, evaluating, and approving Monitored Natural Attenuation remedies. This
Interim 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). EPA will review and evaluate
additional comments received on this Interim Final version before issuing the Final Directive.

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
                                  Decembers, 1997

      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:
      Allison Abernathy—Federal Facilities       (202) 260-9925
      Dianna Young—Federal Facilities          (202) 260-8302
      Ken Lovelace—Superfund                (703) 603-8787
      Felicia Wright—Superfund                (703) 603-8775
      Guy Tomassoni—RCRA                 (703) 308-8622
      Dana Tulis—UST                       (703) 603-7175
      Hal White—UST                        (703) 603-7177
      Linda Fiedler—Technology Innovation     (703) 603-7194

Office of Research and Development:
      John Wilson—RMRL, Ada, OK           (405) 436-8532
      Fran Kremer—NRMRL, Cincinnati, OH    (513) 569-7346
      Fred Bishop—NRMRL, Cincinnati, OH     (513) 569-7629

Groundwater Forum:
      Ruth Izraeli—RCRA, Superfund           (212) 63 7-4311

Region 1
      Joan Coyle—UST                       (617) 573-9667
      Ernie Waterman—RCRA                 (617) 223-5511
      Richard Willey—Superfund               (617) 573-9639
      Bill Brandon—Federal Facilities           (617) 573-9629
      Meghan Cassidy—Federal Facilities         (617) 573-5785

Region 2
      Derval Thomas—UST                    (212) 637-4236
      Ruth Izraeli—Superfund                  (212) 63 7-4311
      Jon Josephs—ORD Technical Liaison       (212) 637-4317
      Carol Stein—RCRA                     (212) 63 7-4181

Region 3
      Jack Hwang—UST                      (215)566-3387
      Kathy Davies—Superfund                (215) 566-3315
      Deborah Goldblum—RCRA              (215) 566-3432

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Region 4
      David Ariail—UST                      (404) 562-9464
      Kay Wischkaemper—Technical Support     (404) 562-4300
      Donna Wilkinson—RCRA                (404)562-4300
      Robert Pope—Federal Facilities            (404) 562-4300

Region 5
      Gilberto Alvarez—UST                  (312) 886-6143
      Tom Matheson—RCRA                  (312) 886-7569
      Luanne Vanderpool—Superfund           (312) 353-9296

Region 6
      Lynn Dail—UST                        (214) 665-2234
      John Cernero—UST                     (214) 665-2233

Region 7
      William F. Lowe—RCRA                (913)551-7547
      Dave Drake—Superfund                  (913) 551-7626

Region 8
      Sandra Stavnes—UST                    (303) 312-6117
      Randy Breeden—RCRA                  (303) 312-6522
      Rich 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
      David Domingo—RCRA Permits Team     (206) 553-8582
      Mary Jane Nearman—Superfund           (206) 553-6642

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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 9200.4-17
                November, 1997

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                                                        OSWER Directive 9200.4-17


                USE OF MONITORED NATURAL ATTENUATION
                 AT SUPERFUND, RCRA CORRECTIVE ACTION,
                  AND UNDERGROUND STORAGE TANK SITES

                                    Contents

PURPOSE AND OVERVIEW  	  1

BACKGROUND	  3
      Transformation Products 	  4
      Petroleum-Related Contaminants	  4
      Chlorinated Solvents 	  5
      Inorganics  	  6
      Advantages and Disadvantages of Monitored Natural Attenuation	  7

IMPLEMENTATION	  8
      Role of Monitored Natural Attenuation in OSWER Remediation Programs 	  8
      Demonstrating the Efficacy of Natural Attenuation through Site Characterization  .... 10
      Sites Where Monitored Natural Attenuation May Be Appropriate  	13
      Reasonableness of Remediation Time Frame 	15
      Remediation of Contamination Sources and Highly Contaminated Areas  	16
      Performance Monitoring  	17
      Contingency Remedies	18

SUMMARY 	19

REFERENCES CITED	20

ADDITIONAL REFERENCES	22

OTHER SOURCES OF INFORMATION	25

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                                                       OSWER Directive 9200.4-17
NOTICE: This document provides guidance to EPA 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 9200.4-17

PURPOSE AND OVERVIEW

       The purpose of this Directive is to clarify EPA's policy regarding the use of monitored
natural attenuation for the remediation of contaminated soil and groundwater at sites regulated
under Office of Solid Waste and Emergency Response (OSWER) programs. These include
programs administered under the Comprehensive Environmental Response, Compensation, and
Liability Act (CERCLA or Superfund), the Resource Conservation and Recovery Act (RCRA),
the Office of Underground Storage Tanks (OUST), and the Federal Facilities Restoration and
Reuse Office (FFRRO).

