Directive 9283.1-12
EPA 540/R-96/023
PB96-963508
October 1996
Pre-Publication Copy
FINAL GUIDANCE:
PRESUMPTIVE RESPONSE STRATEGY AND EX-SITU TREATMENT
TECHNOLOGIES FOR CONTAMINATED GROUND WATER
AT CERCLA SITES
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
Washington, DC 20460
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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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TABLE OF CONTENTS
PREFACE vii
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 7
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 10
2.3 Post-Construction Refinements 10
2.3.1 Types of Refinements 12
2.3.2 Documenting Refinements 12
2.4 Integrating Response Actions 12
2.4.1 Integrating Source Control and Ground-Water Actions 13
2.4.2 Combining Ground-Water Restoration Methods 13
2.5 Strategy for DNAPL Sites 14
2.5.1 Site Characterization 14
2.5.2 Early Actions 14
2.5.3 Long-Term Remedy 15
2.6 Areas of Flexibility in Cleanup Approach 15
2.6.1 Beneficial Uses and ARARs 16
2.6.2 Remediation Timeframe 17
2.6.3 Technical Impracticability 17
2.6.4 Point of Compliance 18
2.6.5 Natural Attenuation 18
2.6.6 Alternate Concentration Limits 19
3.0 PRESUMPTIVE TECHNOLOGIES 20
3.1 Presumptive Technologies for Ex-Situ Treatment 20
3.1.1 Design Styles within Presumptive Technologies.20
3.1.2 Benefits of Presumptive Technologies 21
3.1.3 Consideration of Innovative Technologies 21
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3.2 Basis for Presumptive Technologies 22
3.2.1 Sources of Information 22
3.2.2 Rationale for Indentifying Presumptive Technologies 22
3.3 Remedy Selection Using Presumptive Technologies 23
3.3.1 Use of Technologies in Treatment Systems 23
3.3.2 This Guidance Constitutes the FS Screening Step 24
3.3.3 Deferral of Final Technology Selection to RD 24
3.4 Information Needed for Selecting Technologies 25
3.4.1 When Should this Information be Collected? 25
3.4.2 Extraction Flow Rate 26
3.4.3 Discharge Options and ARARs 27
3.4.4 Water Quality of Treatment Influent 27
3.4.5 Treatability Studies 28
3.5 Treatment Technologies for Aquifer Tests 28
3.5.1 Treatment Needs during Aquifer Tests 28
3.5.2 Treatment Technologies for Aquifer Tests 28
4.0. REFERENCES 29
LIST OF FIGURES
Figure 1: Examples of Factors Affecting Ground-Water Restoration Potential 4
Figure 2: Phased Ground-Water Actions: Early Action Followed by Long-Term Remedy 9
Figure 3: Phased Ground-Water Actions: Long-Term Remedy Implemented in Phases 11
LIST OF HIGHLIGHTS
Highlight 1: Presumptive Response Strategy 6
Highlight 2: Early Actions that Should be Considered 7
Highlight 3: Remedy Refinements for Extraction/Treatment Remedies 12
Highlight 4: Presumptive Technologies for Treatment of Extracted Ground Water 21
Highlight 5: Summary of Site Information Needed for Treatment Train Design 26
IV
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LIST OF APPENDICES
APPENDIX
A:
Additional Background Information
Al:
Background on DNAPL Contamination
A2:
Contaminants Most Frequently Reported in Ground Water at CERCLA NPL Sites
A3:
Examples of In-Situ Treatment Technologies
A4:
Definition and Discussion of Pulsed Pumping
APPENDIX
B:
ROD Language Examples For Selected Remedy
Bl:
Phased Implementation of Ground-Water Remedy
B2:
Phased Implementation of Extraction Component of Remedy at a DNAPL Site
B3:
Deferring Selection of Treatment Components to Remedial Design
B4:
Suggested ROD Language from 1990 OSWER Directive
APPENDIX
C:
Ex-Situ Treatment Technologies for Ground Water
CI:
Ex-Situ Technologies Considered in Sample of 25 Sites
C2:
Other Components Needed for Treatment Trains
C3:
Information Needed for Selection of Technologies and Design of Treatment Train
C4:
Advantages and Limitations of Presumptive Treatment Technologies
APPENDIX
D:
Descriptions of Presumptive Treatment Technologies
Dl:
Air Stripping
D2:
Granular Activated Carbon
D3:
Chemical/UV Oxidation
D4:
Aerobic Biological Reactors
D5:
Chemical Precipitation
D6:
Ion Exchange/Adsorption
D7:
Electrochemical Methods
D8:
Aeration of Background Metals
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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
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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, 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 ง300.430(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
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for all sites with contaminated
ground water;
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; ง300.430
(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
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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 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
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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
Biotic/Aboitic Decay
High
