United States
Environmental Protection
Agency
Superfund
Office of
Solid Waste and
Emergency Response
EPA 540-R-96-023
OSWER 9283.1-12
PB96-963508
October 1096
Presumptive Response Strategy
and Ex-Situ Treatment
Technologies for Contaminated
Ground Water at CERCLA Sites
Final Guidance
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Directive 9283.1-12
EPA 540/R-96/023
PB96-963508
October 1996
PRESUMPTIVE RESPONSE STRATEGY AND EX-SITU TREATMENT
TECHNOLOGIES FOR CONTAMINATED GROUND WATER
AT CERCLA SITES
FINAL GUIDANCE
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
Washington, DC 20460
llllllllllllllllllllllllllllllllllllllll
174592
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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 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 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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CONTENTS
Section Pane
FIGURES iii
HIGHLIGHTS iii
ACRONYMS USED IN THIS GUIDANCE iv
PREFACE v
1.0 INTRODUCTION 1
1.1 Purpose of Guidance 1
1.2 Expectations and Objectives for Ground-Water Cleanup 2
1.2.1 Program Expectations 2
1.2.2 Objectives for Site Response Actions 2
1.3 Lessons Learned 3
1.3.1 Sources and Types of Contaminants 3
1.3.2 Factors Limiting Restoration Potential 3
1.3.3 Assessing Restoration Potential 5
2.0 PRESUMPTIVE RESPONSE STRATEGY 5
2.1 Definition and Basis for Strategy 5
2.1.1 Benefits of Phased Approach 6
2.1.2 Early Actions 6
2.1.3 Monitoring 8
2.2 Phased Response Actions 8
2.2.1 Two Separate Actions 8
2.2.2 Phasing of a Single Action 8
2.3 Post-Construction Refinements 11
2.3.1 Types of Refinements 11
2.3.2 Documenting Refinements 11
2.4 Integrating Response Actions 12
2.4.1 Integrating Source Control and Ground-Water Actions 12
2.4.2 Combining Ground-Water Restoration Methods 12
2.5 Strategy for DNAPL Sites 13
2.5.1 Site Characterization 14
2.5.2 Early Actions 14
2.5.3 Long-Term Remedy 14
2.6 Areas of Flexibility in Cleanup Approach 15
2.6.1 Beneficial Uses and ARARs 15
2.6.2 Remediation Timeframe 16
2.6.3 Technical Impracticability 17
2.6.4 Point of Compliance 17
2.6.5 Natural Attenuation 18
2.6.6 Alternate Concentration Limits 18
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3.0 PRESUMPTIVE TECHNOLOGIES 19
3.1 Presumptive Technologies for Ex-Situ Treatment 19
3.1.1 Design Styles within Presumptive Technologies 20
3.1.2 Benefits of Presumptive Technologies 20
3.1.3 Consideration of Innovative Technologies 20
3.2 Basis for Presumptive Technologies 21
3.2.1 Sources of Information 21
3.2.2 Rationale for Indentifying Presumptive Technologies 21
3.3 Remedy Selection Using Presumptive Technologies 22
3.3.1 Use of Technologies in Treatment Systems 22
3.3.2 This Guidance Constitutes the FS Screening Step 23
3.3.3 Deferral of Final Technology Selection to RD 23
3.4 Information Needed for Selecting Technologies 24
3.4.1 When Should this Information be Collected? 24
3.4.2 Extraction Flow Rate 25
3.4.3 Discharge Options and ARARs 26
3.4.4 Water Quality of Treatment Influent 26
3.4.5 Treatability Studies 26
3.5 Treatment Technologies for Aquifer Tests 27
3.5.1 Treatment Needs during Aquifer Tests 27
3.5.2 Treatment Technologies for Aquifer Tests 27
4.0. REFERENCES 28
APPENDICES
A. Additional Background Information
A1 Background on DNAPL Contamination A-2
A2 Contaminants Most Frequently Reported in Ground Water at CERCLA NPL Sites .... A-4
A3 Examples of In-Situ Treatment Technologies A-6
A4 Definition and Discussion of Pulsed Pumping A-8
B. ROD Language Examples For Selected Remedy
B1 Phased Implementation of Ground-Water Remedy B-l
B2 Phased Implementation of Extraction Component of Remedy at a DNAPL Site B-3
B3 Deferring Selection of Treatment Components to Remedial Design B-5
B4 Suggested ROD Language from 1990 OSWER Directive B-7
C. Ex-Situ Treatment Technologies for Ground Water
CI Ex-Situ Technologies Considered in Sample of 25 Sites C-l
C2 Other Components Needed for Treatment Trains C-3
C3 Information Needed for Selection of Technologies and Design of Treatment Train C-4
C4 Advantages and Limitations of Presumptive Treatment Technologies C-9
D. Descriptions of Presumptive Treatment Technologies
D1 Air Stripping D-l
D2 Granular Activated Carbon D-3
D3 Chemical/UV Oxidation D-4
D4 Aerobic Biological Reactors D-7
D5 Chemical Precipitation D-9
D6 Ion Exchange/Adsorption D-ll
D7 Electrochemical Methods D-l3
D8 Aeration of Background Metals D-l 5
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FIGURES
Figure Page
1 Examples of Factors Affecting Ground-Water Restoration Potential 4
2 Phased Ground-Water Actions: Early Action Followed by Long-Term Remedy 9
3 Phased Ground-Water Actions: Long-Term Remedy Implemented in Phases 10
Al-1 Components of DNAPL Sites A-2
A1-2 Types of Contamination and Contaminant Zones of DNAPL Sites (Cross-Section) A-2
HIGHLIGHTS
Highlight Page
1 Presumptive Response Strategy 6
2 Early Actions that Should be Considered 7
3 Remedy Refinements for Extraction/Treatment Remedies 12
4 Presumptive Technologies for Treatment of Extracted Ground Water 20
5 Summary of Site Information Needed for Treatment Train Design 25
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ACRONYMS USED IN THIS GUIDANCE
ACL Alternate Concentration Limit
ARAR Applicable or Relevant and
Appropriate Requirement
CERCLA Comprehensive Environmental
Response, Compensation, and
Liability Act of 1980, as amended by
SARA
CERI Center for Environmental Research
Information
CFR Code of Federal Regulations
CSGWPP Comprehensive State Ground Water
Protection Program
DNAPL Dense Nonaqueous Phase Liquids
EPA Environmental Protection Agency
ESD Explanation of Significant
Differences
FS Feasibility Study
GAC Granular Activated Carbon
LNAPL Light Nonaqueous Phase Liquids
MCL Maximum Contaminant Level
MCLG Maximum Contaminant Level Goal
NAPL Nonaqueous Phase Liquid
NCP National Oil and Hazardous
Substances Pollution Contingency
Plan
NPL National Priorities List
OERR Office of Emergency and Remedial
Response
ORD Office of Research and Development
OSWER Office of Solid Waste and Emergency
Response
PCB Polychlorinated Biphenyl
Compounds
POTW Publicly Owned Treatment Works
RARA Resource Conservation and Recovery
Act
RD Remedial Design
RD/RA Remedial Design/Remedial Action
RI Remedial Investigation
RI/FS Remedial Investigation/Feasibility
Study
ROD Record of Decision
SACM Superfund Accelerated Cleanup
Model
SARA Superfund Amendments and
Reauthorization Act of 1986
UV Ultra Violet (light)
VOC Volatile Organic Compound
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PREFACE
Presumptive Remedies Initiative. The objective of the presumptive remedies initiative is to use the
Superfund program's past experience to streamline site investigations and speed up selection of cleanup
actions. Presumptive remedies are expected to increase consistency in remedy selection and implementation,
and reduce the cost and time required to clean up similar types of sites. The presumptive remedies approach
is one tool within the Superfund Accelerated Cleanup Model (SACM) (EPA, 1992d).
Presumptive remedies are preferred technologies for common categories of sites, based on historical
patterns of remedy selection and EPA's scientific and engineering evaluation of performance data on
technology implementation. Refer to EPA Directive, Presumptive Remedies: Policy and Procedures (EPA,
1993d) for general information on the presumptive remedy process and issues common to all presumptive
remedies. This directive should be reviewed before utilizing a presumptive remedy and for further
information on EPA expectations concerning the use of presumptive remedies. "Presumptive remedies
are expected to be used at all appropriate sites," except under unusual site-specific circumstances (EPA,
1993d).
Other Presumptive Remedy Guidance. Previous fact sheets from EPA's Office of Solid Waste and
Emergency Response (OSWER) have established presumptive remedies for municipal landfill sites (EPA,
1993f), for sites with volatile organic compounds in soils (EPA, 1993e) and for wood treater sites (EPA,
1995g). A presumptive response selection strategy for manufactured gas plant sites is under development.
Additional fact sheets are in progress for sites contaminated with polychlorinated biphenyl compounds
(PCBs), metals in soils and for grain storage sites.
Relation of this Guidance to Other Presumptive Remedies. The fact sheets mentioned above provide
presumptive remedies (or a strategy for selecting remedies) for "source control" at specific types of sites.
With respect to ground-water response, source control refers to containment or treatment of materials that
may leach contaminants to ground water, or a combination of these approaches. In general, treatment is
expected for materials comprising the principal threats posed by a site, while containment is preferred for low
level threats (EPA, 1991c). Where contaminants have reached ground water and pose an unacceptable risk to
human health or the environment, a ground-water remedy will generally be required in addition to the source
control remedy and this guidance should be consulted.
Instead of establishing one or more presumptive remedies, this guidance defines a presumptive response
strategy. EPA expects that some elements of this strategy will be appropriate for all sites with contaminated
ground water and all elements of the strategy will be appropriate for many of these sites. In addition, this
guidance identifies presumptive technologies for the ex-situ treatment component of a ground-water
remedy, that are expected to be used for sites where extraction and treatment is part of the remedy. (The term
presumptive technology is used in this guidance to denote only the ex-situ treatment component of a ground-
water remedy.) Other remedy components could include methods for extracting ground water, enhancing
contaminant recovery or degradation of contaminants in the subsurface, discharging treated water, preventing
contaminant migration, and institutional or engineering controls to prevent exposure to contaminants.
Applicability to RCRA Corrective Action Program. EPA continues to seek consistency between cleanup
programs, especially in the process of selecting response actions for sites regulated under the Comprehensive
Environmental Response, Compensation and Liability Act (CERCLA or Superfund program) and corrective
measures for facilities regulated under the Resource Conservation and Recovery Act (RCRA). In general,
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even though the Agency's presumptive remedy guidances were developed for CERCLA sites, they should
also be used at RCRA Corrective Action sites to focus RCRA Facility Investigations, simplify evaluation of
remedial alternatives in the Corrective Measures Study, and influence remedy selection in the Statement of
Basis. For more information refer to the RCRA Corrective Action Plan (EPA, 1994c), the proposed Subpart
S regulations (Federal Register, 1990b), and the May 1, 1996 RCRA Corrective Action Advance Notice of
Proposed Rulemaking (Federal Register, 1996).
Use of this Guidance. The presumptive response strategy, described in Section 2.1, integrates site
characterization, early actions, remedy selection, performance monitoring, remedial design and remedy
implementation activities into a comprehensive, overall response strategy for sites with contaminated ground
water. By integrating these response activities, the presumptive strategy illustrates how the Superfund
Accelerated Cleanup Model (SACM) can be applied to ground-water cleanup. Although this response
strategy will not necessarily streamline the remedial investigation/feasibility study (RI/FS) phase, EPA
expects that use of the presumptive strategy will result in significant time and cost savings for the overall
response to contaminated ground water. By providing a mechanism for selecting achievable remediation
objectives, the presumptive strategy will minimize the need for changing these objectives during remedy
implementation. By optimizing the remedy for actual site conditions during implementation, the effectiveness
of the selected remedy can be greatly increased, which will reduce the time and cost required to achieve
remediation objectives.
The presumptive technologies for treating extracted ground water, identified in Section 3.1, are the
technologies that should generally be retained for further consideration in the Detailed Analysis portion of the
feasibility study (or in the remedial design as explained in Section 3.3.3). This guidance and its associated
Administrative Record will generally constitute the Development and Screening of Alternatives portion of the
feasibility study (FS) for the ex-situ treatment component of a ground-water remedy (see Section 3.3.2). In
this respect, the presumptive technologies will streamline the FS for this component of a ground-water
remedy in the same way that other "presumptive remedies" streamline the FS for the overall remedy for their
respective site types (see EPA, 1993d).
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1.0 INTRODUCTION
In implementing the Superfund and other
remediation programs, cleanup of contaminated
ground water has proven to be more difficult than
anticipated. For many sites, the program
expectation of returning ground waters to their
beneficial uses (see Section 1.2.1) often requires
very long time periods and may not be practicable
for all or portions of the site. Thus, the ultimate
cleanup goal for ground water may need to be
different over different areas of the site (see
Section 1.3.1). For sites where achieving the
ultimate goal will require a long time period,
interim remediation objectives will generally be
appropriate, such as preventing further plume
migration. Therefore, a critical first step in the
remedy selection process is to determine the
full range of remedial objectives that are
appropriate for a particular site.
This guidance is intended to emphasize the
importance of using site-specific remedial
objectives as the focus of the remedy selection
process for contaminated ground water. Those
remedy components that influence attainment of
remedial objectives should receive the greatest
attention. For example if restoring the aquifer to
beneficial use is the ultimate objective, remedy
components that influence attainment of cleanup
levels in the aquifer include: methods for
extracting ground water, enhancing contaminant
recovery, controlling subsurface contaminant
sources (e.g., nonaqueous phase liquids or
NAPLs, discussed in Appendix A1) or in-situ
treatment of contaminants. Some or all of these
remedy components should be included in
remedial alternatives that are developed and
evaluated in detail in the feasibility study (FS)
when aquifer restoration is a remedial
objective.
Although the technologies employed for treating
extracted ground water and the types of discharge
for the treated effluent are important aspects of a
remedy, they have little influence on reducing
contaminant levels or minimizing contaminant
migration in the aquifer. In developing this
guidance, historical patterns of remedy selection
and available technical information were reviewed
in order to identify presumptive technologies for
ex-situ treatment of ground water. By providing
presumptive technologies, this guidance
attempts to streamline selection of these
technologies and shift the time and resources
employed in remedy selection to other, more
fundamental aspects of the ground-water
remedy.
Although extraction and treatment has been and
will continue to be used as part of the remedy for
many sites with contaminated ground water, it
may not be the most appropriate remediation
method for all sites or for all portions of a given
contaminant plume. Also, remedial alternatives
that combine extraction and treatment with other
methods, such as natural attenuation (defined in
Section 2.6.5) or in-situ treatment, may have
several advantages over alternatives that utilize
extraction and treatment alone (see Section 2.4.2).
(Remedial alternatives are evaluated against
remedy selection criteria defined in the National
Contingency Plan at §300.430(e)(9)(iii) (Federal
Register, 1990a).) In general, the remedy
selection process should consider whether
extraction and treatment can achieve remedial
objectives appropriate for the site and how this
approach can be most effectively utilized to
achieve these objectives. This guidance also
describes a presumptive response strategy
which facilitates selection of both short and
long-term remediation objectives during
remedy selection, and allows the effectiveness
of the remedy to be improved during
implementation.
1.1 Purpose of Guidance
In summary, this guidance is intended to:
Describe a presumptive response
strategy, at least some elements of which
are expected to be appropriate for all sites
with contaminated ground water;
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Identify presumptive technologies for
treatment of extracted ground water (ex-
situ treatment) that are expected to be
used (see EPA, 1993d) for sites where
extraction and treatment is part of the
remedy;
Simplify the selection of technologies for
the ex-situ treatment component of a
ground-water remedy, and improve the
technical basis for these selections; and
Shift the time and resources employed
in remedy selection from ex-situ
treatment to other, more fundamental
aspects of the ground-water remedy, as
discussed above.
1.2 Expectations and Objectives for Ground-
Water Cleanup
Careful consideration should be given to national
program expectations as well as site-specific
conditions when determining cleanup objectives
that are appropriate for a given site.
1.2.1 Program Expectations. Expectations for
contaminated ground water are stated in the
National Oil and Hazardous Substances Pollution
Contingency Plan (NCP), as follows:
"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. 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." (Federal Register, 1990a;
§300.430 (a)(l)(iii)(F), emphasis added.)
The Preamble to the NCP explains that the
program expectations are not "binding
requirements." "Rather, the expectations are
intended to share collected experience to guide
those developing cleanup options" (Federal
Register, 1990a; at 8702).
1.2.2 Objectives for Site Response Actions.
The program expectations can be used to define
the following overall objectives for site response
actions, which are generally applicable for all sites
with contaminated ground water:
Prevent exposure to contaminated ground
water, above acceptable risk levels;
Prevent or minimize further migration of
the contaminant plume (plume
containment);
Prevent or minimize further migration of
contaminants from source materials to
ground water (source control); and
Return ground waters to their expected
beneficial uses wherever practicable
(aquifer restoration).
In this guidance the term "response action" is used
to indicate an action initiated under either
CERCLA removal or remedial authority.
"Response objective" is the general description of
what a response action is intended to accomplish.
Source control is included as an objective because
the NCP expectation of aquifer restoration will
not be possible unless further leaching of
contaminants to ground water is controlled, from
both surface and subsurface sources. The
objectives, given above, are listed in the
sequence in which they should generally be
addressed at sites.
Monitoring of ground-water contamination is not
a separate response objective, but is necessary to
verify that one or more of the above objectives has
been attained, or will likely be attained (see
Section 2.1.3). Other response objectives may
also be appropriate for some sites, depending on
the type of action being considered and site
conditions (e.g., maximizing the reuse of extracted
ground water may be an appropriate objective for
some sites). Response objectives may be
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different over different portions of the
contaminant plume, as discussed in Section
1.3.1.
1.3 Lessons Learned
The most important lesson learned during
implementation of Superfund and other
remediation programs is that complex site
conditions are more common than previously
anticipated, including those related to the source
and type of contaminants as well as site
hydrogeology. As a result of these site
complexities, restoring all or portions of the
contaminant plume to drinking water or similar
standards may not be possible at many sites using
currently available technologies.
1.3.1 Sources and Types of Contaminants.
Approximately 85 percent of sites on the
CERCLA National Priorities List (NPL sites)
have some degree of ground-water contamination.
Contaminants have been released to ground water
at a wide variety of site types and can include a
variety of contaminants and contaminant
mixtures. Sources of contaminants to ground
water not only include facilities from which the
original release occurred (e.g., landfills, disposal
wells or lagoons, storage tanks and others) but
also include contaminated soils or other
subsurface zones where contaminants have come
to be located and can continue to leach into ground
water (e.g., NAPLs, see Appendix Al). Thus, the
plume of contaminated ground water may
encompass NAPLs in the subsurface (sources of
contamination) as well as dissolved contaminants.
In this case, different response objectives may be
appropriate for different portions of the plume.
For example, source control (e.g., containment)
may be the most appropriate response objective
for portions of the plume where NAPLs are
present and can not practicably be removed, while
aquifer restoration may be appropriate only for the
remaining portions of the plume (see Section
2.5.3).
Although originating from a variety of sources,
contaminants which reach ground water tend to be
those that are relatively mobile and chemically
stable in the subsurface environment (e.g., less
likely to sorb to soil particles or degrade above the
water table). Organic and inorganic contaminants
most frequently found in ground water at
CERCLA sites are listed in Appendix A2.
Sixteen of the 20 most common organic
contaminants are volatile organic compounds
(VOCs). Of the 16 VOCs, 12 are chlorinated
solvents and four are chemicals found in
petroleum fuels. Petroleum fuels are light
nonaqueous phase liquids (LNAPLs, with a
density lighter than water); while most chlorinated
solvents are dense nonaqueous phase liquids
(DNAPLs) in pure form (see Appendix A1).
1.3.2 Factors Limiting Restoration Potential.
At many sites, restoration of ground water to
cleanup levels defined by applicable or relevant
and appropriate requirements (ARARs) or risk-
based levels may not be possible over all or
portions of the plume using currently available
technologies. Two types of site conditions inhibit
the ability to restore ground water:
Hydrogeologic factors, and
Contaminant-related factors.
Recent studies by EPA and others have concluded
that complex site conditions related to these
factors are more common at hazardous waste sites
than originally expected (EPA, 1989a, 1992b,
1992g, and 1993b; and the National Research
Council, 1994). Examples of hydrogeologic or
contaminant-related factors affecting the difficulty
of restoring ground water are given in Figure 1.
These types of site conditions should be
considered in the site conceptual model, which is
an interpretive summary of the site information
obtained to date (not a computer model). Refer to
EPA, 1993b and 1988a for additional information
concerning the site conceptual model. For every
site, data should be reviewed or new data
should be collected to identify factors that
could increase (or decrease) the difficulty of
restoring ground water.
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Figure 1. Examples of Factors Affecting Ground-Water Restoration Potential
Certain site characteristics may limit the effectiveness of subsurface remediation. The examples listed below are highly
generalized. The particular factor or combination of factors that may critically limit restoration potential will be site specific.
(Figure 1 is taken from EPA, 1993b with minor modifications.)
Site/Contaminant
Characteristics
Generalized Remediation Difficulty Scale
Increasing difficulty
Small Volume
Large Volume
Nature of Release
Short Duration
Long Duration
Slug Release
Continual Release
10"2 cm/sec)
~ Low (< 1 Cf cm/sec)
of Aquifer
Temporal Variation
Little/None
High
of Flow Regime
Vertical Flow
Little
Large Downward Flow
Component
o
"5
CD
X
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1.3.3 Assessing Restoration Potential.
Characterizing all site conditions that could
increase the difficulty of restoring ground water is
often not possible. As a result, the likelihood that
ARAR or risk-based cleanup levels can be
achieved (restoration potential) is somewhat to
highly uncertain for many sites, even after a
relatively complete remedial investigation. This
uncertainty can be reduced by using remedy
performance in combination with site
characterization data to assess the restoration
potential. By implementing a ground-water
remedy in more than one step or phase (as two
separate actions or phasing of a single action as
described in Section 2.2), performance data from
an initial phase can be used to assess the
restoration potential and may indicate that
additional site characterization is needed. In
addition to providing valuable data, the initial
remedy phase can be used to attain short-term
response objectives, such as preventing further
plume migration. Phased implementation of
response actions also allows realistic long-term
remedial objectives to be determined prior to
installation of the comprehensive or "final"
remedy.