       EPA remains fully committed to its goals of protecting human health and the environment,
remediating contaminated soils and groundwater, and protecting uncontaminated groundwaters
and other environmental resources1 at all sites being remediated under OSWER programs. EPA
does not consider monitored natural attenuation to be a "presumptive" or "default" remedy—it is
merely one option that should be evaluated with other applicable remedies. EPA advocates using
the most appropriate technology for a given site. EPA does not view monitored natural
attenuation to be a "no action" or "walk-away" approach, but rather considers it to be an
alternative means of achieving remediation objectives that may be appropriate for a limited set of
site circumstances where its use meets the applicable statutory and regulatory requirements. As
there is often a variety of methods available for achieving a given site's remediation objectives2,
monitored natural attenuation 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, monitored natural attenuation should be selected
only where it meets all relevant remedy selection criteria, where it will be fully protective of
human health and the environment, and where it will meet site remediation objectives, within a
time frame that is reasonable compared to that offered by other methods. In the majority of cases
where monitored natural  attenuation 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. Monitored natural attenuation should be used very cautiously as the sole
remedy at contaminated sites.  Furthermore, the availability of monitored natural attenuation 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.
    Environmental resources to be protected include groundwater, drinking water supplies, surface waters, ecosystems
and other media (air, soil and sediments) that could be impacted from site contamination.

    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, minimizing further migration of contaminants from source areas, minimizing 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.

                                             1

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                                                              OSWER Directive 9200.4-17

       Use of monitored natural attenuation does not signify a change in OSWER's remediation
objectives, including the control of source materials and restoration of contaminated
groundwaters, where appropriate (see Section 1, under "Implementation").  Thus, EPA expects
that source control measures will be evaluated for all  sites under consideration for any proposed
remedy.  As with other remediation methods, selection of monitored natural attenuation 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 monitored natural
attenuation toward a site's remediation objectives should be carefully monitored and compared
with expectations. Where monitored natural attenuation'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 rapidly.
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
monitored natural attenuation may 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
monitored natural attenuation to meet remedial objectives that are protective of human health and
the environment, source control and performance monitoring are fundamental components
of any monitored natural attenuation remedy.

       This  Directive is  not intended to provide detailed technical guidance on evaluating
monitored natural attenuation remedies. At present, there is a relative lack of EPA guidance
concerning appropriate implementation of monitored  natural attenuation remedies.  With the
exception of Chapter IX  in OUST's guidance manual (USEPA, 1995a),  EPA has not yet
completed and published specific technical guidance to support the evaluation of monitored
natural attenuation for OSWER sites.  However, technical resource documents for evaluating
monitored natural attenuation in groundwater, soils, and sediments are currently being developed
by EPA's Office of Research and Development (ORD).  In addition, technical information
regarding the evaluation  of monitored natural attenuation as a remediation alternative is available
from a variety of 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
information  can be obtained. Although non-EPA documents may provide regional and state site
managers, as well as the  regulated community, with useful technical information, these non-EPA
guidances are not officially endorsed by EPA, 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.

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                                                               OSWER Directive 9200.4-17

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 remedial 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; 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
contaminants,  and for this reason, EPA expects that monitored natural attenuation will be most
appropriate at  sites that have a low potential for plume generation and migration (see Section 3
under "Implementation"). Other terms associated with natural attenuation in the literature include
"intrinsic remediation", "intrinsic bioremediation", "passive bioremediation", "natural 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)    The contaminant may be converted to a  less toxic form through destructive
              processes such as biodegradation or abiotic transformations;

       (2)    Potential exposure levels may be reduced by lowering  of concentration
              levels (through destructive processes, or by dilution or dispersion); and

       (3)    Contaminant mobility and bioavailability may be reduced by sorption to 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 (see Section 3 under "Implementation" for a discussion of favorable site conditions).
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, monitored natural attenuation will be used in conjunction with active
remediation measures.  For example, monitored natural attenuation could be employed in lower

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                                                                OSWER Directive 9200.4-17

concentration areas of the dissolved plume and as a follow-up to active remediation in areas of
higher concentration.  EPA also encourages the consideration of innovative approaches which
may offer greater confidence and reduced remediation time frames at a modest additional cost.

