EPA 600/R-12/712 I December 2012 I www.epa.gov/ada
United States
Environmental Protection
Agency
Framework for Site Characterization
for Monitored Natural Attenuation
of Volatile Organic Compounds in
Ground Water
Office of Research and Development
National Risk Management Research Laboratory, Ada, Oklahi
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EPA/600/R-12/712
December 2012
for
for
of in
by
Bruce E. Pivetz
Shaw Environmental & Infrastructure, Inc.
EP-C-08-034
David Abshire
t/.S. EPA Region 6
Dallas, JX
William Brandon and Stephen Mangion
U.S. EPA Region 1
Boston, MA
Brad Roberts
U.S. EPA Region 7
Kansas City, KS
Bruce Stuart
Missouri Department of Natural Resources
Luanne Vanderpool
U.S. EPA Region 5
Chicago, IL
Barbara Wilson
Dynamac Corporation
Steven D. Acree
Ground Water and Ecosystems Restoration Division
National Risk Management Research Laboratory
Ada, OK
Ground Water and Ecosystems Restoration Division
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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NOTICE
The U.S. Environmental Protection Agency through its Office of Research and Development
funded and managed the research described here under contract to Shaw Environmental &
Infrastructure, Inc. (EP-C-08-034). It has been subjected to the Agency's peer and administrative
review and has been approved for publication as an EPA document. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.
Nothing in this document changes Agency policy regarding remedial selection criteria, remedial
expectations, or the selection and implementation of MNA. This document does not supersede
any previous guidance and is intended for use in conjunction with other documents, including
the OSWER Directive 9200.4-17P, Use of Monitored Natural Attenuation at Superfimd, RCRA
Corrective Action, and Underground Storage Tank Sites (U.S. EPA, 1999).
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CONTENTS
NOTICE ii
LIST OF FIGURES v
LIST OF TABLES v
ACKNOWLEDGMENTS vii
ABSTRACT ix
1.0 INTRODUCTION 1
1.1 Purpose 1
1.2 Scope and Limitations 2
1.2.1 Contaminants 2
1.2.2 Nonaqueous Phase Liquids 3
1.2.3 Geologic Setting 3
2.0 MNA SITE CHARACTERIZATION OBJECTIVES AND VARIABLES 4
2.1 Introduction 4
2.2 MNA Site Characterization Objectives 5
2.3 Conceptual Site Model 7
2.3.1 Systems Engineering Approach 8
2.3.2 Representative Zones 10
2.4 MNA Site Characterization Variables - Introduction 11
2.4.1 Site Characterization and Professional Judgement 14
2.5 Geological Variables 14
2.6 Hydrogeological Variables 16
2.6.1 Flow Paths 18
2.6.2 Background Ground Water 19
2.7 Anthropogenic Variables and Receptors 20
2.7.1 Receptor Identification and Vulnerability 21
2.8 Contaminant Variables 21
2.8.1 Source Area Contamination 23
2.8.2 Source Control History 26
2.8.3 Transformation Products and Byproducts 26
2.8.4 Tracers 27
2.9 Geochemical Variables 27
2.10 Biological Variables 29
2.10.1 Direct Approaches to Evaluation of Biodegradation 34
2.10.1.1 Microbiological and Molecular Techniques 34
2.10.1.2 Stable Isotope Evaluation 35
2.10.1.3 Microcosm Studies 35
3.0 THE MNA SITE CHARACTERIZATION PROCESS 37
3.1 Introduction 37
3.2 MNA Site Characterization Activities 38
3.2.1 Site Characterization Methods 43
3.2.2 Site Characterization Locations 43
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4.0 INTEGRATING AND EVALUATING SITE DATA 49
4.1 Introduction 49
4.2 Data Analysis Considerations 49
4.2.1 Data Quality Assessment (DQA) Process 49
4.2.2 Statistical Considerations 50
4.3 Data Analysis 51
4.3.1 Data Comparisons 51
4.3.2 Hydrogeologic and Contaminant Transport Calculations 52
4.3.3 Contaminant Mass Loss Calculations 52
4.3.4 Plume Stability 53
4.3.5 Attenuation Rates 54
4.3.5.1 Concentration vs. Distance Attenuation Rates 55
4.3.5.2 Concentration vs. Time Attenuation Rates 56
4.3.5.3 Biodegradation Rate Constants 56
4.3.6 Modeling 56
4.3.7 Remediation Time Frames 58
4.4 Site Characterization, Decision-Making, and Remedy Selection 58
5.0 REFERENCES 60
References Cited 60
Annotated Additional References 64
6.0 GLOSSARY 68
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LIST OF FIGURES
Figure 1. Elements of a Conceptual Site Model for Monitored Natural Attenuation 8
Figure 2. Systems Engineering Approach to the Conceptual Site Model for Monitored Natural
Attenuation 9
Figure 3. Conceptual Site Model. Example of Representative Zones 11
Figure 4. Variation in Ground-Water Flow Paths, Directions, and Gradients 19
Figure 5. Contaminant Distribution 25
Figure 6. Transect Development 47
Figure 7. Site Characterization Sampling Locations 48
Figure 8. Conceptual Approach to Data Analysis for Attenuation Rates 57
LIST OF TABLES
Table 1. MNA Performance Monitoring Objectives and Their Relationship to Site Characterization. ... 6
Table 2 MNA Site Characterization Variables 13
Table 3 Approach to and Sequence of MNA Site Characterization Activities 40
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ACKNOWLEDGMENTS
The lead author was Bruce Pivetz (Shaw Environmental & Infrastructure, Inc.). Extensive review
comments and additional text were provided by David Abshire (U.S. EPA/Region 6), William
Brandon (U.S. EPA/Region 1), Stephen Mangion (U.S. EPA/ORD/Region 1), Brad Roberts (U.S.
EPA/Region 7), Bruce Stuart (Missouri Department of Natural Resources), and Luanne Vanderpool
(U.S. EPA/Region 5). Additional text was provided by Steven D. Acree (U.S. EPA/ORD/NRMRL/
GWERD). A contributing author was Barbara Wilson (formerly with Dynamac Corporation).
Contributors of text to early development efforts for this document included Kelly Hurt (formerly
with Dynamac Corporation), Daniel F. Pope (Shaw Environmental & Infrastructure, Inc.), Wayne
Kellogg (formerly with Dynamac Corporation), Gary A. Robbins (University of Connecticut),
Michael J. Barcelona (Western Michigan University), Richard J. Brainerd (MACTEC Engineering
and Consulting, Inc.), and Kenneth Banks (formerly with Dynamac Corporation).
Kathy Tynsky (SRA International, Inc., a contractor to U.S. EPA at the R.S. Kerr Environmental
Research Center in Ada, Oklahoma) prepared the final figures for publication. Martha Williams
(SRA International, Inc.) assisted with final editing and formatting for publication.
Project management and review was provided by Steven D. Acree (U.S. EPA/ORD/NRMRL/
GWERD). The work assignment manager was David Burden (U.S. EPA/ORD/NRJVIRL/GWERD).
Peer review comments were provided by Mark Malinowski (California Department of Toxic
Substances Control), Mark Ferrey (Minnesota Pollution Control Agency/Environmental Outcomes
Division), and R. Ryan Dupont (Utah State University/Utah Water Research Laboratory).
Additional review comments were provided by Hal White (U.S. EPA/OSWER/OUST), John T.
Wilson (U.S. EPA/ORD/NRMRL/GWERD), James Weaver (U.S. EPA/ORD/NRMRL/GWERD),
Dominic DiGiulio (U.S. EPA/ORD/NRMRL/GWERD), and members of the U.S. EPA Ground
Water Forum.
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ABSTRACT
Monitored Natural Attenuation (MNA) is unique among remedial technologies in relying entirely
on natural processes to achieve site-specific objectives. Site characterization is essential to provide
site-specific data and interpretations for the decision-making process (i.e., to determine if site
remedial goals can be met with MNA in appropriate remedial time frames), and to provide site-
specific data and interpretations to design a performance monitoring system (i.e., to determine the
necessary monitoring parameters, locations, and frequency for monitoring).
This publication provides a framework for site characterization in the context of MNA and is
intended primarily for project managers to use during the planning, tasking, implementation, and/
or review of site characterization where MNA may be considered as a potential remedial technol-
ogy. This document presents a broad overview of technical issues including development of a
conceptual site model, characterization variables, sampling locations and frequencies, problematic
issues encountered at MNA. sites and approaches to overcome them, and the interpretations related
to the MNA decision-making process.
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1.0
NTRODUCTION
1.1
The Office of Solid Waste and Emergency
Response (OSWER) Directive 9200.4-17P
(U.S. EPA, 1999) defines monitored natural
attenuation (MNA) as "the reliance on natural
attenuation processes (within the context of a
carefully controlled and monitored site cleanup
approach) to achieve site-specific remediation
objectives within a time frame that is reason-
able compared to that offered by other more
active methods. " Natural attenuation processes
"include a variety of physical, chemical, or
biological processes that, under favorable
conditions, act without hitman intervention
to reduce the mass, toxicity, mobility, volume,
or concentration of contaminants in soil or
ground water. These in-situ processes include
biodegradation; dispersion; dilution; sorption;
volatilization; radioactive decay; and chemi-
cal or biological stabilization, transformation,
or destruction of contaminants" (U.S. EPA,
1999). MNA is unique among remedial
technologies in relying entirely on natural
processes to achieve site-specific objectives.
Effective evaluation of these natural processes
often requires a thorough and detailed three-
dimensional characterization and understanding
of subsurface conditions and contaminant trans-
port and fate. Site characterization is essential
to provide site-specific data and interpretations
for the decision-making process on the appli-
cability and selection of MNA as a remedial
technology for a site (i.e., to determine if
site remedial goals can be met with MNA in
appropriate remedial time frames and with an
allowable spatial extent of contaminants), and
to provide site-specific data and interpretations
to design and initiate the MNA remedial tech-
nology (i.e., to determine the necessary param-
eters, locations, and frequency for monitoring).
This publication provides a framework for site
characterization in the context of MNA. and is
intended primarily for project managers to use
during the planning, tasking, implementation,
and/or review of site characterization for sites
where MNA may be considered as a potential
remedial technology. This document presents
a broad overview of technical issues including
development of a conceptual site model, char-
acterization variables, sampling locations and
frequencies, problematic issues encountered at
MNA sites and approaches to overcome them,
and the interpretations required for the MNA
decision-making process. It outlines strategies
and concepts regarding how site characteriza-
tion fits into the overall scope of investigation
and remediation of ground-water contamination
sites where MNA may be considered.
This publication is intended to be easily
utilizable by project managers, by providing
sufficient explanation and detail to under-
stand the scope and interpretation of the site
characterization for a potential MNA site, in
a relatively short and easily read (yet com-
prehensive) document. Other MNA protocols
and technical guidance documents have been
previously published (see National Research
Council, 2000, for a list of such documents)
and can provide valuable additional scientific
discussion on specific topics, yet may be too
lengthy or detailed for ease of use by a project
manager. Some of the existing documents may
not sufficiently describe the overall framework
of site characterization in the context of MNA,
or may not sufficiently address some aspects of
the site characterization activities.
Nothing in this document changes Agency
policy regarding remedial selection criteria,
remedial expectations, or the selection and
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implementation of MNA. This document does
not supersede any guidance. It is a technical
reference to be used in conjunction with other
documents, including:
OSWER Directive 9200.4-17P, Use of
Monitored Natural Attenuation at Super fund,
RCKA Corrective Action, and Underground,
Storage Tank Sites (U.S. EPA, 1999). The
Directive clarifies EPA's policy regarding the
role of MNA in remediation of contaminated
soil and ground water, provides background
information on MNA, and discusses imple-
mentation of MNA.
Technical Protocol for Evaluating Natural
Attenuation of Chlorinated Solvents in
Ground Wafer (Wiedemeier et al., 1998).
This document provides technical back-
ground primarily on the biological processes
responsible for natural attenuation for
chlorinated solvents, specific information
on the data collection and analysis for MNA
evaluation, and a protocol for the overall
evaluation of MNA. It does not specifically
address in detail the overall site character-
ization or long-term monitoring for MNA
remedies.
Performance Monitoring of MNA Remedies
for VOCs in Ground Water (Pope et al.,
2004). This document provides technical
recommendations on the design and imple-
mentation of monitoring to evaluate the
effectiveness of natural attenuation. It is to
be used after site characterization has been
conducted and MNA has been selected as a
component of a remedy (although reference
to it can be useful during remedy selection
to help with estimating potential costs of
performance monitoring).
Region 5 Framework for Monitored Natural
Attenuation Decisions for Ground Water
(U.S. EPA, 2000). This concise document
provides a framework outlining the data
needed for an MNA remedy decision, a
decision-making flowchart for MNA, tables
summarizing NA processes and the param-
eters used for indicating NA, and a glossary
of relevant terms.
1.2
For this document to be accessible and use-
ful to a wide audience with varying levels of
expertise, discussions on some topics are kept
general. Details of particular methodologies for
sampling, analysis, or modeling are beyond the
scope and are not provided. Detailed informa-
tion on such topics is readily available in the
technical and scientific literature; a selection of
the literature is referenced in the text.
1,2,1
This document focuses on characterization of
sites where MNA is being considered for re-
mediation of the portion of the site with dis-
solved-phase volatile organic compound (VOC)
ground-water contamination. It will be most
appropriate for VOC-contaminated sites with
chlorinated solvent compounds and/or petro-
leum hydrocarbon compounds, as these con-
taminants are known to be readily susceptible to
degradative natural attenuation processes (under
the appropriate conditions). It may be less
applicable to sites with other types of contami-
nants such as wood-treating chemicals, pesti-
cides, or energetics (e.g., explosives, propel-
lants). Natural attenuation processes for these
other contaminants may be less understood or
be less effective than those occurring with the
chlorinated solvent and petroleum hydrocarbon
contaminants.
This document is not intended for characteriza-
tion of sites where MNA is being considered
for remediation of inorganic contaminants.
However, inorganic compounds are discussed
to the extent that they impact, or are impacted
by, natural attenuation processes related to the
VOCs. MNA of a variety of inorganic com-
pounds (metals, non-metals, and radionuclides)
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is discussed in Ford et al. (2007s1 and 2007b)
and Ford and Wilkin (2010).
1.2,2
Much of the site characterization discussed
in this document focuses on the portion of
the site with dissolved-phase contamina-
tion (this portion of the site will also have
sorbed-phase contamination that results from
contaminant partitioning). However, many
VOC-contaminated sites have a source zone
containing nonaqueous phase liquids (either
dense nonaqueous phase liquids (DNAPLs2 ) or
light nonaqueous phase liquids (LNAPLs)) as
the source of the dissolved-phase contamina-
tion. Site characterization for consideration of
MNA as a remedial technology should include
sufficient characterization of the contribution
of contamination from the source (i.e., mass
flux (rate of flow) from the source zone) to
permit evaluation of the use of MNA as part of
the overall site cleanup approach. Extensive
characterization of the NAPL source area itself
is not a primary focus of this document.
Key Point
Caution - Many NAPL source areas are
unlikely to be effectively remediated in a
reasonable time frame using MNA alone.
The portions of the site with significant
contaminant mass flux from the source area
may not allow remediation solely by MNA
in a reasonable time frame either.
V J
attenuation processes and contaminant distribu-
tion, fate, and transport in the saturated zone.
For example, ground-water fluctuations can
release contamination that may be in previously
unsaturated media, and water infiltrating down-
ward from the unsaturated zone can alter the
characteristics of the ground water. MNA site
characterization should include such portions of
the unsaturated zone.
Site characterization for MNA in karst or
fractured rock with fracture-dominated flow is
beyond the scope of this document due to the
significant difficulty in determining contami-
nant transport and fate pathways and processes
in such settings. Specialized characterization
techniques may be required for those sites, in
addition to those techniques discussed in this
document for use in porous media.
Cross-media transfer pathways, such as ground
water to surface water, or ground water to
soil gas to indoor air, are not addressed in this
document; however, site characterization for
any remedy typically would include character-
izing all significant pathways by which con-
taminants may move away from source areas
and ground-water plumes to impact receptors
(e.g., surface water and indoor air).
1.2.3
The site characterization discussed here focuses
on the saturated porous media zone. However,
the unsaturated zone can influence natural
1 Reference citations in bold font are included in the Annotated
Additional References.
2 Terms indicated using underlined bold italic font when the
term's first significant usage appears in the text are defined and
further discussed in the Glossary. The reader is strongly encour-
aged to read the entry in the Glossary before proceeding.
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2.0
MNA SITE CHARACTERIZATION OBJECTIVES AND
VARIABLES
2.1 Introduction
The unique aspect of MNA as a remedial
technology is its reliance on naturally occur-
ring processes (as opposed to active engineered
intervention) to achieve remedial goals within
a plume and to control the plume before any
receptors are reached. Determination of the
flow paths, rates of contaminant migration, and
rates of attenuation processes, is necessary to
understand the current behavior and stability of
the plume, and to predict the future behavior of
the plume with an acceptable degree of cer-
tainty. Site characterization at potential MNA
sites provides the information necessary for this
detailed understanding of the plume behavior
and stability, and the attenuation rate estimates.
The initial site characterization that typically
occurs at any contaminated site generally lacks
sufficient detail or the specific information
required for an evaluation of MNA, but can
serve as a starting point for MNA-related site
characterization activities.
A three-tiered approach to an evaluation of
MNA may be followed to demonstrate that
natural attenuation is occurring at a site. In this
approach, successively more detailed informa-
tion is collected as required to document a net
loss of contaminants and the natural attenua-
tion processes responsible for this loss, and to
determine rates of attenuation. Three catego-
ries of site-specific information are commonly
referred to (U.S. EPA, 1999) and used as "lines
of evidence". The three lines of evidence are:
« Ground-water and/or soil chemistry data
(i.e., contaminant measurements) that dem-
onstrate a reduction in concentration and/or
mass of contaminants.
Hydrogeologic and geochemical data that
indirectly indicate the processes causing
contaminant reduction.
« Field or laboratory microcosm data (e.g.,
reactants involved in contaminant loss) that
directly demonstrate the processes causing
the contaminant loss.
The uncertainty for any one line of evidence
may be managed by using more than one
approach for that line of evidence. Uncertainty
is diminished if the multiple approaches yield
comparable conclusions. Uncertainty may also
be managed by using more than one of the
three lines of evidence, by seeking comparable
conclusions from the various lines of evidence.
In some cases, the first line of evidence may be
sufficient if the "historical data are of sufficient
quality and duration" (U.S. EPA, 1999); how-
ever, generally the first two lines of evidence
are needed. The third line of evidence is used
if the other two lines of evidence are inconclu-
sive. The need for multiple lines of evidence
will be dependent on the scale, complexity, and
level of concern at the site.
There are a number of variables required
to evaluate and judge these three lines of
evidence. This document will use the term
"variable" in a generic manner to refer to data,
information, or concepts that can be qualita-
tively described or quantitatively measured for
the subsurface properties and processes. This
usage of the term "variable" follows from its
first and broadest dictionary definition "n. I.
Something that varies or is prone to variation. "
(Houghton Mifflin Co., 1997). The variables
are items to be measured or evaluated during
the site characterization. It is important to
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understand that quantitative estimates for
the values of variables will always likely be
subject to some uncertainty, for example, due
to an incomplete understanding of flow paths.
Key Point
In this document, the term "variables"
refers to the data, information, or con-
cepts that qualitatively describe or quanti-
tatively measure the subsurface properties
and processes.
V J
Site characterization provides data for MNA
remedy decision-making. The challenge in
evaluating MNA is not merely demonstrat-
ing that natural attenuation processes are
occurring; this can be a relatively easy task.
Rather, the evaluation of MNA. as a remedial
alternative also requires making the determina-
tion that the natural attenuation processes are
occurring at an acceptable rate to meet site
remedial goals in a timely manner, and that
there is a reasonable expectation that these
processes are sustainable and can be relied
upon as a long-term solution. For example,
MNA may not be sustainable when it relies
on the presence of two or more contaminants
(or other variables) that are each required in
combination to facilitate contaminant degrada-
tion. In this case, it is necessary to estimate if
the quantity of each of the contaminants (or of
the other variables) is adequate to sustain the
required reactions.
2.2
Objectiwes
The objectives of MNA site characterization
are to:
« Provide information for the evaluation of
MNA as a remedial technology.
* Provide information for planning and
implementing performance and long-term
monitoring of MNA.
Specific objectives (i.e., intermediate steps)
during the MNA site characterization are to:
« Define the geology of the site.
Define and quantify the hydrogeology and
the ground-water flow field and flow paths.
Define and quantify the contaminant,
geochemical, and biological variables.
Identify the source and nature of the
contaminant(s), and estimate the source
mass and mass flux.
« Measure and understand the subsurface
physical (geological and hydrogeological),
geochemical, and biological processes.
Determine and understand the three-
dimensional nature and spatial variability
of conditions and processes at the site (i.e.,
the spatial distribution of the values of the
variables).
Determine and understand the seasonal
and longer-term temporal variability of the
subsurface conditions and processes at the
site.
« Estimate attenuation rates.
« Evaluate plume behavior (including the
potential for future plume migration).
Site characterization activities for MNA
differ from the site characterization activities
routinely conducted at contaminated sites,
in requiring collection of more specific data
on fate and transport of contaminants and
other solutes, especially on the biological and
geochemical processes leading to attenuation.
MNA site characterization should produce
a detailed understanding of site conditions
and processes in three dimensions and of any
changes that might occur with time (season-
ally and longer-term). The subsurface con-
tains varying degrees of heterogeneity, the
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biological, hydrogeological, and geochemical
variables that define contaminant migration
and degradation can vary both spatially and
temporally. Beneficial microbial processes
may not occur in all portions of a plume;
attenuation rates may vary within a plume and
with time. Identifying and understanding in
detail these spatial and temporal variations is
of fundamental importance for the character-
ization and assessment of MNA.
Site characterization provides the information
for evaluating MNA as a potential remedial
technology. If MNA is selected as part of the
remedy, performance monitoring will then be
conducted as the remedy is implemented (refer
to Pope et al. (2004) for a discussion of per-
formance monitoring). Although site char-
acterization occurs initially, and performance
monitoring is a subsequent activity, the two
are linked, complementary activities. Much
of the site characterization information will be
useful in planning the performance monitoring
and evaluating the monitoring results. Thus,
it is important to understand the relationship
of performance monitoring objectives to site
characterization (Table 1). If monitoring of
the plume needs to extend over a multi-year
period as part of the site characterization in
order to understand the plume behavior prior to
decision-making, some of the site characteriza-
tion activities may transition into performance
monitoring.
Table 1 MNA Performance Monitoring Objectives and Their Relationship to Site Characterization.
M.NA Performance .Monitoring
Objectives (U.S. EPA, 1999)
Relationship to MNA Site Characterization
Demonstrate that natural attenuation is
occurring according to expectations.
Expectations of performance are largely based on the
data, calculations, assumptions, and estimates developed
during site characterization. If expectations are not being
met (which might occur due to changing conditions or an
incorrect conceptual model), the conceptual model may
need updating and an additional iteration of field activi-
ties mav be necessary.
Detect changes in environmental condi-
tions (e.g., hydrogeologic, geochemical,
microbiological, or other changes) that
may reduce the efficacy of any of the
natural attenuation processes.
Site characterization provides the baseline conditions
used to detect changes.
Identify any potentially toxic and/or
mobile transformation 'products.
Transformation products (e.g., daughter products) should
be identified during site characterization. Transformation
products can also include naturally occurring compounds
(such as metals) that may become mobilized due to
changes in geochemical conditions.
