r/EPA
United States
Environmental Protection
Agency
Performance Monitoring of
MNA Remedies for VOCs in
Ground Water
Non-Hazardoys
Degradation
Prod ycts& Other
Low
Concentration
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EPA/600/R-04/027
April 2004
Performance Monitoring of MNA
Remedies for VOCs in Ground Water
Daniel F. Pope
Dynamac Corporation
3601 Oakridge Boulevard
Ada, OK 74820
Steven D. Acree
U.S. EPA, Office of Research and Development
National Risk Management Research Laboratory
Ground Water and Ecosystems Restoration Division
Ada, OK 74820
Herbert Levine
U.S. EPA, Region 9
Superfund Division
San Francisco, CA 94105
Stephen Mangion
U.S. EPA, Region 1
Office of Research and Development
Boston, MA 02114
Jeffrey van Ee
U.S. EPA, Office of Research and Development
National Exposure Research Laboratory
Environmental Sciences Division
Las Vegas, NV 89193
Kelly Hurt and Barbara Wilson
Dynamac Corporation
3601 Oakridge Boulevard
Ada, OK 74820
Prepared under contract to Dynamac Corporation
Contract Numbers 68-C-99-256 and 68-C-02-092
Project Officer
David S. Burden
Ground Water and Ecosystems Restoration Division
National Risk Management Research Laboratory
Ada, OK 74820
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
w*
Recycled/Recyclable
Printed with vegetable-based ink on
paper that contains a minimum of
50% post-consumer fiber content
processed chlorine free.
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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 EPA Contract Nos. 68-C-99-256 and
68-C-02-092 to Dynamac Corporation, Ada, Oklahoma. 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.
All research projects making conclusions or recommendations based on environmental data and
funded by the U.S. Environmental Protection Agency are required to participate in the Agency
Quality Assurance Program. This project did not involve the collection or use of environmental
data and, as such, did not require a Quality Assurance Plan.
it
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FOREWORD
The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's
land, air, and water resources. Under a mandate of national environmental laws, the Agency
strives to formulate and implement actions leading to a compatible balance between human
activities and the ability of natural systems to support and nurture life. To meet this mandate,
EPA's research program is providing data and technical support for solving environmental
problems today and building a science knowledge base necessary to manage our ecological
resources wisely, understand how pollutants affect our health, and prevent or reduce
environmental risks in the future.
The National Risk Management Research Laboratory (NRMRL) is the Agency's center for
investigation of technological and management approaches for preventing and reducing risks
from pollution that threatens human health and the environment. The focus of the Laboratory's
research program is on methods and their cost-effectiveness for prevention and control of
pollution to air, land, water, and subsurface resources; protection of water quality in public
water systems; remediation of contaminated sites, sediments and ground water; prevention and
control of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with both
public and private sector partners to foster technologies that reduce the cost of compliance and to
anticipate emerging problems. NRMRL's research provides solutions to environmental problems
by: developing and promoting technologies that protect and improve the environment; advancing
scientific and engineering information to support regulatory and policy decisions; and providing
the technical support and information transfer to ensure implementation of environmental
regulations and strategies at the national, state, and community levels.
Effective performance monitoring for remedies that rely on the natural attenuation of
contaminants is a crucial element of remedial design and implementation. Effective monitoring
system designs are formulated from an enhanced understanding of the migration and ultimate
fate of the contaminants in the site-specific environment. This document provides technical
recommendations regarding the types of monitoring parameters and analyses useful for evaluating
the effectiveness of the natural attenuation component of ground-water remedial actions. The
information -will be helpful during the design of the performance monitoring plan as well as
during its implementation.
Stephen G. Schmelling, Director
Ground Water and Ecosystems Restoration Division
National Risk Management Research Laboratory
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IV
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TABLE OF CONTENTS
Page
NOTICE ii
FOREWORD iii
ACRONYMS AND ABBREVIATIONS x
ACKNOWLEDGMENTS xi
ABSTRACT xii
CHAPTER 1 INTRODUCTION 1
1.1 Purpose 1
1.2 Scope 1
CHAPTER 2 PERFORMANCE MONITORING SYSTEM DESIGN 3
2.1 Introduction 3
2.2 Objectives of Performance Monitoring 3
2.3 Developing Site-Specific Monitoring Objectives and Performance
Criteria for MNA 5
2.4 The MNA Conceptual Site Model 8
2.4.1 Hydrogeology 9
2.4.2 Contaminant Distribution, Migration, and Fate 13
2.4.3 Geochemistry 15
2.4.4 Receptor Locations 16
2.5 Monitoring Network Design 17
2.5.1 Introduction 17
2.5.2 Monitoring Locations 18
2.5.2.1 Typical Target Zones 19
2.5.2.2 Screen Lengths 22
2.5.3 Monitoring Parameters 23
2.5.4 Monitoring Frequency 25
2.6 Demonstrating MNA Effectiveness with Respect to
Remedial Objectives 30
2.6.1 #1 - Demonstrate that Natural Attenuation is
Occurring According to Expectations 30
2.6.1.1 Temporal Trends in Individual Wells 31
2.6.1.2 Estimation of Contaminant Mass Reduction 33
2.6.1.3 Comparisons of Observed Contaminant
Distributions with Predictions and Required
Milestones 34
2.6.1.4 Comparison of Field-Scale Attenuation Rates 35
2.6.2 #2 - Detect Changes in Environmental Conditions that May Reduce
the Efficacy of Any of the Natural Attenuation Processes 35
2.6.2.1 Geochemical Parameters 36
2.6.2.2 Hydrogeologic Parameters 37
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Page
2.6.3 #3 - Identify Any Potentially Toxic and/or Mobile
Transformation Products 37
2.6.4 #4 - Verify that the Plume is Not Expanding Downgradient,
Laterally, or Vertically 38
2.6.5 #5 - Verify No Unacceptable Impacts to Downgradient
Receptors 39
2.6.6 #6 - Detect New Releases of Contaminants 40
2.6.7 #7 - Demonstrate the Efficacy of Institutional Controls 41
2.6.8 #8 - Verify Attainment of Remediation Objectives 42
2.7 Monitoring Plan Contents 42
2.7.1 Introduction 42
2.7.2 Background and Site Description 44
2.7.3 Conceptual Site Model for Natural Attenuation 44
2.7.4 Objectives and Decision Points 44
2.7.5 Monitoring Network and Schedule 44
2.7.6 Monitoring of Institutional Controls 45
2.7.7 Evaluations of Remedy Effectiveness 45
2.7.8 Plan for Verifying Attainment of RAOs 45
2.7.9 Sampling and Analysis Plan 46
2.7.10 Quality Assurance Project Plan 46
CHAPTER 3 ANALYSIS OF PERFORMANCE MONITORING DATA 47
3.1 Introduction 47
3.2 The DQA Process 47
3.3 Interpreting the Data 49
3.3.1 Introduction 49
3.3.2 Preliminary Presentation and Evaluation of the Data 50
3.3.3 Data Comparisons 50
3.3.3.1 Comparisons of Concentrations Within and
Outside the Plume 51
3.3.3.2 Trend Analyses 51
3.3.3.3 Comparisons with Existing Literature and Laboratory
Studies 51
3.3.3.4 Comparisons with Threshold Values 52
3.3.4 Statistics 52
3.4 Elements of a Performance Monitoring Report 52
3.4.1 Introduction 52
3.4.2 Summary 53
3.4.3 Background and Site Description 53
3.4.4 Monitoring Network and Schedule 53
3.4.5 Evaluation of New Data 55
3.4.6 Evaluation of Institutional Controls 55
3.4.7 Conceptual Site Model Evaluation 55
3.4.8 Recommendations 56
VI
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Page
CHAPTER 4 APPLICATION OF MONITORING DATA TO
REMEDIAL DECISIONS 57
4.1 Introduction 57
4.2 Decision 1 - Continue Monitoring Program Without Change 57
4.3 Decision 2 - Modify the Monitoring Program 57
4.4 Decision 3 - Modify Institutional Controls 59
4.5 Decision 4 - Implement a Contingency or Alternative Remedy 59
4.5.1 Decision Criterion 1: Contaminant Concentrations in
Soil or Ground Water at Specified Locations Exhibit an
Increasing Trend Not Originally Predicted During
Remedy Selection 60
4.5.2 Decision Criterion 2: Near-Source Wells Exhibit Large
Concentration Increases Indicative of a New or Renewed
Release 61
4.5.3 Decision Criterion 3: Detection of a Contaminant in
Monitoring Wells Located Outside of the Original Plume
Boundary or Other Compliance Monitoring Boundaries 61
4.5.4 Decision Criterion 4: Contaminant Concentrations
Are Not Decreasing at a Sufficiently Rapid Rate to
Meet the Remediation Objectives 62
4.5.5 Decision Criterion 5: Changes in Land and/or
Ground-Water Use that Have the Potential to Reduce
the Protectiveness of the MNA Remedy 62
4.5.6 Decision Criterion 6: Contaminants Are Identified in
Locations Posing or Having the Potential to Pose
Unacceptable Risk to Receptors 62
4.6 Decision 4 - Terminate Performance Monitoring 62
REFERENCES 63
GLOSSARY 71
APPENDIX A VARIABILITY IN MEASURED PARAMETERS AND THE EFFECTS
ON PEFORMANCE MONITORING A-l
A.I Introduction A-2
A.2 Spatial and Temporal Variability A-2
A.3 Measurement Variability A-3
A.4 Variability in Data Interpretation A-4
VII
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LIST OF FIGURES
Page
1. Steps in the establishment of data quality objectives
(modified from U.S. EPA, 2000a) 7
2. Elements of a conceptual site model for monitored natural attenuation 10
3. Geologic block diagram and cross section depicting a stream environment
in -which sediments have accumulated as valley fill 12
4. Example of a network design for performance monitoring, including
target zones for monitoring effectiveness with respect to specific remedial
objectives 19
5. Cross section A-A' through monitoring network in general direction of
ground-water flow 20
6. Cross section B-B' through monitoring network perpendicular to ground-
water flow 21
7. Examples of possible changes in monitoring frequency over the monitoring
life cycle 27
8. Monitoring frequency effects on sampling data collection and
interpretation 29
9. Potential effects of changes in ground-water flow direction on
temporal trends in contaminant concentrations 32
10. Conceptual monitoring network for verifying lack of impact to surface
water from ground-water discharge 41
Vlll
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LIST OF TABLES
Page
1. Objectives for Performance Monitoring of MNA (U.S. EPA, 1999a) 4
2. Examples of MNA-Relevant Decisions to be Addressed Using the
DQO Process 8
3. Source Characterization Information for Conceptual Site Model
Development 14
4. Elements of a Performance Monitoring Plan 43
5. Elements of a Performance Monitoring Report 54
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LIST OF ACRONYMS AND ABBREVIATIONS
Acronym
BTEX
CERCLA
COCs
DCE
DNAPL
DQA
DQO
EPA
LNAPL
MCL
MTBE
MNA
NAPL
OSWER
PCE
PRGs
QA/QC
RAOs
RCRA
ROD
TCE
TEA
TICs
VC
Definition
Benzene, Toluene, Ethylbenzene, Xylenes
Comprehensive Environmental Response Compensation and
Liability Act
Contaminants of Concern
Dichloroethene
Dense Nonaqueous Phase Liquid
Data Quality Assessment
Data Quality Objectives
U.S. Environmental Protection Agency
Light Nonaqueous Phase Liquid
Maximum Contaminant Level
Methyl-t-Butyl Ether
Monitored Natural Attenuation
Nonaqueous Phase Liquid
Office of Solid Waste and Emergency Response
Perchloroethene (tetrachloroethene)
Preliminary Remediation Goals
Quality Assurance/Quality Control
Remedial Action Objectives
Resource Conservation and Recovery Act
Record of Decision
Trichloroethene
Terminal Electron Acceptor
Tentatively Identified Compounds
Vinyl Chloride
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ACKNOWLEDGMENTS
The authors express their appreciation for helpful comments received from numerous
organizations and individuals including the U.S. EPA Ground Water Forum, the U.S. EPA Federal
Facilities Forum, Ken Lovelace, Guy Tomassoni, Hal White, Terry Evanson, Dr. Aristeo M. Pelayo,
Patricia Ellis, Todd Wiedemeier, Dr. Frank Chapelle, Dr. Ryan Dupont, Dr. John Wilson, Dr. Jim
Weaver, and Dr. Robert Ford. The authors would also like to acknowledge Carol House for her
support in creating the graphics and layout for this report.
XI
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ABSTRACT
Environmental monitoring is the major component of any remedy that relies on natural
attenuation processes. The objective of this document is to identify data needs and evaluation
methods useful for designing monitoring networks and determining remedy effectiveness.
Effective monitoring of natural attenuation processes involves a three-dimensional approach to
network design and clearly defined performance criteria based on site-specific remedial action
objectives. Objectives for the monitoring program -will be met through routine evaluations of
institutional controls and measurements of contaminant, geochemical, and hydrologic parameters.
These data are used to evaluate changes in three-dimensional plume boundaries, contaminant
mass and concentration, and hydrological and geochemical changes that may indicate changes in
remedy performance.
Data interpretation focuses on detection of spatial and temporal changes, and assessment of their
impacts on the achievement of site-specific goals. Particular changes of interest include:
• Progress toward contaminant removal objectives and indications of additional contaminant
releases,
• Contaminant detections at the horizontal and vertical plume boundaries that may indicate
plume expansion,
• Geochemical changes (e.g., oxidation-reduction (redox) conditions) indicative of possible
changes in contaminant transformation rates,
• Changes in ground-water flow rates or directions such that contaminants may move into
previously unimpacted areas, and
• Changes in land and resource uses that threaten the effectiveness of institutional controls.
Decisions regarding remedy effectiveness and the adequacy of the monitoring program will
generally result in either continuation of the program, program modification, implementation
of a contingency or alternative remedy, or termination of the performance monitoring program.
Such decisions are appropriately based on specific, quantifiable performance criteria defined in
the monitoring plan. Continuation of the program without modification -would be supported
by contaminant concentrations behaving according to remedial expectations -while ground-
water flow and geochemical parameters remain -within acceptable ranges. Modification of the
program, including increases or decreases in monitoring parameters, frequency, or locations, may
be -warranted to reflect changing conditions or improved understanding of natural attenuation
processes at the site. Situations that may trigger implementation of a contingency or alternative
remedy include:
• Increasing contaminant concentrations or trends not predicted during remedy selection or
indicative of new releases,
• Contaminant migration beyond established plume or compliance boundaries,
• Contaminants not decreasing at a rate sufficient to meet remediation objectives,
• Changes in land or ground-water use that have the potential to reduce the protectiveness of
the remedy, and
• Contaminants observed at locations posing or having the potential to pose unacceptable
risks to receptors.
xtt
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Chapter 1
INTRODUCTION
1.1 Purpose
The term "monitored natural attenuation," as used in this document and in the Office of Solid
Waste and Emergency Response (OSWER) Directive 9200.4-17P (U.S. EPA, 1999a), refers to "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 reasonable compared to that offered by other more active methods." Performance
monitoring will be an essential component of the remedy to ensure site-specific objectives are
achieved.
This document is designed to be used during preparation and review of long-term monitoring
plans for sites where MNA has been or may be selected as part of the remedy. Performance
monitoring system design depends on site conditions and site-specific remedial objectives;
this document provides information on technical issues to consider during the design process.
Discussions include details of issues concerning monitoring parameters, locations (i.e., three-
dimensional monitoring locations relative to the plume), and monitoring frequencies. This
document does not provide details of particular methodologies for sampling, analysis, modeling, or
other characterization tools.
Nothing in this document changes Agency policy regarding remedial selection criteria, remedial
expectations, or the selection and implementation of MNA. This document does not supercede
any guidance. It is intended for use as a technical reference in conjunction with other documents,
including OSWER Directive 9200.4-17P, "Use of Monitored Natural Attenuation at Superfund,
RCRA Corrective Action, and Underground Storage Tank Sites" (U.S. EPA, 1999a), and
"Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water"
(U.S. EPA, 1998a).
1.2 Scope
This document focuses on chlorinated solvent compounds and common fuel-related aromatic
compounds (i.e., benzene, toluene, ethylbenzene, and xylenes (BTEX)) dissolved in ground water
within porous media. These compounds comprise a significant portion of current ground-water
pollution problems, and there is a considerable body of information available concerning their
behavior in the subsurface. It is necessary to have a detailed understanding of the behavior of a
contaminant in the subsurface in order to properly evaluate natural attenuation processes and
develop an adequate monitoring plan. The limited data that may be available on subsurface
behavior of other contaminants increase the difficulty of conducting adequate evaluations of
remedy performance. Contaminants such as inorganic compounds, radionuclides, fuel oxygenates
{e.g., methyl-t-butyl ether (MTBE)), explosives, and wood treating chemicals are not specifically
addressed in this document. Many of these contaminants behave differently from those on
which this document is focused. Such compounds may not be attenuated at rates sufficient to
be protective of human health and the environment while other compounds associated with a
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given release may be rapidly attenuated. For example, MTBE has been used as a common fuel
component (U.S. EPA, 2003a). At many sites, the potential for significant migration of MTBE
may be greater than that of the BTEX contaminants. The determination of conditions under
-which MTBE may be readily attenuated in the subsurface is an area of ongoing research. Although
the details of an appropriate performance monitoring program are specific to the contaminant
under consideration, the type of monitoring and evaluation methods that are discussed will often
be applicable to contaminants such as MTBE that are not explicitly considered in this document.
Soils and/or sediment-only remedies are discussed only in reference to contaminant sources for
ground water.
This document will focus on contaminants in the aqueous-phase plume. As stated in U.S.
EPA (1999a), it is expected that source control will be a fundamental component of any MNA
remedy. The degree to -which contaminant sources are removed or controlled directly affects the
effectiveness and remedial time frame for MNA remedies. In general, the discussions provided in
this document -will be most applicable to sites -with controlled sources or no materials that provide
continuing sources for ground-water contamination (e.g., NAPL). Accordingly, the contaminant
release scenario used in figures throughout the document is one in -which only a small volume
of NAPL -was released to the subsurface and effective source removal actions -were subsequently
implemented.
This document is limited to evaluations performed in porous-media settings. Detailed discussion
of performance monitoring system design in fractured rock, karst, and other such highly
heterogeneous settings is beyond the scope of this document. Ground -water and contaminants
often move preferentially through discrete pathways (e.g., solution channels, fractures, and joints)
in these settings. Existing techniques may be incapable of fully delineating the pathways along
-which contaminated ground -water migrates. This greatly increases the uncertainty and costs
of assessments of contaminant migration and fate and is another area of continuing research.
As noted in OSWER Directive 9200.4-17P (U.S. EPA, 1999a), "MNA will not generally be
appropriate -where site complexities preclude adequate monitoring." The directive provides
additional discussion regarding the types of sites -where the use of MNA may be appropriate.
This document focuses on monitoring the saturated zone, but site characterization and monitoring
for MNA or any other remedy typically -would include monitoring of all significant path-ways by
-which contaminants may move from source areas and contaminant plumes to impact receptors
(e.g., surface water and indoor air).
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Chapter 2
PERFORMANCE MONITORING SYSTEM DESIGN
2.1 Introduction
Designing a monitoring system to assess natural attenuation processes and the effectiveness
of those processes with respect to achieving remedial objectives involves making site-specific
decisions regarding:
• Monitoring parameters,
• Number and location of monitoring points,
• Monitoring frequency, and
• Methods to analyze and interpret the data obtained from monitoring points.
Sections 2.2 and 2.3 of this chapter discuss typical objectives of performance monitoring and
ways to develop site-specific objectives and performance criteria such that an MNA remedy can be
evaluated based on measurable criteria.
Sections 2.4 and 2.5 discuss general principles of development of the MNA conceptual site model
and the monitoring network design, based on data collected during the site characterization
efforts. These general principles provide a framework for understanding the controlling factors
that guide the monitoring decisions listed above. The data that generally should be available from
the site characterization effort are discussed in detail.
Section 2.6 of this chapter addresses the specific application of these general principles to
demonstrating effectiveness of MNA for attaining the performance monitoring objectives in
Table 1.
The final section of this chapter (Section 2.7) provides suggested content and format for
performance monitoring plans.
2.2 Objectives of Performance Monitoring
The OSWER Directive 9200.4-17P (U.S. EPA, 1999a) provides eight specific objectives for the
performance monitoring program of an MNA remedy (Table 1). This document will discuss the
technical aspects of monitoring systems typically used to meet these and similar objectives. The
objectives usually will be met by implementing a performance monitoring program that routinely
evaluates the effectiveness of institutional controls and measures contaminant concentrations,
geochemical parameters (e.g., oxidation-reduction (redox) parameters, dissolved organic carbon,
pH), and hydrologic parameters. These data will be used to evaluate the dynamic behavior of the
plume over time, including:
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Table 1. Objectives for Performance Monitoring of MNA (U.S. EPA, 1999a)
1) Demonstrate that natural attenuation is occurring according to expectations,
2) Detect changes in environmental conditions (e.g., hydrogeologic, geochemical,
microbiological, or other changes) that may reduce the efficacy of any of the natural
attenuation processes,
3) Identify any potentially toxic and/or mobile transformation products,
4) Verify that the plume(s) is not expanding downgradient, laterally or vertically,
5) Verify no unacceptable impact to downgradient receptors,
6) Detect new releases of contaminants to the environment that could impact the
effectiveness of the natural attenuation remedy,
7) Demonstrate the efficacy of institutional controls that were put in place to protect
potential receptors, and
8) Verify attainment of remediation objectives.
