xvEPA
United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
Superfund
EPA540-R-97-013
OSWER 9355.0-69
PB97-963301
August 1997
RULES OF THUMB FOR
SUPERFUND REMEDY SELECTION
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EPA540-R-97-013
OSWER 9355.0-69
PB97-963301
August 1997
N
RULES OF THUMB FOR SUPERFUND REMEDY SELECTION
U S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
Washington, DC 20460
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ABSTRACT
This guidance document describes key principles and expectations, interspersed with "best
practices" based on program experience, that should be consulted during the Superfund
remedy selection process. These remedy selection "Rules of Thumb" are organized into
three major policy areas: 1) risk assessment and risk management, 2) developing remedial
alternatives, and 3) ground-water response actions. The purpose of this guide is to briefly
summarize key elements of various remedy selection guidance documents and policies in
one publication. EPA believes that consistent application of national policy and guidance
is an important means by which we ensure the reasonableness, predictability, and cost-
effectiveness of our decisions. Gathering these remedy selection policy expectations into
one document will support our ongoing efforts to promote these important objectives. For
more detailed discussions of these policy areas, consult the National Contingency Plan
(NCP) and the guidance documents listed at the end of each section. This guide has been
developed as one of the Superfund administrative reforms announced by Administrator
Carol Browner on October 2, 1995.
To Obtain Documents:
EPA employees can obtain additional copies of this guidance, or copies of
documents referenced in this guidance, by calling the Superfund Document
Center at 703-603-9232, or by sending an e-mail request to
superfund.documentcenter@epamail.gov. Non-EPA employees can obtain
these documents by contacting the National Technical Information Service
at 703-487-4650. This document is also available on the Internet at
http://www.eDa.gov/suDerfiind.
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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
the National Contingency Plan. The guidance is designed to communicate 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 cannot 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.
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INTRODUCTION
The Superfund program's remedy selection process links the analysis of site cleanup alternatives,
conducted in a remedial investigation/feasibility study (RJ/FS), with the documentation of the
selected remedy in a Record of Decision (ROD). Section 121 of the Superfund statute (the
Comprehensive Environmental Response, Compensation and Liability Act (CERCLA)) established
five principal requirements for the selection of remedies. Remedies must: 1) protect human health
and the environment; 2) comply with applicable or relevant and appropriate requirements (ARARs)
unless a waiver is justified; 3) be cost-effective; 4) utilize permanent solutions and alternative
treatment technologies or resource recovery technologies to the maximum extent practicable; and
5) satisfy a preference for treatment as a principal element, or provide an explanation in the ROD
as to why this preference was not met.
In the Superfund regulations (the National Oil and
Hazardous Substances Contingency Plan (NCP)),
EPA established a national goal and a series of
expectations to reflect the principal requirements of
Section 121 and to help focus the RI/FS on
appropriate waste management options (Exhibit 1).
EPA also developed nine criteria for evaluating
remedial alternatives to ensure that all important
considerations are factored into remedy selection
decisions (Exhibit 2). These criteria are derived
from the statutory requirements of Section 121, as
well as technical and policy considerations that
have proven to be important for selecting among
remedial alternatives. The nine criteria analysis
comprises two steps: an individual evaluation of
each alternative with respect to each criterion; and
a comparison of options to determine the relative
performance of the alternatives and identify major
trade-offs among them (i.e., relative advantages and
disadvantages).
Applicability to the RCRA
Corrective Action Program
The Superfund and Resource Conservation
and Recovery Act (RCRA) Corrective Action
programs generally should yield similar remedies in
similar circumstances. Therefore, the Agency believes
that many of the principles conveyed in this document
are applicable to the RCRA Corrective Action
program, except where justified based on clear
programmatic differences. For example, although
RCRA Corrective Action incorporates risk-based
decision making, formal "baseline risk assessments"
are not always conducted as they are for Superfund
sites. Superfund project managers using these
principles can be confident that remedies selected
generally will satisfy RCRA Corrective Action;
likewise, RCRA Corrective Action project managers
are encouraged to use these principles, as appropriate,
to promote cost-effective remedial decision making
and consistency with Superfund. For more
information see: Coordination between RCRA
Corrective Action and Closure and CERCLA Site
Activities (OSWER Directive 9200.0-25, September
24, 1996); and Advance Notice of Proposed
Rulemaking (ANPR) for RCRA Corrective Action (61
FR 19432, May 1, 1996).
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Exhibit l
Superfund Program Goal and Expectations
Program Goal (40 CFR 300.430(a)(l)(i))
The national goal of the remedy selection process is to select remedies that are protective of human
health and the environment, that maintain protection over time, and that minimize untreated waste.
Program Expectations (40 CFR 300.430(a)(l)(iii)(A-F))
EPA generally shall consider the. following expectations in developing appropriate remedial
alternatives:
• EPA expects to use treatment to address the principal threats posed by a site, wherever practicable.
• EPA expects to use engineering controls, such as containment, for waste that poses a relatively low
long-term threat or where treatment is impracticable.
• EPA expects to use a combination of methods, as appropriate, to achieve protection of human health
and the environment.
• EPA expects to use institutional controls, such as water use and deed restrictions, to supplement
engineering controls as appropriate for short- and long-term management to prevent or limit exposure
to hazardous substances, pollutants or contaminants.
• EPA expects to consider using innovative technology when such technology offers the potential for
comparable or superior treatment performance or implementability, fewer or lesser adverse impacts
than other available approaches, or lower costs for similar levels of performance than demonstrated
technologies.
• EPA expects to return usable ground waters to their beneficial uses wherever practicable, within a
timeframe that is reasonable given the particular circumstances of the site.
EPA established the RI/FS process for gathering the information necessary to select a remedy that
is appropriate for the site and fulfills these statutory mandates. The RI includes sampling and
analysis to characterize the nature and extent of site contamination, performance of a baseline risk
assessment to assess the current and potential future risks to human health and the environment
posed by that contamination, and the conduct of treatability studies (where appropriate) to evaluate
the potential costs and effectiveness of treatment (or recovery) technologies to reduce the toxicity,
mobility, or volume of specific site waste. The FS includes the development and screening of
alternative remedial actions, and the detailed evaluation and comparison of the final candidate
cleanup options. Typically, a range of options is developed during the FS concurrently with the RI
site characterization, with the results of each influencing the other in an iterative fashion. (See RI/FS
Guidance for a more complete discussion.)