Verify that the plume(s) is not expand-
ing (either downgradient, laterally or
vertically).
The baseline extent of the plume should be defined
during the initial site characterization activities. This
baseline plume definition, when coupled with longer-term
plume monitoring during follow-up site characterization
activities, should be used to evaluate the plume stability.
Verify no unacceptable impact to down-
gradient receptors.
The location of, and flow paths to, downgradient recep-
tors, and suitable monitoring locations, should be identi-
fied during site characterization.
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Detect new releases of contaminants to
the environment that could impact the
effectiveness of the natural attenuation
remedy
* Demonstrate the efficacy of institutional
controls that were put in place to protect
potential receptors.
* Verify attainment of remediation
objectives.
* Source areas and potential source areas should be
identified and delineated during site characterization.
and monitored for new releases of site contaminants.
Naturally occurring compounds (such as metals) may
become mobilized due to changes in geochemical condi-
tions, and represent new contamination.
This is a performance monitoring task, but based on
expectations developed during site characterization.
This is a performance monitoring task, but based on
expectations developed during site characterization.
2.3
Development of a conceptual site model
(CSM), or "conceptual model" is an important
means to integrate all the information known
and collected for the site. A CSM is "a three-
dimensional representation that conveys what
is known or suspected about contamination
sources, release mechanisms, and. the transport
and fate of those contaminants" (U.S. EPA,
1999).
The subsurface is complex, containing many
different materials and having numerous
interacting processes. The typical investigative
and characterization activities conducted by
various parties at a site produce a large amount
of information on the subsurface materials
and processes that is presented in numerous
reports, tables, figures, graphs, diagrams, etc.
In order to effectively use and understand all
the site information, it is necessary to first
compile, organize, and distill it into a coher-
ent mental, written, and visual picture, in
which all the information converges to yield
a scientifically valid and internally consistent
interpretation of the subsurface. A good CSM
equates to a comprehensive, clear, logical,
three-dimensional understanding of site
conditions and processes. Figure 1 is a visual
representation of a CSM:, presenting with one
glance important concepts about the site, such
as physical setting, ground-water flow, extent
of contamination, and source of contamination.
Additional visual representations can present
other aspects of the CSM (e.g., Figures 2, 3,
4, and 5) and the CSM can be expressed in
concise, well-written summary text. Once the
CSM: is developed and understood, the sup-
porting documentation such as figures, maps,
tables, logs, text, etc. can be referred to as
needed for the detailed data and information
about the site.
A conceptual site model is developed by
placing each piece of information about the
site variables in its proper position and context
within the three-dimensional volume of the
site, and visualizing the distribution of the
information, while recognizing that the
information may also change with time. A
CSM can be constructed step-wise. First, the
basic physical framework of the site is estab-
lished based on the geology and then the
hydrogeology. The problem at the site (i.e.,
the contamination) is described, and then more
complexity is added regarding processes active
at the site (i.e., the geochemistry and biology).
The interactions of the variables with each
other also need to be incorporated into the
CSM. These interactions have to be scientifi-
cally consistent, following the physical laws
which govern them (e.g., two processes that
are mutually exclusive cannot be assumed to
occur in the same place at the same time; or
two variables that are physically interrelated,
such as bulk density and porosity, must be
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Contaminated site
-Leaking drums
-Leaking UST
-Leaking AST
Pumping Well:
Capture zones at pumping rates
Q (larger), Q2 (smaller)
Ground-water elevations
t, - lower
t2 - higher
GROUND-WATER
FLOW DIRECTION
Figure 1. Elements of a Conceptual Site Model for Monitored Natural Attenuation. Typically, the subsur-
face will be much more heterogeneous than shown here; the illustration presents a simplified
view.
consistent). Conservative assumptions (based
on sound scientific principles) regarding the
values or impacts of some variables may need
to be made when those variables cannot be
adequately quantified or described.
Key Point
Development of the conceptual site model
should be an iterative process. The CSM
should be updated as new site characteriza-
tion information is collected. Examination
of the current version of the CSM may indi-
cate gaps in knowledge where additional
characterization data should be collected.
2.3.1 Systems Engineering Approach
Many environmental professionals (e.g.,
geologists, hydrogeologists, and environmental
engineers) are likely to be familiar with the
terminology and approaches associated with
the development and use of a CSM for the
subsurface. Others may find it helpful to use
a systems engineering approach, which uses a
different terminology and analytical approach
(Figure 2). Simply stated, this approach
identifies pertinent constant elements to a site
(i.e., "system architecture") as well as relevant
dynamic elements which influence the site
system (i.e., "system dynamics"). Using the
systems approach, one seeks to understand
system behavior, including site-specific outputs
(e.g., contaminant flux, biodegradation, etc.)
as a result of the interplay between system
dynamics and system architecture. When
considered as a system, it can be easier to
discern when inputs and/or outputs have been
neglected in the conceptual site model. The
goal of any systems evaluation is to capture,
to a reasonable extent, a detailed knowledge
-------
System Boundary
Input Throughput Output
Water g5^» B^^> I \
b^ - ^
Conta I
Figure 2. Systems Engineering Approach to the Conceptual Site Model for Monitored Natural Attenua-
tion.
of system architecture and dynamics, thereby
enabling a comprehensive understanding
of system behavior. In this application, the
system evaluation approach allows for predic-
tion of water flow and contaminant transport,
and facilitates identification of the most impor-
tant elements of the site, such as principal
hydrogeologic units, dominant flow pathways,
and most representative geochemical zones.
In the site characterization context, "system
architecture" is generally defined by the sur-
face and subsurface flow pathways, comprised
of both natural (i.e., geologic) pathways and
man-made pathways in the built environment,
(e.g., subsurface drains, impermeable barriers,
utility conduits, etc). System architecture also
includes the way that various geologic and
non-geologic pathways interact hydraulically.
System architecture can be conceptualized in
most cases by understanding the nature and
three-dimensional configuration of subsurface
geologic units in concert with a knowledge
of as-built conditions for man-made systems
within the subject aquifer(s). For example, an
understanding of depositional geology allows
one to anticipate the degree of heterogeneity
that may be present as well as likely forms,
sizes, and shapes of the geologic units which
define the aquifer. Similarly, a thorough
knowledge of location and depth of an engi-
neered drainage system may point to obvious
preferential pathways and/or locations where
exchange with the ambient ground-water
system may occur.
"System dynamics" simply represents the vari-
ability in outputs resulting from the interaction
of dynamic forces with the system architec-
ture. For example, input of water (e.g., from
-------
precipitation - infiltration) and withdrawals
from the aquifer (e.g., from pumping), are both
dynamic elements which vary with time, and
are influenced by the inherent system architec-
ture. System dynamics may also be impacted
by ground-water withdrawals or additions
which are caused by ongoing site operations
where a business remains on-site (e.g., pump-
ing footing tile drains to protect foundations,
leaking water or sewer lines). The complex
interaction of these multiple site-specific inputs
and outputs, in the context of the site-specific
system architecture, determines the resulting
throughput and output variability, such as
ground-water flux through the aquifer. For
instance, system dynamics revealed by a
monitoring system at a site located adjacent
to a river may have wide variability in flow
and head based on changes in river stage and
the relative connection of various geologic
units with the river. There may be significant
changes in horizontal gradients to and from
the river, in response to rising or falling river
stage. Vertical gradients may also reverse
direction (e.g., from upward to downward) if
different geologic units react to stimulus (e.g.,
pumping or change in river stage) at differ-
ent rates. The reversals in gradient might
be perplexing without consideration of the
system. This example demonstrates the value
of the systems approach, suggesting that a
more simplistic approach, such as collection
and evaluation of limited data from a single
point in time could be misleading or erroneous.
In this manner, the systems approach provides
a feedback loop which assists in understanding
current information as well as streamlining
future data collection needs in consideration of
the particular system dynamics.
2.3.2
A useful component of the CSM for MNA site
characterization is the representative zone.
The representative zone is a tool or means (not
an end), and is the term used in this docu-
ment to denote and conceptualize the different
portions or zones (each with its own set of
variable values) that make up the subsurface.
At each sampled point in the subsurface,
information is collected or assumed for the
values of the variables. Evaluation of the
data relative to contaminant fate, contaminant
transport, and natural attenuation processes,
will indicate which variables are most critical,
predominant, or descriptive of the conditions
most affecting plume migration and attenuation
at each particular location. That set or com-
bination of these predominant variables will
"represent" the conditions at that location. All
the subsurface locations with that same set of
predominant variables are grouped together in
a representative zone. A representative zone is
defined here as a three-dimensional portion of
the subsurface in which a unique set or com-
bination of the predominant variables has the
same value or range of values for each variable
at all locations within that three-dimensional
volume. Different portions of the subsurface
are likely to have different conditions (i.e., dif-
ferent sets of variables, and/or different values
for the sets of variables); thus, the subsurface
can be divided up into more than one represen-
tative zone. Different representative zones are
different from each other in some manner (e.g.,
they may be hydraulically distinct, geochemi-
cally distinct, or microbiologically distinct).
Figure 3 illustrates, conceptually, a subsurface
volume divided up into different representative
zones. The entire subsurface of the site could
be divided up into many different zones so that
each zone has different sets of variables and
specific values for those variables. However, it
is likely that there will be a limited number of
different predominant sets of variables (i.e., a
small number of representative zones neces-
sary to adequately define the site). Further, the
important representative zones are those that
contain the contamination plume, those that
are downgradient between the plume and any
receptors, and/or those in surrounding areas
where shifts in ground-water flow direction
might expand the plume. The number of
-------
Figure 3. Conceptual Site Model, Example of Representative Zones. The subsurface here has been
divided into four different geologic units (clay, sandy silt, fine-grained sand, and gravel), based
on lithology grain size, and resultant hydraulic conductivity. The subsurface was also subdi-
vided based on the geochemical electron-acceptor conditions (methanogenic, sulfate-reduc-
ing, iron-reducing, and aerobic). Five different representative zones were identified based on
different combinations of the geologic units and geochemical electron-acceptor conditions.
representative zones, the sizes of the represen-
tative zones, the predominant variables, and
the range of values of those variables are all
site-specific; some general discussion of this
issue is provided in Section 2.4.1.
It is important to define representative zones
because it allows the dividing of a poten-
tially large amount of data and information
into smaller more manageable data sets that
describe the three-dimensional distribution of
the important conditions affecting contaminant
migration and attenuation. Recognizing the
predominant conditions in different portions
of the subsurface is important for understand-
ing and evaluating the overall effectiveness of
MNA.
2.4 MNA Site Characterization
Variables - Introduction
The fate and transport of a contaminant plume
depend on the subsurface physical, chemical,
and biological properties and processes. These
properties and processes are described by a
large number of variables. Physical variables
describe or quantify the physical nature of the
porous media (geological variables) and the
ground water flowing through it (the hydrogeo-
logical variables). The naturally occurring fea-
tures of a site are likely to have been modified
through human activity (beyond the obviously
human-caused contamination itself). These
modifications will be referred to as anthropo-
genic variables, and may most often reveal
themselves as changes to the natural physical
-------
setting of a site. Other naturally occurring
features can be of concern to human activity;
these will be called anthropocentric variables.
Chemical variables describe or quantify the
contamination (contaminant variables), as well
as the geochemistry of the ground water and
porous media (geochemical variables) and the
geochemical interactions with the contami-
nant. Biological variables describe or quantify
the subsurface microbial community and its
interaction with the contaminant, and the
porous media and ground-water geochemistry.
Measurements of the hydrogeological and
contaminant variables are used to determine
the direction and rate of migration of con-
taminants (i.e., transport). Measurements of
the contaminant, geochemical, and biological
variables are used to evaluate the destruction
of contaminants (i.e., fate).
Table 2 lists the descriptive or quantifiable
variables associated with the subsurface
physical, chemical, and biological properties
and processes that are most commonly mea-
sured at potential MNA sites. Some variables
may overlap the categories in Table 2 since
they may interact among physical, chemical,
and biological processes. The use of these
variables for an MNA evaluation is discussed
in following sections. The sequential order
of collecting information on the variables
during an actual site characterization will be
discussed in Chapter 3. Additional discussion
of some variables is provided in the glos-
sary, and further information on the variables
can be obtained from the extensive technical
literature on hydrogeology, microbiology, and
geochemistry.
The subsurface is often heterogeneous and
anisotropic. and the subsurface variables
can vary spatially. This variability may
significantly impact plume behavior or change
plume behavior from one location to another.
This requires characterization efforts to be
conducted longitudinally, laterally, and verti-
cally (i.e., three-dimensionally) relative to the
plume, for example, using transects (i.e., lines
of sampling locations). Transects are typically
transverse (perpendicular to the ground-water
flow direction), or longitudinal (parallel to the
ground-water flow direction). Having multiple
vertical and horizontal sampling points in these
various transects provides three-dimensional
characterization of the subsurface. Further,
there can be variation in time (temporal vari-
ability) in the values of many of the variables
on a seasonal basis or during the migration and
lifespan of the plume. Time-series data for the
hydrogeologic, geochemical, and biological
variables should be collected and assessed for
changes with time.
Key Point
Characterization of the site and contaminant
plume in three dimensions is often critical
due to the heterogeneity and spatial vari-
ability of the subsurface.
-------
Table 2 MNA Site Characterization Variables. Categorization and listing of variables for MNA site char-
acterization. The variables are discussed in the corresponding sections of the text.
Physical Variables
Geological Variables
Lithology
* Mineralogy
* Stratigraphy
* Depositional environments/features
* Structural features
* Texture (grain-size distribution)
« Porosity
* Bulk density
* Particle (solid) density
Hydrogeological Variables
* Hydraulic head
* Hydraulic conductivity
* Porosity
* Ground-water recharge and discharge (location
and extent)
« Surface water bodies, and interactions with site
ground water (location and extent)
* Precipitation
* Dilution
« Dispersion
Chemical Variables
Contaminant Variables
* Identity of contaminant(s)
* Contaminant concentrations
* Contaminant solubility
Contaminant density
* Contaminant mixtures (i.e., commingling)
* Partition (or Distribution) Coefficients (soil/water,
NAPL/water)
* Henry's Law Constant
« Source area contamination
« Source control history
Geocfaeinical Variables
* Oxidation-reduction potential (ORP) or redox
potential
Dissolved oxygen
* Nitrate
* Manganese
Iron
* Sulfate
« Methane
« Dissolved hydrogen
Metals and metalloids (as site-specific contaminants,
such as arsenic, and as teactants with contaminants)
pH
* Alkalinity
Soil organic carbon (Total organic carbon (TOC),
fraction of organic carbon (f ), or soil organic matter
(OM))
* Temperature
* Additional major ions
« Isotopes (relevant stable isotopes and radioisotopes)
Anthropogenic and Anthrooocentric Variables
Biological Variables
Engineered features
Nearby wells
Human-caused ground-water recharge and
discharge
Receptors
Contaminant concentrations
Daughter products
Byproducts
Oxidation-reduction potential (ORP) or redox
potential
Ground-water organic carbon (Total organic carbon
(TOC) and dissolved organic carbon (DOC))
PH
Temperature
Alkalinity-
Dissolved oxygen
Nitrate
Manganese
Iron
Sulfate
Methane
Dissolved hydrogen
Microbial community
Stable isotopes (2H/'H and 13C/12C)
-------
2.4.1
An important part of the site characterization
effort involves deciding the level of detail and
at what scale and frequency the sampling is to
be conducted (which translates into the number
of samples to be collected). Each site is
different, so guidelines on specific numbers of
sampling locations and samples, or frequency
of sampling, cannot be provided that would be
applicable to all sites. There are no widely-
accepted protocols for deciding the particular
level of detail that is necessary for site char-
acterization. This document does not propose
any hard-and-fast rules for deciding the level
of detail, but rather, provides a discussion of
the factors that affect the choice of the level
of detail. In general, the sampling density
(including the number of transects) and level
of detail investigated should match the scale,
complexity, and level of concern at the site.
Different numbers of samples may need to be
collected for different variables, depending
on the subsurface heterogeneity and resultant
potential wide range of measured values for
each specific variable. For example, grain-size
distribution is likely to vary much more across
the site than does ground-water temperature.
The sampling frequency can be influenced by
the frequency of significant events at the site
(e.g., seasonal changes). Professional knowl-
edge and experience are used to determine
what variables are the most critical, where data
gaps may be, or where and how many addi-
tional samples are required. Subsequent site
characterization can then focus on these more
critical variables.
This document lists the variables that may
need to be characterized in an evaluation of
MNA. Existing knowledge and data for the
site can be used to modify the number of
variables requiring data collection during the
MNA-related site characterization. However,
it is often not possible a priori to determine
which variables will be most critical at a
particular site. Evaluation of the previously
existing data and the initial MNA site charac-
terization data may identify which variables
are most critical, and which are to be focused
on. This can also indicate how to group the
predominant and/or most critical variable
values together into representative zones. The
evaluation will also identify if and how many
additional data points may be needed for
further investigation of each of these variables.
2.5
Geological variables impact the direction,
magnitude, and variability of ground-water,
dissolved-contaminant, and NAPL-phase flow.
The geological variables are discussed below.
Geological variables include:
« Lithology. The lithology can provide
indications on how water and contaminants
can flow and react in a consolidated porous
medium.
Mineralogy. The properties of the miner-
als that make up the porous media impact
the subsurface geochemistry, and how the
porous media physically and geochemically
interacts with contaminants and microbes.
Reactive iron and sulfur minerals have been
shown to contribute to abiotic degradation
of halogenated hydrocarbon contaminants in
ground water (He et al., 2009). Weathering
and solubilization of the various minerals
can release different constituents into the
ground water, resulting in varying aqueous
geochemistries. Minerals of different densi-
ties, and varying proportions of those miner-
als, can result in different average particle
densities and overall bulk densities.
Stratigraphy. The nature of the layering
of consolidated and unconsolidated porous
media can impact the direction and rate of
ground-water and contaminant flow.
« Depositional environments/features.
Site-specific definition of the depositional
-------
environments and features aids in under-
standing the potential for heterogeneity in
sediment characteristics and in determin-
ing if there are potential preferential flow
zones. For example, a site that includes
subsurface sediments deposited by a
stream may have areas of higher-hydraulic-
conductivity gravel that act as preferential
pathways for faster contaminant migration.
« Structural features. Features such as
cracks, joints, faults, worm holes, root
openings, or other preferential flow paths
affect the direction of ground-water and
contaminant flow.
Texture (grain-size distribution). The
size, range of sizes (i.e., distribution), and
arrangement of the solid grains at a site
affect how water and contaminants flow
through the porous media, and can impact
geochemical and biological processes.
Porosity. The porosity can be used in an
equation to calculate ground-water velocity
and contaminant retardation factor (which
are then used to calculate contaminant
velocities and travel times). It can also
be used to calculate the volumes of water,
solid phase, and/or NAPL phase in a given
volume of the subsurface.
Bulk density. The bulk density can be used
to derive the value of porosity. It is also
used in the equation to calculate the con-
taminant retardation factor.
Particle /solid) density. The particle
density is used, along with bulk density, to
calculate the porosity.
A review of the regional geology using exist-
ing information or by conducting a recon-
naissance of the surrounding area provides
a starting point for developing the geologic
components of the conceptual model. At
a regional scale, the variables that must be
defined are the regional lithology, stratigraphy,
depositional environments, and structural fea-
tures, which will indicate what type of geology
might be expected at the site. This informa-
tion will help to determine which investigative
tools and techniques will be appropriate (e.g.,
direct push methods may not work well in
very rocky till). Knowledge of the regional
depositional environment will help to assess
the level of effort needed to characterize the
subsurface geology (e.g., a meandering stream
depositional environment would be expected
to exhibit more variability than a beach-like
depositional environment). Knowledge of
regional stratigraphy may also point to the type
of aquifer(s) that make up the site.
Site-specific definition of the lithology,
mineralogy, and stratigraphy is also critical,
including the type, thickness, lateral continuity,
and orientation of geologic units and bedding
features. The lithologic and stratigraphic
information can be used to determine if
there may be any potential barriers to flow
(such as a thick, laterally continuous clay
layer). Stratigraphic barriers or lithologic
interfaces may be sites of DNAPL accumula-
tion. Detailed definition is important because
lithologic differences or bedding (even when
subtle) can impact flow paths or DNAPL
migration. DNAPL constituents may dif-
fuse into lower-permeability media, where
they represent a long-term ongoing source
of contamination as the contaminants diffuse
back out into the ground water in the higher-
permeability media. Texture (grain size dis-
tribution), porosity, bulk density, and particle
(solid) density should be determined for each
of the geologic units involved in or influencing
contaminant transport (this can include aquita-
rds in addition to contaminated aquifers).
Site-specific geologic information can be
obtained from subsurface core samples, surface
geophysical measurements, and/or geophysical
logging of boreholes. Literature or "typical"
values are sometimes used for some of the
variables, such as bulk density or porosity.
-------
However, actual site-specific measurements are
recommended for bulk density and grain-size
distribution, due to the importance of these
variables in hydrogeologic calculations. For
variables that are harder to measure, values
can be assumed (e.g., for particle density) or
calculated (e.g., porosity) based on measured
related variables (e.g., bulk density).
2.6
Hydrogeology provides the foundation for
understanding the behavior of subsurface
water. Knowledge of the site-specific hydroge-
ology is used in conjunction with the site-
specific geologic and contaminant variables to
determine the ground-water and contaminant
flow paths, contaminant migration rates, and
subsurface variability. Ground-water and
contaminant velocity calculations (Chapter 4)
involve the use of some hydrogeologic vari-
ables. The hydrogeologic variables included
in MNA site characterization, in rough order of
more important to less important are:
* Hydraulic head. Hydraulic heads are used
to determine the direction of ground-water
flow (from higher head to lower head), the
horizontal and vertical hydraulic gradients
(for determining the ground-water flow
velocities), and to define the piezometric
surface for each hydrologic unit (aquifer or
aquitard) at the site. The hydraulic heads
should be determined within each geologic
unit involved in or influencing contaminant
transport (this can include aquitards in addi-
tion to contaminated aquifers). Hydraulic
gradients are likely to be different within
different hydrogeologic units and should
be calculated using measurements from
within the different units. Water levels
(i.e., hydraulic heads) are typically variable
with time (e.g., water-table fluctuations are
frequently observed), and it is important
to make regular, periodic measurements.
The frequency of the water-level measure-
ments will be determined by site-specific
dynamics, which are affected by both
natural and anthropogenic events. The
variations in hydraulic heads and gradients
in response to external influences on site
hydrology (e.g., seasonal or longer term pre-
cipitation patterns, tidal cycles, and changes
in patterns of ground-water withdrawal or
irrigation) should be determined.|fyrfrflif|ic_
conductivity. Hydraulic conductivity
should be determined for each geologic
unit involved in or influencing contaminant
transport (this can include aquitards in addi-
tion to contaminated aquifers). Hydraulic
conductivity data are used in conjunction
with hydraulic gradients to determine the
magnitude of ground-water flow.
Porosity. Porosity is a geological variable
determined as part of the geological charac-
terization. It is a critical variable for hydro-
geological calculations of ground-water and
contaminant flow velocities. Total porosity
defines the entire pore space (i.e., space not
occupied by solid material), is designated
as a fraction of a unit volume, and in the
saturated zone is occupied by ground water.
The effective porosity is the pore space
through which ground-water flow actually
occurs. A more accurate understanding of
ground-water flow (and associated calcula-
tions) can be obtained by using the value of
effective porosity rather than the value of
total porosity.
Ground-water recharge and discharge.