Low
Potential
Volatility
High
Low
Contaminant
Low
~ High
Retardation (Sorption)
Potential
E
o
sz
U
Contaminant Phase
Volume of
Contaminated Media
Contaminant Depth
Aqueous, Gaseous
-Sorbed
ฆ LNAPLs
Small
Shallow '
-DNAPLs
" Large
Deep
Hydrogeologic
Characteristics
Stratigraphy
Simple Geology,
e.g., Planar Bedding
Strata
Complex Geology,
e.g., Interbedded and Discontinuous
Texture of
Unconsolidated Deposits
Sand
Clay
Degree of Heterogeneity
Homoaeneous
Heteroaeneous e.a.. interbedded sand and
e.g., well-sorted sand
silts, clays, fractured media, karst
Hydraulic Conductivity
of Aquifer
High (>10"2cm/sec)
~ Low (< 104 cm/sec)
Temporal Variation
of Flow Regime
Little/None
~ High
Vertical Flow
Little
Component
^ Large Downward Flow
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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.
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).
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Highlight 1. Presumptive Response
Strategy
For sites with contaminated ground
water, site characterization
should be coordinated with
response actionsand 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).
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 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
Response (OSWER), including EPA, 1989a,
1990, 1992b, and 1993b. These directives
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;
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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 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
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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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 potential should
be assessed prior to establishing objectives
for the long-term remedy (see Section 1.3.3).
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
8
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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 objectives (e.g., restoring ground-water) over all or portions of the plume.
)
Decision
Documents
Remedy
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 Action,
Design & Construct Action
Continue Site Characterization as
Required
Evaluate Alternatives,
Select Remedy & Likely Refinements,
Design & Construct Remedy
9
-------
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 in one ROD. A second decision
document could still be required if evaluation
of the first phase 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
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 data are sufficient to determine that the likelihood
of attaining long-term objectives is relatively high.
Decision
Documents
Remedy
Phase
Remedy Selection/ Implementation Steps
ROD
Phase I
Are
Data Sufficient to
Determine Likelihood of
Attaining Long-Term Objectives
(e.g., Ground-Water
^\Resto ration)?/'^
No
Yes
Are Long-Term
Objectives Attainable?
No-
ROD
Amendment
orESD
Yes
Memo to
Admin.
Record
orESD
Phase
Are Refinements Needed?
Yes-
Remedy
Refinement
No
Monitor Remedy Until
Objectives Attained
Implement Changes
Design & Construct Phase I
Modify Long-Term
Objectives
Evaluate Alternatives
Select Remedy
Select & Implement
Refinements
Design & Construct Phase
Monitor Remedy & Evaluate
Performance
Monitor Phase I & Evaluate
Performance
Complete Remedial Investigation
Determine Long-Term Objectives
for Different Portions of Plume
Evaluate Alternatives
Select Remedy & Likely
Refinements
Determine Phases I & II
11
-------
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
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.
Sven if an ESD is not
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.
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 andPost-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
12
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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 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
13
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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, 1995f and EPA, 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.
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);
14
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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; ง300.430 (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);
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
15
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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 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
16
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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; ง300.430(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 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; ง300.430
(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 ง121(d)(4)(C)).