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Exhibit 2
RELATIONSHIP OF THE NINE CRITERIA
TO THE STATUTORY FINDINGS
NINE CRITERIA
STATUTORY FINDINGS
PROTECTION OF HUMAN HEALTH
AND THE ENVIRONMENT
COMPLIANCE WITH ARARs
PROTECTION OF HUMAN HEALTH
AND THE ENVIRONMENT
•>• COMPLIANCE WITH ARARs OR
JUSTIFICATION OF A WAIVER
LONG-TERM EFFECTIVENESS
AND PERMANENCE
TOXICITY, MOBILITY, OR
VOLUME REDUCTION
THROUGH TREATMENT
SHORT-TERM EFFECTIVENESS
IMPLEMENTABILITY
COST
STATE AGENCY ACCEPTANCE
COMMUNITY ACCEPTANCE
->• COST-EFFECTIVENESS
UTILIZATION OF PERMANENT
SOLUTIONS AND TREATMENT OR
RECOVERY TO THE MAXIMUM
EXTENT PRACTICABLE ("MEP")
PREFERENCE FOR TREATMENT AS A
PRINCIPAL ELEMENT (AS A CONSIDERATION
IN BALANCING THE NINE CRITERIA)
EPA also established a two-step remedy selection process, in which a preferred remedial action is
presented to the public for comment in a Proposed Plan, which summarizes preliminary conclusions
as to why that option appears most favorable based on the information available and considered
during the FS. Following receipt and evaluation of public comments on the Proposed Plan (which
may include new information), EPA makes a final decision and documents the selected remedy in
a ROD. (See Remedy Selection Guidance for a more complete discussion.)
EPA has issued numerous guidance documents that complement and clarify the remedy selection
framework presented in the NCP. Rules of Thumb for Superfund Remedy Selection summarizes the
remedy selection policy expectations contained in these guidance documents as well as in the
Superfund statute and regulations. By summarizing this information in a single document, EPA
expects to assist Remedial Project Managers and other program implementers in applying remedy
selection principles in an appropriately consistent manner.
Please note that this guidance document is not a comprehensive guide to every Agency policy that
might affect remedy selection, nor is it a replacement for the careful application of regulatory and
statutory requirements to individual sites. Rather, the document is a synopsis of the principles and
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expectations that are likely to have the most bearing on a wide range of site remedy selections.
While this document should help expedite and focus the remedy selection process, it is not a
substitute for a careful review and application of CERCLA, the NCP, and relevant guidance
documents at individual sites.
Primary source documents for policy statements have been identified in parentheses in italics
following each Rule of Thumb and full citations are included at the end of each major section.
Specific page citations are provided, where appropriate, and represent the beginning of a relevant
section in the document. These source documents should be obtained and consulted for more
information.
For Additional Information on the Remedy Selection Process:
NCP: National Oil and Hazardous Substances Pollution Contingency Plan (The NCP):
With the Preambles of 1988 and 1990 and the New Index of Key Terms (OSWER
Publication 9200.2-14, January 1992).
RI/FS Guidance: Guidance for Conducting Remedial Investigations and Feasibility Studies
under CERCLA (EPA 540-G-89-004, October 1988).
ROD Guidance: Interim Final Guidance on Preparing Superfund Decision Documents: The
Proposed Plan, The Record of Decision, Explanation of Significant Differences, The Record of
Decision Amendment: Interim Final (EPA 540-G-89-007, July 1989). (Revision anticipated in
1998.)
Remedy Selection Guidance: A Guide to Selecting Superfund Remedial Actions
(OSWER Publication 9355.0-27FS, April 1990).
SACM Guidance: Guidance on Implementation of the Superfund Accelerated Cleanup Model
(SACM) under CERCLA and the NCP (OSWER Directive 9203.1-03, July 7, 1992). Five
additional fact sheets also describe SACM and are available by citing the following reference:
OSWER Directive 9203.1-051 (Volume 1, Numbers 1-5), December 1992.
Non-Time Critical Removal Guidance: Guidance on Conducting Non-Time Critical Removal
Actions under CERCLA (EPA 540-R-93-057, August 1993).
Role of Cost Directive: The Role of Cost in the Superfund Remedy Selection Process
(EPA 540-F-96-018, September 1996).
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RISK ASSESSMENT AND RISK MANAGEMENT
Background
The mandate of the Superfund program is to protect human health and the environment from current
and potential threats posed by uncontrolled hazardous waste sites. The NCP established the RI/FS
process to characterize the nature and extent of site risks, develop and evaluate cleanup options, and
gather other information necessary to select a remedy that is appropriate for a site. A baseline risk
assessment is performed as part of the RI/FS to evaluate the potential threat to human health and the
environment in the absence of any remedial action. EPA uses the results of the RI/FS and baseline
risk assessment to make a series of site-specific risk management decisions in the Superfund remedy
selection process.
Presented below is a summary of key principles and expectations for risk assessment and risk
management that have been developed for the Superfund program. Consideration of these principles
will help ensure that remedies are both cost-effective and appropriately consistent with national
policy and guidance.
Risk Assessment Rules of Thumb
The following principles should be consulted when developing the baseline risk assessment. If the
RI/FS only addresses a portion of the site or specific medium (e.g., ground water), these principles
apply to the baseline risk assessment developed in support of that specific operable unit. Additional
efforts may be required to relate the specific action to the overall risk posed by the site as a whole.
1) Conceptual Site Model: Evaluate available data and develop a well-defined conceptual site
model (CSM) in the earliest stages of the baseline risk assessment. The CSM is a three-
dimensional "picture" of site conditions that illustrates contaminant sources, release
mechanisms, exposure pathways, migration routes, and potential human and ecological
receptors. The CSM documents current and potential future site conditions and is supported
by maps, cross sections, and site diagrams that illustrate what is known about human and
environmental exposure through contaminant release and migration to potential receptors.