A detailed discussion of factors to consider for
assessing restoration potential is provided in
Guidance for Evaluating the Technical
Impracticability of Ground-Water Restoration
(EPA, 1993b; Section 4.4.4). An especially
important tool for this evaluation is the site
conceptual model, which should integrate data
from site history, characterization and response
actions. This assessment could provide
justification for waiving ARARs due to technical
impracticability from an engineering perspective
over all or portions of a site (EPA, 1993b). It is
recommended that technical assistance be enlisted
from regional technical support staff or the
Technical Support Project (EPA, 1994d) when
evaluating technical impracticability.
Data from remedy performance are not always
necessary to justify an ARAR waiver due to
technical impracticability (see Section 2.6.3).
At the completion of the remedial investigation
(RI), site conditions may have been characterized
to the extent needed for EPA (or the lead agency)
to determine that ground-water restoration is
technically impracticable from an engineering
perspective (EPA, 1993b; EPA 1995b). For this
case, an ARAR waiver request can be submitted
to EPA (or the lead agency), and if approved,
included in the Record of Decision (ROD). It will
often be appropriate to include an ARAR waiver
in the ROD for portions of a site where DNAPLs
have been confirmed in the aquifer (see Section
2.5.3).
2.0 PRESUMPTIVE RESPONSE
STRATEGY
2.1 Definition and Basis for Strategy
Key elements of the presumptive strategy are
summarized in Highlight 1. In the presumptive
response strategy, site characterization and
response actions are implemented in a several
steps, or in a phased approach. In a phased
response approach, site response activities are
implemented in a sequence of steps, or phases,
such that information gained from earlier phases is
used to refine subsequent investigations,
objectives or actions (EPA, 1989a, 1992b,
1993b).
In general for sites with contaminated ground
water, site characterization should be
coordinated with response actions and both
should be implemented in a step-by-step or
phased approach.
Performance data from an initial response action
are also used to assess the likelihood that ARAR
or risk-based cleanup levels can be attained by
later, more comprehensive actions. Although it is
recognized that phased implementation may not
be appropriate for all ground-water remedies, EPA
expects that some elements of this strategy will be
appropriate for all sites with contaminated ground
water and that all elements will be appropriate for
many of these sites. For this reason, the
response approach given in
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Highlight 1. Presumptive Response
Strategy
For sites with contaminated ground
water, site characterization
should be coordinated with
response actions and both should
be implemented in a phased
approach (Sections 1.3.3 and 2.1).
Early or interim actionsshould be
used to reduce site risks (by
preventing exposure to and further
migration of contaminants) and to
provide additional site data (Section
2.1.2).
Site characterization and
performance data from early or
interim ground-water actions should
be used to assess the likelihood
of restoring ground waterto
ARAR or risk-based cleanup levels
(restoration potential). (Sections
1.3.3 and 2.1.2.)
The restoration potential should be
assessed prior to establishing
objectives for the long-term
remedy (Sections 1.3.3 and 2.1.2).
All ground-water actions should
include provisions for monitoring
and evaluating their performance
(Section 2.1.3).
Ground-water response actions,
especially those using extraction
and treatment, should generally be
implemented in more than one
phase - either as two separate
actions or phasing of a single action
(Sections 2.2.1 and 2.2.2).
In addition to phasing, post-
construction refinementswill
generally be needed for long-term
remedies, especially those using
extraction and treatment (Section
2.3.1).
Highlight 1 is a presumptive strategy for
contaminated ground water.
Also, this response strategy is considered
presumptive because the basic elements were
included in all previous policy directives
concerning ground-water remediation from EPA's
Office of Solid Waste and Emergency
recommended use of a phased approach for site
characterization and response actions, and more
frequent use of early actions to reduce site risks.
Better integration of site activities and more
frequent use of early actions are also essential
components of the Superfund Accelerated
Cleanup Model (SACM), defined in EPA, 1992d.
2.1.1 Benefits of Phased Approach.
Implementing investigations and actions in phases
provides the following major benefits:
Data from earlier response actions are
used to further characterize the site and
assess restoration potential;
Attainable objectives can be set for each
response phase;
Flexibility is provided to adjust the
remedy in response to unexpected site
conditions;
Remedy performance is increased,
decreasing remediation timeframe and
cost; and
Likely remedy refinements are built into
the selected remedy, better defining the
potential scope and minimizing the need
for additional decision documents.
2.1.2 Early Actions. "Early" refers to the timing
of the start of an action with respect to other
response actions at a given site. For Superfund
sites, early actions could include removal actions,
interim remedial actions, or early final remedial
actions (EPA, 1992b and EPA, 1991b). Although
initiated prior to other actions, some early ground-
water actions may need to operate over a long time
6
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period (e.g., hydraulic containment actions). In
this guidance the later, more comprehensive
ground-water action is called the "long-term
remedy, " consistent with SACM terminology
(EPA, 1992e). Early actions that should be
considered in response to contaminated ground
water are listed in Highlight 2, categorized by
response objective. Early or interim actions
should be used to reduce site risks (by
preventing exposure to contaminated ground
water and further migration of contaminants)
and to provide additional site data.
Factors for determining which response
components are suitable for early or interim
actions include: the timeframe needed to attain
specific objectives, the relative urgency posed by
potential or actual exposure to contaminated
ground water (e.g., likelihood that contaminants
will reach drinking water wells), the degree to
which an action will reduce site risks, usefulness
of information to be gained from the action, site
data needed to design the action, and compatibility
with likely long-term actions (EPA, 1992e).
Whether to implement early response actions and
whether to use removal or remedial authority for
such actions should be determined by the
"Regional Decision Team" defined under SACM
(EPA, 1992f) or similar decision-making body for
the site.
Early or interim actions should be integrated as
much as possible with site characterization and
with subsequent actions in a phased approach.
Once implemented, early actions will often
provide additional site characterization
information, which should be used to update the
site conceptual model. Also, treatability studies
(see Section 3.4.5) needed for selection or design
of the long-term remedy should be combined with
early actions whenever practical. Site
characterization and performance data from early
or interim ground-water actions should be used to
assess the likelihood of restoring ground water to
ARAR or risk-based cleanup levels (restoration
potential). The restoration
;
Highlight 2. Early Actions That Should
Be Considered
Prevent exposure to contaminated ground
water:
Plume containment
Alternate water supply
Well head treatment
Use restrictions
Prevent further migration of contaminant
plume:
Plume containment
Contain (and/or treat) plume "hot
spots"
Prevent further migration of contaminants
from sources:
Source removal and/or treatment
Excavate wastes or soils
and remove from site
Excavate soils and treat ex-
situ
Treat soils in-situ
Extract free-phase NAPLs
(see Appendix A1)
Source containment
Contain wastes or soils
Contain subsurface NAPLs
Provide additional site data:
Assess restoration potential
Combine actions with treatability
studies
7
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potential should be assessed prior to
establishing objectives for the long-term
remedy (see Section 13 3) 2.1.3 Monitoring.
Monitoring is needed to evaluate whether the
ground-water action is achieving, or will achieve,
the intended response objectives for the site (see
Section 1.3.1) and other performance objectives
for the action (e.g., discharge requirements). All
ground-water actions should include
provisions for monitoring and evaluating their
performance. A monitoring plan should be
developed for both early and long-term actions. In
general, the monitoring plan should include:
Response objectives and performance
requirements for the ground-water action;
Specific monitoring data to be collected;
Data quality objectives;
Methods for collecting, evaluating and
reporting the performance monitoring
data; and
Criteria for demonstrating that response
objectives and performance requirements
have been attained.
Flexibility for adjusting certain aspects of
monitoring during the life of the remedy should be
included in the monitoring plan, such as changes
in the monitoring frequency as the remedy
progresses or other changes in response to remedy
refinements (see Section 2.3.1). A detailed
discussion of the data quality objectives process is
provided in EPA, 1993j. Methods for monitoring
the performance of extraction and treatment
actions are discussed in EPA, 1994e.
2.2 Phased Response Actions
In general, ground-water response actions,
especially those using extraction and
treatment, should be implemented in more
than one phase. There are two options for
phasing response actions - implementation of two
separate actions, or implementation of a single
action in more than one phase. It is recognized
that phased implementation may not be
appropriate for all ground-water remedies. In
some cases, it may be more appropriate to install
the entire remedy and then remove from service
those components that later prove to be unneeded.
2.2.1 Two Separate Actions. In this approach an
early or interim ground-water action is followed
by a later, more comprehensive action (the long-
term remedy). A flow chart of this approach is
given in Figure 2. Earlier ground-water actions
are used to mitigate more immediate threats, such
as preventing further plume migration. Response
objectives for the long-term remedy are not
established until after performance of the earlier
action is evaluated and used to assess the
likelihood that ground-water restoration (or other
appropriate objectives) can be attained. Two
separate decision documents are used, in which
response objectives are specified that are
appropriate for each action. The earlier decision
document could be an Action Memorandum or an
Interim Record of Decision (Interim ROD), since
the early action could be initiated under either
CERCLA removal or remedial authority. This
approach should be used when site
characterization data are not sufficient to
determine the likelihood of attaining long-term
objectives (e.g., restoring ground water) over
all or portions of the plume, which will be the
case for many sites. In order to provide
sufficient data for assessing the restoration
potential, the early or interim action may need to
operate for several years.
2.2.2 Phasing of a Single Action. In this
approach the long-term remedy for ground water
is implemented in more than one design and
construction phase. A flow chart of this approach
is given in Figure 3. Response objectives for the
long-term remedy are specified in a single Record
of Decision (ROD) prior to implementing the
remedy. Provisions for assessing the attainability
of these objectives using performance data from
an initial remedy phase are also included in the
ROD. Thus, phased remedy implementation and
assessment of remedy performance are specified
8
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in one ROD. A second decision document could
still be required if evaluation of the first phase
9
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Figure 2. Phased Ground-Water Actions: Early Action Followed by Long-Term Remedy
[ This approach should be used when site characterization data are not sufficient to determine the likelihood of attaining long-term
I objectives (e.g., restoring ground-water) over all or portions of the plume.
Decision Remedy
Documents Phase Remedy Selection/ Implementation Steps
Continue Site Characterization
Interim
ROD or
Action
Memo
Early or
Interim
Action
No
Yes
ROD
Memo to
Admin. Record
orESD
Long-Term
Remedy
Remedy
Refinement
.Yes
Are Refinements Needed?
No
Monitor Remedy Until
Objectives Attained
Are
Data Sufficient to ^
Determine Liklihood of
Attaining Long-Term Objectives
(e.g., Ground-Water
Restoration)?
Select & Implement
Refinements
Determine Early Action Objectives
Monitor Action & Evaluate Performance
Complete Remedial Investigation
Monitor Remedy & Evaluate
Performance
Determine Long-Term Objectives for
Different Portions of Plume
Evaluate Alternatives,
Select Remedy & Likely Refinements,
Design & Construct Remedy
Continue Site Characterization as
Required
Evaluate Alternatives,
Select Action,
Design & Construct Action
10
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Figure 3. Phased Ground-Water Actions: Long-Term Remedy Implemented in Phases
This approach should be used when site characterization dataare sufficient to determine that the likelihood
of attaining long-term objectives is relatively high.
Decision
Documents
Remedy
Phase
Remedy Selection/ Implementation Steps
^ Complete
Investigation
Remedial
ROD
Phase I
Are
Data Sufficient to
Determine Likelihood of
Attaining Long-Term Objectives
(e.g., Ground-Water
Restoration)?
No
Yes
Are Long-Term
Objectives Attainable?
No
ROD
Amendment
or ESD
Yes
Memo to
Admin. Record
or ESD
Phase
Are Refinements Needed?
Yes
Remedy
Refinement
No
Monitor Remedy Until
Objectives Attained
Implement Changes
Design & Construct Phase I
Evaluate Alternatives
Select Remedy
Select & Implement
Refinements
Modify Long-Term
Objectives
Design & Construct Phase
Monitor Remedy & Evaluate
Performance
Monitor Phase I & Evaluate
Performance
Evaluate Alternatives
Select Remedy & Likely Refinements
Determine Phases I & II
Determine Long-Term Objectives for
Different Portions of Plume
11
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indicates that long-term objectives or other
aspects of the remedy require modification, and
the modified remedy differs significantly from the
selected remedy in terms of scope, performance or
cost (EPA, 1991a). This approach should be
used when site characterization data indicate
that the likelihood of attaining long-term
objectives is relatively high.
When phased remedy implementation is specified
in a ROD, the Agency should ensure that the
proposed plan contains sufficient information
regarding the nature, scope timing and basis of
future decision points and alternatives that the
public is able to evaluate and comment on the
proposed remedy. Example language illustrating
how such an approach can be specified in the
selected remedy portion of the ROD is included in
Appendices B1 and B2 for hypothetical sites.
These examples follow the suggested ROD
language given in EPA, 1990b, although the
wording has been updated to reflect this and other
recent guidance (EPA, 1993b). For comparison,
suggested ROD language from the EPA, 1990b is
included as Appendix B4.
Phased implementation of a remedy can often be
beneficial even for relatively simple ground-water
actions. For example, one extraction well could
be installed as the initial phase and the
performance of this well would be used to
determine whether any additional wells are needed
and whether long-term objectives need to be re-
evaluated.
Phased implementation of an extraction and
treatment remedy will require that the treatment
system be designed to accommodate phased
installation of the extraction system. Presumptive
technologies for the treatment system and other
design considerations are discussed in Section 3.
Use of modular treatment components, which can
be easily added or removed from the treatment
system, may facilitate phased implementation or
other changes in flow or contaminant
concentration that may occur during the life of a
remedy. Another approach is to design the
treatment system for the higher flows expected
from all phases of the extraction system. Some
components of the remedy, such as buried
portions of the piping distribution system, are
difficult to install in phases and should be
designed to carry the highest expected flows.
2.3 Post-Construction Refinements
Even after phased implementation of a ground-
water remedy, post-construction refinements will
generally be needed because of the long time
period over which the remedy will operate,
especially for extraction and treatment remedies.
The refinement portion of the long-term remedy,
after phased design and construction, is shown in
both Figures 2 and 3.
2.3.1 Types of Refinements. Post-construction
refinements that should be considered for
extraction and treatment remedies are given in
Highlight 3. These refinements are intended to be
relatively minor changes to the remedy (i.e., for
which an Explanation of Significant Differences
(ESD) or ROD Amendment would generally not
be required). For example, adding a new
extraction or reinjection well, or a few additional
monitoring wells should be considered a minor
modification to a remedy that includes a relatively
large number of such wells, because the overall
scope, performance and cost of the remedy are not
significantly changed (EPA, 1991a). One or more
such refinements should generally be implemented
when the results of a remedy evaluation indicate
that they are needed to increase the performance
of the remedy or to decrease the remediation
timeframe.
2.3.2 Documenting Refinements. Potential post-
construction refinements should be included in the
ROD as part of the selected remedy. Listing
specific remedy refinements in the ROD serves to
communicate the anticipated full scope of the
remedy to all concerned parties at an early date,
and also minimizes the likelihood that a
subsequent ESD or ROD Amendment will be
needed. When remedy refinements are specified
in a ROD, the Agency should ensure that the
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Highlight 3. Remedy Refinements for
Extraction/Treatment Remedies
Change the extraction rate in some
or all wells.
Cease extraction from some wells.
Initiate "pulsed pumping" (see
Appendix A4).
Add or remove extraction or
reinjection wells, or drains.
Add or remove monitoring wells.
Refine source control components
of remedy.
Refine enhanced recovery or in-situ
degradation components of remedy
(see Note).
Refine ex-situ treatment
components
NOTE: A ground-water remedy could
include both extraction and treatment and in-
situ treatment methods.
proposed plan contains sufficient information
regarding the nature, scope timing and basis of
future decision points and alternatives that the
public is able to evaluate and comment on the
proposed remedy. Example ROD language
specifying likely post-construction refinements for
the extraction portion of the selected remedy is
given in Appendices B1 and B2. Even if an ESD
is not required, a letter or memorandum should be
included in the post-ROD portion of the
Administrative Record explaining the minor
remedy modifications and the reasons for them.
Additional information concerning documentation
of remedy modifications can be found in the EPA
fact sheet entitled Guide to Addressing Pre-ROD
and Post-ROD Changes (EPA, 1991a).
2.4 Integrating Response Actions
In general, actions in response to contaminated
ground water should be planned and implemented
as part of an overall strategy. Earlier actions (see
Highlight 2 for examples) should be compatible
with and not preclude implementation of later
actions. For example, permanent facilities should
not be constructed which could interfere with
possible later actions (e.g., structures that would
interfere with later construction of extraction wells
or of a cap).
2.4.1 Integrating Source Control and Ground-
Water Actions. Restoration of contaminated
ground water generally will not be possible unless
contaminant sources have been controlled in some
manner. Source control is a critical component for
active restoration remedies (e.g., extraction and
treatment and in-situ methods) as well as for
natural attenuation (defined in Section 2.6.5).
Selection of appropriate source control actions
should consider whether other contaminant
sources (i.e., NAPLs) are likely to be present in
addition to contaminated soils. If NAPLs are
present, the vast majority of contaminant mass
will likely reside in the subsurface NAPLs rather
than in the surficial soils. Therefore, for this case
source control actions that are intended to
minimize further contamination of ground water
should focus on controlling migration of
contaminants from the subsurface NAPLs. Also,
capping or treatment of surficial soils may be
needed to prevent exposure to contaminants from
direct soil contact or inhalation, but these actions
alone would be ineffective in preventing further
contamination of ground water at sites where
NAPLs are present.
2.4.2 Combining Ground-Water Restoration
Methods. A remedy could include more than one
method for restoring ground water to its beneficial
uses, such as combining extraction and treatment
with natural attenuation or in-situ-treatment with
extraction and treatment. Extraction and
treatment is especially useful for providing
hydraulic containment of those portions of the
13
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plume where contaminant sources are present
(e.g., subsurface NAPLs or contaminated soils), or
for containing or restoring those plume areas with
relatively high concentrations of dissolved
contamination ("hot spots"). However, extraction
and treatment may not be the best method for
restoring large areas of the plume with low
contaminant levels.
Once source areas are controlled, natural
attenuation may be able to restore large
portions of the plume to desired cleanup levels
in a timeframe that is reasonable (see Section
2.6.2) when compared with the timeframe and
cost of other restoration methods. Thus,
natural attenuation of some plume areas combined
with extraction and treatment to contain source
areas and/or plume "hot spots" may be the most
appropriate restoration approach for many sites
with relatively large, dilute plumes. Whether or
not natural attenuation is used alone or combined
with other remediation methods, the Agency
should have sufficient information to demonstrate
that natural processes are capable of achieving the
remediation objectives for the site. EPA is
currently preparing a directive that will provide
more detailed discussion of EPA policy regarding
the use of natural attenuation for remediation of
contaminated ground water (EPA, 1996c).
By combining in-situ treatment and extraction and
treatment methods it may be possible to
significantly increase the effectiveness with which
contaminants are removed from the aquifer. In
this guidance, in-situ treatment methods for
ground water are divided into two types:
Methods that can be used to enhance
contaminant recovery during extraction
and treatment (e.g., water, steam or
chemical flooding; hydraulic or pneumatic
fracturing); and
Methods for in-situ degradation of
contaminants generally involve adding
agents to the subsurface (i.e., via wells or
treatment walls) which facilitate chemical
or biological destruction, and have the
potential to be used as an alternative to
extraction and treatment for long-term
restoration of ground water.
Examples of both types of in-situ treatment
methods are given in Appendix A3. Reinjection
of treated ground water can be used as a method
for enhancing contaminant recovery as well as a
discharge method, if the reinjection is designed for
this purpose as part of an extraction and treatment
remedy. When considering enhanced recovery
methods for sites with subsurface NAPLs,
potential risks of increasing the mobility of
NAPLs should be evaluated. Methods of in-situ
degradation of contaminants most frequently used
at Superfund sites include air sparging, various
types of in-situ biological treatment and
permeable treatment walls or gates (EPA, 1995e).
Additional information concerning air sparging
and permeable treatment walls is available in
EPA, 1995fandEPA, 1995d, respectively. EPA
encourages the consideration, testing and use of
in-situ technologies for ground-water remediation
when appropriate for the site.
2.5 Strategy for DNAPL Sites
Dense nonaqueous phase liquids (DNAPLs) pose
special cleanup difficulties because they can sink
to great depths in the subsurface, continue to
release dissolved contaminants to the surrounding
ground water for very long time periods, and can
be difficult to locate. Due to the complex nature
of DNAPL contamination, a phased approach to
characterization and response actions is especially
important for sites where DNAPLs are confirmed
or suspected. A recent EPA study concluded that
subsurface DNAPLs may be present at up to 60
percent of CERCLA National Priorities List sites
(EPA, 1993c). Refer to Appendix A1 for
additional background information on DNAPLs.
Two types of subsurface contamination can be
defined at DNAPL sites, the:
DNAPL zone, and the
Aqueous contaminant plume.
14
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The DNAPL zone is that portion of the
subsurface where immiscible liquids (free-phase
or residual DNAPL) are present either above or
below the water table. Also in the DNAPL zone,
vapor phase DNAPL contaminants are present
above the water table and dissolved phase below
the water table. The aqueous contaminant
plume is that portion of the contaminated ground
water surrounding the DNAPL zone where
aqueous contaminants derived from DNAPLs are
dissolved in ground water (or sorbed to aquifer
solids) and immiscible liquids are not present.
2.5.1 Site Characterization. If DNAPLs are
confirmed or suspected, the remedial investigation
(RI) should be designed to delineate the:
Extent of aqueous contaminant plumes,
and the
Potential extent of DNAPL zones.
Methods and strategies for characterizing DNAPL
sites as well as suggested precautions are
discussed in other guidance (EPA, 1992a and
1994b) and by Cohen and Mercer, 1993. The
reason for delineating these areas of the site is that
response objectives and actions should generally
be different for the DNAPL zone than for the
aqueous contaminant plume. It is recognized that
for some sites complete delineation of the
DNAPL-zone may not be possible.