       While monitored natural attenuation is often dubbed "passive" remediation because it
occurs 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 monitored
natural attenuation 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 investigation and cleanup must still be addressed as required under the particular
OSWER program, regardless of the remedial approach selected.

Transformation Products

       It also should be noted that some natural attenuation processes may result in the creation
of transformation products3 that are more toxic 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 monitored natural
attenuation remedy is appropriate and protective in the long term.  Additionally, some natural
attenuation processes may 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).  Such cross-media transfer is not desirable, and generally not acceptable except
under certain site-specific circumstances, and would likely require an evaluation of the potential
risk posed by the contaminant(s) once transferred to that medium.

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, ethyl benzene, 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 may  eventually decrease to  levels
below regulatory standards.  Following degradation of a dissolved BTEX plume, a residue
    The term "transformation products" in the Directive includes biotically and abiotically formed products described
above (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 and sediment.

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                                                                 OSWER Directive 9200.4-17

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, monitored natural attenuation
alone is generally not sufficient to remediate even a petroleum release site. Implementation of
source control measures in conjunction with monitored natural attenuation is almost always
necessary. Other controls (e.g., institutional controls4), in accordance with applicable state and
federal requirements, may also be necessary to ensure protection of human health and the
environment.  Furthermore, while BTEX contaminants tend to biodegrade with relative ease,
other chemicals (e.g., methyl tertiary-butyl ether [MTBE]) that are more resistant to biological or
other degradation processes may also be present in petroleum fuels. In general, monitored natural
attenuation is not appropriate as a sole remediation option at sites where non-degradable and
nonattenuated contaminants are present at levels that pose an unacceptable risk to human health
or the environment.  Where non-degradable contaminants are present, all processes (listed on
page 4) which contribute  to natural attenuation should be evaluated to ensure protection of
human health and the environment.

Chlorinated Solvents

       Chlorinated solvents, such as trichloroethylene, represent another class of common
contaminants that may also biodegrade under certain environmental conditions. 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 may not often occur. Because of the nature and the
distribution of these compounds, natural attenuation may not be effective as a remedial option.  If
they are not adquately addressed through removal or containment measures,  source materials can
continue to contaminate groundwater for decades or even centuries. Cleanup of solvent spills is
also complicated by the fact that a typical spill includes multiple contaminants, including some that
are essentially non-degradable.5  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.
    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.

    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 are other solvents under the same environmental conditions .

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                                                                 OSWER Directive 9200.4-17

Inorganics

       Monitored natural attenuation 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 sorption6 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
because some mechanisms are more desirable than others. 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 are key components of a sufficiently protective monitored natural attenuation
remedy.

       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.  It is
critical that the near surface or surface soil pathways be carefully evaluated and eliminated as
potential sources of radiation exposure.

       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 are less mobile, not bioavailable, and/or non-toxic. Therefore, natural attenuation
    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 9200.4-17

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

       Monitored natural attenuation has several potential advantages and disadvantages, and its
use should be carefully considered during site characterization and evaluation of remediation
alternatives. Potential advantages of monitored natural attenuation 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 contaminated media;

       •      Less intrusion as few surface structures are required;

       •      Potential for application to all or part of a given site,  depending  on site
              conditions and cleanup objectives;

              Use in conjunction with, or as a follow-up to, other (active) remedial
              measures; and

       •      Lower overall remediation costs than those associated with active
              remediation.

       The potential disadvantages of monitored natural attenuation include:

       •      Longer time frames  may be required to achieve remediation objectives,
              compared to active remediation;

              Site characterization may be more complex and costly;

              Toxicity of transformation products may exceed that of the parent
              compound;

       •      Long term monitoring will generally be necessary;

       •      Institutional  controls may be necessary to ensure long term protectiveness;

       •      Potential exists for continued contamination migration, and/or cross-media
              transfer of contaminants;

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                                                              OSWER Directive 9200.4-17

              Hydrologic and geochemical conditions amenable to natural attenuation are
              likely to change over time and could result in renewed mobility of
              previously stabilized contaminants, adversely impacting remedial
              effectiveness; and

       •       More extensive education and outreach efforts may be required in order to
              gain public acceptance of monitored natural attenuation.

