Low-Risk Site Closure
Guidance Manual to Accelerate Closure of Conventional
and Performance Based Contract Sites
S.K. Farhat  C.J. Newell
M. Vanderford  T.E. McHugh  N.T. Mahler
GSI ENVIRONMENTAL INC.
HOUSTON, TEXAS
J.L. Gillespie

P.N. Jurena

A.A. Bodour
AIR FORCE CENTER FOR ENGINEERING & THE ENVIRONMENT
LACKLAND AFB, TEXAS
JULY 2012
DISCLAIMER
The Low-Risk Closure Guidance Manual is made available on an as-is basis without guarantee or
warranty of any kind, expressed or implied. The United States Government, GSI Environmental
Inc., the authors, and reviewers accept no liability resulting from the use of this documentation.
Implementation and interpretation of the predictions of the manual are the sole responsibility of
the user.
Cover Art:
Cover photograph courtesy of Dr. Thomas Sale, Colorado State University, Fort Collins,
Colorado.
For Citation:
Farhat, S.K., C.J. Newell, M. Vanderford, T.E. McHugh, N.T. Mahler, J.L. Gillespie, P.N. Jurena,
and A.A. Bodour, “Low-Risk Site Closure Guidance Manual to Accelerate Closure of
Conventional and Performance Based Contract Sites”, developed for the Air Force Center for
Engineering and the Environment by GSI Environmental Inc., Houston., Texas, July 2012.
Contacts: Dr. Shahla Farhat – skfarhat@gsi-net.com
Dr. Chuck Newell – cjnewell@gsi-net.com
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Low-Risk Site Closure Guidance Manual to Accelerate
Closure of Conventional and PBC Sites
AIR FORCE CENTER FOR ENGINEERING AND THE ENVIRONMENT
TABLE OF CONTENTS
Section
Page No.
EXECUTIVE SUMMARY ............................................................................................ ES-1 1.0 INTRODUCTION ...................................................................................................... 1 1.1 1.2 1.3 1.4 What is a Low-Risk Site? .................................................................................. 1 What is Site Closure? ....................................................................................... 1 What is Low-Risk Site Closure?........................................................................ 2 Goals of this Document..................................................................................... 2 2.0 NEW THINKING ABOUT CLOSING GROUNDWATER SITES .............................. 4 2.1 New Technical Concepts .................................................................................. 4 2.2 New Low-Risk Closure Regulatory Approaches ............................................... 5 3.0 DO I HAVE A LOW-RISK SITE? ............................................................................. 8 3.1 QUESTION I. Do You Have a Complete CSM That Reflects Key Low-Risk
Closure Concepts? ........................................................................................... 8 3.1.1. Question I.1. Have all of the components of the CSM been
evaluated?............................................................................................. 8 3.2 QUESTION II. Are Sources Controlled? ........................................................ 13 3.2.1. Question II.1. Are there any significantly mobile source
materials?............................................................................................ 13 3.2.2. Question II.2. Is the source zone free of any environmentally
significant quantity of NAPL? .............................................................. 14 3.2.3. Question II.3. Is it possible that any further source zone cleanup
will be constrained by matrix diffusion processes? ............................. 16 3.2.4. Question II.4. Are sources relatively small? ......................................... 18 3.2.5. Question II.5. Are source zone concentrations stable or
decreasing?......................................................................................... 19 3.2.6. Question II.6. Is there evidence of on-going source attenuation
processes? .......................................................................................... 21 3.2.7. Question II.7. Will future source remediation only marginally
improve site conditions?...................................................................... 23 L O W - R I S K
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TABLE OF CONTENTS
3.3. QUESTION III. Will Residual Contamination Have No Adverse Effect on
Present and Future Land and Water Uses? ................................................... 26 3.3.1. Question III.1. Is the groundwater plume stable, decreasing, or
probably decreasing? .......................................................................... 26 3.3.2. Question III.2. Is there evidence of on-going natural attenuation
processes in the plume? ..................................................................... 26 3.3.3. Question III.3. Are conditions protective of potential and future
receptors? ........................................................................................... 29 3.3.4. Question III.4. Is there a near-term need for the impacted
groundwater resource or any impacted land uses? ............................ 31 4.0 REDUCING LONG-TERM MONITORING INTENSITY ......................................... 34 5.0 REFERENCES ....................................................................................................... 36 6.0 CASE STUDIES – FIELD APPLICATION OF LoRSC MANUAL ......................... 39 APPENDICES Appendix A. Summary Of State Programs For Site Exit/Closure ................................ 101 Appendix B. Low-Risk Site Quick Reference Checklist and Blank Forms ................... 102 Appendix C. Conceptual Site Model ............................................................................ 109 Appendix D. 14 Compartment Model Step-by-Step Guide and Template ................... 110 TABLES
Table ES.1. LoRSC Manual Decision Logic
Table 1.
Application of the Plume Magnitude Classification System
Table 2.
Summary of Natural Attenuation Footprints at MNA Case Study Sites
(NRC, 2000)
FIGURES
Figure ES-1. LoRSC Manual Decision Logic Flow Chart
Figure 1.
Example of site with source excavation, but where groundwater plume
remains
Figure 2.
Criteria for low-threat closure of chlorinated solvent sites, San Francisco
Bay California Regional Water Quality Board (Figure from CRWQCB,
2009)
Figure 3.
Example of a CSM for a monitored natural attenuation remedy (USEPA,
2004)
Figure 4.
Depiction of a low-risk site using Sale’s 14 Compartment Model
Figure 5a.
Example of DNAPL mobility
Figure 5b.
Example of LNAPL mobility
Figure 6a.
Example of significant quantity of DNAPL
Figure 6b.
Example of significant quantity of LNAPL
Figure 7.
Conceptual model of matrix diffusion effects as part of plume response
(AFCEE, 2007)
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TABLE OF CONTENTS
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
MAROS plume trend classification system
Method for assessing the geochemical environment for groundwater
chlorinated solvent MNA (Truex et al., 2006)
Qualitative Decision Chart on the merits of source depletion (Sale et al.,
2008; Kavanaugh et al., 2003)
RBCA analyses for both potential and actual receptors (Figure A.3 from
GSI, 2007)
Economic value normalization methodology of groundwater in the SRT
(Newell et al., 2008)
Architecture of the Groundwater Sensitivity Toolkit (GSI, 2002)
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LIST OF ABBREVIATIONS
AFCEE
CDPHE
CF
cis-DCE
COV
CRWQCB
CSM
DNAPL
ESTCP
ITRC
GSI
GTS
LIF
LNAPL
LoRSC
LTMO
LUFT
MAROS
MCL
MNA
NAPL
NRC
NSZD
PBC
PCE
POE
PWS
RBCA
SERDP
SRT
TCE
TCEQ
TDS
USAF
USEPA
UST
WQP
Air Force Center for Engineering and the Environment
Colorado Department of Public Health and Environment
Confidence Factor
cis-Dichloroethene
Coefficient of Variation
California Regional Water Quality Control Boards
Conceptual Site Model
Dense Non-Aqueous Phase Liquid
Environmental Security Technology Certification Program
Interstate Technology and Regulatory Council
GSI Environmental Inc.
Geostatistical Temporal/Spatial
Laser Induced Fluorescence
Light Non-Aqueous Phase Liquid
Low-Risk Site Closure
Long-Term Monitoring Optimization
Leaking Underground Fuel Tank
Monitoring and Remediation Optimization System
Maximum Contaminant Level
Monitored Natural Attenuation
Non-Aqueous Phase Liquid
National Research Council
Natural Source Zone Depletion
Performance Based Contracting
Tetrachloroethene
Point of Exposure
Public Water Supply
Risk Based Corrective Action
Strategic Environmental Research and Development Program
Sustainable Remediation Toolkit
Trichloroethene
Texas Commission on Environmental Quality
Total Dissolved Solids
U.S. Air Force
U.S. Environmental Protection Agency
Underground Storage Tank
Water Quality Protection
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EXECUTIVE SUMMARY
EXECUTIVE SUMMARY
To help provide United States Air Force (USAF) site managers and site consultants a
roadmap for effective exit strategies, the Air Force Center for Engineering and the
Environment (AFCEE) has funded the development of a comprehensive decision
support tool, the Low-Risk Site Closure (LoRSC) Manual. While the LoRSC Manual can
be applicable to any type of groundwater contaminant, such as petroleum fuels,
chlorinated solvents, pesticides, and metals, some of the decision logic is based on key
processes at hydrocarbon and chlorinated solvent sites. The Manual is also designed
for sites managed under Performance Based Contracting (PBC) approaches, as well as
other contracting methods.
This guide was developed to help site managers determine if they have a low-risk site by
combining key concepts, information, and experience into one dynamic decision support
tool. This information can then be used to assist site managers build effective exit
strategies for closing low-risk sites and/or reducing long-term monitoring intensity. An
exit strategy for a given site can be strengthened by using multiple lines of evidence;
therefore, this guide provides weight-of-evidence decision logic to build consensus
between site stakeholders.
The LoRSC Manual was developed to provide site stakeholders with a specific, focused,
technology transfer roadmap that can be used to support regulatory decision making by
outlining:
1) how low-risk sites work,
2) why they won’t cause a future environmental problem,
3) why they should be closed, or at a minimum, should be monitored only on a very
limited basis,
The Manual is intended to provide a methodology that can be used by site personnel to
identify the type of USAF site and its probability for potential closure (e.g. gasoline spill
on shallow soil only, TCE under 500 feet of fractured rock), and evaluate and prioritize
sites based on threat criteria grouping sites as LoRSC Type A, B, or C:
 LoRSC Type A Sites: Strongest case for low-risk closure or reduced monitoring;
 LoRSC Type B Sites: Moderately good case for low-risk closure or reduced
monitoring;
 LoRSC Type C Sites: More difficult for low-risk closure or reduced monitoring.
The decision logic is based on identifying and examining three main categories of data:
a comprehensive Conceptual Site Model (CSM), control of sources, and adverse effects
of residual contamination. The low-risk site decision logic is presented below.
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Low-Risk Sites FAQ
1. Is this concept just for solvents and fuels or are there any other types of
contaminants for which this Manual can be used?
The AFCEE LoRSC Manual can be applied to any type of groundwater contaminant
including chlorinated solvents, petroleum fuels, metals, and pesticides.
2. What are the key types of data I need to apply the LoRSC Approach?
Almost all of the data collected to characterize and remediate a site are used: the
Conceptual Site Model, presence and mobility of NAPL, site history, hydrogeology,
trends in groundwater concentration data, lines of evidence for natural attenuation
processes, receptor information, and need for the impacted groundwater.
3. Can this occur just for residential or also for industrial levels? Does this
change the concept of low-risk?
Low-risk closure will apply for both residential and industrial levels as long as the
conditions are met.
4. This sounds too risky, what if I don’t want to apply any risky technology to my
site?
The AFCEE LoRSC methodology is not a “risky technology”. It is a methodology for
examining and analyzing data that should have already been collected. Additionally,
the methodology provides a pathway for identifying critical missing gaps in the data.
5. Can this be applied to metals sites?
Low-risk closure will apply to metals as long as the conditions are met.
6. Can the Manual be used to identify what additional work needs to be done to
close a site?
The AFCEE LoRSC methodology provides a pathway for identifying critical missing
gaps in the data.
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Table ES.1
LoRSC Manual Decision Logic
Low-Risk Decision Questions
I.
Do You Have A Complete Conceptual Site Model (CSM) That Reflects Key Low-Risk Closure Concepts?
1.
Have all of the components of the Conceptual Site
Model been evaluated?
II.
Are Sources Controlled?
Answers For
“Supporting”
Questions
Answers For “Must
Have” Questions
Key Low-Risk Decision Criteria
Manual Reference
Conceptual Site Model checklist.
Yes ⧠
No ⧠
Section 3.1.1
1. Are there any significant mobile source materials?
DNAPL sites: no mobile DNAPL observed. LNAPL sites: no expanding LNAPL zone and zero
or low LNAPL transmissivity.
Yes ⧠
No ⧠
Section 3.2.1
2. Is the source zone free of any environmentally
significant quantity of NAPL?
Little or no DNAPL observed in transmissive zones, and no significant LNAPL accumulation
based on specific volume calculations.
Yes ⧠
No ⧠
Section 3.2.2
3. Is it possible that any further source zone cleanup will
be constrained by matrix diffusion processes?
Qualitative evaluation of matrix diffusion processes based on geology, chemical properties,
timing of initial release, and remediation efforts.
Yes ⧠
No ⧠
Section 3.2.3
4. Are sources relatively small?
Plume is classified as a Mag 4 Plume Magnitude Category or less based on mass discharge
estimates, OR maximum source concentrations are < 20x Maximum Contaminant Level (MCL).
Yes ⧠
No ⧠
Section 3.2.4
5. Are source zone concentrations stable or decreasing?
Representative source zone concentrations over time are shown to be stable, decreasing, or
probably decreasing.
Yes ⧠
No ⧠
Section 3.2.5
6. Is there evidence of on-going natural attenuation
processes in the source zone?
Footprints of source zone attenuation are seen (such as generation of daughter products or
consumption of electron acceptors).
Yes ⧠
No ⧠
Section 3.2.6
7. Will future source remediation only marginally improve
site conditions?
There is “Less Need For Source Treatment” based on the Qualitative Decision Chart.
III.
Yes ⧠
No ⧠
Section 3.2.7
Will Residual Contamination Have No Adverse Effect on Present and Future Land and Water Uses?
1. Is the groundwater plume stable or shrinking?
Plume trend analyses showing decreasing plume over time.
Yes ⧠
No ⧠
Section 3.3.1
2. Is there evidence of on-going natural attenuation
processes in the plume?
Analyses of natural attenuation processes and footprints of natural attenuation in the plume.
Yes ⧠
No ⧠
Section 3.3.2
3. Are conditions protective of potential and future
receptors?
Analyses showing all exposure pathways for receptors are incomplete or present acceptable
risks, and that future exposure will not occur.
Yes ⧠
No ⧠
Section 3.3.3
Yes ⧠
No ⧠
Section 3.3.4
4. Is there a near-term need for the impacted groundwater Evaluation of future needs for groundwater resource and associated overlying land uses.
resource or any impacted land uses?
KEY:
“Must Have” Data: Critical Line of evidence for low-risk site closure - necessary to demonstrate these criteria at almost all sites if applicable.
“Supporting” Data: Supporting line of evidence, with 0-4 of the supporting lines recommended for low-risk site closure.
MUST HAVE:
All Yes?
Yes
(Type A or B)
No
(Type C)
SUPPORTING:
How Many “Yes”?
Type A if 3-4 Yes
Type B if 0-2 Yes
WHAT IT MEANS
LoRSC Site Type A (Strongest case for low-risk closure or reduced monitoring) = All “Must Have” Questions = Yes AND 3 or 4 of the “Supporting” Questions = Yes
LoRSC Site Type B (Moderately good case for low-risk closure or reduced monitoring) = All “Must Have” Questions = Yes AND 0 to 2 of the “Supporting” Questions = Yes
LoRSC Site Type C (More difficult for low-risk closure or reduced monitoring) = Any “Must Have” Questions = No
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FIGURE ES.1
LoRSC Manual Decision Logic Flow Chart
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INTRODUCTION
1.0
INTRODUCTION
1.1
What is a Low-Risk Site?
Complete cleanup of contaminated groundwater sites is often difficult, and consequently,
clean closure in the immediate future is unattainable at many sites. This problem is
particularly acute at sites with releases of chlorinated solvents, but hydrocarbon and
other releases can also result in persistent groundwater concentrations in excess of
closure criteria. While contaminant concentrations at such sites may decrease
significantly due to remediation and/or natural attenuation, persistent low-levels of
groundwater contamination above closure criteria can preclude objectives such as
reaching background concentrations or drinking water standards. However, this type of
contamination, when combined with other key factors, can mean that the site actually
poses very little risk to human health and the environment. Such a Low-Risk site may
be amenable for complete closure in some regulatory jurisdictions, or a conditional
closure where limited monitoring is required while the site attenuates.
Figure 1. Example of site with source excavation, but where groundwater plume remains
(photograph courtesy of Dr. Thomas Sale, Colorado State University).
1.2
What is Site Closure?
Site closure has different meanings under different regulatory programs. For example,
hydrocarbon sites regulated by the Texas Commission on Environmental Quality
(TCEQ) can be closed by meeting the following criteria that indicate a low-risk site:





No impacted or threatened water wells are present within 0.5 mile radius of the
site.
The affected groundwater zone is not considered part of a state designated
major/minor aquifer.
The affected groundwater is unlikely to be used in the future.
There is no discharge of the affected groundwater to a surface water body used
for human drinking water, contact recreation, or habitat to a protected or listed
endangered plant and animal species located within 0.25 mile radius of the site.
A depth to water greater than 15 feet (or depth to utilities that is greater than 15
feet) and an affected aquifer that is not part of a karst or fractured bedrock
geology.
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INTRODUCTION
Other sites in Texas can obtain a reduction in natural attenuation monitoring under the
Texas Risk Reduction Rules if the site is shown to have a stable plume in a designated
“Plume Management Zone” institutional control. Furthermore, groundwater monitoring
may be terminated if the plume is shown to be shrinking and there is no threat of future
impact on downgradient locations.
Likewise, the California Regional Water Quality Board allows:
 No Further Action closure of Underground Storage Tanks (UST). Under specific
conditions, concentrations at the site may be greater than the water quality
objectives at the time of closure.
 The San Francisco Bay Region allows low-risk/low-threat closure at both
petroleum fuel and chlorinated solvent sites under specific conditions. Under this
guidance, concentrations at the site may be greater than the water quality
objectives at the time of site closure.
Seven different state regulatory programs were found that currently have some type of
program that appears to address low-risk sites. Section 2.2 below discusses these in
detail.
1.3
What is Low-Risk Site Closure?
For certain sites, the risk posed by residual, hard-to-remove groundwater contamination
is very low. Depending on the particular regulatory program, this class of site might be
suitable for:


Complete Closure: No further action; or
Conditional Closure: Some type of conditional closure where future site
maintenance requirements (such as long-term monitoring) are greatly reduced.
In theory, low-risk closure could apply to either residential or industrial land uses,
although applying a low-risk type closure would likely be easier for industrial land uses.
1.4
Goals of this Document
The goal of this document is to provide site consultants, site managers, and regulators
tools and new information to:





Better understand the lifecycle of sites with groundwater contamination (for
example, sites contaminated with chlorinated solvents, petroleum hydrocarbons,
metals, pesticides, etc.) and how low-risk sites work.
Learn about previously under-appreciated groundwater fate and transport
processes.
Balance what can and what cannot be achieved with existing groundwater
remediation technologies.
Bring together key pieces of site information to build a comprehensive CSM and
determine if the site can be categorized as a low-risk site.
Explain why low-risk sites won’t cause future environmental problems.
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At sites with the right characteristics and at the right stage of the plume lifecycle,
build a technically sound, science-based case for some type of Low-Risk
Closure.
Provide a framework useful for both PBC projects and other approaches for
closing sites.
It should be emphasized that this document is not regulatory guidance, does not
establish policy, nor does it replace any existing state or federally mandated programs or
requirements.
This guide is intended to help site managers determine if they have a low-risk site by
providing key concepts, information, and experience in one dynamic decision support
tool. This information can then be used to assist site managers build effective exit
strategies for closing low-risk sites and/or reducing long-term monitoring intensity. The
exit strategy for a given site can be effectively strengthened by using multiple lines of
evidence; therefore, this guide provides weight-of-evidence decision logic to build
consensus between site stakeholders.
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2.0
NEW THINKING ABOUT CLOSING GROUNDWATER SITES
Over the past several years, there has been an increased focus on the “end game” of
site remediation projects, and how to get sites to closure, both from a technical and
regulatory perspective.
2.1
New Technical Concepts
The difficulties experienced at hundreds of these sites has led to a more detailed look at
the performance of remediation technologies and on previously-underappreciated
environmental processes that now appear to be a major constraint in our ability to close
sites. A series of scientific and engineering studies, many of them funded by the Air
Force Center for Engineering and the Environment (AFCEE) and the Department of
Defense’s Strategic Environmental Research and Development Program (SERDP) and
Environmental Security Technology Certification Program (ESTCP) have shed new light
on:

Expectations for in-situ remediation technology performance such as thermal
remediation, chemical oxidation, bioremediation, and chemical reduction.