2.5.2 Early Actions. The early actions listed in
Highlight 2 should be considered. Also, the
following early actions are specifically
recommended for DNAPL sites (EPA 1992b,
1993b):
Prevent further spread of the aqueous
plume (plume containment);
Prevent further spread of hot spots in the
aqueous plume (hot spot containment);
Control further migration of contaminants
from subsurface DNAPLs to the
surrounding ground water (source
control); and
Reduce the quantity of source material
(free-phase DNAPL) present in the
DNAPL zone, to the extent practicable
(source removal and/or treatment).
At DNAPL sites, hot spots in the aqueous plume
often are associated with subsurface DNAPLs.
Therefore, the second and third actions listed
above are essentially the same.
2.5.3 Long-Term Remedy. The long-term
remedy should attain those objectives listed above
for the DNAPL zone, by continuing early actions
or by initiating additional actions. Although
contaminated ground waters generally are not
considered principal threat wastes, DNAPLs
may be viewed as a principal threat because they
are sources of toxic contaminants to ground water
(EPA, 1991c). For this reason EPA expects to
remove or treat DNAPLs to the extent practicable
in accordance with the NCP expectation to "use
treatment to address the principal threats posed by
a site, wherever practicable" (Federal Register,
1990a; §300.430 (a)(l)(iii)(A)). However,
program experience has shown that removal of
DNAPLs from the subsurface is often not
practicable, and no treatment technologies are
currently available which can attain ARAR or
risk-based cleanup levels where subsurface
DNAPLs are present. Therefore, EPA generally
expects that the long-term remedy will control
further migration of contaminants from
subsurface DNAPLs to the surrounding
ground water and reduce the quantity of
DNAPL to the extent practicable.
For the aqueous plume, the long-term remedy
should:
Prevent further spread of the aqueous
plume (plume containment);
15
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Restore the maximum areal extent of the
aquifer to those cleanup levels
appropriate for its beneficial uses (aquifer
restoration).
In general, restoration of the aquifer to ARAR
or risk-based cleanup levels in a reasonable
timeframe will not be attainable in the DNAPL
zone unless the DNAPLs are removed. For this
reason, it is expected that ARAR waivers due to
technical impracticability will be appropriate for
many DNAPL sites, over portions of sites where
non-recoverable DNAPLs are present (EPA,
1995c). Also, EPA generally prefers to utilize
ARAR waivers rather than ARAR compliance
boundaries for such portions of DNAPL sites (see
Section 2.6.4). A waiver determination can be
made after construction and operation of the
remedy or at the time of remedy selection (i.e., in
the ROD), whenever a sufficient technical
justification can be demonstrated (EPA, 1993b;
EPA 1995b). For further information refer to
Section 2.6.3 of this guidance and EPA's
Guidance for Evaluating the Technical
Impracticability of Ground-Water Restoration
(EPA, 1993b). Restoration of the aqueous plume
may also be difficult due to hydrogeologic factors,
such as sorption of dissolved contaminants to
solids in finer grained strata. For some sites,
ARAR waivers may also be appropriate for all or
portions of the aqueous plume when supported by
adequate justification.
2.6 Areas of Flexibility in Cleanup Approach
The current response approach to contaminated
ground water, as defined in the NCP and other
guidance, includes several areas of flexibility in
which response objectives and the timeframe in
which to meet them can be adjusted to meet site
specific conditions. These are briefly discussed
below.
2.6.1 Beneficial Uses and ARARs. Since EPA
generally expects to return contaminated ground
waters to their beneficial uses wherever
practicable, the required cleanup levels for a given
site should be determined from applicable or
relevant and appropriate requirements (ARARs)
based on the current and expected future
beneficial uses of the ground water at that site.
Depending on state requirements and water
quantity or quality characteristics, some ground
waters are not expected to provide a future source
of drinking water (e.g., EPA Class III ground
waters (EPA, 1986) or similar state designations).
In general, drinking water standards are relevant
and appropriate cleanup levels for ground waters
that are a current or future source of drinking
water, but are not relevant and appropriate for
ground waters that are not expected to be a future
source of drinking water (Federal Register, 1990a;
Preamble at 8732). (Drinking water standards
include federal maximum contaminant levels
(MCLs) and/or non-zero maximum contaminant
level goals (MCLGs) established under the Safe
Drinking Water Act, or more stringent state
drinking water standards.) Ground waters may
have other beneficial uses, such as providing base
flow to surface waters or recharging other
aquifers. For contaminated ground waters that
discharge to surface water, water quality criteria
established under the Clean Water Act, or more
stringent state surface water requirements, may
also be cleanup level ARARs (Federal Register,
1990a; Preamble at 8754). Thus, the beneficial
uses of contaminated ground water at a particular
site will generally provide the basis for
determining which federal or state environmental
requirements are applicable or relevant and
appropriate cleanup levels. For additional
information on the determination of cleanup
levels, refer to EPA, 1988b, Chapter 4.
Determination of current and expected future
beneficial uses should consider state ground-water
classifications or similar designations. Several
states have developed ground-water use or priority
designations as part of a Comprehensive State
Ground Water Protection Program (CSGWPP),
defined in EPA, 1992h. EPA is currently
developing a directive (EPA, 1996a) which will
recommend that EPA remediation programs
should generally defer to state determinations of
future ground-water use ~ even when this
determination differs from the use that would
16
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otherwise have been determined by EPA ~ when
such determinations are:
Developed as part of an CSGWPP that is
endorsed by EPA, and
Based on CSGWPP provisions that can
be applied at specific sites (EPA, 1996a).
This provision of the directive, when final, is
intended to supersede previous guidance contained
in the Preamble to the NCP (Federal Register,
1990a; at 8733). Refer to EPA, 1996a for
additional information concerning the role of
CSGWPPs in the selection of ground-water
remedies. When information concerning
beneficial uses is not available from a CSGWPP,
ground-water classifications defined in EPA, 1986
(i.e., EPA Classes I, II or III) or "more stringent"
state ground-water classifications (or similar state
designations) should generally be used to
determine the potential future use, in accordance
with the NCP Preamble (Federal Register, 1990a;
at 8732-8733). Regardless of the ground-water
use determination, remedies selected under
CERCLA authority must protect human
health and the environment and meet ARARs
(or invoke an ARAR waiver).
Many states have antidegradation or similar
regulations or requirements that may be potential
ARARs. Such requirements typically focus on 1)
prohibiting certain discharges, 2) maintaining
ground-water quality consistent with its beneficial
uses, or 3) maintaining naturally occurring
(background) ground-water quality. Regulations
of the third type do not involve determination of
future ground-water use, and often result in
cleanup levels that are more stringent than the
drinking water standard for a particular chemical.
Such requirements are potential ARARs if they
are directive in nature and intent and established
through a promulgated statute or regulation that is
legally enforceable (see Federal Register, 1990a;
Preamble at 8746). For further information
concerning issues related to state ground-water
antidegradation requirements, refer to EPA,
1990a.
2.6.2 Remediation Timeframe. "Remediation
timeframes will be developed based on the
specific site conditions" (Federal Register, 1990a;
Preamble at 8732). Even though restoration to
beneficial uses generally is the ultimate objective,
a relatively long time period to attain this
objective may be appropriate for some sites. For
example, an extended remediation timeframe
generally is appropriate where contaminated
ground waters are not expected to be used in the
near term, and where alternative sources are
available. In contrast, a more aggressive remedy
with a correspondingly shorter remediation
timeframe should generally be used for
contaminated ground waters that are currently
used as sources of drinking water or are expected
to be utilized for this purpose in the near future
(Federal Register, 1990a; at 8732). A state's
CSGWPP may include information helpful in
determining whether an extended remediation
timeframe is appropriate for a given site, such as
the expected timeframe of use, or the relative
priority or value of ground-water resources in
different geographic areas.
A reasonable timeframe for restoring ground
waters to beneficial uses depends on the particular
circumstances of the site and the restoration
method employed. The most appropriate
timeframe must be determined through an analysis
of alternatives (Federal Register, 1990a; Preamble
at 8732). The NCP also specifies that:
"For ground-water response actions, the
lead agency shall develop a limited
number of remedial alternatives that
attain site-specific remediation levels
within different restoration time periods
utilizing one or more different
technologies." (Federal Register, 1990a;
§300.430(e)(4).)
Thus, a comparison of restoration alternatives
from most aggressive to passive (i.e., natural
attenuation) will provide information concerning
the approximate range of time periods needed to
attain ground-water cleanup levels. An
excessively long restoration timeframe, even with
17
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the most aggressive restoration methods, may
indicate that ground-water restoration is
technically impracticable from an engineering
perspective (see Section 2.6.3). Where restoration
is feasible using both aggressive and passive
methods, the longer restoration timeframe
required by a passive alternative may be
reasonable in comparison with the timeframe
needed for more aggressive restoration
alternatives. The most appropriate remedial
option should be determined based on the nine
remedy selection factors defined in the NCP
(Federal Register, 1990a; §300.430 (e)(9)(iii)).
Although restoration timeframe is an important
consideration in evaluating whether restoration of
ground water is technically impracticable, no
single time period can be specified which would
be considered excessively long for all site
conditions (EPA, 1993b). For example, a
restoration timeframe of 100 years may be
reasonable for some sites and excessively long for
others.
2.6.3 Technical Impracticability. Where
restoration of ground water to its beneficial uses is
not practicable from an engineering perspective,
one or more ARARs may be waived by EPA (or
the lead agency) under the provisions defined in
CERCLA § 121(d)(4)(C)). The types of data used
to make such a determination are discussed in
Guidance for Evaluating the Technical
Impracticability of Ground-Water Restoration
(EPA, 1993b). Alternative remedial strategies, to
be considered when restoration ARARs are
waived, are also discussed in EPA, 1993b. A
finding of technical impracticability may be made
in the Record of Decision (ROD) prior to remedy
implementation, or in a subsequent decision
document after implementation and monitoring of
remedy performance.
2.6.4 Point of Compliance. The area over which
ARAR or risk-based cleanup levels are to be
attained is defined in the NCP as follows:
"For ground water, remediation levels
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" (Federal
Register, 1990a; Preamble at 8713).
Thus, the edge of the waste management area can
be considered as the point of compliance, because
ARAR or risk-based cleanup levels are not
expected to be attained in ground water within the
waste management area. In general, the term
"waste left in place" is used in the NCP to refer to
landfill wastes that, at the completion of the
remedy, will be contained or otherwise controlled
within a waste management area.
For the purposes of ARAR compliance, EPA
generally does not consider DNAPLs as "waste
left in place." DNAPLs are typically not located
in a waste management area, as envisioned in the
NCP. This is because the full extent of DNAPL
contamination is often not known, DNAPLs can
continue to migrate in the subsurface, and
measures for controlling their migration are either
unavailable or have uncertain long-term reliability.
Also, as discussed in Section 2.5.3, restoration of
the aquifer to ARAR or risk-based cleanup levels
generally will not be attainable in a reasonable
timeframe unless the DNAPLs are removed. For
these reasons, EPA generally prefers to utilize
ARAR waivers rather than an alternate point
of compliance over portions of sites where non-
recoverable DNAPLs are present in the
subsurface (EPA, 1995c).
The NCP Preamble also acknowledges that "an
alternative point of compliance may also be
protective of public health and the environment
under site-specific circumstances" (Federal
Register, 1990a; at 8753). For example, where
the contamination plume is "caused by releases
from several distinct sources that are in close
geographical proximity...the most feasible and
effective cleanup strategy may be to address the
problem as a whole, rather than source by source,
and to draw the point of compliance to encompass
the sources of release" (Federal Register, 1990a;
at 8753). The NCP Preamble goes on to say that
"...where there would be little likelihood of
exposure due to the remoteness of the site,
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alternate points of compliance may be considered,
provided contamination in the aquifer is controlled
from further migration" (Federal Register, 1990a;
at 8734). The Agency has not developed
additional guidance on the use of alternate points
of compliance at Superfund sites.
2.6.5 Natural Attenuation. Natural attenuation
is defined in the NCP as "biodegradation,
dispersion, dilution, and adsorption" of
contaminants in ground water (Federal Register,
1990a; Preamble at 8734). The NCP goes on to
explain that natural attenuation may be a useful
remedial approach if site-specific data indicate
that these processes "will effectively reduce
contaminants in the ground water to
concentrations protective of human health [and the
environment] in a timeframe comparable to that
which could be achieved through active
restoration." This approach differs from the "no
action" alternative because natural attenuation is
expected to attain cleanup levels in a reasonable
timeframe (discussed in Section 2.6.2). The NCP
recommends use of natural attenuation where it is
"expected to reduce the concentration of
contaminants in the ground water to the
remediation goals [ARAR or risk-based cleanup
levels] in a reasonable timeframe."
Natural attenuation may be an appropriate
remedial approach for portions of the contaminant
plume when combined with other remedial
measures needed to control sources and/or
remediate "hot spots" (also see Section 2.4.2).
Whether or not natural attenuation is used alone or
combined with other remediation methods, the
Agency should have sufficient information to
demonstrate that natural processes are capable of
achieving the remediation objectives for the site.
One caution is that natural attenuation may not be
appropriate for sites where contaminants
biodegrade to intermediate compounds that are
more toxic and degrade more slowly.
Additional EPA policy considerations regarding
the use of natural attenuation for remediation of
contaminated ground water are provided in EPA,
1996c. Although currently in draft, this EPA
directive recommends that remedies utilizing
natural attenuation should generally include: 1)
detailed site characterization to show that this
approach will be effective; 2) source control
measures to prevent further release of
contaminants to ground water; 3) performance
monitoring to assure that natural attenuation is
occurring as expected; and 4) institutional
controls and other methods to ensure that
contaminated ground waters are not used before
protective concentrations are reached. Also,
contingency measures may be needed in the
event that natural attenuation does not progress as
expected.
2.6.6 Alternate Concentration Limits.
Alternate concentration limits (ACLs) are
intended to provide flexibility in establishing
ground-water cleanup levels under certain
circumstances. In the Superfund program, EPA
may establish ACLs as cleanup levels in lieu of
drinking water standards (e.g., MCLs) in certain
cases where contaminated ground water
discharges to surface water. The circumstances
under which ACLs may be established at
Superfund sites are specified in CERCLA
§121 (d)(2)(B)(ii), and can be summarized as
follows:
The contaminated ground water must
have "known or projected" points of entry
to a surface water body;
There must be no "statistically significant
increases" of contaminant concentrations
in the surface water body at those points
of entry, or at points downstream; and
It must be possible to reliably prevent
human exposure to the contaminated
ground water through the use of
institutional controls.
Each of these criteria must be met and must be
supported by site-specific information. Such
information also must be incorporated into the
appropriate portions of the Administrative Record
(e.g., the RI/FS and ROD).
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The NCP Preamble also advises that ACLs not be
used in every situation in which the above
conditions are met, but only where active
restoration of the ground water is "deemed not to
be practicable" (Federal Register, 1990a; at
8754). This caveat in the Preamble signals that
EPA is committed to the program goal of
restoring contaminated ground water to its
beneficial uses, except in limited cases. In the
context of determining whether ACLs could or
should be used for a given site, the term
"practicability" refers to an overall finding of the
appropriateness of ground-water restoration,
based on an analysis of remedial alternatives using
the Superfund remedy selection criteria, especially
the "balancing" and "modifying" criteria (EPA,
1993b). (These criteria are defined in part
§3 00.43 0(e)(9)(iii) of the NCP (Federal Register,
1990a.) This is distinct from a finding of
"technical impracticability from an engineering
perspective," which refers specifically to an
ARAR waiver and is based on the narrower
grounds of engineering feasibility and reliability
with cost generally not a major factor, unless
ARAR compliance would be inordinately costly
(see Section 2.6.3 and EPA, 1993b). Where an
ACL is established, such an ARAR waiver is not
necessary. Conversely, where an ARAR is waived
due to technical impracticability, there is no need
to establish CERCLA ACLs, as defined above.
When establishing an ACL, a detailed site-specific
justification should be provided in the
Administrative Record which documents that the
above three conditions for use of ACLs are met,
and that restoration to ARAR or risk-based levels
is "not practicable" as discussed above.
Although alternate concentration limits are also
defined in the RCRA program, users of this
guidance should be aware of several important
differences in the use of ACLs by the RCRA
and Superfund programs. For "regulated units"
(defined in 40 CFR 264.90) ACLs are one of the
three possible approaches for establishing
concentrations limits of hazardous constituents in
ground water. Those options are described in 40
CFR 294.94(a). Factors considered when
determining whether an ACL is appropriate for a
particular facility are provided in 40 CFR
264.94(b). The use of RCRA ACLs is not strictly
limited to cases where contaminated ground water
discharges to surface water, or to cases where
ground-water restoration is considered "not
practicable" (as is the case in Superfund).
However, the factors considered in the RCRA
ACL decision are meant to ensure that
establishment of ACLs will be protective of
human health and the environment.
A specific reference to ACLs is not made in the
existing framework for implementing RCRA
Corrective Action at "non-regulated units"
(Federal Register, 1990b and 1996). However,
the Corrective Action framework recommends
flexibility for the development and use of risk-
based cleanup standards, based on considerations
similar to those used for establishing ACLs under
40 CFR 264.94.
3.0 PRESUMPTIVE TECHNOLOGIES
3.1 Presumptive Technologies for Ex-Situ
Treatment
Presumptive technologies for the treatment
portion of an extraction and treatment remedy (ex-
situ treatment) are identified in Highlight 4.
Descriptions of each of the presumptive
technologies are presented in Appendices D1
through D8. These technologies are presumptive
for treatment of contaminants dissolved in
ground water that has been extracted from the
subsurface, and are expected to be used for this
purpose at "all appropriate sites." (Refer to the
Preface of this guidance and EPA, 1993d for
further information concerning the Agency's
expectations concerning the use of presumptive
treatment technologies.)
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Highlight 4. Presumptive Technologies
For Treatment Of Extracted Ground
Water
For treatment of dissolved organic
contaminants, volatiles, semivolatiles and
others (see Note):
Air stripping
Granular activated carbon (GAC)
Chemical/UV oxidation (for cyanides
also)
Aerobic biological reactors
For treatment of dissolved metals:
Chemical precipitation
Ion exchange/adsorption
Electrochemical methods (when
only metals are present)
Aeration of background metals
For treatment of both organic and
inorganic constituents:
A combination of the technologies
listed above
NOTE: A given treatment train could include
a combination of one or more of the
presumptive technologies for treatment of
dissolved contaminants as well as other
technologies for other purposes (e.g.,
separation of solids) as indicated in
Appendix C2.
3.1.1 Design Styles within Presumptive
Technologies. The presumptive technologies
identified in Highlight 4 refer to technology types
rather than specific designs (design styles). Each
presumptive technology represents a single
process falls within one of these technology types
(e.g., innovative air stripper designs, or
innovative media for ion exchange/adsorption of
metals). A listing of design styles of the
presumptive technologies typically considered
during Superfund remedy selection are listed in
Appendix CI.
3.1.2 Benefits of Presumptive Technologies.
Use of the presumptive technologies identified in
this guidance will simplify and streamline the
remedy selection process for the ex-situ treatment
portion of a ground-water remedy by:
Simplifying the overall selection process,
since the large number and diverse
assortment of these technologies have
been reduced to relatively few technology
types;
Eliminating the need to perform the
technology screening portion of the
feasibility study (FS), beyond the analysis
contained in this guidance and its
associated Administrative Record. (See
Section 3.3.2);
Allowing, in some cases, further
consideration and selection among the
presumptive technologies to be deferred
from the FS and ROD to the remedial
design (RD), which prevents duplication
of effort and allows selection to be based
on additional data collected during the RD
(see Section 3.3.3);
Shifting the time and resources employed
in remedy selection from ex-situ
treatment to other, more fundamental
aspects of the ground-water remedy (see
Section 1.0); and
Facilitating the use of extraction and
treatment for early actions, where
appropriate, since selection of the
treatment component is simplified.
3.1.3 Consideration of Innovative
Technologies. Use of presumptive technologies
for treatment of extracted ground water is
intended to simplify the remedy selection process,
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but does not preclude the consideration of
innovative technologies for this purpose in the FS
or RD. Refer to the EPA fact sheet, Presumptive
Remedies: Policy and Procedures (EPA, 1993d),
for additional information. Many innovative or
emerging technologies for ex-situ treatment are
actually design variations of one of the
presumptive technology types, as discussed above,
and others may be considered on a site-specific
basis. In addition, EPA encourages consideration
of in-situ treatment technologies for ground-water
remedies, either when combined with extraction
and treatment or as an alternative to such methods
(see Section 2.4.2).
3.2 Basis for Presumptive Technologies
3.2.1 Sources of Information. Three sources of
information were used to determine which
technologies should be identified as presumptive
for ex-situ treatment of ground water:
Review of the technologies selected in all
RODs signed from fiscal years 1982
through 1992;
Review of capabilities and limitations of
ex-situ treatment technologies from
engineering and other technical literature;
and
Detailed evaluation of the technologies
considered in the FS and selected in the
ROD or RD for a sample of 25 sites for
which at least one ex-situ treatment
technology was selected.
The above information is summarized in a
separate report entitled Analysis of Remedy
Selection Results for Ground-Water Treatment
Technologies at CERCLA Sites (EPA, 1996b). A
total of 427 RODs selected at least one ex-situ
technology for treatment of ground water, as of
September 30, 1992. From these RODs, a sample
of 25 sites were selected for detailed evaluation of
the rationale used to select these technologies as
part of the ground-water remedy.
3.2.2 Rationale for Indentifying Presumptive
Technologies. At least one of the eight
presumptive technologies, identified in Highlight
4, was selected as part of the ground-water remedy
in 425 of 427 RODs, or 99.5 percent of the time.
In only five RODs were technologies other than
the presumptive technologies selected as part of
the treatment train. Therefore, presumptive
technologies were the only technologies selected
for ex-situ treatment of dissolved ground-water
contaminants in 420 of the 427 RODs.
More importantly, all the presumptive
technologies are well understood methods that
have been used for many years in the
treatment of drinking water and/or municipal
or industrial wastewater. Engineering Bulletins
or Technical Data Sheets have been developed by
EPA and the Naval Energy and Environmental
Support Activity, respectively, for five of the eight
presumptive technologies. These publications
generally include site specific performance
examples, and are included as references, along
with other publications, with the description of
each technology in Appendix D.