IMPLEMENTATION

       The use of monitored natural attenuation is not new in OSWER programs.  For example,
in the Superfund program, selection of natural attenuation as an element in a site's groundwater
remedy goes as far back  as 1985.  Use of monitored natural attenuation in OSWER programs has
continued since that time, slowly increasing 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 soil and groundwater cleanup objectives.  However,
complete reliance on monitored natural attenuation is appropriate only  in a limited  set of
circumstances at contaminated sites.  The sections which follow seek to clarify OSWER program
policies regarding the use of monitored natural attenuation. Topics addressed  include site
characterization; the types of sites where monitored natural attenuation may be appropriate;
reasonable remediation time frames; the importance of source control;  performance monitoring;
and contingency remedies where monitored natural attenuation will be  employed.

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 monitored natural
attenuation 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 monitored natural attenuation, can be a complex
process.  Therefore, monitored natural attenuation should be carefully evaluated along with other
viable remedial approaches or technologies (including innovative technologies) within the
applicable remedy selection framework.  Monitored natural attenuation should not be
considered a default or presumptive remedy at any contaminated  site.

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                                                                     OSWER Directive 9200.4-17

        Each OSWER program has developed regulations and policies to address the particular
types of contaminants and facilities within its purview7. Although there are differences among
these programs, they share several key principles that should generally be considered during
selection of remedial measures, including:

        •        Source control actions 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.8

               Contaminated groundwaters should be returned to "their beneficial uses9
               wherever practicable, within a time frame  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" (which may be appropriate).10

        •       Contaminated soil should be remediated to achieve an acceptable level of
               risk to human and environmental receptors, and to prevent any transfer of
    Existing program guidance and policy regarding monitored natural attenuation 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).

    Principal threat wastes are those source materials (e.g., non-aqueous phase liquids [NAPL], saturated soils) that are
highly toxic or highly mobile that generally cannot be reliably contained (USEPA, 1991). Low level threat wastes are
source materials that can be reliably contained or that would pose only a low risk in the event of exposure.
Contaminated groundwater 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, such
as a drinking water supply, or as a source of recharge to surface water, 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  1997b, or contact your state implementing agency .

      This is a general expectation for remedy selection in the Superfund program, as  stated in the National
Contingency Plan (USEPA, 1990a, §300.430 (a)(l)(m)(F)). 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."

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                                                              OSWER Directive 9200.4-17

              contaminants to other media (e.g., surface or groundwater, air, sediments)
              that would result in an unacceptable risk or exceed required cleanup levels.

       Consideration or selection of monitored natural attenuation as a remedy or remedy
component does not in any way change or displace these (or other) remedy selection principles.
Nor does use of monitored natural attenuation diminish EPA's or the regulated party's
responsibility to achieve protectiveness or to satisfy long-term site cleanup objectives.
Monitored natural attenuation is an appropriate remediation method only where its use
will be protective of human health and the environment and it will be capable of achieving
site-specific remediation objectives within a time frame that is  reasonable compared to
other alternatives.  The effectiveness of monitored natural attenuation in both near-term and
long-term time frames should be demonstrated to EPA (or other regulatory authority) through:
1) sound technical analysis which provides confidence in natural attenuation's ability to achieve
remediation objectives; 2) performance monitoring; and 3) backup  or contingency remedies where
appropriate  In summary, use of monitored natural attenuation does not imply that EPA or
the responsible parties are "walking away" from the cleanup or financial responsibility
obligations at a site.

       It also should be emphasized that the selection of monitored natural attenuation as a
remedy does not imply that active remediation measures are infeasible, or are "technically
impracticable." Technical impracticability (TI) determinations, which EPA makes based on the
inability to achieve required cleanup levels using available remedial technologies and approaches,
are used to justify a change in the remediation objectives at Superfund and RCRA sites (USEPA,
1993a). A TI determination does not imply that there will be no active remediation at the  site, nor
that monitored natural attenuation will be used at the site.  Rather, a TI determination simply
indicates that the cleanup levels and objectives which would otherwise be required cannot
practicably be attained within a reasonable time frame 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 containment for an area contaminated with dense non-aqueous
phase liquids (DNAPL), in addition to approaches intended to restore to beneficial uses the
portion of the plume with dissolved contaminants.  Several remedial approaches could be
appropriate to address the dissolved plume, one of which could be  monitored natural attenuation
under suitable conditions. However, the evaluation of natural attenuation processes and the
decision to rely upon monitored natural attenuation for the dissolved plume should be distinct
from the recognition that restoration of a portion of the plume is technically impracticable  (i.e.,
monitored natural attenuation 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 monitored natural attenuation as a remedy or remedy
component should be thoroughly and adequately supported with site-specific
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                                                                   OSWER Directive 9200.4-17

characterization data and analysis.  In general, the level of site characterization necessary to
support a comprehensive evaluation of natural attenuation 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; 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 natural attenuation can be appropriately applied at a site.
Demonstrating the efficacy of this remediation approach likely will require analytical or numerical
simulation of complex attenuation processes.  Such analyses,  which are critical to demonstrate
natural attenuation's ability to meet remedial action objectives, generally require a detailed
conceptual site model as a foundation11.