The importance of “matrix diffusion”: stored contaminant mass resulting from
diffusion of dissolved groundwater contaminants into low-permeability zones.

Understanding that many groundwater source zones naturally attenuate over
time, even ones containing light non-aqueous phase liquids (LNAPL) and dense
non-aqueous phase liquids (DNAPL), due to natural flushing and degradation
processes within the source zone itself.

How simple groundwater tools and models can help understand and account
for key source and plume processes for site closure purposes. Examples include
AFCEE’s SourceDK model (Farhat et al., 2004), the U.S. Environmental
Protection Agency’s (USEPA) REMChlor model (Falta et al., 2007), ESTCP’s
Mass Flux Toolkit (Farhat et al., 2006), and the upcoming ESTCP Matrix
Diffusion Toolkit (Farhat et al., 2012).

The “14 Compartment Model”, which guides users to consider where the mass
and mass fluxes are located at the site and in what phase (i.e., vapor, NAPL,
aqueous, and sorbed). This model is centered on matrix-diffusion effects, where
7 of the 14 different compartments are “low permeability” compartments.
Each one of these major points is discussed in the Low-Risk Closure approach
presented in the following pages.
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NEW THINKING ABOUT CLOSING GROUNDWATER SITES
2.2
New Low-Risk Closure Regulatory Approaches
There is increasing emphasis on managing and closing low-risk sites. Several state
regulatory programs currently allow the closure of low-risk sites. Example programs are
listed below with details of documents currently available in Appendix A.