Identification of regional and local ground-
water recharge and discharge areas provides
a preliminary indicator for the general
direction of ground-water movement. The
amount and variation of recharge and
discharge can impact the hydraulic gradient,
ground-water and contaminant velocities
and flux, and ground-water flow direc-
tion (e.g., changing river stage or tides
can reverse ground-water flow direction).
Anthropogenic water additions (e.g., from
septic systems or leaking water utility pipes)
-------
or withdrawals (e.g., by nearby production
wells, footing tile drains, etc.) can act as
localized recharge or discharge, respec-
tively, and can alter local-scale ground-
water gradients and flow directions.
Surface water bodies, and interactions
with site ground water. Surface water
bodies (e.g., lakes, streams, or marine
waters) can interact with ground-water
flow at a site and influence contaminant
migration. They can affect ground-water
geochemistry, as with saline water near a
seashore. Surface water bodies are often
discharge areas for site ground water;
however, some are recharge areas. Due
to spatial and temporal variability, they
may be both recharge and discharge areas
at different times (e.g., seasonally or with
tidal fluctuations) or in different locations.
Information required on surface water
bodies can include the position relative to
the site, water elevations and fluctuations,
water movement and biogeochemical pro-
cesses within the hyporheic zone, sediment
characteristics, and water chemistry. Stream
bottom leakage will sometimes be important
information to obtain for a stream at or
near the site, involving measurement of
streambed thickness and vertical hydraulic
conductivity, and calculation of leakance.
Precipitation. A portion of rainfall at a
site will infiltrate into the subsurface and
recharge the ground water, and could lead to
some dilution of contamination. Infiltrating
rainfall can also leach additional contami-
nant mass from the unsaturated zone into
the ground water. Aquifer recharge from
infiltrated water might push contaminant
plumes downward, rather than diluting
contaminant concentrations.
Dilution. Mixing of the plume with
infiltrating uncontaminated water can lead
to non-destructive decreases in contaminant
concentration. Quantifying the amount of
dilution from this mechanism can help in
properly attributing concentration decreases
to either non-destructive or destructive
processes. Dilution of contaminant con-
centrations in a ground-water sample, or in
the plume in the local vicinity of a monitor-
ing well, can also occur in the well by the
mixing of water from both contaminated
and uncontaminated zones.
* Dispersion. Dispersion is the spreading and
mixing of contaminants into uncontami-
nated water flow paths via several mecha-
nisms related to the motion of the ground
water and the properties of the porous
media through which it flows. This has
essentially a diluting effect, and can lead to
non-destructive decreases in contaminant
concentration. Assessing dispersion can
help in properly attributing concentra-
tion decreases to either non-destructive or
destructive processes. However, it may
not be possible to distinguish the effects of
dispersion from those of any dilution caused
by infiltration of uncontaminated water.
A review of the regional hydrogeology pro-
vides a starting point for developing the hydro-
geologic components of the conceptual model.
The regional ground-water flow can suggest
the direction of site-specific ground-water
flow, and indicate potential areas for initial
investigations. However, since site-specific
ground-water flow directions may vary signifi-
cantly from the regional patterns, the detailed
three-dimensional ground-water flow direction
or directions at the site need to be confirmed.
Key Point
The direction of ground-water flow and
plume migration in portions of a site
may be different from what might be
expected by examining regional or even
local ground-water flow directions, due to
subsurface heterogeneity and preferential
flow paths.
-------
Additional details on hydrogeological char-
acterization are provided in many references,
including U.S. EPA (1986).
2.6.1
A flow path is an imaginary line that traces the
path a water molecule would follow as it flows
through the aquifer. Contaminants dissolved in
that ground water would also follow this flow
path. A plume centerline is the flow path along
the longitudinal central axis of the plume, and
in an idealized conceptual plume, would define
the highest concentration portion of the plume
at each distance downgradient. Plume behav-
ior and attenuation rates are determined, in
large part, through identifying and monitoring
flow paths with time. Attenuation rate calcula-
tions (discussed in Chapter 4) use data from
monitoring points along the flow paths, ideally,
along the plume centerline. In practice,
however, a plume centerline cannot be defini-
tively identified, for the reasons discussed
below. Identification of a broader region along
the longitudinal axis of the plume extending
from the source to the end (toe) of the plume is
more likely.
Temporal variations in ground-water flow
direction may occur, which result in temporal
variations in contaminant flow paths and com-
plicate attempts to define the flow paths and
calculate the attenuation rates. For example,
if the ground-water flow direction changed
slightly each year, a given molecule of water
and a dissolved contaminant emanating from
a specific location at the contaminant source
would be carried along a curving and twisting
flow path, and a hypothetical, ideal plume cen-
terline would appear to curve and twist, rather
than being straight. Further, in subsequent
years, other molecules of water and dissolved
contaminants emanating from the same spe-
cific location at the contaminant source would
follow different flow paths, shifted slightly
from each other. There would not be a unique,
spatially and temporally stable plume cen-
terline, and the plume centerline would shift
away from a given set of monitoring wells that
might sample it one year (but not the next).
Figure 4 conceptually illustrates changes in
ground-water flow paths and directions.
During characterization, the potential for
temporal flow direction variations should be
investigated, by examining historic ground-
water head data and by measuring ground-
water levels at different times of the year. For
some sites, tidal fluctuations can also be a con-
founding factor in determining ground-water
flow and flow path directions. For other sites,
the intermittent or changing operation of well
fields can cause variations in ground-water
flow direction. Any such flow direction varia-
tions can be integrated to produce an average
or generalized direction of flow, resulting in a
somewhat wider central region of the plume
rather than a narrow unique plume centerline
flow path. A more accurate longitudinal
transect (line of monitoring points extending
longitudinally along the central axis of the
plume) can then be based on this understand-
ing of flow directions, especially when coupled
with contaminant concentration data that may
help to define the plume.
Definition of flow paths can also be difficult if
preferential flow paths are present. Preferential
flow paths are flow paths where ground water
and contaminants travel at increased veloci-
ties and/or in different directions compared
to the ground water in the surrounding area.
Preferential flow paths can be due to natural
features such as highly permeable stream
deposits or to anthropogenic features such as
buried utility lines, heterogeneous fill, drain-
age ditches, etc. The potential for preferential
flow paths should be investigated during the
site characterization by assessing site informa-
tion for anthropogenic features and by careful
logging of subsurface borings in conjunction
with an understanding of geologic preferential
pathways potentially present in the area.
Given the state of site characterization tools
and technology, and further difficulties due to
-------
Time t,
Time t2
Pumping Well (Q, > Q2)
Larger capture zone at Q1 pumping rate
Smaller capture zone at Q2 pumping rate
/
PREeiPlWlpN (at t2)
Ground-water elevations
t, - lower
t2 - higher
.4 -
J INFILTRATION /
k- X5,154M.G I FALLING
±WATER--J-WATER.
fTT3*-*^fc:
^\^r-/^
SANDY SILT SILT
FINE-GRAINED
SAND
GROUND-WATER
FLOW DIRECTION
UNFRACTURED POROUS BEDROCK
MEDIUM-GRAINED SA
Figure 4. Variation in Ground-Water Flow Paths, Directions, and Gradients. The ground-water flow
direction (lateral direction is shown by the larger solid arrows) may change from time to time
(e.g., f, and t2) due to natural causes (e.g., changes in precipitation and infiltration) or anthro-
pogenic causes (e.g., changes in ground-water pumping, Q1 and Q2). Contamination carried
by the ground water will follow the changing ground-water flow paths. The contaminant flow
path resulting from changing ground-water flow directions will differ from a straight-line flow
path (a presumed plume centerline) that is based on the flow direction at any given time, or
that is based on an average flow direction. Monitoring well or transect placement needs to
take into account the changing ground-water flow directions. Contaminant concentrations
measured at monitoring locations also need to be interpreted based on an understanding of
the changing ground-water flow directions.
the possible temporal variations in flow direc-
tions, it is more realistic to attempt to delineate
general zones of similarity along transects
instead of attempting to define discrete flow
paths. Although transect sampling cannot
overcome all temporal and spatial variability
issues, acquisition and integration of multiple
laterally and vertically discrete data yields an
assessment more representative of the entire
range of conditions.
2.6.2 Background Ground Water
Background ground water, or background.
generally refers to the ground water that is
found upgradient of the contaminated ground
water. Knowledge of the background ground-
water quality is critical, for comparison to the
contaminated site ground water. Differences
in water quality and geochemical variables
between the background (i.e., "background
levels" of measured variables) and the plume
can indicate the changes that may have
occurred due to MNA fate processes, and help
in estimating potential changes that may occur
in the future.
Establishing background may not be straight-
forward, and often involves more than just
one well upgradient of a plume (U.S. EPA,
1986). The determination of background
-------
includes consideration of many things (depths
and thicknesses of geologic units, ground-
water flow directions, screen lengths, etc.).
Background samples need to be taken from
areas representative of ground water not
impacted by contaminants from the site, pref-
erably upgradient (rather than sidegradient),
and from the originally same geological and
geochemical environment as the downgradient
contaminated ground water. If ground-water
contamination exists regionally in the vicinity
of the site, and/or the areas upgradient of the
plume have naturally different geological and
geochemical conditions, it may not be possible
to collect a suitable upgradient background
sample. A background sample might need
to come from a different area of the site, and
comparisons of differences between samples
should recognize the less-than-ideal sample.
For adequate comparisons, background well
screens need to be placed to sample ground
water at the depths and in the geologic units
that correspond to those in the plume. Well
clusters may be necessary. To compare
electron acceptor and electron donor concen-
trations, the background samples need to be
taken from ground water that will subsequently
move through the site (i.e., be in the upgradi-
ent portion of the flow paths through the site).
This consideration also applies to other water
quality/geochemistry variables, such as metals.
It is likely that multiple background wells will
be required to reflect the subsurface geologic
variability. There may also be natural spatial
or temporal variability in the uncontaminated
ground-water geochemistry. An understanding
of natural geochemical variability is needed
so that geochemical variations observed in
the plume can be accurately assessed and to
avoid misinterpreting the relationships between
observed geochemical variations in the plume
and levels of microbial activity.
2.7
Receptors
Anthropogenic or human-related variables, in
addition to the natural geological and hydro-
geological variables discussed above, can
affect contaminant fate and transport at a site.
These anthropogenic variables include:
Engineered features. The engineered
features or "built environment" of the site
may influence the movement of surface
drainage, infiltration, and ground water.
Engineered features such as storm drains,
other subsurface utilities, catch basins, and
paved or other impervious areas should
be identified during the site characteriza-
tion. Subsurface engineered features such
as buried utility corridors may often form
preferential flow paths disrupting the natural
ground-water flow direction and magnitude.
Interpretations of ground-water flow paths
and subsequent calculations of contaminant
travel time and attenuation rates may be
inaccurate if influences of preferential flow
paths are not identified.
* Wells. The location and characteristics of
nearby water-supply, irrigation, or injection
wells should be determined. Pumping from
or injection into nearby wells can change
the ambient ground-water flow direction,
ground-water flow rates, and ground-water
gradients, and impact contaminant migra-
tion. The significance of these wells often
depends on their proximity to the potential
MNA site and on their extraction or injec-
tion rates.
« Anthropogenic ground-water recharge
and discharge. As mentioned in
Sections 2.3.1 and 2.6, small-scale localized
ground-water recharge and discharge can
occur due to anthropogenic features. Water
from leaking water utility pipes or from
septic systems can not only alter ground-
water gradients and flow directions locally,
but could result in anomalous ground-water
-------
chemistry (either from potable water enter-
ing the subsurface or from septage contami-
nants and nutrients). Removal of ground
water by anthropogenic drainage systems
can also locally alter ground-water gradients
and flow directions.
2.7.1 and
Potential receptors, receptor locations, and
exposure routes should be identified to deter-
mine if MNA can be applied to a site without
the risk of exposing a receptor to adverse envi-
ronmental or health conditions during the time
period of the MNA remediation. Receptors,
receptor locations, and exposure routes
relevant to MNA of a ground-water plume of
dissolved contaminants may include:
* Nearby water supply wells (public or pri-
vate), which may represent the most likely
pathway of human exposure to subsurface
contaminants.
Nearby surface water bodies, including
lakes, streams, springs, or wetlands, which
may be discharge locations for ground water
from the site. Risks may be environmental,
or human if the water body is used for
recreation.
Nearby residents or onsite commercial
workers, who may be exposed through
vapor intrusion into a residence or commer-
cial building, or through ingestion of ground
water from a private water supply well.
« Remedial workers, who may be exposed
during the ground-water and soil monitoring
that may occur during MNA, which may
occur over a longer period of time than with
active remediation technologies.
On-site construction workers, who may
be exposed through vapor inhalation or
dermal contact with contaminated soil or
ground water when working on utilities or
excavations.
The locations of these potential receptors
should be assessed to determine if they are
within or downgradient of the plume flow
paths, and could be reached during the lifespan
of the plume.
2.8
The transport of contaminants in the subsur-
face is affected by media and contaminant
properties. Further, the hydrodynamic, abiotic,
and biotic processes that affect the fate of
the chemicals during transport are strongly
influenced by the contaminant properties:
Contaminant identity. Contaminant
identity is an obvious, yet critical, vari-
able. Characterization should include not
only specified "contaminants of concern"
(COCs) and compounds with regulatory
remedial goals, but other compounds that
might have an impact on the subsurface fate
and transport of the COCs. Emerging or
overlooked contaminants can also impact
natural attenuation, and should be consid-
ered during characterization. Also of inter-
est may be tentatively identified compounds
(TICs), which may have been indicated by
some analyses to be possibly present, but
not positively identified. These compounds
should be positively identified to determine
their risk potential and effect on remediation
alternatives.
* Contaminant concentrations. The concen-
trations of dissolved contaminants are used
to delineate the contaminant distribution.
It may be desirable to prepare figures that
show the distribution of contaminants pres-
ent at levels greater than regulatory accep-
tance. Real-time data acquisition (obtained
by using field tests, or field GC and GC/
MS) coupled with laboratory analysis of
samples from temporary wells and evalu-
ation of the results can be used to select
appropriate locations for permanent moni-
toring points. Contaminant distribution may
also imply additional sources, previously
unidentified.
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Contaminant solubility. The contaminant
solubility in water is an approximate
indicator of the expected maximum levels
of dissolved contaminants. However, the
aqueous solubility of constituents of a
NAPL is a function of their concentration
within the NAPL. In relative terms, source
contaminants with higher water solubility
will be capable of producing higher con-
centrations of dissolved contaminants in the
plume.
Contaminant (NAPL) density. Contaminant
density is generally not an issue for sites
without NAPL and with just dissolved con-
taminants (although, in rare cases, a dense
aqueous phase plume (DAPL) may be pres-
ent at a site). Knowledge of the contami-
nant density is important for NAPL sites,
with LNAPL sites often having dissolved-
phase contamination near the water table,
and DNAPL sites often having NAPL-phase
and dissolved-phase contamination extend-
ing below the water table (potentially to
significant depths).
Contaminant mixture. Information about
different NAPL-phase contaminants that
may occur mixed together is used to deter-
mine solubility, density, and persistence of
such mixed contaminant sources, and can
serve as an indicator of contaminants that
will be encountered in the dissolved plume.
Some contaminants are more soluble when
co-dissolved with other classes of con-
taminants. Each contaminant present in the
source area should be included in plume
investigations. In addition, commingling of
different NAPL source contaminants may
result in a neutrally buoyant source area
contaminant mixture and/or produce unex-
pected dissolved contaminant distributions.
For example, commingling of tetrachloro-
ethene (PCE) (a DNAPL) and gasoline (an
LNAPL) source contaminants can lead to
the unexpected presence of a large portion
of dissolved PCE near the water table due
to PCE in the LNAPL phase. This would
be in addition to PCE in the dissolved phase
found considerably below the water table
due to the downward movement of PCE in
the DNAPL phase (some PCE NAPL phase
might be found at the water table along with
PCE in the dissolved phase; the distribution
of PCE also depends on the mass spilled).
A mixture of different contaminants can also
occur in the dissolved phase. Knowledge
of the different compounds in this type of
contaminant mixture is also important in
cases where one contaminant or type of
contaminant may be an electron donor for
other contaminants. For example, BTEX
compounds can be electron donors for
biodegradation of chlorinated compounds.
Partition (or Distribution) Coefficient.
Partition coefficients of each contaminant
in the dissolved-phase plume indicate how
compounds partition between different
subsurface phases, such as between soil
organic carbon and ground water. The soil
organic carbon/water partition coefficient
is used (in conjunction with the fraction of
organic carbon or fraction of organic matter
present in the soil) to estimate how much
of a compound might be sorbed to the soil
organic matter if the ground-water concen-
tration is known. This partition coefficient
is also used to calculate the contaminant's
retardation factor and velocity. If the
source area contains NAPL, it is important
to understand the partitioning of individual
compounds between the NAPL phase and
the dissolved phase.
Henry's Law Constant The Henry's Law
constant for a compound is a distribution
coefficient that is used to estimate vapor
phase concentration values if the water
phase concentration is known, and to
estimate the degree to which a contaminant
might partition from ground water to the
unsaturated zone vapor phase.
-------
Source Area Contamination. Sorbed
organic contaminants and N A PL in the
un saturated and saturated zones are
common sources of continuing dissolved-
phase ground-water contamination. NAPLs,
if present, generally represent the greatest
fraction of total contaminant mass at a site,
and can represent a very significant source
of continuing dissolved-phase ground-water
contamination. Since natural attenua-
tion processes do not reduce NAPL mass
rapidly, NAPL contamination and sources
might introduce insurmountable technical
challenges to MNA if there is significant
NAPL mass and resultant flux of dissolved
contaminants. Prediction and characteriza-
tion of the movement and distribution of
DNAPL is especially problematic, par-
ticularly if it has moved into the saturated
zone. It is important to carefully evaluate
the nature and distribution of source area
contamination, because they strongly
impact the nature and characteristics of
the dissolved-phase contaminant plume,
potential effectiveness of MNA, and the
design of an MNA performance monitoring
program. Characterization of source area
contamination requires a broad set of site
characterization variables that encompasses
a variety of types of information, discussed
below in Section 2.8.1. The source area
contamination variables include all infor-
mation regarding the contaminant source,
such as the source release location, history,
type, size, dimensions, volume, mass, and
distribution.
Source Control History. It is important to
carefully evaluate any source control activi-
ties, because they (as do the characteristics
of the source) strongly impact the nature
and characteristics of the dissolved-phase
contaminant plume, potential effective-
ness of MNA, and the design of an MNA
performance monitoring program. Source
control history is a broad site characteriza-
tion variable that encompasses a variety of
types of information, discussed below in
Section 2.8.2. The source control history
includes all information regarding activities
for removing and/or containing the source
area contamination.
2.8.1 Source Area Contamination
A number of items should be considered when
attempting to develop an understanding of the
spatial distributions of contaminants in subsur-
face source areas (i.e., source architecture).
and the resulting dissolved-phase plumes:
The location of the source area is used as a
basis for developing the conceptual model
of contaminant distribution. Identification
and delineation of the point of entry into
ground water of infiltrating (leaching)
dissolved contamination or of NAPL, along
with knowledge of the ground-water flow
direction, can help delineate the dissolved-
phase plume and provide some general
indication of the approximate NAPL
migration pathways. Initial dissolved
contaminant investigations are directed
downgradient of source areas. If DNAPL
is present, dissolved contamination may
occur downgradient of whatever (potentially
complex) path the DNAPL took. However,
the direction of the DNAPL flow pathway
may differ significantly from the direction
of ground-water flow (Cohen and Mercer,
1993).
The source area size and dimensions are
influenced by the source history, which
includes the spill amount, source release
date (i.e., time since release), spill type
and duration (e.g., a slow release or a
catastrophic event), and spill phase (free
product and/or dissolved phase).
The source area size and dimensions can
also depend on the chemical properties of
the spilled compounds; subsurface finger-
ing, relative permeabilities, and heteroge-
neities of the subsurface media; amount of
-------
recharge (infiltration); the geologic media
(see Section 2.5), and ground-water veloci-
ties. The size and duration of the contami-
nant release may also influence the move-
ment of NAPLs and the pore sizes which
the NAPL can enter (e.g., due to the head
pressure of the NAPL acting on movement
into the pores).
The amount of surface area of the source
area contaminant mass exposed to contact
with the ground water also affects source
persistence. For example, if ground-water
flow bypassed much of a NAPL source area
(and was in contact with the surface area of
just the outer portion of the NAPL source
mass) dissolution of the NAPL could be
slower and the source might persist longer
compared to ground water flowing through
a source area of dispersed small NAPL
globules where the ground water contacts
more of the NAPL mass.
The source mass (estimated amount of the
release) and flux (i.e., rate of contaminant
mass migration from the source) are used to
estimate source persistence and to define the
dimensions and persistence of a dissolved
plume. Plume length, plume mass, plume
concentration, plume persistence, and source
persistence will be, in general, directly
proportional to source mass and flux. For
DNAPL sites, even when using the best
available sampling technology, detection of
the liquid NAPL phase is difficult; at many
sites, it may be unlikely that the DNAPL
phase will be found and its distribution
adequately characterized. Estimates of
DNAPL mass in the source zone will have
a great deal of uncertainty due to geological
heterogeneity and the spatial heterogeneities
in DNAPL distribution.
Phase distribution of contamination.
Knowing what contaminant phases are
present helps in understanding the con-
taminant distribution and potential amount
of contaminant mass. Following a con-
taminant release, the contaminant may be
distributed among three phases (Figure 5):
(1) Sorbed onto the subsurface matrix.
(2) Dissolved in pore water. (3) Unsaturated
zone soil vapor. If NAPL is present, it will
represent a fourth phase: (4) Pure free-phase
NAPL. Free-phase NAPL may be pres-
ent at sufficiently high saturations to be
mobile or potentially mobile or as "residual
saturation" where disconnected globules are
trapped by capillary forces (and which is
not mobile under the prevailing conditions).
Type of NAPL and hydrogeologic setting.
Knowing what type of NAPL is present and
the media through which it migrates helps
in understanding the contaminant distribu-
tion and development of the dissolved-
phase plume. The NAPL may be lighter
than water (LNAPL) or denser than water
(DNAPL). NAPL will migrate from the
point of entry differently and in a more
complex manner than does dissolved-phase
contamination (i.e., leachate). LNAPLs
will tend to migrate downward through
the unsaturated zone until they approach
the water table and then spread laterally
at the water table. DNAPLs will also
migrate downward through the unsaturated
zone, but may continue their downward
migration even when they reach the water
table. As NAPL migrates through either
the unsaturated or saturated zone, a frac-
tion of the hydrocarbon will be retained by
capillary forces in the soil pores, potentially
serving as a source for continuing ground-
water contamination. Movement of NAPLs
through the subsurface into ground water is
complex and strongly influenced by large
and small-scale features of the subsurface
geologic environment (Mercer and Cohen,
1990; Cohen and Mercer, 1993). In both
the unsaturated and saturated zones, changes
in permeability and geologic discontinui-
ties such as bedding will impact the NAPL
pathway. NAPL will tend to spread laterally
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Figure 5. Contaminant Distribution. Contaminant mass can occur in the dissolved, sorbed, and vapor
phases, and in the NAPL phase if the release included NAPL. The contaminant distribution
will vary with depth, with the type of subsurface geologic media, and with the type of NAPL
released (i.e., LNAPL orDNAPL).
-------
on low permeability layers. Migration of
the NAPL will depend, in part, on subsur-
face topography and permeability of the low
permeability layer or permeability transition
zone, and the NAPL will tend to flow down-
slope by gravity.