       Site characterization should include collecting data to  define (in three spatial dimensions
over time) the nature and distribution of contamination sources as well as the extent of the
groundwater plume and its potential impacts on receptors. However, where monitored natural
attenuation will be considered as a remedial approach, certain aspects of site characterization may
require more detail or additional elements. For 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 required. 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.

       Monitored natural attenuation 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
     A conceptual site model 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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                                                                OSWER Directive 9200.4-17

ensure with a high degree of certainty 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 channels (e.g., solution channels, foliations, fractures,
joints).  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 channels that carry contaminated groundwater through the
subsurface.  Monitored natural attenuation will not generally be appropriate where site
complexities preclude adequate monitoring.  Although in some situations 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 monitored natural attenuation is high
compared to the cost of other remedial alternatives. Under such circumstances, natural
attenuation would not necessarily be the low-cost alternative.

       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. Therefore, the impacts of any ongoing or proposed remedial actions should be
factored into the analysis of natural attenuation's effectiveness.  When considering source
containment/treatment together with natural attenuation of chlorinated solvents, the potential for
cutting off sources of organic carbon (which are critical to biodegradation of the solvents)  should
be carefully evaluated.

       Once the site characterization data have been collected and a conceptual model developed,
the next step is to evaluate the efficacy of monitored natural attenuation as a remedial approach.
Three types of site-specific information or "evidence" should be used in such an evaluation:

       (1)    Historical groundwater and/or soil chemistry data that  demonstrate a clear
              and meaningful trend12 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
     For guidance on the statistical analysis of environmental data, please see USEPA, 1989 and 1992b, listed in the
"References Cited" section at the end of this Directive.

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                                                             OSWER Directive 9200.4-17

              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 implementing state agency determines that historical data
(Number 1 above) are of sufficient quality and duration to support a decision to use
monitored natural attenuation, EPA expects that 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 monitored natural attenuation at those  sites with contaminants which
do not readily degrade through biological processes (e.g., most non-petroleum compounds,
inorganics), at sites with contaminants that transform into more toxic and/or mobile forms than
the  parent contaminant, or at sites 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 physical
characteristics of the environmental setting (e.g., hydrogeology, ground cover, or 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 monitored natural attenuation
are  collected and evaluated by capable technical specialists with expertise in the relevant sciences.
Further, EPA expects that the results will be provided in a timely manner to EPA or to the state
implementing agency for evaluation and approval.

Sites Where  Monitored Natural Attenuation May Be Appropriate

       Monitored natural attenuation is appropriate as a remedial approach only where it can be
demonstrated capable of achieving a site's remedial  objectives within a time frame that is
reasonable compared to that offered by other methods and where it meets the applicable remedy
selection criteria for the particular OSWER program.  EPA expects that monitored natural
attenuation  will be most appropriate when used  in conjunction with active remediation
measures (e.g., source control), or as a follow-up to active remediation measures that have
already been implemented

       In determining whether monitored natural attenuation is an appropriate remedy for soil or
groundwater at given site, EPA or other regulatory authorities should consider the following:
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                                                               OSWER Directive 9200.4-17

       •       Whether the contaminants present in soil or groundwater can be effectively
              remediated by natural attenuation processes;

       •       Whether the resulting transformation products present a greater risk than
              do the parent contaminants;

              The nature and distribution of sources of contamination and whether these
              sources have been or can be adequately controlled;

       •       Whether the plume is relatively stable or is still migrating and the potential
              for environmental conditions to change over time;

              The impact of existing and proposed active remediation measures upon the
              monitored natural attenuation component of the remedy;

       •       Whether drinking water supplies, other groundwaters, surface waters,
              ecosystems,  sediments, air, or other environmental resources could be
              adversely impacted as a consequence of selecting monitored natural
              attenuation as the remediation option;

              Whether the estimated time frame of remediation is reasonable (see below)
              compared to time frames required for other more active methods (including
              the anticipated effectiveness of various remedial approaches on different
              portions of the contaminated soil and/or groundwater);

       •       Current and projected demand for the affected aquifer over the time period
              that the remedy will remain in effect (including the availability of other
              water supplies and the loss of availability of other groundwater resources
              due to contamination from other sources); and

       •       Whether reliable  site-specific vehicles for implementing institutional
              controls (i.e., zoning ordinances) are available, and if an institution
              responsible for their monitoring and enforcement can be identified.