• Changes in three-dimensional plume boundaries,
• Changes in the geochemical setting (i.e., as indicated by the geochemical parameters,
especially the redox parameters such as redox potential, dissolved oxygen, nitrate/nitrite,
manganese (II), iron (II), sulfate, and methane) that maybe indicative of changes inbiotic
or abiotic processes affecting the rate and extent of natural attenuation, and
• Contaminant mass and/or concentration reductions indicative of progress toward
contaminant reduction objectives.
Plume behavior can then be evaluated to judge the effectiveness of the MNA remedy, the adequacy
of the monitoring program, and the adequacy of the conceptual site model for MNA. On the basis
of these judgements, decisions may be made for subsequent phases of site operations, including:
• Continue the performance monitoring program -without change,
• Modify the performance monitoring program,
• Modify the institutional controls,
• Implement a contingency or alternative remedy, or
• Terminate performance monitoring.
As is the case for other remedies, performance monitoring for MNA remedies continues until all
remedial action objectives have been met (e.g., contaminant concentrations throughout the site
meet remedial requirements). The last phase of performance monitoring (verification monitoring)
may involve changes to the performance monitoring program as appropriate to verify remedial
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goals have been met (e.g., a period of more frequent monitoring and/'or more spatially dense
monitoring, especially if monitoring locations or frequency had been reduced over an extended
performance monitoring period).
2.3 Developing Site-Specific Monitoring Objectives and Performance Criteria for MNA
Site-specific performance monitoring objectives for MNA are derived from site-specific remedial
action objectives (RAOs) and preliminary remediation goals (PRGs). RAOs provide a general
description of what the cleanup will accomplish (e.g., restoration of ground water). PRGs are the
more specific statements of the desired endpoint concentrations or risk levels, for each exposure
route, that are believed to provide adequate protection of human health and the environment
based on preliminary site information. For guidance concerning remedial objectives refer to
current, program-specific documents (e.g., U.S. EPA, 1997a; U.S. EPA, 2004).
The site-specific performance monitoring objectives are general statements of what is to be
required of the monitoring network (e.g., detect plume expansion). Performance criteria are
detailed statements that set forth standards or requirements based on specific measurements.
For instance, it might be specified that the monitoring system be able to detect a contaminant
concentration of 1 ng/L in ground water in the area between the known contaminated aquifer
and a receptor. Clearly stated performance monitoring objectives, accompanied by specific,
quantifiable performance criteria, are useful for designing and evaluating the performance
monitoring system, and assessing MNA remedy effectiveness. RAOs, PRGs, performance
monitoring objectives, and performance criteria may be specified in remedy decision documents
(e.g., Record of Decision, Corrective Action Plan) to provide the basis for development of the
performance monitoring plan.
Common remedial objectives include lack of plume expansion and reduction of contaminant
concentrations to established limits. Examples of performance criteria for monitoring such
objectives include:
• The ability of the monitoring network to detect a specified contaminant at a specified
concentration at specified sampling locations (e.g., detect the occurrence of vinyl chloride
at a transect of wells located between the plume boundaries and potential receptors at a
concentration of 2 ng/L), and
• The ability of the monitoring network to detect a specified decrease in contaminant
concentrations throughout the site within a specified time frame (e.g., detect a 50 %
decrease in concentrations throughout the plume by the end of ten years).
In order to develop such performance criteria and realistic means of evaluating performance with
respect to these criteria, a systematic process should be followed. This process should take into
account the data needs and the methods available to obtain the data. For example, if statistical
tests are to be used to assess MNA performance, a systematic process can be used to select the
appropriate tests, and to choose the type of data and data collection techniques necessary to obtain
data required for the test. One such systematic development process that is highly effective and is
endorsed in current guidance (U.S. EPA, 2000a) is the Data Quality Objectives (DQO) Process.
The DQO process is a systematic planning approach for data collection that is based on the
scientific method. The DQO process identifies goals of the data collection and decision-making
process, and assesses consequences of incorrect decisions. Although the process is typically
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described in linear terms (Figure 1), it is really a flexible process that relies on iteration and
modification as the planning team -works through each step, allowing earlier steps to be revised
considering new information. The basic steps in the process begin with identification of the
problem to be solved and the resources available to support the solution. Based on a statement of
the problem, a specific decision that requires the acquisition of new data is formulated. Once the
specific question to be answered has been stated, the data needed to make the decision can then
be specified. Decisions regarding the boundaries of the study required to answer the question
of concern are then made. Such boundaries may include spatial boundaries (e.g., the boundaries
of the contaminated aquifer) and time boundaries (e.$., the time frame for data collection). The
decision to be made is then simplified through development of a logical "if-then" statement
describing the conditions under which different alternatives would be chosen. At this point,
acceptable limits on errors in the decision and methods for limiting uncertainty in the data are
established. Finally, cost-effective sampling designs to provide the needed data are produced.
Although these steps are largely intuitive, steps may be overlooked if a specified framework is not
used.
The DQO process provides a framework for addressing the issues of subsurface heterogeneity
and data variability (Appendix A) that cause uncertainty about site characteristics, and often
present obstacles to development of monitoring plans and data interpretation. Subsurface
geology, hydrology, geochemistry, biology, and contaminant distribution may be highly variable
and interact in complex ways. Therefore, it is important to have a defined process for dealing
with the variability in order to design an appropriate monitoring system, and interpret the data
derived from the monitoring system within an acceptable range of uncertainty. Uncertainty
may be expressed mathematically using statistical techniques, if feasible, and it may be expressed
qualitatively as "professional judgement" concerning the reliability of a certain interpretation
of the data. The DQO process involves identification of data gaps that may cause an erroneous
decision to be made, and assessment of the cost-benefit ratio of filling those gaps to reduce
uncertainty.
When the DQO process identifies additional data needed to reduce uncertainty and facilitate
development of the performance monitoring plan, further investigation of a site is warranted. Use
of the Triad approach (U.S. EPA, 2001a) to planning and conducting the investigation can
provide the information rapidly and cost-effectively, allowing the performance monitoring plan
to be developed and deployed expeditiously. The Triad approach combines systematic planning
to ensure that the characterization goals are clearly defined with dynamic work plans (U.S.
EPA, 2003b) and quick-turnaround analytical techniques, including field analytical techniques
(U.S. EPA, 2003c), to provide data meeting site-specific requirements in a condensed time frame.
The approach promotes the use of new science and technology tools to identify and manage
information gaps (i.e., uncertainties) that could lead to unacceptable decision errors. The Triad
approach is particularly effective where site heterogeneity increases uncertainty about the
representativeness of isolated data points. The Triad approach is intended to produce an accurate
conceptual site model. Using the dynamic work plan element of the Triad approach allows
efficient refinement of the model in real time by specifying sampling locations and analyses based
on data from previous samples.
Examples of typical problems and concerns that may be addressed using the DQO process during
development of a performance monitoring plan are listed in Table 2. Important outcomes
from the DQO process include the spatial and temporal scales for the collection of data, sample
collection methods, acceptable decision error rates, and number of samples needed to support
decision-making.
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1. STATE THE PROBLEM
Summarize the problem that will require new data,and
identify the resources available; develop site-specific
conceptual model for monitored natural attenuation.
2. IDENTIFY THE DECISION
Identify the decision that requires new data
(e.g., Is the plume expanding?).
3. IDENTIFY INPUTS TO THE DECISION
Identify the information needed to support the decision
and specify which inputs require new measurements.
4. DEFINE THE STUDY BOUNDARIES
Specify the spatial and temporal aspects of the media that
the data must represent to support the decision.
5. DEVELOP A DECISION RULE
Develop a logical "if.,.then..."statement that defines the
conditions that would cause the decision maker to
choose among alternative actions.
6. SPECIFY LIMITS ON DECISION ERRORS
Specify the decision maker's acceptable limits on decision
errors, which are used to establish performance goals
for limiting uncertainty in the data.
7. OPTIMIZE THE DESIGN FOR OBTAINING DATA
Identify the most resource-effective sampling and analysis design
for generating data that are expected to satisfy the DQOs.
Figure 1. Steps in the establishment of data quality objectives (modified from U.S. EPA, 2000a).
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Table 2. Examples of MNA-Relevant Decisions to be Addressed Using the DQO Process
Is natural attenuation occurring according to expectations?
Q Determine that plume is behaving as specified in decision documents.
Q Determine that contaminant mass reduction is proceeding as specified in decision documents.
Q Determine that contaminant concentrations in source areas and the downgradient plume are
declining at sufficient rates to meet RAO's.
Are there any changes in conditions that may affect the efficacy of natural attenuation ?
Q Determine that there are no changes in ground-water flow rates and directions that would
impact plume stability.
Q Determine that there is no change in the geochemical environment (e.g., redox conditions) that
would impact plume stability or contaminant reduction.
Are there potentially toxic and/or mobile transformation products ?
Q Determine if naturally-occurring arsenic, manganese or other potentially problematic species
are being transformed and mobilized due to changes in the geochemical environment.
Q Determine if previously unrecognized toxic and/or mobile transformation products are present.
Is the plume expanding?
Q Determine that the plume is not migrating beyond current horizontal or vertical boundaries.
Q Determine that no contaminants are found in the wells placed between the downgradient
edge of the plume and receptors at concentrations above a specified limit.
Are there any unacceptable impacts to receptors?
Q Determine that there is no unacceptable impact to surface-water bodies, wetlands, or other
ecological receptors.
Q Determine that there is no impact to indoor air in adjacent buildings.
Q Determine that there is no impact to water-supply wells.
Are there any new contaminant releases that may impact remedy effectiveness?
Q Determine that there are no new releases of contaminants from the source area.
Are the specified institutional controls effective?
Q Determine that institutional controls are currently effective in eliminating exposure to
contaminants.
Have remedial objectives been met?
Q Verify that contaminant concentrations are below required levels.
2.4 The MNA Conceptual Site Model
The monitoring network design is based on the data derived from site characterization and any
other site studies. These data are used to develop a conceptual model of the site before remedy
selection; the conceptual model can then be used to facilitate the performance monitoring
design process. The conceptual site model for natural attenuation is the site-specific qualitative
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and quantitative description of the migration and fate of contaminants with respect to possible
receptors and the geologic, hydrologic, biologic, geochemical, and anthropogenic factors
that control contaminant distribution. Essentially, the conceptual site model expresses an
understanding of the site structure, processes, and factors that affect plume development and
behavior. It is built on assumptions and hypotheses that have been evaluated using site-specific
data, and is continually reevaluated as new data are developed throughout the site lifetime. The
conceptual model typically should be developed and evaluated using a team approach (i.e., using a
team of subject-matter experts including hydrogeologists, microbiologists, statisticians/modelers,
chemists and other experts as appropriate for the specific site).
A three-dimensional conceptual site model that incorporates temporal changes is often needed
to provide a framework for interpreting the site data, judging the significance of changes in
site conditions, and predicting future behavior of the source and plume. Understanding plume
formation and behavior is the basis for predicting future plume behavior, and therefore, predicting
whether the MNA remedy will be able to achieve site remedial goals within specified time frames.
Conceptual models are expressed tangibly in text, site maps (e.g., contaminant isoconcentration
maps and potentiometric surface maps), cross sections (e.g., hydrogeologic and chemical
distributions), and other graphical presentations, and in terms of mathematical calculations
describing the plume and site.
The development of quantitative models (i.e., mathematical representations of site conditions)
based on the conceptual site model generally is an essential part of site characterization, remedy
selection, and performance monitoring for MNA. Quantitative models may be as simple as linear
regressions for estimation of contaminant attenuation rates, or as complex as numerical models
of ground-water flow and contaminant fate and migration. These mathematical representations
are used to help understand site processes, locate monitoring points, estimate attenuation rates,
and evaluate possible effects of different conditions on plume behavior. Quantitative assessments
require particular types of data. The data collection effort should be designed with the chosen
evaluation methods, calculations, and model(s) in mind.
The data and analyses necessary for formulation of an adequate three-dimensional conceptual
site model for MNA depend on site-specific conditions. However, the types of information and
analyses that are generally needed for model development are illustrated in Figure 2 and described
below.
2.4.1 Hydrogeology
Hydrogeology is the foundation of the conceptual site model for natural attenuation (National
Research Council, 2000). Detailed knowledge of site hydrogeology is crucial to understanding
how ground water flows and contaminants may be transported in the subsurface. A general
discussion of the effects of geology and hydrology on ground-water flow follows.
Much of the spatial variability in observed contaminant concentrations is the result of geologic
heterogeneity. In a sedimentary geologic setting, spatial changes in geology are present at scales
that can vary from fractions of an inch to miles. Sedimentary facies (i.e., sedimentary bodies that
are internally similar in characteristics) determine the three-dimensional geometries, connectivity,
and heterogeneity (i.e., variability) of transmissive zones and barriers to flow (Galloway and
Sharp, 1998). These characteristics and the resulting variability in hydraulic properties (e.g.,
hydraulic conductivity and porosity) are generally the result of the original geologic depositional
processes. In similar fashion, anthropogenic features such as buried utility corridors and
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Contaminant Source and Source
Control Information
D Location, nature, and history of contaminant releases
or sources
D Locations and characterizations of sources for ground-
water contamination [ec?.,nQnaqueous phase liquid
(NAPL)]
D Locations and descriptions of source control and other
ongoing and proposed remedial actions
Geologic and Hydrologic Information
D Regional and site geologic and hydrologic settings,
including controls on ground-water flow
D Analyses of deposltional environments and geologic
features that may serve as Zones of preferential flow or
barriers to flow, including geometry and physical
properties of geologic facies (e.g., texture, porosity, buIk
density) and their variability
D Stratigraphy, including thickness and lateral continuity
of geologic units, and bedding features
D Anthropogenic features (e.g., buried corridors and
heterogeneous fill materials) that control ground-water
flow.and may serve as migration pathways or barriers
D Depth to ground water and temporal variation
D Characteristics of surface water bodies (e.g., locations,
depths,and flow rates),their interactions with ground
water,and temporal variations
D Ground-water recharge and discharge locations, rates
and temporal Variability
D Hydraulic gradients, including horizontal and vertical
components, and their variations in response to
fluctuations in site hydrology (e.g., seasonal or longer
term precipitation patterns and changes in patterns of
ground-water withdrawal or irrigation)
D Hydraulic properties (e.g.,hydraulic conductivities,
storage properties, and effective porosities) and their
variability and anisotropy within geologic units
d Quantitative description of the ground-water flow field
D Chemical properties of the subsurface matrix including
mineralogy and organic matter
Receptor Information
D Aquifer classification, current usage information, and
reasonably anticipated future Usage
D Locations and production data for water-supply wells
D Locations and information on human and ecological
receptors under current and reasonably anticipated
future conditions
D Areas susceptible to impact by vapor-phase
contaminants (e.g., indoor air)
D Information on local historical and cultural uses of land,
water, and other resources used to identify receptor
populations
D Descriptions of institutional controls currently in place
Contaminant Distribution, Transport and Fate
Distribution of each contaminant phase (i.e., gaseous,
aqueous, sorbed, NAPL) and estimates of mass
D Mobility of contaminants in each phase
d Temporal trends in Contaminant mass and
concentrations
O Sorption information, including retardation factors,
sorption mechanisms, and controls
D Contaminant attenuation processes and rate estimates
Q Assessment of facilitated transport mechanisms (eg.,
complexation or colloidal transport)
D Geochemical characteristics that affect or are indicative
of contaminant transport and fate, and mineralogyjf
needed
D Potential for mobilization of secondary contaminants
(e.g., arsenic)
D Effects of other proposed or ongoing remedial
activities on contaminant transport,fate and natural
attenuation processes
Figure 2. Elements of a conceptual site model for monitored natural attenuation.
heterogeneous fill materials may also result in the formation of transmissive zones and flow
barriers. Transmissive zones are subsurface units where ground water flows in paths constrained
or bounded by lower hydraulic conductivity materials (i.e., geologic impediments to flow) or
hydrologic barriers (e.g., hydraulic head boundaries). An example of a transmissive zone is a
deposit of discontinuous, interbedded sands and silt bounded above by the water table and below
by a thick, locally continuous clay of low hydraulic conductivity. These bounds act to keep ground
10
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water flowing within the transmissive zone. However, even -within a given transmissive zone,
ground water may move in sinuous paths due to small-scale differences in hydraulic conductivity
resulting from heterogeneous geologic materials or to temporally variable, three-dimensional
hydraulic gradients.
Transmissive zones may be separate and distinct pathways for contaminant movement. For
example, the degree of hydrologic connection between different sedimentary facies depicted in
Figure 3 is small. Monitoring points in different sedimentary facies may appear to be similar and
may be hydraulically downgradient of one another -without significant ground-'water flow and
contaminant migration from one unit to the next. A reduction in contaminant concentrations
between two monitoring points that are not in the same flowpath may not accurately represent
contaminant attenuation in either flowpath. Inferences about natural attenuation based on
apparent decreases in contaminant concentration in the downgradient direction are likely to
be incorrect in these situations unless ground-'water flow paths are determined and monitored.
Determining flow paths is often difficult to accomplish, especially -when using small numbers of
monitoring points.
Differentiation of the ground-water flow field by means of a detailed characterization of site
geology is crucial for effective performance monitoring of a natural attenuation remedy. In this
respect, evaluation of sedimentary depositional environments is an especially useful framework
for the understanding of site stratigraphy and the distribution of lithologic controls on ground-
'water flow. Three-dimensional characterization is used to evaluate and predict the effects of
natural attenuation processes ($$., advection, dispersion, sorption, and transformation processes)
on contaminants. Requirements for monitoring transmissive zones of interest typically should be
considered in the development of the site-specific DQOs for a performance monitoring plan.
Temporal variations in the ground-'water flow field due to natural or anthropogenic changes in
ground-'water recharge or withdrawal may also be important influences on contaminant migration.
These fluctuations may lead to changes in the geometry of the plume that will affect monitoring
system design and operation. For example, there can be seasonal changes in water elevations that
depend upon the temporal patterns of rainfall, and, in the north, snowfall and snowmelt. Because
there can be fixed controls on -water levels (i.e., the fixed elevation of the oceans or large water
bodies), hydraulic gradients can also change seasonally. In addition, there can be longer term
fluctuations in water elevations associated with sequences of unusually wet or dry years. With
increasing development, land use changes can alter patterns of recharge, discharge, or withdrawal
that in turn may impact contaminant migration.
Available ground-'water elevation data are used to determine, if possible, the expected range of
variation of velocities over the life of the plume that is being treated by MNA. For example,
seasonal variations in precipitation may change hydraulic gradients and the depth to ground water
resulting in fluctuations in horizontal and vertical plume boundaries, changes in contaminant
concentrations in individual wells, changes in direction of plume migration, and plume discharge
to intermittent streams and wetlands. Anthropogenic influences on site hydrology such as
changes in ground-'water withdrawal or irrigation rates and patterns may have similar effects
on plume behavior but may occur on frequencies and timing other than those corresponding to
precipitation patterns. Temporal variations in plume behavior affect the choice of performance
monitoring -well locations and the analyses of data obtained from the well network.
Geologic and hydrologic data and interpretations that are used in the development of the
performance monitoring plan are shown on Figure 2. The level of detail needed will be site
11
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Area of Former Solvent Tank
A and Ongoing Source Controls
Target Monitoring Zones
1. Source area I _l
2. Contaminated zones of highest
concentrations and mobility
3. Plume fringes exhibiting low
contaminant concentrations
4. Plume boundaries
5. Recalcitrant zone determined
from historical trends
6. Upgradient locations
mam
Legend
Gravel, gravel-sand mixtures
Medium to coarse-grained sand
Fine-grained silty sand
Bedrock
Dissolved Plume
Figure 3. Geologic block diagram and cross section depicting a stream environment in which sediments have
accumulated as valley fill. In the figure, there are numerous coarse-grained deposits as well as finer-
grained materials with lower hydraulic conductivity.
12
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specific. Much of the geologic information is obtained from geologic cores and supplemented
with information from surface and, particularly, borehole geophysical methods. Innovative
characterization technologies, such as the cone penetrometer and geologic sampling using direct-
push methods, offer the potential for cost-effectively evaluating the geologic controls on ground-
water flow and their variability in greater detail than previously possible. Hydrologic information
will generally be obtained through hydraulic testing in the field (e.g., pumping tests, slug tests, and
tracer tests) and through the measurement of hydraulic heads in wells and piezometers that are
appropriately screened in individual hydrostratigraphic units. Additional information regarding
geologic and hydrologic site characterization concepts and techniques may be obtained from a
variety of sources (e.g., Butler, 1998; Kruseman and de Ridder, 1989; U.S.' EPA, 1991a, 1993a,
1993b, 1997b, 2002a).
At a minimum, the hydrogeologic database generally should be sufficient to:
• Define geologic and hydrologic controls on the ground-water flow field (e.g., transmissive
units, barriers to flow, and the horizontal and vertical components of hydraulic gradients),
• Quantify ground-water flow rates and directions and their spatial and temporal variations
within transmissive units, and
• Support identification of possible receptors and characterization of natural attenuation
rates and the relative effects of dominant natural attenuation processes.
It is desirable that the hydrogeologic database be developed and kept in electronic form, for
ease of adding, sorting, analyzing and transferring data, developing and publishing reports, and
interfacing with geographic information systems. There are no widely-recognized standard
formats for such databases, but the interested parties may adhere to a particular format for a given
site.
The scale and intensity of the characterization are determined by the variability in site geology
and hydrology and the acceptable level of uncertainty in the outcome of the evaluations. Spatial
and temporal variability in these parameters and their effects on the performance monitoring
network and sampling frequencies typically should be explicitly considered. This process
generally should consider the observed variability, the sources of that variability, and the degree to
which the variability affects decisions regarding the monitoring network design and monitoring
strategies.
2.4.2 Contaminant Distribution, Migration, and Fate
Contaminant distribution and behavior should be well characterized within both former source
areas and the downgradient plume (i.e., the plume downgradient of all known source materials).