The types of data used to make such a
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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, 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
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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
ง121 (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).
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 ง300.430(e)(9)(iii)
of the NCP (Federal Register, 1990a.) This is
distinct from a finding of "technical
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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.)
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 treatment process, which
may be accomplished by one or more design
styles. For example, the air stripping process
separates dissolved organic chemicals from
water by volatilizing them to an air stream,
which can be achieved using packed towers,
aeration trays or other design styles.
Innovative design styles are intended to be
included within the presumptive technologies
listed in Highlight 4, as long as the treatment
process falls within one of these technology
types (e.g., innovative air stripper designs, or
innovative media for ion exchange/adsorption
of
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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.
metals). A listing of design styles of the
presumptive technologies typically considered
during Superfund remedy selection are listed in
Appendix C1.
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, 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
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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.
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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 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;
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4. Separation of solids generated during
treatment;
5. Final treatment of dissolved
contaminants prior to discharge
(polishing); and
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
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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 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
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Highlight5. 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.
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 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
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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).
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
27
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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
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
28
-------
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 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.
29
-------
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.
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.
30
-------
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.
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
31
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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.
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APPENDIX A
Additional Background Information
Appendix A1: 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
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Appendix Al: 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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Insert Figures Al-1 and A1-2 (one page)
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, February 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, January 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 Sites1
Organic Contaminants:
Chemical2 Halo-2 No.1
Rank Organic Contaminants (Other Names) Group genated? DNAPL?3 Sites
1
Trichloroethylene, 1,1,2- (TCEf
Volatile
Yes
Yes
336
2
Tetrachloroethene (perchloroethene; PCEJS
Volatile
Yes
Yes
170
3
Chloroform (trichloromethane)3
Volatile
Yes
Yes
167
4
Benzenepr
Volatile
No
No
164
5
Toluenepr
Volatile
No
No
159
6
Trichloroethane, 1,1,1- (methyl chloroform; 1,1,1-TCA)S
Volatile
Yes
Yes
155
7
Polychlorinated biphenyls
PCB
Yes
Yes
139
8
Trans-Dichloroethylene, 1,2- (trans-l,2-DCEJs
Volatile
Yes
Yes
107
9
Dichloroethane, 1,1- (1,1-DCAf
Volatile
Yes
Yes
105
10
Dichloroethene, 1,1- (vinylidene chloride; 1,1-DCE)S
Volatile
Yes
Yes
95
11
Vinyl chloride (chloroethylene)3
Volatile
Yes
No
82
12
Xylenepr
Volatile
No
No
76
13
Ethylbenzenepr
Volatile
No
No
68
14
Carbon tetrachloride (tetrachloromethaneJs
Volatile
Yes
Yes
68
15
Phenol
Semivol.
No
No
61
16
Methylene chloride (dichloromethaneJs
Volatile
Yes
Yes
58
17
Dichloroethane, 1,2- (ethylene dichloride; 1,2-DCA)S
Volatile
Yes
Yes
57
18
Pentachlorophenol (PCP)
Semivol.
Yes
Yes
53
19
Chlorobenzene (benzene chlorideJs
Volatile
Yes
Yes
48
20
Benzo(A)Pyrene
Semivol.
No
Yes
37
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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:
1 Number of CERCLA National Priorities List (NPL) sites for which the chemical was reported inground 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).
2 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.
3 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).
4 In pure form mercury is also a DNAPL.
cs These organic contaminants are chlorinated solvents. A total of 12 are listed.
pf These organic contaminants are constituents ofpetroleum fuels. A total of four are listed.