The CSM is initially developed during the scoping phase of the RI/FS and should be
modified as additional information becomes available. (RI/FS Guidance; DQO Guidance;
Soil Screening Guidance; and RAGS I Part A)
2) Exposure Pathways: Evaluate all relevant exposure pathways related to the site (e.g., direct
ingestion, inhalation), for both current and reasonably anticipated future land uses as well
as current and potential future ground-water and surface water uses. (Land Use Directive;
RAGS I Part A; and Soil Screening Level Guidance}
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3) Data Needs: Collect sufficient contaminant concentration data from each relevant medium
to adequately characterize the nature and extent of contamination and to develop sound
estimates of risk associated with each exposure pathway. (DQO Guidance)
4) Site-Specific Risk Calculation: The following principles apply to site-specific risk
calculations in the baseline risk assessment:
• Calculate the cumulative risks to an individual for chronic exposures, using
reasonable maximum exposure (RME) assumptions by combining a
statistically sound, arithmetic average, exposure-point concentration with
reasonably conservative values for intake and duration. The most current
updates on exposure assumptions, methods, and models for the residential
exposure scenario can be obtained from the Soil Screening Guidance.
• Use the most current toxicity values provided by the Integrated Risk
Information System (IRIS) or the Health Effects Assessment Summary
Tables (HEAST). Call the Superfund Technical Support Center in Cincinnati
at (513) 569-7300 if toxicity values are not reported in IRIS or HEAST.
(RAGS I Part A)
• Include estimates of risk for current and reasonably anticipated future land
uses and potential future ground-water and surface water uses, without
institutional controls. The baseline risk assessment is essentially an
evaluation of the "no action" alternative (i.e., an assessment of the risk
associated with a site in the absence of any remedial action or control).
While institutional controls do not actively clean up the contamination at a
site, they can control exposure and, therefore, are considered to be limited
action alternatives that may be evaluated during the remedy selection process.
(1990 NCP Preamble at 55 FR 8710)
• Include a discussion that identifies major sources of uncertainty or variability
and their influence on the risk estimates. Probabilistic methods may aid in
evaluating uncertainty at some sites. (RAGS I Part A; and EPA's Risk
Characterization Policy)
5) Other Measures of Risk: Other measures of risk (e.g., central tendency) can be used to
describe site risks more fully. However, RME risk generally should be the principal basis
for evaluating potential risks at Superfund sites. (1990 NCP Preamble at 55 FR 8711;
RAGS I Part A, page 6-4; and EPA's Risk Characterization Policy)
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6) Exposed Populations: The risk analysis should clearly identify the population, or population
sub-group (e.g., highly exposed or susceptible individuals), for which risks are being
evaluated. (RA GSI Part A, page 6-6; and EPA's Risk Characterization Policy)
7) Ecological Risk Assessment: Include an assessment of ecological risk in the baseline risk
assessment in order to support EPA's mission to protect the environment. A screening
ecological risk assessment generally should be conducted to identify those chemicals, media,
and portions of the site requiring more detailed study and analysis. Use site-specific toxicity
tests, field studies, and food-chain models whenever appropriate. (ECO Risk Guidance; and
RAGS II)
Risk Management Rules of Thumb
The following principles should be consulted when making risk management decisions in the
Superfund program. Unless otherwise noted, the Rules of Thumb presented in this section are
derived from the Role of Baseline Risk Assessment Directive.
1) Basis for Action: A response action is generally warranted if one or more of the following
conditions is met:
• The cumulative excess carcinogenic risk to an individual exceeds lO"4 (using
reasonable maximum exposure assumptions for either the current or
reasonably anticipated future land use);
• The non-carcinogenic hazard index is greater than one (using reasonable
maximum exposure assumptions for either the current or reasonably
anticipated future land use);
• Site contaminants cause adverse environmental impacts; or
• Chemical-specific standards or other measures that define acceptable risk
levels are exceeded and exposure to contaminants above these acceptable
levels is predicted for the RME. Examples include: drinking water standards
that are exceeded in ground water when that ground water is a current or
potential source of drinking water; or water quality standards that are
exceeded in surface or ground waters that support the designated uses of
these waters (e.g., support aquatic life).
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2)
3)
4)
5)
Preliminary Remediation Goals for Carcinogens: In the absence of ARARs for chemicals
that pose carcinogenic risks, PRGs generally should be established at concentrations that
achieve 10~6 excess cancer risk, modifying as appropriate based on exposure, uncertainty, and
technical feasibility factors.
Preliminary Remediation Goals for
Non-Carcinogens: In the absence of
ARARs for chemicals that pose non-
carcinogenic risks, PRGs. generally should
be established at concentrations that
achieve a hazard quotient of one.
Cumulative noncancer risks are
determined by adding hazard quotients for
chemicals with the same toxic endpoint or
mechanism of action (e.g., the toxic
endpoint for both ethylbenzene and
styrene is liver toxicity and so these
hazard quotients can be summed). In
establishing PRGs for chemicals that
affect the same target organ/system, PRGs
for individual chemicals should be divided
by the number of chemicals present in this
group (Soil Screening Guidance, page 32).
Chemical-Specific ARARs: When a
single ARAR for a specific chemical (or in
some cases a group of chemicals) defines
an acceptable level of exposure,
compliance with the ARAR generally will
be considered protective even if it is
outside the risk range (unless there are
extenuating circumstances, such as
exposure to multiple contaminants or pathways).
Background Concentrations: EPA does not generally clean up below natural background
levels. However, where anthropogenic (i.e., man-made) background levels exceed
acceptable risk-based levels, and EPA has determined that a response action is appropriate,
EPA's goal is to develop a comprehensive response to address area-wide contamination.
This will help avoid response actions that create "clean islands" amid widespread
contamination (Soil Screening Guidance, page 8).
RAOs, PRGs and Final Cleanup Levels
Remedial action objectives (RAQsl provide
a general description of what the cleanup will
accomplish (e.g., restoration of groundwater).
Preliminary remediation goals (PRGs) are the more
specific statements of the desired endpoint
concentrations or risk levels, for each exposure
route, that are believed to provide adequate
protection of human health and the environment
based on preliminary site information. Initial PRGs
are developed early in the RI/FS process and are
based on ARARs and other readily available
information, such as concentrations associated with
10"* cancer risk or a hazard quotient equal to one for
noncarcinogens calculated from EPA toxicity
information. Initial PRGs may also be modified
based on exposure, uncertainty, and technical
feasibility factors. As data are gathered during the
baseline risk assessment and RI/FS, PRGs are
refined into final contaminant-specific cleanup
levels. Based on consideration of factors during the
nine criteria analysis and using the PRG as a point
of departure, the final cleanup level may reflect a
different risk level within the acceptable risk range
(10"4 to 10"6 for carcinogens) than the originally
identified PRG. The final cleanup levels, not PRGs,
are documented in the Record of Decision.