2.8.2
In general, MNA would be more likely to be
effective at sites where any remaining sources
(such as NAPL) are controlled so as to prevent
further input of contaminants to ground water,
or where there are no source area contami-
nants that could provide continuing sources of
dissolved contaminants to site ground water
(perhaps through previous removal of source
area contaminants). At some sites, such as
with deep DNAPL sources, source control may
not be achievable. At such sites, assessment of
MNA effectiveness will depend on a thorough
analysis of the site characterization data, espe-
cially of data indicating that source mass is
minor and not mobile, dissolved contaminant
flux is minor, and/or the dissolved-phase plume
is stable or receding.
The history of contaminant releases and
implementation/effectiveness of source con-
trols affect contaminant distribution and the
temporal trends in dissolved contaminants at
any given monitoring point downgradient of
the source. For example, the current plume
configuration at a site may have been strongly
influenced by characteristics of the source
(e.g., source contaminant mass) before any
source removal or control activities had been
conducted, especially if a significant amount
of time had elapsed between the contaminant
release and the source control activities (note
that plume configuration could also have been
influenced by previous flow regimes different
from the current regime). Interpreting plume
development based on post-source control
source characteristics could be misleading.
Thus, it is important to assess any historical
information about the nature and distribu-
tion of the source prior to source control,
in addition to the current characteristics of
any remaining source area contaminants and
associated source control mechanisms.
Knowledge of the location, approximate time,
and relative mass of contaminant releases and
the timing and effectiveness of previously
implemented source controls is important for
the evaluation of observed temporal trends. It
can be problematic to use pre- and post-source
control data (such as contaminant levels or
MNA indicators) from wells located downgra-
dient of source areas for trend analyses and/or
degradation rates unless the potential impacts
of the source control activities are recognized.
For example, a declining trend in contaminant
concentrations downgradient of a source area
may be predominantly due to the effects of a
previous removal action rather than natural
attenuation processes (due to reduced dissolved
contaminant flux from the source, rather than
degradation of dissolved contaminants in the
plume). Estimates of contaminant transport
rates based on the hydrogeologic data, com-
parisons with the behavior of more conserva-
tive solutes found in the source material, and
other lines of evidence may aid in distinguish-
ing the effects of source history and controls
from those of natural attenuation processes.
If modeling of the plume is conducted, the
modeling simulations should take into account
any source removal activities.
2.8.3 Transformation and
Byproducts
Physical and biological processes associated
with natural attenuation can result in changes
to ground-water geochemistry, especially in pH
and Eh. Such changes can lead to the release
and mobilization of metals or non-metals
found in naturally occurring minerals in the
subsurface soils and sediments. For example,
naturally occurring arsenic and manganese
may be released at sites where the subsurface
system is driven to anaerobic conditions by
biological degradation of organic compounds
such as petroleum hydrocarbons. Mobilization
-------
may also occur for metals introduced during
site activities; in either case, the metals may
be of concern as contaminants. In addition,
the biodegradation of PCE and trichloroethene
(TCE) can result in the formation of c/'s-l,2-di-
chloroethene (cis-DCE) and vinyl chloride
(Wiedemeier et al., 1998). At some sites, cis-
DCE may accumulate since the biodegradation
of PCE and TCE may occur more readily than
biodegradation of cis-DCE. Similarly, vinyl
chloride may accumulate under anaerobic
conditions that favor PCE, TCE, and cis-DCE
biodegradation (whereas the vinyl chloride
could be more readily biodegradable under aer-
obic conditions). The transformation product
vinyl chloride is more toxic and mobile than
the parent compound (U.S. EPA, 1999). The
characterization of such degradation products
should be part of the site evaluation (Pope et
al., 2004). Some reaction products (such as
acetylene) are unique to abiotic reactions (He
et al., 2009), and monitoring of ground water
for these products can indicate the occurrence
of abiotic, rather than biological, degradation.
Due to potential temporal changes or spatial
variability, it may be necessary to continue
to monitor for transformation products and
byproducts or to monitor in different locations
along a flow path.
2.8.4
Measuring the concentration of conservative
tracers along a flow path may help to deter-
mine the contribution of adsorption, dilution,
and volatilization to contaminant loss. The
apparent reduction in the conservative tracer
along the flow path is assumed to be due to
those non-destructive processes. Loss of
contaminant greater than the loss of the conser-
vative tracer is assumed to be due to degrada-
tion, if the geochemical evidence indicates that
degradation is occurring. The conservative
tracer should have chemical and physical
properties as similar to the contaminants as
possible, while being resistant to degradation
under the prevailing environmental conditions.
Some inorganic or organic compounds pres-
ent in the ground water or in the contaminant
might be used as tracers. Organic tracers have
included 1,2,3,4-tetramethylbenzene present in
a light-crude-oil-contaminated ground water
(Cozzarelli et al., 1990) and 2,3-dimethyl-
pentane in a gasoline-contaminated ground
water (Wilson et al., 1994). Chloride can also
be a tracer, for example, by examining the
changes along a flow path from a source in
the proportions of inorganic chloride produced
and organic chloride lost during reductive
dechlorination of chlorinated solvents. Tracer
use can be problematic if there are external
sources (such as for chloride in areas of road
salting) or if there are multiple sources along a
flow path.
2.9
The geochemistry of the subsurface porous
media and ground water affects the transport
and fate of contaminants via abiotic and biotic
reactions and mechanisms (e.g., sorption and
degradation). Degradation reactions primar-
ily occur in the dissolved phase; however, the
solid-phase mineral content can enable certain
biological reactions (e.g., Fe(III) may be a
terminal electron acceptor(TEA)). The solid
phase geochemistry may also be important in
abiotic degradation of certain contaminants
such as chlorinated VOCs. He et al. (2009)
indicate that reactions occurring at the surface
of reactive iron and sulfur minerals present
in the solid phase can increase the rate of
reductive dechlorination of some chlorinated
contaminants, with the mineral surfaces acting
as electron donors and/or reaction mediators.
Characterization of the relative significance
of abiotic processes in natural attenuation is
a promising area of research, and biological
processes have been the focus at almost all
MNA sites. However, with increasing aware-
ness of abiotic natural attenuation processes,
the geochemical variables relevant to abiotic
degradation may become more routinely incor-
porated into MNA site characterization.
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Some of the geochemical variables listed
below are relevant for biologically-mediated
reactions; additional discussion of these
variables is in the section on biological vari-
ables. Biodegradation processes may cause
changes in geochemical variables, leaving an
observable geochemical "footprint" that can
be related to biodegradation processes (such
geochemical variables have been referred to
as "indicatorparameters", i.e., variables that
are indicative of biodegradation of contami-
nants). For example, petroleum compounds
usually serve as electron donors during
microbial degradation of the compounds (i.e.,
the compounds are oxidized during microbial
metabolism). During this process, TEAs
(e.g., oxygen, Fe(III), sulfate) are used, and
form reduced products (e.g., carbon dioxide,
Fe(II), sulfide, respectively). The decreases
in TEA concentrations can be determined,
and in many cases, the reduced products can
also be measured; the resulting data can be
used to indirectly evaluate biodegradation.
Geochemical variables can also indicate
whether redox conditions or other geochemi-
cal conditions could enhance the mobility of
inorganic anthropogenic or naturally occurring
compounds such as manganese or arsenic.
The geochemical variables to be characterized
for the hydrogeological units of interest and in
background samples are:
* Dissolved oxygen. Dissolved oxygen is
a TEA for contaminant biodegradation. It
also inhibits reductive processes (i.e., reduc-
tive dechlorination).
Nitrate (NO;). Nitrate is a TEA for
contaminant biodegradation. It may inhibit
reductive processes.
Manganese. Mn(IV) may act as a
TEA. Mn(II) may become mobilized as
a reduction product of Mn(IV). Mn(II)
measurement may be conducted at sites
where manganese has the potential to be a
ground-water contaminant, or for assessing
contaminant biodegradation where Mn(IV)
has acted as a TEA (e.g., in DCE degrada-
tion under manganese-reducing conditions).
Iron. Solid-phase ferric iron (Fe(III)) is
frequently present, may be bioavailable, and
can be used as a TEA by many microorgan-
isms. Measurement of Fe(II) in ground
water can be used for assessing contaminant
biodegradation where Fe(III) has acted as
an electron acceptor. Reactive iron minerals
can facilitate abiotic degradation of chlori-
nated organic compounds.
Sulfate (SO42-). Sulfate is a TEA for
contaminant biodegradation.
Methane (CH4). Methane can be a
byproduct of the biodegradation of petro-
leum hydrocarbons and other contaminants,
where carbon dioxide is used as a TEA
under strongly reducing conditions, and
can be indicative of reducing conditions in
ground water.
Dissolved hydrogen. Dissolved hydrogen
can be used to help identify the terminal
electron-accepting process occurring in
the ground water. It can also be a primary
electron donor (this is discussed further in
Section 2.10).
Oxidation-reduction potential (ORP)
or redox potential Important chemi-
cal changes in contaminants or naturally
occurring compounds can occur through
oxidation-reduction reactions, involving
transfer of electrons and changes in oxida-
tion states. The ORP can be an indicator
of the transfer of electrons between com-
pounds, and of the tendency for particular
transformations to occur (e.g., reductive
dechlorination takes place at low redox
potentials). However, ORP field measure-
ments may not correlate well with other
redox reaction data.
Metals and metalloids. Some metals (e.g.,
-------
chromium) may be of particular concern as
contaminants or potential contaminants at
a site. They may be anthropogenic con-
taminants at the site, or may occur naturally
and become mobilized by the changing
conditions occurring during natural attenua-
tion (e.g., manganese or arsenic). Reactive
solid-phase iron minerals can increase the
rate of reductive dechlorination of some
chlorinated contaminants.
pH. The ground-water pH can affect what
species of compounds occur and what reac-
tions may occur. Also, the activity of many
microorganisms is affected by pH.
Alkalinity. Ground-water alkalinity can be
indicative of the dissolution of carbonate
minerals by dissolved CO2 (carbonic acid)
and can be indicative of microbial activity.
Alkalinity also serves to buffer pH.
Soil organic carbon (Total organic carbon
(TOO or fraction of organic carbon
(foc)). Organic contaminants can sorb
to carbon-containing organic matter in
the subsurface; thus, measurement of the
organic carbon contained in the subsurface
matrix is important for calculations of
contaminant sorption, velocities, and travel
times. Some porous media analyses might
measure organic matter (%OM); this
quantity can be converted to foc.
Temperature. Different ground-water flow
zones may have slightly different water tem-
peratures; different ground-water tempera-
tures may be useful in distinguishing flow
paths and representative zones.
Additional major ions. Major ion (or
element) geochemistry involves analysis
and interpretation of concentrations and
ratios of common dissolved ions in ground
water (e.g., Ca2+, Mg2+, K', Na+, Cl", SO42-,
CO,2*, HCO3~). Major ion geochemistry can
be used to determine if ground waters in
different portions of the site have similar
geochemistry and possibly similar sources,
to differentiate waters derived from dif-
ferent semi-confined aquifers, to detect
ground water influenced by surface water
infiltration from a contaminated pond, or to
evaluate mixing of different ground waters.
Some of these major ions are also discussed
elsewhere in this document if they have
other specific uses.
Isotopes. Stable isotope and radioisotope
geochemistry evaluates concentrations of
isotopes of elements to indicate ground-
water sources and mixing. The abundance
patterns of different isotopes are influenced
by their sources and movement through
the ecosphere. Isotope geochemistry has
been applied to determine ground-water
age (i.e., when meteoric water entered the
subsurface) and evaluate mixing of waters
from subsurface aquifers and surface waters
(see Clark and Fritz, 1997, for a discussion
of isotopes for these uses). Stable isotope
geochemistry has also been used to identify
contaminant sources and evaluate contami-
nant degradation (see Section 2.10.1.1).
2.10
Biodegradation of contaminants has primarily
been evaluated indirectly by determining con-
taminant mass or concentration changes, and
by determining changes in the geochemistry of
the contaminated media caused or influenced
by biodegradation or biodegradation-related
processes. Generally, at many sites, in-situ
biodegradation of contaminants has not been
measured directly, although direct methods
continue to be developed and the use of these
methods has become more established (as
discussed in Section 2.10.1).
Biodegradation of contaminants through
redox reactions requires utilization of electron
acceptors and electron donors. Contaminants
that are oxidized during the biodegradation
(e.g., BTEX) are electron donors, and require
suitable terminal electron acceptors. Other
-------
contaminants that are reduced during biodeg-
radation (e.g., chlorinated solvents undergoing
reductive dechlorination) are electron accep-
tors and require a suitable electron donor.
Collection and interpretation of data on elec-
tron donor and TEA identity, concentration,
mass, and behavior is a vital part of MNA site
characterization. This information addresses
the second line of evidence for evaluating
MNA (introduced in Section 2.1).
Subsurface TEAs are utilized in the sequence
dissolved oxygen (under aerobic conditions),
then nitrate, Mn(IV), Fe(III), sulfate, and
carbon dioxide (under anaerobic conditions).
This sequence will generally not be well-
defined or distinct temporally or spatially, as
different terminal electron-accepting processes
may occur in close proximity in time or space
(and some overlapping of the processes may
occur).
Data indicating the supply of TEAs are used
to determine the sustainability of the oxida-
tive biodegradation processes. Sufficient
TEA supply must be available for the mass of
contaminant present that is to undergo oxida-
tive biodegradation. The amount of a specific
TEA present will limit the extent and/or rate of
biodegradation occurring under that electron-
accepting microbial process. Huling et al.
(2002) discuss the methodology for examining
the balance between available TEA and the
TEA required for natural attenuation at a site
with a dissolved-phase BTEX plume resulting
from an LNAPL source area. The presence of
degradable organic carbon compounds other
than contaminants can lead to an overestima-
tion of the amount of contaminant biodegra-
dation that will occur, as TEAs are used to
oxidize non-contaminant organic compounds,
causing a decline in TEAs that is not accompa-
nied by contaminant degradation.
A continuous supply of electron donor is
required to support sustained reductive dechlo-
rination. As the electron donor is degraded,
the concentrations of the naturally occurring
subsurface TEAs may change if these TEAs
are also being used during degradation of the
electron donors. Characterizing changes in
these TEAs (including a contaminant acting
as an electron acceptor) and electron donors
provides information used to determine if
the subsurface conditions are suitable for the
biodegradation of the contaminant. In addi-
tion to decreasing concentrations of TEAs
and electron donors, microbial metabolism of
TEAs and electron donors produces metabolic
byproducts that may also be easily detected
and quantified as additional evidence of the
nature and extent of contaminant biodegrada-
tion at a site.
The biological variables, and relevant chemical
or geochemical variables related to biodegra-
dation, include:
Contaminant concentrations. Decreasing
contaminant (parent compound) concentra-
tions are a primary line of evidence for
natural attenuation. However, for the most
effective natural attenuation, decreases in
contaminant concentration should be linked
to contaminant destruction rather than to
non-destructive processes such as dilution.
At high concentrations, the contaminant
toxicity may be an impediment to microbial
degradation of a compound.
« Daughter products. Degradation of a
contaminant produces degradation prod-
ucts (daughter products). The increase in
daughter product concentrations should be
proportional to the decrease in parent com-
pound concentrations. The parent/daughter
product proportionality can be determined
by examining the stoichiometry of the
degradation reaction. Daughter products
should be measured for evaluating the rate
and extent of biodegradation of chlorinated
solvents. Reductive dechlorination of PCE
yields TCE, which then degrades to DCE,
which then degrades to VC, which finally
-------
degrades to ethene/ethane (Wiedemeier
et al., 1998). The relative distribution of
daughter products along the ground-water
flow path will indicate the presence or
absence of required metabolic processes
for subsequent reductive pathways, and
allows the determination of rates of reduc-
tive dechlorination taking place under field
conditions. Daughter products of petroleum
compounds such as BTEX are fermentation
products (alcohols, fatty acids, etc.) that are
considered to be relatively nontoxic and
readily biodegradable and non-persistent
under most aquifer conditions (Wiedemeier
etal., 1998).
Byproducts. Byproducts may result from
the biodegradation of a contaminant, such as
chloride in the case of chlorinated solvents.
Oxidation-reduction potential (ORP) or
redox potential. The ORP is a general
field measurement sometimes used as a
rough indication of petroleum hydrocarbon
biodegradation (and thus, approximating
the plume location (Wiedemeier et al.,
1999)) and type of microorganism likely to
be present and active (e.g., methanogens at
low ORP). Highly negative redox readings
typically are indicative of the biodegrada-
tion of electron-donating contaminants
(e.g., readily degradable contaminants such
as petroleum hydrocarbons). Oxidation-
reduction potential decreases as oxygen, and
then other TEAs (i.e., nitrate, manganese,
sulfate) are removed from the system during
biodegradation).
Ground-water organic carbon (Total
organic carbon (TOC) and dissolved
organic carbon (DOC)). Both anthropo-
genic organic carbon and natural organic
carbon can serve as electron donors, as in
the reductive dechlori nation of chlorinated
compounds (Wiedemeier et al., 1998). If
anthropogenic organic carbon is lacking,
collection of a ground-water sample and
measurement of the organic carbon content
can indicate if natural organic carbon may
be present to serve as an electron donor
(Wiedemeier et al., 1998). The heterotro-
phic bacteria responsible for much of the
biodegradation occurring in natural attenu-
ation use organic carbon as their carbon
source; inorganic carbon is not used in this
process, although heterotrophs have been
reported to assimilate small quantities of
CO2 (Alexander, 1977). Inorganic carbon,
however, does have a role in the carbon
cycle, which impacts the ground-water
geochemistry.
pH. Biodegradation processes are pH-
sensitive (Wiedemeier et al., 1998); for
example, reductive dechlorination of PCE
by Dehalococcoides ethenogenes strain 195
is optimum at pH 6.8 to 7.5 (Maymo-Gatell,
1997). Changes in pH may occur due to
production of organic acids during biodeg-
radation of organic compounds.
Temperature. The rate of microbial
activity (e.g., biodegradation) generally
increases with increasing temperature
(although, if there are other limiting factors,
increased microbial activity may not occur
with increasing temperatures (Alexander,
1994)). Increased microbial activity with
increasing temperature is likely only within
the range of temperatures tolerated by the
biodegrading microbes. Most soil bacteria
are able to grow at temperatures between
15 °C and 45 °C, and have optimal activ-
ity between 25 °C and 35 °C (Alexander,
1977). Temperatures above those ranges
can adversely impact microbial activity and
survival.
At most sites and under normal conditions,
however, temperature would rarely have
a significant impact on natural attenuation
(geothermal water perhaps being an excep-
tion). Significant temperature variations are
unlikely since ground-water temperatures
are relatively constant. However, for near-
surface soils and ground water in colder
-------
northern regions, seasonal effects may
occur, such as with the warmer temperatures
during summer (Alexander, 1994).
Alkalinity. Increases in ground-water
alkalinity are expected in response to the
production of carbonic acid as CO2 from
microbial activity (in areas with carbonate
minerals, dissolution of the minerals by the
carbonic acid also contributes to alkalinity).
Dissolved oxygen. Microorganisms
preferentially use DO as a TEA when using
organic carbon contaminants as electron
donors. Areas of petroleum hydrocarbon or
other organic carbon contamination will be
expected to have DO depletion compared
to background DO levels. If background
DO levels are high, little or no DO at a
site is usually indicative of the presence of
contamination, which may or may not be
the contaminants of primary concern, acting
as electron donors for a viable population
of aerobic microbes. Some biodegradative
processes and microorganisms are inhibited
by oxygen; for example, the presence of
oxygen prevented reductive dechlorination
of PCE by Dehalococcoides ethenogenes
strain 195 (Maymo-Gatell, 1997).
Nitrate. Nitrate depletion with respect to
background indicates that nitrate is serv-
ing as a TEA when contaminants or other
sources of organic carbon are acting as
an electron donor. If background nitrate
levels are high, low levels of nitrate at the
site may be indicative of the presence of
contamination acting as electron donors for
nitrate-reducing microbes.
Manganese. Mn(II) may be present as
a reduction product of the TEAMn(IV).
Manganese has been a less commonly
measured TEA and transformation product;
however, it should be routinely measured
to provide additional information on the
electron-accepting processes occurring at a
site.
Iron. The TEA ferric iron (Fe(III)) is
seldom measured, as it occurs in the solid
phase rather than the dissolved phase.
However, the soluble ferrous iron, Fe(II) is
a reduction product of Fe(III) and is more
easily measured. Elevated levels of ferrous
iron with respect to background levels typi-
cally indicate that contaminants and other
sources of organic carbon are being utilized
by iron-reducing microbes. If background
ferrous iron concentrations are low, then
higher concentrations on site may be used
as an indication of petroleum hydrocarbon
biodegradation and of the relationship
between the plume and the metabolic by-
product Fe(II) (Wiedemeier et al., 1998).
Sulfate. Depletion of sulfate with respect
to background sulfate concentrations
indicates that sulfate is serving as a TEA
when contaminants or other organic materi-
als are acting as a carbon source. If back-
ground sulfate levels are high, low levels
of sulfate at the site may be indicative of
contamination biodegradation accomplished
by sulfate-reducing bacteria. Sulfide is
a product from the reduction of sulfate;
sulfide is readily reoxidized to sulfate in
the presence of oxygen, or precipitated as
metal sulfides, so sulfide may not be present
in high concentrations even if significant
sulfate reduction is taking place.
Methane. Methane is produced during
methanogenesis, when CO,, is used as a
TEA during biodegradation of petroleum
hydrocarbons and other organic compounds.
The presence of methane above background
levels may indicate that microbes have
depleted all other TEAs. Elevated levels
of methane may sometimes be used as an
indication of petroleum hydrocarbon bio-
degradation. However, methane may also
be formed under natural conditions (e.g.,
"swamp gas" in wetlands). The presence of
naturally formed methane may pose difficul-
ties in attributing methane to the presence
-------
and natural attenuation biodegradation of
contamination.
Dissolved hydrogen. Dissolved hydrogen
is an electron donor for halorespiration
of some chlorinated compounds, and is
produced by fermentation of various organic
compounds such as petroleum hydrocar-
bons and other organic carbon compounds
(Wiedemeier et al., 1999). The dissolved
hydrogen concentration can be used to
determine whether reductive dechlorina-
tion is possible. Also, dissolved hydrogen
can be used to more accurately identify
the actual terminal electron-accepting
processes when other indicators are incon-
clusive (Lovley et al., 1994 and Loffler et
al., 1999). This is a specialized, and less
common measurement, although there are
commercial labs that provide these analyses
on a routine basis.
Microbial community. The subsurface
microorganisms can be classified based
on their characteristics, the environmental
conditions under which they live, or on
their effects. In general, aerobes (aerobic
populations) live under aerobic condi-
tions, anaerobes (anaerobic populations)
under anaerobic conditions, and faculta-
tive microorganisms can live under either
aerobic or anaerobic conditions. The
microbes may also be classified as to the
geochemical impact they have (i.e., sulfate
reducers or methanogens), or by their
genus or species (e.g., Dehalococcoides
sp., or Dehalococcoides ethenogenes).
Some general identification, measurement,
and classification of the subsurface micro-
organisms as to their behavior or effects
will provide greater understanding of the
microbial processes occurring at the site.
This characterization can include the com-
position and physiological capabilities of
the microbial community, and the density
(population) of its various components.