A-5
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Appendix A3: Examples of In-Situ Treatment Technologies1
I. Enhanced Recovery Methods
Recirculation/flooding:
Water flooding
(physical)
Steam flooding
(physical)
Chemical flooding2
(chemical)
Nutrient flooding2
(biological)
Thermal enhanced reovery:
Radio frequency
Electrical resistance
(AC or DC)
Enhancement of secondary permeability:
Induced fracturing with water or
or air pressure (physical)
Other methods:
Electromigration (electrical)
Treatment Agents
(and process type)
Water
Heated water
- Steam
Surfactants
Solvents
Redox agents
Nitrate
Other
- Heat
- Heat
Not applicable
- Electric current
Agent Delivery Methods
- Injection wells
- Injection wells
- Injection wells
Injection wells
Injection wells
Injection wells
Injection wells
Electrodes in wells
Electrodes in wells
Not applicable
- 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
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
Treatment Agents
- Air
Iron filings
Other agents
Hydrogen peroxide
Oxy gen / surf actant
(microbubbles)
Nitrate
Other
Methane
Other
Methane and/or
Oxygen
Agent Delivery Methods
- Injection wells
- Permeable walls/gates3
- Permeable walls/gates3
- Injection wells4
- Injection wells4
- Injection wells3
- Injection wells
- Injection wells
NOTES:
3 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.
4 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 ofphase one is
completed, the extraction system will be carefully monitored on a regular basis and its
performance evaluated. Operation and monitoring of phase 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 of phase
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 20 years,
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Appendix Bl. Phased Implementation of Ground-Water Remedy (continued)
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 ofphase 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 of phase 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
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Appendix B2. Phased Implementation of Extraction Component of Remedy at a DNAPL Site
(continued)
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. 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 9283.1-12 from 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.
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Appendix B3. Deferring Selection of Treatment Components to Remedial Design (continued)
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.
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
ifpossible, 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:
Appendix C2:
Appendix C3:
Appendix C4:
Ex-Situ Technologies Considered in Sample of 25 Sites
Other Components Needed for Treatment Trains
Information Needed for Selection of Technologies and Design
of Treatment Train
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 Trains1
Solid or Liquid Separation
Technologies
Oil/grease separation4
F iltration5
Coagulation5
(or flocculation)
Clarification5
(or sedimentation)
Effluent Polishing Technologies2
Activated carbon
Ion exchange
Neutralization
Vapor Phase Treatment
Technologies3
Activated carbon
Resin adsorption
Catalytic oxidation
Thermal incineration
Acid gas scrubbing
Condensation
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
NOTES:
i
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.
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.
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.).
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.
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
(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
of Technologies and Design of Treatment Train
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 treatmentof 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 testsinstead 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 othertechnologies (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.
C-9
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Appendix C4 (Continued)
Technology
Advantages
Limitations
Chemical/ UV Where oxidation is complete, organic contaminants are
Oxidation destroyed and not transferred to
other media; minimal residuals generated.
Effective on a wide variety of volatile and semivolatile
organics, including chlorinated organics, as well as
cyanide and some metals.
Operating costs can be competitive with air stripping and
activated carbon.
Incomplete oxidation will leave original contaminants and possibly toxic oxidation products;
activated carbon polishing may be required.
Capital costs may preclude small-scale applications, especially for ozone systems.
Metals may precipitate during oxidation, requiring filtration post-treatment and residuals
disposal.
UV light sources are subject to fouling and scaling from solids, iron compounds, carbonates,
etc. Pretreatment may be required to remove these substances.
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 Organic contaminants degraded, often with minimal
Biological cross-media environmental impacts.
Reactors Proven effective for many organic compounds.
Some systems (e.g., trickling filters and rotating
biological contactors) have minimal energy requirements
and generally low capital and operating costs.
Can be designed to require a minimum of operator
attention.
Relatively simple, readily available equipment.
Trained contractors available to implement the
technology.
A residual organic sludge is generated that must be disposed of properly.
Some compounds are difficult or impossible to degrade (recalcitrant) or slow to degrade.
Difficulties acclimating microorganisms to contaminants are possible; requires longer startup
time than other technologies to achieve effective steady-state performance
Volatile organics may require air emission controls or pretreatment to remove them.
Variations in flow or concentration may require significant operator attention to prevent
microorganisms from being killed.