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6) Selecting Remedial Action: In the absence of ARARs, remedies should reduce the risks
from carcinogenic contaminants such that the excess cumulative individual lifetime cancer
risk for site-related exposures falls between 10"4 and 10'6. The Agency has expressed a
preference for cleanups achieving the more protective end of the risk range (i.e., 10'6).
(NOTE: The upper boundary of the risk range is not a discrete line at 1 x 10"4, although EPA
generally uses 1 x 10"4 in making risk management decisions. A specific risk estimate
around 10"4 may be considered acceptable if justified based on site-specific conditions.) For
non-carcinogens, remedies generally should reduce contaminant concentrations such that
exposed populations or sensitive sub-populations will not experience adverse effects during
all or part of a lifetime, incorporating an adequate margin of safety (i.e., a hazard index at or
below one).
7) Timing: A "phased approach" to site investigation and cleanup generally will accelerate risk
reduction and provide additional technical site information on which to base long-term risk
management decisions. Phased cleanup approaches should be employed wherever
practicable (40 CFR 300.430(a)(l)(ii)(A)\ For more information about the use of early
actions to expedite site cleanup, see SACM Guidance and the Ground-Water Presumptive
Strategy.
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For Additional Information on Risk Assessment and Risk Management:
RI/FS Guidance: Guidance for Conducting Remedial Investigations and Feasibility Studies under CERCLA
(EPA 540-G-89-004, October 1988).
RAGS I Part A: Risk Assessment Guidance For Superfund, Volume 1, Part A: Human Health Evaluation
Manual, Interim Final (EPA 540-1-89-002, March 1989).
RAGS II: Risk Assessment Guidance for Superfund, Volume II, Environmental Evaluation Manual, Interim
Final (EPA 540-1-89-001, March 1989).
RAGS I Part B: Risk Assessment Guidance for Superfund, Volume I, Part B: Human Health Evaluation
Manual, Development of Risk-Based Preliminary Remediation Goals, Interim Final (EPA 540-1 -89-002,
December 1991). [Note: Soil Screening Guidance provides improvements in inhalation and ground water
exposure pathway discussions.]
RAGS I Part C: Risk Assessment Guidance for Superfund, Volume I, Part C: Human Health Evaluation
Manual, Risk Evaluation of Remedial Alternatives, Interim (EPA 540-R-92-004, December 1991).
Role of Baseline Risk Assessment Directive: Role of the Baseline Risk Assessment in Superfund Remedy
Selection (OSWER Directive 9355.0-30, April 22, 1991).
SACM Guidance: Guidance on Implementation of the Superfund Accelerated Cleanup Model (SACM)
under CERCLA and the NCP (OSWER Directive 9203.1-03, July 7, 1992).
POO Guidance: Data Quality Objectives Process for Superfund: Interim Final Guidance (EPA 540-R-93-
071, 1993).
EPA's Risk Characterization Policy: Policy for Risk Characterization at the U.S. Environmental Protection
Agency (Memorandum from Administrator Carol Browner, March 21, 1995).
Land Use Directive: Land Use in the CERCLA Remedy Selection Process (OSWER Directive 9355.7-04,
May 25, 1995).
Soil Screening Guidance: Soil Screening Guidance: Users Guide (EPA 540-R-96-018, April 1996).
Ground-Water Presumptive Strategy: Presumptive Response Strategy and Ex-Situ Treatment Technologies
for Contaminated Ground Water at CERCLA Sites (EPA 540-R-96-023, October 1996).
ECO Risk Guidance: Ecological Risk Assessment Guidance for Superfund: Process for Designing and
Conducting Ecological Risk Assessments (EPA. 540-R-97-006, June 1997).
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DEVELOPING REMEDIAL ALTERNATIVES
Background
The national goal of the remedy selection process is to "select remedies that are protective of human
health and the environment, that maintain protection over time, and that minimize untreated waste"
(40 CFR 300.430(a)(l)(i)). While protection of human health and the environment can be achieved
through a variety of methods, this goal reflects CERCLA's emphasis on achieving protection
through the aggressive, but realistic, use of treatment. Remedies that rely on engineering and
institutional controls as a major component, in addition to being less permanent than treatment
remedies, may place constraints on the productive re-use of land.
To accomplish this goal, the NCP describes six
alternatives. These expectations, derived from the
mandates of CERCLA Section 121 and based on
previous Superfund experience, were developed as
guidelines to communicate the types of remedies
that EPA generally will find appropriate for
specific types of waste. Although remedy
selection decisions are site-specific determinations
based on analyses of remedial alternatives using
the nine evaluation criteria, these expectations
help to focus the RI/FS on appropriate waste
management options. This section discusses the
first four of the six NCP expectations presented in
Exhibit 1. Unless otherwise noted, this section
has been derived from the Principal Threats
Guidance.
Identifying Principal and Low-level
Threat Wastes
The concept of principal threat waste and low-
level threat waste, as developed by EPA in the
NCP and expanded in subsequent guidance,
should be applied on a site-specific basis when
characterizing "source material." Source material
is defined as material that includes or contains
hazardous substances, pollutants, or contaminants
that act as a reservoir for migration of
expectations for the development of remedial
Reasonably Anticipated Future Land
Use and Principal Threat Waste
Identification
The reasonably anticipated future land use at
a site is significant in defining principal threat waste
areas. Pursuant to the NCP and the 1995 land use
guidance, current land use and reasonably
anticipated future land use should be considered in
identifying realistic exposure scenarios for
estimating site risks. When the baseline risks
associated with the reasonably anticipated future
land use trigger action, the definition of principal
threat wastes may be determined by the reasonably
anticipated future land use scenario as well. For
example, soil contamination that could be
considered a principal threat under a residential
exposure scenario might be considered a low-level
threat under a non-residential exposure scenario.