In the 25 site sample, the presumptive
technologies, identified in Highlight 4, were the
only technologies selected in the ROD for all sites
and the only technologies implemented in the RD
for 24 sites. Other technologies were consistently
eliminated from further consideration, usually in
the technology screening step, based on technical
limitations which were verified by the engineering
literature. As part of this evaluation the large
number and diverse assortment of technologies
considered for ex-situ treatment of ground water
were categorized according to the underlying
treatment process. A complete listing of the
technologies considered in the FS, ROD or RD for
the 25 sites is given in Appendix CI, categorized
by process type and with the presumptive
technologies identified.
Some technologies are identified as presumptive
even though they were selected in relatively few
RODs. Aeration of background metals was
identified as presumptive because this technology
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is often used for removal of iron and manganese,
and was considered and selected for this purpose
at two of the 25 sample sites. Electrochemical
methods for metals removal were also identified
as presumptive because these methods were
considered at all three sample sites where metals
were the only contaminants of concern, and were
selected at two of these sites. Chemical/UV
oxidation and aerobic biological reactors were
identified as presumptive technologies for treating
organic contaminants for the following technical
reasons:
A range of chemical, physical and
biological treatment methods should be
included in the presumptive technologies,
because air stripping and granular
activated carbon, alone or combined, may
not provide cost effective treatment (see
Section 3.4.5) for all organic
contaminants.
These methods destroy organic
contaminants as part of the treatment
process instead of transferring them to
other media, which reduces the quantity
of hazardous treatment residuals (e.g.,
spent carbon) that will require further
treatment.
Ongoing research and development
efforts, by EPA and others, are expected
to increase the cost effectiveness of these
treatment methods.
3.3 Remedy Selection Using Presumptive
Technologies
Selection of technologies for long-term treatment
of extracted ground water requires an
understanding of the types of technologies that
will be needed, how they will be used in the
treatment system and site-specific information for
determining the most appropriate and cost-
effective technologies. The presumptive
technologies for treating dissolved
contaminants in extracted ground water,
identified in Highlight 4, are the technologies
that should be retained for further
consideration in the Detailed Analysis portion
of the feasibility study (FS). This guidance and
its associated Administrative Record will
generally constitute the Development and
Screening of Alternatives portion of the FS for the
ex-situ treatment component of a ground-water
remedy, as discussed in Section 3.3.2.
Site information needed to select cost-effective
treatment technologies (see Section 3..4) is often
not collected until the remedial design (RD) phase.
In such cases, it will generally be appropriate
to specify performance requirements for the
treatment system in the ROD, but defer
selection of specific technologies until the RD,
as discussed in Section 3.3.3.
3.3.1 Use of Technologies in Treatment
Systems. Complete treatment of extracted ground
water generally requires that units of more than
one technology, or multiple units of a single
technology (unit processes), be linked together in
a treatment train. A given treatment train could
include some combination of treatment
technologies for the following purposes:
1. Separation of mineral solids and/or
immiscible liquids from the extracted
ground water during initial treatment
(pretreatment);
2. Treatment of dissolved contaminants;
3. Treatment of vapor phase contaminants
from the extracted ground water or those
generated during treatment;
4. Separation of solids generated during
treatment;
5. Final treatment of dissolved
contaminants prior to discharge
(polishing); and
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6. Treatment of solids generated during
treatment.
Presumptive technologies for treatment of
dissolved contaminants in extracted ground
water (No. 2 and 5, above) are identified in
Highlight 4. Examples of the types of
technologies used for other purposes are given in
Appendix C2, along with a listing of the general
sequence of unit processes used in a treatment
train. Solid residuals (such as sludges from
chemical or biological processes, or spent carbon
media) will generally require additional treatment
or disposal, either as part of the treatment train or
at a separate facility. Presumptive technologies
for purposes other than for treatment of dissolved
contaminants have not been identified in this
guidance.
Use of modular treatment components, which can
be easily added or removed from the treatment
system, may facilitate phased implementation or
other changes that may occur during the life of a
remedy. Phased implementation of the extraction
portion of a remedy may require that some
components of the treatment system also be
installed in stages. Also, modification of the
treatment system over time may be needed in
response to changes in the inflow rate or
contaminant loadings, or to increase the
effectiveness or efficiency of the treatment system.
3.3.2 This Guidance Constitutes the FS
Screening Step. This guidance and its associated
Administrative Record will generally constitute
the "development and screening of alternatives"
portion of the feasibility study (FS), for the ex-situ
treatment component of a ground-water remedy.
When using presumptive technologies, the FS
should contain a brief description of this approach
(see fact sheet entitled Presumptive Remedies:
Policy and Procedures (EPA, 1993d)), and refer
to this guidance and its associated Administrative
Record. Such a brief description should fulfill the
need for the development and screening of
technologies portion of the FS for the ex-situ
treatment component of the remedy.
3.3.3 Deferral of Final Technology Selection to
RD. Although EPA prefers to collect the site
information needed for technology selection prior
to the ROD, it is sometimes impracticable to
collect some of the necessary information until the
remedial design (RD) phase. (See Section 3.4 for
a summary of site information generally needed
for selection of these technologies.) In reviewing
remedy selection experience for a sample of sites,
EPA found that at seven of 25 sites (28 percent)
the type of technology selected in the ROD for
treatment of extracted ground water was later
changed in the RD because of additional site
information obtained during the design phase
(EPA, 1996b). Where EPA lacks important
information at the ROD stage, it may be
appropriate to defer final selection among the
presumptive ex-situ treatment technologies (as
well as selection of specific design styles) to the
RD phase.
In this approach, EPA would identify and evaluate
the technologies and provide an analysis of
alternative technologies in the FS (this guidance
and its associated administrative record will
generally constitute that discussion). The
proposed plan would identify the technologies that
may be finally selected and specify the timing of
and criteria for the future technology selection in
sufficient detail that the public can evaluate and
comment on the proposal. The ROD would also
identify all ARARs and other performance
specifications and information associated with
discharge and treatment of the extracted ground
water, including the types of discharge, effluent
requirements, and specifications developed in
response to community preferences. Specifying
the performance criteria and other requirements in
the ROD (using a type of "performance based
approach") ensures that the remedy will be
protective and meet ARARs. Overall, the ROD
should be drafted so that the final selection of
technologies at the RD phase follows directly
from the application of criteria and judgments
included in the ROD to facts collected during the
RD phase. If the ROD is drafted in this fashion,
documenting the final technology selection can
generally be accomplished by including a
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document in the post-ROD portion of the
Administrative Record, which explains the basis
of technology selection (e.g., Basis of Design
Report, or memorandum to the RD file).
Advantages of deferring selection of ex-situ
treatment technologies to the RD include:
The remedy selection process is further
streamlined, since final selection and the
accompanying detailed analysis for these
technologies is performed only in the RD
not in both the FS and the RD,
minimizing duplication of effort;
Site information collected during the RD
can be used to make final technology
selections as well as to design the
treatment train, which facilitates selection
of the most cost effective technologies
(see Section 3.4.5);
The likelihood that changes in the
treatment train will be made during the
RD is explicitly recognized in the ROD;
and
The time and resources employed in the
FS can focus on other components of the
ground-water remedy that have more
direct influence on attainment of
remedial objectives for contaminated
ground water (see Section 1.0).
Cost estimates for remedial alternatives,
including the ex-situ treatment component, will
need to be included in the FS regardless of
whether or not technology selection is deferred to
the RD. For cost estimating purposes when
deferring technology selection to the RD,
reasonable assumptions should be made
concerning the treatment system, including
assumptions concerning the presumptive
technologies and likely design styles to be used.
To assist in making such assumptions, advantages
and limitations for the presumptive technologies
are summarized in Appendix C4. Also, brief
descriptions of the presumptive technologies and
references for additional information are provided
in Appendix D. Assumptions used for estimating
treatment costs should be consistent across all
remedial alternatives. All assumptions should be
clearly stated as such in the FS and ROD.
Example ROD language for deferring technology
selection to the RD is given in Appendix B3 for a
hypothetical site. This language is only for the ex-
situ treatment portion of an extraction and
treatment remedy and should appear in the
selected remedy portion of the ROD when
following this approach.
3.4 Information Needed for Selecting
Technologies
The site information listed in Highlight 5 is
generally needed to determine the treatment
components of a complete treatment train for
extracted ground water and to select the most
appropriate technology type and design style for
each component. Further detail regarding site data
needed and the purpose of this information is
provided in Appendix C3. Much of this
information is also needed for design of the
extraction component of an extraction and
treatment remedy.
3.4.1 When Should this Information be
Collected? The information listed in Highlight 5
is needed for design of the treatment train.
Therefore, it must be collected prior to or during
the design phase, for either an early action or long-
term remedy. Much of this information should
also be available for selecting among the
presumptive technologies, since it is generally
needed to determine the technologies most
appropriate for site conditions. The timing of
information needed during remedy selection is
different when deferring technology selection to
the RD than when selecting technologies in the
ROD, as discussed in Section 3.3.3. However,
much of this information can be collected along
with similar data gathered during the remedial
investigation (RI). In general, it is recommended
that as much of this information as possible be
obtained prior to the RD in order to minimize the
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Highlight 5. Summary of Site Information
Needed For Treatment Train Design
Total extraction flow rate
Discharge options and requirements
Target effluent concentrations
Contaminants
Degradation products
Treatment additives
Natural constituents
Other requirements
Regulatory
Operational
Community concerns or
preferences
Water quality of treatment influent
Contaminant types and
concentrations
Naturally occurring constituents
Other water quality parameters
Treatability information
NOTE: Further detail is provided in Appendix
C3.
need for additional site investigations during the
RD and to accelerate the RD phase,
much of this information can be collected along
with similar data gathered during the remedial
investigation (RI). In general, it is recommended
that as much of this information as possible be
obtained prior to the RD in order to minimize the
need for additional site investigations during the
RD and to accelerate the RD phase.
3.4.2 Extraction Flow Rate. Inflow to the
treatment system is the total flow from all
extraction wells or drains. Estimates of total
extraction flow rate often have a high degree of
uncertainty (i.e., one or more orders of
magnitude), depending on type of data and
estimation method used. Expected flow rates
from extraction wells are typically estimated from
hydraulic properties of the aquifer. Aquifer
hydraulic properties may have considerable
natural variation over the site and accurate
measurement of these properties is often difficult.
In order to reduce uncertainty during design of the
treatment system, aquifer properties used in
estimating the inflow should generally be
obtained from pumping-type aquifer tests and
not from "slug tests," laboratory measurements on
borehole samples or values estimated from the
literature.
Pumping-type aquifer tests provide a much better
estimate of average aquifer properties than other
methods, because a much larger volume of aquifer
is tested. For the same reason, ground water
extracted during pumping tests is more
representative of that which will enter the
treatment system, and should generally be used for
treatability studies of ex-situ treatment
technologies instead of samples obtained from
monitoring wells. Suggested procedures for
conducting pumping-type aquifer tests are given
in EPA, 1993i. Methods for treatment of
contaminated ground water extracted during
pumping-type aquifer tests are discussed in
Section 3.5.
The likely variability in the total extraction rate
during the life of the remedy should also be
estimated. Variability in the extraction rate could
result from addition or removal of extraction
wells, short-term operational changes in the
system (e.g., changing the pumping rates) or
seasonal fluctuations in the water table. The
number of extraction wells could change as a
result of implementing the remedy in phases or
from post-construction refinement of the remedy
(see Section 2.3.1).
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3.4.3 Discharge Options and ARARs. All
options for discharge of ground water after
extraction and treatment should be identified and
considered in the FS, especially options that
include re-use or recycling of the extracted ground
water. Water quality requirements for the treated
effluent (i.e., effluent ARARs) may be different
for each discharge option. Examples of regulatory
requirements include those promulgated under the
federal Safe Drinking Water Act and Clean Water
Act, which would apply to discharges to a
drinking water system or to surface waters,
respectively; and state requirements for these
types of discharge. Effluent requirements could
also include those for chemicals added during
treatment, contaminant degradation products, and
naturally occurring constituents (e.g., arsenic), in
addition to those for contaminants of concern. In
general, one or more types of discharge for
extraction and treatment remedies should be
selected in the ROD, not deferred to the RD.
ARARs for the treated effluent will determine the
overall level of treatment needed, which in turn
determines the type of components needed in the
treatment train (see Section 3.3.1) and is a critical
factor in selecting appropriate treatment
technologies.
In some cases it may be appropriate to select more
than one type of discharge for the selected remedy.
One type of discharge may be preferred, but may
not be capable of accepting the entire flow of
treated effluent. For example, it may be possible
to re-use or recycle a portion but not all of the
discharge. It may also be desirable to reinject a
portion of the treated effluent for enhanced
recovery of contaminants (aquifer flushing) but
prohibitively costly to reinject the entire discharge.
In addition to the types of discharge, ARARs
and other specifications related to technology
selection or operating performance of the
treatment system should be specified in the
ROD. Regulatory requirements for all waste
streams from the treatment system should be
specified, including those for the treated effluent;
releases to the air; and those for handling,
treatment and disposal of solid and liquid
treatment residuals. Other specifications could
include those preferred by the affected community,
such as requirements to capture and treat
contaminant vapors (even though not required by
ARARs) or limits on operating noise. Other
specifications may also be needed to maintain
continued operation of the system, such as water
quality conditions necessary to minimize chemical
and/or biological clogging of injection wells or
drains.
3.4.4 Water Quality of Treatment Influent. In
order to design the treatment system, contaminant
types and concentrations and other water quality
parameters must be estimated for the total flow
entering the system. Since some technologies are
more effective than others in removing certain
contaminant types, this is an important technology
selection factor. Concentrations of naturally
occurring constituents as well as background and
site-related contaminants in the extracted ground
water should also be measured, as discussed in
Appendix C3.
3.4.5 Treatability Studies. Treatability studies
involve testing one or more technologies in the
laboratory or field to assess their performance on
the actual contaminated media to be treated from a
specific site. These studies may be needed during
the RI/FS to provide qualitative and/or
quantitative information to aid in selection of the
remedy, or during the RD to aid in design or
implementation of the selected remedy. Three
tiers of testing may be undertaken: 1) laboratory
screening, 2) bench-scale testing, or 3) pilot-scale
testing. Treatability studies may begin with any
tier and may skip tiers that are not needed (EPA,
1989c).
For treatment of extracted ground water,
treatability studies are generally needed to
accurately predict the effectiveness and total cost
of a technology for a given site, including
construction and operating costs; and the costs of
other components that may be needed in the
treatment train (see Section 3.3.1). Optimizing
the cost effectiveness of the treatment train is
especially important for systems designed to
27
-------
operate over a long time period. (In this guidance,
optimizing the cost effectiveness of the treatment
system is defined as meeting all treatment and
other performance requirements while minimizing
total costs per unit volume of water treated.)
Treatability studies may also indicate that some
technologies provide cost effective treatment when
all of the above factors are considered, even
though these technologies were infrequently
selected in past RODs (e.g., chemical/UV
oxidation or aerobic biological reactors). For
these reasons treatability studies will be helpful in
selecting among the presumptive technologies.
Similarly, a presumptive treatment technology
should not be eliminated from further
consideration in the FS or RD simply because a
treatability study is required to determine its
applicability for a given site. In general, some
type of treatability study should be performed
prior to or during the design of any system
expected to provide long-term treatment of
extracted ground water, including systems using
presumptive technologies.
3.5 Treatment Technologies for Aquifer Tests
Although pumping-type aquifer tests are the
preferred method of determining average aquifer
properties (see Section 3.4.2) and this information
is useful for remedy selection, such testing is often
deferred to the RD phase because of the need to
determine how to treat and/or dispose of the
extracted ground water. To facilitate use of such
tests earlier in the site response, ex-situ treatment
technologies most suitable for this application are
discussed below.
3.5.1 Treatment Needs during Aquifer Tests.
In comparison to an extraction and treatment
remedy, pumping-type aquifer tests (see Section
3.4.2) generate relatively small flows of
contaminated ground water over a short period of
time. At the time of such tests, the estimated
pumping rates and contaminant loadings generally
have a high degree of uncertainty. Often the total
volume of ground water extracted during testing is
held in storage tanks or lined ponds to prevent the
discharge from affecting water levels in
observation wells and interfering with the test.
Storage of the extracted ground water also allows
subsequent flow to a treatment system to be
controlled and optimized. For example, if storage
vessels are used for both the untreated and treated
water, the extracted water can be routed through
the treatment system as many times as necessary
to meet discharge and/or disposal requirements.
Therefore, the cost effectiveness of treatment
technologies (see Section 3.4.5) is less important
for aquifer testing than for the long-term remedy,
because of the much smaller volume of ground
water to be treated and the much shorter period of
operation.
3.5.2 Treatment Technologies for Aquifer
Tests. Technologies for treating ground water
extracted during aquifer tests should be able to
treat a wide range of contaminant types, be
available in off-the-shelf versions (short lead time
for procurement), have a short on-site startup
time, be relatively simple to operate, and be
available in easily transportable units. Of the
presumptive technologies identified above, the
three most suitable for this application are:
Granular activated carbon,
Air stripping, and
Ion exchange/adsorption.
Granular activated carbon can effectively remove
most dissolved organic contaminants and low
concentrations of some inorganic compounds. Ion
exchange/adsorption can remove most metals. Air
stripping may be applicable for volatile organic
contaminants (VOCs) and generally is more cost
effective than granular activated carbon for
treating VOCs when flow rates are greater than
about three gallons per minute (Long, 1993).
Granular activated carbon may still be needed in
conjunction with air stripping, for treating
dissolved semivolatile organic contaminants, or
for reaching stringent effluent requirements for
VOCs. Granular activated carbon may also be
needed for treatment of vapor phase contaminants
separated by an air stripper. Also, treatability
28
-------
studies generally are not required for the above
three technologies, especially for short-term
applications. Additional information regarding
the availability and field installation of skid or
trailer mounted treatment units (package plants) is
available in EPA, 1995a.
Other presumptive ex-situ treatment technologies
(chemical/UV oxidation, aerobic biological
reactors, chemical precipitation, and
electrochemical methods) generally are less
suitable for aquifer testing purposes. In general,
these other technologies require longer lead times
for procurement and longer time on-site for
startup; and have more complex operating
requirements and higher capital costs.
4.0. REFERENCES
Cohen, R.M., and J.W. Mercer, 1993. DNAPL
Site Evaluation. C.K. Smoley, Boca Raton, FL
and ORD Publication EPA/600/R-93/022.
EPA, 1986. "Guidelines for Ground-Water
Classification Under the EPA Ground-Water
Protection Strategy, Final Draft," November,
1986.
EPA, 1988a. "Guidance for Conducting Remedial
Investigations and Feasibility Studies Under
CERCLA, Interim Final," OSWER Directive
9355.3-01, EPA/540/G-89/004, October 1988.
EPA, 1988b. "Guidance on Remedial Actions for
Contaminated Ground Water at Superfund Sites,"
OSWER Directive 9283.1-2, EPA/540/G-88/003,
December 1988.
EPA, 1989a. "Considerations in Ground Water
Remediation at Superfund Sites," OSWER
Directive 9355.4-03, October 18, 1989.
EPA, 1989b. "Interim Final Guidance on
Preparing Superfund Decision Documents,"
OSWER Directive 9335.3-02, October 1989.
EPA, 1989c. "Guide for Conducting Treatability
Studies Under CERCLA, Interim Final,"
OERR/ORD Publication EPA/540/2-89/058,
December 1989.
EPA, 1990a. "ARARs Q's & A's: State Ground-
Water Antidegradation Issues," OSWER
Publication 9234.2-11/FS, July 1990.
EPA, 1990b. "Suggested ROD Language for
Various Ground Water Remediation Options,"
OSWER Directive 9283.1-03, October 10, 1990.
EPA, 1991a. "Guide to Addressing Pre-ROD and
Post-ROD Changes," OSWER Publication
9355.3-02FS-4, April 1991.
EPA, 1991b. "Guide to Developing Superfund
No Action, Interim Action, and Contingency
Remedy RODs," OSWER Publication 9355.3-
02FS-3, April 1991.
EPA, 1991c. "Guide to Principal Threat and Low
Level Threat Wastes," OSWER Publication
9380.3-06FS, November 1991.
EPA, 1992a. "Estimating Potential for
Occurrence of DNAPL at Superfund Sites,"
OSWER Publication, 9355.4-07FS, January
1992.
EPA, 1992b. "Considerations in Ground-Water
Remediation at Superfund Sites and RCRA
Facilities - Update," OSWER Directive 9283.1-
06, May 27, 1992.
EPA, 1992c. "Guidance on Implementation of the
Superfund Accelerated Cleanup Model (SACM)
under CERCLA and the NCP," OSWER Directive
9203.1-03, July 7, 1992.
EPA, 1992d. "The Superfund Accelerated
Cleanup Model (SACM)," OSWER Publication
9203.1-021, November 1992.
EPA, 1992e. "Early Action and Long-Term
Action Under SACM - Interim Guidance,"
OSWER Publication 9203.1-051, December 1992.
29
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EPA, 1992f. "SACM Regional Decision Teams -
Interim Guidance," OSWER Publication 9203.1-
051, December 1992.
EPA, 1992g. "Evaluation of Ground-Water
Extraction Remedies: Phase II, Volume 1
Summary Report," OSWER Publication 9355.4-
05, February 1992.
EPA, 1992h. "Final Comprehensive State Ground
Water Protection Program Guidance," Publication
EPA 100-R-93-001, December 1992.
EPA, 1993a. "Guidance on Conducting Non-
Time-Critical Removal Actions Under CERCLA,"
OSWER Publication 9360.0-32, EPA/540-R-93-
057, August 1993.
EPA, 1993b. "Guidance for Evaluating Technical
Impracticability of Ground-Water Restoration,"
OSWER Directive 9234.2-25, EPA/540-R-93-
080, September 1993.
EPA, 1993c. "Evaluation of the Likelihood of
DNAPL Presence atNPL Sites, National Results,"
OSWER Publication 9355.4-13, EPA/540-R-93-
073, September 1993.
EPA, 1993d. "Presumptive Remedies: Policy and
Procedures," OSWER Directive 9355.0-47FS,
EPA/540-F-93-047, September 1993.
EPA, 1993e. "Presumptive Remedies: Site
Characterization and Technology Selection For
CERCLA Sites With Volatile Organic
Compounds In Soils," OSWER Directive 9355.0-
48FS, EPA/540-F-93-048, September 1993.