Characterization of the microbiological
community is important primarily for sites
with chlorinated solvents, since complete
biodegradation of the toxic chlorinated
compounds requires the presence of spe-
cific microbes that are not always present
at a site.
isotopes (2H/'H and 13C/12C). Stable
isotope geochemistry has been used to
evaluate contaminant biodegradation (dis-
cussed in more detail in Section 2.10.1.1).
For example, stable isotope geochemistry
has been used to indicate if a particular
chlorinated compound is a daughter product
resulting from biodegradation or whether
it was a compound present in the original
contamination source.
Changes in various geochemical indicators
accompany biodegradation, and help not only
to show that biodegradation is occurring but
also indicate what primary TEA processes
are occurring throughout a site. Geochemical
data and trends can be used to provide the
following kinds of information related to
biodegradation:
* Whether ambient redox conditions and
processes favor biodegradation of the
contaminants, as well as identifying the
dominant degradation processes and long-
term monitoring parameters indicative of
the continuing effectiveness of the biodegra-
dation processes.
Whether stoichiometric relationships
between TEA (oxygen, nitrate, sulfate, etc.)
utilization and contaminant biodegradation
are observable. If a clear relationship is
observed, this may help provide an indica-
tion of the rate of contaminant loss during
biodegradation; Dupont et al. (1998) state
that the rate and extent of microbial utiliza-
tion of TEAs during biodegradation should
correspond to observed contaminant loss.
* Identification of zones beyond the current
-------
plume boundaries where soluble electron
acceptors or donors are depleted or non-
hazardous reaction products are enriched
with respect to ambient ground water but
contaminants are not detected. The water in
these zones has been called "treated water"
(i.e., water that once was contaminated
but has been remediated through natural
attenuation biodegradation). Monitoring
the geochemistry of such zones helps to
determine plume stability.
Geochemical variables and trends that may be
useful indicators of biodegradation processes
are discussed in more detail in Wiedemeier
et al. (1998; 1999) and Wiedemeier and Haas
(2002). As discussed in these references, the
individual variables diagnostic of dominant
processes and most useful depend on site-
specific conditions.
^" ^
Key Point
Measurement and interpretation of the
biological and geochemical variables
provide indirect indications that subsurface
processes are occurring to reduce contami-
nation. This is the second line of evidence
mentioned in the OSWER Directive on
MNA(U.S. EPA, 1999).
v, s
2.10.1 Direct Approaches to of
The site characterization methods and vari-
ables discussed above generally involve
indirect indicators of subsurface biodegrada-
tion processes. Other, more direct methods of
evaluating biodegradation have been devel-
oped, such as analysis of stable isotopes and
evaluation of microbial community structure
and dynamics.
2.10.1.1 Microbiological and Molecular
Techniques
Techniques have been developed to evaluate
microbial community structure and dynam-
ics. These techniques are used to determine
whether the microorganisms have the capabil-
ity to produce specific contaminant-degrading
enzymes, or to categorize the microorganisms
relative to functionality (e.g., methanotrophs)
or community structure (e.g., population
dynamics). For example, the presence and
activity of populations of particular species
known to carry out biotransformations (e.g.,
Dehalococcoides sp., associated with reductive
dehalogenation of halogenated solvents) can be
measured. These relatively more recent meth-
ods may become more widely used in MNA
site characterization as experience is gained
in their use, and as they start providing direct
evidence of naturally occurring biodegrada-
tion at sites where current information may be
inconclusive. Weiss and Cozzarelli (2008) and
ITRC (2011) provide thorough and valuable
overviews of numerous existing and emerging
microbial and molecular methods that can be
used for evaluating natural attenuation. They
provide examples and discussion of how these
methods have been used to investigate subsur-
face biogeochemistry at contaminated sites and
how they can provide information to evaluate
the potential for natural attenuation at a site, or
to identify indicators of contaminant biodeg-
radation. Microbial and molecular methods
include:
* Nucleic acid techniques employing the
polymerase chain reaction (PCR) can be uti-
lized to detect and enumerate specific gene
sequences (Weiss and Cozzarelli, 2008).
Quantitative PCR (qPCR) can provide quan-
titative information on the genes involved in
biodegradation of specific contaminants.
« Fluorescence in-situ hybridization (FISH),
which uses fluorescent molecules to mark
genes or chromosomes, allows identification
of microorganisms containing genes for
specific enzymes of interest (e.g., enzymes
that remove Cl~ from chlorinated solvents).
Yang and Zeyer (2003) report on the use
of FISH for detecting Dehalococcoides
species.
-------
Terminal restriction fragment length poly-
morphism (T-RFLP) analysis is a technique
that allows rapid profiling of diverse forms
of an individual gene (such as 16S rRNA
genes (i.e., 16S rDNA)) to help assess the
diversity, structure, and population changes
of the various functional groups of bacteria
(e.g., autotrophic ammonia oxidizers, deni-
trifiers, methanotrophs, and methanogens).
This may help to determine if particular
microorganism groups are present at a
site. Lorah et al. (2003) illustrate the use
of T-RFLP, in an investigation of anaerobic
degradation of 1,1,2,2-tetrachloroethane
during natural attenuation of chlorinated
VOCs in wetland sediments.
* Phospholipid fatty acid analyses (PLFA)
measure the lipids present in the viable
microbial biomass and can be used to
help differentiate between various groups
within microbial communities, and moni-
tor changes in the microbial community as
contaminant and geochemical concentra-
tions and conditions change. PLFA was one
of the methods used by Davis et al. (2002)
to estimate total biomass for an evaluation
of natural attenuation of chlorinated ethenes.
2.10.1.2 Stable Isotope Evaluation
Stable isotope fractionation techniques (i.e.,
compound-specific isotope analysis, or CSIA)
can be used to evaluate the biodegradation of
organic contaminants such as PCE, TCE, DCE,
and VC. Biological degradation of organic
compounds using enzymatic processes causes
changes in the ratio of hydrogen (2H and 1H)
or carbon (13C and 12C) isotopes in the parent
compounds and daughter products. The light
isotopes are enriched in the daughter products,
and the heavy isotopes are enriched in the
parent compound as the parent is transformed
to the daughter by biological processes. For
example, 12C has a smaller mass, forms weaker
bonds, and is more reactive than 13C. As TCE
is dechlorinated by microorganisms, 12C bonds
are preferentially broken, causing an isotopic
enrichment of the remaining TCE in 13C. The
relative abundances of 12C and 13C in the
supposed parent and daughter compounds can
indicate whether the presence of a daughter
product is due to biodegradation or whether
it was present as an initial contaminant, and
can also provide information about the rates
of degradation. The use of stable isotopes for
investigating biodegradation is discussed in
Hunkeler et al. (2008). Further examples of
the usage of stable carbon isotopes are pro-
vided by Hunkeler et al. (2005), McKelvie et
al. (2007), Song et al. (2002), and Wilson et
al. (2005).
2.10.1.3 Microcosm Studies
The third line of evidence for evaluating MNA
is data from field or laboratory microcosms
that can directly demonstrate the processes
causing contaminant loss. If the first two lines
of evidence are inconclusive (i.e., if informa-
tion collected during site characterization of
the variables discussed above cannot conclu-
sively demonstrate the occurrence of NA),
then microcosms studies can be conducted.
Laboratory or in-situ microcosms consist of
small amounts of environmental media (soil,
sediments, ground water) that are isolated
or partially isolated from the environment
and studied to determine how contaminants
degrade in the media.
Microcosm studies may be useful in cases
where the contaminant biodegradation path-
ways are not well known or where specific
site conditions are considered likely to inhibit
biodegradation. Biodegradation of common
petroleum contaminants is well documented
and therefore microcosm studies are not likely
to be required. Degradation pathways of
chlorinated solvents are fairly well understood,
and microcosm studies may not be useful
unless the usual site-specific field-derived
evidence for degradation is equivocal. In
some cases, microcosm studies may be useful
to indicate likely ranges of contaminant
-------
biodegradation rates, although degradation
rates achieved in laboratory microcosms often
differ dramatically from those rates calculated
from field data. Transformation products may
be identified using microcosm studies. The
use of appropriate experimental conditions
and controls, mass balance measurements, and
identification of transformation products may
assist in distinguishing and elucidating biotic
and abiotic loss mechanisms. The result from
microcosm studies conducted by Ferrey et
al. (2004) suggested that abiotic degradation
processes were responsible for the observed
loss of c/.v-DCE that had been produced by
biological reductive dechlorination of TCE.
Microcosm studies can also help determine
the need for, or impacts of, additives such as
nutrients, electron donors, electron acceptors,
or bioaugmentation cultures, and to detect
toxicity problems (i.e., inhibition of degraders
by contaminants or environmental conditions).
Ex-situ microcosms, consisting of small
amounts of site media brought to the labora-
tory, have been widely used for these purposes.
In-situ microcosms, involving some method
of partially isolating the environmental media
(e.g., by driving a pipe into the subsurface to
partially isolate the media forced into the pipe
from the rest of the subsurface), have been less
commonly used.
Ex-situ microcosm studies suffer from prob-
lems associated with removal of samples
from the site (e.g., shock to microorganisms
upon sampling and removal to laboratory
conditions) and the general differences
in environmental conditions between the
laboratory and field (e.g., contaminant and
geochemical fluxes, temperature cycles, and
other natural fluctuations under field condi-
tions). In-situ microcosms are considered
more likely to be representative of field
conditions, but may involve more expense
for preparation and monitoring than ex-situ
microcosms. Both types of microcosms
suffer from the fact that the media used in the
microcosm represent a very small portion of
the subsurface, considering the well-known
temporal and spatial variability of subsurface
conditions. Therefore, although microcosm
studies can provide a general indication of the
items discussed in the preceding paragraph,
the results may not apply to all portions of
the subsurface. However, for contaminants
where there may be some uncertainty regard-
ing their biodegradability, a microcosm study
may provide one additional piece of evidence
in deciding whether or not to continue the site
characterization and MNA evaluation.
Key Point
Microcosm studies are part of the third
line of evidence mentioned in the OSWER
Directive on MNA (U.S. EPA, 1999).
Information from this line of evidence may
be particularly useful if the information
from the other two lines of evidence is
inconclusive.
v J
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3.0
THE MNA SITE CHARACTERIZATION PROCESS
3.1
Contaminated sites generally undergo the fol-
lowing sequential stages of investigative and
remedial activities:
Recognition of a contamination problem.
* Initial assessment of currently available
information about the site.
« Site characterization, encompassing the
collection of more detailed, current site
information through field investigations and
monitoring (e.g., ground-water data).
Assessment and screening of potential
remedial approaches.
Collection of detailed data specific to a par-
ticular issue and/or to designing, implement-
ing, and monitoring a particular remedial
approach.
« Design, implementation, and monitoring of
the remedial approach.
At some point during these activities (e.g.,
during the site characterization stage), the
potential for the use of MNA as a remedial
technology may be suggested if it is noted
that NA processes may be occurring at the
site, if it is realized that the site may have
characteristics common to other MNA sites,
or if the dissolved contaminant plume is stable
or receding. At that point, specific activities
relevant to and constituting the MNA site
characterization can be undertaken. For sites
with contaminants and conditions poten-
tially amenable to MNA (e.g., chlorinated
solvents sites with anaerobic ground water,
or petroleum-hydrocarbon fuel sites), collec-
tion of information relevant to MNA (such as
TEA concentrations) at the beginning of the
site characterization process could speed the
assessment of MNA.
A significant portion of site characterization
activities consists of mobilization to the field
and collection of soil and ground-water quality
data. An expedited approach, such as the EPA
Triad approach (discussed in Section 3.2.1),
can be a powerful and cost effective strategy
in conducting these activities. However, in
a broad sense, site characterization involves
acquisition of information that will help in
understanding the contamination and potential
remediation of a site; this occurs during all site
activities and not just during one field mobili-
zation. Thus, some site characterization could
occur and be useful during any of the investi-
gative and remedial stages mentioned above.
In this broad sense, site characterization should
be an iterative process, in which additional
information is collected to address additional
questions or potential approaches that arise.
On-going development and refinement of a
detailed conceptual site model is especially
important during MNA site characterization, as
collection and interpretation of the numerous
types of field data provide new insights on the
subsurface conditions and processes relevant
to effective MNA. Uncertainties regarding
specific values of variables can be addressed
by making conservative assumptions for those
values, and conducting sensitivity analyses to
determine the impact of those assumed values.
This allows the site characterization and
the MNA decision-making process to move
forward.
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3.2
Activities
The general likelihood of the potential for
successful MNA can be assessed during the
initial MNA site characterization activities.
Initial activities for assessing the potential for
MNA and the scope of further characterization
should address the following issues:
Identification of unacceptable current
impacts on receptors, or a realization that
contaminants are very unlikely to degrade
to acceptable levels before they reach the
potential receptors, suggests that MNA will
not be effective and that some active reme-
dial technology would be needed to stop the
current or impending receptor impact.
« The types of contaminants present (based
on the historical records and sample
analytical results from monitoring wells)
should be identified to assess if they are
amenable to MNA. Sites with a single
contaminant (e.g., benzene or PCE) or
single class of contaminant (e.g., petro-
leum hydrocarbons or chlorinated ethenes)
often are more appropriate for MNA than
those sites with multiple different types of
compounds (e.g., a site with chlorinated
ethenes, chlorinated ethanes, heavy metals,
and PAHs). However, sites with both
petroleum hydrocarbons and chlorinated
solvents are especially amenable to MNA.
Further, some VOCs are more recalcitrant
than others and less easily attenuated under
certain site conditions. The initial identifi-
cation of VOCs at a site may indicate that
potentially more recalcitrant byproducts
(such as c/s-DCE and vinyl chloride at chlo-
rinated ethene sites) have accumulated; this
indicates that natural attenuation processes
may not be occurring sufficiently to meet
remedial goals.
* Sites with significant geologic complex-
ity will likely have a lower probability
than sites with less complex geology of
definitively documenting success using
MNA, and will likely be more difficult and
expensive to accurately characterize.
* The tools and techniques used to character-
ize the site for MNA should be evaluated.
The costs for the methods used should be
weighed against the amount and quality of
information that can be obtained. Sampling
and analysis costs will also affect any
assessment of costs for MNA characteriza-
tion methods, especially if some longer-term
monitoring is needed to collect sufficient
data for evaluating plume behavior as part
of the MNA site characterization.
« Ground-water flow should be assessed by
monitoring a sufficient number of piezom-
eters and/or ground-water monitoring wells
(including wells in transects) on a sufficient
basis to determine temporal variations of
the flow pattern over seasonal changes
throughout the year.
The geochemistry required to promote
natural attenuation needs to be addressed.
If the geochemistry of the site is shown
to be highly variable (either temporally,
spatially, or both), then it may be difficult
to correlate TEA and electron donor (e.g.,
petroleum hydrocarbon contaminant) use to
demonstrate that the MNA will achieve site
remedial goals across the site and through-
out time.
If MNA appears to have only a limited chance
for success, those site characterization activi-
ties relevant only to MNA can be stopped, and
other remedies can be considered. However,
the MNA site characterization may indicate
that while MNA might not be effective,
enhanced bioremediation technologies perhaps
might be effective, and the MNA site charac-
terization could continue as characterization
for those technologies. In cases where MNA
appears to be a potential alternative, MNA site
characterization would continue.
-------
Table 3 lists the MNA site characterization requirements of any other site activities need
activities, in a general sequential order from to be considered.
initial steps (recognizing that MNA might have
potential applicability at a site) through final
steps (making recommendations for imple-
menting MNA). The activities are discussed
throughout the rest of this document. Not all
of these activities will be necessary at every
site. The activities to be conducted depend on
the scale and complexity of the site, the type
and amount of existing information, and other
activities at the site; they can also be influ-
enced by cost and time constraints. Additional
discussions regarding the site characterization
process can be found in Wiedemeier et al.
(1998).
MNA site characterization is not likely to be a
stand-alone activity; it occurs in conjunction
with other activities at a site. Some of the site
characterization activities would be conducted
even if MNA were not being investigated, for
example, during the Remedial Investigation
(RI) phase of the CERCLA process. The
variables that may be unique or specific to an
MNA site characterization are the microbio-
logical and geochemical variables, which relate
to biodegradation processes. However, those
variables may be investigated even if MNA is
not being proposed, for example, during evalu-
ation of enhanced bioremediation.
MNA may be evaluated as a potential remedial
technology, for example, during the Feasibility
Study (FS) phase of the CERCLA process.
For some sites, it may be a stand-alone tech-
nology; however, for many sites where MNA
appears potentially effective, it is likely to be
part of a set of combined technologies. This
may involve using MNA for the downgradi-
ent, lower-concentration portion of a dissolved
plume and/or as a longer-term polishing step in
an area that is first treated with a more aggres-
sive technology. The role of MNA in overall
site remediation can affect the type, location,
and duration of the MNA site characterization
activities, since the impacts or monitoring
-------
Table 3 Approach to and Sequence of MNA Site Characterization Activities.
PRELIMINARY ACTIVITIES
Review Existing Information
* Review existing conceptual site model.
« Review site setting, history of site uses, contaminant characteristics, and waste release history.
Review existing contaminant concentration data.
Obtain and review existing literature on local and site-specific geology and hydrogeology.
* Review previous characterization activities and data on geology, hydrogeology, contaminants, and
geochemistry.
Review past and present remedial activities.
Review potential receptor locations.
Preliminary Evaluation of Natural Attenuation
Examine preliminary evidence for the occurrence of natural attenuation processes, based on existing
information.
Establish preliminary remedial goals.
Conduct literature review of the efficacy of biodegradation or other attenuation processes for the contaminants
of concern.
* Develop preliminary MNA conceptual site model.
Make recommendations for continuing MNA-specific site characterization and investigation.
FIELD SITE CHARACTERIZATION ACTIVITIES
Planning
« Evaluate suitability of existing monitoring locations for MNA data collection.
Determine if there is suitable recent hydrogeological information (to include piezometric surface elevations);
evaluate this information for ground-water flow directions.
* Assess site geological conditions to determine appropriate subsurface boring techniques and methods, subsur-
face contaminant delineation techniques, and geophysical measurement techniques.
Plan monitoring point or monitoring well locations and construction details.
Plan location, number, and description of transects.
* Establish monitoring parameters, analytical methods, and sampling frequency.
Prepare Sampling and Analysis Plan.
Prepare Quality Assurance Project Plan.
* Prepare Health and Safety Plan, as needed.
Implementation
Some of these items may require additional iterations, as data are evaluated, although the use of the Triad
approach (expedited characterization) should minimize such iterations.
* Conduct site-specific reconnaissance and surface mapping.
Conduct comprehensive synoptic round of piezometric surface elevation measurements, contaminant and
geochemical sampling, and laboratory analyses using existing monitoring locations (only if suitable for use for
MNA variables).
« Conduct surface geophysical measurements.
Conduct direct-push borings, temporary well installation, logging, soil sample collection, ground-water sample
collection, and subsurface contaminant delineation.
* Install monitoring wells, based on direct-push results and/or laboratory results from temporary wells.
Conduct geophysical logging of newly installed wells.
Conduct comprehensive synoptic MNA-spccific baseline round of piezometric surface elevation measure-
ments, contaminant and geochemical sampling, and laboratory analyses using all suitable monitoring locations
(including newly installed wells).
Conduct real-time evaluation of sample results and assessment for additional sampling locations.
Perform hydrogeological measurements (e.g., aquifer tests), as needed
* Conduct periodic synoptic rounds of piezometric surface elevation measurements in monitoring wells.
Conduct periodic synoptic rounds of monitoring well contaminant and geochemical sampling and laboratory
analyses.
ED]
-------
Table 3 continued...
As samples arc collected, conduct laboratory analyses and measurements of soil and ground water for
geological, hydrogeological, contaminant, and geochemical variables (e.g., grain size, hydraulic conductivity,
contaminant concentrations. TOC. and DOC).
MICROBIOLOGICAL LABORATORY (As Needed)
Microbiological and molecular techniques to identify microbes and determine if suitable microbes are present.
« Stable isotope evaluation.
Conduct microcosm studies.
CHARACTERIZATION DATA ORGANIZATION FOR UPDATING CONCEPTUAL SITE
MODEL
Geological and Hydrogeological Variables Information
* Describe stratigraphy, lithology, geological structures, geological units, and hydrostratigraphic units.
« Prepare tables of geologic and hydrogeological data (e.g., grain size, hydraulic conductivity), with appropriate
statistical measures.
Prepare maps and transect cross-sections showing the geologic and hydrogcologic setting of the site.
* Prepare piezometric surface maps for each hydrostratigraphic unit.
Anthropogenic and Anthropocentric Variables Information
* Describe engineered features, wells, and receptors.
« Assess impact of anthropogenic variables on contaminant fate and transport.
Contaminant Variables Information
Prepare tables of contaminant concentration data, with appropriate statistical measures.
* Prepare maps of contaminant concentration data for each hydrostratigraphic unit.
Prepare cross-sections with contaminant concentration data for each transect (transverse to plume, and longitu-
dinal/plume centerline.).
Geochemical Variables Information
Prepare tables of geochemical data (i.e., TOC/DOC. electron acceptor, electron donor, daughter product, and
reaction byproduct concentrations), with appropriate statistical measures.
Prepare maps of geochemical data for each hydrostratigraphic unit.
« Prepare cross-sections with geochemical data for each transect (transverse and longitudinal/plume centerline.).
Biological Variables Information
* Prepare tables of biological and microbiological data, with appropriate statistical measures.
Prepare maps of biological and microbiological data for each hydrostratigraphic unit.
Summarize relevant literature on biodegradation of site contaminants.
CHARACTERIZATION DATA ANALYSIS AND INTERPRETATION
Update conceptual site model (an iterative process as data are collected).
Evaluate contaminant properties and applicable attenuation processes.
* Evaluate contaminant phase distributions (dissolved, sorbed, NAPL), and controls on the phase distribution.
Evaluate and describe contaminant source(s).
Identify microbial populations, applicable degradative processes, and the necessary environmental conditions
for microbial activity.
« Conduct statistical evaluation of data, to assess uncertainty.
Identify the three-dimensional nature, spatial variability, and temporal variability of conditions and processes at
the site, and their impact on the MNA interpretations
-------
Table 3 continued...
1 Prepare maps and cross sections showing spatial distribution and relationships of hydrogeology, contaminant
concentrations, and geochemical data.
Identify and map representative zones and flow paths.
Calculate contaminant velocities in ground water, retardation, and travel times to receptors.
Estimate contaminant mass or mass flux, and spatial or temporal changes at or between transects.
Analyze trends in contaminant, geochemical, and biological data and relevance to natural attenuation
processes.
Calculate attenuation rates (Concentration vs. Distance, Concentration vs. Time, and biological).
Conduct mathematical modeling of contaminant fate and transport.
Evaluate impact on MNA of other actual or potential remedial activities, and other near-site activities.
Calculate MNA remedial time frames, relative to the remedial goals for the site.
Prepare MNA site characterization report and recommendations.
RECOMMENDATIONS (AS WARRANTED)
* Provide recommendations on the need for longer-term characterization and trend evaluation prior to MNA
remedy decision-making.
Provide recommendations on the applicability and use of MNA as a stand-alone technology or as part of a set
of combined technologies for all or portions of the site.
* Provide recommendations on monitoring locations, frequencies, parameters, sampling methods, and analytical
methods for longer-term characterization and trend evaluation prior to MNA remedy decision-making, and/or
for MNA performance monitoring.