Although no "threshold level" of risk has been
established to identify principal threat waste, a
general rule of thumb is to consider as a principal
threat those source materials with toxicity and
mobility characteristics that combine to pose a
potential risk several orders of magnitude greater
than the risk level that is acceptable for the current
or reasonably anticipated future land use, given
realistic exposure scenarios. (For more information,
see Principal Threats Guidance and Land Use
Directive.)
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contamination to ground water, to surface water, to air, or acts as a source for direct exposure.
Contaminated ground-water plumes are generally not considered to be source material, although
nonaqueous phase liquids (NAPLs) in the ground water generally would be viewed as source
material.
Identifying principal and low-level threat wastes combines concepts of both hazard and risk. In
general, principal threat wastes are those source materials considered to be highly toxic or highly
mobile which generally cannot be contained in a reliable manner and/or would present a significant
risk to human health or the environment should exposure occur. Conversely, low-level threat wastes
are those source materials that generally can be reliably contained and that would present only a low
risk in the event of exposure. The manner in which principal threats are addressed generally will
determine whether the statutory preference for treatment as a principal element is satisfied.
Rules of Thumb for Developing Appropriate Remedial Alternatives for Source Materials
1) Combination of Methods of Protection: An appropriate combination of treatment
technologies, engineering controls, and institutional controls should be considered when
developing remediation approaches that will be protective of human health and the
environment. Federal or state ARARs (e.g., land disposal restrictions under RCRA) may
limit containment and treatment options.
2) Treatment of Principal Threats: "EPA expects to use treatment to address the principal
threats posed by a site, wherever practicable" (40 CFR 300.430(a)(l)(iii)(A)).
3) Containment of Principal Threats: In some situations, it may be appropriate to contain
rather than treat principal threat wastes due to difficulties in treating the wastes. The
following situations generally should result in a determination that treatment is not
practicable under the nine remedy selection criteria (Exhibit 2). For example, when:
• Treatment technologies are not technically feasible or are not
available within a reasonable time frame;
• The extraordinary volume of materials or complexity of the site may
make implementation of the treatment technologies impracticable
(e.g., large landfills);
• Implementation of a treatment-based remedy would result in greater
overall risk to human health and the environment due to risks posed
to workers, the surrounding community, or impacted ecosystems
during implementation (to the degree that these risks cannot be
otherwise addressed through implementation measures); and
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• Implementation of the treatment technology would have severe
effects across environmental media.
4) Containment of Low Level Threats: "EPA expects to use engineering controls, such as
containment, for waste that poses a relatively low long-term threat or where treatment is
impracticable" (40 CFR 300.430(a)(l)(iii)(B)).
5) Treatment of Low-Level Threats: In some situations, it may be appropriate to treat rather
than contain low-level threat wastes. For example:
• Once a decision has been made to treat some wastes onsite,
economies of scale may make it cost-effective to treat more than just
principal threat wastes, to alleviate or minimize the need to maintain
engineering or institutional controls over time.
• In some circumstances, treatment of more than principal threat wastes
may be appropriate in order to make the whole site consistent with
the reasonably anticipated future land use (e.g., where there are plans
for residential development). See the Land Use Directive to obtain
additional information for considering land use in remedy selection
decisions.
6) NAPLs as Principal Threat Wastes: Although nonaqueous phase liquids (NAPLs) are
generally viewed as principal threat wastes, program experience has shown that removal
and/or in-situ treatment of NAPLs may not be practicable. Hence, EPA generally expects
that the quantity of free-phase NAPL (i.e., "free product") should be reduced to the extent
practicable and that an appropriately designed containment strategy should be developed for
NAPLs that cannot be removed from the subsurface. This policy applies to both dense
nonaqueous phase liquids (DNAPLs) and light nonaqueous phase liquids (LNAPLs),
although of the two, it is generally more difficult to remove or treat DNAPLs in the
subsurface. (See Ground-Water Rule of Thumb #10 for more complete discussion of
DNAPLs.)
7) Use of Institutional Controls: Institutional controls (such as easements, well drilling
prohibitions, building permit restrictions, land use zoning restrictions, or fishing bans)
generally shall not substitute for more active measures (e.g., treatment and/or containment
of source material) as the sole remedy unless active measures are not practicable, based on
the balancing of trade-offs among alternatives that is conducted during the remedy selection
process. Institutional controls typically will be used in conjunction with engineering controls
when the remedy results in long-term waste management onsite. (40 CFR
300.430(a)(l)(iii)(D))
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For Additional Information on Developing Remedial Alternatives:
RI/FS Guidance: Guidance for Conducting Remedial Investigations and Feasibility Studies
Under CERCLA (EPA 540-G-89-004, October 1988).
Principal Threats Guidance: A Guide to Principal Threat and Low Level Threat Wastes (OSWER
Directive 9380.3-06FS, November 1991).
Land Use Directive: Land Use in the CERCLA Remedy Selection Process (OSWER Directive
9355.7-04, May 25, 1995).
Ground-Water Presumptive Strategy: Presumptive Response Strategy and Ex-Situ Treatment
Technologies for Contaminated Ground Water at CERCLA Sites (EPA 540-R-96-023, October
1996).
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GROUND-WATER RESPONSE ACTIONS
Background
Contaminated ground water exists at over 85 percent of the sites on the National Priorities List
(NPL). The goal of ground-water remediation at Superfund sites is to protect human health and the
environment through a combination of short-term measures (e.g, provision of alternate water
supplies) and long-term measures to restore ground-water quality appropriate for its beneficial uses.
Remedial action for contaminated ground water generally is warranted when EPA determines, based
on the results of the baseline risk assessment, that the contamination poses a current or potential
threat to human health or the environment (CERCLA §104(a)(l)). Additionally, where the ground
water is currently used (or is potentially usable) as a drinking water supply, exceedance of Maximum
Contaminant Levels (MCLs) and non-zero Maximum Contaminant Level Goals (MCLGs)
established under the Safe Drinking Water Act also may be used as the basis for taking a remedial
action. The goals of the long-term ground-water cleanup program are summarized in the NCP as
follows:
"EPA expects to return usable
ground waters to their beneficial
uses wherever practicable, within
a time frame that is reasonable
given the particular circumstances
of the site. When restoration of
ground water to beneficial uses is
not practicable, EPA expects to
prevent further migration of the
plume, prevent exposure to the
contaminated ground water, and
evaluate further risk reduction"
(40 CFR 300.430(a)(l)(iii)(Fj),
Rules of Thumb for Ground-Water
Response Actions
The rules of thumb for ground-water response
actions are organized into four sequential
steps. EPA recognizes that site investigation
and analysis is a dynamic process that evolves
as more information is gathered during the RI
5Zf
Key Ground-Water Response Questions
What are the current and potential future
beneficial uses of the contaminated ground
water?