Source characterization data (Table 3) are used to help identify appropriate performance
monitoring constituents and monitoring locations and depths as well as interpret historical data
and predict future behavior of the dissolved plume. Some of the data, such as the history of source
release, may be unavailable.
Some of the more common sources for continued ground-water contamination by organic
compounds are NAPL and sorbed organic contaminants within the vadose and, especially,
saturated zones (Palmer and Fish, 1992; U.S. EPA, 1993c). NAPL infiltration into the subsurface
is complex and is influenced by physical properties of the NAPL and by macro- and micro-scale
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Table 3. Source Characterization Information for Conceptual Site Model Development
Data Type
Utility
Three-dimensional distribution of physical contaminant
phases (e.g., nonaqueous phase liquid (NAPL) and sorbed
materials) that are continuing sources for ground-water
contamination.
Contaminant release and source removal/control
histories (i.e.,timing and descriptions).
Constituents that were released, those that are currently
present, and toxic transformation products.
Conceptualize,in conjunction with hydrogeologic data,
three-dimensional migration pathways of both NAPL and
aqueous-phase contaminants.
Project possible behavior of dissolved plume.
Identify need for source controls and estimate range of
possible time frames for restoration of the plume.
Identify areas where contaminant attenuation rate may
not be sufficient to meet contaminant reduction
objectives (;.e.,"recalcitrant zone").
Aid in identifying appropriate monitoring locations and
depths.
Constrain interpretations of contaminant migration and
historical trends in observed contaminant concentrations.
Identify potentially dominant transport and fate
processes based on chemical properties.
Identify monitoring constituents.
geologic features (Mercer and Cohen, 1990; Cohen and Mercer, 1993). Careful evaluation of the
extent of NAPL infiltration and migration is needed because NAPL may have migrated both
vertically and horizontally far from the original site of the release. Source materials may greatly
affect MNA remedy performance with respect to attainment of contaminant reduction goals, the
geometry of the associated dissolved plume, and design of the performance monitoring program. It
is expected that source control will be a fundamental component of any MNA remedy (U.S. EPA,
1999a).
Detailed definition of contaminant distribution throughout the three-dimensional boundaries
of the contaminated aquifer provides data that can be correlated with the hydrogeologic
characterization data to determine the effects of the hydrogeologic controls on contaminant
migration. The data generally should be sufficient to define the zones of greatest contamination,
rapid contaminant migration and greatest risk to possible receptors in addition to defining the
plume boundaries in order to target these zones during the performance monitoring program.
The effects of the dominant attenuation processes may be evaluated and field-scale attenuation
rates estimated (U.S. EPA, 1998a; Wiedemeier et al, 1999; U.S. EPA, 2002b) in order to identify
and monitor the controls on plume stability and project progress toward remedial action objectives
of reduction in contaminant concentrations to target levels. Several processes may control the
fate of the dissolved plume (e.g., the processes that are the components of the attenuation rate:
dispersion, dilution, sorption processes, volatilization, and chemical and biological degradation).
The need for determining the contribution of each component of the attenuation rate will vary
depending on remedial goals. For instance, only chemical and biological degradation actually
destroy contaminant mass; the other processes that only lower contaminant concentration,
retard contaminant migration, or move contaminants to other media may not produce acceptable
remedial results at all sites.
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The controls on each process and the potential for continuation of the processes at current rates
throughout the remediation time frame typically should be considered because the effectiveness of
some of the processes may vary over time, invalidating predictions of future effectiveness based on
historical rates. For example, continued biodegradation of chlorinated solvent contamination may
be contingent on the continued supply of readily degradable organic carbon compounds to serve
as electron donors in the biotransformation processes (Wiedemeier et al., 1999; Leahy and Shreve,
2000).
Data on the shape and dynamic behavior of the dissolved contaminant plume collected over a
period of several years are helpful in order to evaluate natural attenuation processes and develop
the monitoring plan. Three to five years of periodic monitoring (during the site characterization
and remediation effort) may, in many cases, be sufficient to form a conceptual model of plume
behavior adequate for developing the performance monitoring plan. However, more complex
or highly variable sites may require longer-term characterization to adequately evaluate the
range of plume behavior due to variations in factors such as ground-water flow and biological
activity (Barcelona et al., 1989). For the purposes of performance monitoring plan development,
these data are used to determine whether significant temporal {e.g., seasonal) fluctuations in
plume boundaries are occurring. This information is needed to site wells to monitor long-
term plume stability and to trigger implementation of contingency remedies based on observed
plume migration. The data may also indicate portions of the plume where the progress toward
contaminant reduction goals may be slow and enhanced monitoring may be warranted to
determine the cause and any necessary remedy modifications. It is important to note that a few
years of site characterization monitoring data are not reasonably sufficient to accurately predict
plume behavior (and performance monitoring needs) for decades. However, such data can
appropriately be used to refine a conceptual model sufficient for initial design of a performance
monitoring program. The monitoring program in operation can then provide data for continual
refinement of the conceptual model and, subsequently, of the monitoring program itself.
2.4.3 Geochemistry
Characterization of subsurface geochemical environments and their variability provides important
insights into the types of bio tic and abiotic processes that may be affecting plume behavior.
Many of the processes driving plume behavior cannot be measured directly (e.g., biological
transformation of contaminants). However, the processes may cause changes in geochemical
parameters, leaving an observable geochemical "footprint" that can be related qualitatively and
quantitatively to the biotic and abiotic processes (National Research Council, 2000).
In general, fuels serve as electron donors during microbial degradation of the fuels (i.e., the
fuels are oxidized during microbial metabolism). More oxidizing redox conditions and greater
availability of electron acceptors at a site lead to more efficient biodegradation processes for
fuel degradation (Ludwig et al., 2000). In contrast, chlorinated solvents may serve as electron
acceptors in microbial metabolism (i.e., the solvents can be reduced during microbial metabolism),
and more reducing redox conditions lead to more efficient biodegradation (Chapelle, 1996). Note
that some chlorinated solvents such as trichloroethene (TCE), dichloroethene (DCE), and vinyl
chloride (VC) can also act as electron donors, and degrade under more oxidizing conditions.
Because the degradation of fuels and solvents is influenced by redox conditions, assessment of
ambient redox conditions is an important component of any monitoring program for monitored
natural attenuation of fuels or solvents (U.S. EPA, 2000b). The nature of this assessment maybe
15
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as simple as delineating the distribution of oxic/anoxic ground water, or it may be more in depth
requiring the delineation of oxic, iron (III) -reducing, sulfate-reducing, and methanogenic zones
at the site (Chapelle et al., 2000). The role of Eh measurements in redox assessments is subject
to numerous uncertainties (Barker, 2000), but Eh measurements are qualitative indicators of
redox conditions (Westall, 2000). The list of redox parameters given by Wilson (2000) can be
used to develop a site-specific monitoring program for redox parameters. The appropriate level of
monitoring can only be determined on a site-by-site basis (Wilson, 2000).
Detailed discussions of biogeochemical reactions pertinent to fuel and chlorinated solvent
contamination in ground water and the geochemical patterns associated with these processes
may be found in Azadpour-Keeley et al. (1999), Azadpour-Keeley et al. (2001), National Research
Council (2000), U.S. EPA (1998a), and Wiedemeier et al. (1999).
Geochemistry can provide the following kinds of information:
• Whether ambient redox conditions and processes favor the natural attenuation of the
contaminants of concern, as well as identifying the dominant degradation processes
and long-term monitoring parameters indicative of the continuing effectiveness of those
processes,
• Whether stoichiometric relationships between electron acceptor (oxygen, nitrate, sulfate,
etc.) utilization and contaminant degradation are observable,
• Whether redox conditions or other geochemical conditions could enhance the mobility of
certain contaminants of concern (e.g., manganese or arsenic), or
• Identify zones beyond the current plume boundaries where soluble electron acceptors
or donors are depleted or nonhazardous 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 by natural attenuation). Because the plume would travel into these zones if it
expands, such zones can serve as target zones for monitoring plume stability.
Geochemical parameters and trends that are often useful indicators of biotransformation
processes at sites with fuel and chlorinated solvent contamination are discussed in U.S. EPA
(1998), Wiedemeier et al. (1999), and Wiedemeier and Haas (2002). The individual parameters
diagnostic of dominant processes and most useful in a performance monitoring program depend
on site-specific conditions. Parameters to be measured are chosen -with regard to their potential
to affect site-related decisions (i.e., it would not be useful to measure a given parameter if the
information -would not be used to change site-related decisions).
2.4.4 Receptor Locations
For the purpose of specifying performance monitoring locations, identification of human and
ecological receptors that may be affected by the contaminant plume under current and reasonably
anticipated future conditions is a critical element. In addition to possible receptors -within the
areas that are currently contaminated, locations outside of the plume that maybe subject to
impact during the remedy performance period should be identified. Such receptor locations may
include, but are not limited to, -water supply -wells, buildings, aquifers, -wetlands, sediments, and
surface-water bodies. In many instances, this list may include aquifers that are below zones
16
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of shallow contamination. The vapor intrusion to indoor air pathway is also of considerable
importance (U.S. EPA, 2002c). This document focuses on monitoring the saturated zone, but
site characterization and monitoring for any remedy should include appropriate monitoring of
all significant pathways by which contaminants may migrate and impact receptors. Sources of
additional information concerning receptor identification include U.S. EPA (1997c) and U.S. EPA
(1998b).
Information on receptors and possible pathways for impact is used to identify appropriate
locations for monitoring points between the plume and the possible receptor as well as locations
where impact may occur. Monitoring at these locations provides confidence that no unacceptable
impacts occur.
2.5 Monitoring Network Design
2.5.1 Introduction
The media of primary concern and, therefore, the focus of the monitoring network design, will
be ground water at many sites. However, monitoring of other media {e.g., indoor air, soil gas,
soils, surface water, and sediments) may be necessary to determine possible impacts to receptors
and other measures of remedy effectiveness. Guidance regarding the assessment of the indoor
air exposure pathway is provided in U.S. EPA (2002c). Recent discussion of the assessment of
the interactions of ground water and surface water useful in formulating effective monitoring
networks may be found in U.S. EPA (2000e). At many sites, the assessment of such cross-media
transfers of contaminants is essential to adequate performance monitoring for MNA remedies.
The focus of the following discussion is primarily on the monitoring of ground water as this is
often an exposure pathway of significant concern. Performance monitoring is extended to other
media as warranted by site conditions.
A monitoring system designed for evaluating the performance of an MNA remedy with respect
to specific remedial action objectives may be very different from the network established during
earlier phases of site characterization, feasibility studies, or interim actions. Existing wells were
often incrementally installed for different purposes, such as defining the extent of contamination
during the remedial investigation or evaluating an engineered remedy during a feasibility study.
The location and number of these wells may not be well suited for MNA performance monitoring.
Specification of a monitoring network design should be based on consideration of all available
information concerning the processes and factors expected to control contaminant distribution.
The network is designed to provide data to demonstrate attainment of all the remedial action
objectives for an MNA remedy.
A plume is a dynamic, three-dimensional distribution of contaminants in ground water, that
generally necessitates three-dimensional monitoring. Plume shape is influenced by many factors,
including original source distribution, geology, hydrology, and biologic processes. The resulting
spatial and temporal variability significantly impact choice of monitoring locations and frequencies
and necessitate continual reevaluation of the performance monitoring network. Appendix A
provides a brief discussion of some issues regarding variability.
The density of sampling points in a monitoring network will depend on the geology and
hydrology, the spatial scales at -which contaminant distribution varies horizontally, vertically and
temporally, and the desired level of confidence in the evaluation. Plumes often vary significantly
in concentration in transverse and vertical cross sections (e.g., Cherry, 1996) making evaluation
17
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of contaminant distribution and remedy performance difficult. In these cases, a dense network
of monitoring points will often be needed to support many of the performance monitoring
evaluations described below.
One approach to this problem is to define and monitor the plume using clustered monitoring
points positioned in transects across and through the plume, perpendicular to the direction of
ground-water flow (Figures 4, 5 and 6). The horizontal and vertical spacings of the monitoring
points in each transect are determined by the hydrogeologic conditions that control contaminant
migration and the dimensions and spatial heterogeneity of the resulting contaminant distribution.
The horizontal distance between transects is generally based on changes in contaminant
concentration along the plume, and the location of the source and distal portions of the plume.
For example, transects may be placed:
• Immediately upgradient of the source area to monitor contaminant and electron acceptor
flux into the plume,
• Immediately downgradient of the source area to monitor contaminant flux to ground
water,
• Near the downgradient and sidegradient plume boundaries to monitor contaminant
concentration increases indicating possible plume expansion,
• Immediately downgradient of the plume or other compliance boundary to monitor for
plume expansion, and
• Along the plume to provide information on plume configuration and contaminant
attenuation. Horizontal spacing of the transects for determination of contaminant
attenuation may be based on the number of locations needed for attenuation rate
calculations, and changes in contaminant concentration along the plume {e.g., spacing
conforming to order-of-magnitude decreases in contaminant concentration).
The elevations of sampling intervals are generally based on stratigraphy (i.e., sampling the
different stratigraphic intervals), the vertical component of hydraulic gradients, and contaminant
distribution (i.e., sampling the top, bottom, and "core" of the plume, and, possibly, above and
below the plume to vertically bound it).
The use of a transect-based approach to monitoring may greatly reduce the uncertainty in
performance monitoring evaluations at many sites by improving the definition of contaminant
distribution and its variability. The transect approach helps to locate ground-water flow lines
and contaminant migration paths. Transects also provide a better definition of contaminant
distribution under conditions of changing hydraulic gradients. A detailed example of the use
of a transect-based approach in the evaluation of natural attenuation processes at a site where
petroleum products were released may be found in Kao and Wang (2001).
2.5.2 Monitoring Locations
Generally, each distinct zone of contaminant migration and geochemical regime is monitored to
assess its impact on remediation. For instance, if part of a plume of tetrachloroethene is anaerobic
with high levels of electron donors available and another part of the plume is aerobic with few
electron donors available, degradation of the tetrachloroethene may be very active in the anaerobic
zone but nonexistent in the aerobic zone. For each zone with distinctly different conditions or
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Site (Map View)
Upgradient
Transect
Lateral {Side Gradient) {
\
BS
Source
(T) Area
Recalcitrant High
Zone Concentration
Plume Core
Low
Concentration
Plume Fringe
Plume
Boundaries
Non-Hazardous
Degradation
Products & Other
Geochemical Indicators
Ground-Water Flow
Target Monitoring Zones
1. Source area
2. Contaminated zones of highest
concentrations and mobility
3. Plume fringes
4. Plume boundaries
5. Recalcitrant zone determined
from historical trends
6. Upgradient and sidegradient
locations
x Monitoring well cluster
A Piezometer
-*- Transect of well clusters
Figure 4. Example of a network design for performance monitoring, including target zones for monitoring
effectiveness with respect to specific remedial objectives. In this example, monitoring network
design is based on transects of wells oriented perpendicular to the ground-water flow direction.
Sampling locations for target monitoring zones were chosen based on site characterization.
Piezometers provide additional data for evaluation of changes in potential ground-water flow
direction.
controls on contaminant migration and fate, the following locations would be monitored: areas
hydraulically upgradient and sidegradient to the plume, source area, main body of the plume, and
distal portions and boundaries of the plume.
2.5.2.1 Typical Target Zones
Typical target zones for monitoring a contaminant plume (Figures 4, 5 and 6) include:
• Source areas - within and immediately downgradient of remediated source areas
The monitoring objectives are to determine and demonstrate whether any further
contaminant releases to ground water occur and to estimate contaminant reduction
over time. In situations where the source is contained, increased contamination or new
contaminants could be indicative of such conditions as cap failure, buried drums that
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Area of Former Solvent
Tank and Ongoing
Source Controls
0-
Monitoring Well
Cluster
150
Target Monitoring Zones
1. Source area L _ 1
2. Contaminated zones of highest
concentrations and mobility
3. Plume fringes exhibiting low
contaminant concentrations
4. Plume boundaries
5. Recalcitrant zone determined
from historical trends
6. Upgradient locations
Legend
Gravel, gravel-sand mixtures
Medium to coarse-grained sand
Fine-grained silty sand
Dissolved Plume
Figures. Cross section A-A'through monitoring network in general direction of ground-water flow.
Placement of monitoring points within target zones is based on geologic controls and contaminant
distribution characterized prior to remedy selection and is periodically modified, as warranted,
based on evaluation of performance monitoring data. In this scenario,detailed site characterization
data would be used to define the limits of the source area, the distribution of any NAPLand
aqueous-phase contaminants, and the effectiveness of source removal and control actions. Source
control activities and monitoring associated with the release from the former solvent tank are not
pictured.
rupture, a rise in the water table transferring additional contaminants from the
vadose zone, or slurry -wall failure. These new contaminant releases could be greater
than the capacity of the subsurface to attenuate concentrations without significant
plume expansion or could include contaminants not effectively remediated by MNA.
Transmissive zones with highest contaminant concentrations or hydraulic conductivity
A change in conditions in these zones, such as an increase in contaminant mass
from source areas or increased ground-water velocity, may lead to a relatively rapid
impact to a downgradient receptor.
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Monitoring Well
Cluster
OJ
OJ
Q.
OJ
Q
100-
150
Legend
Gravel, gravel-sand mixtures
Medium to coarse-grained sand
Fine-grained silty sand
Dissolved Plume
Figures. Cross section B-B'through monitoring network perpendicular to ground-water flow. Monitoring
points are placed to define the plume horizontally and vertically, as well as monitor contaminant
concentration zones and geochemical zones.
Distal or fringe portions of the plume
These are areas where reduction of contaminant concentrations to levels required by
remedial action objectives may be attained most rapidly or -where plume expansion may be
observed.
Plume boundaries and, other compliance boundaries
Multilevel monitoring points typically -would be placed at the sidegradient, downgradient,
and vertical plume boundaries, and bet-ween these boundaries and possible receptors.
Multilevel monitoring generally should also be performed at any other compliance
boundaries specified in remedy decision documents. Results from these monitoring
locations may directly demonstrate unacceptable plume expansion and changes in ground-
water flow directions.
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• Zones in which contaminant reduction rates appear to he lower than required to meet
remediation goals (i.e., recalcitrant zone)
These are the areas where attaining cleanup standards -within accepted time frames
may be impeded due to site conditions (e.g., presence of previously undetected source
materials or low flux of electron acceptors). Such areas, if present, will be delineated
through evaluation of data obtained throughout the performance monitoring period.
These areas may require additional characterization to determine if additional
remedial actions are necessary to reduce contaminant concentrations to desired
levels.
• Areas representative of contaminated and uncontaminated geochemical settings
Sampling locations for monitoring the geochemical setting include monitoring
points that are hydraulically upgradient and sidegradient with respect to the plume.
Because assumptions concerning the geochemical setting and naturally occurring
changes in geochemical parameters affect interpretation of data from the plume, such
assumptions should be tested and evaluated like other parts of the conceptual site
model. Therefore, multiple monitoring points generally should be used to determine
the variability of geochemical conditions outside the plume. Data concerning the
movement of electron acceptors, donors, and any contaminants into the plume aid
in understanding and interpreting data from the plume. These geochemical data
are used to determine whether the observed differences in geochemical parameter
concentrations within the plume are due to contaminant transformation processes
rather than natural variations in the background geochemical conditions. The
locations sidegradient to the plume help to evaluate changes in plume geochemistry
with time as ground water migrates through uncontaminated aquifer materials.
Changes in geochemistry within the plume may not be directly related to attenuation
of the contaminants, so geochemical changes outside the plume generally should
be assessed and compared to geochemical changes taking place within the plume.
If upgradient and lateral monitoring points show geochemical changes similar to
changes in the plume, such changes may not be attributed solely to contaminant-
related processes (i.e., degradation), and, therefore, may not serve as supporting
evidence for degradation processes.
Areas
s supporting the monitoring of site hydrology
At some sites, monitoring of ground-water elevations at locations additional
to those used for the monitoring of chemical parameters may be needed to
determine hydraulic gradients. At such sites, appropriate locations for placing
piezometers will often include positions that are upgradient and sidegradient
to the contaminant plume, as well as in zones above and below the plume.
Piezometers are usually spaced across the site so that ground-water elevation
measurement errors are relatively small compared to the difference in ground-
water elevations between piezometers.
2.5.2.2 Screen Lengths
The screen length for a given -well constitutes an important part of the three-dimensional
monitoring location. The screen is sized to sample the interval of interest. The interval
22
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may be defined by stratigraphy, contaminant loadings, or geochemistry, based on the site
characterization data. Factors to consider in determining screen length include the following:
• Well screens can be matched to stratigraphic intervals if the intervals are relatively
small, and the geochemical and contaminant values are similar throughout the vertical
extent of the interval intersected by the well screen. For instance, a five-foot thick
sand layer within which contaminated water migrates could be sampled with a five-
foot screen to intersect the entire interval. Any interval of significantly different
conductivity could be targeted by a specific screen length. Well clusters can be used to
provide coverage of the entire contaminated unit, as needed.
• Monitoring wells typically should screen comparable intervals. For example, well
screen lengths can be matched to contaminant loadings. Suppose that dissolved
petroleum contaminants (e.g., benzene, toluene, ethylbenzene, and xylenes) were
transported primarily within a 5 ft to 10 ft thick interval of a 30-ft thick sandy unit. In
this situation, a monitoring well screen could be sized to sample the most contaminated
part of the unit to help determine attenuation of the contaminants in that interval.
The use of longer screens may result in artificially lower measured containment
concentrations, or even lack of detections, due to the mixing of water with different
chemical compositions. Calculated attenuation rates or estimated plume boundaries
may reflect variations in screen length and placement rather than actual attenuation
if monitoring well screen intervals are not matched to contaminant distribution in the
contaminated stratigraphic interval.