Cold weather can cause operational difficulties.
Treatability studies are needed for selection and design.
Pretreatment may be needed to remove contaminants toxic to the microorganisms, such as
heavy metals.
Low organic loading and the potential for supplementary nutrients and food sources must be
considered.
C-10
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Technology
Advantages
Limitations
Treatment Technologies for
the Removal of Inorganic Contaminants
Chemical
Most commonly used method for
A residual sludge is generated that must be treated and/or disposed of properly; metals are
Precipitation
removing soluble heavy metal
not usually easy to recover from sludge.
ions from contaminated water.
Up to four times stoichiometric chemical additions may be required, especially for sulfide
Pretreatment for solids and iron
precipitation (see below).
generally not required.
Hvdroxide Precipitation
Hvdroxide Precipitation
Organics or complexing ions may form chelates/complexes instead of insoluble metal
Reliable method, chemicals
hydroxides.
relatively easy to handle, and not
Optimum pH is different for each metal hydroxide, one pH may not effectively treat all
costly.
soluble metal ions; successive treatments may be required.
Carbonate Precipitation
pH must be controlled within a narrow range.
Reliable method, calcium
Naturally occurring sulfate in ground water may react with lime to form gypsum, which
carbonate easy to handle, and not
increases sludge, can clog filters, and can coat pipelines (caustic soda addition can reduce
costly.
this problem but increases costs and dissolved solids [sodium salts] that must be removed
Effectively removes a variety of
from treated ground water).
soluble metals.
Carbonate Precipitation
Sulfide Precipitation
Calcium carbonate is not effective for ground water with high alkaline content.
Reliable method.
Pretreatment to remove organic, chelating, or oil and grease contaminants may be required.
High removal efficiency over a
Sulfide Precipitation ("Soluble Sulfide)
broader pH range.
Excess sulfide ions that are not precipitated remain in solution. They may be removed by
Relatively insensitive to most
using aeration to convert them from ionic to oxide form (sulfate).
chelating agents.
pH control between 8 and 9.5 is required to avoid release of hydrogen sulfide gas.
Can remove chromates and
Cost is high compared to hydroxide and carbonate precipitation
dichromates without reducing
Sulfide Precipitation (Insoluble Sulfides)
hexavalent chromium to trivalent
Ferrous sulfide is used in amounts greater than that required by stoichiometric
form if ferrous ions are present or
considerations.
added.
Produces more sludge than soluble sulfide or hydroxide processes.
C-ll
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Technology
Advantages
Limitations
Treatment Technologies for the Removal of Inorganic Contaminants (continued):
Ion Exchange/
Adsorption
High removal efficiencies for
heavy metals.
Suitable for use as a polishins
step after other technologies.
Technology is reasonably well
understood.
On-site backflushing of exchange
media allows immediate reuse.
Resins are usually costly and may not be cost-effective for large treatment loadings.
Generates large volume of backflush solution (approximately 2.5 to 5% of the original
ground-water flow rate) that is concentrated in the metals removed and requires treatment or
disposal.
Requires bench-scale testing to determine operational requirements and suitability of
prospective resins.
Beds can be fouled by particulate matter, oxidizing agents, oils, greases, biological growths,
and intra-bed precipitates; therefore, pretreatment may be needed.
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 cyanide
Hexavalent chromium reduction generates a heavy metal precipitate that must be removed
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:
1 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.
C-12
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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 D. Description of Presumptive Treatment Technologies
D1. Air Stripping
Air stripping uses volatilization to transfer contaminants from ground water to air. In general, waters
contacted with an air stream to volatilize dissolved contaminants into the air stream. Stripping ofa
specific chemical depends on the equilibrium vapor pressureof 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 moฎ
difficult to air strip. Air stripping is potentially applicable to certain halogenated semi-volatile organc
compounds (SVOCs). It is not applicable to nonhalogenated SVOCs; heavy organics such as PCBง
dioxins/furans and pesticides; or inorganic metal compounds (U.S. EPA, 1991).