       For example, evaluation  of a given site may determine that, once the source area and
higher concentration portions of the plume are effectively contained or remediated, lower
concentration portions of the plume could achieve cleanup standards within a few decades
through monitored natural attenuation, if this time frame is comparable to those of the more
aggressive methods evaluated for this site. Also, monitored  natural attenuation would more  likely
be appropriate if the plume is not expanding, nor threatening downgradient wells or surface water
bodies, and where ample potable water supplies are available. The remedy for this site could
include source control, a pump-and-treat system to mitigate only the highly-contaminated plume
areas, and monitored natural attenuation in the lower concentration portions of the plume. In
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                                                                OSWER Directive 9200.4-17

combination, these methods would maximize groundwater restored to beneficial use in a time
frame consistent with future demand on the aquifer, while utilizing natural attenuation processes
to reduce the reliance on active remediation methods (and  reduce cost).

       Of the above factors, the most important considerations regarding the suitability of
monitored natural attenuation as a remedy include whether the groundwater contaminant plume is
growing, stable, or shrinking, and any risks posed to human and environmental receptors by the
contamination  Monitored natural attenuation should not be used where such an approach
would result in significant contaminant migration or unacceptable impacts to receptors.
Therefore, sites where the contaminant plumes are no longer increasing in size, or are shrinking in
size, would be the most appropriate candidates for monitored natural attenuation remedies.

Reasonableness of Remediation Time Frame

       The longer remediation time frames typically associated with monitored natural
attenuation should be compatible with site-specific land and groundwater use scenarios.
Remediation time frames generally should be estimated for all remedy alternatives undergoing
detailed analysis, including monitored natural attenuation13. Decisions regarding the
"reasonableness" of the remediation time frame for any given remedy alternative should then be
evaluated on a site-specific basis.  While it is expected that monitored natural attenuation may
require somewhat longer to achieve remediation objectives than would active remediation, the
overall remediation time frame for a remedy which relies in whole or in part on monitored natural
attenuation should not be excessive compared to the other  remedies considered. Furthermore,
subsurface conditions and plume stability can change over  the extended timeframes that are
necessary for monitored natural attenuation.

       Defining a reasonable time frame is a complex and site-specific decision.  Factors that
should be considered when evaluating the length of time appropriate for remediation include:

              Classification of the affected resource (e.g., drinking water source,
              agricultural water source) and value of the resource14;
     EPA recognizes that predictions of remediation time frames may involve significant uncertainty; however, such
predictions are very useful when comparing two or more remedy alternatives.

     In determining whether an extended remediation time frame 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.

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                                                              OSWER Directive 9200.4-17

       •       Relative time frame in which the affected portions of the aquifer might be
              needed for future water supply (including the availability of alternate
              supplies);

       •       Uncertainties regarding the mass of contaminants in the subsurface and
              predictive analyses (e.g., remediation time frame, 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 extended time for remediation; and

       •       Provisions by the responsible party for adequate funding of monitoring and
              performance evaluation over the period required for remediation.

       Finally, individual states may provide information and guidance relevant to many of the
factors discussed above as part of a Comprehensive State Groundwater Protection Program
(CSGWPP). (See USEPA, 1992a)  Where a CSGWPP has been developed, it should be
consulted for groundwater resource classification and other information relevant to determining
required cleanup levels and the urgency of the need for the groundwater.  Also, EPA remediation
programs generally should defer to state determinations of current and future groundwater uses,
when based on an EPA-endorsed CSGWPP that has provisions for site-specific decisions
(USEPA, 1997b).

       Thus, EPA or other regulatory authorities should consider a number of factors when
evaluating reasonable time frames for monitored natural attenuation at a given site. These factors,
on the whole, should allow the regulatory agency 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
returned to  beneficial uses within a reasonable time frame. When these conditions cannot be met
using monitored natural attenuation, a remedial alternative that does meet these expectations
should be selected instead.

Remediation of Contamination Sources and Highly Contaminated Areas

       The need for control measures for contamination sources and other highly contaminated
areas should be evaluated as part of the remedy decision process at all sites, particularly where
monitored natural attenuation is under consideration as the remedy or as a remedy component.
Source control measures include removal,  treatment, or containment measures (e.g., physical or
hydraulic control of areas of the plume in which NAPLs are present in the subsurface).  EPA
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                                                              OSWER Directive 9200.4-17

prefers remedial options which remove or treat contaminant sources when such options are
technically feasible.