• Well screen lengths can be matched to geochemistry, to sample a zone where a particular
geochemistry prevails. Because attenuation of some contaminants is highly sensitive to
the geochemical environment, it is often desirable to be able to accurately identify and
discretely sample locations in the plume where a particular geochemistry prevails.
2.5.3 Monitoring Parameters
The primary classes of parameters to be monitored during these evaluations generally will be
the contaminants of concern (COCs), geochemical indicators of transformation processes, and
hydrogeologic parameters. Contaminants of concern are those chemicals identified during site
investigations that are required to be addressed by the response action proposed in the remedy
decision documents.
Contaminants of concern may be identified from:
• Ground-water and soil monitoring data,
• Contaminant source histories, and
• Evaluation of contaminants that potentially may be formed or mobilized as a result of
biotransformation processes or changes in the geochemical environment. For example,
toxic products such as dichloroethene and vinyl chloride would be measured at sites
contaminated with chlorinated solvents where they might be expected to be present.
At some sites, it may be necessary to identify and include chemicals that have only been
tentatively identified in previous sampling {e.g., Tentatively Identified Compounds (TICs)).
23
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Because the effectiveness of natural attenuation processes varies markedly for different
contaminants, it is often inappropriate to assume that other components of the plume will be
adequately remediated along -with the primary contaminants of concern during MNA. Potentially
hazardous plume components should be positively identified, evaluated to determine if they will
be sufficiently mitigated by MNA, and monitored, as appropriate, if they are a threat to human
or environmental health. For instance, MTBE (commonly found in gasoline-sourced plumes),
was overlooked at most sites for many years because sampling and analysis at most such sites
emphasized BTEX. However, at many such sites, BTEX components may be attenuated to meet
standards before MTBE, so monitoring only BTEX may result in a mistaken interpretation
of the effectiveness of MNA. Another example of an overlooked contaminant is 1,4-dioxane,
which may be associated with certain plumes of volatile organic contaminants. Recent advances
in analytical methods allow detections in ground-water samples at much lower concentrations
(e.g., 1.5 M-g/L) than previously possible. 1,4-Dioxane has been used as a solvent stabilizer in
1,1,1-trichloroethane at concentrations of 2 % to 8 % by volume (Mohr, 2001) and is a class II-
B probable carcinogen. This contaminant is highly soluble in and mobile in ground water. At
sites where this contaminant is present, it may be expected to migrate further than the associated
plume of chlorinated solvent compounds (Fetter, 1994). At such sites, a monitoring program that
does not include 1,4-dioxane would likely result in a mistaken interpretation of the effectiveness
of MNA.
At a well-characterized site, the DQO process can be used to choose parameters and monitoring
frequencies for each monitoring location based on the value of the data to monitoring MNA, as
an alternative to the measurement of all parameters at all locations. For example, if several years
of monitoring indicate that the geochemistry in the central portion of a BTEX plume is stable,
it may not be necessary or useful to continue to analyze samples from these locations for all of
the geochemical parameters at each sampling event. However, all contaminants normally would
be measured in all samples on a periodic basis to verify the conceptual site model. The most
important monitoring parameters are derived from a thorough understanding of site history,
ambient geochemistry, contaminant sources, and geochemical changes induced by contaminant
degradation.
The purposes for long-term monitoring of geochemical parameters in addition to ground-water
contaminants include:
• Confirmation of dominant biotransformation processes,
• Evaluation of the potential for continued transformation, and
• Identification of the zones of contaminant migration (e.g., where geochemistry indicates
contaminant degradation has taken place).
These purposes are discussed in Azadpour-Keeley et al. (1999), Azadpour-Keeley et al. (2001),
U.S. EPA (1998a), Wiedemeier et al. (1999), and Wiedemeier and Haas (2002).'
The specific parameters useful at a given site depend on site contaminants, natural geochemical
settings, and dominant biotransformation processes. In most cases, a select group of parameters
that indicate the geochemical environment (e.g., oxidation-reduction potential, dissolved oxygen,
pH), identify geochemical regimes affecting contaminant degradation (e.g., nitrate, iron (II),
sulfate, methane), or are products of contaminant degradation (e.g., ethane, ethene) will be
measured in most samples. In addition to the measurement of geochemical parameters in ground-
water samples, periodic monitoring of solid-phase electron acceptors, such as bioavailable iron,
24
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in aquifer materials may be useful at some sites to evaluate the supply of such materials relative
to the mass of contaminants to be degraded (Ruling et at, 2002). The geochemical parameters
measured generally should be chosen based on the utility of the data for affecting site-related
decisions (i.e., if no decisions would be changed based on the data, then the data need not be
collected).
The primary hydrologic parameter of interest is the elevation of ground water in monitoring -wells
and piezometers. Estimates of hydraulic gradients and changes in the elevation of the -water table
with respect to locations of residual source materials (i.e., any source materials that remain after
all appropriate source control actions have been completed in accordance -with OSWER Directive
9200.4-17P (U.S. EPA, 1999a)) are fundamental to all evaluations conducted during performance
monitoring. The -wells and piezometers used in these evaluations should be discretely screened
in individual hydrogeologic units within and adjacent to the contaminant plume. Additional data
that are often essential in assessing ground-water flow include surface-water elevations, local
rates and schedules of irrigation, local precipitation data, and pumping rates and schedules for
nearby -wells. The evaluations should be three-dimensional in nature, including both horizontal
and vertical components of hydraulic gradients that control three-dimensional contaminant
migration. In many cases, the vertical component of the hydraulic gradient may be larger and
may display more variability than the horizontal component. Depending on the anisotropy in
the hydraulic conductivity of aquifer materials and the magnitude, direction, and duration of the
hydraulic gradients, vertical movement of ground -water and contaminants may be significant and
fluctuations in both the horizontal and vertical components of ground-water flow may exist.
2.5.4 Monitoring Frequency
Monitoring frequency affects the ability of the performance monitoring program to:
• Provide timely -warning of impact to receptors,
• Detect contaminant releases to ground -water that -warn of possible plume expansion,
• Detect changes in plume size/concentration,
• Determine temporal variability of data,
• Detect changes in geochemistry that -warn of changes in attenuation, and
• Yield data necessary to reliably evaluate progress toward contaminant reduction objectives.
The most appropriate frequency for ground-water sampling depends on the rate -with -which
contaminant concentrations change due to ground-water flow and natural attenuation processes,
the degree to -which the causes of this variability are known, the types of evaluations to be
performed, the location(s) of possible receptors, and the RAOs for the site. Based, in part, on
previous studies (e.g., Barcelona et at, 1989), quarterly monitoring will often be an appropriate
frequency to establish baseline conditions over a period of time sufficient to observe seasonal
trends, responses to recharge, and to estimate attenuation rates for key contaminants. Quarterly
monitoring for several years provides baseline data to determine trends at new monitoring points
and test key hypotheses of the conceptual site model. In situations where hydrologic, geochemical
and contaminant trends are stable and the conceptual site model is verified by measured site
data, reductions in sampling frequency may be -warranted. In situations -where variability is high,
25
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increases in monitoring frequency and additional investigations to determine the source of the
variability may be warranted. More frequent monitoring may be appropriate under circumstances
where ground-water flow is rapid and/or contaminant travel time to receptors is short.
More frequent monitoring of ground-water elevations may be warranted, particularly during
the establishment of baseline conditions, to improve the characterization of ground-water flow
patterns. In addition, more frequent monitoring may be needed to observe changes in ground-
water flow patterns in response to other site activities, such as the start or cessation of ground-
water extraction, source control activities, and other significant changes in the hydrologic system.
In some cases, monitoring of this parameter on a very frequent basis using automated recording
equipment maybe needed to determine the effects of variability in recharge and discharge rates
or locations on ground-water flow patterns. This will aid in specification of appropriate long-term
monitoring frequencies and locations for evaluating fluctuations in hydraulic gradients and trends
in chemical parameters. Based on the results of such assessments, initial monitoring frequencies
may be adjusted to adequately capture the fundamental features of the trends. In situations
where temporal trends in hydraulic gradients are absent or -well characterized, monitoring of
ground-water elevations at a frequency no less than that of the chemical parameters is generally
appropriate.
Several years of monitoring data are typically necessary for estimation of the site variability and
expected rates of change in ground-water flow, contaminant concentrations, and geochemistry
(Barcelona et at, 1989). Once site characterization and initial performance monitoring activities
have provided these data, reevaluation of the monitoring frequency may be warranted if trends
are established and the remedy is progressing as expected. Increases and decreases in monitoring
frequency may occur several times over the life of the remedy in response to changes in site
conditions and monitoring needs (Figure 7). For example, note in Figure 7 that monitoring
frequency was decreased in response to stable contaminant concentration declines, increased
in response to a sudden increase in contaminant concentration, decreased again based on the
reestablishment of stable contaminant concentration declines, and increased yet again -with the
implementation of the verification monitoring program.
Specifically, monitoring frequency generally should be related to detecting changes in site
parameters that indicate ability of the MNA remedy to achieve site-related remedial action
objectives, and to provide early warning of possible impact to receptors. The monitoring
frequency may be adjusted to capture information regarding trends of interest -while eliminating
unnecessary redundancy. However, monitoring frequency determinations should take into
account that apparently stable site parameters may sometimes change due to natural or
anthropogenic causes. For example, contaminant releases or remediation activities on adjacent
properties may cause chemical or hydrologic changes in plume properties at the site. Increased
rainfall can change recharge rates, ground-water extraction can cause changes in ground-water
flow, and air sparging for volatiles stripping or hydrogen peroxide injection for contaminant
oxidation could change the geochemical regime possibly changing biodegradation rates.
Water-table variations and changes in velocity are common in flood plains, particularly near large
rivers that have major changes in the stage of the river. In some climates, most of the recharge
to aquifers due to precipitation occurs in only a few months of the year. In some settings, water
extracted by trees from shallow water table aquifers can influence the direction of ground-water
flow. Irrigation or municipal water supply wells that pump intermittently can change ground-
water flow in an irregular manner.
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D_
C
o
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Examples of factors influencing specification of the monitoring frequency are provided below.
Each example considers only the point under discussion. The actual monitoring frequency
selected would be based on consideration of all the factors bearing on the given parameter and
monitoring location.
• Monitoring Frequency Determination Based on Possible Contaminant Travel Time to
Receptors
Consider a site with the following conditions: the travel time for ground water from
the downgradient plume boundary to a receptor -was expected to be two years, based on
analysis of the ground-water flow field. The time required to design, construct and bring
an alternative remedy online to intercept the contaminant and protect the receptor would
be added to the ground-water travel time. In this hypothetical situation, monitoring
contaminant concentrations at wells placed downgradient of the current plume boundary
to detect plume expansion would be conducted at least annually, and, in most cases,
more frequently (e.g., quarterly) to allow time for confirmation of detections and other
contingencies. Monitoring may be more frequent for -wells near and downgradient of the
plume boundary or other compliance boundaries than for interior -wells. In this example,
the travel time for a conservative constituent -was used in the calculations to allow a more
conservative estimate of the minimum contaminant migration time than that obtained
through use of an estimated transport velocity for a contaminant that may undergo
sorption or biotransformation reactions.
• Monitoring Frequency Determination Based on Evaluation of a Cyclic Change
Some sites, such as sites -where the climate produces a pronounced -wet season/dry season
with associated changes in recharge rates and ground-water flow characteristics, may
display pronounced cyclical trends in contaminant concentrations and plume boundaries.
For example, seasonal changes may involve seasonal inputs of contaminant from the
vadose zone, and may require more frequent monitoring or changes in monitoring
timing relative to cyclical changes (i.e., the particular dates when samples are taken)
(Figure 8). By reviewing the historical variability in -water levels for the site and recorded
climatic variability (i.e., drought frequencies, periods of above-average rainfall), a plan for
specifying the monitoring frequency may be established. The objective of such a plan is
to increase monitoring frequency sufficiently to prevent unmonitored expansion of the
plume -while avoiding collection of unnecessary data. Typically, data gathered over several
years are necessary to evaluate seasonal trends and determine -what frequency and time
of monitoring are appropriate to capture changes related to seasonal trends (Barcelona et
at, 1989). Examination of prevailing annual -weather patterns (available from National
Weather Service historical data) may be helpful for determining appropriate sampling
frequencies.
• Monitoring Frequency Determination Based on Relevance of Parameters
If a parameter is not expected to significantly influence evaluations of remedy performance
at a site, then monitoring frequency for that parameter could be greatly reduced or
eliminated. If information gained from frequent sampling for a given parameter -would
not be reasonably expected to change site-related decisions, sampling for that parameter
could be reduced in frequency. However, absent specific reasons for excluding a parameter,
28
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O
.
U
R 10.0
Annual
Biannual
\
Quarterly 1, Starting in January
Quarterly 2, Starting in February
0°
EFFECTS OF MONITORING FREQUENCY
The following example is used to illustrate the effect of different
monitoring frequencies and timing on monitoring data and data
interpretation.
Site Characteristics
The site had a leaking underground storage tank that contained gasoline.
The tank has been removed and source control actions implemented, but
some undetected petroleum product remains at residual saturation in the
vadose zone and saturated zone extending downgradient of the source
area. The water table varies about four feet in elevation throughout the
year.
This example is focused on one monitoring well from the multi-well
performance monitoring network. The well is constructed with a ten-
foot screen and located 100 feet downgradient from the LNAPL release
area. Travel time for contamination to move from the release area to the
monitoring well varies, but averages about five months. The top of the well
screen is located at 20 feet below ground surface (bgs)(/.e., about one foot
above the highest observed water table elevation). The bottom of the well
screen is at 30 feet bgsand is terminated at a regionally extensive, uniform
clay layer.
The ground water is contaminated from the water table to a depth of
about 28 feet bgs. There is an uncontaminated zone about two feet thick
over the clay confining layer, so that some clean water is intercepted by the
lower part of the well screen.
During the rainy season in the fall (November-December) and spring
(March-May),the watertable reaches its"maximum"elevation at about
22 feet bgs, which is about two feet belowthetopofthe well screen. The
higher watertable intercepts more of the previously undetected residual
LNAPL and sorbed contaminants in the vadose zone in the release area,
causing a pulse of contaminants to enter the ground water.
From December to March and May to June, the water table declines to its
"average"elevation,at about 24 feet bgs. During July,a nearby irrigation
well comes online, and continues pumping until sometime in September,
causing the water table to decline to elevations as low as 26 feet bgs,
its"minimum"elevation.Also,ground-water flow is shifted so that the
monitoring well is not directly downgradient of the source zone.
During periods of high water table elevations, flushing and interception
of contaminated vadose zone by infiltration and the elevated water table
cause pulses of contaminant to move toward the monitoring well,arriving
several months after the rain. During periods of low water table
elevations, less residual NAPL is intercepted by the ground water, so water
with lower contaminant concentrations moves toward the monitoring well.
Also,the shift in ground-water flow direction caused by the irrigation well
causes the monitoring well to intercept the fringe of the plume, rather than
the more central, highly contaminated volume of the plume.
Results
The concentrations of a contaminant of concern (COC) from the
monitoring well are shown in the adjacent panels. The trend line in each
graph shows the estimated linear trend of contaminant concentration
based on the data in the chart. It can be seen that at sites where
conditions cause significant changes in the contaminant concentration
throughout the year,the frequency of sampling and the timing of the
sampling cycle relative to the contaminant concentration cycle can
strongly affect the data obtained and the interpretation of the data.
These data highlight the importance of site characterization in choosing
monitoring frequency and, in this example, identifying the need for
additional source controls for effective remediation.
Figure 8. Monitoring frequency effects on sampling data collection and interpretation. The relationship of
monitoring frequency and timing to cyclical changes in contaminants may significantly affect data
collection and interpretation.
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it generally would be expected that the entire suite of geochemical parameters relevant to
the site, as well as contaminants and hydrogeological parameters, would be measured at all
monitoring points on a set schedule. This allows an evaluation of continued stability in
the geochemical setting and the potential for changes in biotransformation processes and
attenuation rates to be performed.
• Monitoring Frequency Determination Based on Information Redundancy
If, over a period of several years, data trends are stable, a reduction in monitoring
frequency maybe warranted (Figure 7). Also, if two or more wells sampling the same
zone are located close together, and consistently produce similar data, changes in the
monitoring frequency of one or more of the wells may be considered. For example, if
the geochemical indicators at a given monitoring location are stable over a long period
(e.g., within a range indicating suitable conditions for degradation of the contaminants),
then monitoring for these geochemical indicators could be reduced in frequency, to the
point of being measured annually, biennially, or at even greater intervals. However, the
possibility of a rapid change in a previously stable pattern may affect determination of the
monitoring frequency. For instance, nitrate concentrations, that can 1) change quickly due
to anthropogenic inputs and 2) strongly inhibit reductive dechlorination of the chlorinated
solvent compounds, generally should continue to be monitored frequently at sites where
reductive dechlorination is an important component of the MNA remedy.
Contrastingly, for example, if a sudden change in contaminant concentrations were noted
(Figure 7), an increase in monitoring frequency maybe warranted to provide information
to facilitate understanding of the change and provide earlier warning of further change.
2.6 Demonstrating MNA Effectiveness with Respect to Remedial Objectives
Representative techniques for demonstrating the effectiveness of MNA, with respect to the
remedial objectives provided in Table 1, are discussed below. The discussions are intended to
provide general suggestions on the types of assessments, monitoring network designs, parameters,
and evaluation frequencies. However, specific study designs depend on site conditions and the
site-specific limits on decision errors. For additional information concerning remedial objectives,
refer to current program-specific guidance.
2.6.1 # 1 - Demonstrate that Natural Attenuation is Occurring According to Expectations
Although remedial expectations and, consequently, appropriate performance monitoring analyses
are site specific in nature, reduction in contaminant concentrations to specified levels is a general
expectation for most selected remedies. Other common goals, such as the prevention of additional
contaminant migration, are discussed in the following sections. Data analyses useful in evaluating
progress toward contaminant reduction objectives include evaluation of temporal trends in
contaminant concentrations or mass, comparisons of observed contaminant distributions with
predictions or required milestones, and, in some cases, comparison of calculated attenuation
rates with the range of rates required to meet remedial objectives within the required time frame.
Evaluations of adequate progress toward restoration objectives are difficult due, in large measure,
to subsurface variability and to a lesser extent, measurement variability (Appendix A). This
will often necessitate use of multiple lines of evidence (e.g., temporal trends and estimates of
contaminant mass loss, as discussed in the following sections) and relatively dense monitoring
networks to reduce uncertainty to acceptable levels.
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2.6.1.1 Temporal Trends in Individual Wells
Temporal trends in the concentrations of all contaminants of concern measured in ground-water
samples are essential indicators of plume stability and progress toward contaminant reduction
objectives. Temporal trends in an individual well can sometimes be used to estimate the potential
lifetime of the plume at that location (U.S. EPA, 2002b). However, contaminant trends at
monitoring points located throughout the plume will be needed to adequately interpret progress
toward most contaminant reduction goals.
Wells used for analysis of temporal trends generally should be screened in portions of the
plume representative of areas where different processes may be dominant (e.g., different
biotransformation processes or processes that mobilize naturally occurring toxic metals) and
representative of the range of contaminant concentrations observed at the site. Such areas
(Figures 4, 5, and 6) include:
• In and immediately downgradient of suspected or former source areas,
• In the most contaminated zones downgradient of these areas,
• In areas (recalcitrant zones) where contaminants may be attenuating at rates too low to
meet remediation goals due to differences in geochemistry or other factors,
• In fringe areas of the plume,
• In locations surrounding the plume or other compliance boundaries,
• In contaminated zones with the highest ground-water flow or contaminant migration rates
that may serve as pathways for rapid migration.
Statistical techniques, such as those described in Gibbons (1994), Gilbert (1987), Helsel (1995),
and U.S. EPA (2000c), maybe used to objectively determine whether contaminant concentrations
are increasing or declining -with time, and to compare trends between -wells. A trend may be
assessed to determine if the trend would be sufficient to meet remedial goals in the desired time
frame.
Temporal trends observed in -wells may result from processes such as biotransformation that
reduce contaminant mass or concentrations. However, temporal trends may be the result of
contaminant release history and effects of source control actions. Trends may also result from
processes such as changes in ground-water elevations (Figure 8) or ground-water flow directions
(Figure 9) that merely change the shape of the plume or retard its movement, rather than
degrading contaminants. For instance, changes in ground-water flow patterns may cause a change
in plume shape, perhaps causing some -wells to have lower contaminant concentrations, and other
-wells to have higher concentrations. Ground-water flow patterns vary in response to temporal
differences in the rates and locations of recharge (e.g., precipitation or irrigation) and discharge
(e.g., pumping of potable or agricultural wells or operation of drainage systems) at many sites.
Variations in ground-water flow directions and rates may occur on scales of hours to years and
may influence contaminant concentration trends in individual -wells.
Variations in recharge may cause pulses of contaminants to be released from poorly controlled
sources, or may provide pulses of relatively uncontaminated ground -water. For instance, rain
31
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Contaminant Monitoring Well
Concentration Clusters
/in ug/[_
«
Ground-Water Flow
Time 1
Active Irrigation Well
Ground-Water Flow
Time 2
Active Irrigation Well ®
New Monitoring Wells
&
Ground-Water Flow
Time 3
Well Cluster
W3
Well Cluster
W3
(Offset)*
Irrigation
Well
Well Cluster
W3
(Offset)*
Irrigation
Well
• Offset from plane of cross section
o
QJ
O
s
20 -
10 -
W2-B
W1-B
W3-B
W4-B
W5-B
Figure 9. Potential effects of changes in ground-water flow direction on temporal trends in contaminant
concentrations.
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may infiltrate through a vadose zone source, or higher -water tables may intercept vadose zone
contamination, causing increased contaminant concentrations in ground water. As pulses of
contaminants or uncontaminated ground water move downgradient, contaminant concentrations
in monitoring -wells may increase or decrease temporarily, perhaps leading to false inferences
about contaminant attenuation.