-------
3.2,1 Site
Site characterization for MNA will often
require the acquisition of relatively large
amounts of vertically and laterally discrete
data. The speed and ease of direct-push
sampling, if geologic conditions permit its use,
allow data to be gathered from many discrete
locations (and some direct-push equipment has
continuous logging capabilities). The direct-
push characterization data, as well as data from
temporary wells, can also be used to select
the most representative permanent sampling
locations for MNA performance monitor-
ing. At sites where direct-push equipment
cannot be used due to geological conditions,
nests of conventional monitoring wells can be
installed at depth intervals appropriate to the
site geology, hydrogeology, and contaminant
distribution.
In some cases, innovative technologies can
be used to expedite the site characterization.
Surface geophysical techniques, soil conduc-
tivity probes, and contaminant sensors such as
membrane interface probes (MIP) may aid in
determining the location for monitoring points,
in the collection of information on the MNA
variables, and in initial delineation of repre-
sentative zones. They can aid in delineation
of the source area, providing information on
the source area variables to be used in attenu-
ation time frame calculations. However, they
may not provide data of quality comparable
to laboratory analytical data for items such as
ground-water contaminant concentrations.
Accomplishing a detailed MNA site char-
acterization investigation in a cost-effective
manner often requires the use of rapid sample
acquisition and analysis technologies by
highly trained field personnel, and the flex-
ibility to adjust the field activities based on
real-time data. EPA and other Federal and
State agencies developed the "Triad" approach
(Crumbling, 2001), a set of strategies to use
new sampling and analysis techniques along
with real-time communication with interested
parties to enable investigators to support
field-based decision making. This approach
was developed to decrease the total costs of
site characterization, as well as of subsequent
remediation and performance monitoring. The
Triad approach consists of three fundamental
elements:
Systematic planning based on an evolving
conceptual site model.
* Dynamic work strategies (i.e., a work plan
that is modified in the field based on field
results).
Real-time measurement technologies, using
rapid sampling techniques such as direct-
push technologies, fast-turnaround fixed
laboratory analyses, and/or on-site analysis,
which allow rapid use of the results to influ-
ence the field activities.
The expedited site assessment (ESA) approach
and specific relevant methods and techniques
are discussed in U.S. EPA (1997), focusing
on use at underground storage tank locations.
U.S. EPA (1997) describes the ESA approach
and how it compares to conventional site
assessment approaches. It provides detailed
information on surface geophysical methods,
soil-gas surveys, direct push technologies, and
field analysis of petroleum hydrocarbons. A
variety of sampling and analysis techniques are
described in numerous other technical docu-
ments, such as U.S. EPA (1986; 1991; 1993)
and Wiedemeier et al. (1998).
3,2.2
The overall objective of MNA site character-
ization is to provide site information regarding
natural attenuation processes that will allow
evaluation of MNA as a potential remedial
technology. This requires selecting locations
for sampling and monitoring, discussed below.
The MNA site characterization activities may
gradually transform into longer-term monitor-
ing if MNA is implemented as a remedial
-------
technology. The monitoring locations selected
and used during the site characterization and
initial evaluation of MNA might be utilized
during early performance monitoring and
longer-term monitoring (i.e., during long-term
stewardship of the MNA site), since, presum-
ably, they have been carefully selected to
yield information specific to MNA. However,
the total number and density of transects and
monitoring locations may be different during
these different stages of monitoring. For
example, fewer transects might be used during
the longer-term monitoring if the focus is
on compliance boundaries rather than three-
dimensional monitoring of the entire plume.
In some cases, new transects might be needed
for the longer-term monitoring, for example,
if the site characterization has better identified
the location of a critical flow path. In any
case, the site characterization will provide data
to establish the detailed three-dimensional
CSM so that monitoring point locations and
sampling frequency for performance monitor-
ing can be determined.
MNA site characterization locations are based
on the following considerations:
Plume lateral (i.e., sidegradient) boundaries
should be defined. A cluster of wells can be
placed on each side of the plume in uncon-
taminated ground water, for each transverse
transect across the plume. These boundary
monitoring wells can be placed a short
distance outside each lateral plume bound-
ary and, ideally, coupled with another well
just inside the plume boundary (i.e., paired
well clusters to define the plume bound-
ary). Variations or trends in contaminant
concentrations in these coupled wells can
be used to monitor for lateral expansion or
shifting of the plume. If the plume bound-
ary location shifts, new paired well clusters
can be installed.
Plume vertical boundaries should be
defined, by well clusters or points extending
to an uncontaminated interval beneath the
plume.
The downgradient extent of the plume
should be defined and monitored, using a
transect of wells or points placed downgra-
dient of the plume and in its flow path, and
upgradient of potential receptors. These
downgradient locations can be used to allow
detection of an expanding plume and initia-
tion of an alternate remedial action prior to
contaminants impacting the receptors.
Background wells or a transverse transect of
background wells upgradient of the source
area will provide the background geochemi-
cal variable data necessary for comparison
to information collected within the plume.
A longitudinal transect (i.e., a transect along
the plume centerline, which is the longitudi-
nal axis of the plume) can provide the con-
taminant, geochemical, and microbiological
variable information necessary to calculate
attenuation rates. The individual locations
in this longitudinal transect ideally are
placed in the highest concentration portion
of successive downgradient cross-sections
(i.e., transverse transects) of the plume. The
initial presumed plume centerline location
can be estimated based on the initial site
data. This estimated location of the plume
centerline may need to be reconsidered
and/or refined as additional data are col-
lected and as a better understanding of the
site develops through interpretation of the
data. A well-defined plume centerline may
not exist: rather, there may be a somewhat
broader region encompassing the center of
the plume.
Multiple transverse transects across the
plume at different locations along its flow
path will provide information on the spatial
variability of, and changes in, the MNA
variables. Comparison of flux rates across
these transects should allow the determina-
tion of contaminant degradation rates in
-------
the plume. Each transverse transect should
encompass the entire plume width at that
location. A transverse transect located
immediately downgradient of the source
area provides information on the contamina-
tion leaving the source area (i.e., the source
area flux) and moving downgradient into the
dissolved-phase plume. The use of three or
more transverse transects will help in accu-
rately understanding the three-dimensional
hydrogeological and geochemical environ-
ments of the site. For example, transects
could be spaced so that each transect
measures contaminants at significantly
lower concentrations (such as at order of
magnitude changes) than the preceding
upgradient transect. Correct placement of
these transects to achieve such a spacing
may require some initial sampling along the
longitudinal axis of the plume.
The spacing and number of locations
laterally for each transverse transect should
be based on the size, heterogeneity, and
geologic complexity of the site. The initial
sampling location can be near the initially
presumed plume centerline. Once sampling
locations have been established to define the
lateral boundaries, additional lateral transect
sampling locations within each side of the
plume can be placed halfway between the
plume centerline location and the boundary
locations. Three locations within the plume
would be considered the minimum number
needed within the plume for each transverse
transect (in addition to two outside the
plume to define the plume boundaries).
Sites with broad plumes might need addi-
tional sampling locations in a given trans-
verse transect. Additional transect sampling
locations can continue to be placed halfway
between each previous set of adjoining
sampling locations, resulting in a progres-
sively denser network of sampling points.
In general, the density of lateral sampling
points is increased until a consistent and
identifiable pattern is evident in contaminant
or geochemical concentrations along the
transverse transect. Figure 6 provides an
example of transect development.
The depth, location, and length of well
screens, and the number of sampling loca-
tions vertically at each transect location,
depend on the geological and hydrogeologic
complexity of the subsurface. At least one
sampling location is necessary within each
discrete vertical zone of interest (i.e., zone
of contaminated ground water). Sites with a
number of different relatively thin geologic
units and flow zones (and thin flow zones
in thick geologic units) might necessitate
more intensive sampling and shorter well
screens to differentiate the different units.
Relatively thick or homogeneous geologic
units could be characterized with longer
well screens or more widely spaced vertical
or lateral sampling locations.
« All hydrostratigraphic units in contaminated
and adjacent areas at the site should be
represented in the transects and sampling
locations (areally and vertically).
Figure 7 illustrates a hypothetical ideal set of
transects and sampling locations for an MNA
site. Transect development and sample spac-
ing are discussed by Guilbeault et al. (2005)
and Kao Wang (2001). In general, the
number and spacing of both lateral and vertical
sampling locations (i.e., sampling density and
transect complexity) need to match the geo-
logical complexity of the site. For example,
interbedded sands, silts, and clays may require
a higher lateral and vertical density of sam-
pling points since the units may not be laterally
continuous and are likely to be relatively thin.
Further, it may be necessary to factor in the
potential for multiple sources, multiple path-
ways, and natural or anthropogenic preferential
pathways. In many cases, there is a focus on
defining the boundary of a plume, and poten-
tial additional sources inside the plume could
-------
be missed if the sampling points inside the
plume are too sparse.
Due to site conditions (e.g., engineered
features) or constraints on resources or time, it
may not be possible to have an ideal number
of transverse transects, with their resulting
increased number of lateral and vertical data
points. The transect approach, however, even
with fewer transects, will often provide more
valuable information than the use of more
scattered sampling points or the use of existing
monitoring wells that may not permit adequate
delineation of ground-water flow paths or
estimation of contaminant flux.
-------
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sequential manner until the areal and vertical extent of the plume is well-defined and the
three-dimensional nature of the subsurface is understood, (a) An initial transverse transect
is placed near the presumed source area, (b) Additional locations are investigated along the
initial transect, (c) The results from subsurface sampling provide definition of the vertical and
transverse extent of contamination along the first transverse transect, (d) Second and third
transverse transects are placed downgradient of the first transect, (e) Sampling locations are
placed along the presumed longitudinal axis of the plume, (f) A transverse transect downgra-
dient of the previous transects defines the downgradient extent of the plume.
-------
(a) Site (Map View)
Upgradient
Transect (5
Lateral (Side Gradient)
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Ground-Water Flow
(b) Cross Section A - A'
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Target Monitoring Zones
J) Source area
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' concentrations and mobility
Plume fringes
Plume boundaries
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from historical trends
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locations
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-------
4.0
INTEGRATING AND EVALUATING SITE DATA
4.1
Data collected during initial site characteriza-
tion activities and during subsequent more
detailed site characterization activities focusing
on MNA are used to develop the conceptual
site model, an understanding of subsurface
processes, and attenuation rate estimates, in
order to assess the potential for MNA as a site
remedial technology.
Assessment and integration of the data involve
several basic steps:
* Placing the data in a context of time, loca-
tion, sampling protocols, and analytical
protocols. The time context includes data
collected during the site characterization,
supplemented by historical data (although
the historical data must be scrutinized for
comparability to the data collected during
the MNA site characterization). The loca-
tion context includes the geological (i.e.,
stratigraphic) setting, as well as the lateral
and vertical position of the data within the
site. Sampling protocol context includes
the lengths and positions of well screens,
as well as sampling methods. Analytical
protocol context includes analytes measured
and detection limits.
« Calculation of basic statistical measures
(means, ranges). In calculating means from
data, one should consider whether the data
fit normal distributions (e.g., porosity data),
log normal distributions (e.g., hydraulic
conductivity data), or some other form of
distribution.
Presentation and visualization of the data
using tables, graphs, charts, maps, time-
series plots, and cross-sections. Use of
a variety of visualization methods can
enhance the understanding and interpreta-
tion of the relationships and patterns among
time, location (distribution), physical
processes, biological processes, and geo-
chemical processes in the subsurface.
Applying appropriate statistical tests to
detect changes and trends, to assess the
potential for attainment of goals.
Making decisions based on the data.
The conceptual model for MNA, the site
characterization program, and the data assess-
ment are modified in an iterative manner as
necessary during the data evaluation process to
reflect new data and new understandings of the
site architecture and processes.
4.2
4.2,1
Process
Established standards of practice should
apply to MNA site characterization data
analysis, such as the U.S. EPA's Data Quality
Assessment (DQA). DQA is "the scientific
and statistical evaluation of environmental
data to determine if they meet the planning
objectives of the project, and thus are of the
right type, quality, and quantity to support
their intended use" (U.S. EPA, 2006). Given
the large amount and numerous types of data
developed during MNA site characterization,
a thorough evaluation of the data type, qual-
ity, and quantity is critical. The DQA process
is discussed fully in U.S. EPA (2006) and
is discussed relative to MNA performance
monitoring in Pope et al. (2004).
The DQA process provides a means for
determining changes and trends. Data on
contaminant concentrations, geochemical
-------
variables, and ground-water flow variables
are assessed with respect to levels, changes,
and trends. Contaminant changes are direct
measures of remediation goals, and geochemi-
cal and ground-water flow data are important
for assessing contaminant transport, fate, and
attenuation processes. The data must be inter-
preted in light of data variability so that real
changes or trends in data values can be distin-
guished from data variability. This is particu-
larly important in visualizing and quantifying
three-dimensional concentration distributions,
in estimating rates of natural attenuation, and
in making predictions of the time frame to
achieve remediation goals. Evaluations should
be performed to determine the uncertainties
and variability associated with data or analyses
(and the degree of variability associated with
an individual measurement, or the means of
measurements, should be indicated).
4.2.2
It is prudent to consult a statistician or
appropriate literature on statistics during the
planning phase (before collecting data) with
respect to statistical considerations. This will
help ensure that a sufficient amount of data,
and the right type of data, are collected to
address any hypotheses that are established
up front. Detailed discussions on the statisti-
cal issues mentioned below is provided in
U.S. EPA (2009).
In order to use characterization data from indi-
vidual locations or sets of locations to make
scientifically-defensible inferences about other
parts of the site (i.e., the rest of the plume), it
is necessary to use statistical procedures that
provide a systematic way to estimate plume
characteristics from individual data points or
sets of data points, or to evaluate changes and
trends.
In choosing and applying an appropriate
statistical test for a particular analysis, it is
necessary to consider:
The purpose of the test (i.e., detect a trend,
compare means to a threshold value, etc.).
MNA characterization data are primarily
used to investigate trends, examine correla-
tions between variables, and estimate rates
of degradation.
Sampling design (sampling location). Most
commonly used statistical tests assume that
the data are the result of random sampling
(i.e., that the data are taken from randomly
chosen sampling locations). However,
monitoring well location is often based on
professional judgement, and sampling tran-
sects for MNA site characterization are not
randomly located. In those cases, results
from random-sample-based statistical tests
should be viewed with caution; preferably,
more appropriate statistical tests should be
used.
Sampling design (sampling frequency).
Statistical tests can require sample statistical
independence. Contaminant concentra-
tions in a well at different sampling events
are often related to each other because
they are derived from the same source and
are transported by the same processes. If
the data are not independent, some basic
analyses may be readily adjusted to account
for non-independence using methods in
U.S. EPA (2009; 1992). Spacing sampling
events appropriately in time can help attain
physical independence of samples from a
well. Discussion and guidance on taking
physically independent samples is found in
U.S. EPA (2009; 1992).
Data characteristics. Probability-based
statistical test (the two major types are
parametric and nonparametric) selection
is based on the probability distribution of
the data. Parametric tests rely on certain
underlying assumptions about the form and
parameters of the data distribution (e.g.,
normal or log-normal distribution). Testing
for normality and equal variance (see
-------
U.S. EPA (2009; 2006; 2002a)), and data
transformation may be needed for paramet-
ric tests. Nonparametric tests do not require
any assumptions about the distribution of
the data.
Outliers are data values that are extremely
large or extremely small when compared
to the bulk of the data. They may be true
data or a result of error. Excluding true data
or including erroneous data in a statistical
test will distort the results of the statisti-
cal analysis. Identification of outliers (see
U.S. EPA (2009; 2006; 2002a)) and a course
of action if outliers are identified may
be necessary. The first step in assessing
potential errors in data would be to conduct
a data quality review, prior to applying
statistical tests to the data set.
Non-detects. Quantitative values must be
established to conduct statistical tests using
data sets that contain any "non-detect"
values. Methods to do so are discussed in
U.S. EPA (2009), Chapter 13 ofHelsel and
Hirsch (2002), and in Gibbons and Coleman
(2001).
Multiple comparisons. Statistical analyses
of data sets often involve making many
comparisons within and among data sets.
However, multiple comparisons for many
probability-based statistical methods can
increase the possibility of a false posi-
tive (Type I error), and using methods to
control for Type I error can increase the
possibility of a false negative (Type II
error). U.S. EPA (2009; 2006; 2002a; 1992)
contain methods for assessing the effects of
Type I and Type II errors.
For a detailed discussion of the considerations
for choosing statistical tests, as well as step-
by-step methods or guidance for calculations,
see U.S. EPA (2009; 2006; 2002a; 1992).
Statistical software resources are listed at
http://www. epa. sov/0 UALITY/ga links.
html#software . If necessary, a statistician
with experience in working with contaminated
sites should be consulted to determine the most
appropriate statistical methods for analysis of
the data.
4.3 Data Analysis
4.3.2 Data Comparisons
Data comparisons are often made as part of
the performance monitoring phase of an MNA
remedy (Pope et al., 2004); these data com-
parisons can also have utility during the site
characterization phase. The data comparisons
include:
Comparisons with background levels.
Differences in parameter values between the
plume and background locations can indi-
cate the occurrence of biological processes,
as discussed in Section 2.7.1.2.
Comparisons of contaminant levels with
regulatory requirements can be used to
identify and map the portion of a plume that
exceeds regulatory levels, and to identify
sampling and transect locations.
Comparisons to determine if contaminant
or daughter product levels are increasing
or decreasing. These comparisons include
both trend tests to determine upward or
downward concentration trends through
time, and intrawell comparison tests
(Gibbons, 1994), which compare recent
data at a monitoring location to historical or
previously collected (e.g., baseline) data at
the same location. The monitoring period
required to reliably determine if contami-
nant levels are increasing or decreasing is
likely to be longer than the time allotted for
the site characterization, so trends may not
be as readily apparent during the site char-
acterization as during performance monitor-
ing. It may be possible to integrate site
characterization data with prior, historical
data to assess trends, although care must be
taken to ensure that data are comparable. In
-------
particular, care must be taken when compar-
ing ground-water data pre- and post-source
zone actions. Another approach would be
to conduct an extended monitoring period
up to several years following an initial
period of intensive site characterization,
before final decisions are made on the use
of MNA as a remedial technology.
« Comparisons with existing literature and
laboratory studies (e.g., how the attenuation
rates compare with those at other sites or
in laboratory studies). Comparison of site
characterization data with laboratory or
literature data can indicate if the character-
ization data are in the range that could be
expected, or that are realistic. However,
it should be remembered that site-specific
conditions can vary significantly from other
sites or from laboratory results.
« Spatial trends are also important in the data
evaluation process. For example, compari-
sons of contaminant concentrations and
geochemical indicators of microbial activity
as seen in different portions of subsurface
cross sections can indicate the lateral dis-
tribution of contaminants and can delineate
areas of varying microbial activity.
4.3,2 Contaminant
Transport Calculations
One of the basic uses for the hydrogeologic
characterization data is the calculation of the
contaminant velocities in ground water. The
contaminant velocities help in estimating how
much time there is until a potential receptor
might be impacted, which is one factor in
developing a remedial strategy. As an under-
standing of the attenuation processes (e.g.,
biodegradation) and rates is developed, the
contaminant travel time to the receptor and
expected concentrations at the receptor can
be estimated (and the remedial time frame
estimated).
The contaminant velocity calculations involve
the use of some measured or derived variables.
Variables used:
« Hydraulic conductivity (horizontal and/or vertical).
* Hydraulic gradient (horizontal and/or vertical).
* Total or effective porosity.
« Fraction of soil organic carbon.
« Partition coefficient.
The values used for these variables in the cal-
culations should reflect the ground-water and
contaminant flow paths of most significance at
the site. The use of values averaged arithmeti-
cally from all site-wide locations is likely to
result in misleading results.
The calculated values are:
« Specific discharge, also known as the Darcy
flux, Darcy velocity, and apparent velocity,
calculated using Darcy's Law.
* Seepage velocity, also known as average
linear velocity.
Retardation factor (R). The higher the R
value, the slower the contaminant travels in
ground water relative to the ground-water
velocity and the longer it will take to travel
a given distance.
The equations for calculating these quantities
are given in general texts on ground water
such as Freeze and Cherry (1979), Fetter
(1993), and Domenico and Schwartz (1998).
4,3,3
The dissolved-phase mass flux through a
cross-section (transect) can be estimated using
the contaminant concentrations and calculated
ground-water discharge at and through the
transect. The mass flux across a given transect
can be compared to that of a downgradient
transect to estimate the mass of contami-
nant being degraded between the transects.
However, changes in concentrations could also
be due to non-degradative processes such as
dilution or sorption, and errors in the measure-
ment of the ground-water variables lead to
-------
uncertainties in the calculated ground-water
flux. Further, the two transects would each
have to sample the same ground-water flow
paths.
Total contaminant mass in the plume can be
calculated from contaminant concentration
results, interpolating concentrations between
sampling locations, and making assumptions
about the three-dimensional volumes con-
taining given concentrations. Contaminant
mass estimates can be made for each of the
dissolved, sorbed, and NAPL phases (a mass
estimate for a source area with NAPL would
include all three phases; a mass estimate for
the dissolved phase would include the dis-
solved and/or sorbed phases). These mass esti-
mates are "mass-in place" estimates (ideally,
representing the entire contaminant mass in the
plume). If prepared in a consistent manner,
mass estimates from one sampling event can
be compared to those from subsequent sam-
pling events to establish trends and determine
the amount of mass loss due to destructive
processes (as long as any apparent losses due
to non-destructive processes such as dilution
or sorption are understood). However, it is
extremely difficult to know that the mass esti-
mates represent the mass actually in the plume
or in the source, especially if there is signifi-
cant subsurface heterogeneity or variability in
contaminant distribution. NAPL presence in a
source area makes accurate estimates virtually
impossible for the source area. Contaminant
mass estimates for the dissolved phase
plume might be better for a slightly-sorbing
dissolved-phase contaminant, especially if the
mass estimates use only the dissolved-phase
concentrations and neglect the sorbed-phase
concentrations. These mass and mass change
estimates are useful as one piece of derived
information and should be attempted; however,
their limitations and assumptions must be
recognized.
4,3,4
One criterion for determining whether MNA is
an appropriate remedial technology at a given
site is "whether or not the contaminant plume
is stable" (U.S. EPA, 1999). Information is
also necessary on whether or not the environ-
mental conditions that influence plume stabil-
ity may change with time, and thus change the
stability condition of the plume. Geochemical
data collected during the site characterization
(within the plume and in background locations)
are useful for assessing the environmental
conditions and the potential for change.
Plume stability assessment is an important
aspect of evaluating the potential use of MNA
at a site, as the OSWER Directive (U.S. EPA,
1999) indicates that "sites where the contami-
nant plumes are no longer increasing in extent,
or are shrinking, would he the most appropri-
ate candidates for MNA remedies ". The MNA
site characterization activities provide informa-
tion used in this assessment. The state of a
plume (i.e., the plume trend) is often described
using the terms expanding, stable, or shrink-
ing. It may be very difficult to conclusively
show that a plume is stable (and incorrect to
use statistics to prove stability), thus, "a plume
that is stable" may be more accurately consid-
ered to be "a plume that appears stable" or "a
plume that is showing relatively little change".
However, statistics can be used to prove (or
show) that a plume is not stable.
The underlying foundation for assessing the
state of a plume (regardless of the specific
plume stability method used) is, essentially, the
use of ground-water contaminant concentra-
tions (from monitoring wells located through-
out and around the plume) and examinations
of changes (trends) over a period of time.