       Contaminant sources which are not adequately addressed complicate the long-term
cleanup effort. For example, following free product recovery, residual contamination from a
petroleum fuel spill may continue to leach significant quantities of contaminants into the
groundwater. Such a lingering source can unacceptably extend the time necessary to reach
remedial 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 and
threaten 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.

Performance Monitoring

       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 monitored natural attenuation than for other types of
remedies due to the longer remediation time frames, potential for ongoing contaminant migration,
and other uncertainties associated with using monitored natural attenuation.  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 remedy performance as well as
define the anticipated performance objectives of the remedy.  In addition, all monitoring programs
should be designed to accomplish the following:

             Demonstrate that natural attenuation is occurring according to
              expectations;

       •      Identify  any potentially toxic transformation products resulting from
             biodegradation;

             Determine if a plume is expanding (either downgradient, laterally or
             vertically);

       •      Ensure no impact to downgradient receptors;
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                                                               OSWER Directive 9200.4-17

              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;

              Detect changes in environmental conditions (e.g., hydrogeologic,
              geochemical, microbiological, or other changes) that may reduce the
              efficacy of any of the natural attenuation processes15; and

              Verify attainment of cleanup objectives.

       Performance monitoring should continue as long as contamination remains above
required cleanup levels. Typically, monitoring is continued for a specified period (e.g.., one to
three years) after cleanup levels 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 State implementing
agency as part of any proposed monitored natural attenuation 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.

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 and
enhancement 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 employed where the selected technology is not proven for the specific site application, where
    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 monitored
natural attenuation because of the lack of engineering controls to control contaminant migration.

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                                                               OSWER Directive 9200.4-17

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.

       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 measures.  Such criteria might
include the following:

       •       Contaminant concentrations in soil or groundwater at specified locations
              exhibit an increasing trend;

       •       Near-source wells exhibit large concentration increases indicative of a new
              or renewed release;

       •       Contaminants are identified in sentry/sentinel wells located outside of the
              original plume boundary, indicating renewed contaminant migration;

              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 monitored natural attenuation remedy.

       In establishing triggers or contingency remedies, however, care is needed to ensure that
sampling variability or seasonal fluctuations do not set off a trigger inappropriately.  For example,
an anomalous spike in dissolved concentration(s) at a well(s), which may set off a trigger, might
not be a true indication of a change in trend.

       EPA recommends that remedies employing monitored natural attenuation be evaluated to
determine the need for including one or more contingency measures that would be capable of
achieving remediation objectives.  EPA believes that a contingency measure may be particularly
appropriate for a monitored natural attenuation remedy which has been selected based primarily
on predictive analysis (second and third lines of evidence discussed previously) as compared to
natural attenuation remedies based on historical trends of actual monitoring data (first line of
evidence).

SUMMARY

       The use of monitored natural attenuation does not signify a change in OSWER's
remediation objectives; monitored natural attenuation should be selected only where it will be
fully protective of human health and the environment. EPA does not view monitored natural
attenuation to be a "no action" remedy,  but rather considers it to be a means of addressing
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                                                              OSWER Directive 9200.4-17

contamination under a limited set of site circumstances where its use meets the applicable
statutory and regulatory requirements.  Monitored natural attenuation 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 monitored natural attenuation should include a
comprehensive site characterization, risk assessment where appropriate, and measures to control
sources.  Also, monitored natural attenuation should not be used where such an approach would
result in significant contaminant migration or unacceptable impacts to receptors and other
environmental resources.  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 time frame that is reasonable compared to
time frames associated with other methods.  Where monitored natural attenuation'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 monitored natural attenuation will be, at many sites, a reasonable
and protective component of a broader remedial strategy.  However, EPA believes that there will
be many other sites where uncertainties too great or a need for a more rapid remediation will
preclude the use of monitored natural attenuation as a stand-alone remedy. This Directive should
help promote consistency in how monitored natural attenuation remedies are proposed, evaluated,
and approved.
REFERENCES CITED

United States Environmental Protection Agency (USEPA).  1988a.  Section 5.3.3.1. Natural
attenuation with monitoring.  Guidance on remedial actions for contaminated groundwater at
Super/and sites, 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
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                                                            OSWER Directive 9200.4-17

United States Environmental Protection Agency. 1991. A guide to principal threat and low level
threat wastes, Superfund Publication 9380.3-06FS (Fact Sheet), Office of Emergency Remedial
Response. Washington, D.C.