Air stripping is most effective for contaminants witha dimensionless (molar volume) Henry's law constant
greater than 0.01 (or 2.4 x 104 atm-m3/gmol at 25ฐ 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-wate
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, tla
contaminated air stream may need further treatment. Additional polishing treatment of the aqueos
effluent also may be necessary, depending on discharge requirements.
Design
Air strippers are designed for a ^Decific target chemical (either the predominant contaminant or the most
difficult-to-strip contaminant) with a desired target removalefficiency. 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 taver equipped with an air blower. The ground water
is fed into the top of the stripper and theair is introduced at the bottom, creating a countercurrent gas-
liquid contact. Random plastic packing is frequently used to improve gas-liquid contact. Structurel
packing and steel packing may also be used. Packed-tower air stripper design involves specification of
stripper column diameter and packing heght 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 wate
primarily used to cool water using countercurrent ambient air flow) may provide a cost-effective
alternative to conventional packed towers.
Shallowtray air strippers or dffused tank aeration units are less susceptible to fouling problems
than packed towers and may be preferable where the water to be treated contains hitj
concentrations of certain inorganics (e.g., iron).
D-l
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Appendix D. Description of Presumptive Treatment Technologies
D1. Air Stripping (continued)
Alternative Techniques/Enhanced Methods (continued )
Because the efficiency of air stripping increases at higher temperatures, increasing the influeti
ground-water temperature (typically about 55ฐ 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 ofsteam. In this guidance, these methods are not considered a type of air
stripping and are not identfied as a presumptive technology for ex-situ treatment of ground water.
Pre/Post-treatment
Pretreatmentto remove iron and other metals and to contol 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 stripperdi
further reduce organic contaminant levels and meet discharge requirements.
Contaminants in the air discharge may be reduced by activated carbon adsorption, catalyti
oxidation, or incineration to meet air emission requirements.
Selected References
Lamarre, B. 1993. Selectingan 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., Boa
Raton, FL. 214 pp.
Okoniewski, B.A. 1992. Remove VOCs from wastewater by ar 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/45D/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 D. Description of Presumptive Treatment Technologies
D2. Granular Activated Carbon (continued)
Activated carbon removes contaminants fran ground water by adsorption. The adsorption process takes
place in three steps: (1) contaminant migration to the external sorbent surface; (2) diffusion into tie
sorbent pore structure; and (3) adsorption onto the sorbent surface. The principal form of activate!
carbon used for ground-water treatment is granular acivated carbon (GAC). GAC is an excellent sorbent
due to its large surface area, which generally ranges from 500 to 2,000 riVg.
Applicability
GAC is applicable to a wide variety of contaminants including: halogenated volatile and semivolatft
organics, nonhalogenated volatile and semivolatile organics, PCBs, pesticides, dioxins/furans, mob
organic corrosives, metals, radioactive materials, inorganic cyanides, and certain oxidizers. GACa
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 emissionsfrom 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 tie
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 spentcarbon 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 6
contaminated ground water but can be used for spent G\C from air emission control devices. GAC used
for metals sorption may require disposal. If disposed of, spent GAC may have to be managed asa
hazardous waste.
Design
Activated carbon is a well-developed, widely used technology with many successful ground-wate
treatment applications, especiallyfor 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 flowconfiguration 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 6
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 carbors'
adsorption capacity for these constituents, and the carbon reactivation (or regeneration) frequency
Depending on the ground-water suspended sdids 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 pjlsed bed (carbon beds operated with nearly
continuous "pulsed" addition of fresh carbon and withdrawal of spent carbon) designs can le
used if higher removal efficiencies are required.
D-3
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Appendix D. Description of Presumptive Treatment Technologies
D2. Granular Activated Carbon (continued)
Alternative Techniques/Enhanced Methods (continued)
Because the adsorption capacity of GAC is much higher for gฎ phase treatment than for liquid
phase treatment, it is often more economical to usean 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^
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 humi
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 GfC
during treatment, which could result in potential exposure hazards for operatjg
personnel and the spent carbon may require treatient and/or disposal as hazardous
waste.