While the simplest interpretation of changes
in ground-water contaminant concentrations in
the monitoring wells would be that increases
in concentrations signify an expanding plume,
decreases in concentrations signify a shrinking
plume, and relatively constant concentrations
-------
signify a stable plume, the evaluation of plume
stability is less straightforward and there are
a number of issues to consider. These simple
apparent trends may not accurately indicate the
plume behavior since contaminant concentra-
tions in a well (or wells) can be affected by:
* Fluctuations in ground-water levels leading to
exposure of ground water to a greater or a lesser
thickness interval of contaminated media or uncon-
laminatcd media.
« Changes in ground-water flow direction.
"Hot spots" of contamination passing by the well.
Changes in the TEA supply that change the extent
of biodegradation.
« Sampling and laboratory error and variability.
Collection of the site characterization informa-
tion discussed throughout this document will
help in understanding the potential impacts of
these issues, and will help prevent misinter-
pretation of the monitoring well contaminant
trends. It is important to obtain information
from a network of monitoring wells that ade-
quately defines the three-dimensional extent
of the plume and which allows a well-defined
plume boundary region. Robust monitoring
data are needed. It is also important to rec-
ognize that since there is often uncertainty in
subsurface conditions and processes, periodic
review of the data and data interpretations may
be needed.
Under certain conditions, the state of the
plume (expanding or shrinking) can be
assessed using temporal trends in contaminant
concentrations at a number of individual moni-
toring locations around the leading edge of
the plume. The state of the plume (expanding,
stable, or shrinking) can also be determined by
changes in well-defined plume boundaries.
A variety of methods have been used to evalu-
ate plume stability, examining either the tem-
poral changes in contaminant concentrations
throughout the network of monitoring wells, or
examining the temporal changes in contami-
nant mass throughout the plume. Wiedemeier
et al. (1999) discuss visual and statistical tests
using concentration data; Dupont et al. (1998)
describe the use of Thiessen polygons for the
estimation of plume mass and plume centroid
of mass; Looney et al. (2006) discuss empiri-
cal and deterministic approaches for quantify-
ing plume stability, and present a mass balance
concept for documenting the relative stability
of a plume; and Ricker (2008) describes visu-
alization, numerical, and statistical techniques
for assessing plume stability in terms of plume
area, average concentration, contaminant mass,
and center of mass. It can be advantageous to
use more than one method to evaluate plume
stability, especially where there is uncertainty
about the plume being stable or expanding.
4,3,5
Decisions on MNA as an appropriate remedial
technology for a given site generally incorpo-
rate estimates of the rates of natural attenua-
tion processes, expressed with respect to either
time or distance from the source, and based on
site characterization data (decisions on MNA
also incorporate other estimates, such as the
source area mass flux and source persistence).
U.S. EPA (1999) indicates that time-based
estimates are used to predict remedial time
frames and distance-based estimates to evalu-
ate potential plume expansion, stability, or
shrinkage. One objective of MNA site char-
acterization is the calculation of one or more
of the three types of attenuation rate constants
(concentration vs. time (C vs. T), concentration
vs. distance (C vs. D), and biodegradation);
U.S. EPA guidance on the use of rate constants
for MNA is provided and thoroughly discussed
in Newell et al. (2002).
Methods for estimating attenuation rate
constants include numerical models, or simple
empirical methods that entail regressions of
concentration vs. distance, relative concentra-
tion vs. distance, concentration vs. time, or
mass vs. time data.
Beyer et al. (2007) used numerically-simulated
contaminant plumes and different hypothetical
-------
monitoring networks to evaluate strategies for
estimating first-order degradation rate con-
stants. They compared plume centerline rate
constant methods and a rate constant method
using all simulated data. Their findings
illustrate that significant potential uncertain-
ties in estimated attenuation rate constants can
arise, which are due to monitoring locations
being off the plume centerline and inadequate
estimates of hydrogeologic variables, as well
as source and plume width. These uncertain-
ties are also significant in a method presented
by Buscheck and Alcantar (1995), in which
an overall attenuation rate (encompassing
contaminant concentration decreases from all
attenuation mechanisms) was estimated using
a regression of contaminant concentration
data as a function of distance along a plume
centerline. They then calculated a degradation
rate (encompassing contaminant concentra-
tion decreases from destructive mechanisms,
presumably due to biodegradation) by subtract-
ing the estimated contaminant concentration
changes due to hydrodynamic effects (diffu-
sion, dilution, etc.) from concentration changes
indicated by the overall attenuation rate.
However, this method incorporated numerous
assumptions, such as a steady-state distribution
of contaminant concentrations downgradient
of a continuous source. An evaluation of the
influence of input parameters on the Buscheck
and Alcantar method is provided by McNab
and Dooher (1998). The uncertainties and
difficulties in determining the attenuation rates
indicate that site characterization activities for
MNA need to obtain sufficient suitable infor-
mation and data on the relevant variables, to
lessen the impact of these uncertainties.
Newell et al. (2002) discuss the calculation
and use of attenuation and biodegradation
rate constants. Concentration vs. distance
(C vs. D) attenuation rate constants (obtained
by examining contaminant concentration data
along a flow path extending away from the
source) indicate how rapidly dissolved-phase
contamination is diminished after leaving the
source area. Concentration vs. time (C vs. T)
attenuation rate constants (obtained by examin-
ing contaminant concentration with time at
individual monitoring locations within the
plume) are used to assess natural attenuation
because such rates are directly applicable
for determinations of plume lifetime in the
locations where the data were collected.
Biodegradation rate constants are used in
solute transport models. The analyses for the
three types of rate constants are shown concep-
tually in Figure 8.
4.3.5.1 Concentration vs. Distance
Attenuation
C vs. D attenuation rate constants track the
change in contaminant concentration or mass
along a plume, using data from a series of
sampling points at different distances from the
source along the longitudinal axis (centerline)
of the plume. The calculation and use of
C vs. D attenuation rate constants assumes a
stable plume. However, clues to plume stabil-
ity can be gleaned from these rate constants.
The change in such a rate constant, calculated
from data taken at different sampling times
from the same group of sampling locations,
can suggest whether the plume is relatively
stable, expanding, or shrinking. For example,
a lower rate constant at the later sampling
time would suggest that the plume might be
expanding. However, such an interpretation
is not definitive; rather the apparently lower
rate might have been due to other factors, such
as a shift in the plume direction or a changed
ground-water flow velocity. Newell et al.
(2002) provide a method using the C vs. D rate
constant to estimate if a plume is "showing
relatively little change".
Collection of data for use in the C vs. D
calculation requires knowledge of site geol-
ogy and hydrogeology since the data should
be from sequential points along a flow path.
The site hydrogeology should be understood
sufficiently so that flow paths can be identified
and sampled to produce representative data.
-------
Beyer et al. (2007) discuss these issues and
uncertainties.
Historically, a single C vs. D rate constant
has been used to quantify the applicability
of MNA at a site; however, this implicitly
assumes that:
An individual flow path can be identified
and monitored over time with a static moni-
toring network (i.e., assuming temporally
stable flow paths).
An individual flow path is representative of
the entire plume (e.g., in terms of attenua-
tion of contaminants, and potential impact
to receptors).
Sampling methods provide discrete data
from only the flow path of concern.
Dissolved contaminant concentration data
from the series of sampling points used
for the C vs. D attenuation rate constant
are comparable over time (e.g., assuming
temporally stable source loading). Source
strength changes with time have to be
understood (Amerson and Johnson, 2003).
« Dissolved contaminants emanate from a
single discrete source (e.g., there are not
multiple sources along the plume).
Natural attenuation processes (especially,
microbial processes) are temporally stable
and spatially consistent throughout the
plume.
It is unlikely that all or even most of these
assumptions are true at a given site, so use
of a single C vs. D attenuation rate constant
to assess the potential for MNA at a site is
problematic. A more suitable approach is cal-
culation of C vs. D attenuation rate constants
from a variety of flow paths representing the
identified representative zones.
4.3.5,2 Concentration vs. Time Attenuation
Concentration vs. time (C vs. T) attenuation
rate constants (point decay rate constants)
track the change in contaminant concentra-
tions or mass with time at one sampling point
in the plume, using data taken at multiple
times from the single sampling point. If the
mass flux from the source is significantly
decreasing and the plume is shrinking, the
source lifetime can be estimated using these
rate constants, because the rate of source
attenuation is a significant factor in determin-
ing contaminant concentration changes with
time at a given point in the plume. However,
proper evaluation of source lifetime using C
vs. T rate constants requires rate constants
calculated from data taken from multiple
points distributed throughout the entire plume
to integrate the effects of variations in source
strength and longevity across the width of the
source area.
C vs. T rate constants can be used to estimate
the time needed to achieve a specific contami-
nant concentration at the location where the
data were collected. The data for C vs. T rate
constant calculations must be compiled over
several years (three to five years or longer) so
that longer-term contaminant concentration
trends can be differentiated from source varia-
tions, seasonal changes in ground-water flow
paths, changes in microbial activity, etc.
4.3.5.3 Biodegradation Rate Constants
Biodegradation rate constants track the rate
of biological degradation of a contaminant,
unlike the C vs. D and C vs. T rate con-
stants that lump together all the attenuat-
ing processes, including biodegradation.
Biodegradation rate constants are most often
used as an input parameter to models that inte-
grate the major fate and transport parameters.
4.3.6
Computer models integrate information on
hydrogeological, geochemical, and biological
-------
(a)-
o
= Slope
Time
(b)-
= V^ Slope
Distance from Source
(c)-
Comparison of Contaminant Transport and Tracer Transport
Contaminant
^A
Tracer^
Computer Solute Transport Model
~~ >. i n Model with no
**^ biodegradation
Find 'k Calibrate model to data,
assuming biodegradation
Figure 8. Conceptual Approach to Data Analysis for Attenuation Rates, (a) Plotting contaminant con-
centrations from one location at different times provides an estimate of the concentration vs.
time attenuation rate constant, (b) Plotting contaminant concentrations from different loca-
tions along the longitudinal axis of the plume at one time provides an estimate of the concen-
tration vs. distance attenuation rate constant, (c) Comparing migration of a tracer to migration
of the contaminant, or calibration of a solute transport model provides information on the
biodegradation rate constant. (Modified from Newell et al., 2002.)
-------
variables collected during MNA site character-
ization (such as observed flow data, contami-
nant distribution data, and changes in concen-
trations with time). Screening-level contami-
nant fate and transport models can be used
to simulate and compare plume behavior at
different times and under different conditions
(under certain sets of assumptions specific to
each model). The model simulations can be
used to assess attenuation rates and remedia-
tion time frames. Numerous models have been
developed and are in use to simulate natural
attenuation of contaminants such as petroleum
hydrocarbons or chlorinated solvents. Among
these are BIOPLUME III, BIO SCREEN, and
BIOCHLOR (U.S. EPA, 2011).
4.3.7 Time
Projections of the time frame to achieve
remediation goals can either involve estimates
based on observations of mass or concentration
dissipation, or simulations using calibrated fate
and transport models. Mass and concentration
dissipation-based estimates involve monitoring
mass or concentration changes as a function
of time, fitting some function to the mass-time
or concentration-time data and extrapolat-
ing the function to determine the time when
a remediation goal is achieved. Computer
models can be used to simulate concentration
distributions with time to estimate when a
remediation goal is achieved. The site-specific
validity of the assumptions underlying each
estimation method should be evaluated prior to
application.
The empirical estimation methods use data and
provide estimates representing an averaging
of site conditions that influence contami-
nant migration and are inherently biased by
monitoring locations, ground-water conditions
during sampling, concentration averaging if
monitoring wells are used, and other factors.
Furthermore, they assume underlying condi-
tions that influence contaminant concentrations
remain constant with time (e.g., microbial
degradative activity remains constant as the
contamination diminishes, ground-water flow
directions do not change). Similar issues
plague estimates based on transport models.
However, to evaluate uncertainties in simula-
tion estimates, sensitivity analyses can and
should be used with these models.
The time required to reach a specific concen-
tration of a specific contaminant is likely to
differ from the time frame to reach the same or
a different specific concentration of a different
contaminant, due to the varied contaminant,
geological, geochemical, and biological condi-
tions present throughout a site. In addition,
these time frame estimates may vary for
different portions of a site. Thus, the estimates
of contaminant attenuation time frames may
produce a range of time frames. In estimating
the remedial time frame for MNA at the site
(i.e., the estimated time required to achieve
remedial goals for all the contaminants of
interest), all of these time frame estimates will
need to be examined for the longest time frame
(i.e., the last contaminant to reach its remedial
goal).
4.4
At all sites, a variety of physical, chemical,
and biological processes have the capacity to
attenuate environmental contaminants. The
success of a decision to rely on natural attenu-
ation processes as part of a site-remediation
strategy depends on both the occurrence of
those natural attenuation processes and on their
ability to meet site-specific remediation goals
within an acceptable remediation timeframe.
MNA decisions are supported by a detailed
understanding of subsurface conditions and
contaminant transport and fate. Site charac-
terization provides the site-specific data and
interpretations for making a decision whether
site remedial goals can be met with MNA.
The decision-making process evaluates the
applicability of MNA as a remedy for a site
and determines whether to: (1) select MNA as
the remedy, (2) select MNA as a component
-------
of the remedy in conjunction with one or more
other remedial technologies or enhancements
(i.e., a treatment train), or (3) reject MNA and
select another remedial technology. ITRC
(2008) provides information on potential
enhancements to complement MNA.
MNA should be carefully evaluated along with
other viable remedial approaches or technolo-
gies (including innovative technologies) within
the applicable remedy selection framework.
The evaluation of MNA as a remedial alterna-
tive requires making the determination that
natural attenuation processes are taking place
at a rate that is protective of human health and
the environment, that there is a reasonable
expectation that these processes will continue
at acceptable rates for the required remedial
time frame, and that the MNA remedy is
capable of achieving the site-specific reme-
diation objectives within a time frame that
is reasonable compared to other remedial
alternatives.
An informed decision as to the applicability
of MNA as a remedy or portion of a remedy
at a site requires that the site characterization
provides information on the occurrence, type,
and extent of natural attenuation processes; the
stability and sustainability of these processes;
and estimates of the past, present, and future
effects of these processes on contaminant
fate and transport (including rates and time
frames). The assessment of the potential
effectiveness of MNA for a set of site-specific
conditions allows a decision to either move
forward with MNA or to rule out MNA as a
component of the remedy.
The preceding sections of this document
present discussions on technical aspects of
site characterization for sites where MNA is
a potential remedy, including development
of a site conceptual model, characterization
variables, sampling locations and frequen-
cies, and the interpretations required for the
MNA decision-making process. It is the site
characterization discussed in this document
which provides the framework for the evalua-
tion of MNA as a remedial alternative and for
making remedy selection decisions involving
MNA.
-------
5.0
REFERENCES
References Cited
Alexander, M. 1977. Introduction to Soil
Microbiology. John Wiley & Sons, New
York, NY. 467pp.
Alexander, M. 1994. Biodegradation and
Bioremediation. Academic Press, San
Diego, CA. 302 pp.
Amerson, I, and R. Johnson. 2003. Natural
gradient tracer test to evaluate natural
attenuation of MTBE under anaerobic
conditions. Ground Water Monitoring
&Remediation. 23(1):54-61.
Bates, R.L., and J.A. Jackson. 1984.
Dictionary of Geological Terms, Third
Edition. Anchor Books, Doubleday, New
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cations.html
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Designing monitoring programs to effec-
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Annotated Additional References
Ford, R.G., R.T. Wilkin, and R.W. Puls.
2007a. Monitored Natural Attenuation of
Inorganic Contaminants in Ground Water,
Volume 1, Technical Basis for Assessment.
EPA/600/R-07/139. U.S. Environmental
Protection Agency, Office of Research and
Development, National Risk Management
-------
Research Laboratory, Cincinnati, OH. 77
pp. Find link for document under "Year"
tab at: http://www.epa.gov/nrmrl/gwerd/
publications, html
Site characterization for MNA of inorgan-
ics emphasizes different aspects than site
characterization for MNA of organics, with
more focus on the solid phase; geochemistry;
and contaminant speciation, adsorption and
precipitation. However, there are common
elements (see Table 1.1). Section 1C discusses
a tiered approach to characterization. Section
IIIA discusses and emphasizes the importance
of hydrogeological characterization.
Guilbeault, M.A., B.L. Parker, and J.A. Cherry.
2005. Mass and flux distribution from
DNAPL zones in sandy aquifers. Ground
Water. 43(l):70-86.
The site characterization focused on tech-
niques for delineation of DNAPL in sandy
aquifers and was oriented on researching the
factors governing DNAPL distribution, rather
than on a typical MNA site characterization.
However, the work describes in detail some
methods and considerations for the small-scale
delineation that might be needed at MNA sites
for characterizing spatial heterogeneities and
aquifer variability, flow paths, and dissolved-
phase contamination (here, resulting from
DNAPL source areas). One of the sites was
being investigated for natural attenuation of
chlorinated compounds. Direct-push methods
(using the Waterloo Profiler) and field-based
analytical methods are described. Discussion
on determining the appropriate sample spac-
ing (especially vertically) is included. The
text and figures illustrate and describe vertical
contaminant profiles in transects downgradient
of the source areas, for three different sites.
Contaminant plume mass-flux calculations are
described and their use is discussed.
Hunkeler, D., R. Aravena, K. Berry-Spark, and
E. Cox. 2005. Assessment of degradation
pathways in an aquifer with mixed chlo-
rinated hydrocarbon contamination using
stable isotope analysis. Environmental
Science & Technology. 39:5975-5981.
This paper discusses data from a field site
contaminated with at least 14 different chlo-
rinated hydrocarbons. Stable carbon isotopes
were used to identify the specific degradation
pathways that were occurring at the site that
contained a complex mixture of chlorinated
compounds. Stable carbon isotope data were
used to confirm that TCE was present due to
dehydrochlorination of 1,1,2,2-PCA and not
reductive dechlorination of PCE. Isotope data
were also used to confirm that vinyl chloride
and ethene present in the ground water were
due to dichloroelimination of 1,1,2- trichlo-
roethane and 1,2-dichloroethane rather than
reductive dechlorination of PCE, TCE, or
1,2-DCE.
ITRC (Interstate Technology & Regulatory
Council). 2008. Enhanced Attenuation:
Chlorinated Organics. EACO-1.
Washington, DC: Interstate Technology &
Regulatory Council, Enhanced Attenuation:
Chlorinated Organics Team. http://www.
itrcweb. org/Guidance/ListDocuments ? Topi
cID=8&SubTopicID=4
ITRC (2008) discusses and provides a proto-
col for "enhanced attenuation" (EA). EA is a
strategy for plume remediation transitioning
between source-zone treatment and MNA, or
between MNA and slightly more aggressive
technologies. The EA concept complements
MNA by utilizing possible enhancements,
intended for sites where MNA may not be
sufficient to meet regulatory goals. The docu-
ment provides information on plume stability
evaluation, enhancement technologies, a
flowchart with direction on how to incorporate
EA into the site remediation, an appendix on
calculating plume mass balance, an extensive
glossary, and discussion of regulatory issues.
Kao, C.M., and Y.S.Wang. 2001. Field
investigation of the natural attenuation and
intrinsic biodegradation rates at an under-
ground storage tank site. Environmental
Geology. 40(4-5):622-630.
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This paper describes a transect approach
used in evaluating MNA rates in a petroleum
hydrocarbon plume at a gasoline spill site.
Sampling locations are discussed on pages
623-625, and three map and cross-section
figures illustrate the transect and sampling
locations. The remainder of the paper
discusses the analysis of the geochemical data
and evaluation of biodegradation rates.
McKelvie, J.R., S.K. Hirschorn, G. Lacrampe-
Couloume, J. Lindstrom, J. Braddock, K.
Finneran, D. Trego, and B. Sherwood-
Lollar. 2007. Evaluation of TCE and
MTBE in situ biodegradation: integrating
stable isotope, metabolic intermediate, and
microbial lines of evidence. Ground Water
Monitoring & Remediation. 27(4):63-73.
This paper illustrates the integration of
multiple characterization techniques to clarify
the meaning of potentially ambiguous site
information. At a TCE-contaminated site and
a MTBE-contaminated site, the geochemi-
cal conditions and intermediate compounds
suggested that biodegradation had occurred;
however stable isotope analyses indicated that
the presence of presumed metabolic interme-
diates was due to the presumed intermediate
being present as an initial co-contaminant
rather than to biodegradation. This integrated
characterization approach revealed that
natural attenuation biodegradation was not a
significant process at the site.
Song, D.L., M.E. Conrad, K.S. Sorenson, and
L. Alvarez-Cohen. 2002. Stable carbon
isotope fractionation during enhanced in
situ bioremediation of trichloroethene.
Environmental Science & Technology.
36:2262-2268.
Stable carbon isotope data were collected at
a field site (Idaho National Engineering and
Environmental Laboratory Test Area North)
contaminated with chlorinated solvents under-
going reductive dechlorination. Multiple
sources and variable concentrations made
contaminant concentration data difficult to use
for evaluation of biological processes. Stable
carbon isotope analysis of the chlorinated
solvents allowed the attenuation effects of
ground-water transport and bioremediation to
be separated from each other. The complete
biological conversion of TCE to ethene was
confirmed using stable carbon isotopes.
U.S. EPA. 1986. RCRA Ground-Water
Monitoring Technical Enforcement
Guidance Document. OSWER-9950.1.
U.S. Environmental Protection Agency,
Office of Waste Programs Enforcement
and Office of Solid Waste and Emergency
Response, Washington DC. 208 pp.
http://nepis.epa.gov/Exe/ZyPURL.
cgi?Dockev=20012BRN. txt
The introductory Chapter 1 of the "TEGD"
covers many aspects of the geological
and hydrogeological characterization of a
site, focusing on the need to identify and
characterize contaminant pathways. It
provides details on appropriate observations,
measurements, and interpretations needed
to understand the subsurface geology and
ground-water flow. Section 2.2 (pages 66 to
69) discusses important considerations for the
placement and number of background wells.
It should be noted that the discussion is
specific to RCRA; however, there are general
recommendations that can be applied to MNA
sites. Chapter 5, which discusses statistical
analyses, and Appendix A, with a checklist for
hydrogeological site characterization, are also
useful.
U.S. EPA. 2006. Data Quality Assessment:
Statistical Methods for Practitioners.
EPAQA/G-9S, EPA/240/B-06/003. U.S.
Environmental Protection Agency, Office
of Environmental Information, Washington
DC. 190 pp. http://www.epa.gov/qualityl/
qs-docs/g9s-ftnal.pdf
This document was prepared by the U.S. EPA
Quality Staff as a quality management guid-
ance document to assist users in implementing
the U.S. EPA Quality System. It discusses the
scientific and statistical evaluation of environ-
mental data sets to determine if they meet the
planning objectives of the project, and thus
are of the right type, quality, and quantity to
-------
support their intended use (i.e., Data Quality
Assessment (DQA)). It also describes the
Data Quality Objectives (DQO) Process,
used to define criteria for determining the
number, location, and timing of samples to
be collected in order to produce a result with
a desired level of certainty. The guidance
document provides background information
and statistical tools for assessing data qual-
ity. It is organized around the five steps of
the iterative DQA process: (1) reviewing
the project objectives and sampling design;
(2) conducting a preliminary data review;
(3) selecting the statistical method; (4) verify-
ing the assumptions of the statistical method;
and (5) drawing conclusions from the data.
Appendices contain statistical tables, refer-
ences, and information on publications for
in-depth statistical analyses.
Wilson, J.T., P.M. Kaiser, and C. Adair.
2005. Monitored Natural Attenuation of
MTBE as a Risk Management Option at
Leaking Underground Storage Tank Sites.
EPA/600/R-04/179. U.S. Environmental
Protection Agency, Office of Research and
Development, National Risk Management
Research Laboratory, Cincinnati, OH.