What is the approximate timing of the future
need for ground water?
Is restoration of ground water to beneficial uses
technically practicable within a reasonable
timeframe?
What is the range of remedial alternatives that
restore ground water in different but
"reasonable" time periods?
If restoration is not technically practicable,
what remedial activities are necessary to
prevent exposure to contaminants and prevent
further plume migration?
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and a cleanup strategy is developed and refined in accordance with the best available site data.
Hence, this framework is not strictly sequential, and should be tailored to address site-specific
situations.
Step 1: Identifying Remedial Action Objectives and Preliminary Remediation Goals Based
on Current and Potential Future Ground-Water Uses
Once it has been determined that there is a basis for taking a ground-water response action, the first
step should be to identify RAOs and PRGs for the ground water. These RAOs and PRGs should
reflect current and potential future uses of the ground water and exposure scenarios that are
consistent with these uses. Several factors should be considered when identifying ground-water
RAOs and PRGs:
1) Current Ground-Water Uses: RAOs and PRGs must reflect current human use (e.g.,
drinking water supply) as well as current environmental receptors (e.g., surface waters that are
recharged by ground water and that are used by sensitive environmental receptors). (Ground-
Water Presumptive Strategy, page 15)
2) Potential Future Ground-Water Uses: Where available, potential future ground-water uses
should be determined from a Comprehensive State Ground-Water Protection Program
(CSGWPP) that has been endorsed by EPA and has provisions for site-specific use
determinations. In the absence of such a CSGWPP, determination of potential future uses
should consider State ground water classifications or other designations and Federal ground-
water guidelines (e.g., Class I, II, and HI ground waters). The Federal classification system
can be found in "Guidelines for Ground-Water Classification Under the EPA Ground-Water
Protection Strategy" hereafter referred to as the Federal Guidelines. Where State and Federal
classifications result in different ground-water use scenarios, the "use classification" leading
to the more "stringent" RAOs should be used. Thus, ground waters at a given site are generally
assumed to be a potential future source of drinking water if designated as such by the State or
if considered to be a potential source of drinking water under the Federal Guidelines.
(CSGWPP Directive; Federal Guidelines; and 1990 NCP Preamble at 55 FR 8733)
3) PRGs for Drinkable Waters: Generally, drinking water standards (Federal MCLs, non-zero
MCLGs, or more stringent State drinking water standards) are relevant and appropriate as
PRGs, and ultimately as final cleanup levels, for ground waters that are determined to be a
current or potential future source of drinking water. (40 CFR 300.430(e)(2)(i)(B and C); and
Ground-Water Presumptive Strategy, page 15)
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4) Limitation of Using Drinking Water Standards: Generally, drinking water standards should
not be chosen as PRGs for ground waters that are not current or potential future sources of
drinking water (1990 NCP Preamble at 55 FR 8733). The Federal Guidelines define "non-
potable," or Class EQ, ground-water aquifers as those that: contain more than 10,000 mg/liter
total dissolved solids; yield less than 150 gallons per day; or are so contaminated by naturally
occurring conditions (e.g., salinity) or broad-scale human activity not related to a specific
contaminant source that cleanup is not practicable. State classification systems may also
identify ground waters that are not considered to be a potential future source of drinking water
(see Ground Water Rule of Thumb #2 above). Where non-potable ground water has been
contaminated, non-restoration RAOs may be appropriate (e.g., source control, plume
containment). Establishment of PRGs for non-potable ground-water should consider any
surface or ground-water bodies to which such non-potable ground waters discharge, and any
current or potential future uses of the non-potable ground water such as for livestock watering,
agricultural irrigation, industrial uses, or other purpose that might result in human or
environmental exposures. (Ground-Water Presumptive Strategy, page 15)
5) Consideration of More Stringent Ground-Water Standards: Many states have anti-
degradation standards or other requirements that are more stringent than the Federal drinking
water standards for a given constituent. Where such a state requirement is determined to be
an ARAR, it should be used as the PRG. (Ground-Water Presumptive Strategy, page 16)
6) Relationship between Ground-Water RAOs and Soil PRGs: At many sites, soil PRGs are
set at levels that are needed to achieve long-term RAOs for ground water. As a result, these
soil PRGs may need to be more stringent than would otherwise be necessary given the
reasonably anticipated future land use. However, stringent soil PRGs intended to protect
ground water generally will be inappropriate for site areas where the primary source of ground-
water contamination is located below the soil (e.g., DNAPLs below the water table) and
restoration of ground water is determined to be technically impracticable (see Ground-Water
Rule of Thumb #9 below). Therefore, both reasonably anticipated future land uses and
potential future ground-water uses must be considered when developing the overall site
remediation strategy. (Ground-Water Presumptive Strategy, page 12)
7) Relationship Between Ground Water and Other Water Resources: Contaminated ground
waters may discharge and pose a risk to environmental resources such as streams, lakes,
wetlands, or other uncontaminated aquifers. Therefore, ground-water PRGs should be set at
levels that are protective of these other resources as well. For example, cleanup of
contaminated ground waters that discharge to surface water should consider whether water
quality criteria established under the Clean Water Act, or more stringent state surface water
requirements, are ARARs. (1990 NCP Preamble at 55 FR 8754)
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Step 2: Establishing Remedial Action Objectives and Final Cleanup Levels Based on Site-
Specific Conditions
Once ground-water RAOs have been identified (in accordance with current and potential future
uses), PRGs associated with those objectives should be identified and factored into the remedy
selection process. PRGs developed during the RI/FS, and in accordance with the principles
presented in the previous section, are the starting point for determining final cleanup levels which
are documented in the ROD. The following rules of thumb relate to flexibilities in existing EPA
policy which allow for selection of achievable RAOs and their associated cleanup levels and points
of compliance.