EPA, 1993f. "Presumptive Remedy for CERCLA
Municipal Landfill Sites," OSWER Directive
9355.0-49FS, EPA/540-F-93-035, September
1993.
EPA, 1993g. "Innovative Treatment
Technologies: Annual Status Report (Fifth
Edition)", Publication EPA 542-R-93-003,
September 1993.
EPA, 1993h. "In-situ Treatment of Contaminants:
An Inventory of Research and Field
Demonstrations and Strategies for Improving
Ground Water Remediation," OSWER
Publication EPA/500/K-93/001, January 1993.
EPA, 1993i. "Ground Water Issue, Suggested
Operating Procedures for Aquifer Pumping
Tests," OSWER Publication EPA/500/S-93/503,
February 1993.
EPA, 1993j. "Data Quality Objectives Process
for Superfund, Interim Final Guidance" OSWER
Publication 9355.9-01, EPA/540/R-93/071,
September 1993.
EPA, 1994a. "Alternative Methods for Fluid
Delivery and Recovery," ORD/CERI Publication
EPA/625/R-94/003, September 1994.
EPA, 1994b. "DNAPL Site Characterization,"
OSWER Publication 9355.4-16FS, EPA/540/F-
94/049, September 1994.
EPA, 1994c. "RCRA Corrective Action Plan,"
OSWER Directive 9902.3-2A, EPA/520/R-
94/004, May 1994.
EPA, 1994d. "Technical Support Project, Direct
Technical Assistance for Site Remediation,"
OSWER Publication EPA/542-F-94/004, October
1994.
EPA, 1994e. "Methods for Monitoring Pump-
and-Treat Performance," ORD Publication
EPA/600/R-94-94/123, June 1994.
EPA, 1995a. "Manual: Ground Water and
Leachate Treatment Systems," ORD/CERI
Publication EPA/625 R-94/005, January 1995.
EPA, 1995b. "Consistent Implementation of the
FY 1993 Guidance on Technical Impracticability
of Ground-Water Restoration at Superfund Sites,"
OSWER Directive 9200.4-14, January 19, 1995.
30
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EPA, 1995c. "Superfund Groundwater RODs:
Implementing Change This Fiscal Year," OSWER
Memorandum from Elliott P. Laws to Regional
Administrators and others, July 31, 1995 (no
publication number).
EPA, 1995d. "In-Situ Remediation Technology
Status Report: Treatment Walls," OSWER
Publication EPA/542 K-94-004, April 1995.
EPA, 1995e. "Innovative Treatment
Technologies: Annual Status Report (Seventh
Edition)," OSWER Publication EPA-542-R-95-
008 Number 7, Revised, September 1995.
EPA, 1995f. "Soil Vapor Extraction (SVE)
Enhancement Technology Resource Guide,"
OSWER Publication EPA/542-B-95-003,
October 1995.
EPA, 1995g. "Presumptive Remedies for Soils,
Sediments and Sludges at Wood Treater Sites,",
OSWER Directive 9200.5-162, EPA/540-R-
95/128, December 1995.
EPA, 1996a. "Consideration of'Comprehensive
State Ground Water Protection Programs' by EPA
Remediation Programs," Draft OSWER Directive
9283.1-09 dated June 1996. Final Directive
expected by November 1996.
EPA, 1996b. "Analysis of Remedy Selection
Experience for Ground Water Treatment
Technologies at CERCLA Sites," Draft Final
Report dated July 1996. Final Report expected by
November 1996.
EPA, 1996c. "Use of Natural Attenuation at
Superfund, RCRA Corrective Action, and
Underground Storage Tank Sites," Draft OSWER
Directive dated September 1996. Final
Directive expected by February 1997.
Federal Register, 1990a. Volume 55, No. 46,
March 8, 1990; 40 CFR Part 300, "National Oil
and Hazardous Substances Pollution Contingency
Plan; Final Rule" (NCP).
Federal Register, 1990b. Volume 55, No. 145,
July 27, 1990; 40 CFR Parts 264, 265, 270 and
271, "Corrective Action for Solid Waste
Management Units at Hazardous Waste Facilities;
Proposed" (proposed Subpart S regulations).
Federal Register, 1996. Volume 61, No. 85, May
1, 1996; "Corrective Action for Releases from
Solid Waste Management Units at Hazardous
Waste Management Facilities, Advance Notice of
Proposed Rulemaking."
Long, G. M., 1993. "Clean up Hydrocarbon
Contamination Effectively," Chemical
Engineering Progress, Vol. 89, No. 5.
National Research Council, 1994. Alternatives
for Ground Water Cleanup. National Academy
Press, Washington, DC.
31
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APPENDIX A
Additional Background Information
Appendix Al: Background on DNAPL Contamination
Appendix A2: Contaminants Most Frequently Reported in Ground Water at CERCLA
NPL Sites
Appendix A3: Examples of In-Situ Treatment Technologies
Appendix A4: Definition and Discussion of Pulsed Pumping
-------
Appendix A1: Background on DNAPL Contamination
DNAPL Background
A nonaqueous phase liquid (NAPL) is a chemical that is a liquid in its pure form, which does not readily
mix with water but does slowly dissolve in water. Dense NAPLs (DNAPLs) sink while light NAPLs
(LNAPLs) float in water. When present in the subsurface NAPLs slowly release vapor and dissolved phase
contaminants, resulting in a zone of contaminant vapors above the water table and a plume of dissolved
contaminants below the water table. The term NAPL refers to the undissolved liquid phase of a chemical or
mixture of compounds and not to the vapor or dissolved phases. NAPLs may be present in the subsurface as
either "free-phase" or as "residual-phase." The free-phase is that portion of NAPL that can continue to
migrate and which can flow into a well. The residual-phase is that portion trapped in pore spaces by capillary
forces, which can not generally flow into a well or migrate as a separate liquid. Both residual and free-phase
NAPLs are sources of vapors and dissolved contaminants.
LNAPLs tend to pose less of a cleanup problem than DNAPLs. The most common LNAPLs are petroleum
fuels, crude oils and related chemicals, which tend to be associated with facilities that refine, store or
transport these liquids. Since LNAPLs tend to be shallower, are found at the water table and are associated
with certain facilities, they are generally easier to locate and clean up from the subsurface than DNAPLs.
DNAPLs pose much more difficult cleanup problems. These contaminants include chemical compounds and
mixtures with a wide range of chemical properties, including chlorinated solvents, creosote, coal tars, PCBs,
and some pesticides. Some DNAPLs, such as coal tars, are viscous chemical mixtures that move very slowly
in the subsurface. Other DNAPLs, such as some chlorinated solvents, can travel very rapidly in the
subsurface because they are heavier and less viscous than water. A large DNAPL spill not only sinks
vertically downward under gravity, but can spread laterally with increasing depth as it encounters finer
grained layers. These chemicals can also contaminate more than one aquifer by penetrating fractures in the
geologic layer which separates a shallower from a deeper aquifer. Thus, large releases of DNAPLs can
penetrate to great depths and can be very difficult to locate and clean up.
The contamination problem at DNAPL sites has two different components, as shown in Figures Al-1 and
Al-2, the:
DNAPL zone, and the
Aqueous contaminant plume.
The DNAPL zone is that portion of the subsurface where immiscible liquids (free-phase or residual DNAPL)
are present either above or below the water table. Also in the DNAPL zone, vapor phase DNAPL
contaminants are present above water table and dissolved phase below water table. The aqueous
contaminant plume is that portion of the contaminated ground water surrounding the DNAPL zone where
aqueous contaminants derived from DNAPLs are dissolved in ground water (or sorbed to aquifer solids) but
immiscible liquids are not present. Depending on the volume of the release and subsurface geology, the
DNAPL zone may extend to great depths and over large lateral distances from the entry location, as discussed
above.
A-l
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A-2
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Appendix Al: Background on DNAPL Contamination (continued)
Planning of site investigation and remedial activities at sites with subsurface DNAPLs should include certain
precautions, to minimize the potential for further DNAPL migration resulting from such activities. Further
detail on characterization of DNAPL sites is provided in EPA, 1994 and in Cohen and Mercer, 1993 (see
below).
DNAPL References
Additional information concerning DNAPL contamination can be obtained from the following references:
Cohen, R.M., and J.W. Mercer, 1993. DNAPL Site Evaluation. C.K. Smoley, Boca Raton, FL, 1993; and
EPA/600/R-93/022, Februaiy 1993.
EPA, 1991. "Ground Water Issue: Dense Nonaqueous Phase Liquids," OSWER Publication EPA/540/4-91-
002, March 1991.
EPA, 1992a. "Estimating Potential for Occurrence of DNAPL at Superfund Sites," OSWER Publication
9355.4-07FS, Januaiy 1992.
EPA, 1992b. "Dense Nonaqueous Phase Liquids ~ A Workshop Summary, Dallas, Texas, April 16-18,
1991," Office of Research and Development Publication EPA/600/R-92/030, February 1992.
EPA, 1992c. "Considerations in Ground-Water Remediation at Superfund Sites and RCRA Facilities -
Update," OSWER Directive 9283.1-06, May 27, 1992.
EPA, 1993b. "Guidance for Evaluating Technical Impracticability of Ground-Water Restoration," OSWER
Directive 9234.2-25, EPA/540-R-93-080, September 1993.
EPA, 1994. "DNAPL Site Characterization," OSWER Publication 9355.4-16FS, EPA/540/F-94/049,
September 1994.
A-3
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Appendix A2: Contaminants Most Frequently Reported in Ground Water at CERCLA NPL Sites 1
Organic Contaminants:
Rank Organic Contaminants (Other Names)
9
10
11
12
13
14
15
16
17
18
19
20
Trichloroethylene, 1,1,2- (TCE)CS
Tetrachloroethene (perchloroethene; PCE)CS
Chloroform (trichloromethane)cs
Benzenepf
Toluenepf
Trichloroethane, 1,1,1- (methyl chloroform;
1,1,1-TCA)CS
Poly chlorinated biphenyls
Trans-Dichloroethylene, 1,2- (trans-1,2-DCE)C
Dichloroethane, 1,1- (1,1-DCA)CS
Chemical2
Group
Volatile
Volatile
Volatile
Volatile
Volatile
Volatile
PCB
Volatile
Volatile
Dichloroethene, 1,1- (vinylidene chloride; 1,1-DCE)CS Volatile
Vinyl chloride (chloroethylene)cs Volatile
Xylenepf Volatile
Ethylbenzenepf Volatile
Carbon tetrachloride (tetrachloromethane)cs Volatile
Phenol Semivol.
Methylene chloride (dichloromethane)cs Volatile
Dichloroethane, 1,2- (ethylene dichloride; 1,2-DCA)CS Volatile
Pentachlorophenol (PCP) Semivol.
Chlorobenzene (benzene chloride)05 Volatile
Benzo(A)Pyrene Semivol.
Halo-2
genated?
Yes
Yes
Yes
No
No
Yes
Yes
Yes
Yes
Yes
Yes
No
No
Yes
No
Yes
Yes
Yes
Yes
No
No.1
DNAPL?3 Sites
Yes
Yes
Yes
No
No
Yes
Yes
Yes
Yes
Yes
No
No
No
Yes
No
Yes
Yes
Yes
Yes
Yes
336
170
167
164
159
155
139
107
105
95
82
76
68
68
61
58
57
53
48
37
A-4
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Appendix A2: Contaminants Most Frequently Reported in Ground Water at CERCLA NPL Sites
(continued)1
Inorganic Contaminants:
No.1
Rank Inorganic Contaminants Sites
1 Lead 307
2 Chromium and compounds 215
3 Arsenic 147
4 Cadmium 127
5 Mercury4 81
6 Copper and compounds 79
7 Zinc and compounds 73
8 Nickel and compounds 44
9 Cyanides (soluble salts) 39
10 Barium 37
NOTES:
Number of CERCLA National Priorities List (NPL) sites for which the chemical was reported in ground water as
a contaminant of concern in the Superfund Site Assessment, for either proposed or final NPL sites. This data was
obtained from the Superfund NPL Assessment Program (SNAP) data base, as of August 30, 1994. At that time
total of 1294 sites were listed on the NPL (64 proposed and 1230 final).
Classification of organic contaminants as volatile, semivolatile, PCB, or pesticide; and as halogenated or
nonhalogenated is from EPA Publication, "Technology Screening Guide for Treatment of CERCLA Soils and
Sludges," EPA/540/2-88/004, September 1988.
Classification of whether or not a chemical is a dense nonaqueous phase liquid (DNAPL) in pure form is from
Cohen and Mercer, 1993 (see References).
In pure form mercury is also a DNAPL.
These organic contaminants are chlorinated solvents. A total of 12 are listed.
These organic contaminants are constituents of petroleum fuels. A total of four are listed.
A-5
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Appendix A3: Examples of In-Situ Treatment Technologies 1
I. Enhanced Recovery Methods
Recirculation/flooding:
Water flooding
(physical)
Steam flooding
(physical)
Chemical flooding2
(chemical)
Nutrient flooding2
(biological)
Thermal enhanced recovery:
Radio frequency
Electrical resistance
(AC or DC)
Enhancement of secondary permeability:
Induced fracturing with water or
or air pressure (physical)
Other methods:
Treatment Agents
(and process type)
- Water
- Heated water
- Steam
Surfactants
Solvents
Redox agents
Nitrate
Other
- Heat
- Heat
Agent Delivery Methods
Not applicable
- Injection wells
- Injection wells
- Injection wells
- Injection wells
- Injection wells
- Injection wells
- Injection wells
- Electrodes in wells
- Electrodes in wells
Not applicable
Electromigration (electrical)
- Electric current
- Electrodes in wells
NOTES:
1 List of technologies and technology status is from EPA, 1993h (see References section of guidance).
2 Chemicals or nutrients for micro-organisms, respectively, are added to reinjection water.
A-6
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Appendix A3: Examples of In-Situ Treatment Technologies (continued )'
II. In-situ Treatment Processes
Treatment Agents
Agent Delivery Methods
Physical/chemical treatment:
Volatilization and oxygen
enhancement by air sparging
Reductive dehalogenation by
metal catalysts (abiotic)
Biological treatment:
Oxygen enhancement of aerobic
organisms (also includes air
sparging, above)
Nutrient enhancement of aerobic
organisms
Nutrient enhancement of anaerobic
organisms to produce enzymes that
degrade contaminants (cometabolism)
Sequential anaerobic-aerobic
treatment
- Air
Iron filings
Other agents
Hydrogen peroxide
Oxygen/ surfactant
(microbubbles)
Nitrate
Other
Methane
Other
Methane and/or
Oxygen
- Injection wells
- Permeable walls/gates3
- Permeable walls/gates3
- Injection wells4
- Injection wells4
- Injection wells3
- Injection wells
- Injection wells
NOTES:
In permeable treatment walls/gates, treatment agents are added with trench backfill materials or are injected via
perforated pipes placed in the backfill. These walls are placed in the subsurface across the natural flow path of
the contaminant plume. They can be combined with impermeable flow barriers in a "funnel and gate"
arrangement, in which flow is directed through the treatment walls/gates.
Use of permeable treatment walls/gates to deliver treatment agents for these methods may also be feasible.
A-7
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Appendix A4: Definition and Discussion of Pulsed Pumping
Pulsed Pumping
In pulsed pumping, some or all extraction pumps are turned off and then back on for specified periods of time
(e.g., one or more monitoring periods). The on and off cycles can be continued or the extraction and
treatment remedy can be returned to continuous pumping. Although not widely used in remedies to date, this
method may be effective in enhancing the recovery of contaminants from the aquifer . Pulsed pumping
can recover contaminants located in the following portions of the aquifer that are relatively unaffected during
pumping:
Upper portions of the aquifer that have been dewatered by pumping, and
Zones with minimal ground-water flow during pumping (flow stagnation zones).
Pulsed pumping may also enhance contaminant recovery for aqueous phase contaminants that are sorbed to
the aquifer matrix. Therefore, pulsed pumping can be initiated as a post-construction refinement of an
extraction and treatment remedy (see Section 2.4), when an evaluation of remedy performance indicates that
this technique may increase the recovery of contaminants from the aquifer.
Pulsed pumping can also be used as a method of evaluating the effectiveness of an extraction and
treatment remedy and/or the effectiveness of source control actions. For example, if contaminant levels
increase substantially when pumping is stopped, it is an indication that contaminants continue to be derived
from source materials, and that additional remedial measures (e.g., source control/removal) may be necessary.
These source materials could include aqueous contaminants sorbed to aquifer solids in finer-grained aquifer
layers, NAPLs (refer to Appendix A1), contaminated soils, or other sources.
Pulsed pumping should generally not be initiated until after sufficient monitoring data has been obtained
from continuous pumping to establish a statistically valid performance trend. Also, the influence of pulsed
pumping on plume containment should be considered; and extraction wells used primarily for containment
(i.e, at plume leading edge) should generally not be pulsed .
A-8
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APPENDIX B
ROD Language Examples For Selected Remedy
Appendix B1: Phased Implementation of Ground-Water Remedy
Appendix B2: Phased Implementation of Extraction Component of Remedy at a
DNAPL Site
Appendix B3: Deferring Selection of Treatment Components to Remedial Design
Appendix B4: Suggested ROD Language from 1990 OSWER Directive
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Appendix Bl: Phased Implementation of Ground-Water Remedy
Site Conditions:
At hypothetical Site 1 (an LNAPL site) surficial soils and the underlying ground water in Aquifer C are
contaminated with volatile organic compounds (VOCs). At this site, Aquifer C is currently used as a source
of drinking water, with several wells located on-site and in the estimated path of the contaminant plume.
Early actions were used for exposure prevention and source control. Under Superfund removal authority,
an alternate water supply was provided to several residences, and leaking drums and heavily contaminated
soils were excavated and taken off-site for disposal. A soil vapor extraction system was installed as an
interim remedial action . No further source control actions are planned. DNAPLs are not likely to be
present in the subsurface because most of the contaminants are LNAPLs rather than DNAPLs in pure form.
The selected ground-water remedy relies on extraction and treatment for preventing further migration of the
contaminant plume and for restoration of Aquifer C. The selected remedy will be implemented in two
construction phases.
ROD Language for Extraction Component of Remedy :
The following, or similar language, should appear in the Selected Remedy section of the ROD:
The ultimate goal for the ground-water portion of this remedial action is to restore Aquifer C to
its beneficial uses. At this site, Aquifer C is currently used as a source of drinking water. Based
on information obtained during the remedial investigation and on a careful analysis of all
remedial alternatives, EPA and the State of believe that the selected remedy will achieve this
goal.
The extraction portion of the ground-water remedy will be implemented in two phases. In phase
one, a sufficient number of extraction wells will be installed with the objective of minimizing
further migration of the contaminant plume. It is currently estimated that two to four extraction
wells will be required for phase one.1 After construction of phase one is completed, the extraction
system will be carefully monitored on a regular basis and its performance evaluated. Operation
and monitoring ofphase one for a period of up to one year may be needed to provide sufficient
information to complete the design of phase two.
In phase two, additional extraction wells will be installed with the objective of restoring Aquifer
C for use as a source of drinking water, in addition to maintaining the remedial objectives for
phase one. Restoration is defined as attainment of required cleanup levels in the aquifer, over the
entire contaminant plume. Cleanup levels for each ground-water contaminant of concern are
specified in Table of the ROD. Current estimates indicate that an additional two to four
extraction wells may be required to attain these cleanup levels within a timeframe of
approximately 20 years.1 However, monitoring and evaluation of the performance ofphase one
will be used to determine the actual number and placement of wells for phase two.
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Appendix Bl: Phased Implementation of Ground-Water Remedy (continued)
The selected remedy will include ground-water extraction for an estimated period of 20 years,
during which the system's performance will be carefully monitored, in accordance with the
monitoring plan defined in Section of the ROD, and adjusted as warranted by the
performance data collected during operation. Refinement of the extraction system may be
required, if EPA determines that such measures will be necessary in order to restore Aquifer C in
a reasonable timeframe, or to significantly reduce the timeframe or long-term cost of attaining
this objective. Refinement of the extraction system may include any or all of the following:
1) Adjusting the rate of extraction from some or all wells;
2) Discontinuing pumping at individual wells where cleanup goals have been
attained;
3) Pulsed pumping of some or all extraction wells to eliminate flow stagnation
areas, allow sorbed contaminants to partition into ground water, or otherwise
facilitate recovery of contaminants from the aquifer; and
4) Installing up to two additional ground-water extraction wells to facilitate or
accelerate cleanup of the contaminant plume.1
It is possible that performance evaluations of the ground-water extraction system - after
completion of phase one, during implementation or operation of phase two, or after subsequent
refinement measures - will indicate that restoration of Aquifer C is technically impracticable
from an engineering perspective. If such a determination is made by EPA, the ultimate
remediation goal and/or the selected remedy may be reevaluated.2
NOTES:
1. Although not required in a ROD, the estimated number of wells is included in this example for the
following reasons, to:
Provide a basis for estimating the cost of the selected remedy, including upper
and lower costs for phase one, phase two and the potential refinement measures;
Provide some specificity regarding how the extraction component of the
remedy will be used in the overall remediation strategy , because changes in the
extraction system directly influence the time period required to attain the remedial
objectives for this site; and to
Provide some bounds for the scope, performance and cost of the selected
remedy, which will assist in determining whether future, post-ROD remedy
modifications require an Explanation of Significant Differences (see Section 2.4 of
this guidance).
2. Reevaluation of the ultimate remediation goal and/or the selected remedy would generally require an
ESD or ROD amendment.
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Appendix B2: Phased Implementation of Extraction Component of Remedy at a DNAPL Site
Site Conditions:
At hypothetical Site 2 (a DNAPL site), ground water in Aquifer A is contaminated with volatile and
semivolatile organic contaminants (no metals as contaminants of concern). DNAPLs have also been
observed in this aquifer. At this site, Aquifer A is not currently used as source of drinking water, but several
wells are located off-site in the estimated path of the contaminant plume.
The selected remedy includes extraction and treatment for hydraulic containment of the likely DNAPL-zone
(see Appendix A1 of this guidance) and for restoration of the aquifer outside the DNAPL-zone. Reinjection
of a portion of the treated ground water will be used to enhance recovery of contaminants from the aquifer.