74 pp. Find link for document under
"Year" tab at: http://www. epa.gov/nrmrl/
gwerd/publications. html
The report reviews the current state of knowl-
edge on the transport and fate of MTBE in
ground water, emphasizing the processes that
could be used to evaluate monitored natural
attenuation of MTBE, or to manage the risk
associated with MTBE in ground water. The
report provides recommendations concern-
ing data required for site characterization for
evaluation of natural attenuation or manage-
ment of risk. It demonstrates data procedures,
including stable carbon isotope information,
that can be used to assess MTBE risk. The
fundamentals of stable carbon isotopes usage
are presented.
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6.0
GLOSSARY
Note: Some terms in this Glossary may have
definitions that were developed or modified for
specific use only within this document.
Anisotropic This term indicates that the
hydrogeological properties of the aquifer are
different in different directions. For example,
if the horizontal hydraulic conductivity is
greater than the vertical hydraulic conductivity,
the aquifer would be considered anisotropic
with respect to hydraulic conductivity.
Background Background refers to the ground
water found upgradient of the contaminated
ground water. In some ground-water inves-
tigations, background ground-water samples
may have been collected in any uncontami-
nated ground water outside of a plume (includ-
ing in sidegradient locations). However, for
MNA site characterization the background
samples should be taken from the upgradi-
ent, uncontaminated portion of flow paths of
ground water that will eventually flow through
the MNA zone of the plume whenever pos-
sible. This is because the geochemistry of that
upgradient ground water needs to be compared
to the geochemistry of the water within the
MNA zone to investigate the presence of
electron acceptors, donors, and transforma-
tion products. This upgradient ground water
could be the source of the dissolved electron
acceptors or electron donors that will eventu-
ally flow into the plume and sustain any MNA
occurring there.
Bulk density The bulk density is the mass
of a porous media per unit volume. Bulk
density can be measured and expressed after
the sample has been dried to remove the soil
water (a dry weight bulk density) or using the
sample as received, with the soil water present
(a wet weight bulk density). The dry weight
bulk density is the preferred form for many
evaluations.
Byproduct A compound formed during
degradation (i.e., a fragment resulting from
breakdown of another compound, e.g., chloride
from chlorinated solvents), or a compound
formed or transformed as a result of a geo-
chemical change (e.g., Fe(II) resulting from
reduction of Fe(III)).
Conceptual Site Model (CSM) The CSM
is a three-dimensional representation of the
subsurface and contaminant plume. It incorpo-
rates all the information about the subsurface
conditions and processes regarding the pres-
ence, transport, and fate of the contaminant
plume.
Contaminant concentration (C) Of major
importance in any site characterization is the
delineation of contamination at different con-
centration levels. Regulatory limits may be set
at a specific concentration for a given contami-
nant at a given site. The concentration of a
particular contaminant may be used to estimate
the required amount of electron acceptor or
electron donor, using the appropriate reaction
stoichiometry.
Contaminant (NAPL) density Evaluation of
NAPL needs to consider that the density of
the NAPL phase in-situ may have been altered
from that of the original pure-phase product,
due to "weathering" (i.e., loss of some con-
stituents of the NAPL) or through dissolution
in a co-solvent.
Contaminant identity (single compound
or mixtures) Identification of contaminants
should include tentatively identified com-
pounds (TICs) and emerging or overlooked
contaminants.
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Contaminant toxicity This term is used here
specifically to indicate the adverse impact of a
compound on useful, biodegrading subsurface
microbes. The contaminant concentrations
relative to a toxic level are important for
predicting where biodegradation may or may
not occur. For example, high contaminant
concentrations near NAPL source areas may
inhibit biodegradation.
Darcy's Law Darcy's Law is an empirical
equation expressing the volumetric discharge
of water moving through a porous medium:
Q = -KA(dh/di)
where Q = discharge [L3/T]
(volume per time, or flow rate, of water)
K = proportionality constant, defined as
hydraulic conductivity [L/T]
A = cross-sectional area through which
the water flows [L2]
dh/dl = hydraulic gradient [L/L]
where dh = change in the
hydraulic head between
measurement points
dl = distance between
measurement points
Daughter product A compound produced
from degradation of an initial compound
(the initial compound changes into the
daughter compound). Daughter products of
some common contaminants can occur in a
sequence, for example, PCE degrades to TCE,
which then degrades to DCE, followed by
further degradation to VC. The relative con-
centrations of parent and daughter products for
the chlorinated ethenes can indicate the extent/
completeness of reductive dechlorination, and
the presence of appropriate microorganisms.
Deposltional environments/features The
environment in which sediments were depos-
ited influences the types of sediments depos-
ited and how those sediments will be oriented
and organized.
Dilution Dilution is quantified using a
"dilution factor", which indicates the relative
amount of uncontaminated water that is mixed
with the contaminated ground water.
Dispersion Hydrodynamic dispersion is the
movement of dissolved contamination through
both mechanical mixing of ground-water
flow and any molecular diffusion. Dispersion
is quantified using a dispersion coefficient,
which gives the sum of the molecular diffusion
and the mechanical mixing. The mechanical
mixing portion of the dispersion coefficient is
the mathematical product of the ground-water
velocity and the dispersivity (a physical char-
acteristic of the porous medium having units
of length). Dispersion is typically not evalu-
ated as part of the characterization; however,
if ground-water modeling is conducted or the
dispersion coefficient needs to be calculated,
literature values of dispersivity and molecular
diffusion generally can be assumed and used
along with the site-specific ground-water
velocity to estimate a dispersion coefficient.
Dissolved hydrogen (H ) Dissolved hydro-
gen is an electron donor that is used directly by
some microorganisms to biodegrade chlori-
nated solvent compounds. Some microbes
may use it preferentially over other electron
donors. Different electron-accepting processes
generally occur at different ranges of dissolved
hydrogen concentrations; thus, measurement
of the dissolved hydrogen concentration can
provide information on the dominant electron-
accepting process in a specific region at a
site and help in delineating a representative
zone. Collection and analysis of samples for
dissolved hydrogen analysis can be difficult,
and must be done carefully, due to a number of
complicating issues (e.g., samples should not
be collected from wells with metal screens or
casing, as hydrogen may be produced by the
presence of the metal).
DNAPL Dense nonaqueous phase liquid,
such as chlorinated solvents, coal tar, or
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creosote. The density of DNAPL is greater
than the density of water.
Effective porosity (ne ) Effective porosity is
the pore space (i.e., porosity) through which
flow can actually occur (some water in the
pore spaces is held in a thin film by capillary
forces and does not move through the aqui-
fer; in addition, some pores may be closed).
Effective porosity is always less than total
porosity. Though more difficult to achieve,
determination of effective porosities can refine
estimates of ground-water and contaminant
velocities, and may be the desired value to be
used in modeling.
Electron acceptor An electron acceptor is
a compound that receives (gains) an electron
during the microbially mediated transfer of
electrons from a second compound (the micro-
organism gains energy during this transfer).
The electron acceptor is reduced during the
coupled oxidation-reduction reactions. The
electron acceptors important in biodegradation
of petroleum hydrocarbons are oxygen, nitrate,
manganese (IV), iron (III), sulfate, and carbon
dioxide (each of these is the terminal electron
acceptor (TEA) for a specific redox reac-
tion). Chlorinated ethenes such as PCE, TCE,
DCE, and vinyl chloride can also be electron
acceptors.
Electron donor An electron donor is a
compound that provides (loses) an electron
during the microbially mediated transfer of
electrons to a second compound (the microor-
ganism gains energy during this transfer). The
electron donor is oxidized during the coupled
oxidation-reduction reactions. Common
electron donors can be petroleum hydrocarbon
contaminants, naturally occurring organic
carbon compounds, dissolved hydrogen,
or organic carbon compounds added to the
ground water during active remediation.
Flow path A flow path is the subsurface
pathway followed by a water molecule or
solute as it travels in the ground water.
Ground-water discharge Discharge is
the ground water leaving the saturated zone,
primarily when it enters a surface water body.
A ground-water discharge area is the end point
of ground-water flow paths.
Ground-water recharge Water that is
added to a saturated media is termed recharge.
The ground water can be replenished by pre-
cipitation or surface water infiltrating through
the unsaturated zone down into the saturated
zone, or by surface water directly in contact
with the saturated zone. A recharge area is the
area where ground-water flow paths begin.
Henry's Law Constant The Henry's Law
Constant gives the proportionality between the
concentration of a dissolved component in the
aqueous phase and its concentration in the gas
above the aqueous phase at equilibrium. The
value of the constant is temperature-dependent;
thus, accurate measurement of the ground-
water temperature is important if calculations
of vapor-phase contaminants are conducted.
Heterogeneity Heterogeneity is "A charac-
teristic of a medium in which material prop-
erties vary from point to point" (U.S. EPA,
2002b). The subsurface is generally not
uniform or homogeneous throughout. Even
small portions of a site that appear homoge-
neous are likely to have some variations that
affect contaminant fate and transport. Many of
the calculations for values of some variables,
estimates, rates, etc., discussed here (as well as
the use of some models) assume homogeneous
subsurface conditions. Since the subsurface
is heterogeneous, it should be recognized that
such calculations and model simulations will
always have some degree of uncertainty.
Hydraulic conductivity (K) Hydraulic con-
ductivity is the capacity of a material to trans-
mit water and, as expressed in Darcy's law,
is a proportionality constant for the volume
of ground water flowing in unit time through
a unit area of porous media under a unit
hydraulic gradient. Hydraulic conductivity is
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commonly expressed with units of L/T. The
hydraulic conductivity of the aquifer can be
determined from aquifer tests (pumping tests,
slug tests, or tracer tests). Due to heterogene-
ity of the subsurface media, hydraulic conduc-
tivity values generally vary spatially within a
site.
Horizontal hydraulic conductivity (Kh ) and
vertical hydraulic conductivity (Ky ) values
are likely to differ, as most subsurface units
are anisotropic (i.e., the unit's properties differ
depending on the direction, such as horizontal
vs. vertical). Kh and Kv data may be used to
estimate the potential for "plume diving" (i.e.,
downward migration), as low Kh/Kv ratios may
indicate that the plume may be more likely to
migrate downward.
Hydraulic gradient (dh/dl, or i) Hydraulic
gradient is the change in total hydraulic head
between two points, divided by the distance
between the points.
Hydraulic (piezometric) head (h)Hydraulic
head is the sum of the elevation head and
pressure head of ground water at a point. The
hydraulic heads are determined using water
level gauging data in monitoring wells or
piezometers (water level data from pumping
wells generally should not be used), preferably,
having discrete points or very short vertical
screened intervals. In theory, hydraulic head
is a point measurement. However, in most
circumstances this is not possible as a practi-
cal matter, and short (5 to 10 foot length) well
screens are commonly used. At sites where
specific hydrogeologic units have substantial
thicknesses (e.g., thick bedrock units in the
midwestern US), slightly longer screen lengths
may be used. However, long screen lengths
are not commonly used and are not recom-
mended, since long well screens could span
hydrogeologic units of substantially different
properties (the heads are likely to vary within
the different units).
Hydrogeology The science of subsurface
waters and related geologic aspects of surface
water (Bates and Jackson, 1984). It encom-
passes the solid porous media (i.e., the geol-
ogy) and the ground water.
Hydrostratigraphic unit A geologic unit,
group of units, or part of a unit that has similar
hydrogeologic characteristics throughout
(modified from Domenico and Schwartz,
1998).
Indicator parameter (variable) A vari-
able that is indicative of the degradation of a
contaminant. Its presence or concentration
indicates that a chemical, geochemical, and/or
microbiological process is likely to be contrib-
uting to a change (generally a reduction) in the
contaminant concentration or mass.
Lithology This refers to the composition of
the solids in the porous media, and helps to
describe the physical character of rock units at
the site.
LNAPL Light nonaqueous phase liquid, such
as gasoline. The density of LNAPL is less
than the density of water.
Major ions Hem (1985) defines major
constituents in ground water as those dissolved
constituents "commonly present in concentra-
tions exceeding 1.0 mg/L ". These constituents
are primarily cations (Ca2+, Mg2+, Na*, and K+)
and anions (SO42-, Cl', F, NO3-, HCO3-, and
CO32" ), but also includes nonionic Si (reported
as the oxide Si(X). Major ion geochemistry
analysis is not always necessary for natural
attenuation characterization; however, it can
be important when investigating and differ-
entiating different ground waters in order to
understand ground-water flow paths. Further,
some of the major ions can be electron accep-
tors (SO42~ and NO3~) or byproducts (e.g.,
Cl~ resulting from reductive dechlorination of
chlorinated solvents).
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Microbial community Microbiologists
classify microorganisms in a variety of ways,
for example, based on their characteristics, the
environmental conditions under which they
live, or on their effects. General categories
are used in this document. In general, aerobes
(aerobic populations) live under aerobic condi-
tions, and anaerobes (anaerobic populations)
under anaerobic conditions. The microbes
may also be classified as to the geochemical
impact they have (i.e., sulfate reducers or
methanogens), or by their genus or species
(e.g., Dehalococcoides sp., or Dehalococcoides
ethenogenes).
Organic matter (OM) Organic material that
exists within a soil or aquifer material. Soil
analyses may report the percent of organic
matter (%OM) in a sample. This can be
converted to a fraction of organic carbon using
the approximation fc = 0.59(%OM/100).
Oxidation-reduction potential (ORP) or
redox potential Microbial biodegradation
requires transfer of electrons. The ORP
measured in a sample (relative to a refer-
ence electrode) provides an indication of the
expected electron acceptor.
Particle (solid) density Particle density is
used to calculate porosity. For most sites, the
solid density can be assumed to be 2.65 g/cm3.
Partition (or Distribution) coeff cient (Kd )
Contaminants can be found in the sorbed
phase on solid material or in the dissolved
phase in ground water. The relative amount
of the contaminant in each of these phases
is described by a distribution coefficient or a
partition coefficient (the names are often used
interchangeably):
K, = S/C
where Kd = partition coefficient
[L-YM1 (cnrVg, mL/g)
S = sorbed concentration
[M/M] (g/g)
C = dissolved concentration
[M/L3] (g/mL)
This is often written as:
S = KdC
This equation assumes a linear adsorption
isotherm, which occurs for many organic com-
pounds. The partition coefficient Kd can be
estimated using the K value and the fraction
° oc
of organic carbon in the subsurface:
- Kocfoc
where Kd = partition coefficient
[L3/M] (cm3/g, mL/g)
Koc = the organic carbon
partition coefficient
[L3/M] (cni3/g, mL/g)
foc = the fraction of organic
carbon
Piezometric surface (also called poten-
tiometric surface) This is a surface defined
by the value of total hydraulic head (i.e., the
piezometric head) at each point in a subsurface
unit. For an unconfined aquifer, it is defined
by the elevation of the water table at all loca-
tions. For a confined aquifer, the piezometric
surface is defined by the piezometric head in
short-screened wells in that aquifer.
Porosity (n) (see also the Glossary entry
for Effective porosity) Porosity (or pore
space) is that portion of the total subsurface
volume that is not occupied by solid particles
(i.e., the void space (open space) in the porous
media). The solid particles or grains making
up a porous media will have empty space
between them. In a saturated porous medium,
the pore space will be fully occupied by
water. Porosity is commonly expressed as a
fraction of the total volume (e.g., a porosity
of 0.30 means that 30% of a unit volume will
be empty space - though filled with water in a
saturated porous medium - and 70% of the unit
volume will be occupied by solid particles).
One pore volume of a specific subsurface
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volume will be that specific subsurface volume
multiplied by the porosity.
The value of porosity can be calculated using
the following equation:
n =
t/Ps
where n = porosity
pb = soil bulk density
ps = average soil particle
density (solid density),
typically 2.65 g/cm3
The void ratio, used in engineering, is a similar
concept for describing the subsurface solid and
empty volumes, but expressed on a different
basis. The void ratio (e) is the ratio of the
volume of void (empty) space to the volume of
solids, expressed as a percentage (e.g., 30%) or
as a fraction (e.g., 0.30):
Porosity and void ratio are related by:
e = n/(l-n)
and n = e/( 1 + e)
Preferential iow Ground-water flow in
a porous medium is generally idealized to
flow throughout an entire porous medium in
a uniform manner. However, the subsurface
often has areas of greater hydraulic conductiv-
ity in which ground water may "preferentially"
flow at a greater rate than predicted based on
the idealized ground-water flow. Subsurface
gravel lenses, fractures, and buried utility lines
are examples of preferential flow paths. Water
and contamination flowing in preferential flow
paths may appear at greater distances and at
shorter times than predicted based on bulk
properties of the porous medium.
Receptors A receptor is an "ecological
entity exposed to a stressor" (U.S. EPA, 1997).
Receptors or potential receptors may be human
(i.e., people using downgradient drinking water
wells or using downgradient water bodies for
recreation) or environmental (i.e., downgradi-
ent water bodies such as wetlands or streams,
and the plants or animals living there). The
prevention of contamination from reaching
or contacting receptors is the primary driving
force behind a site remediation.
Representative zone A representative zone
is a three-dimensional portion of the subsur-
face throughout which the value or range of
values is similar for each individual variable
within a given set of predominant variables.
For example, if 20 locations were sampled and
there were 15 locations with a fine-to-medium
sand and 5 locations with silt, then the subsur-
face could be divided into two representative
zones ((1) sand and (2) silt) based on this
predominant variable of grain size. If 10 of
the 15 sand locations had sulfate-reducing
conditions and 5 had methanogenic condi-
tions, and the five silt locations had sulfate-
reducing conditions, then the subsurface would
be divided into three representative zones
((1) sand with sulfate-reducing conditions, (2)
sand with methanogenic conditions, and (3) silt
with sulfate-reducing conditions). Generally,
the predominant variables used to distinguish
representative zones include grain size (i.e.,
texture or lithology, which often relates to
hydraulic conductivity), contaminant type and
concentration, and electron acceptor.
Retardation factor (or coefficient) (R)
A common measure of contaminant travel in
ground water in a saturated porous media,
relative to the average linear ground-water
velocity (i.e., seepage velocity), is the retarda-
tion factor, calculated as:
R=l+pbKd/n
where R = retardation factor
(dimensionless)
pb = bulk density (g/cm3)
Kd = partition coefficient
(cm3/g)
n = porosity (dimensionless)
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Calculating the retardation factor allows an
estimation of the contaminant velocity in
ground water:
vc= v/R
where v = average linear ground-
water velocity (L/T)
v ^ = contaminant velocity
(L/T)
The retardation factor can also be expressed as:
R = v/vc
If Kd > 0, or R > 1, the contaminant travel
velocity will be retarded relative to the average
velocity of the ground water. The higher the
R value, the slower the contaminant travels
in ground water relative to the ground-water
velocity and the shorter distance it will migrate
in a given time (compared to contaminants
with a lower R value).
Seepage velocity (v)The seepage velocity of
moving ground water is the calculated aver-
age velocity at which the ground water would
move in a straight line, such as between two
points on a map. It is calculated by dividing
the specific discharge by the effective poros-
ity. The seepage velocity is also referred to
as average linear velocity. This is the veloc-
ity that is used when calculating how fast a
contaminant will travel at a site. The value of
seepage velocity may vary for different flow
paths throughout a site. When calculating the
velocity of a contaminant using the seepage
velocity to estimate travel time to a receptor,
the maximum value of the range of site seep-
age velocities should be used in order to be
conservative.
Soil organic carbon (sometimes referred
to as total organic carbon or TOC) or frac-
tion of organic carbon (foc) Organic
carbon in the aquifer matrix affects sorption
of organic contaminants. The soil organic
carbon can be measured through laboratory
analysis of the solid phase, and expressed as
foc, the weight fraction of organic carbon in a
subsurface solid sample. The foc can also be
derived using the generally more commonly
reported percent organic matter (%OM),
using fc = 0.59(%OM/100). f c is used when
calculating a partition coefficient (Kd) from an
organic carbon partition coefficient (Koc), using
Kd = focKoc, where Koc is the organic carbon
partition coefficient.
Source The source can refer to both the
type and the initial location of the contamina-
tion released to the subsurface.
Source architecture The term "architecture"
has been used to refer to the spatial arrange-
ment of the contaminant source, including the
source distribution, surface area, and location
relative to ground water.
Source release(s) The entry of the
contaminant(s) into the subsurface. The
release(s) could have been intentional or acci-
dental; slow or rapid; one-time, intermittent,
or continuous; small or large (catastrophic); in
one or more discrete locations; and with any
number or mixture of contaminants.
Specific discharge (q) The specific discharge
is the volume of ground water flowing per unit
time through a given cross-section (i.e., a volu-
metric flow rate per unit area). It is calculated
as the discharge (volume of water flow per
unit time) divided by the cross-sectional area
through which the water flows. It has units of
L/T, the same as a velocity. Specific discharge
is calculated using Darcy's Law. The specific
discharge is also referred to as Darcy flux,
Darcy velocity, apparent velocity, or discharge
velocity. The symbols "V" or "vs" are used by
some authors to denote the specific discharge;
however, this may create confusion with the
use of "V" for the seepage velocity.
Stratigraphy This refers to the layering of
the solids in the porous media. Description of
the stratigraphy includes the thicknesses and
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sequencing of the layers, and the nature of the
interfaces between the layers.
biologically (biotic) mediated reaction. It may
be a daughter product or a byproduct.
Structural features Lenses; orientation of
layers; preferential flow paths; fractures; faults.
Synoptic This refers to an overall view of
the whole of a site, at a given time. When
used here in terms of ground-water sampling,
it means that all monitoring wells would be
sampled at one time.
Texture (grain-size distribution) This refers
to the proportions of the different size fractions
of the particles in the porous media (i.e., sand,
silt, and clay). Grain size distributions are
also used to select an appropriate well screen
opening size.
Total organic carbon (TOC) Organic
carbon in the ground water can be measured to
estimate the concentration of electron donors.
The total organic carbon (TOC) is measured
using an unfiltered ground-water sample, and
dissolved organic carbon (DOC) is measured
using a filtered sample.
The organic carbon in soil (i.e., in the solid
phase) can be measured (see the Glossary
entry on soil organic carbon). In some usage,
this has been referred to as total organic carbon
(or TOC) when discussing the solid phase (i.e.,
soil).
Transect A transect is a line of sampling loca-
tions. Transects are typically oriented perpen-
dicular to the ground-water flow direction (a
transverse transect, across the plume width)
or parallel to the ground-water flow direc-
tion (a longitudinal transect). Each location
in a transect would ideally have two or more
sampled depths. The sampling locations in a
transect form a plane, and represent a slice or
cross-section of the plume.
Transformation product A general term for
a compound produced from or due to an initial
compound during a geochemically (abiotic) or
Variable This document will use the generic
term "variable" to refer to data, information,
or concepts that can be qualitatively described
or quantitatively measured for the subsurface
properties and processes, yet which are subject
to variation. A "variable" can range from a
broad descriptive concept (e.g., the variable of
"depositional environment" could be described
as "unconsolidated floodplain deposits") to a
specific quantifiable property (e.g., the variable
of "porosity" for a given geologic unit within
those unconsolidated floodplain deposits could
have a measured value of 0.3). A variable
may also be the numerical value resulting
from an equation containing one or more other
variables. The term is not meant in the formal,
strictly quantitative mathematical sense. The
term "variable" is intended to replace similar
or related terms such as parameter, factor,
property, element, component, etc. Further,
the term reinforces the important concept that
whatever is being described by a particular
"variable" is subject to change (i.e., vary)
spatially or with time.
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