Thermal reactivation, using heatalone 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 reactivatia
process.
Selected References
Long, G.M. 1993. Clean up hydrocarbon contamination effectively. Chemical Engineering Progress,
89(5):58-67.
Stover, E.L. 1988. Treatrrent 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. Ofice of Emergency and Remedial Response, Washington, D.C. 8 pp.
D-4
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Appendix D. Description of Presumptive Treatment Technologies
D3. Chemical/UV Oxidation
Chemical oxidation uses chemical oxidizing agents to destroy toxic organic chemicals and cyanid
compounds (CN) in groundwater. Commoriy 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 toxi
byproducts (e.g., HCI, chlorinated organics). Utraviolet light (UV) is often used in conjunction with ozone
and/or hydrogen peroxide to promote faster and more complete destruction of organic compound
(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 cyanic
compounds. Chemical oxidaticn is potentially applicable to PCBs, dioxins/furans, and metals (oxidation
can be used to precipitate metals under certain conditions). Chemical oxidation isnot 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 fo
certain compounds at concentrations ranging up to several thousand mg/L. UV can enhance tie
oxidation of compounds that are resistant to chemical oxidation alone (e.g., PCBs). Iron or coppe
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 chlorinate!
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, o
intermediate degradation products can be formed that may be toxic. These toxic substances may b
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 d
introduction into the reaction vessel (reactor). Tie 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 storaje 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. Tb
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 drasticajl
reduced.
Site-specific treatability studies are generally recommended for chemical oxidation systems. Extensive
pretreatment may be required to condition ground waterfor effective oxidation. If UV lamps are used, the
studies must evaluate the potential for fouling or scaling of the quartz tubes at the ground-wate
composition, oxidant concentration, and UV intensity conditions anticipated for long-term systen
operation. If fouling or scaling is likely, pretreatnent 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 forremoving precipitated metal sludges also may
be necessary.
D-5
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Appendix D. Description of Presumptive Treatment Technologies
D3. Chemical/UV Oxidation (continued)
Alternative Techniques/Enhanced Methods
UV radiation can be used in combination with a chemical oxidizing agent to increase tie
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.
Hydrodynamiccavitation is an innovative technology recently demonstrated under EPA's SITE
program that uses forced cavitation of gas to enhance destruction of organics during lil/
oxidation processes.
Pre/Post-treatment
Pretreatment may be necessary to remove solids, microorganisms, calcium carbonate, ira
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 D. Description of Presumptive Treatment Technologies
D4. Aerobic Biological Reactors
Biological reactors use microorganisms to degrade organic contaminants in ground water in ex sii
reactors. There are two basic types of ex situ biological treatment processes: aerobic reactors ad
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 aerobri
biological treatment because anaerobic treatment processes are not widely used for ground-wate
treatment.
Applicability
Aerobic biological reactors are applicable to a wide variety of halogenated and nonhalogenated volatile
and semivolatile organics. Aeobic 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 asa
competing mechanism. Microbial growth produces an excess organic sludge (biomass) that must b
disposed of properly. This sludge may concentrate metals and recalcitrant organic compounds that are
resistant to degradation. Biodegadation 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 wateris 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 mechanic^
aerators or diffused air systems. These aeration systems also keep the solution well mixed,
improving contact between microbes and dissolvedcontaminants 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 6
substrate media available for biomass growth. Examples include trickling filter, rotatiig
biological contactor, fluidized bed, fixed bed, and roughing filter designs.
Alternative Techniques/Enhanced Methods
Direct addition of powdered activated carbcn (PAC) into suspended growth bioreactors can both
improve removal efficiency and reduce the likelihood of process upsets by buffering tie
concentrations of toxic compounds at levels amenable to biodegradation.