8) Restoration Potential: If ground-water restoration is determined to be the RAO, MCLs, non-
zero MCLGs, or other risk-based cleanup levels will have to be achieved in order for the
ground-water resource to be restored. Site-specific information should be analyzed to
determine the likelihood that ground water can be restored to these levels using available
technologies (i.e., to determine the aquifer's "restoration potential"). (TI Guidance, page 13;
and Ground-Water Presumptive Strategy, page 5)
9) Technical Impracticability: An ARAR waiver should be invoked for those portions of the
contaminated soil or ground water where it has been demonstrated that attainment of one or
more ARARs are technically impracticable from an engineering perspective. The "TI" waiver
must be justified by site-specific information developed for the Administrative Record in
accordance with EPA guidance. In the event that a TI waiver is invoked, an "alternative
remedial strategy" must be developed that will ensure protection of human health and the
environment. This strategy should be incorporated into the decision document along with the
waiver justification and should define achievable RAOs and final cleanup levels for the site.
At a minimum, the alternative strategy should prevent human exposure to the contaminated
ground water, prevent further contaminant migration, and define any other appropriate risk
reduction measures. Note that the waiver should be invoked only for that portion of the
contaminated ground water for which restoration to ARARs is technically impracticable. As
a result, RAOs and final cleanup levels may be different for different portions of the
contaminant plume. (TI Guidance; and Ground-Water Presumptive Strategy, page 17)
10) DNAPL Sites: The likelihood of the presence of dense nonaqueous phase liquids (DNAPLs)
should be evaluated wherever DNAPL-type compounds (e.g., chlorinated solvents such as
TCE) are found in significant concentrations in the ground water or are known to have been
managed or disposed of at the site. The presence of DNAPLs can significantly impact the
restoration potential of the site. Where DNAPLs (or other persistent contamination sources)
are present in the subsurface and cannot practicably be removed, containment of such sources
may be the most appropriate remediation goal. In such cases, a TI waiver should be invoked
for the DNAPL zone. Where significant quantities of potentially mobile DNAPL are
identified, extraction should be considered in conjunction with containment. Extraction of
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potentially mobile DNAPLs will increase the long-term reliability of the containment remedy
as well as remove source material from the aquifer. Containment of the DNAPL zone will
increase the likelihood that the remaining portion of the aqueous phase plume can be restored
to a beneficial use. (Ground-Water Presumptive Strategy, page 13; and TI Guidance, page 6)
11) Point of Compliance: Final cleanup levels for contaminated ground water generally should
be attained throughout the entire contaminant plume, except when remedies involve areas
where waste materials will be managed in place. In the latter case, cleanup levels should be
achieved "at and beyond the edge of the waste management area when waste is left in place"
(1990 NCP Preamble at 55 FR 8713). In some cases, such as where several distinct sources
are in close geographic proximity, it may be appropriate to move the point of compliance to
"encompass the sources of release." In such cases, the point of compliance may be defined to
address the problem as a whole, rather than source by source. (1990 NCP Preamble at 55 FR
8753; and Ground-Water Presumptive Strategy, page 17)
12) Alternate Concentration Limits (ACLs): Under limited circumstances specified in CERCLA
§121(d)(2)(B)(ii), ACLs may be established in lieu of cleanup levels that would otherwise be
ARARs (e.g., MCLs). The conditions under which ACLs may be considered are where: 1)
contaminated ground water discharges to surface water; 2) such ground-water discharge does
not lead to "statistically significant" increases of contaminants in the surface water; and 3)
enforceable measures can be implemented to prevent human consumption of the contaminated
ground water. In general, ACLs may be used where the preceding conditions are satisfied, and
where restoration of the ground water is found to be impracticable, based on a balancing of the
remedy selection criteria. (1990 NCP Preamble at 55 FR 8732 and 8754; and Ground-Water
Presumptive Strategy, page 18)
Step 3: Evaluating Remedial Technologies and Cleanup Time Frames
Following the establishment of achievable remedial action objectives, cleanup levels, and areas of
compliance, a remediation technology (or combination of technologies) should be selected from
those identified in the Feasibility Study. The principal factors to consider at this stage are how
quickly the remedial action objectives need to be achieved, and what remediation strategies and
technologies should be used to achieve them. These factors will have a significant impact on the
type of remedy chosen for the site, as well as the cost of that remedy.
13) Using Early Actions: Early actions, such as a removal or interim remedial action taken before
the final remedy is selected, should be used where appropriate to reduce site risks early in the
site remediation process. In addition to reducing site risks and controlling further contaminant
migration, these activities will also provide additional site characterization information that
greatly improves the ability to make sound long-term remedy decisions. (Ground-Water
Presumptive Strategy, page 16; and SACM Guidance)
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14) Restoration Time Frames: Where the contaminated ground water is not currently used or an
alternate water source is readily available, and there is no near-term future need for the
resource, it will likely be appropriate to consider a longer time frame for achieving restoration
cleanup levels. Where longer remediation time frames are appropriate, less aggressive
remediation methods and/or more passive remediation approaches (such as source control
combined with monitored natural attenuation) should be considered. Restoration time frames
should be estimated for all viable remedial alternatives being considered for the site (40 CFR
300.430(e)(4)). Comparison of aggressive and passive remedial alternatives can provide a
helpful basis for identifying the range of time periods that will be needed to attain remediation
objectives, and will provide the basis for determining the remediation timeframe and
technologies appropriate for the site. (1990 NCP Preamble at 55 FR 8732; and Ground-Water
Presumptive Strategy, page 16)
15) Innovative Technologies: New and emerging technologies should be evaluated in the FS if
such technologies offer "the potential for comparable or superior performance or
implementability; fewer or lesser adverse impacts than other available approaches; or lower
costs for similar levels of performance than demonstrated treatment technologies" (40 CFR
300.430(e)(5)).
16) Monitored Natural Attenuation: At some sites, data gathered during the RI/FS may indicate
that physical or biological processes (unassisted by human intervention) may effectively reduce
contaminant concentrations such that remedial objectives in the contaminant plume (or certain
portions of the plume) may be achieved in a reasonable time frame without active remediation.