Trends in contaminant concentrations may also result from measurement variability. For
example, changes in sample collection and analysis procedures and personnel can introduce
variability into contaminant data. Consideration of these sources of variability may be helpful for
interpretation of patterns in the data, especially -when abrupt changes in established patterns are
noted.
Due to the variety of possible causes, it may be difficult to isolate or identify specific causes of
contaminant concentration trends either in individual -wells or groups of-wells. The range of
viable hypotheses that may explain observed trends generally should be incorporated into the
conceptual site model and consistently used for evaluating site data in order to obtain accurate
interpretations of the data. Contaminant concentration changes in -wells ordinarily should
be evaluated in the context of the conceptual site model and sampling history before they are
attributed to natural attenuation processes or other possible causes.
A site-wide trend cannot be satisfactorily elucidated from monitoring one sampling location, or
one target monitoring zone -within a plume. Significant reductions in contaminant concentrations
from all or most sampling locations may indicate decreases in contaminant mass and imply
progress toward restoration. Mixed trends in different portions of the aquifer (Figure 9) may
imply temporal changes in plume migration and an inadequate conceptual site model rather than
contaminant mass loss.
2.6.1.2 Estimation of Contaminant Mass Reduction
Estimation of the reduction in total contaminant mass is another tool that may be used to evaluate
progress toward achieving contamination reduction objectives. The production of daughter
products from parent contaminants is generally primary evidence of biotransformation processes,
particularly at chlorinated solvent sites. However, certain compounds that can be daughter
products (e.g., trichloroethene) may be or have been present as a substantial portion of the source
materials at many sites. Thus, source characterization information is needed to ensure their
appearance is not incorrectly attributed to natural attenuation processes.
Qualitatively, simple comparisons of the stoichiometric ratios of parent contaminants to daughter
products in different parts of the plume provide important evidence of the effects of degradation
processes with migration distance and should be considered during evaluations of contaminant
mass reduction. However, data for quantitative assessments of contaminant mass reduction -would
consist of the concentrations of all contaminants of concern in samples obtained throughout the
contaminated aquifer. As with all methods used to evaluate subsurface remediation performance,
the most appropriate spacing of monitoring points for evaluation of mass reduction depends on
the variability in contaminant distribution. Uncertainties in these methods normally result from
use of sparse data sets relative to in-situ variability. Application of transect-based approaches
with monitoring at multiple depths may be needed to provide adequate data for evaluation of
mass reduction. Given a sufficient density of monitoring points, evaluation of contaminant
mass reduction may provide a more consistent indication of MNA effectiveness than analysis of
temporal contaminant concentrations trends in individual monitoring -wells.
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Cohen et at. (1994) describe methods for estimating total contaminant mass in the aquifer if
no NAPL or other source material is present. Although the authors describe this methodology
for evaluation of pump-and-treat performance, the techniques would be identical for estimating
total contaminant mass -within the plume during MNA performance evaluations. Depending on
site conditions and contaminant properties, periodic sampling of both ground water and aquifer
materials may be needed to better determine total contaminant (dissolved and sorbed phase) mass
trends.
Data from transects of clustered sampling points, placed perpendicular to the longitudinal axis of
the plume, may also be used to estimate contaminant flux within the plume, and, therefore, mass
loss along the plume axis (Weaver etal., 1996). Such transects provide two-dimensional cross
sections of contaminant distribution. Use of multiple transects can provide a three-dimensional
view of the plume. If used with hydrogeologic data, contaminant mass flux through each cross
section can be calculated. Contaminant mass loss and, consequently, attenuation rates may be
estimated using the differences in the fluxes between transects. Sampling density and frequency
for transects generally should be based on the factors identified in Sections 2.5.2 Monitoring
Locations and 2.5.4 Monitoring Frequency.
As noted in other sections, evaluations of cross-media transfers of contaminants are important
elements of performance monitoring for MNA. Subsurface contaminant mass within one
medium may be reduced due to contaminant migration into other media, so the potential for such
loss should be evaluated in order to provide a comprehensive assessment of contaminant mass
reduction (e.g., evaluate the potential for cross-media transfer into soil gas). Contaminant mass
in all physical phases (e.g., NAPL, aqueous phase, vapor phase, sorbed) should be assessed within
each medium to estimate the actual contaminant mass reduction.
2.6.1.3 Comparisons of Observed Contaminant Distributions with Predictions and Required
Milestones
Contaminant distribution data obtained during performance monitoring may periodically be
compared with previous predictions or specified milestones (e.g., at least 50 % contaminant
reduction at all monitoring points within a specified number of years) to ensure that suitable
progress toward contaminant reduction goals is achieved. Quantitative models may be used to
help set or evaluate the milestones. For example, models may be used to estimate the range of
time frames for achieving remedial objectives (e.g., contaminant concentrations below Maximum
Contaminant Levels (MCLs) in all wells within 20 years) and set intermediate milestones (e.g.,
concentrations in all wells at the current plume periphery below MCLs in five years). Although
these comparisons may be as simple as visual comparisons of contoured maps or comparisons of
predicted and observed total contaminant mass estimates, the use of statistical methods provides a
more objective basis for evaluation. Types of data comparisons for such evaluations are discussed
in Chapter 3.
The goal of these comparisons is to identify any significant deviations from predictions or other
progress requirements that may signify flaws in the conceptualization of site conditions. For
instance, a gradual long-term trend of increasing water levels may cause increased dissolution of
inadequately removed or contained vadose zone contaminants, causing increased ground-water
contaminant concentrations in the source area and, potentially, resulting in plume expansion.
Comparison of the observed contaminant concentrations with predicted concentrations or
milestones would highlight the change and trigger reevaluation of the conceptual site model. If
the potential impact on the remedy is negligible, revision of the conceptual site model may be the
only outcome of such evaluations. However, if the potential impact is significant, implementation
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of alternative remedies may be triggered or, at a minimum, additional characterization to better
define the factors limiting contaminant reduction and determine methods for mitigating the
magnitude of their impact on the remedy would be warranted.
Monitoring parameters for these periodic comparisons may include all contaminants of
concern but, at some sites, may be limited to the most mobile, toxic, or recalcitrant compounds
as indicators of the behavior of other site contaminants. The most appropriate frequency for
performing this type of evaluation depends on the expected rate of change in contaminant
concentrations and the ability of the monitoring network to reliably measure significant changes
(Section 2.5.4.). It is likely that performance of such evaluations at a frequency coincident with
major, comprehensive site reviews (e.g., every few years) may be appropriate for most sites.
2.6.1.4 Comparison of Field-Scale Attenuation Rates
Information obtained during periodic performance monitoring may also be used to estimate
field-scale attenuation rates and compare these estimates with the range of rates determined to
be necessary to meet remediation objectives within specified time frames. Methods that have
been used to estimate attenuation rates and the biodegradation component of the attenuation rate
are described in U.S. EPA (1998a), Wiedemeier et al. (1999), and U.S. EPA (2002b). However,
commonly used calculation methods (e.g., Buscheck and Alcantar, 1995) often suffer from
significant uncertainty regarding the applicability of assumptions required in the calculations as
well as the limited data available from many sites (McNab and Dooher, 1998). Careful evaluation
of site conditions and the assumptions inherent in the method of analysis is required to produce
useful results. Due to the increased uncertainties inherent in the estimation of attenuation rates,
it is anticipated that periodic evaluation of such rates during performance monitoring often may
have few advantages over the direct comparisons of contaminant distributions described above.
2.6.2 #2 - Detect Changes in Environmental Conditions that May Reduce the Efficacy of
Any of the Natural Attenuation Processes
Changes in hydrogeological and geochemical conditions may affect microbiological populations,
the transformation processes that result in contaminant destruction, and rates or directions of
ground-water flow and contaminant migration. Reductions in the effective rates of contaminant
transformation, sustained increases in hydraulic gradients, or, in some cases, exceedence of
sorptive capacity may lead to plume expansion. Sustained changes in ground-water flow
directions may result in risk to receptors that were not originally identified at the time of remedy
selection. The monitoring network should be designed to detect changes in such parameters to
allow determination of their effects on remedy performance.
It is particularly important to monitor and evaluate the effects of other remedial actions that
may be taking place on or near the site. Many remedial actions (e.g., capping of the site, pump
and treat, air sparging, chemical oxidation) -would be expected to change hydrogeologic and/or
geochemical conditions, potentially adversely affecting the MNA remedy. For instance, a site cap
could reduce the flux of oxygen to a shallow aquifer, potentially slowing degradation of petroleum
hydrocarbons, or a pump and treat remedy could alter the flow of ground water at the site, and
potentially change the geochemical environment. In addition, the potential effect of planned
remedial actions should be taken into account -when predicting the efficacy of MNA during the
remedy selection phase, and designing and implementing the performance monitoring system.
Routine monitoring parameters for these determinations are primarily geochemical indicators
of the contaminant transformation reactions that may occur within the aquifer, and ground-
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water elevations in -wells and piezometers that are used to estimate hydraulic gradients in each
hydrogeologic zone.
2.6.2.1 Geochemical Parameters
The values and patterns of geochemical parameters other than ground-water contaminants
provide evidence of the potential for continued biotransformation. The usefulness of geochemistry
for performance monitoring comes from these factors:
• Degradation of some contaminants {e.g., some highly chlorinated ethenes, highly
chlorinated methanes, and highly chlorinated chlorobenzenes) appears to occur only under
fairly specific geochemical conditions, and
• Degradation of contaminants causes specific geochemical changes that may be, at least
qualitatively, correlated with microbial activity and contaminant degradation.
Though rates may differ, some contaminants (e.g., BTEX) can be degraded under a variety of
geochemical conditions, but others (e.g., tetrachloroethene (PCE)) may require a more narrow
range of conditions (Wiedemeier etal., 1999). The controls on biotransformation processes at
chlorinated solvent sites are very complex and not as well understood as at sites contaminated
solely by more readily degradable hydrocarbons such as the aromatic components of fuels (i.e.,
BTEX). Appreciable amounts of a biodegradable carbon compound can produce appropriate
geochemical conditions for reductive dechlorination (Wiedemeier et al., 1999; Leahy and Shreve,
2000). Therefore, depletion of the carbon source may limit the extent of biotransformation
of chlorinated ethenes if reductive dechlorination is a significant biodegradation process.
Biotransformation of some contaminants, such as MTBE and other ethers that may be present
in fuel contamination, is not well understood and is the subject of continuing research (National
Research Council, 2000).
Most of the geochemical signatures seen at sites of chlorinated solvent contamination are
reflections of biological processes utilizing various carbon sources and often are not directly
caused by biotransformation of the chlorinated solvent contaminants. However, these data have
value as continuing indicators of appropriate geochemical environments for biotransformations
of contaminants in many situations. The analyses provide evidence regarding the continued
effectiveness of MNA and the adequacy of the conceptual site model. If biotransformation is
a major natural attenuation mechanism, then the geochemistry generally should be monitored
to determine that biotransformation continues. If a strong correlation exists between changes
in geochemical indicators, decreasing contaminant levels, and the products of known biological
transformation processes in a particular portion of the plume, this may be considered supporting
evidence for continued microbial destruction of contaminants. However, in areas of decreasing
contaminant concentration that do not bear a strong geochemical signature, other processes
typically should be considered. The relationship between geochemical patterns and degradation of
organic carbon other than the contaminants of concern should also be considered.
Geochemical parameters generally should be measured throughout the plume in order to establish
a correlation between reductions in contaminant concentrations and microbial activity (U.S.
EPA, 1998a; Wiedemeier et al., 1999). In addition, geochemical samples generally should be
obtained upgradient and sidegradient of the plume (Figure 4) to determine the range and variation
in ambient conditions, in order to differentiate changes in geochemistry related to microbial
metabolism of contaminants from unrelated processes. The specific parameters useful at a given
site include:
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• Parameters displaying correlations with contaminant distribution that are
indicative of dominant biotransformation processes,
• Parameters showing conditions that are necessary for contaminant degradation,
and
• Parameters whose appearance may indicate changes detrimental to continued
biotransformation.
Discussion of these parameters and the interpretation of geochemical data is found in U.S. EPA
(1998a) and Wiedemeier et al. (1999).
At sites where the total available supply of electron acceptors utilized in contaminant
biotransformation is predominantly composed of solid phase material (e.g., bioavailable iron
species), periodic monitoring of bioavailable electron acceptors in soil samples may provide
information concerning their rates of depletion (Huling et at, 2002) and the potential for renewed
plume migration. This type of monitoring may be useful where a large mass of biodegradable
contaminant relative to the total electron acceptor supply is present. Where warranted, it is
anticipated that this type of assessment would be performed on a relatively infrequent basis
determined by the possible time frames over which significant changes are expected to occur.
2.6.2.2 Hydroqeoloqic Parameters
Long-term changes in ground-water flow directions due to sustained changes in recharge or
discharge locations or rates may result in risks to receptors that were not originally identified
at the time of remedy selection. In addition, increases in ground-water flow rates may result in
expansion of the plume. Such changes may result from many factors including land use changes
(e.g., irrigation of adjacent areas), installation of new pumping wells or increase in pumping rates
of existing wells, and installation of drainage systems, as well as climatic patterns.
Monitoring of ground-water and surface-water elevations should be performed at a frequency
sufficient to determine significant variations in hydraulic gradients. The frequency would usually
be no less than the monitoring frequency for the chemical parameters. Other hydrologic data
such as rates of local ground-water pumping, irrigation, and precipitation may also be needed at
some sites. As previously noted, the evaluations generally should be three-dimensional in nature,
monitoring horizontal and vertical components of hydraulic gradients. The monitoring network
should be dense enough to accurately estimate spatial variations in gradients. The number of
monitoring points needed at any given site depends in large measure on the sources of aquifer
recharge and discharge in the vicinity of the site and the degree of geologic heterogeneity.
2.6.3 #3 - Identify Any Potentially Toxic and/or Mobile Transformation Products
Contaminant transformation processes may result in the formation of new chemicals that may
be more toxic or more mobile than their "parent" compounds. Transformation products may
appear sequentially along the direction of ground-water flow and all possible products may not
be present throughout the plume. Potentially toxic daughter products should be evaluated based
on information regarding contaminant transformation pathways and monitoring data. All such
products typically should be included as contaminants of concern in the monitoring program
and monitored at the same frequency as other contaminants of concern. Mobile but nontoxic
transformation products may be monitored as an aid to determining flowpaths and demonstrate
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continued biotransformation processes, as discussed in Sections 2.6.1, Demonstrate that Natural
Attenuation is Occurring According to Expectations, and 2.6.2, Detect Changes in Environmental
Conditions that May Reduce the Efficacy of Any of the Natural Attenuation Processes.
Minerals in the aquifer matrix have specific chemical compositions and crystalline structure
and are in dynamic equilibrium -with surrounding pore water. Large changes in oxidation-
reduction potential caused by biological processes within the contaminant plume may result
in transformation of some naturally occurring mineral species into more soluble and mobile
forms that pose risks to human health or the environment (Brady et at, 1999, U.S. EPA, 1998a,
Wiedemeier et at, 1999). Mobilization of arsenic and manganese species has been observed at
some sites. Where such mobilization is possible, these species generally should be monitored as
potential contaminants, and also for interpretation of their possible use as electron acceptors.
Metals that are possible contaminants of concern due to human health or surface-water quality
criteria should be included in the ground-water monitoring program at the same frequency
as other contaminants. Removal of these constituents from the monitoring program may be
warranted if data demonstrate lack of appropriate mineral species or that such mobilization does
not occur under site conditions.
2.6.4 #4 - Verify that the Plume is Not Expanding Downgradient, Laterally, or Vertically
Plume expansion may result in a plume extending to areas that were previously uncontaminated,
migrating beyond established compliance boundaries, degrading additional ground-water
resources, and potentially increasing risk to receptors. Previous site characterization and
performance monitoring data obtained using a three-dimensional monitoring network will
likely be needed to assess hydrogeological changes, temporal variability in the lateral and vertical
extent of the plume, and to determine the effective boundary for demonstrating compliance with
the nonexpansion objective. Assessment of plume expansion is complicated because ground-
water flow systems, like all natural systems, are temporally dynamic. For example, precipitation
may vary seasonally and/or over longer time frames. This may result in changes in -water table
elevations and possible changes in hydraulic gradients and contaminant inputs to a plume.
Anthropogenic effects {e.g., variations in ground-water extraction rates or loss of water from
distribution systems) also can be a source of changes in subsurface conditions. Such changes
may result in three-dimensional changes in plume geometry, causing the plume boundary to be
in continual flux. This constant variation in plume geometry means that, in some cases, it may
require several years of monitoring to definitively determine if a plume is not expanding. In such
cases, particularly where receptors are immediately susceptible to impact, implementation of
alternative remedies may be more appropriate than waiting to verify a lack of plume expansion.
Plume expansion may be caused by many geochemical and biologic processes as well as variability
in site hydrology. Examples of these processes include:
• Increases in contaminant inputs to the plume that exceed the capacity of transformation
and hydrologic processes to attenuate the concentrations within the current plume
boundaries, and
• Changes in oxidation-reduction potential due to factors such as the depletion of electron
donors or acceptors that reduce the effective rates of transformation processes. For
example, depletion of a degradable carbon source prior to complete biotransformation of
tetrachloroethene to lesser chlorinated products may result in additional tetrachloroethene
migration.
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A three-dimensional performance monitoring network (Figures 4, 5 and 6) will often be necessary
to meet this objective. Monitoring of points throughout the plume, including locations in or
near existing or suspected source areas and in the zones of highest contaminant concentrations,
generally will be needed to evaluate changes that may lead to plume expansion. Monitoring
points of the most immediate concern will often be those points located near the horizontal and
vertical plume boundaries and any other compliance boundaries specified in the remedy decision.
Trends of increasing contaminant concentrations in these wells will often be direct evidence of
plume expansion. In addition, evaluations of temporal trends in individual monitoring points and
comparisons with trends observed in different areas of the site may aid in confirming the roles of
the dominant processes (e.g., advection, dispersion, and transformation) controlling contaminant
migration. The appropriate number and locations of monitoring points will be dependent on such
factors as the size of the plume, ground-water velocity, proximity to receptors, and presence of
preferential pathways for contaminant migration.
In practice, wells located at the plume and other compliance boundaries, and between the
boundaries and receptors, may often be used to detect contaminant increases and trigger the
implementation of contingency or alternative remedies to prevent impact to receptors. Such -wells
-would be located sufficiently upgradient of receptors to allow adequate time for implementation
of the contingency remedy and demonstration of its effectiveness given the ground-water velocity
within the most transmissive contaminated zones. Monitoring at the receptor point generally
should also be included in all settings. For example, monitoring of public and private water supply
-wells that -would be at risk if the plume expanded should be incorporated into the performance
monitoring program, as -warranted. Wells chosen for sampling should be based on evaluations of
the capture zone for each -well. In similar fashion, monitoring at locations of possible impact to
ecological or other receptors should also be included.
At some sites, the geochemical fingerprint of ground -water can be established and used to trace
-water downgradient to distinguish ground -water that has never been contaminated from ground
-water that -was previously contaminated (National Research Council 2000, Weidemeier and Haas
2002). Such information may be used to site -wells near the current plume boundary in zones
-where contaminant migration -would be expected if plume expansion occurred. Depletion of
electron acceptors, and presence of metabolic by-products and nonhazardous daughter products
may be used as indicators of appropriate monitoring locations. The most useful parameters
for sites with hydrocarbon contamination may include nitrate, sulfate, iron, methane, and
dissolved oxygen. The most useful tracers in plumes of chlorinated solvent compounds are
often their reduced transformation products, particularly ethane or ethene, but also include the
same parameters as for petroleum hydrocarbon plumes. The most appropriate parameters for
determining locations for monitoring -wells downgradient of a contaminant plume depend on site-
specific correlations of contaminants and geochemical indicators.
2.6.5 #5 - Verify No Unacceptable Impacts to Downgradient Receptors
Impacts to receptors may result from plume migration to -wells used for drinking and other
domestic purposes, irrigation or industrial purposes; contaminant migration to indoor air; or
discharge of ground -water to -wetlands or surface-water bodies. Prevention of unacceptable
impacts to receptors includes continuing verification of plume stability and the reliability of
institutional controls. Adequate demonstration of the lack of unacceptable impact to receptors
will often require monitoring of-wells located bet-ween the plume boundaries and the receptors,
in the transmissive zones -where contaminants may migrate if plume expansion occurs. In the
case of potential impacts to receptors associated with water production wells, it would generally
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include periodic monitoring of the production -wells. The potential for impacts to both human
and ecological receptors should be evaluated, and the performance monitoring system designed to
verify that no unacceptable impacts occur.
Performance monitoring -with respect to this objective also includes monitoring of the cross-media
transfer of contaminants. Cross-media transfers of concern include transfers between ground
water and soil gas, air, sediments, and surface water. For example, in some situations, particularly
-where volatile contaminants are found in the vicinity of existing buildings or other structures,
the potential may exist for contaminant migration through the vadose zone to indoor air. In such
situations, periodic monitoring of soil gas and indoor air may be needed to determine that no
impacts occur. Guidance regarding techniques for the monitoring of soil gas may be found in U.S.
EPA (1988, 1993b). Fate and migration of volatile contaminants from the subsurface to indoor air
is a rapidly evolving area of research and guidance/policy development. The reader is referred to
U.S. EPA (2002c) for further information.
Another major cross-media transfer process of concern is movement of contaminants bet-ween
ground-water and surface -water/sediments. The ground-water/surface-water transition zone,
-where ground -water and surface -water mix in the saturated sediment beneath and beside surface
-water, may exert a major influence on input of contaminants to surface -water because of the
enhanced chemical and biological activity often found in these zones. The hydrologic and
geochemical conditions in areas -where ground -water interacts with surface -water often differ
markedly from those in the main body of the plume and may require more intensive monitoring to
determine the effect on remedial goals (Winter, 2000).