United States Environmental Protection Agency. 1992a. Final comprehensive state ground
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United States Environmental Protection Agency.  1992b. Methods for evaluating attainment of
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United States Environmental Protection Agency. 1993a. Guidance for evaluating the technical
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United States Environmental Protection Agency. 1994a. Proceedings of Symposium on natural
attenuation ofgroundwater, EPA/600/R-94/162, Office of Research and  Development.
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United States Environmental Protection Agency. 1994b. Methods for monitoring pump-and-
treat performance, EPA/600/R-94/123, Office of Research and Development.  Washington, D.C.

United States Environmental Protection Agency. 1995a. Chapter IX: Natural attenuation. How
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United States Environmental Protection Agency. 1996a. Presumptive response strategy and
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United States Environmental Protection Agency. 1996b. Corrective action for releases from
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United States Environmental Protection Agency. 1997a. Proceedings of the symposium on
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EPA/540/R-97/504, Office of Research and Development. Washington, D.C.
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                                                            OSWER Directive 9200.4-17

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
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American Academy of Environmental Engineers. 1995. Innovative site remediation technology,
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American Society for Testing and Materials. (Forthcoming). Provisional standard guide for
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American Society for Testing and Materials. (Forthcoming). Standard guide for remediation of
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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
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Hinchee, R.E., J.T. Wilson, and D.C. Downey.  1995. Intrinsic bioremediation. Columbus,
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Klecka, G.M., J.T. Wilson, E. Lutz, N. Klier, R. West, J. Davis, J. Weaver, D. Kampbell, and B.
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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.

New Jersey Department of Environmental Protection.  1996. Site remediation program,
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Norris, R.D., R.E. Hinchee, R.A. Brown, P.L. McCarty, L.  Semprini, J.T. Wilson, D.H.
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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.
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                                                            OSWER Directive 9200.4-17

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
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United States Environmental Protection Agency.  1988b. Groundwater modeling: an overview
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United States Environmental Protection Agency.  1992c. Quality assurance  and control in the
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United States Environmental Protection Agency.  1994c. The hydrocarbon spill screening model
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United States Environmental Protection Agency.  1994d. Assessment framework for ground-
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United States Environmental Protection Agency.  1994e. Ground-water modeling compendium,
EPA 500-B-94-004, Office of Solid Waste and Emergency Response. Washington, D.C.

United States Environmental Protection Agency.  1994f A technical guide to ground-water
model selection at sites contaminated with radioactive substances, EPA 402-R-94-012, Office of
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United States Environmental Protection Agency.  1994g. Guidance for conducting external peer
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United States Environmental Protection Agency.  1994h. Report of the agency task force on
environmental regulatory modeling, EPA 500-R-94-001, Office of Air and Radiation.
Washington, D.C.
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                                                             OSWER Directive 9200.4-17

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
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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
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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.
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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
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United States Environmental Protection Agency.  1997c.  (Draft) Geochemicalprocesses
affecting sorption of selected contaminants, Office of Radiation and Indoor Air. Washington,
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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.
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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.

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.
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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
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Wisconsin Department of Natural Resources.  1993. ERRP issues guidance on natural
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OTHER SOURCES OF INFORMATION

USEPA Internet Web Sites

       http://www.epa.gov/ORDAVebPubs/biorem/
       Office of Research and Development, information on passive and active bioremediation

       http://www.epa.gov/ada/kerrlab. html
       Office of Research and Development, R.S. Kerr Environmental Research Laboratory

       http://www.epa.gov/OUST/cat/natatt.htm
       Office of Underground Storage Tanks, information on natural attenuation

       http ://www. epa.gov/swerffrr/chlorine.htm
       Federal Facilities Restoration and Reuse Office, fact sheet on natural attenuation of
       chlorinated solvents

       http://www.epa.gov/swerffrr/petrol.htm
       Federal Facilities Restoration and Reuse Office, Fact sheet on natural attenuation of
       petroleum  contaminated sites

       http://www.epa.gov/hazwaste/ca/subparts.htm
       Office of Solid Waste, information on RCRA Subpart S

       http://www.epa.gov/swerosps/bf/
       Office of Outreach Programs, Special Projects, and Initiatives, information on Brownfields

Other Internet Web Sites

       http://clu-in.com
       Technology Innovation Office, information on hazardous site cleanups
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