It has been determined that aquifer restoration within the DNAPL-zone is technically impracticable from an
engineering perspective, as explained in the Statutory Determinations section of the ROD. The remedy
will be implemented in two construction phases.
ROD Language for Extraction Component of Remedy :
The following, or similar language, should appear in the Selected Remedy section of the ROD:
The ultimate goal for the ground-water portion of this remedial action is to restore the maximum
areal extent of Aquifer A to its beneficial uses. At this site Aquifer A is potentially useable as a
source of drinking water and is currently used off-site for this purpose. Based on information
obtained during the remedial investigation and on a careful analysis of all remedial alternatives,
EPA believes that the selected remedy will achieve this goal.
The extraction portion of the ground-water remedy will be implemented in two phases. In phase
one, a sufficient number of extraction wells will be installed to achieve two remedial objectives
for Aquifer A: 1) minimizing further migration of contaminants from suspected subsurface
DNAPL areas to the surrounding ground water; and 2) minimizing further migration of the
leading edge of the contaminant plume. It is currently estimated that three to five extraction
wells will be required for phase one.1 After construction of phase one is completed, the
extraction system will be carefully monitored on a regular basis and its performance evaluated.
This evaluation may provide further information concerning the extent of the DNAPL-zone.
Operation and monitoring ofphase one for a period of up to two years may be needed to
provide sufficient information to complete the design of phase two.
In phase two, additional extraction wells will be installed with the objective of restoring the
maximum areal extent of Aquifer A for use as a source of drinking water, in addition to
maintaining phase one objectives. Reinjection wells and related pumping equipment for flushing
a portion of the treated ground water through the aquifer (water flooding) will also be installed
in order to enhance the recovery of contaminants. Restoration is defined as attainment of
required cleanup levels in the aquifer, over the portion of the contaminant plume outside the
DNAPL-zone. Cleanup levels for each ground-water contaminant of concern are specified in
Table ; although cleanup level ARARs within the DNAPL-zone have been waived by EPA due
technical impracticability from an engineering perspective, as discussed in Section of the
ROD. Current estimates indicate that these cleanup levels can be attained in the portion of
Aquifer A outside the DNAPL-zone within a timeframe of approximately 25 years.
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Appendix B2: Phased Implementation of Extraction Component of Remedy at a DNAPL Site
(continued)
Current estimates also indicate that an additional two to six extraction wells and two to four
reinjection wells may be required for phase two.1 However, monitoring and evaluation of the
performance ofphase one will be used to determine the actual number and placement of wells
for phase two.
The selected remedy will include ground-water extraction for an estimated period of 25 years,
during which the system's performance will be carefully monitored, in accordance with the
monitoring plan defined in Section of the ROD, and adjusted as warranted by the
performance data collected during operation. Refinement of the extraction system may be
required, if EPA determines that such measures will be necessary in order to restore the maximum
areal extent of Aquifer A in a reasonable timeframe, or to significantly reduce the timeframe or
long-term cost of attaining this objective. Refinement of the extraction system may include any
or all of the following:
1) Adjusting the rate of extraction from some or all wells;
2) Discontinuing pumping at individual wells where cleanup goals have been
attained;
3) Pulsed pumping of some or all extraction wells to eliminate flow stagnation
areas, allow sorbed contaminants to partition into ground water, or otherwise
facilitate recovery of contaminants from the aquifer;
4) Installing up to two additional ground-water extraction wells to facilitate or
accelerate cleanup of the contaminant plume; and1
5) Installing up to two additional reinjection wells.1
It is possible that performance evaluations of the ground-water extraction system - after
completion of phase one, during implementation or operation of phase two, or after subsequent
refinement measures - will indicate that restoration ofportions or all of Aquifer A is technically
impracticable from an engineering perspective. If such a determination is made by EPA, the
ultimate remediation goal and/or the selected remedy may be reevaluated.2
NOTES:
1. The reasons for including the estimated number of wells in this example are discussed in the Notes
section of the previous example, Appendix B2.
2. Reevaluation of the ultimate remediation goal and/or the selected remedy would generally require an
ESD or ROD amendment.
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Appendix B3: Deferring Selection of Treatment Components to Remedial Design
Site Conditions:
Hypothetical Site 2 is the same site used in the previous example, Appendix B2. Most of the treated
ground water will be discharged to the nearby Muddy River, although a portion (20 to 30 percent) will be
reinjected to Aquifer A to enhance contaminant recovery. Contaminant-specific and other water quality
requirements for discharge to the Muddy River were specified by the state and are listed in Table of the
ROD. Other specifications for the treatment system are also listed in the ROD, which include filtering of
suspended mineral solids to minimize clogging of reinjection wells; and treatment of vapor phase organic
contaminants from air stripping or other processes, as requested by the local community.
ROD Language for Treatment Component of Remedy :
The ex-situ treatment component of the ground-water remedy will utilize presumptive
technologies identified in Directive 9283.1-12from EPA's Office of Solid Waste and Emergency
Response (OSWER), included as Attachment of the ROD. Since contaminants of concern
include volatile and semivolatile organic compounds, one or more of the presumptive
technologies - air stripping, granular activated carbon (GAC), chemical/UV oxidation and
aerobic biological reactors - will be used for treating aqueous contaminants in the extracted
ground water. Other technologies will also be needed in the treatment system for removal of
suspended mineral solids and treatment of vapor phase contaminants. The actual technologies
and sequence of technologies used for the treatment system will be determined during remedial
design. Final selection of these technologies will be based on additional site information to be
collected during the remedial design. (See Section 3.4 and Appendix C3 of OSWER Directive
9283.1-12for a discussion of site information needed for selection and design of the ex-situ
treatment system.) Based on this additional information and sound engineering practice the
treatment system shall be designed to:
Attain the chemical-specific discharge requirements and other performance
criteria specified in Table and Section of the ROD; and
Treat, or be easily modified to treat, the expected flow increase from phase one
to phase two of the extraction system.
Other design factors shall include:
Maximizing long-term effectiveness,
Maximizing long-term reliability (i.e., minimize the likelihood of process upsets),
and
Minimizing long-term operating costs.
Additional information concerning presumptive technologies for the ex-situ treatment component
of the remedy is provided in OSWER Directive 9283.1-12. Descriptions of each of the
presumptive technologies are presented in Appendices Dl through D8, and advantages and
limitations of each of these technologies are listed in Appendix C4 of this directive.
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Appendix B3: Deferring Selection of Treatment Components to Remedial Design (continued)
For the purpose of estimating the approximate cost of the treatment component of the selected
remedy, the following treatment sequence is assumed for aqueous contaminants: flow
equalization tanks, a gravity oil-water separator, an air stripper, followed by GAC units. GAC
will also be used to treat vapor phase contaminants from the air stripper. The GAC units will be
thermally reactivated at an off-site facility. Separated DNAPL compounds will be recycled if
possible, but since the actual composition of the recovered liquids is unknown, costs for
incineration at an off-site facility were used for the cost estimate.
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Appendix B4: Suggested ROD Language from 1990 OSWER Directive
Recommended language for the Selected Remedy section of the ROD was given in OSWER Directive
9283.1-03, entitled "Suggested ROD Language for Various Ground-Water Remediation Options," dated
October 10, 1990. For the RODs in which the final remedy without a contingency is selected, this Directive
recommended that "the following type of language should appear in the Selected Remedy section of the
ROD:"
The goal of this remedial action is to restore ground water to its beneficial use, which is, at this
site, (specify whether this is a potential or actual drinking water source, or is used for non-
domestic purposes). Based on information obtained during the remedial investigation and on a
careful analysis of all remedial alternatives, EPA < (optional) and the State/Commonwealth of
> believe that the selected remedy will achieve this goal. It may become apparent, during
implementation or operation of the ground-water extraction system and its modifications, that
contaminant levels have ceased to decline and are remaining constant at levels higher than the
remediation goal over some portion of the contaminated plume. In such a case, the system
performance standards and/or the remedy may be reevaluated.
The selected remedy will include ground-water extraction for an estimated period of
years, during which the system's performance will be carefully monitored on a regular basis and
adjusted as warranted by the performance data collected during operation. Modifications may
include any or all of the following:
a) at individual wells where cleanup goals have been attained, pumping may be
discontinued;
b) alternating pumping at wells to eliminate stagnation points;
c) pulse pumping to allow aquifer equilibration and to allow adsorbed
contaminants to partition into ground water; and
d) installation of additional extraction wells to facilitate or accelerate cleanup of
the contaminant plume.
To ensure that cleanup goals continue to be maintained, the aquifer will be monitored at those
wells where pumping has ceased on an occurrence of every years following
discontinuation of ground-water extraction.
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APPENDIX C
Ex-Situ Treatment Technologies for Ground Water
Appendix CI: Ex-Situ Technologies Considered in Sample of 25 Sites
Appendix C2: Other Components Needed for Treatment Trains
Appendix C3: Information Needed for Selection of Technologies and Design of
Treatment Train
Appendix C4: Advantages and Limitations of Presumptive Treatment Technologies
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Appendix CI: Ex-Situ Technologies Considered in Sample of 25 Sites
Technologies that were considered for treatment of extracted ground in the sample of 25 sites reviewed in
detail (EPA, 1996b) are listed below. These technologies were either considered in the feasibility study
(FS), or considered and/or selected in the record of decision (ROD) or remedial design. The technologies are
listed according to overall process type, and by design style within each type. Those technologies identified
as presumptive technologies are also indicated. For further information on how presumptive technologies
were identified, refer to Section 3.2 of this guidance and EPA, 1996b.
For Treatment of Organic Contaminants:
For Treatment of Metals:
Presumptive Technologies:
Air stripping:
Packed tower
- Ambient temperature
- Higher temperature
Aeration methods
- Ambient temperature
- Higher temperature
Cascade falls
Granular activated carbon (GAC)
Chemical/UV oxidation:
Chemical oxidation alone
- Ozone
- Hydrogen peroxide
- Chlorine compounds
- Potassium permanganate
Chemical with UV oxidation
- Ozone
- Hydrogen peroxide
UV oxidation alone (photolysis)
Alkaline chlorination (for cyanide)
Unspecified oxidation methods
Aerobic biological reactors:
Attached growth
- Trickling filter
- Rotating biological contactors
- Fixed bed
Suspended growth
- Activated sludge
- Sequencing batch reactors
- Aeration ponds/lagoons
- Unspecified suspended growth
Unspecified aerobic reactors
Chemical precipitation:
Hydroxide precipitants
- Sodium hydroxide
- Lime
- With prior chemical reduction
Sulfide precipitants
- Sulfur dioxide
- Sodium sulfide
- Sodium bisulfide/bisulfites
- With prior chemical reduction
- Unspecified sulfide precipitant
Other precipitation methods
- Ferrous sulfate
- Potassium permanganate
- Activated consumable element
- Unspecified chemical precipitation
Ion exchange/adsorption:
Fixed bed
- Impregnated/synthetic resin
- Activated alumina
Electrodialysis
Unspecified ion exchange
Electrochemical methods:
Electrochemical reduction
Magnetically activated
Aeration of Background Metals:
Aeration basin
Cascade aeration
Other aeration methods
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Appendix CI: Ex-Situ Technologies Considered in Sample of 25 Sites (continued)
For Treatment of Organic Contaminants:
Other Technologies Considered:
Chemical treatment:
Hydrolysis
Catalytic dehydrochlorination
Catalytic dechlorination
Chlorinolysis
Thermal Destruction:
Incineration
Calcination
Wet air oxidation
Supercritical water oxidation
Microwave discharge/plasma
High temperature separation:
Steam stripping
Distillation
Membrane filtration:
Reverse osmosis
Ultrafiltration
Anaerobic biological treatment:
Anaerobic biological reactor
Enzymatic degradation
Liquid-liquid extraction:
Solvent extraction
Liquid carbon dioxide extraction
Evaporation:
Evaporation basin
Land treatment:
Surface spreading
Spray irrigation
For Treatment of Metals:
Granular activated carbon (for metals)
Reverse Osmosis
Biological treatment of metals
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Appendix C2: Other Components Needed for Treatment Trains 1
Solid or Liquid Separation
Technologies
Effluent Polishing Technologies2
Vapor Phase Treatment
Technologies3
Activated carbon
Ion exchange
Neutralization
Oil/grease separation4
Filtration5
Coagulation5
(or flocculation)
Clarification5
(or sedimentation)
General Sequence of Unit Processes Used in Aqueous Treatment Trains
Sequence Unit Treatment Process Treatment Stage
Begin
End
Equalize inflow
Separate solid particles
Separate oil/grease (NAPLs)
Remove metals
Remove volatile organics
Remove other organics
Polish organics2
Polish metals
Adjust pH, if required
Pretreatment
Pretreatment
Pretreatment
Treatment
Treatment
Treatment
Post-treatment
Post-treatment
Post-treatment
Activated carbon
Resin adsorption
Catalytic oxidation
Thermal incineration
Acid gas scrubbing
Condensation
NOTES:
1 In addition to the presumptive technologies listed in the guidance, other treatment components are needed either prior to (pretreatment) or subsequent to
(post-treatment) the presumptive technologies. This listing is not intended to be presumptive. Not listed are technologies that may be required for treatment
residuals, such as spent carbon.
2 Effluent polishing technologies are those used for the final stage of treatment prior to discharge, and can include pH adjustment (neutralization) as well as
additional removal of aqueous constituents.
3 Vapor phase contaminants released during water treatment may need to be contained and treated. This includes organic contaminants volatilized during air
stripping, from biological treatment, or other gases released from chemical oxidation, reduction or biologic processes (e.g., hydrochloric acid, hydrogen
sulfide, methane, etc.).
4 Methods for separation of oil and/or grease from water include, but are not limited to, gravity separation and dissolved air floatation. These methods can be
used to remove NAPLs from the extracted ground water.
5 These technologies can be used to remove solid particles at the beginning of the treatment train or for removal of other solids resulting from chemical
precipitation, chemical/UV oxidation or biological treatment.
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Appendix C3: Information Needed for Selection of Technologies and Design of Treatment Train
Information Needed
1. Total extraction flow rate:
Total extracted flow
Flow variability
Uncertainty of estimate
Purpose of Information
Inflow to the treatment system is the total flow from
all extraction wells. Since this flow must also be
discharged, large flows may determine the availability
of some discharge options. Flow rate and
concentration determines the mass loading (mass per
unit water volume) of each contaminant entering the
treatment system. The mass loading determines the
dimensions and capacities of treatment vessels, and
whether continuous flow or batch design are used for
each treatment unit. Flow is also a factor for selecting
among the presumptive treatment technologies
because some are less cost effective for high or low
flows.
Variable inflow rates may require use of flow
equalization tanks, batch instead of continuous flow
operation or use of modular treatment units that can
be added or subtracted from the treatment train. Some
technologies can handle variable flow more easily
than others. Variable extraction rates may result from
short-term operational changes, seasonal changes or
phased well installation.
Uncertainty in the flow estimate can result from
natural variability of aquifer properties over the site,
and from the method used to measure these properties.
Since flow is a critical design parameter, additional
characterization may be needed to reduce the level of
uncertainty. Estimates of the total extraction rate
should be based on pumping type aquifer tests,
since this method provides a much better estimate
of average aquifer properties than other methods.
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Appendix C3: Information Needed for Selection of Technologies and Design of Treatment Train
(continued)
Information Needed
2. Discharge options and effluent requirements:
Options available
Target effluent concentrations, each
option
- Contaminants
- Contaminant degradation
products
- Treatment additives
- Natural constituents
- Water quality parameters
Other requirements, each option
- Regulatory
- Operational
Community concerns or preferences
Purpose of Information
Options for discharge of treated ground water could
include: discharge to surface waters; discharge to a
drinking water system; reuse or recycling for other
purposes (e.g., industrial processes); infiltration or
reinjection to shallow subsurface or reinjection to the
same aquifer; or discharge to POTW. Target effluent
concentration levels for both contaminants and
naturally occurring constituents may be markedly
different for each discharge option.
Effluent requirements could include those for
chemicals added during treatment, contaminant
degradation products, naturally occurring constituents
(e.g., arsenic), and water quality parameters (e.g.,
suspended solids) in addition to maximum
concentration levels for chemicals of concern. These
requirements will determine the overall level of
treatment needed, which in turn determines the type of
components needed in the treatment train and is a
critical factor in selecting appropriate treatment
technologies.
Each discharge option may have different water
quality requirements for the treated effluent, from both
a regulatory and operational standpoint. For
example, reinjection to the subsurface must meet
substantive federal and/or state requirements for
underground injection (regulatory) as well as
minimize chemical and biological clogging of
injection wells or infiltration lines (operational). Use
of the best available technology (BAT) could also be a
regulatory requirement. The affected community
may also have concerns or preferences regarding the
type of discharge.
Target effluent concentrations determine the overall
removal efficiency the treatment train must attain for
each constituent. For example, if the target effluent
level is 10 mg/L and the inflow concentration is 1000
mg/L, then the treatment train must attain an overall
removal efficiency of 99.0 percent (1000 - 0.99(1000)
= 10). The treatment train may need to include more
than one type of technology, or multiple units of a
single technology, in order to attain the required
overall removal efficiency.
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Appendix C3: Information Needed for Selection of Technologies and Design of Treatment Train
(continued)
Information Needed
Purpose of Information
3. Water quality of treatment influent:
Contaminant types and concentrations:
- Inorganic chemicals
- Organic chemicals
- Concentration changes over time
- Nonaqueous phase liquids
(NAPLs)
Naturally occurring constituents:
- Major cations (metals) and
anions
- Organic chemicals
- Radionuclides
Contaminant types and concentrations must be
estimated for the total flow entering the treatment
system. Since some technologies are more effective in
removing certain contaminant types, this is an
important technology selection factor. Inflow
concentrations are needed to determine the removal
efficiency of the treatment train, as discussed above.
The design should consider the potential for inflow
concentrations to change over time. Contaminant
concentrations usually decrease as remediation
progresses. Also, short term increases may occur if a
"hot spot" of more highly contaminated ground water
is captured by the extraction system. Samples
obtained from pumping type aquifer tests provide
better estimates of average contaminant
concentrations, because such samples are obtained
from a relatively large aquifer volume.
If present, subsurface NAPLs (refer to Appendix Al)
may become entrained in the extracted ground water.
These immiscible liquids should be removed in a
pretreatment step (process used prior to other
treatment methods). Also, a specialized extraction
system may be needed to remove free-phase NAPLs
from the subsurface.
Naturally occurring or non-site related constituents
may need to be removed to prevent interference with
treatment processes and may be a factor in technology
selection. Metals such as iron, manganese, and
calcium can leave mineral deposits (scaling) on air
stripper packing and on activated carbon or other
treatment media. If not accounted for, these metals
can also cause premature exhaustion of ion exchange
capacity and increased consumption of reagents in
chemical oxidation or precipitation processes. Iron
also promotes biological fouling in air strippers.
Heavy metals (e.g., lead, mercury) and cyanides can
be toxic to microorganisms in biological reactors.
Metals can also form deposits on well screens of
extraction or reinjection wells (encrustation) or
promote biological fouling (clogging) on well screens.
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Appendix C3: Information Needed for Selection of Technologies and Design of Treatment Train
(continued)
Information Needed
3. Water quality of influent (continued):
Other water quality parameters:
- Indicator parameters
- Design parameters
Purpose of Information
Dissolved organic constituents (e.g., from decay of
organic materials or from landfill leachate) can
interfere with adsorption of targeted compounds and
can cause premature exhaustion of activated carbon.
Metal-organic complexes can interfere with chemical
oxidation or precipitation processes.
If present, naturally occurring radionuclides can
accumulate in treatment media or residuals (e.g.,
activated carbon or chemical sludges) resulting in
potential exposure hazards for personnel and
additional transportation and disposal considerations.
Other water quality parameters are used as effluent
quality standards, indicator parameters, or design
parameters for treatment processes. Indicator
parameters are used to indicate the presence of other
constituents. For example, total dissolved carbon
(TDC) is a measure of the relative level of dissolved
organic constituents. Gross alpha and gross beta
particle activity are relatively simple measurements
that indicate the relative abundance of naturally
occurring radionuclides. Other indicator parameters
include: total dissolved solids (TDS), chemical
oxygen demand (COD), biological oxygen demand
(BOD) and total suspended solids (TSS).
Temperature and pH are design parameters for most
treatment processes.
Also, high levels of total suspended solids (TSS) in
extracted ground water may indicate that extraction
wells are not properly designed or developed. Most
treatment technologies require that suspended solids
in excess of certain level be removed during
pretreatment, where acceptable levels may differ for
each technology.
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Appendix C3: Information Needed for Selection of Technologies and Design of Treatment Train
(continued)
Information Needed
Purpose of Information
4. Treatability information:
From technical literature
Treatability studies
- Laboratory screening
- Bench-scale testing
- Pilot-scale testing
Modeling predictions
Projections of effluent quality
Treatability information is needed to select technology
types and design styles from among the presumptive
technologies; and for selection and design of other
components of the treatment train. The particular mix of
contaminants and naturally occurring constituents can
vary considerably for different sites. Treatability
information is available in the technical literature for
some technologies, including air stripping and granular
activated carbon (GAC).
Treatability studies include 1) laboratory screening, 2)
bench-scale testing, or 3) pilot-scale testing. These
studies may begin with any tier and skip tiers that are not
needed (see Section 3.4 of guidance). Computer models
for predicting treatment performance are available for
some technologies.
In general, treatability studies should be performed prior
or during the design of any system expected to provide
long-term treatment of extracted ground water,
including systems using presumptive technologies.
Treatability studies are needed to accurately predict the
effectiveness and cost of a technology for a given site,
including construction and operating costs; and the costs
of other components of the treatment train. Optimizing
the cost effectiveness of the treatment train (i.e.,
minimizing the total cost per unit volume of water
treated) is especially important for systems designed to
operate over a long time period.
Treatability studies may reveal unexpected site
conditions, such as the presence of naturally occurring
compounds that interfere with the planned treatment
process or that metal contaminants can be effectively
removed by removing mineral solids. Such studies are
also needed to determine pretreatment requirements, and
requirements for treating aqueous, vapor and solid waste
streams resulting from a particular treatment process.
Treatability studies are needed to determine optimum
chemical reagents and reagent quantities for pH
adjustment; oxidation, reduction or precipitation of
contaminants; and parameters for design of biological
and other reactors.