The locations and characteristics of contaminated ground-water discharges generally should be
determined. Areas of discharge and interaction may vary rapidly both temporally and spatially.
Plume discharge to the surface-water body or -wetlands may not be at the immediate shoreline or
channel edge, and the discharge may vary spatially through time. In addition, contaminant plumes
may also migrate beneath streams and drainage features in some settings. Monitoring generally
should occur in the interface bet-ween ground -water and surface -water as -well as in the surface-
water column (U.S. EPA, 1991b).
Tools to characterize the hydraulic relationships bet-ween ground -water and surface -water (Figure
10) include piezometers, pore -water sampling devices, devices for the in situ measurement of
ground-water velocity, and certain geophysical techniques. Such tools may be used to define
ground-water flow from the plume into the surface-water bodies and to aid in siting monitoring
points for determining the impact of the discharging -water on the sediments and surface-
water quality. It should be noted that mobilized metals may be a primary concern during these
investigations due to the relatively low thresholds for unacceptable ecological impact. Multilevel
monitoring is generally needed to identify contaminant discharge locations. Once the hydraulic
relationships are characterized, -water and sediment sampling locations may be specified.
2.6.6 #6 - Detect New Releases of Contaminants
Increases in contaminant concentrations or detection of new contaminants at monitoring points
located within and immediately downgradient of source areas may be indications of new releases.
Releases may result from such causes as the failure of source control measures, increased -water
infiltration or -water table rise into contaminated portions of the vadose zone, or new releases to
the environment. In many cases, it may be difficult to determine -whether increased contaminant
concentrations are due to new sources or other causes. Increases in contaminant mass may be
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Sediment
Sampling
Locations
Lakebed
Piezometer
Monitoring
Wells
Pore Water
Sampler
Contaminant
Plume
Ground-Water Flow
Figure 10. Conceptual monitoring network for verifying lack of impact to surface water from ground-water
discharge.
sufficient to result in expansion of the plume and/or extension of time frames required to attain
contaminant reduction objectives, warranting implementation of additional source control/
removal measures, other remedy modifications, or alternative remedies.
2.6.7 #7 - Demonstrate the Efficacy of Institutional Controls
Institutional controls may be put in place to prevent access to or use of contaminated ground
water. These controls are an integral and necessary part of an MNA remedy because they are
relied upon to ensure that the remedy is protective to human health and the environment. For
this reason they should be monitored with the same degree of thoroughness as other components
of the remedy. Institutional controls (U.S. EPA, 2000d) include:
• Governmental controls (e.g., zoning restrictions, ordinances, statutes, building
permits),
• Proprietary controls (e.g., easements, covenants),
• Enforcement and permit tools (e.g., administrative orders or consent decrees with
institutional control), and
• Informational devices (e.g., state registries, deed notices, hazard advisories).
At a minimum, the following questions should be answered during development of the monitoring
plan to allow an initial assessment of the potential efficacy of institutional controls:
1) Are institutional controls in place?
2) What are the criteria for determining that institutional controls are effective and operating
as intended?
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3) Who is responsible for monitoring and reporting on the integrity and effectiveness of
institutional controls for the site?
4) Are institutional controls being monitored regularly and at an appropriate frequency?
5) Are institutional control monitoring results included in either a performance monitoring
report or a separate report? Are these reports completed at an appropriate frequency and
forwarded to the appropriate regulator?
6) What procedures are in place to report breaches or failures of the institutional controls
to the appropriate U.S. EPA and/or state regulator, local or tribal government, and the
designated party or entity responsible for reporting?
7) Is the property likely to be transferred in the near future, and if so, who will be responsi-
ble for the future monitoring and reporting and enforcement of the institutional controls?
8) Are there provisions in place to notify regulators of any impending property transfer such
as in a consent decree or order?
The MNA performance monitoring plan should include a description of activities to initiate and
periodically monitor institutional controls, or refer to a separate plan for monitoring institutional
controls. The assumptions and uncertainties associated with the site-specific institutional controls
should be carefully considered in designing the monitoring plan and determining effectiveness.
Compliance monitoring may include field inspection of affected areas, particularly if affected
properties have been sold or land use patterns have changed, as well as determinations that the
specified controls have been enacted. Each control should be periodically investigated to determine
whether the control continues to be implemented as specified.
2.6.8 #8 - Verify Attainment of Remediation Objectives
One of the fundamental cleanup objectives for most sites is the reduction of contaminant
concentrations in subsurface media (i.e., soil and/or ground water) to specified levels. The
remedial action objective of attaining permitted standards throughout the plume should be
demonstrated before monitoring is terminated to ensure that the required standards are actually
achieved. The techniques for determining compliance with RAOs under a natural attenuation
remedy are similar to those used for determining compliance following application of other
remediation methods. The demonstration of the attainment of cleanup objectives should include
sufficient verification monitoring (e.g., three to five years) once the standards are met to evaluate
the effects of natural variations in site conditions, based on objective statistical analyses of the
data. Statistical methods useful in these evaluations include analyses of temporal trends in
contaminant concentrations and comparisons with the specified concentration standard. Guidance
regarding verification of compliance with cleanup objectives is provided in Cohen et al. (1994) and
U.S. EPA (1992a).
2.7 Monitoring Plan Contents
2.7.1 Introduction
The following material has been prepared to provide suggestions regarding appropriate formats
and components of performance monitoring plans. Although there is no standard format that
is universally accepted, most monitoring plans make extensive use of maps, cross sections, and
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Table 4. Elements of a Performance Monitoring Plan
Background and Site Description
The following information typically would be discussed or incorporated by reference:
Q Site setting, history,and characteristics
Q Remedial goals
Q Past and present remedial actions and institutional controls
Conceptual Site Model for Natural Attenuation
The following information typically would be discussed or incorporated by reference:
Q Descriptions and locations of potential receptors (text and maps)
Q Geologic and hydrologic controls on ground-water flow (text, maps, and cross sections)
Q Contaminant sources,distribution, migration,and fate (text, maps,and cross sections)
Q Relationship of geochemistry to attenuation processes
Objectives and Decision Points
Q Remedial Action Objectives (RAOs)
Q Monitoring Objectives
Q Performance Criteria
Q Decisions to be made based on monitoring data (e.g., monitoring network changes, implementation
of contingency/alternative remedy, implementation of verification monitoring, terminate performance
monitoring) and the criteria for making each decision
Monitoring Network and Schedule
Q Detailed discussion of relationship of conceptual model, supporting data, and analyses to design of the
monitoring network
Q List and map of monitoring locations
Q Description and construction details for each monitoring point
Q Monitoring schedule specifying monitoring parameters,analytical methods, sampling frequency
Q Maps, cross sections, and other visual aids to show where, when, and by what methodologies samples are to be
taken and analyzed
Q Statistical methods and test designs to be used for data interpretation
Monitoring of Institutional Controls (ICs)
Q Descriptions of ICs and the procedures for their implementation
Q Procedures for verifying establishment of ICs
Q Frequency for monitoring ICs
Q Procedures for routine monitoring of the effectiveness of ICs and parties responsible for the monitoring
Evaluations of Remedy Effectiveness
Q Description of evaluations to be performed to demonstrate effectiveness of natural attenuation with respect to
site-specific remedial action objectives
Q Determination of temporal and spatial trends in contaminant concentrations or mass
Q Comparisons of contaminant concentrations with previous predictions or milestones
Q Comparisons of contaminant concentrations in areas outside the previous plume boundaries or other
compliance boundaries with specified action levels
Q Frequency (e.g., quarterly,annually) on which each evaluation is to be conducted and the data that are to be
used
Q Data presentations to be used (e.g., tables, maps,cross sections, and other figures)
Methodology or Plan for Verifying Attainment of Remedial Objectives and Initiating Termination of Performance
Monitoring
Sampling and Analysis Plan
Quality Assurance Project Plan
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figures to convey monitoring requirements and approaches. Ideally, monitoring data would be
acquired, transmitted, evaluated and presented in an electronic form for ease of transmission
and analysis. The format and content of a monitoring plan -will vary according to site-specific
needs, but most plans would contain or reference the elements listed in Table 4 and briefly
discussed in the following sections.
2.7.2 Background and Site Description
Background information on the site, including a discussion of the site setting, history,
characteristics, remedial goals, past and present remedial actions, and any institutional
controls, typically would be provided in abbreviated fashion or incorporated by reference to
existing documents. Essential aspects (e.g., hydrogeologic setting, contaminant distribution,
remedial goals and actions) generally should be addressed at sufficient length that the technical
reviewer can judge the adequacy of the monitoring efforts. The geologic and hydrogeologic
controls on ground-water flow at the regional scale and site scale should be discussed in detail
with references to the supporting data or provided by reference to specific portions of existing
documents.
2.7.3 Conceptual Site Model for Natural Attenuation
Essential elements of the conceptual site model for natural attenuation include:
• Descriptions and locations of possible receptors,
• Geologic and hydrologic controls on ground-water flow,
• Contaminant distribution and behavior, and
• Relationship of geochemistry and anthropogenic factors to attenuation processes
The monitoring plan typically would provide a detailed discussion of how the conceptual site
model and the supporting data and analyses were used to design the network.
2.7.4 Objectives and Decision Points
The monitoring plan typically should specify the RAOs, monitoring objectives, and
performance criteria used to guide the performance monitoring plan development and provide
decision points for continuing, modifying, or terminating performance monitoring. The
decisions to be made should be thoroughly discussed in relation to site objectives, specific
decision points, methods of determining attainment of goals, and how site activities would
proceed based on decisions made. The criteria upon which each decision is to be based should
be explicitly stated.
2.7.5 Monitoring Network and Schedule
The plan should include information on the monitoring network including a list and map
of monitoring locations; a description of the construction of each monitoring point; and
a monitoring schedule specifying monitoring parameters, analytical methods, sampling
frequency, and statistics to be used for data interpretation. Information regarding Data
Quality Objectives (DQO's) should be provided or incorporated by reference. It is particularly
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important that maps, cross-sections, and other visual aids be extensively used so that the reviewer
can determine exactly where, when, and by what methodologies samples are to be taken and
analyzed. In situations where different hydrogeologic units are monitored, separate maps
with -wells/sampling locations for each major lithologic unit may provide additional clarity in
visualization of the network design.
2.7.6 Monitoring of Institutional Controls
The monitoring plan should include detailed descriptions of the institutional controls and their
implementation procedures. Procedures for verifying that institutional controls are put in
place, for monitoring the effectiveness of the controls, and for reporting monitoring results to
appropriate government entities should be described. The frequency for monitoring and reporting
effectiveness should be specified. The parties responsible for implementing, verifying, and
monitoring effectiveness of the institutional controls should be listed, along -with complete contact
information.
2.7.7 Evaluations of Remedy Effectiveness
The monitoring plan should specify the evaluations that will be performed to demonstrate the
effectiveness of natural attenuation -with respect to the site-specific remedial action objectives.
The frequency (e.g., quarterly, annually) on -which each evaluation is to be conducted and the data
that are to be used would also be specified. The evaluations should be designed to objectively
determine performance in relation to specific performance criteria established in remedy decision
documents. Pertinent evaluations include:
• Determination of temporal and spatial trends in contaminant concentrations or mass,
• Comparisons of observed contaminant concentrations with previous predictions or
established milestones, and
• Comparisons of contaminant concentrations in areas outside of previous plume boundaries
with specified action levels (e.g., drinking-water standards)
In addition, the plan should specify the data presentation methods (e.g., tables, maps, cross
sections, and other figures). Submission of data in electronic formats should be considered for
ease of manipulation and independent analysis.
2.7.8 Plan for Verifying Attainment ofRAOs
The performance monitoring plan should include discussion of the methodology or plan for
verifying attainment of all remedial action objectives and initiating termination of performance
monitoring. In general, the plan developed at the initiation of performance monitoring should be
flexible and allow for future modifications based on observations of actual remedy performance.
The methodology should include:
• Proposed time period for verifying attainment of objectives, considering any observed
seasonal or other temporal trends,
• Description of monitoring locations, parameters, and frequency, including discussion of
any possible changes that may be warranted, and
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• Specification of statistical methods of data analysis to be used in demonstrating
attainment.
It is recommended that the plan for the verification of RAOs be reevaluated prior to its
implementation to insure that the verification methodology initially proposed is still valid
considering the additional data obtained and any changes in the conceptual site model that
occurred during the performance monitoring period.
2.7.9 Sampling and Analysis Plan
The sampling and analysis plan typically should include a description of:
• Sample collection methods,
• Sample preservation and handling,
• Chain-of-custody procedures,
• Analytical procedures, and
• Field and laboratory quality assurance/quality control.
Guidance regarding the development and specific elements of sampling and analysis plans is found
in a variety of sources including U.S. EPA (1986a, 1986b, 1992b, 1993a, 2002a).
2.7.10 Quality Assurance Project Plan
The quality assurance project plan (QAPP) documents the manner in -which quality assurance and
quality control activities will be implemented throughout the performance monitoring program.
The QAPP is composed of the following elements:
• Description of project tasks, data quality objectives, and management,
• Description of data acquisition and management,
• Description of assessments, responses, and oversight, and
• Description of data validation, verification, and usability.
Detailed guidance regarding preparation of quality assurance project plans is available in U.S. EPA
(1998c).
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Chapter 3
ANALYSIS OF PERFORMANCE MONITORING DATA
3.1 Introduction
Data interpretation involves these basic steps of the data quality assessment (DQA) process :
• Placing the data in context of time, location, sampling and analytical methods,
• Preliminary assessment of the data with basic statistical measures (e.g., means and ranges),
and graphs, charts, maps, time-series plots, and cross-sections,
• Applying appropriate statistical tests to detect changes, trends, and assess attainment of
goals, and
• Making decisions based on the data.
The conceptual site model for MNA and the monitoring program are continually refined as
new data are gathered and interpreted. Each new round of data should help develop a better
understanding of the site and site processes, including plume shape, location, stability, and
dynamics; attenuation rates; geochemical regimes; and site hydrogeology.
The criteria used for site-related decision-making are usually based on either presence of specific
contaminant concentrations (e.g., MCLs at a specified group of wells), or spatial and temporal
trends in concentrations (e.g., a decreasing trend in contaminant concentrations that indicates
progress toward contaminant reduction objectives). Other site-related data (e.g., geochemical
and ground-water flow data) are important for corroborating contaminant migration, fate, and
attenuation processes. These data may be used to provide evidence for contaminant degradation
and the continuation of appropriate conditions for attenuation at acceptable rates.
3.2 The DQA Process
The Data Quality Assessment (DQA) process is an iterative procedure used to evaluate, analyze,
and interpret data. Guidance for the DQA process is found in U.S. EPA (2000c). The assessment
methods used at each site should be specified in the performance monitoring plan.
Briefly, the DQA process for performance monitoring of an MNA site is as follows:
• Review the Data Quality Objectives (DQOs) and the sampling design to ensure that
the design is suited to achieving the DQOs. The data derived from the monitoring
should have the proper characteristics for their intended use; that is, data should
be of the correct type, quality, and quantity in order to be useful for making site-
related decisions.
• Conduct a preliminary review of the data, such as calculating means and other
basic statistics, graphing the data, and mapping the data to identify obvious
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patterns, relationships, and anomalies, such as possible "outliers". Simple scatter
plots, for example, may show relationships between two variables that are not
evident using purely numerical methods. Mapping the data in plan view and
cross-section is helpful for understanding patterns of contaminant distribution and
migration, and for understanding relationships between geochemical zones and
contaminant attenuation.
• Verify the selection of the appropriate statistical test(s) (if needed) for analyzing
the data, considering the types of decisions that are to be made (e.g., attainment
of contaminant reduction objectives) and the sampling design. Identify the
assumptions that underlie the chosen statistical test (e.g., normally distributed
data). The appropriate statistical tests are identified through the application of
the DQO process during monitoring system design, because the design is chosen
considering the requirements of the particular tests to be used.
• Verify that the data meet the assumptions of the statistical test, and whether any
departures from the assumptions are acceptable. Given the nature of many sites,
it is possible that data will not always meet the assumptions of either parametric
or nonparametric statistical tests. Ground-water monitoring data typically exhibit
extremely high variability and skewed distributions. Although certain assumptions
of particular tests may be in question, statistical tests are often still a worthwhile
guidance tool for decision-making. Statistical analyses should be considered as a
way to bring a measure of objectivity to the decision-making framework.
As the design of the sampling and analysis plan can minimize or eliminate
violations of statistical assumptions, it is recommended that statisticians be
included in the performance monitoring planning phase. The data analyst,
however, bears the ultimate responsibility of determining if the assumptions of
a particular statistical test are violated and how these violations may affect site
decisions.
• Interpret and draw conclusions from the data analysis by performing the
appropriate statistical tests. The sampling design should be evaluated to determine
if any changes are needed to enhance the usability of the data.
• Interpret and draw conclusions from the data in the context of the conceptual site
model for MNA and site remediation goals.
In addition to the steps presented here for assuring the quality of the data, it is recommended
that a process be implemented to assure the quality of the conceptual site model for MNA. With
each new round of sampling, the conceptual site model for MNA should be reevaluated and, if
necessary, modified. In particular, the conceptual site model for MNA should be reevaluated from
the standpoint of its power to explain observed plume shape, location, stability, and dynamics;
ground-water now patterns; and geochemical regimes.
At many sites, source control and interim remedial activities will occur between the original site
characterization process and implementation of performance monitoring. Because these activities
are expected to change the site's characteristics and behavior, it is possible that performance
monitoring -will require development of a conceptual site model for MNA that is substantially
different from the MNA conceptual site model that was based on the initial site characterization
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data. Development and implementation of the new conceptual site model for MNA may require
collection of additional data to better depict existing site characteristics and parameter variability.
These data are used to determine trends, determine plume characteristics, and assess the
likelihood of meeting site remediation goals.
3.3 Interpreting the Data
3.3.1 Introduction
Data analysis and interpretation focus on two fundamental aspects: detection of changes or trends
in the data, and assessment of the changes or trends in the context of their impact on the potential
for MNA to achieve site-related goals. Data can be interpreted with regard to variability so that
attenuation-related changes and trends can be distinguished from other sources of variation in
the data. In addition, once changes or trends are identified, the importance of the trend should be
considered (i.e., Does the trend indicate that MNA -will or -will not meet site-specific goals?).
Particular changes that are of interest include:
• Changes in ground-water flow rates or directions that indicate contaminants may move
farther downgradient, laterally or vertically into previously unimpacted areas,
• Changes in contaminant concentrations -within the plume, that may indicate new releases,
significant changes in the rate of release from any source materials that remain following
implementation of all appropriate source control measures, or changes in the attenuation
rate,
• Detections of contaminants outside the known plume or other compliance boundaries,
indicating unacceptable plume expansion or additional source areas, and
• Changes in geochemistry that may indicate changes in attenuation rates, such as changes
in availability of electron donors and electron acceptors, changes in oxidation-reduction
potential, or other primary geochemical indicators; or changes in sorption characteristics
(e.g., adsorption, precipitation).
Data interpretation is complicated by data variability (Appendix A). This variability is inherent
in the measurement process, in which case it may be evaluated in a carefully designed and
implemented sampling and analysis-related QA/QC program, and it is also associated with natural
causes. Due to the dynamic nature of natural processes, data typically exhibit fluctuations of
various magnitudes. For example, there is an expected degree of natural variability -within a plume
that may include small-scale expansion and shrinkage in response to changes in ground-water
flow rates and biological degradation rates throughout the year. Contaminant concentrations
in individual -wells may fluctuate -with changes in plume configuration caused by oscillations in
ground-water flow. Also, changes in ground-water elevations can cause changes in contaminant
concentrations measured in monitoring -wells if there are vertical differences in -water quality or
other factors.
Examination of historical and spatial patterns may provide information useful in assessing the
source and significance of variability in the measured parameters. For instance, if there are
multiple -wells in the same hydrogeologic setting that show a consistent trend for a number of
monitoring variables across two monitoring events, then one may suspect a natural variation,
such as recharge differences, and investigate a correlation of recharge or other external hydrologic
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factors with concentration as indicative of the likely cause of the change. If, on the other hand,
a specific well shows an unusually large change in concentration between monitoring events,
relative to nearby monitoring wells, then one may suspect a measurement factor is involved in that
localized change and look at the possible influences of sampling technique or other such factors on
the concentration change.
Because plume variability should be taken into account when interpreting monitoring data, it is
important to have a good estimate of natural variability derived from the site investigation. In
addition, it is recommended that performance monitoring data be continually evaluated with
respect to variability, so that there is an ever-expanding database for the site that can be used
for comparison with the changes in parameters noted between monitoring events as well as for
comparison to predicted, acceptable trends.
Statistical procedures are available that allow assessment of site characteristics and trends. If a
change in site characteristics or a departure from a predicted trend is found, it can be evaluated
to determine what impact the change may have on MNA's ability to achieve site-related goals.
Conceptual models for MNA can be used to conduct sensitivity analyses to help determine
what changes may be significant in the context of the site-related goals. For instance, sensitivity
analyses could be used to determine if a two-fold or three-fold change in attenuation rate will
significantly affect attainment of remedial goals. Departures from expected trends may not mean
remedy failure, because there may be a range of attenuation rates that will achieve the desired
remedial goals in the allotted time.