California
(both
chlorinated
solvent
and
petroleum
http://www.swrcb.ca.gov/),
Alaska (petroleum fuel sites, www.dec.alaska.gov),
Florida (low yield/poor quality sites, www.dep.state.fl.us),
Texas (petroleum fuel sites, www.tceq.state.tx.us),
North Carolina (underground storage tank sites, www.ncdenr.gov),
Wisconsin (petroleum fuel sites, www.dnr.wi.gov), and
Wyoming (storage tanks, www.deq.state.wy.us).
fuel
sites,
Several states (California, Florida, North Carolina, and Wisconsin) also allow
groundwater site closure with contaminants in place under specific conditions.
Two programs that are particularly relevant were developed in California and Colorado.
In 2009, the San Francisco Bay Region of the California Regional Water Quality Control
Board (CRWQCB), developed the “Assessment Tool for Closure of Low-Threat
Chlorinated Solvent Sites” built upon their 1996 guidance for low-risk closure of fuelimpacted sites. In this document,
The Groundwater Committee, a staff committee of the San Francisco Bay
Regional Water Quality Control Board (S.F. Bay Water Board) embarked on a
project to develop criteria for evaluating if and when chlorinated solvent sites that
pose little threat to human and ecological health, water quality, and beneficial
uses but do not yet meet cleanup standards at all locations, could be closed. This
process is referred to as “low-threat closure.”
Under this system, nine separate criteria1 must be met for a “low-threat closure”
(CRWQCB, 2009) (Figure 2).
On May 1, 2012, the California State Water Board adopted the statewide “Low-Threat
Underground Storage Tank Case Closure Policy”:
With the knowledge and experience gained over the last 25 years of investigating
and remediating petroleum UST releases, site conditions and characteristics
have been identified that if met, will generally ensure the protection of human
health, safety and the environment. This Policy identifies those standardized
criteria. The Policy is necessary to establish consistent, statewide case closure
criteria for low-threat petroleum UST sites in California.
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NEW THINKING ABOUT CLOSING GROUNDWATER SITES
Figure 2. Criteria for low-threat closure of chlorinated solvent sites,
San Francisco Bay California Regional Water Quality Board (CRWQCB, 2009, Table ES-1).
The Colorado Department of Public Health and Environment (CDPHE) also issued a
draft “Guidance for the Closure of Low-Threat Sites with Residual Ground Water
Contamination” in August 2010. This methodology is based on six lines of evidence that
must be met for a “low-threat closure”:
1. Adequate characterization of the site.
2. Remediation of source areas.
3. No exposure to contaminants.
4. Demonstration of natural attenuation processes.
5. Definition of the timeframe for achieving remediation goals.
6. Ability to enact, implement and maintain institutional controls over time.
The Colorado guidance does not have specific criteria for determining what is needed to
have “adequate” site characterization; it is site and regulatory program specific (CDPHE,
2010). The document states:
Division personnel will apply professional judgment in each case, factoring in
such elements as: the cause of the suspected release, the chemicals of concern,
the complexity of the site hydrology and hydrogeology, the magnitude of the
problem, and the potential for future exposures.
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The CRWQCB (2009) specifies this type of characterization work is needed to apply
their low-threat closure guidance:
Site characterization work should be designed to minimize uncertainty and
maximize accuracy to 1) effectively characterize pollutant distribution and
migration pathways in all media, including soil, soil-gas, and groundwater, and 2)
identify potential migration pathways to allow for appropriate decision-making
pertaining to risk, monitoring and remediation.
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DO I HAVE A LOW-RISK SITE?
3.0
DO I HAVE A LOW-RISK SITE?
This section provides a variety of methodologies, calculations, graphics, scientific
literature, and modeling approaches/tools that can be used to determine if a site could
be considered low-risk, and be a candidate for closure and/or reduction in intensity of
long-term site care.
The methodology relies partially on key concepts presented in the “Assessment Tool for
Closure of Low-Threat Chlorinated Solvent Sites” (CRWQCB, 2009) and the “Draft
Guidance for the Closure of Low-Threat Sites with Residual Ground Water
Contamination” (CDPHE, 2010).
A quick reference checklist for the LoRSC Manual decision logic is provided in
Appendix B.
3.1
QUESTION I. DO YOU HAVE A COMPLETE CSM THAT REFLECTS KEY
LOW-RISK CLOSURE CONCEPTS?
3.1.1. Question I.1. Have all of the components of the CSM been evaluated?
Criteria: CSM checklist is complete.
Development of the CSM should start in the early stages of site investigation and it
should be updated and refined continuously as additional information becomes
available. The CSM is typically supported by visual aids such as tables, diagrams, maps,
figures, and hydrogeologic cross-sections. Critical information contained in a
comprehensive CSM should include, where available:
1. Site Information – including historical, current, and future property use or
industrial activities.
2. Site Investigations – including dates of investigations, soil borings, geophysical
investigations, site geochemistry, presence of off-site affected groundwater,
evidence of NAPL, and dates of most recent NAPL observation.
3. Source Characterization – including primary (e.g., tank, drum, sump, etc.) and
secondary (e.g., NAPL, contaminated soil, etc.) source locations, release
mechanisms (e.g. spills, landfill), size and boundary, substance(s) released,
date(s) of release, volume and mass of substance(s) released, and source
control measures taken.
4. Constituents of Concern – including chemical constituents of regulatory concern;
identification of those most likely to pose some risk due to their presence, toxicity
and mobility at the site.
5. Nature and Extent of Contamination – including the horizontal and vertical
distribution of the contamination and concentrations.
6. Hydrogeology – including stratigraphy, vadose (unsaturated) and saturated zone
types, aquifer properties such as hydraulic conductivity, gradient and porosity,
confining unit soil type, depth to top of aquifer, depth to groundwater, direction of
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groundwater flow including preferential pathways, recharge, proximity to surface
waters, and interaction between groundwater and surface water.
7. Geochemistry – geochemical parameters and conditions such as oxygen
concentrations, nitrate, sulfate, and iron.
8. Migration and Exposure Pathways – including groundwater, surface water, soil,
air, sediment, and biota, and identification of both complete and incomplete
exposure pathways.
9. Contaminant Attenuation Pathways – including advection, dispersion, chemical
and biological transformation mechanisms, sorption, and dilution.
10. Receptors – including identification of and mitigation activities protecting actual
and potential a) human receptors (e.g., well locations, groundwater-to-surface
water discharge locations, underground utilities, etc.), b) ecological receptors, c)
sensitive receptors (e.g., day-care centers, schools, residences, hospitals, etc.),
and d) current and future groundwater and surface water resources.
Identification of any potential adverse effects should also be included.
11. Soil Remediation – including date(s) of soil remediation initiated and completed,
remediation technology employed, soil volume treated (or removed), results of
treatment/removal, and adequacy of treatment in meeting regulatory standards.
12. Groundwater Remediation - including date(s) of groundwater remediation
initiated and completed, remediation technology employed, information on NAPL
recovery, results of treatment, and adequacy of treatment in meeting regulatory
standards.
13. 14
Compartment
Model
–
diagram
of
different
contaminant
phases/compartments at the site. The 14 Compartment Model is discussed
below.
14. Stakeholders – including regulatory agencies, property owners, developers,
municipalities, and adjacent communities.
If possible these elements should be described in a graphic, such as a block diagram
showing key parts of a CSM. Figure 3 shows an example CSM for a Monitored Natural
Attenuation (MNA) remedy (USEPA, 2004). A template for a CSM is provided in
Appendix C.
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Figure 3. Example of a CSM for a monitored natural attenuation remedy (USEPA, 2004).
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A very useful conceptual framework is the 14 Compartment Model developed by Sale et
al. (2008) (Figure 4). The model provides a means for 1) accounting for the relative
distribution of contaminant mass at a site, 2) assessing the stage of plume maturity, and
3) evaluating site response to remedial treatment.
Source Zone
Zone/
Phases
Plume
Low
Permeability
Transmissive
Transmissive
Low
Permeability
Vapor
IP
IP
IP
IP
DNAPL
0
0
NA
NA
Aqueous
2
1
1
2
Sorbed
2
1
1
2
Legend:
Figure 4. Depiction of a low-risk site using Sale’s 14 Compartment Model. Solid arrows
represent reversible mass transport between compartments, while dashed arrows
represent irreversible transport. See Appendix D for instructions on completing the 14
Compartment Model for a specific site. (NA = Not Applicable; IP = Incomplete Pathway.)
As described by Sale and Newell (2011):
“It is important to realize that the 14 Compartment Model is a useful tool, but it is only
part of a conceptual site model. Explicitly considering the 14 Compartment Model helps
ensure that all of the different phases and transmissive zones are considered when
making management decisions. But it is also important that a conceptual site model
include a mass balance that addresses the spatial distribution of the mass of
contaminants, and the fluxes of contaminants within the site, as well as the
hydrogeologic and biogeochemical information needed to evaluate fate and transport.
The use of the 14 Compartment Model is designed to encourage the development of
integrated strategies, in conjunction with the other aspects of a quantitative conceptual
site model.”
The quantitative application of the 14 Compartment Model is discussed in detail in the
Decision Guide document prepared by Sale and Newell (2011). In general, the user puts
in an order-of-magnitude estimate of the pre-remediation concentration in each box (for
example, 1000 mg/L would be a “3” as shown in 103 mg/l). Some compartments may not
be measured directly, but the concentrations can be inferred by evaluating
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concentrations in adjacent compartments. The change in concentration due to
remediation would then be applied using an order of magnitude approach. If a
remediation technology is thought to reduce concentrations in a particular compartment
by two orders of magnitude (to 0.01 of the pre-remediation concentration, or a 99%
reduction), then the post-remediation version of the 14 Compartment Model would have
a “1” in that box. Similar before-and-after versions can be developed for the mass
discharge (sometimes called mass flux) between compartments. The overall goal is to
show order-of-magnitude changes on a semi-quantitative basis. Detailed measurements
of concentration in each compartment are helpful, but not necessary (Sale and Newell,
2011).
Note there is an increasing interest in the regulatory community to use this model for
regulatory decision making. The Interstate Technology and Regulatory Council’s
(ITRC’s) Integrated DNAPL Site Strategy Technology and Regulatory Guidance (2011),
depends heavily on the 14 Compartment Model to guide accurate decision making about
remediation and management of chlorinated solvent sites.
A template and step-by-step guide for the 14 Compartment Model is provided in
Appendix D. A qualitative application of the model includes identifying all phases/zones
that could potentially contain the contaminants.
If the CSM includes all of items 1-14 (Section 3.1.1) relevant to the Site, and
includes a qualitative 14 Compartment Model, then Question I.1 is answered
“YES”.
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3.2
QUESTION II. ARE SOURCES CONTROLLED?
3.2.1. Question II.1. Are there any significant mobile source materials?
Criteria: No mobile DNAPL. No expanding LNAPL zone.
A controlled chlorinated solvent source zone should have no consistently observed
mobile DNAPL in monitoring wells, should not discharge to surface water, nor exhibit
any other evidence of an expanding DNAPL zone (e.g., Figures 5a and 5b). For
hydrocarbon sites, any LNAPL accumulation must not be expanding spatially. For other
types of NAPL, the NAPL accumulation must not be expanding spatially.
Figure 5a. Example of DNAPL mobility. Top panel depicts no mobility of DNAPL while the
lower panel shows the mobility of DNAPL (brown color). Light red color indicates
dissolved phase plume.
Figure 5b. Example of LNAPL mobility. Top panel depicts no mobility of LNAPL while the
lower panel shows the overall LNAPL footprint increasing in size, indicating mobility of
LNAPL.
Chlorinated solvent sites: DNAPL is extremely mobile, and in most cases any consistent,
observed DNAPL accumulations in wells should be remediated before a site can be
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considered a low-risk site. However, most chlorinated solvent DNAPLs have high density
and low viscosity, so that migration in relatively permeable media can cease within a few
months to a few years following the time of release (USEPA, 2009). Therefore, it is
unlikely that there is a currently expanding DNAPL zone at a chlorinated solvent site if:
1) the most recent release is more than a few years old; 2) the site is a smaller release;
3) DNAPL has not been observed in any monitoring wells; and 4) the site is adequately
characterized.
Hydrocarbon sites: LNAPL presence in monitoring wells can, in certain circumstances,
be caused by low-mobility, low volume LNAPL accumulations. Therefore, the key
criterion is to confirm that the LNAPL zone is not increasing in size by comparing maps
of the observed LNAPL accumulations over time. If consistently LNAPL-free wells on the
periphery of the LNAPL zone change so that LNAPL is consistently observed, then the
LNAPL zone may be expanding. If the overall LNAPL areal footprint is not expanding
over time, then there are no significantly mobile source materials. (Note that the size of
the LNAPL footprint is considered at the scale of the entire site, and that very small scale
changes at the pore level or hypothesized changes between monitoring points would not
be considered as indicating the presence of mobile LNAPL).
If LNAPL is found in monitoring wells, LNAPL transmissivity calculations can be made.
The ITRC LNAPL technical guidance (ITRC, 2009) states that “Beckett and Lundegard
(1997) proposed that appreciable quantities of LNAPL cannot be recovered and that
there is little migration risk associated with a well with an LNAPL transmissivity (Tn) of
0.015 ft2/day. However, ITRC LNAPL Team members’ experience indicates that
hydraulic or pneumatic recovery systems can practically reduce Tn to values between
0.01 and 0.8 ft2/day.” Alternatively, if the LNAPL transmissivity is 0.01 ft2/day then there
are no significantly mobile source materials.
Apparent thickness (the thickness of the LNAPL in monitoring wells) should not be used
as an indicator of mobile LNAPL or of significant LNAPL accumulation because
formation effects in fine-grained soils can greatly magnify the amount of LNAPL in the
well compared to the specific volume of LNAPL in the formation. Adamski et al. (2005)
provide a detailed description of the conceptual model of LNAPL behavior in fine-grained
soils.
LNAPL mobility tracer technology and companion calculations, developed by Colorado
State University, can be used to determine if the LNAPL zone is expanding. The LNAPL
tracer technology utilizes a fluorescent dye that is only visible in an LNAPL. The dye is
injected into a well containing LNAPL and intermittently agitated to obtain uniformly
mixed tracer concentrations at the time of measurement (Smith et al., 2012). The rate of
disappearance of the dye is then used to estimate the LNAPL migration rate (LNAPL
velocity). More importantly, corresponding calculations and simple modeling can be
used to determine if the rate of Natural Source Zone Depletion (NSZD) is enough to
keep the LNAPL body from expanding (Mahler et al., 2012). If tracer tests and NSZD
calculations indicate no expansion for the LNAPL body, then there are no significantly
mobile source materials.
If there is no significantly mobile NAPL in the source zone, then Question II.1 is
answered “YES”.
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3.2.2. Question II.2. Is the source zone free of any environmentally significant
quantity of NAPL?
Criteria: Little or no DNAPL observed in transmissive zones, and no significant LNAPL
accumulation based on specific volume calculations.
Chlorinated solvent sites: To answer yes, DNAPL is either not directly observed in core
samples; or the average saturation (percent of pore spaced filled with DNAPL) of the
DNAPL observed in cores collected from the transmissive zone is less than 1% (e.g.,
see Figures 6a and 6b). For core analyses, dye testing or other enhanced DNAPL
evaluation techniques are preferred to help reduce the occurrence of false negatives.
General indirect rules about DNAPL occurrence (such as the 1% rule) should not be
used by themselves to indicate the presence of DNAPL, but only with other, converging
lines of evidence (see USEPA, 2009 for a discussion of the 1% rule). This is because
the indirect methods have a considerable uncertainty (the USEPA says the 1% rule is a
generality, and that “DNAPL may be present”), and some matrix diffusion experts are
now suggesting that 1% of solubility could be generated by matrix diffusion processes
alone, resulting in false positives. In summary, if DNAPL has never been observed in
core samples, and/or if DNAPL has been observed, but has an average saturation of
less than 1% in the source zone, then there is no environmentally significant quantity of
DNAPL in the source zone.
Figure 6a. Example of significant quantity of DNAPL.
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Figure 6b. Example of significant quantity of LNAPL.
Hydrocarbon sites: For sites with LNAPL observed in monitoring wells, specific volume
(the volume of LNAPL divided by the area) can be used to evaluate the actual amount of
LNAPL present. Average LNAPL specific volumes less than 0.1 feet can be considered
to be relatively low accumulations of LNAPL (this is equivalent to 33,000 gallons of
LNAPL per acre). This value is based on recent research that indicates that NSZD in
LNAPL zones are degrading at the rate of thousands of gallons per year (or a potential
degradation time of about 30 years) (Adamski et al., 2005; Mahler et al., 2012). If LNAPL
has never been observed in core samples, and/or if LNAPL has been observed, but has
a specific volume of less than 0.1 foot in the source zone, then there is no
environmentally significant quantity of LNAPL in the source zone.
Apparent thickness (the thickness of the LNAPL in monitoring wells) should not be used
as an indicator of mobile LNAPL or of significant LNAPL accumulation because
formation effects in fine-grained soils can greatly magnify the amount of LNAPL in the
well compared to the specific volume of LNAPL in the formation. Adamski et al. (2005)
provide a detailed description of the conceptual model of LNAPL behavior in fine-grained
soils.
If there are no environmentally significant quantities of NAPL in the source
zone, then Question II.2 is answered “YES”.
3.2.3. Question II.3. Is it possible that any further source zone cleanup will be
constrained by matrix diffusion processes?
Criteria: Qualitative evaluation of matrix diffusion processes based on geology, chemical
properties, timing of initial release, and remediation efforts.
Most remediation programs specify that source control actions should use treatment to
address "Principal Threat" wastes (or products) wherever practicable (USEPA, 1999).
Principal threat wastes are those source materials that are “highly toxic or highly mobile
that generally cannot be reliably contained or would present a significant risk to human
health or the environment should exposure occur. They include liquids and other highly
mobile materials (e.g., solvents) or materials having high concentrations of toxic
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compounds” (USEPA, 1991). Low-level threat wastes are “source materials that
generally can be reliably contained and that would present only a low risk in the event of
release” (USEPA, 1991). Since contaminated groundwater is not source material, it is
neither a principal nor a low-level threat waste (USEPA, 1991). Furthermore, matrix
diffusion sources are neither highly toxic (low-strength) nor mobile, and can be reliably
contained at most sites via MNA (because they are typically low strength sources).
Consequently, because matrix diffusion sources are not a Principal Threat Waste, there
is no need for immediate or near-term treatment. However, future remediation of matrix
diffusion-dominated sources would be more difficult (i.e., likely more difficult than
removing NAPL from transmissive zones). Therefore, matrix diffusion sources are a
supporting line of evidence for Low-Risk Site designation (it is a “Supporting” question,
not a "Must Have" question).
The potential for matrix diffusion effects can be seen at virtually any site with
heterogeneity in the subsurface, NAPL, and/or where persistent groundwater
contaminant concentrations after source-zone remediation have been observed (Figure
7). Key factors favoring matrix diffusion (adapted from Sale et al., 2008), ordered from
more important to potentially less important, include:
 Presence of Low-Permeability lenses or strata in an affected aquifer in contact
with transmissive zones containing plumes.
 High concentrations of contaminants.
 Older release sites (i.e., significant elapsed time since contaminant release).
 Geologic settings where transmissive zones are only a small fraction of the total
volume of the aquifer.
 Aquifers with relatively slow groundwater flow rates.
 Sediments with high fraction organic carbon content.
 Presence of contaminants that exhibit stability in their physical setting.
 Release of large amounts of contaminants.
Advancing solvent plume
Low permeability silts
Transmissive sand
Expanding diffusion halo in stagnant zone
Simultaneous inward and outward diffusion in stagnant zones
Figure 7. Conceptual model of matrix diffusion effects as part of plume response (AFCEE,
2007).
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Site factors can be evaluated to qualitatively estimate if matrix diffusion effects are
expected to be significant. In general, there is potential for significant matrix diffusion
effects if NAPL, or the aqueous phase contaminant plume in the transmissive unit, has
been in direct contact with Low-Permeability material (i.e. fine-grained sands, silts, or
clays) or sedimentary rock for 20 years or more. Simple planning-level models such as
the “square root model” or the “Dandy Sale model” in the ESTCP Matrix Diffusion Toolkit
develop by GSI Environmental (Farhat et al., 2012), can be used to quantitatively
determine if matrix diffusion could be an important component at a site.
Although most groundwater research to date related to matrix diffusion has focused on
chlorinated solvent sites, other research has indicated potential matrix diffusion effects
for MTBE releases (Rasa, 2011). Interestingly, one of the earliest multiple-site research
studies, the 1995 California Leaking Underground Fuel Tank (LUFT) Historical Case
Analysis (Rice et al., 1995) may have found evidence of matrix diffusion effects when
they identified a category of “exhausted plumes” with low concentration, stable
benzene/toluene/xylenes/ethylbenzene plumes. The cause of the exhausted plumes was
never identified, but is consistent with matrix diffusion effects at old, weathered
hydrocarbon site plume zones.
If it is possible that any further source zone cleanup will be constrained by
matrix diffusion processes, then Question II.3 is answered “YES”.
3.2.4. Question II.4. Are sources relatively small?
Criteria: Plume is classified as a “Mag 4” Plume Magnitude Category or less based on
mass discharge estimates, OR maximum source concentrations are < 20x MCL.
Estimates of mass discharge (mass per time, also called mass flux) have become
increasingly valuable at sites with contaminated groundwater plumes (ITRC, 2010).
However, understanding the broader implication of flux measurements is not always
intuitive. Specifically, because mass discharge values lack context, it can be difficult to
communicate the magnitude and significance of mass flux/mass discharge to
stakeholders and decision makers.
New classification methodology has been developed (Newell et al., 2011) that bases
mass discharge on a “plume magnitude” (“Mag”) scale (see Table 1). Based on 10
different categories of mass discharge ranges, the system provides a simple contextual
method for understanding plume strengths. The classification system can assist site
managers in using site specific mass discharge to refine CSMs, prioritize sites,
determine potential impacts, and evaluate plumes both temporally and spatially. For
example, with this approach, a “Mag 4 Plume” was used to define a low-risk plume
because a Mag 4 plume cannot cause an exceedance of a 5 μg/L MCL in a drinking
water well pumping ≤ 100 gallons per minute due to mixing of clean water and the
plume.
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Table 1. Application of the Plume Magnitude Classification System.
Mass
Discharge
Plume
Low-Risk
(g/day)
Classification
Plume?
Impact*
< 0.001
Mag 1
Limited impact
YES
Could impact a domestic well, pumping at
0.001 to
Mag 2
YES
150 gallons per day (gpd) or less
0.01
Could impact a well pumping at 1 gallons
0.01 to 0.1
Mag 3
YES
per minute (gpm) or less
Could impact a well pumping at 10 gallons
0.1 to 1
Mag 4
YES
per minute (gpm) or less
Could impact a well pumping at 100 gpm
1 to 10
Mag 5
MAYBE
or less
Could impact a stream with a mixing zone
10 to 100
Mag 6
MAYBE
base flow of 1 cubic feet per second (cfs)
or less
100 to
Could impact a stream with a mixing zone
Mag 7
LIKELY NOT
1,000
base flow of 10 cfs or less
1,000 to
Could impact a stream with a mixing zone
Mag 8
LIKELY NOT
10,000
base flow of 100 cfs or less
10,000 to
Could impact a stream with a mixing zone
Mag 9
LIKELY NOT
100,000
base flow of 1,000 cfs or less
Could impact a stream with a mixing zone
>100,000
Mag 10
LIKELY NOT
base flow of >10,000 cfs
* Impact based on a drinking water standard in pumped water or mixing zone of 5 μg/L.
Natural attenuation (both biotic and abiotic) is assumed to be the main mechanism for
residual pollutant concentrations achieving cleanup standards within a reasonable timeframe. Based on a study of low-risk closures in the California San Francisco Bay area,
sites with residual concentrations less than or equal to 20 times the site cleanup
standard (e.g., the MCL), have a greater probability of achieving these standards in a
reasonable timeframe via natural attenuation (CRWQCB, 2009).
Using these two data sources, the LoRSC Manual defines a source as a small source if
the key constituent being discharged from the source is either:
1. A “Mag 4” plume or less based on the Plume Magnitude Classification System in
Table 1, AND/OR
2. The current maximum concentrations of key groundwater constituents in the
source zone are all less than ~20x their MCLs.
If the source is small, then Question II.4 is answered “YES”.
3.2.5. Question II.5. Are source zone concentrations stable or decreasing?
Criterion: Representative source zone concentrations over time are shown to be stable,
decreasing, or probably decreasing.
Representative concentrations could be average, geometric mean, or maximum
observed concentrations from each sampling event. Decreasing trends in source zone
wells can be demonstrated by:
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