This approach is most likely to be appropriate in low concentration portions of the plume,
where source control actions have removed the bulk of the contaminant mass, or where
biodegradation will efficiently destroy the contaminants in situ. In some cases, remediation
alternatives that combine active remediation (in source areas or areas of high concentration)
with monitored natural attenuation (in lower concentration portions of the plume) may be most
appropriate. Sufficient information is necessary to demonstrate that natural processes are
capable of achieving remedial objectives for the site. Performance monitoring is a critical
component of this remediation approach because monitoring is needed to ensure that the
remedy is protective and that natural processes are reducing contamination levels as expected.
Sites with contaminants that do not readily attenuate, or sites that require relatively rapid
cleanup due to the demand for the ground-water resource, generally will not be appropriate
candidates for natural attenuation. (1990 NCP Preamble at 55 FR 8734; Ground-Water
Presumptive Strategy, page 18; and Natural Attenuation Guidance)
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17) Presumptive Treatment Technologies: Generally, selection of technologies for ex-situ
(above ground) treatment of extracted ground water should employ one or more of the
presumptive technologies identified in the Ground-Water Presumptive Strategy. The
engineering capabilities of these presumptive treatment technologies are well understood,
enabling the selection process to be streamlined for the ex-situ treatment component of a
ground-water remedy.
Step 4: Monitoring and Evaluating Remedy Performance
Selection of the site remedy marks the end of a data gathering, study, and decision-making process
and marks the beginning of the cleanup phase, which includes designing, constructhlg, operating and
maintaining the remedy. Since most ground-water remedies are expected to be operated, maintained,
and/or monitored for long periods of time, further opportunities for improving the performance and
cost-effectiveness of the cleanup should be explored and utilized if appropriate. An ongoing
Administrative reform initiative highlights EPA's recent efforts to encourage appropriate changes
to existing remedy decisions to enhance overall remedy effectiveness and cost-effectiveness, without
compromising protectiveness or other objectives of the Superfund program. (See Remedy Updates
Guidance for more information.,) The following principles describe how periodic evaluation of a
remedy can lead to improvements in performance and shortened cleanup time frames.
18) Implementing Remedies in Multiple Phases: Implementation of ground-water remedies in
more than one phase may increase the performance and cost-effectiveness of the long-term
remedy. Performance data from an early phase can be used to refine the design of later phases
so that the ultimate remedy is optimized for actual site conditions (e.g., optimized number,
location, and pumping rate of extraction wells). (Ground-Water Presumptive Strategy, page
5 and page 12)
19) Periodic Review: Performance of the ongoing remedy should be evaluated on a regular basis
(e.g., every 1 to 5 years) to compare anticipated with actual results, to identify any potential
deficiencies in the remedy's protectiveness, and to seek opportunities to improve its
performance over the long term. This is especially important when the selected remedy relies
on monitored natural attenuation. (Ground-Water Presumptive Strategy, page 8; and Natural
Attenuation Guidance)
20) Improving Remedy Performance: Ground-water remedies often involve multiple extraction
and/or injection points, subsurface structures or containment barriers, and other features whose
actual performance in the field may vary from that assumed during design, given uncertainties
about subsurface geology prior to construction. Careful assessment of performance monitoring
data may be used to refine the remedy, such as modifying extraction rates or changing the
pattern of extraction wells. Such improvements are capable of shortening cleanup time frames,
and thus reducing costs. (Ground-Water Presumptive Strategy, page 11)
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21) Revisiting and Modifying Cleanup Goals: At some sites it may be necessary to revisit the
ROD's remedial action objectives or final cleanup levels, if performance data indicate that
attainment of these objectives or levels is not technically practicable. If it is determined that
a TI waiver is appropriate, the waiver generally should be invoked in a ROD amendment,
although in some cases an Explanation of Significant Differences (BSD) may be sufficient.
An BSD may be used where the revised remedy is generally consistent with the "alternative
remedial strategy" discussed in the original ROD and the original ROD: 1) contained detailed
discussions of the potential need for a future TI waiver, and 2) identified an alternative
remedial strategy to be used in the event a TI waiver was determined to be appropriate for the
site. If an BSD is determined to be sufficient, public notice and opportunity for comment
should also be provided. (Ground-Water Presumptive Strategy, page 11; TI Guidance, page
19 and 24; and Remedy Updates Guidance)
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For Additional Information on Ground Water Response Actions:
Federal Guidelines: Guidelines for Ground-Water Classification Under the EPA Ground-Water
Protection Strategy, Final Draft (EPA Office of Water, November 1986).
Ground-Water Guidance: Guidance on Remedial Actions for Contaminated Ground Water at
Superfund Sites (EPA 540-G-88-003, December 1988).
Role of Baseline Risk Assessment Directive: Role of the Baseline Risk Assessment in Superfund
Remedy Selection (OSWER Directive 9355.0-30, April 22, 1991).
DNAPL I: Estimating Potential for Occurrence of DNAPL at Superfund Sites
(OSWER Publication 9355.4-07FS, January 1992).
SACM Guidance: Guidance on Implementation of the Superfund Accelerated Cleanup Model
(SACM) under CERCLA and the NCP (OSWER Directive 9203.1-03, July 7, 1992). Five
additional fact sheets also describe SACM and are available by citing the following reference:
OSWER Directive 9203.1-051 (Volume 1, Numbers 1-5), December 1992.
TI Guidance: Guidance for Evaluating the Technical Impracticability of Ground-Water
Restoration (EPA 540-R-93-080, September 1993).
DNAPL II: DNAPL Site Characterization (OSWER Publication 9355.4-16FS, September
1994).
Remedy Updates Guidance: Superfund Reforms: Updating Remedy Decisions (OSWER
Directive 9200.0-22, September 27, 1996).
Ground Water Presumptive Strategy: Presumptive Response Strategy and Ex-situ Treatment
Technologies for Contaminated Ground Water at CERCLA Sites (EPA 540-R-96-023, October
1996).
CSGWPP Directive: The Role of CSGWPPs in EPA Remediation Programs (OSWER
Directive 9283.1-09, April 4, 1997).
Natural Attenuation Guidance: Use of Monitored Natural Attenuation at Superfund, RCRA
Corrective Action, and Underground Storage Tank Sites (OSWER Directive 9200.4-17,
DRAFT).
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