Treatability studies should be performed on samples
obtained from pumping type aquifer tests instead of
from monitoring wells, because such samples are more
representative of contaminated ground water that will
enter the treatment system. Samples obtained for
treatability studies should be obtained after several hours
of pumping.
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Appendix C4: Advantages and Limitations of Presumptive Treatment Technologies
Technology
Advantages
Limitations
Treatment Technologies for the Removal of Organic Contaminants
Air Stripping
Successfully used in hundreds of groundwater
applications
Low operating cost relative to other technologies
(e.g., energy usage is relatively low).
Operationally simple system requiring a minimum of
operator assistance.
Treatability studies often not required for selection or
design, but are recommended.
Trained contractors available to implement the
technology.
Contaminants transferred to air, and treatment of air emissions may be required.
Pretreatment for metals removal and pH control may be needed to reduce fouling and
corrosion.
Post-treatment (polishing) may be required.
Large surges in influent concentrations can reduce removal efficiency because the efficiency
for an individual compound is fixed regardless of influent concentrations.
Air stripping is not as effective for compounds with low Henry's law constants or high
solubilities b,c
Cold weather can reduce efficiency.
Granular
Activated
Carbon
Successfully used for contaminated ground water at
many Superfund and underground storage tank sites.
Operationally simple system requiring a minimum of
operator assistance.
Reeularlv used as a polishins step followine other
treatment technologies.
Treatability studies generally not required, but are
recommended (information is available from carbon
vendors).
Trained contractors available to implement the
technology.
Generally a cost-effective alternative as single- step
treatment for flows less than about 3 gpm.d
Activated carbon is generally too costly for use as a single-step treatment if ground-water
chemistry requires high carbon usage rates.
Contaminants are not destroyed but are transferred to another media (i.e., spent carbon must
be regenerated or disposed of properly).
Pretreatment for suspended solids removal is often required.
Pretreatment for metals removal and pH control may be needed to reduce fouling and
corrosion.
Organic compounds that have low molecular weight and high polarity are not recommended
for activated carbon (e.g., acetone).
Naturally occurring organic compounds may exhaust carbon bed rapidly and may interfere
with the adsorption of targeted chemicals.
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Appendix C4: Advantages and Limitations of Presumptive Treatment Technologies (continued)
Technology
Advantages
Limitations
Chemical/ UV
.
Where oxidation is complete, organic contaminants
Incomplete oxidation will leave original contaminants and possibly toxic oxidation products;
Oxidation
are destroyed and not transferred to other media:
activated carbon polishing may be required.
minimal residuals generated.
Capital costs may preclude small-scale applications, especially for ozone systems.
Effective on a wide variety of volatile and
Metals may precipitate during oxidation, requiring filtration post-treatment and residuals
semivolatile organics, including chlorinated
disposal.
organics, as well as cyanide and some metals.
UV light sources are subject to fouling and scaling from solids, iron compounds, carbonates,
Operating costs can be competitive with air stripping
etc. Pretreatment may be required to remove these substances.
and activated carbon.
Process must be closely monitored to ensure contaminant destruction and to prevent safety
hazards.
Peroxide and other chemical oxidants must be properly stored and handled.
Site-specific treatability studies are necessary (process may require large quantities of oxidizer
to destroy target compound(s) if reactive nontarget compounds are present).
Aerobic
.
Orsanic contaminants desraded. often with minimal
A residual organic sludge is generated that must be disposed of properly.
Biological
cross-media environmental impacts.
Some compounds are difficult or impossible to degrade (recalcitrant) or slow to degrade.
Reactors
Proven effective for many organic compounds.
Difficulties acclimating microorganisms to contaminants are possible; requires longer startup
Some systems (e.g., trickling filters and rotating
time than other technologies to achieve effective steady-state performance
biological contactors) have minimal energy
Volatile organics may require air emission controls or pretreatment to remove them.
requirements and generally low capital and operating
Variations in flow or concentration may require significant operator attention to prevent
costs.
microorganisms from being killed.
Can be designed to require a minimum of operator
Cold weather can cause operational difficulties.
attention.
Treatability studies are needed for selection and design.
Relatively simple, readily available equipment.
Pretreatment may be needed to remove contaminants toxic to the microorganisms, such as
Trained contractors available to implement the
heavy metals.
technology.
Low organic loading and the potential for supplementary nutrients and food sources must be
considered.
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Appendix C4: Advantages and Limitations of Presumptive Treatment Technologies (continued)
Technology
Advantages
Limitations
Treatment Technologies for the Removal of Inorganic Contaminants
Chemical
Most commonly used method
A residual sludge is generated that must be treated and/or disposed of properly; metals are
Precipitation
for removing soluble heavy
not usually easy to recover from sludge.
metal ions from contaminated
Up to four times stoichiometric chemical additions may be required, especially for sulfide
water.
precipitation (see below).
Pretreatment for solids and iron
Hvdroxide Precipitation
generally not required.
Organics or complexing ions may form chelates/complexes instead of insoluble metal
Hvdroxide Precipitation
hydroxides.
Reliable method, chemicals
Optimum pH is different for each metal hydroxide, one pH may not effectively treat all
relatively easy to handle, and not
soluble metal ions; successive treatments may be required.
costly.
pH must be controlled within a narrow range.
Carbonate Precipitation
Naturally occurring sulfate in ground water may react with lime to form gypsum, which
Reliable method, calcium
increases sludge, can clog filters, and can coat pipelines (caustic soda addition can reduce
carbonate easy to handle, and
this problem but increases costs and dissolved solids [sodium salts] that must be removed
not costly.
from treated ground water).
Effectively removes a variety of
Carbonate Precipitation
soluble metals.
Calcium carbonate is not effective for ground water with high alkaline content.
Sulfide Precipitation
Pretreatment to remove organic, chelating, or oil and grease contaminants may be required.
Reliable method.
Sulfide Precipitation (Soluble Sulfide)
High removal efficiency over a
Excess sulfide ions that are not precipitated remain in solution. They may be removed by
broader pH range.
using aeration to convert them from ionic to oxide form (sulfate).
Relatively insensitive to most
pH control between 8 and 9.5 is required to avoid release of hydrogen sulfide gas.
chelating agents.
Cost is high compared to hydroxide and carbonate precipitation
Can remove chromates and
Sulfide Precipitation (Insoluble Sulfides)
dichromates without reducing
Ferrous sulfide is used in amounts greater than that required by stoichiometric
hexavalent chromium to
considerations.
trivalent form if ferrous ions are
Produces more sludge than soluble sulfide or hydroxide processes.
present or added.
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Appendix C4: Advantages and Limitations of Presumptive Treatment Technologies (continued)
Technology Advantages Limitations
Treatment Technologies for the Removal of Inorganic Contaminants (continued):
Ion Exchange/
High removal efficiencies for
Resins are usually costly and may not be cost-effective for large treatment loadings.
Adsorption
heavy metals.
Generates large volume of backflush solution (approximately 2.5 to 5% of the original
Suitable for use as a polishins
ground-water flow rate) that is concentrated in the metals removed and requires treatment
step after other technologies.
or disposal.
Technology is reasonably well
Requires bench-scale testing to determine operational requirements and suitability of
understood.
prospective resins.
On-site backflushing of
Beds can be fouled by particulate matter, oxidizing agents, oils, greases, biological growths,
exchange media allows
and intra-bed precipitates; therefore, pretreatment may be needed.
immediate reuse.
Resins may be irreversibly harmed by aromatics and certain other organic compounds; and
by iron, manganese, and copper if enough dissolved oxygen is present. Pretreatment may
be needed.
Spent resins require treatment before disposal.
Electro-
High removal efficiencies for
Particulate matter, oxidizing agents, oils, greases, biological growths may reduce process
chemical Methods
certain heavy metals.
efficiency; therefore, pretreatment may be needed.
Can treat both metals and
Hexavalent chromium reduction generates a heavy metal precipitate that must be removed
cyanide simultaneously.
from solution in a subsequent clarification or settling process.
Technology is reasonably well
A heavy metal sludge residual may be generated that may require treatment (dewatering
understood.
and/or fixation) and that will require disposal.
Requires little floor space due to
A spent acid rinse solution may be generated that requires treatment or disposal.
short residence time for
Electrodes must be replaced occasionally.
hexavalent chromium reduction.
Requires minimal operator
attention.
Low operating costs compared
to chemical reduction or
precipitation.
Requires no chemical addition.
NOTES:
s U.S. Environmental Protection Agency. 1991. Engineering Bulletin: Air Stripping of Aqueous Solutions. EPA/540/2-91/022. 8 pp.
b B. Lamarre. 1993. Selecting an air stripper (what to consider!) The National Environmental Journal. 26-29.
c G.M.Long. 1993. Clean up hydrocarbon contamination effectively. Chemical Engineering Progress: 58-66.
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APPENDIX D
Descriptions of Presumptive Treatment Technologies
Appendix Dl:
Air Stripping
Appendix D2:
Granular Activated Carbon
Appendix D3:
Chemical/UV Oxidation
Appendix D4:
Aerobic Biological Reactors
Appendix D5:
Chemical Precipitation
Appendix D6:
Ion Exchange/Adsorption
Appendix D7:
Electrochemical Methods
Appendix D8:
Aeration of Background Metals
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Appendix D1: Air Stripping
Air stripping uses volatilization to transfer contaminants from ground water to air. In general, water is
contacted with an air stream to volatilize dissolved contaminants into the air stream. Stripping of a specific
chemical depends on the equilibrium vapor pressure of that chemical as expressed by its Henry's law
constant.
Applicability
Air stripping is applicable to most of the volatile organic compounds (VOCs) as well as volatile inorganics
such as ammonia and hydrogen sulfide. VOCs with high solubility in water (e.g., acetone) are more difficult
to air strip. Air stripping is potentially applicable to certain halogenated semi-volatile organic compounds
(SVOCs). It is not applicable to nonhalogenated SVOCs; heavy organics such as PCBs, dioxins/furans
and pesticides; or inorganic metal compounds (U.S. EPA, 1991).
Air stripping is most effective for contaminants with a dimensionless (molar volume) Henry's law constant
greater than 0.01 (or 2.4 x 10~4 atm-m3/gmol at 25° C). (Henry's law constants are available in U.S. EPA
[1990]). Removal efficiencies greater than 99 percent are difficult to achieve for certain compounds. In
general, other treatment technologies will be required for such chemicals when ground-water
concentrations are high (e.g., above 10,000 ppm or 1 percent).
Contaminant Fate
Contaminants are not destroyed by air stripping but are physically separated from contaminated ground
water and transferred to air. Depending on the level of contaminants in the air discharge, the contaminated
air stream may need further treatment. Additional polishing treatment of the aqueous effluent also may
be necessary, depending on discharge requirements.
Design
Air strippers are designed for a specific target chemical (either the predominant contaminant or the most
difficuIt-to-strip contaminant) with a desired target removal efficiency. The air stripping process is well
understood and the technology is well developed. Air stripping has an extensive track record in a variety
of applications.
The most frequently used configuration is a packed tower equipped with an air blower. The ground water
is fed into the top of the stripper and the air is introduced at the bottom, creating a countercurrent gas-liquid
contact. Random plastic packing is frequently used to improve gas-liquid contact. Structured packing and
steel packing may also be used. Packed-tower air stripper design involves specification of stripper column
diameter and packing height for a specified ground-water flow rate and air-to-water ratio. Shallow-tray
aeration devices provide an alternative gas-liquid contacting system that provides a more compact, lower
profile system that is less subject to fouling.
Alternative Techniques/Enhanced Methods
For high flow rates (over 1,000 gpm), cooling towers (large structures with cascading water
primarily used to cool water using countercurrent ambient air flow) may provide a cost-effective
alternative to conventional packed towers.
Shallow tray air strippers or diffused tank aeration units are less susceptible to fouling problems
than packed towers and may be preferable where the water to be treated contains high
concentrations of certain inorganics (e.g., iron).
D-l
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Appendix D1: Air Stripping (continued)
Alternative Techniques/Enhanced Methods (continued)
Because the efficiency of air stripping increases at higher temperatures, increasing the influent
ground-water temperature (typically about 55° F) using a heat exchanger can increase the stripper's
removal efficiency, especially for less volatile contaminants.
Steam stripping methods, which use steam rather than air as the stripping medium, can be used to
remove highly soluble contaminants and SVOCs not usually amenable to air stripping. However,
operation costs for steam stripping can be two to three times greater than air stripping, depending on
the cost of steam. In this guidance, these methods are not considered a type of air stripping and are
not identified as a presumptive technology for ex-situ treatment of ground water.
Pre/Post-treatment
Pretreatment to remove iron and other metals and to control hardness may be necessary to reduce
fouling and mineral deposition in packed tower air strippers.
Granular activated carbon is sometimes used to polish the treated water from an air stripper to further
reduce organic contaminant levels and meet discharge requirements.
Contaminants in the air discharge may be reduced by activated carbon adsorption, catalytic
oxidation, or incineration to meet air emission requirements.
Selected References
Lamarre, B. 1993. Selecting an air stripper (what to consider!). The National Environmental Journal: 26-29.
Nyer, E.K. 1985. Groundwater Treatment Technologies. Van Nostrand Reinhold, New York, NY. 187 pp.
Nyer, E.K. 1993. Practical Techniques for Groundwater and Soil Remediation. CRC Press, Inc., Boca Raton,
FL. 214 pp.
Okoniewski, B.A. 1992. Remove VOCs from wastewater by air stripping. Chemical Engineering Progress:
89-93.
U.S. EPA Environmental Protection Agency. 1990. Hazardous Waste Treatment, Storage, and Disposal
Facilities (TSDF) - Air Emission Models. EPA/450/3-87-026. Office of Air Quality Planning and Standards,
Research Triangle Park, NC. Appendix D.
U.S. Environmental Protection Agency. 1991. Engineering Bulletin: Air Stripping of Aqueous Solutions.
EPA/540/2-91/022. Office of Research and Development, Cincinnati, OH. 9 pp.
D-2
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Appendix D2: Granular Activated Carbon
Activated carbon removes contaminants from ground water by adsorption. The adsorption process takes
place in three steps: (1) contaminant migration to the external sorbent surface; (2) diffusion into the sorbent
pore structure; and (3) adsorption onto the sorbent surface. The principal form of activated carbon used
for ground-water treatment is granular activated carbon (GAC). GAC is an excellent sorbent due to its
large surface area, which generally ranges from 500 to 2,000 m2/g.
Applicability
GAC is applicable to a wide variety of contaminants including: halogenated volatile and semivolatile
organics, nonhalogenated volatile and semivolatile organics, PCBs, pesticides, dioxins/furans, most organic
corrosives, metals, radioactive materials, inorganic cyanides, and certain oxidizers. GAC is potentially
applicable to certain organic cyanides, and it is not applicable to asbestos, inorganic corrosives, and
reducers (U.S. EPA, 1991). GAC is sometimes used alone for ground-water treatment. However, GAC
is typically used for polishing aqueous effluents or controlling air emissions from other treatment
technologies.
The adsorption capacity of activated carbon varies for specific organic compounds and for different types
of GAC (based on the origin of coal and the percent binder used in the manufacture of the GAC).
Contaminant-specific adsorption isotherms for a given type of GAC are generally available from the carbon
manufacturer.
Contaminant Fate
Contaminants are not destroyed by carbon adsorption, but are physically separated from contaminated
water and transferred to carbon. After exhaustion, the spent carbon may be reactivated, regenerated,
incinerated, or disposed of. Thermal reactivation and incineration destroy most or all adsorbed organic
contaminants. Steam or hot gas regeneration is not appropriate for spent GAC from treatment of
contaminated ground water but can be used for spent GAC from air emission control devices. GAC used
for metals sorption may require disposal. If disposed of, spent GAC may have to be managed as a
hazardous waste.
Design
Activated carbon is a well-developed, widely used technology with many successful ground-water treatment
applications, especially for secondary polishing of effluents from other treatment technologies.
Contaminated ground water is contacted with a fixed GAC bed in a vessel. Flow direction is generally
vertically downward, although an upward flow configuration is also possible. Fixed-bed configurations are
also used for air emission control.
Adsorber design involves determining total carbon requirements and the number and dimensions of vessels
needed to house the carbon. The amount of carbon required for a given application depends on the
loading of adsorbable constituents in ground water (or contaminated air stream), the carbon's adsorption
capacity for these constituents, and the carbon reactivation (or regeneration) frequency. Depending on the
ground-water suspended solids content, it may be necessary to periodically backwash down flow carbon
beds to relieve pressure drop associated with solids accumulation.
Alternative Techniques/Enhanced Methods
Staged bed (multiple beds operated in series) and pulsed bed (carbon beds operated with nearly
continuous "pulsed" addition of fresh carbon and withdrawal of spent carbon) designs can be used
if higher removal efficiencies are required.
D-3
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Appendix D2: Granular Activated Carbon (continued)
Alternative Techniques/Enhanced Methods (continued)
Because the adsorption capacity of GAC is much higher for gas phase treatment than for liquid
phase treatment, it is often more economical to use an air stripper followed by gas phase GAC
to treat the air stripper exhaust than to use GAC alone for ground-water treatment.
GAC is not identified as a presumptive technology for removal of metals dissolved
extracted ground water. Spent carbon used for metals removal can be difficult to
regenerate and may require treatment and/or disposal as a hazardous waste. Although
GAC can remove low concentrations of certain metals, it has not been widely used for this
purpose (U.S. EPA, 1991).
Pre/Post-treatment
Pretreatment may be required to remove natural organic matter, such as fulvic and humic acids,
that may interfere with the adsorption of the target contaminants or rapidly exhaust the GAC.
Naturally occurring radionuclides, if present in ground water, can accumulate in the GAC
during treatment, which could result in potential exposure hazards for operating personnel
and the spent carbon may require treatment and/or disposal as hazardous waste.
Thermal reactivation, using heat alone or steam, is typically used as a post-treatment method
for the spent carbon. The carbon is reactivated in a high-temperature reactor under reducing
conditions. Most organic contaminants are thermally degraded during the reactivation process.
Selected References
Long, G.M. 1993. Clean up hydrocarbon contamination effectively. Chemical Engineering Progress,
89(5):58-67.
Stover, E.L. 1988. Treatment of herbicides in ground water. Ground Water Monitoring Review: 54-59.
Stenzel, M.H. 1993. Remove organics by activated carbon adsorption. Chemical Engineering Progress:
36-43.
U.S. Environmental Protection Agency. 1991. Engineering Bulletin: Granular Activated Carbon
Treatment. EPA/540/2-91/024. Office of Emergency and Remedial Response, Washington, D.C. 8 pp.
D-4
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Appendix D3: Chemical/UV Oxidation
Chemical oxidation uses chemical oxidizing agents to destroy toxic organic chemicals and cyanide
compounds (CN) in ground water. Commonly used oxidizing agents include: ozone, hydrogen peroxide,
hypochlorites, chlorine, and chlorine dioxide. Ozone and hydrogen peroxide are generally preferred for
removing organics and CN from ground water because chlorine-based oxidants can produce toxic
byproducts (e.g., HCI, chlorinated organics). Ultraviolet light (UV) is often used in conjunction with ozone
and/or hydrogen peroxide to promote faster and more complete destruction of organic compounds
(reaction rates may be increased by factors of 100 to 1,000).
Applicability
Chemical oxidation is applicable to both volatile and semivolatile organic compounds and cyanide
compounds. Chemical oxidation is potentially applicable to PCBs, dioxins/furans, and metals (oxidation
can be used to precipitate metals under certain conditions). Chemical oxidation is not applicable to
asbestos and radioactive materials (U.S. EPA, 1991).
Chemical oxidation generally is effective for concentrations less than 500 |jgL, but has been used for
certain compounds at concentrations ranging up to several thousand mg/L. UV can enhance the oxidation
of compounds that are resistant to chemical oxidation alone (e.g., PCBs). Iron or copper catalysts may
be required for efficient destruction of certain organic compounds (e.g., phenols).
Contaminant Fate
Complete oxidation decomposes hydrocarbons into carbon dioxide and water, although chlorinated organic
compounds also yield chloride ions. CN is oxidized to ammonia and bicarbonate by hydrogen peroxide in
an alkaline environment. If oxidation is incomplete, toxic constituents may remain, or intermediate
degradation products can be formed that may be toxic. These toxic substances may be removed using
GAC as a secondary or polishing treatment step.
Design
Chemical oxidation is a proven and effective technology that is carried out in either batch or continuous
reactors. Oxidants are generally added to contaminated ground water in a mixing tank prior to introduction
into the reaction vessel (reactor). The use of ozone as the oxidizing agent requires an onsite ozone
generator and an ozone decomposition unit or other ozone emission control device. The use of hydrogen
peroxide as the oxidizing agent requires storage tanks and special handling protocols to ensure operator
safety. The use of chlorine as the oxidizing agent may produce HCI gas. If HCI is produced, an acid gas
removal system may be necessary.
UV lamps, if used, are typically enclosed in quartz tubes submerged inside the reaction vessel. The tubes
are subject to fouling or scaling from compounds such as iron oxide or calcium carbonate and from
biological floes from microorganisms in ground water. If fouling occurs, oxidation rates are drastically
reduced.
Site-specific treatability studies are generally recommended for chemical oxidation systems. Extensive
pretreatment may be required to condition ground water for effective oxidation. If UV lamps are used, the
studies must evaluate the potential for fouling or scaling of the quartz tubes at the ground-water
composition, oxidant concentration, and UV intensity conditions anticipated for long-term system operation.
If fouling or scaling is likely, pretreatment and/or physical methods for keeping the tubes clean (e.g., wipers)
may be required. If metals are to be removed by oxidation, solids should be removed by clarification or
filtration prior to UV oxidation. Provisions for removing precipitated metal sludges also may be necessary.
D-5
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Appendix D3: Chemical/UV Oxidation (continued)
Alternative Techniques/Enhanced Methods
UV radiation can be used in combination with a chemical oxidizing agent to increase the
effectiveness of oxidation, especially for difficult-to-oxidize compounds.
Metal catalysts, such as iron or copper, can be used in combination with a chemical oxidizing
agent to increase the effectiveness of oxidation for certain types of compounds.
Hydrodynamic cavitation is an innovative technology recently demonstrated under EPA's SITE
program that uses forced cavitation of gas to enhance destruction of organics during UV oxidation
processes.