Graphing natural log concentration (typically in mg/L or μg/L, or molar
concentration for parents+daughter compounds) vs. time for several source zone
wells (see Newell et al., 2002 for a discussion of this method for MNA analysis).
At least five years of temporal data are preferred to ensure enough time to
determine source zone trends. Spatial and temporal trends for both parent
compounds and key breakdown products (if any) should be evaluated.
Data can be analyzed using linear regression or non-parametric tests (such as
the Mann-Kendall test) and the methodology shown in Figure 8 (Aziz et al.,
2003). Several software tools, such as AFCEE’s Monitoring and Remediation
Optimization System (MAROS) program (Aziz et al., 2003) and the GSI MannKendall spreadsheet (Connor et al., 2012) are available to help site managers
make these types of computations.
Note that it is easy to confuse different types of rates in natural attenuation
analysis. A USEPA document is available that describes different types of rates
used in MNA evaluations and how to calculate them (Newell et al., 2002).
Figure 8. MAROS plume trend classification system. CF = Confidence Factor, S = MannKendall Statistic, and COV = Coefficient of Variation (Aziz et al., 2003).
If the average trend in all source zones wells (using a method such as the one employed
in MAROS and shown in Figure 8) is either “Probably Decreasing” or “Decreasing” or
“Stable” then the source zone concentrations indicate natural attenuation processes are
active.
If the source zone concentrations are stable, decreasing or probably
decreasing, then Question II. 5 is answered “YES”.
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3.2.6. Question II.6. Is there evidence of on-going source attenuation processes?
Criteria: Footprints of source zone attenuation are seen (such as generation of daughter
products or consumption of electron acceptors).
Since 2004, there has been increased emphasis on MNA as a remediation technology
for source zones, with the development of field studies, process information, models, and
protocols designed specifically for source zone attenuation. For this low-risk criterion,
“footprints” of natural attenuation describe indicators of change in groundwater other
than a decline in the concentration of the original contaminant in or near the source
zone. Examples include:







Depletion of oxygen, nitrate and sulfate indicate hydrocarbon degradation.
Low oxygen, nitrate, and sulfate concentrations indicate more anaerobic
geochemical conditions that support reductive dechlorination of many chlorinated
solvents such as trichloroethene (TCE) and tetrachloroethene (PCE).
Generation of cis-1,2-dichloroethene (cis-1,2-DCE) and other daughter
products indicates that biodegradation of TCE is occurring in groundwater.
Generation of 1,1-dichloroethene (1,1-DCE) indicates that abiotic degradation
of 1,1,1-trichloroethane is occurring in groundwater. Abiotic degradation is the
chemical transformation that degrades contaminants without microbial facilitation.
This can result in partial or complete degradation of contaminants. Typically, only
halogenated compounds are subject to these mechanisms in the groundwater
environment.
Presence of reactive minerals and soils that can abiotically degrade
chlorinated solvents, e.g., magnetite.
Changes in compound specific isotope ratios can provide supporting
evidence documenting that biodegradation or abiotic transformation processes
are actually destroying contaminants at the site (USEPA, 2008).
Genetic analyses of microbial populations can provide an optional line of
evidence supporting MNA. Members of the Dehalococcoides group of bacteria
are the only organisms known to date to completely degrade chlorinated ethenes
to harmless products. Therefore, for chlorinated solvent sites, the presence or
absence of these organisms can provide information on whether MNA is an
appropriate approach at a specific site (USEPA, 2006). Other metabolic and
genetic indicators can also demonstrate the presence of microbes capable of
cometabolic degradation of compounds.
Key references that discuss footprints and indicators of MNA include:
 The National Research Council’s (NRC) book on Natural Attenuation for
Groundwater Remediation (NRC, 2000).
 Scenarios Evaluation Tool for Chlorinated Solvent MNA (Truex et al., 2006) (e.g.,
see Figure 9).
 USEPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated
Solvents in Groundwater (USEPA, 1998).
 Technical Protocol for Implementing Intrinsic Remediation with Long-Term
Monitoring for Natural Attenuation of Fuel Contamination Dissolved in
Groundwater (Wiedemeier et al. 1999a).
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