3.3.2 Preliminary Presentation and Evaluation of the Data
The data analyst conducts an initial evaluation and review of the data by calculating means,
modes, medians, and ranges, graphing the data, and mapping the data in plan and cross sectional
views to identify obvious patterns, relationships, and anomalies. Simple graphical representations
include site maps and cross sections with posted and contoured data, plots of data as a function of
time, plots of the relationship between two or more variables, and measures of central tendency
(e.g., means, modes, medians) and data dispersion (e.g., standard deviations). It is recommended
that the data analyst consider as many graphical techniques as possible to maximize the amount of
information gained in this step.
As part of this initial evaluation process, it is essential that each data point be interpreted in the
context of its derivation in time and space. For instance, monitoring wells produce data that
should be interpreted in terms of the three-dimensional location of the well screen with respect to
contaminant sources, site stratigraphy, the ground-water flow field, the plume, possible receptors,
season of the year, and, potentially, numerous other factors. The sampling methodology should
also be considered, due to possible differences in contaminant concentrations derived from
different sampling techniques or devices. The analytical methodology may be evaluated in terms
of accuracy, precision, and detection limits. This list of data considerations is not exhaustive, but
merely emphasizes that a number (i.e., a data point) should be placed in context to be interpreted.
The better a data analyst understands the derivation and context of the data, the more likely it is
that the data analyst can make the appropriate decision based on the data.
3.3.3 Data Comparisons
The following types of data comparisons are commonly useful during performance monitoring
evaluations.
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3.3.3.1 Comparisons of Concentrations Within and Outside the Plume
Data obtained -within the plume are compared to data from monitoring points upgradient of
the source area and sidegradient to the plume. Data acquired upgradient of the source area may
be used to monitor for contaminants coming from other sources and for electron donors and
acceptors migrating into the contaminated area (U.S. EPA, 1998a; Wiedemeier et al., 1999).
It often is important to obtain geochemical data from locations sidegradient to the plume
because there may be geochemical changes in the ground water as it flows through the aquifer
that are unrelated to processes in the plume. The range of values and spatial patterns for the
particular parameter should be considered during data interpretation. If the range of values
in the geochemical setting surrounding the plume is similar to (or greater than) the range of
values for the parameter in the plume, then it may be problematic to attach any particular
interpretation to the changes of the parameter in the plume. For instance, suppose sulfate values
in uncontaminated ground water adjacent to a plume vary from less than 1 mg/L to 30 mg/L; then
a drop in sulfate concentration from 20 mg/L to 5 mg/L -within the plume may not necessarily be
conclusive evidence that sulfate reduction is important in plume geochemistry.
3.3.3.2 Trend Analyses
Data are compared to determine if temporal and spatial trends exist within the plume and in
surrounding areas. Trends of interest include trends in contaminant and daughter product
concentrations, electron acceptors and donors, oxidation-reduction potential, and other general
geochemical indicators. These comparisons include trend-to-trend comparisons (e.g., a decreasing
trend in tetrachloroethene compared to an increasing trend in trichloroethene in the same
transmissive zones may indicate degradation of tetrachloroethene to trichloroethene).
Trends at individual sampling points or groups of sampling points may be compared to other
sampling points, or to trends in other groups of sampling points. For instance, contaminant
concentrations at individual sampling points may show different trends. However, evaluating
trends in data from all sampling locations in the plume will determine if the plume exhibits
stability or reduction in contaminant concentrations. Similarly, data from a group of sampling
points at the downgradient limits of a plume may be compared to data from previous sampling
rounds to determine if the plume seems to be stable, shrinking, or expanding.
In some cases, particularly -with petroleum hydrocarbons, known stoichiometric relationships
bet-ween usage of electron acceptors and degradation of contaminants may be used to relate trends
in geochemistry qualitatively or semi-quantitatively to degradation of contaminants.
3.3.3.3 Comparisons with Existing Literature and Laboratory Studies
Parameter values from other sites and the literature may be used to determine how ranges
of values at a particular site compare to the ranges that have been found through research or
experience at other sites or laboratory studies. Typically, these comparisons are made during site
characterization, but they may also be useful in the performance monitoring planning phase. For
instance, a calculated degradation rate for a contaminant may be compared to values found at
other times and places at the same site and to values found at other sites or in laboratory studies.
If literature reports indicate that a compound does not degrade under conditions similar to those
found at the site under investigation, but data from the site seem to indicate the compound is
disappearing at a substantial rate, it would be advisable to carefully reevaluate the monitoring
operations before concluding that the compound is indeed degrading. Note that literature values
cannot substitute for values determined at the site for evaluating and monitoring MNA.
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3.3.3.4 Comparisons with Threshold Values
Contaminant concentrations at the site may be compared to values set by regulation or other
factors. For instance, MNA may be required to reduce the concentration of a contaminant to
an MCL of 5 mg/L at a particular sampling point. Of particular interest for this comparison is
that the threshold value is typically assumed to be a true value, not an estimate of a population
parameter; that is, it has no variability associated with it. This affects the statistical procedures
used for comparing the values, as discussed in the references given below.
3.3.4 Statistics
The previously discussed data comparisons can be conducted by simply comparing measured
values, calculated (or graphed) trends, or set values for contaminants or geochemical parameters.
However, statistical procedures and models provide a formal, quantitative method for assessing the
relationship of sample measurements to characteristics of the sampled system, for using sample
data to make decisions, and for predicting future states of the sampled system.
Statistical procedures are often used to evaluate the variability associated with data, and to use
estimates of variability to guide decision-making processes. For example, if multiple analyses are
performed, statistical procedures can be used to express a measure of the analytical variability
associated with a reported contaminant concentration (e.g., 4.9 mg/L + /- 3.2 mg/L, representing
the 95 % confidence limits on the mean value).
Statistical methods are also available to facilitate analysis and comparison of trends by considering
data variability through time. For instance, changes in contaminant concentrations over space
or time can be used to calculate attenuation rates, and the variability associated with those rates
can be quantified with confidence intervals about the rates. These confidence intervals can
be used to determine the likelihood of attaining site-related remedial goals. If all values of the
attenuation rate falling within the confidence intervals lead to predictions that site remedial goals
will be attained in the desired time frame, then confidence that MNA can attain remedial goals is
increased.
Implementing formal methods that compare data by taking into account data variability is
especially important for decision-making purposes. Gibbons (1994), Gilbert (1987) and Helsel
(1995) contain extensive discussions of the issues concerning use of statistics in environmental
and ground-water monitoring. For a detailed discussion of these points, as well as step-by-step
guidance on calculations for the various types of comparisons mentioned above, see also U.S. EPA
(2000c) and U.S. EPA (1992a).
3.4 Elements of a Performance Monitoring Report
3.4.1 Introduction
Compilation and presentation of monitoring data in an easily usable form that facilitates
interpretation requires significant effort. As was discussed for the monitoring plan, there is no
standard format that is universally acceptable, but, ideally, monitoring data would be acquired,
transmitted, evaluated and presented in an electronic form, and the report would make extensive
use of maps, cross sections, and figures to convey the results of monitoring efforts.
The following material has been prepared to provide suggestions regarding appropriate
components of performance monitoring reports. The format and content of these documents will
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vary according to site-specific needs but most reports would contain or reference the elements
listed in Table 5 and briefly discussed in the following sections.
Elements of a monitoring report (Table 5) include:
• Summary of data interpretations and recommendations,
• Background and site description,
• Monitoring network and schedule description,
• Evaluation of new data and comparisons with previous data and established performance
criteria,
• Interpretation of new data with respect to the conceptual site model for natural
attenuation, and
• Recommendations for action.
3.4.2 Summary
The summary typically would contain a brief description of the site, remedial goals, a narrative
summary of new data and their interpretation, and any recommended actions. This portion of the
report should be written to convey essential findings to both technical and nontechnical readers.
3.4.3 Background and Site Description
Background information on the site, such as a discussion of the site setting, history, characteristics,
remedial goals, past and present remedial activities, and any institutional controls, typically would
be provided in abbreviated fashion or incorporated by reference to existing documents. The most
salient issues (e.g., hydrogeological setting, contaminant distribution, remedial goals and actions)
generally should be addressed at sufficient length that the technical reviewer can judge the
adequacy of the monitoring efforts. The geologic and hydrogeologic setting of the site, including
controls on ground-water flow at the regional scale and site scale, would be illustrated in map and
cross-sectional views.
3.4.4 Monitoring Network and Schedule
The report typically would include information on the monitoring network including a
list and map of monitoring locations; a description of the construction of each monitoring
point; and a monitoring schedule specifying monitoring parameters, analytical methods, and
sampling frequency. Information regarding Data Quality Objectives (DQOs) may be provided
or incorporated by reference. It is particularly important that maps, cross-sections, and other
visual aids be extensively used so that the reviewer can determine exactly where, when, and
by what methodologies each sample was obtained and analyzed. In situations where different
hydrogeologic units are monitored, separate maps with -wells/sampling locations for each major
lithologic unit may provide additional clarity in visualization of the network design.
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Table 5. Elements of a Performance Monitoring Report
Summary of Data Interpretations and Recommendations
Q Brief description of site
Q Remedial goals
Q Narrative summary of new data and their interpretation
Q Recommended actions
Background and Site Description
The following information typically would be discussed or incorporated by reference:
Q Site setting, history,characteristics
Q Remedial goals
Q Past and present remedial activities,and any institutional controls
Q Map and cross-sectional views illustrating geologic and hydrogeologic setting of the site, including controls on
ground-water flow at the regional scale and site scale
Monitoring Network and Schedule
Q List and maps of monitoring locations for each sampled medium and each major hydrogeologic unit
Q Description and construction details for each monitoring point
Q Monitoring schedule specifying monitoring parameters,analytical methods,and sampling frequency for each
monitoring location
Q Data Quality Objectives (DQOs) to be met
Evaluation of New Data
Q Detailed discussion of new results and evaluations
Q Data in tables and electronic files
Q Potentiometric surface maps for each hydrostratigraphic unit
Q Hydrographs of ground-water elevations for key wells in each hydrostratigraphic unit and surface-water
monitoring points
Q Contaminant data posted and contoured on maps for each media and major hydrogeologic unit
Q Hydrochemical cross sections along and perpendicular to ground-water flow directions depicting contaminants,
monitoring points,and hydrogeology
Q Geochemical data posted and contoured on maps for each major hydrogeologic unit
Q Cross sections depicting geochemical data, monitoring points,and hydrogeology
Q Comparison of the new data with previous data and established performance criteria
Q Results of statistical comparisons
Q Discussion of trends and the relation of any trends to remedial goals
Q Assessment of measurement variability from analysis of QA/QC data
Q Observed changes in land use
Evaluation of Institutional Controls (ICs)
Q Description of ICs that are in place with appropriate verification
Q Evaluation of the effectiveness of ICs
Q Discussion of any pending changes in property ownership
Q Observed changes in land or resource uses
MNA Conceptual Site Model Evaluation
Q Evaluation of the conceptual site model incorporating any new data and data trends
Q Discussion of consistency of previous conceptual site model with new data
Q Suspected sources for continued ground-water contamination (e.g., number, location(s),characteristics of
sources)
Q Trends in contaminant and geochemistry values
Q Discussion of any observed changes in site hydrology (e.g., water elevations, ground-water velocities)
Q Discussion of refinements/modifications to conceptual site model
Q Consistency of current data with previous predictions
Q Discussion of changes in land use and potential effects on the conceptual model
Recommendations (as warranted)
Q Recommended changes in monitoring locations
Q Recommended changes in monitoring frequencies
Q Recommended changes in sampling methods
Q Recommended changes in analyses
Q Discussion of new data in relation to performance criteria previously established to trigger implementation of a
contingency remedy
Q Discussion of changes in land use and potential effects on site remedies,and remedy protectivenessfor human
and ecological receptors
Q Recommended remedy modifications (e.g., additional source removal actions)
Q Recommendations for starting verification monitoring,orterminating performance monitoring
Q Rationale for recommended changes
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3.4.5 Evaluation of New Data
The evaluation of new data and comparisons with previous data typically should provide a:
• Detailed discussion of new results and evaluations with presentation of data in tables,
maps, and figures,
• Comparison of the new data with previous data and established performance criteria,
• Discussion of uncertainty with statistical measures of variability, including discussion of
measurement variability assessed through evaluation of QA/QC data, and
• Discussion of trends and the relation of any data trends to the remedial goals.
Tables ordinarily would include hydraulic head data, contaminant data, and geochemical data
in both in-well and between-well comparisons. In other words, all data for each sampling point
would be tabulated to facilitate assessment of the geochemistry at that location, and a tabulation of
data by monitoring parameter would be provided to facilitate comparison of each parameter across
all sampling locations. Tables and figures depicting contaminant concentrations and geochemical
data for individual wells over time would be included to aid in the evaluation of trends. Maps
and cross-sections of the plume would be prepared and compared to previous conditions. Contour
maps of the contaminant concentrations and geochemical parameters are helpful for visualizing
broad trends. These comparisons aid in the evaluation of temporal changes. They may also serve
to identify areas where additional data are needed. Data submission in electronic formats should
also be considered.
3.4.6 Evaluation of Institutional Controls
The performance monitoring report typically would include a full description of all institutional
controls implemented or planned at the site, along with verification of their implementation. The
results of the monitoring activities, including an evaluation of the effectiveness of the individual
institutional controls, should be discussed at length so that the reviewer can judge whether the
controls are likely to continue to meet performance objectives. Any observed or pending changes
in land or resource uses or ownership (e.g., property ownership change, housing developments,
-well installations) should be discussed in view of their current and possible future impact on the
effectiveness of the controls and the performance monitoring operations.
3.4.7 Conceptual Site Model Evaluation
The report typically would contain an evaluation of the conceptual site model incorporating
any new data and data trends. Included in this section -would be a discussion of the following
questions:
(1) Do the new data fit the previous conceptual site model?
Consider:
• Suspected sources for continued ground-water contamination {e.g., number,
location(s), characteristics of sources),
• Trends in contaminant and geochemistry values, and
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• Changes in hydrologic factors (e.g., ground-water elevations, hydraulic
gradients, velocities)
(2) Should the conceptual site model be refined/modified?
(3) Do current data support previous predictions?
Consider uncertainty in previous predictions and implications for the effectiveness of natural
attenuation mechanisms and the current monitoring network.
3.4.8 Recommendations
Recommendations for action, based on interpretation and evaluation of the data in reference
to RAOs, monitoring objectives and performance criteria should be provided and discussed.
Recommendations for action may include changes in monitoring locations, frequencies, methods,
and analyses with a rationale for changes. Recommended actions may also include additional
source removal or other remedy modifications, implementation of contingency or alternative
remedies, advancement to verification monitoring, or termination of performance monitoring
based on achievement of site remedial goals.
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Chapter 4
APPLICATION OF MONITORING DATA TO REMEDIAL DECISIONS
4.1 Introduction
Following data evaluations, decisions are routinely made regarding the effectiveness of the MNA
remedy, the effectiveness of institutional controls, the adequacy of the monitoring program,
and the adequacy of the conceptual site model for MNA. Important decisions that may be made
include:
• Continue monitoring program without change,
• Modify the monitoring program,
• Modify the institutional controls,
• Implement the contingency or alternative remedy, or
• Verify remedial goals have been met and terminate performance monitoring.
Site-specific criteria should be developed to define conditions that indicate the appropriateness
of increased or decreased monitoring, additional characterization, reevaluation of the conceptual
site model, modification of institutional controls, implementation of a contingency or alternative
remedy, or termination of performance monitoring. The following discussion briefly considers
each decision and points relative to why it may be chosen.
4.2 Decision 1 - Continue Monitoring Program Without Change
Evidence leading to this decision would include contaminant concentrations, including toxic
transformation products and any mobilized by-products or secondary contaminants (e.g., arsenic,
manganese) remaining within the bounds of acceptable trends. Ground-water flow parameters
would not have changed outside previously identified acceptable ranges. The geochemistry
would not have changed in such a way as to indicate that the contaminant degradation or other
natural attenuation processes would be significantly affected. Significant changes in geochemistry
include a marked reduction of electron acceptors or donors essential to the important natural
attenuation processes identified at the site, an influx of substances such as oxygen or nitrate that
could interfere with desired processes, or a change in oxidation-reduction potential bringing
the potential into a range not suitable for the identified important natural attenuation processes
(Ferrey et at., 2001). Performance monitoring continues until all remedial action objectives have
been met.
4.3 Decision 2 - Modify the Monitoring Program
Modification of the monitoring program may be warranted to better reflect changing conditions
or increased understanding of natural attenuation processes at the site. Changing conditions,
the need to test key assumptions, or providing final confirmation of remedial goal achievement
57
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may require new sampling points or increases in monitoring parameters or frequency. Examples
include:
• A change in ground-water flow rate and direction caused by installation of an
irrigation -well may necessitate reevaluation of the monitoring frequency and
locations (Figure 9),
• Input of nitrate from fertilizer applications may sufficiently change the
geochemistry to inhibit reductive dechlorination. Increased monitoring frequency
and locations may be necessary to delineate the effect of nitrate influx,
• Periodic pulses of contaminants from the former source area may be caused by
flushing of contaminant from undetected residual material in the vadose zone.
Monitoring in the vadose zone as well as the saturated zone may be needed to
determine the nature and characteristics of the contaminant pulses and provide
data for assessing the appropriate response, and
• Contradictory indications of the potential for plume expansion in some areas or
the data may be so highly variable that interpretation is difficult and unacceptably
uncertain. High data variability, making interpretation difficult, may be caused
by variability inherent in sampling and, in some cases, analysis. In addition,
contaminant concentration changes caused by seasonal changes in ground-water
flow, degradation rates, contaminant releases from poorly controlled source(s), and
influx of electron acceptors can make interpretation difficult unless monitoring is
sufficiently frequent to allow delineation of the changes due to seasonal cycling.
A determination may be made to collect more data sufficient to interpret trends.
However, if the protectiveness of the remedy is in question, implementation of the
specified contingency or an alternative remedy will generally be warranted.
In other instances, decreases in monitoring parameters, frequency, or locations may be
appropriate. For example, decreases in monitoring frequency for certain parameters may be
warranted if the remedy is proceeding according to expectations and trends are stable after
evaluation of data from a sufficient number of monitoring periods (e.g., many years). To
support such a decision, the available data generally should cover a time period sufficient to
allow evaluation of seasonal trends and other long-term cycles and trends. Evidence supporting
reduced monitoring includes trends in contaminant concentrations continuing as expected and
geochemistry and ground-water flow conditions remaining stable (i.e., in the ranges suitable for
continued natural attenuation). Also, the time required for implementation of the contingency
remedy after established decision criteria are met would be considered in any decision to reduce
monitoring frequency. In any case, performance monitoring should continue until all remedial
action objectives have been met (e.g., contaminant concentrations are below levels of concern).
Once performance monitoring data indicate that site remedial goals have been met, a period of
verification monitoring may be initiated. Verification monitoring may include different sampling
frequencies or additional locations from those used during routine performance monitoring,
especially if monitoring locations or frequency had been reduced during the performance
monitoring period. For instance, the number of monitoring locations and monitoring frequency
may have been reduced if data had indicated that the MNA remedy was proceeding as expected.
In such cases, some of the sampling locations or times may have been eliminated from the
sampling program as unnecessary. However, it often may be appropriate to include these sampling
58
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locations and times during verification sampling in order to increase certainty that goals are met at
all locations and times. Once verification monitoring has shown that all MNA remedy goals have
been met, then termination of performance monitoring generally would be warranted.
4.4 Decision 3 - Modify Institutional Controls
Some changes in land or ground-water use have the potential to decrease the effectiveness or
the protectiveness of the MNA remedy. Such changes could include installation of new pumping
-wells in the vicinity of the site. New pumping -wells could change ground-water flow patterns and
increase the potential for plume migration, thereby increasing the risk to receptors. Some changes
in land use could affect subsurface geochemical conditions. For example, excessive application
of nitrogen-containing fertilizers could cause an influx of nitrate to ground -water and possibly
inhibit reductive dechlorination. Changes in land or ground-water use should be monitored and
evaluated to determine potential impacts to the MNA remedy. If existing institutional controls
or procedures for monitoring the controls are not sufficient to maintain the effectiveness or
protectiveness of the MNA remedy, the site manager should consider modifying the institutional
controls or monitoring procedures. If the changes in land or ground-water use resulted from a
breach of the institutional controls established for the site, the procedures used to monitor and
enforce the existing institutional controls should be evaluated and modified to prevent future
breaches. If the changes in land or ground-water use -were allowed by the current institutional
controls, the site manager should consider modifying the institutional controls to minimize future
adverse impacts to the MNA remedy.
4.5 Decision 4 - Implement a Contingency or Alternative Remedy
Remedies relying upon MNA may have an associated contingency remedy in case the MNA
component fails to perform at the desired effectiveness (U.S. EPA, 1999a). In any case, alternative
remedies are usually considered during remedy selection and these or other remedies can be
considered for use if it is determined that the current remedy has failed. Criteria for determining
specific failures and implementing modifications, a contingency, or alternative remedy may be
explicitly stated in the remedy decision documents. If not provided in previous documents, it is
recommended that objective and quantitative decision criteria for determining remedy failure be
developed for use in designing the performance monitoring system and evaluating data from the
system.
Development and evaluation of specific criteria to trigger implementation of a contingency or
alternative remedy are generally based on and related to the purpose behind site RAOs. That
relationship not only provides a rationale for a specific trigger, but also provides context for
evaluating the trigger once it occurs, and deciding on the appropriate response. For example,
a site RAO may require that the MNA remedy control the plume. Suppose that contaminants
-were detected in -wells outside the previously-known plume boundary in 1) an existing -well with
a long history of no contamination, or 2) a newly installed -well. If the aim of the RAO -was that
there should be no contaminant beyond a specific point (the previously-known plume boundary),
then the response to situation 1) or 2) may be the same. However, if the aim of the RAO -was
that the plume should not expand, then the response to situations 1) and 2) may differ, because
contaminants in the existing -well indicate plume expansion, -whereas contaminants in the new
-well may or may not indicate plume expansion (i.e., the contaminants may have been at the new
location for many years, but there -were no samples previously collected in the new location).