Pre/Post-treatment
Pretreatment may be necessary to remove solids, microorganisms, calcium carbonate, iron
oxides, and/or other metals that can interfere with the oxidation process or UV transmission. A
pretreatment sequence of precipitation, flocculation, clarification, and/or filtration steps may be
necessary.
Post-treatment of the aqueous effluent with GAC may be necessary if destruction is not complete
or if toxic byproducts are formed during oxidation.
If toxic metals precipitate during the oxidation process, treatment and/or proper disposal of the
resulting sludge may be required.
Selected References
U.S. Environmental Protection Agency. 1990. CERCLA Site Discharges to POTWs Treatability Manual.
EPA/540/2-90/008. Office of Emergency and Remedial Response. PB91-921269/CCE. NTIS.
Springfield, VA. pp. 11-7 to 11-17.
U.S. Environmental Protection Agency. 1991. Engineering Bulletin: Chemical Oxidation Treatment.
EPA/540/2-91/025. Office of Emergency and Remedial Response, Washington, D.C. 8 pp.
U.S. Environmental Protection Agency. 1993. Superfund Innovative Technology Evaluation Program.
Technology Profiles. Sixth Edition. EPA/540/R-93/526. Office of Research and Development,
Washington, DC.
U.S. Navy. 1993. UV/Oxidation Treatment of Organics in Ground Water. NEESA Document Number
20.2-051.7. Navy Energy and Environment Support Activity, Port Hueneme, CA. 11 pp.
D-6
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Appendix D4: Aerobic Biological Reactors
Biological reactors use microorganisms to degrade organic contaminants in ground water in ex situ
reactors. There are two basic types of ex situ biological treatment processes: aerobic reactors and
anaerobic reactors. Aerobic reactors use oxygen to promote biodegradation and are widely used.
Anaerobic reactors degrade organics in the absence of oxygen. This guidance focuses on aerobic
biological treatment because anaerobic treatment processes are not widely used for ground-water
treatment.
Applicability
Aerobic biological reactors are applicable to a wide variety of halogenated and nonhalogenated volatile and
semivolatile organics. Aerobic biological reactors are potentially applicable to heavy organics, such as
PCBs and certain pesticides, and organic and inorganic cyanides, but are generally not as effective for such
recalcitrant compounds. Aerobic processes are not applicable to metals, asbestos, radioactive materials,
or corrosive or reactive chemicals (U.S. EPA, 1992).
Contaminant Fate
Organic compounds are decomposed to carbon dioxide and water (aerobic processes) or to methane and
carbon dioxide (anaerobic processes). Volatile organics are also removed by volatilization as a competing
mechanism. Microbial growth produces an excess organic sludge (biomass) that must be disposed of
properly. This sludge may concentrate metals and recalcitrant organic compounds that are resistant to
degradation. Biodegradation may produce decomposition byproducts that are emitted to the air or
dissolved in the effluent, and these decomposition byproducts may require additional treatment.
Design
Ex situ biological treatment of ground water is conducted in bioreactors. The primary factors influencing
bioreactor design are the microbial organic utilization rates and the peak organic loading rate (i.e., flow rate
times organic concentration). Treatability tests are necessary to determine these and other design
parameters. Under most circumstances, bioreactors require a significant startup time to acclimate the
microorganisms to the specific contaminants being treated before the bioreactor will operate at optimal
degradation rates. There are two general types of bioreactor design:
In suspended growth reactors, microbes are kept suspended in water using mechanical
aerators or diffused air systems. These aeration systems also keep the solution well mixed,
improving contact between microbes and dissolved contaminants and supplying oxygen to the
system. Activated sludge systems are the most common suspended growth bioreactors. Other
examples include aerated ponds or lagoons, stabilization ponds (using both algae and bacteria),
and sequencing batch reactors.
In attached growth reactors, biomass is attached to a solid substrate, such as sand, rock,
plastic, activated carbon, or resin. Reactor design is dependent upon the surface area of
substrate media available for biomass growth. Examples include trickling filter, rotating
biological contactor, fluidized bed, fixed bed, and roughing filter designs.
Alternative Techniques/Enhanced Methods
Direct addition of powdered activated carbon (PAC) into suspended growth bioreactors can both
improve removal efficiency and reduce the likelihood of process upsets by buffering the
concentrations of toxic compounds at levels amenable to biodegradation.
D-7
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Appendix D4: Aerobic Biological Reactors (continued)
Alternative Techniques/Enhanced Methods (continued)
Microbial augmentation (the addition of specially cultured microorganisms) may be used to
increase the system's removal efficiency for certain difficult-to-degrade contaminants.
Anaerobic reactors (digesters) may be preferred for the treatment of certain ground-wate r
contaminants (e.g., certain chlorinated organics) that are difficult to degrade aerobically .
However, anaerobic reactors have not been identified as a presumptive technology for
the following reasons: 1) anaerobic processes have not been widely used for ground-water
treatment; 2) reaction rates are slower than for aerobic processes, which result in longe r
startup times (for acclimation) and longer treatment times; and 3) such reactors have a
greater sensitivity to process upsets, especially where flow and contaminant concentrations
vary over time. These factors generally result in higher operation and maintenanc e
requirements and costs, and lower performance efficiencies than for aerobic processes i n
ground-water applications.
Pre/Post-treatment
Chemical precipitation (for metals) or other pretreatment (e.g., PAC addition for organics) may
be required to reduce (or buffer) concentrations of compounds that are toxic t o
microorganisms.
Carbon adsorption post-treatment may be used to reduce contaminant concentrations in the
treated water to meet discharge requirements.
Because certain aerated bioreactor designs (e.g., mechanically aerated activated sludg e
systems, aerated ponds and lagoons) present difficulties for direct capture and control of air
emissions, an air stripper (with emission controls) may be a cost-effective treatment prior to
biodegradation if volatile contaminant emissions need to be controlled. For other bioreactor
designs, such as diffu sed-aeration activated sludge and trickling filter systems, air emissions
are more easily captured and can be treated using carbon adsorption, catalytic oxidation, or
incineration.
Selected References
Eckenfelder, W.W., J. Patoczka, and A.T. Watkins. 1985. Wastewater treatment. Chemical
Engineering : 60-74.
Flatman, P.E., D.E. Jerger, and L.S. Bottomley. 1989. Remediation of contaminated groundwater
using biological techniques. Ground Water Monitoring Review: 105-119.
U.S. Environmental Protection Agency. 1979. Selected Biodegradation Techniques for Treatment
and/or Ultimate Disposal of Organic Materials . EPA-600/2-79-006. Office of Research an d
Development, Cincinnati, OH.
U.S. Environmental Protection Agency. 1981. Literature Study of the Biodegradability of Chemicals
in Water (Volume 1. Biodegradability Prediction, Advances in and Chemical Interferences wit h
Wastewater Treatment). EPA/R806699-01. Office of Research and Development, Cincinnati, OH.
U.S. Environmental Protection Agency. 1992. Engineering Bulletin: Rotating Biological Contactors .
EPA/540/S-92/007. Office of Research and Development, Cincinnati, OH. 8 pp.
D-8
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Appendix D5: Chemical Precipitation
Chemical precipitation chemically converts dissolved metal and/or other inorganic ions in ground water into
an insoluble form, or precipitate. Metal ions generally precipitate out as hydroxides, sulfides, or carbonates
and are removed as solids through clarification and filtration. In this guidance, chemical precipitation is
defined to include chemical precipitation of metals by oxidizing or reducing agents, as well as any pH
adjustment (neutralization) and solids removal steps required.
Applicability
Chemical precipitation is applicable to dissolved metal and other inorganic ions (such as arsenate and
phosphate). Chemical precipitation is not applicable to volatile or semivolatile organic compounds (U.S.
Navy, 1993).
Contaminant Fate
Dissolved metals are converted to insoluble forms, which are subsequently removed by flocculation,
clarification, and/or filtration. The solid residue (chemical sludge) containing the metal contaminant then
must be treated and/or disposed of properly.
Design
The process generally takes place at ambient temperatures. Batch reactors are generally favored for lower
flowrates (e.g., up to about 50,000 gpd), and usually use two tanks operating in parallel. Each tank can
act as a flow equalizer, reactor, and settler, thus eliminating separate equipment for these steps.
Continuous systems have a chemical feeder, flash mixer, flocculator, settling unit, filtration system (if used),
and control system for feed regulation. Site-specific treatability tests are required to determine the optimum
type and dosage of precipitation chemicals, necessary pretreatment steps, and post-treatment
requirements for aqueous effluent and sludge residuals.
There are three types of precipitation chemicals:
Metal hydroxides are formed by the addition of alkaline reagents (lime or sodium hydroxide).
Precipitation is then initiated by adjusting pH to the optimum level for the particular metal ion.
Maintaining pH levels within a relatively narrow optimum range is usually necessary to achieve
adequate metal precipitation. Pretreatment with oxidizing or reducing chemicals (e.g., hydrogen
peroxide, ferrous sulfate) may be necessary to precipitate some metals (e.g., iron, manganese,
chromium) in their least soluble form. Natural organic matter can inhibit the formation of
insoluble metal hydroxides by forming metal-organic complexes. Metal hydroxide precipitation
is typically effective for arsenic, cadmium, chromium (+3), nickel, zinc, manganese, copper (+2),
tin (+3), and iron (+3).
Metal sulfides are formed by the addition of either soluble sulfides (e.g., hydrogen sulfide,
sodium sulfide, or sodium bisulfide) insoluble sulfides (e.g., ferrous sulfide). Sodium sulfide and
sodium bisulfide are most commonly used. Sulfur dioxide and sulfur metabisulfite have also
been demonstrated for chromium reduction prior to precipitation. Metal sulfides have lower
solubilities than metal hydroxides, and effective metal removal efficiencies can be achieved over
a broader pH range. The method is mainly used to remove mercury and lead and may be used
to remove arsenic, cadmium, chromium (+3,or +6), silver and others. Sulfide precipitation also
can be used to treat filtered ground water after hydroxide precipitation.
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Appendix D5: Chemical Precipitation (continued)
Alternative Techniques/Enhanced Methods
Metal carbonates are formed by the addition of calcium carbonate or by adding carbon dioxide to
metal hydroxides. Solubilities of metal carbonates are intermediate between the solubilities of metal
hydroxides and metal sulfides. Insoluble metal carbonates are easily filtered from treated ground
water. The method is particularly good for precipitating lead, cadmium, and antimony.
Sodium xanthate has shown promise as a precipitation agent similar to sodium sulfide.
Pre/Post-treatment
Pretreatment to adjust pH is normally required to obtain the lowest precipitate solubility.
Pretreatment may be necessary to oxidize iron or manganese compounds or reduce hexavalent
chromium compounds into forms that can be readily precipitated.
Depending on discharge requirements, the aqueous effluent may need pH adjustment and/or
further polishing. Activated alumina or ion exchange media are regenerable treatment options for
effluent polishing for metals. Activated carbon also may be used but spent carbon may require
treatment and disposal as a hazardous waste.
The sludge may require stabilization treatment by addition of lime/fly ash or portland cement to
reduce permeability and the leachability of metals prior to disposal. In some cases, metals may
be recovered from the residue for reuse, but this is generally not economical.
Selected References
Monopoli, A.V. 1993. Removing dissolved inorganics from industrial wastewater. The National
Environmental Journal: 52-56.
U.S. Environmental Protection Agency. 1987. Handbook on Treatment of Hazardous Waste Leachate.
EPA/600/8-87/006. Office of Research and Development, Cincinnati, OH. pp. 44-45.
U.S. Environmental Protection Agency. 1990. CERCLA Site Discharges to POTWs Treatability Manual.
EPA/540/2-90/008. Office of Emergency and Remedial Response. PB91-921269/CCE. NTIS,
Springfield, VA. pp. 11-23 to 11-36.
U.S. Navy. 1993. Precipitation of Metals from Ground Water. NEESA Document Number 20.2-051.6.
Navy Energy and Environment Support Activity. Port Hueneme, CA. 11 pp.
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Appendix D6: Ion Exchange/Adsorption
Ion exchange removes metal contaminants from water by passing contaminated ground water through
a granular solid or other porous material, usually an impregnated resin, that exchanges sorbed ions
(e.g., H+, OH", Na+, Li+, C03") for contaminants dissolved in ground water. The ion exchange media
are selected to have sorptive affinity for the ionic forms (cation or anion) of the contaminants being
removed. The ion exchange media can therefore be either cationic, anionic, or a mixture of the two.
Because ion exchange is a reversible process, resins can be regenerated by backwashing with a
regeneration solution (e.g., brine; strong or weak acids or bases). Conventional ion exchange resins
are generally too costly for large-scale ground-water treatment and are predominantly used for
polishing of aqueous effluents after other treatment processes.
Applicability
Ion exchange is applicable to ionic contaminants such as dissolved metals or nitrates. Ion exchange is
not applicable to non-ionic contaminants such as most organic compounds.
Contaminant Fate
Contaminants are removed from ground water through sorption onto the exchange media. When most
of the exchange sites of the media become filled, the exchange media are regenerated by backflushing
with a suitable regeneration solution. The concentrated backflush solution must then be disposed of or
stripped of its contaminants. Exchange resins can generally be regenerated many times and have a
relatively long useful life.
Design
Various resin types are available to tailor systems to discrete ionic mixes. For example, acid
exchangers replace cations in water with hydrogen ions and base exchangers replace anions with
hydroxide ions. Weak acid and base exchangers are selective for more easily removed ions while
strong acid and base exchangers are less selective, removing most ions in the ground water.
Generally, ease of cation and anion removal follows an affinity sequence specific to the ions in
question. Synthetic resins are available with unique selectivity sequences. The wide variety of resins
and other ion exchange media (e.g., activated alumina, biological materials) that are available make the
selection of an appropriate exchange media a critical design step. Information on the applicability of
specific resins may be obtained from resin manufacturers. In addition, ion exchange resins generally
have an optimum pH range for effective metals removal. pH control may be required to achieve
maximum removal efficiency from ground water.
A typical ion exchange installation has two fixed beds of resin. While one is in operation, the other is
regenerated. Batch, fixed column, and continuous column bed designs can be used. Downflow
column designs are generally preferred. Continuous column systems eliminate the need for
backwashing but are not commonly used because of the complexity of the resin removal mechanics.
Flow rates up to 7,000 gpm have been reported for ion exchange systems. However, conventional ion
exchange is generally cost-effective for ground-water treatment only at low flow rates or low
contaminant concentrations. It is therefore primarily used as a polishing step following chemical
precipitation or other treatment.
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Appendix D6: Ion Exchange/Adsorption (continued)
Alternative Techniques/Enhanced Methods
Activated alumina is an anionic exchange medium comprised of granulated, dehydrated
aluminum hydroxide. Activated alumina is effective for removing fluoride, selenium, chromium
(+6), and arsenic ions, which are exchanged for hydroxide ions. Adjustment of pH may be
necessary to achieve optimal removal efficiency. The alumina is regenerated with a sodium
hydroxide solution.
Biological materials (e.g., algae, crop residues) have recently shown great promise as an
innovative ion exchange media for metals. Biological media are significantly less costly than
conventional resins (cents per pound vs. dollars per pound), and may become more commonly
used for metals removal from ground water.
Electrodialysis uses alternately placed cation and anion permeable membranes (made of ion
exchange resin) and an electrical potential to separate or concentrate ionic species.
Activated carbon adsorption can also be used to remove inorganics at low concentrations.
However, activated carbon is not identified as a presumptive technology for removal of
metals dissolved extracted ground water. Spent carbon used for metals removal can be
difficult to regenerate and may require treatment and/or disposal as a hazardous waste.
Pre/Post-treatment
Pretreatment may be required to remove suspended solids at concentrations greater than
about 25 mg/L or oil at concentrations greater than about 20 mg/L. Large organic molecules
also can clog resin pores and may need to be removed.
pH adjustment may be necessary to achieve optimal metals removal.
The backwash regeneration solution must be treated to remove contaminants.
Post-treatment of spent ion exchange media may be required to recover concentrated
contaminants or management as a hazardous waste may be required.
Selected References
Clifford, D., Subramonian, S., and Sorg, T.J., 1986. "Removing Dissolved Inorganic Contaminants
from Water," Environmental Science and Technology, Vol. 20, No. 11.
Nyer, E.K. 1985. Groundwater Treatment Technologies. Van Nostrand Reinhold. New York, NY.
187 pp.
U.S. Environmental Protection Agency. 1990. CERCLA Site Discharges to POTWs Treatability
Manual. EPA/540/2-90/008. Office of Emergency and Remedial Response. PB91-921269/CCE.
NTIS. Springfield, VA. pp. 11-102 to 11-112.
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Appendix D7: Electrochemical Methods
Electrochemical processes use direct electrical current applied between two immersed electrodes to
drive chemical oxidation-reduction reactions in an aqueous solution. Historically, electrochemical
processes have been used to purify crude metals or to recover precious metals from aqueous
solutions. Positively charged metal ions are attracted to the negatively charged electrode (the
cathode), where they are reduced. The reduced metals typically form a metallic deposit on the
cathode. Negatively charged ions are attracted to the positively charged electrode (the anode), where
they are oxidized.
For contaminated ground water treatment, electrochemical cells have been used for the reduction (and
subsequent precipitation) of hexavalent chromium to trivalent chromium. In this process, consumable
iron electrodes are used to produce ferrous ions (Fe2+) at the anode and hydroxide ions (OH ) at the
cathode. An oxidation-reduction reaction then occurs between the ferrous, chromium, and hydroxide
ions to produce ferric hydroxide Fe(OH)3 and chromic hydroxide Cr(OH)3, which subsequently
precipitate from solution.
Applicability
Electrochemical processes are applicable to dissolved metals. It is most commonly used in ground
water treatment for the reduction and precipitation of hexavalent chromium. The process also may be
applicable to removing other heavy metals including arsenic, cadmium, molybdenum, aluminum, zinc,
and copper ions. Electrochemical processes have also been used for the oxidation of cyanide wastes
(at concentrations up to 10 percent). Electrochemical processes are not applicable to organic
compounds or asbestos.
Contaminant Fate
Dissolved metals either deposit on the cathode or precipitate from solution. Precipitates form an
inorganic sludge that must be treated and/or disposed of, typically in a landfill. Spent acid solution,
which is used to periodically remove deposits formed on the electrodes, will also require proper
treatment and disposal. Cyanide ions are hydrolyzed at the anode to produce ammonia, urea, and
carbon dioxide.
Design
Electrochemical reactors generally operate at ambient temperatures and neutral pHs. Both batch
reactors and continuous flow reactors are commercially available. Atypical electrochemical cell for
hexavalent chromium reduction consists of a tank, consumable iron electrodes, and a direct current
electrical supply system. An acid solution is used to periodically clean the iron electrodes, which need
to be replaced when they are significantly consumed. Reactor residence times required for treatment
depend on the contaminants present as well as the degree of mixing and current density. Reduction of
hexavalent chromium generally requires short residence times (approximately 10 seconds), whereas
treatment of cyanide compounds requires longer process times.
Pre/Post-treatment
Pretreatment may be necessary to remove suspended solids.
Settling or clarification post-treatment may be necessary to remove the precipitated
trivalent chromic and ferric hydroxides formed during hexavalent chromium
electrochemical reduction.
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Appendix D7: Electrochemical Methods (continued)
Pre/Post-treatment (continued)
The sludge may require stabilization prior to disposal by addition of lime/fly ash or portland cement
to reduce permeability and metal leachability. In some cases, metals may be recovered from the
plated electrode or precipitated residue, but this is generally not economical for typical ground-
water applications.
Selected References
Englund, H.M. and L. F. Mafrica. 1987. Treatment Technologies for Hazardous Waste. APCA Reprint
Series RS-13. Air Pollution Control Association, Pittsburgh, PA. pp. 43-44.
U.S. Environmental Protection Agency. 1990. A Compendium of Technologies Used in the Treatment of
Hazardous Wastes. EPA/625/8-87/014. Office of Research and Development. PB91-90-274093. NTIS.
Springfield, VA. p. 23.
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Appendix D8: Aeration of Background Metals
Aeration (contact with air) removes some metals from water by promoting chemical oxidation and the
formation of insoluble hydroxides that precipitate from the water. Aeration for metals removal differs from
air stripping in that precipitation rather than volatilization is the desired effect of the technology.
Applicability
Aeration techniques are useful for the removal of limited number of dissolved cations and soluble metal
compounds. This method is well suited for the removal of background metals such as iron and manganese
which is necessary as part of a selected remedy such as pretreatment to air stripping. Methods of aeration
for metals include aeration tanks, aeration basins, or cascade aeration. Aeration methods are usually not
sufficient as an independent technology for iron and manganese, but are utilized as a step in the treatment
process. Often, the air-water contact in tank and cascade aeration is not enough to obtain high removal
efficiencies. Spray basins are limited by area, wind, and ice particle formation (Nyer, 1985).
Contaminant Fate
Dissolved metals are oxidized to insoluble hydroxides which precipitate from solution, and can then can be
subsequently removed by flocculation, sedimentation, and/or filtration.
Design
The three types of aeration systems:
Aeration tanks bubble compressed air through a tank of water.
Cascade aeration occurs when air is made by turbulent flow and agitation.
Spray or aeration basins use an earthen or concrete basin with a piping grid and spray nozzles that
spray the water into the air in very fine droplets.
Related methods include aeration used to remove volatile organic contaminants from water are considered
to be a type of air stripping, as discussed in Section 2.1.1. The use of aeration to promote aerobic
biological treatment processes is considered to be an element of biological treatment as discussed in
Section 2.1.4.
Pre/Post Treatment
Aeration is often a pretreatment for other remediation technologies, such as air stripping, to remove
certain metals.
Aeration can be followed by other treatments such as flocculation, sedimentation, and/or filtration to
remove oxidized metals.
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Appendix D8: Aeration of Background Metals
Selected References
Betz. 1962. Betz Handbook of Industrial Water Conditioning. Trevose, PA. pp.19-22
Nyer, E.K. 1985. Groundwater Treatment Technologies. Van Nostrand Reinhold, New York, NY.
187 pp.
Nyer, E.K. 1993. Practical Techniques for Groundwater and Soil Remediation . CRC Press, Inc,
Boca Raton, FL. 214 pp.
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