Natural Attenuation of Fuels and Chlorinated Solvents (Wiedemeier et al.,
1999b).
Frequently Asked Questions about MNA in the 21st Century (Adamson et al,
2012).
Figure 9. Method for assessing the geochemical environment for groundwater chlorinated
solvent MNA (Truex et al., 2006). Note the authors of this document use the term
“anaerobic” for sites with conditions known to support reductive dechlorination, and the
term “anoxic” for more border-line but still low-oxygen conditions.
Note that while most of these protocols have focused on evaluating MNA in the plume, it
is the intent of this low-risk document to apply these specific MNA criteria (footprints of
natural attenuation) in or near the source zone.
If there is evidence of on-going natural attenuation processes in the source
zone, AND there are key footprints of natural attenuation in the source zone,
then Question II.6 is answered “YES”.
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3.2.7. Question II.7. Will future source remediation only marginally improve site
conditions?
Criteria: There is “Less Need for Source Treatment” based on weight of evidence from
the Qualitative Decision Chart (Figure 10).
In 2003, the USEPA convened an Expert Panel to evaluate the state of DNAPL site
remediation. The outcome from the Expert Panel was later modified for use as a chart in
the “Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soil
and Groundwater” document (Sale et al., 2008, p. 26). The chart uses a “weight-ofevidence” logic to resolve the relative need for source treatment. Primary reasons for
considering source treatment include reducing the potential for DNAPL migration,
decreasing source longevity, reducing loading to downgradient plumes, attainment of
MCLs, complying with regulations, and achieving intangible benefits. While developed
for chlorinated solvents, the chart can easily be adapted for hydrocarbon sites by
changing DNAPL to LNAPL in the first row.
One key consideration is that one must have realistic expectations for what source
remediation can provide and at what cost. This topic was covered as “Frequently Asked
Question 13” in Sale et al., (2008) for chlorinated solvent sites where the results from
several multiple-site remediation performance studies have indicated that chemical
oxidation, bioremediation, and thermal treatment projects have, as a very general rule,
reduced source concentrations by one, and sometimes two orders of magnitude (i.e.,
90% to 99%) (Sale et al., 2008). These studies include a 59-site study that included four
different types of in-situ remediation technologies (McGuire et al., 2006), a detailed
state-of-the-practice review of thermal treatment (Johnson et al., 2009), and a
comprehensive survey of chemical oxidation performance (Krembs et al., 2010). The
cost of treatment can be estimated using general unit costs (i.e., see Sale et al., 2008) or
by getting quotes from technology vendors.
Another tool that can be used to evaluate the benefits of source treatment is the
USEPA’s REMChlor model (Falta et al., 2007; Falta et al., 2005a and 2005b). This
simple analytical model can be used to estimate the impact of source zone remediation,
plume remediation, or combined source and plume remediation projects, plume
concentrations, and mass discharge rates.
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Figure 10. Qualitative Decision Chart on the merits of source depletion (Sale et al., 2008;
Adapted from USEPA’s “The DNAPL Remediation Challenge: Is There a Case for Source
Depletion?” (Kavanaugh et al., 2003).
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These same general concepts, as shown in Figure 10, and remediation performance
and cost information, as shown in Sale et al.’s FAQ 13 (2008), can often be applied to
other types of sites besides chlorinated solvent sites.
If the Qualitative Decision Chart in Figure 10, when used with a weight of
evidence approach, indicates “Less Need for Source Treatment”, then Question
II.7 is answered “YES”.
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3.3.
QUESTION III. WILL RESIDUAL CONTAMINATION HAVE NO ADVERSE
EFFECT ON PRESENT AND FUTURE LAND AND WATER USES?
3.3.1. Question III.1. Is the groundwater plume stable, decreasing, or probably
decreasing?
Criterion: Plume trend analysis showing stable, decreasing, or probably decreasing
plume over time using method in Figure 8.
Decreasing trends in plume wells can be demonstrated in a similar fashion as the
techniques for the source zone described in Section 3.2.5:
 Graphing natural log concentration vs. time for several plume zone wells. At least
five years of temporal data are preferred to ensure enough time to determine
source zone trends. Spatial and temporal trends for both parent compounds and
key breakdown products (if any) should be evaluated.
 Total mass and center of mass can be evaluated for plumes over time. Trend
analysis for total mass in the plume can demonstrate overall decreasing trends,
providing strong evidence for a shrinking plume. Data can be analyzed using
linear regression or non-parametric tests (such as the Mann-Kendall test).
Several software tools, such as AFCEE’s MAROS program (Aziz et al., 2003)
and the GSI Mann-Kendall spreadsheet (Connor et al., 2012) are available to
help site managers make these types of computations.
 Note that it is easy to confuse different types of rates in natural attenuation
analysis. A USEPA document is available that describes different types of rates
used in MNA evaluation and how to calculate them (Newell et al., 2002; Wilson
2011).
If the average trend in all plume wells (using a method such as the one employed in
MAROS and shown in Figure 8) is either “Probably Decreasing” or “Decreasing” or
“Stable” then the plume concentrations indicate natural attenuation processes are active.
If the groundwater plume is stable, decreasing, or probably decreasing, then
Question III.1 is answered “YES”.
3.3.2. Question III.2. Is there evidence of on-going natural attenuation processes
in the plume?
Criteria: Analysis of natural attenuation processes and footprints of natural attenuation
in the plume.
“Footprints” of natural attenuation describe indicators of change in groundwater other
than a decline in the concentration of the original contaminant in or near the plume. For
example:


Depletion of oxygen, nitrate and sulfate indicate hydrocarbon degradation.
Low oxygen, nitrate, and sulfate concentrations indicate more anaerobic
geochemical conditions that support reductive dechlorination of many chlorinated
solvents such as TCE and PCE.
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





Generation of cis-1,2-DCE indicates that biodegradation of TCE is occurring in
groundwater.
Generation of 1,1-DCE indicates that abiotic degradation of 1,1,1-trichloroethane
is occurring in groundwater. Abiotic degradation is the chemical transformation
that degrades contaminants without microbial facilitation. This can result in partial
or complete degradation of contaminants. Typically, only halogenated
compounds are subject to these mechanisms in the groundwater environment.
Presence of reactive minerals and soils that can abiotically degrade
chlorinated solvents, e.g., magnetite.
Changes in compound specific isotope ratios can provide supporting
evidence documenting that biodegradation or abiotic transformation processes
are actually destroying contaminants at the site (USEPA, 2008).
Genetic analyses of microbial populations can provide an optional line of
evidence supporting MNA. Members of the Dehalococcoides group of bacteria
are the only organisms known to date to completely degrade chlorinated ethenes
to harmless products. Therefore, for chlorinated solvent sites, the presence or
absence of these organisms can provide information on whether MNA is an
appropriate approach at a specific site (USEPA, 2006). Other metabolic and
genetic indicators can also demonstrate the presence of microbes capable of
cometabolic degradation of compounds.
A reduction in mass flux/mass discharge along the flow path (in both time and
space) can be used to indicate natural attenuation of the plume (USEPA, 1998).
Key references that discuss footprints and indicators of MNA include:







The NRC’s book on Natural Attenuation for Groundwater Remediation (NRC,
2000) (e.g., see Table 2).
Scenarios Evaluation Tool for Chlorinated Solvent MNA (Truex et al., 2006).
USEPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated
Solvents in Groundwater (USEPA, 1998).
Technical Protocol for Implementing Intrinsic Remediation with Long-Term
Monitoring for Natural Attenuation of Fuel Contamination Dissolved in
Groundwater (Wiedemeier et al. 1999a).
Natural Attenuation of Fuels and Chlorinated Solvents (Wiedemeier et al.,
1999b).
An Approach for Evaluating the Progress of Natural Attenuation in Groundwater
(Wilson, 2011).
Frequently Asked Questions about MNA in the 21st Century (Adamson et al,
2012).
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Table 2. Summary of Natural Attenuation Footprints at MNA Case Study Sites (NRC, 2000).
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The NRC (2000) compiled a list of sites where the footprint approach was employed,
and this summary is reproduced as Table 2 above.
If there is evidence of footprints of natural attenuation in the plume, then
Question III.2 is answered “YES”.
3.3.3. Question III.3. Are conditions protective of potential and future receptors?
Criteria: Analysis showing all exposure pathways for actual receptors are incomplete or
do not present excess risk, and that future exposure will not occur at levels above risk
criteria.
An assessment of actual receptors in the area needs to be developed to determine if any
exposure pathways are complete. Key exposure pathways are:



Groundwater ingestion through existing water supply wells.
Discharge to sensitive receiving (surface) waters.
Indoor air impacts results from the groundwater to indoor air pathway for existing
receptors.
Demonstration of protective conditions will usually include an evaluation of current and
possible future property use. The presence of deed notices, zoning restrictions and other
institutional controls limits potential exposure and can be used as one line of evidence
supporting low threat conditions. Additionally, an assessment of future hypothetical
receptors and exposure pathways, or an analysis that demonstrates that there will be no
future complete exposure pathways, supports a conclusion of no adverse effects from
the residual contamination.
Overall, this assessment should ensure that unacceptable risks to water quality, human
health, ecological, and sensitive receptors, both current and future, are identified and
mitigated. It should be demonstrated that the residual contamination present at the site
will not adversely impact current and future receptors. Evaluation of potential impacts to
current and future receptors should include (CRWQCB, 2009):
 Human health.
 Ecological exposure, e.g., aquatic life, wildlife, wetlands, crops, vegetation, and
habitats.
 Sensitive receptors.
 Downgradient groundwater.
 Downgradient surface water.
 Anticipated cross-media transfer exposures, e.g., non-domestic or agricultural
uses, indoor air vapor intrusion through volatilization, surface water or other
aquifer contamination through hydraulic connections.
 Changes in use or potential use of site or surrounding properties.
 Sensitive or vulnerable groundwater basins.
 Discussion of feasibility of existing or future engineering or institutional controls
applied to limit or prevent exposures.
 Vapor intrusion.
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Estimating mass discharge reaching a potential Point of Exposure (POE) such as a
water well or surface water body can be used to demonstrate protective conditions
(Einarson and Mackay, 2001; Newell et al., 2011).
Tools that can be used to estimate risks to receptors include the RBCA Tool Kit (GSI,
2007, see Figure 11 below). Off-site plumes will need special consideration to ensure no
illegal or uncontrolled access to residual contaminated media.
Figure 11. RBCA analyses for both potential and actual receptors (Figure A.3 from GSI,
2007).
Sites where residual contaminant concentrations are above groundwater cleanup
standards pose potentially unacceptable threats or risks, based on current or anticipated
use of land or water resources, and often necessitate risk management measures. Such
measures include institutional controls (e.g., land-use covenants, deed restrictions, and
soil management plans) and engineering controls (e.g., soil capping, fencing, sub slab
venting, and vapor barriers). Measures such as these are necessary to protect human
health and safety, and the environment.
Sites with risk management measures may be eligible for low-risk site closure provided:
 The risk management measure is appropriate for the site circumstances, both
current and future;
 The site meets all other closure criteria;
 Continued oversight from a regulatory agency is not required for the risk
management measure(s); and
 The risk management measures are robust, durable, and sustainable over time.
Placement of any institutional control on a property may require:
 Providing appropriate agencies with property information, e.g., title insurance and
survey of affected property.
 Verification with local municipalities that proposed use prohibitions are consistent
with local zoning requirements.
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
Providing all site stakeholders with notification of intent to install institutional
control. Documentation must be provided to appropriate agencies verifying the
notification of all site stakeholders.
Examples of risk management controls eligible for low-risk site closures include:
 Deed restrictions, for sites where only the drinking water standards have not
been met, prohibiting use of groundwater for drinking water.
 Zoning restrictions limiting property use to commercial or industrial.
 Voluntary protective measures, e.g., vapor barriers preventing potential indoor air
exposures due to soil gas intrusion. However, these protective measures cannot
be required to prevent an existing, eminent, or potential threat.
Conversely, some risk management measures may make a site ineligible for low-risk
closure, e.g.:
 Containment zones or other required waste-containment measure for high
concentration/high risk sites.
 Institutional controls prohibiting sensitive land use or restricting excavation or soil
trenching.
 Active engineering controls required to mitigate exposure to or prevent the
spread of the constituent residual concentrations.
All exposure pathways for actual receptors should be incomplete or present
acceptable risk, and an analysis should show there will be no unacceptable risks
in the future.
If current and future risks are zero or acceptable, then Question III.3 is answered
“YES”.
3.3.4. Question III. 4. Is there a near-term need for the impacted groundwater
resource or any impacted land uses?
Criteria: Evaluation of future needs for groundwater resource and associated overlying
land uses.
Evaluation of potential impacts to current and future water resources should be based on
best professional judgment and include review and documentation of (adapted from
CRWQCB, 2009):
 All relevant water resources publications.
 Local groundwater and surface water management plans.
 Groundwater protection and beneficial use evaluations.
 Domestic and agricultural water well locations.
 Municipal water supply and monitoring well locations.
 Consultations with local water agencies.
 Future beneficial use timeframes.
The current yield and water quality can be important factors in determining the nearterm uses for the groundwater resources. For example, the State of Texas defines
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groundwater with yield <150 gallons per day or total dissolved solids concentrations
>3000 mg/L as not being usable groundwater.
The AFCEE Sustainable Remediation Toolkit (SRT) (AFCEE, 2010) (Figure 12) provides
a method for estimating the economic value of water in a contaminated groundwater
unit, based on the estimated yield of the formation, volume of affected groundwater, and
other factors. Plumes in groundwater with little to no economic value may be considered
at low risk for future exploitation.
The hydraulic communication between a plume in a shallow unit with a deeper unit,
when aquifer data (e.g., pumping tests) are not available or not substantial, needs to be
considered. This can be done by:



Evaluating the local and regional stratigraphy and establishing if a competent
aquitard is present.
Employing tools such as the American Petroleum Institute’s Groundwater
Sensitivity Toolkit (Figure 13) (GSI, 2002) that account for vertical flow across an
aquitard or vertical flow in an artificial penetration such as an abandoned well.
Using indicators such as geochemistry, groundwater age, local discharge points,
and other factors.
Note that in some regions the beneficial uses assigned to groundwater basins and
surface water bodies do not differentiate between shallow and deeper groundwater
aquifers (CRWQCB, 2009). Typically in such cases, the shallow aquifer will have the
same beneficial use designation as the deeper aquifer unless an exception, as allowed
by state regulations, can be demonstrated. In certain instances, resource degradation,
such as pre-existing poor quality or salt-water intrusion due to excessive pumping, may
deem the use of a deeper aquifer impractical as a drinking water source.
Figure 12. Economic value normalization methodology of groundwater in the SRT (Newell
et al., 2008). PWS = Public Water Supply; WQP = Water Quality Protection; TDS = Total
Dissolved Solids.
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Figure 13. Architecture of the Groundwater Sensitivity Toolkit (GSI, 2002).
The residual contamination should show no large adverse economic impact or denial of
large-scale beneficial land or water use if residual contamination remains at the site.
If there are no adverse effects to land and water uses from the residual
contamination, then Question III.4 is answered “YES”.
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REDUCING LONG-TERM MONITORING INTENSITY
4.0
REDUCING LONG-TERM MONITORING INTENSITY
An alternative available when a complete low-risk/low-threat site closure is not justified is
the reduction in the long-term monitoring intensity at the site. Groundwater
characterization and remediation efforts at most sites result in large data sets and a
number of monitoring locations that may or may not be useful in long-term plume
management. Depending on the size and complexity of the plume, several tools and
techniques are available to improve the efficiency of groundwater monitoring networks.
When negotiating reduction in monitoring intensity, results of several qualitative and
quantitative evaluation strategies should be assembled to clarify the evidence for
streamlined data collection.
Qualitative evaluation strategies rely on expert professional opinion and understanding
of site-specific conditions. Tools for qualitative evaluation include decision logic trees,
assembling and comparing site data, and forming a ‘lines of evidence’ approach to site
data. Most long-term monitoring optimization (LTMO) techniques involve either an initial
or final qualitative evaluation of the monitoring program. Quantitative techniques include
statistical, geo-statistical and mathematical optimization methods. Common steps in
developing an optimized long-term monitoring network include:

Identification of site goals and objectives: Sampling strategies should provide
sufficient data to support site reporting and regulatory goals. Each sample should
address one or more monitoring objectives such as demonstrating containment
of the plume, attenuation of mass and protectiveness of the remedy. Sampling
frequency should be proportional to the rate of change of concentrations in the
plume and sufficient to satisfy the reporting frequency of regulatory programs.

CSM: A thorough CSM is essential to developing appropriate monitoring
strategies. LTMO techniques can be beneficial for sites where characterization
efforts and active remedial work are largely complete. Aspects of the CSM that
are of particular importance in LTMO strategies include source control,
delineation of the plume, consistent hydrogeological environment, contaminant
attenuation mechanisms and location of potential receptors.

Minimum data requirements: As a general rule, sites where minimum data
requirements are met (e.g., four to six separate sample events, between six and
15 monitoring locations) and where concentrations have largely stabilized are
good candidates for reduction in monitoring effort.

Plume stability: The determination that a groundwater plume is ‘stable’ can be
the prelude to a reduction in monitoring effort and frequency (USEPA, 1999;
USEPA, 2004; ITRC, 2007). Most methods recommended to demonstrate plume
stability include analyzing historic groundwater data from individual well
locations, preparation of contaminant concentration contour maps, concentration
vs. time, and concentration vs. distance graphs. Quantitative statistical
approaches to stability assessment include a review of summary statistics, data
distributions, and appropriate trend analyses for both individual monitoring
locations and plume-wide measures (Vanderford, 2010).
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
Attenuation Mechanisms: Qualitative approaches to demonstrating the
appropriateness of reduced monitoring efforts can include analyses of
geochemical biodegradation indicators and evaluating the sustainability of mass
destruction mechanisms (Chapelle et al., 2003; TNRCC, 1997).

