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

Recycled/Recyclable

Printed with vegetable-based ink on 

paper that contains a minimum of 

50% post-consumer fiber content 

processed chlorine free.
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.

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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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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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2.7

CHAPTER 3

3.1

3.2

3.3

3.4

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

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

ANALYSIS OF PERFORMANCE MONITORING DATA ......................................47

Introduction.................................................................................................................47

The DQA Process ........................................................................................................47

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

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

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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-1

A.1 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

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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 groundwater 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

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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 Definition

BTEX Benzene, Toluene, Ethylbenzene, Xylenes

CERCLA Comprehensive Environmental Response Compensation and 

Liability Act

COCs Contaminants of Concern

DCE Dichloroethene

DNAPL Dense Nonaqueous Phase Liquid

DQA Data Quality Assessment

DQO Data Quality Objectives

EPA U.S. Environmental Protection Agency

LNAPL Light Nonaqueous Phase Liquid

MCL Maximum Contaminant Level

MTBE Methyl-t-Butyl Ether

MNA Monitored Natural Attenuation

NAPL Nonaqueous Phase Liquid

OSWER Office of Solid Waste and Emergency Response

PCE Perchloroethene (tetrachloroethene)

PRGs Preliminary Remediation Goals

QA/QC Quality Assurance/Quality Control

RAOs Remedial Action Objectives

RCRA Resource Conservation and Recovery Act

ROD Record of Decision

TCE Trichloroethene

TEA Terminal Electron Acceptor

TICs Tentatively Identified Compounds

VC 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.

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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 groundwater 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.

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