D-7
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Appendix D. Description of Presumptive Treatment Technologies
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-water
contaminants (e.g., certain chlorinated organics) that are difficult to degrade aerobically.
However, anaerobe 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 i n
longer 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
maintenance requirements and costs, and lower performance efficiencies than for aerobic
processes in 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 to
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 diffused-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 and
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 D. Description of Presumptive Treatment Technologies
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, d
carbonates and are removed as solids through clarification and filtration. In this guidance, chemicb
precipitation is defined to include chemical precipifetion 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 precipitaton 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 flocculatiop
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 fD
lower flowrates (e.g., up to about 50,000 gpd), aid 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 (i
used), and control system for feed regulation. Sfe-specific treatability tests are required to determine the
optimum type and dosageof precipitation chemicals, necessary pretreatment steps, and post-treatment
requirements for aqueous effluent and sludge residuals.
There are three types of precipitation chemicals:
Metal hydroxidesare formed by the addition of elkaline 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 optimumrange 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 tie
formation of insoluble metal hydroxides by formingmetal-organic complexes. Metal hydroxide
precipitation is typically effective for arsenic, cadmium, chromium (+3), nickel, zing
manganese, copper (+2), tin (+3), and iron (+3).
Metal sulfides are formed by the addition of either solublesulfides (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 haซ
lower solubilities than metal hydroxides, and effective metal removal efficiencies can Is
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 hydroxi<4
precipitation.
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Appendix D. Description of Presumptive Treatment Technologies
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 metalcarbonates are intermediate between the solubilities of metal
hydroxides and metal sulfides. Insoluble metal carbonates are easily filtered from treated groud
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 oxidizeiron 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/o
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 m$r
require treatment and disposal as a hazardous waste.
The sludge may require stabilization treatment by addition of lime/fly ash or portland cement to
reduce permeability and the leachabilty of metals prior to disposal. In some cases, metals may
be recovered from the residue for reuse, but this is generally not economical.
Selected References
Monopoli, A.V. 1993. Removing dissolved inorganics from industrial wastewater. The National
Environmental Journal: 52-56.
U.S. Environmental Protection Agency. 1987. Handbook on Treatment of Hazardous Waste Leachate.
EPA/600/8-87/006. Office of Research and Development, Cincinnati, OH. pp. 44-45.
U.S. Environmental Protection Agency.1990. CERCLA Site Discharges to POTWs Treatability Manual.
EPA/540/2-90/008. Office of Emergency and Remedial Response. PB91-921269/CCE. NTI$
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. 11pp.
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Appendix D. Description of Presumptive Treatment Technologies
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., IT, 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.
Atypical 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 D. Description of Presumptive Treatment Technologies
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 waterSpent 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 D. Description of Presumptive Treatment Technologies
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 (Fฃ+) 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) and chromic hydroxide Cr(OH^, which
subsequently precipitate from solution.
Applicability
Electrochemical processes areapplicable 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 arenot 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 D. Description of Presumptive Treatment Technologies
D7. Electrochemical Methods (continued)
Pre/Post-treatment (continued)
The sludge may require stabilization prior to disposal by addition of lime/fly ash or portlad
cement to reduce permeability and metal leadnability. 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 D. Description of Presumptive Treatment Technologies
D8. Aeration of Background Metals
Aeration (contact with air) removes some metals from water by promoting chemical oxidation and tb
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 aid
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. Aeratio
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 nb
enough to obtain high removal efficiencies. Spray basins are limited by area, wind, and ice particri
formation (Nyer, 1985).
Contaminant Fate
Dissolved metals are oridized 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 basinsuse 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 ae
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 a
discussed in Section 2.1.4.
Pre/Post Treatment
Aeration is often a pretreatrrent for other remediation technologies, such as air stripping, to remove
certain metals.
Aeration can be followed by otha" treatments such as flocculation, sedimentation, and/or filtration to
remove oxidized metals.
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Appendix D. Description of Presumptive Treatment Technologies
D8. Aeration of Background Metals (continued)
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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