Situations that may -warrant implementation of a contingency or other remedy modifications
depend on site conditions and include, but are not limited to, the following examples:
59
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• Contaminant concentrations in soil or ground water at specified locations exhibit an
increasing trend not originally predicted during remedy selection,
• Near-source wells exhibit large concentration increases indicative of a new or renewed
release,
• Contaminants are identified in monitoring -wells located outside the original plume
boundary or other specified compliance boundary,
• Contaminant concentrations are not decreasing at the rate previously determined to be
necessary to meet the remediation objectives,
• Changes in land and/or ground-water use will adversely affect the protectiveness of the
MNA remedy, and
• Contaminants are identified in locations posing unacceptable risk to human or ecological
receptors.
4.5.1 Decision Criterion 1: Contaminant Concentrations in Soil or Ground Water at Specified
Locations Exhibit an Increasing Trend Not Originally Predicted
During Remedy Selection
This criterion involves the identification and assessment of trends in contaminant concentration
data, including toxic transformation products or mobilized inorganic constituents, that indicate
plume expansion, lack of progress toward cleanup objectives, or additional contaminant releases.
Statistical tests such as those described in the references may be used to objectively identify trends
in these data. An example of a criterion based on a trend would be detection of a consistent,
increasing trend in contaminant concentrations in -wells near the downgradient edge of the plume.
Such an event may be evidence of plume expansion, and, if so, may trigger the contingency or
alternative remedy based on the previously-established, site-specific guidelines for assessing trends
and remedy effectiveness.
For some locations in the plume, a detection of a significant change may not trigger immediate
implementation of a contingency or alternative remedy. For instance, a low-level or other-wise
limited contaminant concentration increase near the source area does not necessarily mean that
remediation time frames will be expanded, or that by the time the pulse moves downgradient to
the boundary of the plume there will be a significant plume expansion. Criteria for determining
acceptable limits for increases in contaminant concentrations in locations near the source area
should be based on site conditions and specified in decision documents. Because the contaminant
travel time from the source area to the downgradient edge of the plume may be many years at
some sites, there may be time for further monitoring and assessment before a contingency or
alternative remedy -would have to be implemented. In some cases, if contaminants will not reach
receptors for several years, it may be possible to allow verification procedures to continue until
the next sampling event (i.e., the next quarterly or semiannual sampling event). In contrast, if a
significant trend of increasing contaminant concentration in -wells near the downgradient edge of
the plume -was noted, there may be much less time available to assess the trend if receptors -were
only a short travel time downgradient. In this case, a contingency or alternative remedy may be
triggered immediately.
In some cases, the site characterization may indicate that concentrations in some portions of the
plume are expected to increase temporarily, yet MNA -would still meet remedial objectives. If
-------
so, the temporary increase would not trigger the contingency or alternative remedy as long as
the increase conformed to the predictions and MNA remained protective. Specific criteria for
determining that a significant change in trend has occurred may be developed to ensure that
sampling variability or acceptable seasonal fluctuations do not unnecessarily trigger a contingency
or alternative remedy.
4.5.2 Decision Criterion 2: Near-Source Wells Exhibit Large Concentration Increases
Indicative of a New or Renewed Release
A release may result from such conditions as drum rupture or increased flushing of the source
area caused by changes in hydrologic conditions. The expectation is that the new or renewed
release may increase the time needed to meet remedial objectives or cause the plume to expand.
Contaminant concentration increases beyond those previously predicted and stated in decision
documents could trigger a contingency or alternative remedy, or implementation of sampling
efforts to determine the causes of the increases and the impact on the remedy. In some cases,
detailed characterization and modeling of contaminant fate and migration may indicate that
the increased concentrations would be expected to attenuate without causing plume expansion,
allowing observation for several years to test model predictions, if time is available. In situations
-where evaluation shows the observed contaminant concentration increases are expected to
unacceptably increase the time for plume restoration, or cause unacceptable plume expansion,
triggering the contingency or alternative remedy generally would be warranted.
4.5.3 Decision Criterion 3: Detection of a Contaminant in Monitoring Wells Located Outside
of the Original Plume Boundary or Other Compliance Monitoring
Boundaries
Detections of contaminants outside of the predetermined horizontal or vertical plume boundaries
or other compliance boundaries may indicate unacceptable plume expansion. Procedures for
verifying contaminant detections typically should be included in the monitoring plan. The choice
of procedures depends on many factors including: the locations of the new detections, distance
between these monitoring points and receptors, possible contaminant migration rates to receptors,
time frames required for implementation of the contingency or alternative remedy, and monitoring
procedures. Appropriate verification procedures will often include:
• Verification of the detection, by verifying the analytical procedures showing the
detection and identity of the contaminant. This verification -would be conducted by
the analytical laboratory. It may also be possible for the laboratory to analyze another
aliquot of the same sample, if available, and
• Verification by immediately resampling the -well. Criteria defining the necessary level
of agreement bet-ween results may be developed to facilitate evaluations. For example,
a conservative decision criterion -would be implementation of the contingency remedy
if the contaminant concentration measured in the second sample exceeded the action
level.
If these are verified, this may indicate remedy failure, depending on the RAO for the site. Verified
detections of contaminants in these -wells may be used to trigger implementation of an alternative
or contingency remedy. Similar criteria may be developed to support decisions regarding other
types of compliance boundaries.
61
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4.5.4 Decision Criterion 4: Contaminant Concentrations Are Not Decreasing at a Sufficiently
Rapid Rate to Meet the Remediation Objectives
This assessment will involve such evaluations as the comparison of current data with predictions
used to support the remedy decision and evaluation or projection of temporal trends in
contaminant concentrations. There will generally be significant uncertainty in these evaluations
due to uncertainty in the projection of future conditions and sampling and measurement
variability. Specific, objective criteria and milestones specified in the remedy decision should
be reviewed. If specific criteria and milestones are not stated, they may be developed in the
performance monitoring plan. Specific milestones (e.g., 50 % contaminant concentration decrease
in all wells within a specified number of years) may be developed using such techniques as
projection of calculated attenuation rates. Failure to meet specified milestones or criteria could
result in triggering of the contingency or alternative remedy.
4.5.5 Decision Criterion 5: Changes in Land and/or Ground-Water Use that Have the Potential
to Reduce the Protectiveness of the MNA Remedy
This involves reevaluating the conceptual site model for MNA and determining whether there
are receptors at higher risk than originally conceptualized or whether the change in land or
ground-water use has resulted in hydrologic changes that affect plume stability, or geochemical
changes that affect biotransformation processes. Such changes could increase the potential
for contaminant migration and increased risk to the receptors as noted in Section 4.4. In such
situations where modification of the institution controls is not possible, appropriate, or sufficient
to restore the protectiveness of the remedy, triggering of a contingency or alternative remedy may
be appropriate.
4.5.6 Decision Criterion 6: Contaminants Are Identified in Locations Posing or Having the
Potential to Pose Unacceptable Risk to Receptors
In addition to monitoring contaminants in ground water at locations that indicate unacceptable
risk to human receptors, cross-media transfer of contaminants from ground water to surface
water, to indoor air, or other human or ecological receptors should be monitored based on a site-
specific evaluation of the risk posed by the contaminants. Monitoring may include, for example:
• Sampling in the aquifer near the point of discharge to the surface-water body,
• Pore water or sediment samples where ground water moves into the surface water,
• Samples from the surface-water body, and
• Soil gas and indoor air in enclosed areas near plume or source materials.
Decision criteria may be based on contaminant concentrations exceeding action levels in these
media. Assessment of this decision criterion would use statistical tests and verification procedures
similar to decision criteria 1 and 3.
4.6 Decision 4 - Terminate Performance Monitoring
Specific methods and criteria for demonstrating the attainment of all remedial action objectives
should be developed as noted in Sections 2.6.8 and 2.7.7. In general, once a thorough analysis of
performance/verification monitoring data shows that all MNA-related site remedial goals have
been achieved, termination of performance monitoring would be warranted.
62
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U.S. EPA, 2002a. Site Characterization Library, Volume 1 (Release 2.5) , NERL-LV-02-030, EPA/
600/C-02/002. U.S. Environmental Protection Agency, Office of Research and Development,
National Exposure Research Laboratory, Las Vegas, NV.
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U.S. EPA, 2002b. Calculation and Use of First-Order Rate Constants for Monitored Natural
Attenuation Studies, EPA/540/S-02/500. U.S. Environmental Protection Agency, Office of
Research and Development, National Risk Management Research Laboratory, Cincinnati, OH.
U.S. EPA, 2002c. OSWER Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air
Pathway from Groundwater and Soils (Subsurface Vapor Intrusion Guidance), RCRA-2002-
033. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response,
Washington, DC.
http://www.epa.gov/correctiveact.ion/eis/vapor.htm
U.S. EPA, 2003a. MTBE (methyl tertiary-butyl ether) and Underground Storage Tanks. U.S.
Environmental Protection Agency. OSWER/OUST. Washington, DC .
[Online] Available: http://www.epa.gov/swerustl/mtbe/index.htm (April 2, 2004).
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Managers, OSWER No. 9200.1-40, EPA/540/R-03/002, May 2003. U.S. Environmental
Protection Agency, Office of Solid Waste and Emergency Response, Washington, DC.
http://www.epa.gov/superfund/programs/dfa/guidoc.htm
U.S. EPA, 2003c. Field Analytic Technologies Encyclopedia (FATE). U.S. EPA Technology Innovation
Office, Washington, DC.
[Online] Available: http ://f ate. clu-in. org/ (April 2, 2004).
U.S. EPA, 2004. Handbook of Groundwater Protection and Cleanup Policies for RCRA Corrective
Action, EPA/530/R-01/015. U.S. Environmental Protection Agency, Office of Solid Waste and
Emergency Response, Washington, DC.
Weaver, J.W, J.T. Wilson, and D.H. Kampbell, 1996. Extraction of degradation rate constants from
the St. Joseph, Michigan, trichloroethene site. In: Proceedings of the Symposium on Natural
Attenuation of Chlorinated Organics in Ground Water, Hyatt Regency Dallas, Dallas, TX,
September 11-13, 1996, EPA/540/R-97/504. U.S. Environmental Protection Agency, Office of
Research and Development, Washington, DC.
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Westall, J.C. 2000. Geochemical equilibrium and the interpretation of Eh. In: Workshop on
Monitoring Oxidation-Reduction Processes for Ground-Water Restoration, Workshop Summary,
Dallas, Texas, April 25-27, 2000, EPA/600/R-02/002. U.S. Environmental Protection Agency,
Office of Research and Development, National Risk Management Research Laboratory,
Cincinnati, OH.
Wiedemeier, T.H., H.S. Rifai, C.J. Newell, and J.T. Wilson, 1999. Natural Attenuation of Fuels and
Chlorinated Solvents in the Subsurface. John Wiley & Sons, New York.
Wiedemeier, T.H., P.E. Haas, 2002. Designing monitoring programs to effectively evaluate the
performance of natural attenuation. Ground Water Monitoring & Remediation, 22(3):124-135.
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Wilson, J. 2000. Current state of practice for evaluation of oxidation reduction processes
important to the biological and chemical destruction of chlorinated organic compounds in
ground water. In: Workshop on Monitoring Oxidation-Reduction Processes for Ground-Water
Restoration, Workshop Summary, Dallas, Texas, April 25-27, 2000, EPA/600/R-02/002. U.S.
Environmental Protection Agency, Office of Research and Development, National Risk
Management Research Laboratory, Cincinnati, OH.
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Winter, T.C., 2000. Interaction of ground water and surface water. In: Proceedings of the Ground-
Water and Surface-Water Interactions Workshop, EPA/542/R-00/007. U.S. Environmental
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http://www.epa.gov/tio/tsp/issue.htm
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GLOSSARY
abiotic not relating to living things, not alive.
adsorption process by which molecules collect on and adhere to the surface of an adsorbent solid
because of chemical and/or physical forces.
aerobic living, active, or occurring only in the presence of oxygen.
alkalinity the capacity to accept protons (acid) -while maintaining the pH above a predetermined
value. Ground-water alkalinity may be increased as carbon dioxide emitted during
biodegradation causes bicarbonate production.
alluvial relating to material deposited by moving water.
anaerobic living, active, or occurring only in the absence of oxygen.
anthropogenic man-made.
attenuation a lessening in concentration or mass.
biodegradable refers to a material or compound that can be broken down by natural processes of
living things such as metabolization by microorganisms.
biodegradation act of breaking down material by natural processes of living things such as
metabolization by microorganisms.
contaminants of concern those chemicals identified during site investigations that are required
to be addressed by the response action proposed in the remedy decision documents.
daughter product degradation product of a compound. Vinyl chloride is a daughter product of
the reductive dechlorination of dichloroethene.
diffusion process by which ionic and molecular species move from a region of higher
concentration to a region of lower concentration.
dispersion phenomenon by which a solute in flowing ground water mixes with uncontaminated
water, becoming reduced in concentration. Dispersion is due both to differences in
water velocity at the pore level and differences in the rate at -which -water moves through
different strata. Also refers to statistical measures of how -widely a set of data vary.
dispersivity property that quantifies dispersion in a medium.
electron acceptor a compound capable of accepting electrons during oxidation-reduction (redox)
reactions. Microorganisms obtain energy by transferring electrons from electron donors
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such as organic compounds (or sometimes reduced inorganic compounds such as sulfide)
to an electron acceptor. Electron acceptors are compounds that are relatively oxidized and
include oxygen, nitrate, iron (III), manganese (IV), sulfate, carbon dioxide, or in some
cases the chlorinated aliphatic hydrocarbons such as tetrachloroethene, trichloroethene,
dichloroethene, and vinyl chloride.
electron donor a compound capable of supplying (giving up) electrons during oxidation-
reduction reactions. Microorganisms obtain energy by transferring electrons from electron
donors such as organic compounds (or sometimes reduced inorganic compounds such as
sulfide) to an electron acceptor. Electron donors are compounds that are relatively reduced
and include fuel hydrocarbons and native organic carbon.
hydraulic conductivity relative ability of soil, sediment, or rock to transmit water; a coefficient
of proportionality describing the rate at -which -water can move through a permeable
medium.
hydraulic gradient the change in total hydraulic head with a change in distance in a given
direction.
hydraulic head sum of the elevation head, the pressure head, and the velocity head at a given
point in an aquifer; also referred to as the total head.
hydrostratigraphic unit in which the geologic materials have similar hydrologic properties.
in situ refers to a technology or treatment process that can be carried out in place at the site of
contamination.
metabolic by-product a product of the reaction bet-ween an electron donor and an electron
acceptor. Metabolic by-products can include volatile fatty acids, daughter products of
chlorinated aliphatic hydrocarbons, methane, chloride, carbon dioxide, and water.
oxidation chemical process that results in a net loss of electrons in an element or compound.
porosity the ratio of void volume to total volume of a rock or sediment.
pump and treat treatment method in -which contaminated -water is pumped out of the
contaminated aquifer, then treated.
reduction chemical process that results in a net gain of electrons to the reduced element or
compound.
sorb to remove a substance from the aqueous phase to the solid phase.
sorption movement of a substance from the aqueous phase to the solid phase, whether
by adsorption, absorption, fixation or precipitation. Sorption may be reversible or
irreversible.
substrate substance(s) that provides growth and energy requirements for cells.
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tentatively identified compounds (TICs) compounds appearing in a chemical analysis of
environmental media from a site that have not been definitely identified.
transmissive zones subsurface units where ground-water flow is constrained or bounded by
lower hydraulic conductivity materials (i.e., geologic impediments to flow) or hydrologic
barriers (e.g., hydraulic head boundaries). Transmissive zones may be bounded by
components as obvious as the water table or units with low hydraulic conductivity, or by
conditions as subtle as small differences in grain size, sorting, and packing of seemingly
uniform sands.
uncertainty reduction of confidence in a conclusion -when more than one estimate is available for
a variable.
vadose zone zone between the ground surface and the water table.
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74
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Appendix A
VARIABILITY IN MEASURED PARAMETERS
AND THE EFFECTS ON PERFORMANCE MONITORING
A-l
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Appendix A
VARIABILITY IN MEASURED PARAMETERS
AND THE EFFECTS ON PERFORMANCE MONITORING
A.I Introduction
Monitoring data often exhibit significant variability. The variability in the measured values
reflects both the inherent spatial and temporal variability in the subsurface as well as variability
introduced into the data by the measurement techniques. The variability in the data introduce
uncertainty into the decision-making process, increasing the probability of making incorrect
decisions. Therefore, it is important to be able to assess the nature of data variability to
understand its importance toward achieving remedial goals. Identification and quantitative
assessment of data variability may often be more important in remedies that rely solely on
natural attenuation processes than for engineered remedies as neither contaminant migration nor
attenuation processes are actively controlled.
A.2 Spatial and Temporal Variability
A major source of variability in measured and interpreted contaminant distributions will
generally be related to the placement and screening of monitoring points within the spatially
and temporally heterogeneous subsurface. Current methods for defining subsurface contaminant
distributions rely on techniques that generally sample only a small volume of material immediately
surrounding a well or borehole. Therefore, the locations of sample collection often greatly affect
the interpretation of natural attenuation processes. For example, a vertical series of ground-water
samples collected using short-screened (e.g., 0.5 ft to 1 ft) wells, such as those installed by direct
push technologies, may display differences in contaminant concentrations of several orders of
magnitude over short vertical distances. In comparison, a nearby conventional monitoring -well
screened over the same total interval as the series of short-screened wells may yield samples with
contaminant concentrations that are essentially a flow-weighted average of the samples taken from
the short-screened wells.
The issue of obtaining "representative" samples from heterogeneous media is complex and has
been termed sample "support" (U.S. EPA, 2000a). Proper sample support involves ensuring
that the sample is representative of the original matrix under investigation (U.S. EPA, 2001b).
Sample support and the scale at which remedy performance will be evaluated should be considered
during design of the performance monitoring system because it strongly affects monitoring
network density and the specification of appropriate data evaluation methods. For instance, as
in the example given above, it may be possible to find a narrow zone in the subsurface where
contaminant concentrations exceed allowable limits, but ground water at the receptor point (e.g.,
an irrigation -well) may not exceed limits because the receptor derives ground water from a much
thicker zone in the subsurface. In such a case, the sample support chosen can determine the
remedial decision, because a sample representative of the narrow highly contaminated zone may
lead to a decision different than that derived from consideration of a sample representative of the
thicker zone with a lower mean contaminant concentration.
Three-dimensional spatial variability in dissolved contaminant concentrations occurs due to such
complex factors as the:
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• Nonuniform distribution of the original source materials for ground-water contamination,
• Heterogeneity in site geology, and
• Differences in the geochemical environment.
Source materials in various forms such as sorbed materials and NAPL may be non-uniformly
distributed throughout the vadose and saturated zone. This variability in source distribution
means that source contact with recharge water and ground water (and therefore dissolution of
contaminants into the ground water) is significantly different at varying locations in the aquifer.
Also, the geochemical environment varies throughout the aquifer due to the interactions of
microbial communities with the supply of terminal electron acceptors (TEA) and electron donors
carried by ground water from upgradient locations or released locally from the aquifer matrix.
Therefore, types and rates of contaminant degradation can vary throughout the plume.
After installation of the performance monitoring network, temporal variations in contaminant
distribution may become major influences on observed contaminant trends and data
interpretations. Sources of this variability include:
• Changes in contaminant input to ground water,
• Changes in degradation processes,
• Short-term changes due to seasonal factors (e.g., recharge or other physical factors such as
temperature), and
• Long-term changes due to substrate or TEA depletion, or changes in site hydrology.
Changes in contaminant input to ground water may result from source control actions, additional
releases to the environment, dissolution of vadose zone contamination during infiltration of
precipitation, and dissolution during increases in -water table elevation. Changes in site hydrology,
such as drought, extremely -wet periods, and changes in ground-water extraction or recharge
due to different land uses may change ground-water flow rates, directions, and, correspondingly,
contaminant concentrations at established monitoring points.
A.3 Measurement Variability
Other sources of variability are related to the measurement processes for monitoring parameters
and variability in the interpolation and data interpretations. Numerous factors, such as:
• Differences in sample collection methods (e.g., changes in equipment, pumping volumes or
rates, or pump intake location within a well screen),
• Differences in sample preparation methods (e.g., sample filtration or lack thereof, sample
preservation, and adherence to holding times), and
• Analytical variability (e.g., incorrect instrument calibration, improper operating
parameters)
may result in variability in measured contaminant concentrations. The variability from these
sources is often much less than the variability due to subsurface heterogeneity and temporal
A-3
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variability. However, measurement variability can be significant, and caution should be exercised
in the use of samples taken or analyzed by different methods to assess plume changes (e.g.,
calculating attenuation rates). Measurement variability is generally more readily definable and
controllable than spatial and temporal variability, using proper quality assurance/quality control
procedures such as those discussed in U.S. EPA (1998b). Additional discussion of the sources of
variability in subsurface investigations of contaminant migration and fate is provided by Barcelona
etdl. (1989).
A.4 Variability in Data Interpretation
In addition to spatial and temporal variability, and variability in measured contaminant
concentrations, variability is also introduced during the data interpretation process due to the non-
uniqueness of possible explanations of measured site conditions. That is, there may be a variety of
configurations of the conceptual site model that could explain the available data (e.g., with regard
to source factors, ground-water flow, and components of the attenuation rate). It may be possible
to reduce the number of alternative explanations by designing a focused sampling program to
fill the data gaps. The alternative explanations may be grouped by their probable impact on
attainment of remedial goals, and their likelihood of accurately describing site conditions, in
order to assess the desirability of performing additional work to verify or eliminate important
alternative explanations.
A-4
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