Institutional and Land Use Controls: Restriction of access to groundwater and
elimination of potential exposure factors can be strong support for a reduction in
monitoring effort.
Software tools, such as the peer-reviewed and nationally recognized AFCEE
Geostatistical Temporal/Spatial (GTS) Optimization Algorithm (Cameron and Hunter,
No Date; Hunter, 2011) and MAROS (Aziz et al., 2003), are available to assist site
managers optimize LTMO analyses. A new AFCEE software tool, the 3-Tiered
Monitoring Optimization (3TMO) tool, will be available in the near future (Hunter,
2011). AFCEE has also developed a comprehensive Long-Term Monitoring
Optimization Guide (AFCEE, 2006) for effective identification and application of
appropriate LTMO strategies and optimization.
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REFERENCES
5.0
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Ground Water Monitoring and Remediation 25(1):100-112.
Adamson, D.T. and C.J. Newell, 2012. “Frequently Asked Questions About Monitored Natural
Attenuation in the 21st Century”. Environmental Security and Technology Certification
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AFCEE, 2006.
“Long-Term Monitoring Optimization Guide”. HQ Air Force Center for
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AFCEE, 2007. “AFCEE Source Zone Initiative”. Prepared by T.C. Sale, T.H. Illangasekare, J.
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Sustainable Remediation Tool (SRT), version 2.1, Brooks City-Base, San
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Aziz, J.J.; M. Ling, H.S. Rifai, C.J. Newell, and J.R. Gonzales, 2003. MAROS: a Decision
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Beckett, G. D., and P. Lundegard. 1997. “Practically Impractical—The Limits of LNAPL Recovery
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Connor, J.A. S.K. Farhat, M. Vanderford, and C.J. Newell, 2012. GSI Mann-Kendall Toolkit for
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Chapelle, F. H., M. A. Widdowson, J.S. Brauner, E. Mendez, and C.C. Casey, 2003.
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CDPHE, 2010. “Draft Guidance for the Closure of Low-Threat Sites with Residual Ground Water
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CRWQCB, 2009. “Assessment Tool for Closure of Low-Threat Chlorinated Solvent Sites”.
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Einarson, M.D. and D.M. Mackay. 2001. Predicting the Impacts of Groundwater Contamination,
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Falta, R.W., P.S. Rao, and N. Basu, 2005a. Assessing the Impacts of Partial Mass Depletion in
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Falta, R.W., N. Basu and P.S.C. Rao, 2005b. Assessing the Impacts of Partial Mass Depletion in
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Falta, R.W., M.B. Stacy, A.N.M. Ahsanuzzaman, M. Wang, and R.C. Earle, 2007. REMChlor
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Farhat, S.K., P.C. de Blanc, C.J. Newell, J.R. Gonzales, and J. Perez, 2004. SourceDK
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Farhat, S.K., C.J. Newell, M.A. Seyedabbasi, J.M. McDade, and N.T. Mahler, T.C. Sale, D.S.
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Johnson P., P. Dahlen, J. T. Kingston, E. Foote, and S. Williams, 2009. “Critical Evaluation of
State-of-the-Art In Situ Thermal Treatment Technologies for DNAPL Source Zone
Treatment”. Developed for the Environmental Security Technology Certification Program,
ESTCP Project ER-0314. May 2009.
Kavanaugh, M.C., S.C. Rao, L. Abriola, J. Cherry, G. Destouni, R. Falta, D. Major, J. Mercer, C.
Newell, T. Sale, S. Shoemaker, R. Siegrist, G. Teutsch, and K. Udell. 2003. “The DNAPL
Remediation Challenge: Is There a Case for Source Depletion?”, National Risk
Management Research Laboratory Report EPA/600/R-03/143.
Krembs, F.J., R.L. Siegrist, M. L. Crimi, R.F. Furrer, and B.G. Petri, 2010. Ground Water
Monitoring and Remediation, doi: 10.1111/j1745–6592.2010.01312.x.
Kueper, B. and K. Davies, 2009. “Assessment and Delineation of DNAPL Source Zones at
Hazardous Waste Sites”, U.S. EPA, EPA/600/R-09/119.
McGuire, T.M., McDade, J.M., and Newell C.J., 2006. Performance of DNAPL Source Depletion
Technologies at 59 Chlorinated Solvent-Impacted Sites, Ground Water Monitoring and
Remediation, 26(1): 73-84.
Mahler, N., T. Sale, and M. Lyverse, 2012. A Mass Balance Approach to Resolving LNAPL
Stability, Ground Water, doi: 10.1111/j.1745-6584.2012.00949.x
Newell, C.J., H.S. Rifai, J.T. Wilson, J.A. Connor, and J.J. Aziz, M.P. Suarez, 2002. “Calculation
and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies, USEPA
Remedial Technology Fact Sheet”, U.S. Environmental Protection Agency. EPA/540/S02/500, November 2002. http://www.epa.gov/ada/pubs/issue.html
Newell, C. J., E. Becvar, G. Moore, D. Ruppel, D. Woodward, T.N. Swann, L.M. Beckley, and A.
Rahman, 2008. Building Sustainability into the Air Force Remediation Process: Sustainable
Remediation Tool. Proceedings of the Sustainable Remediation Forum 8, Philadelphia, PA.
Oct 2008.
Newell, C. J., Farhat, S. K., Adamson, D. T. and Looney, B. B., 2011. Contaminant Plume
Classification System Based on Mass Discharge. Ground Water, 49: no. doi: 10.1111/j.17456584.2010.00793.x
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REFERENCES
NRC, 2000. Natural Attenuation for Groundwater Remediation, National Academy Press,
Washington, D.C.
Rasa, E., S. Chapman, B. Bekins, G. Fogg, K. Scow, and D. Mackay, 2011. Role of Back
Diffusion and Biogeochemical Reactions in Sustaining an MTBE/TBA Plume in Alluvial
Media, J. Contaminant Hydrology 126: 235-247.
Rice, D.W., R.D. Grose, J.C. Michaelsen, B.P. Dooher, D.H. MacQueen, S.J. Cullen, W.E.
Kastenberg, L.G. Everett, and M.A. Marino, 1995a. “California Leaking Underground Fuel
Tank (LUFT) Historical Case Analyses”, Environmental Protection Department.
Sale, T., C. Newell, H. Stroo, R. Hinchee, and P. Johnson, 2008. “Frequently Asked Questions
Regarding Management of Chlorinated Solvents in Soil and Groundwater”. Developed for the
Environmental Security Technology Certification Program (ESTCP), July 2008.
Sale, T. and C. Newell, 2011. “A Guide for Selecting REMEDIES FOR Subsurface Releases of
Chlorinated Solvents”. Developed for the Environmental Security Technology Certification
Program, ESTCP Project ER-05 30. March 2011.
Smith, T., T. Sale, and M. Lyverse, 2012. Measurement of LNAPL Flux Using Single-Well
Intermittent Mixing Tracer Dilution Tests, Ground Water, doi: 10.1111/j.17456584.2012.00931.x
TNRCC, 1997. Interim Guidance: Monitoring Natural Attenuation for Verification of Groundwater
Plume Stability, Texas Commission on Environmental Quality (formerly Texas Natural
Resource Conservation Commission): 5.
Truex, M.J., C.J. Newell, B.B. Looney, K.M. Vangelas, 2006. “Scenarios Evaluation Tool for
Chlorinated Solvent MNA”, WSRC-STI-2006-00096, Savannah River National Laboratory,
Aiken, South Carolina.
USEPA, 1991. “A Guide to Principal Threat and Low Level Threat Wastes”, US Environmental
Protection Agency, Washington DC.
USEPA, 1998. “Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in
Ground Water”, US Environmental Protection Agency, Washington DC.
USEPA, 1999. “Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and
Underground Storage Tank Sites”, United States Environmental Protection Agency,
Washington, D.C.
USEPA, 2004. “Performance Monitoring of MNA Remedies for VOCs in Ground Water”, US
Environmental Protection Agency, Cincinnati, OH, EPA/600/R-04/027.
USEPA, 2006. “Evaluation of the Role of Dehalococcoides Organisms in the Natural Attenuation
of Chlorinated Ethylenes in Ground Water”, US Environmental Protection Agency ,
Washington DC, EPA/600/R-06/029, July 2006.
USEPA, 2008. “A Guide for Assessing Biodegradation and Source Identification of Organic and
Ground Water Contaminants using Compound Specific Isotope Analysis (CSIA)”, US
Environmental Protection Agency, Washington DC, EPA/600/R-08/148, December 2008.
Vanderford, M., 2010. A Comprehensive Approach to Plume Stability. Remediation Winter 2010:
21-37.
Wilson, J.T., 2011. “An Approach for Evaluating the Progress of Natural Attenuation in
Groundwater”, US Environmental Protection Agency EPA 600/R-11/204, www.epa.gov/ada
Wiedemeier, T.H., J.T. Wilson, D.H. Kampbell, R.N. Miller, and J.E. Hansen, 1999a. “Technical
Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural
Attenuation of Fuel Contamination Dissolved in Groundwater”, Air Force Center for
Environmental Excellence, San Antonio, TX, March 1999.
Wiedemeier, T.H., H.S. Rifai, C.J. Newell, and J.T. Wilson, 1999b. Natural Attenuation of Fuels
and Chlorinated Solvents in the Subsurface. John Wiley and Sons, Inc., New York.
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CASE STUDIES
6.0
CASE STUDIES – FIELD APPLICATION OF LoRSC
MANUAL
6.1
Site 1. Old Base Landfill Site at Fairchild Air Force Base
6.2
Site 2. Fire Training Area 3 at Offutt Air Force Base
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CASE STUDY 1 – FAIRCHILD AFB
CASE STUDY 1
OLD BASE LANDFILL SITE
FAIRCHILD AIR FORCE BASE
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CASE STUDY 1 – FAIRCHILD AFB
1.0
EXECUTIVE SUMMARY
The AFCEE Low-Risk Site Closure (LoRSC) Manual methodology was applied to
determine if the Old Base Landfill Site, at Fairchild Air Force Base, Washington, could be
classified as a low-risk site. This information could then be used to assist site managers
build effective exit strategies for closing the site and/or reducing long-term monitoring
intensity.
The exit strategy for a given site can be effectively strengthened by multiple lines of
evidence; therefore, the LoRSC Manual provides weight-of-evidence decision logic to
build consensus between site stakeholders. The methodology was applied to the Old
Base Landfill Site. Three main categories of data were examined: 1) a comprehensive
Conceptual Site Model (CSM), 2) control of sources, and 3) adverse effects of residual
contamination on present and future land and water uses.
Based on an evaluation of existing data, the Old Base Landfill Site has:
 a comprehensive CSM,
 control of sources, and
 no potential for adverse effects of residual contamination on present and future
land and water uses.
Consequently, the site may be categorized as a LoRSC Site Type A, “Strongest case
for low-risk closure or reduced monitoring.”
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Table ES.1
LoRSC Manual Decision Logic
Low-Risk Decision Questions
I.
Answers For “Must
Have” Questions”
Key Low-Risk Decision Criteria
Answers For
“Supporting” Questions
Reference
Do You Have A Complete Conceptual Site Model (CSM) That Reflects Key Low-Risk Closure Concepts?
Conceptual Site Model checklist.
Yes ⊠
No ⧠
Section 3.1.1
1. Are there any significantly mobile source materials?
DNAPL sites: no mobile DNAPL observed. LNAPL sites: no expanding LNAPL zone and zero
or low LNAPL transmissivity.
Yes ⊠
No⧠
Section 3.2.1
2. Is the source zone free of any environmentally
significant quantity of NAPL?
Little or no DNAPL observed in transmissive zones, and no significant LNAPL accumulation
based on specific volume calculations.
Yes ⊠
No ⧠
Section 3.2.2
3. Is it possible that any further source zone cleanup will
be constrained by matrix diffusion processes?
Qualitative evaluation of matrix diffusion processes based on geology, chemical properties,
timing of initial release, and remediation efforts.
Yes ⊠
No ⧠
Section 3.2.3
4. Are sources relatively small?
Plume is classified as a Mag 4 Plume Magnitude Category or less based on mass discharge
estimates, OR maximum source concentrations are < 20x Maximum Concentration Limit (MCL).
Yes ⊠
No ⧠
Section 3.2.4
5. Are source zone concentrations stable or decreasing?
Representative source zone concentrations over time are shown to be stable, decreasing, or
probably decreasing.
Yes ⊠
No ⧠
Section 3.2.5
6. Is there evidence of on-going natural attenuation
processes in the source zone?
Footprints of source zone attenuation are seen (such as generation of daughter products or
consumption of electron acceptors).
Yes ⊠
No ⧠
Section 3.2.6
7. Will future source remediation only marginally improve
site conditions?
There is “Less Need For Source Treatment” based on the Qualitative Decision Chart.
1. Have all of the components of the Conceptual Site
Model been evaluated?
II.
III.
Are Sources Controlled?
Yes ⊠
No ⧠
Section 3.2.7
Will Residual Contamination Have No Adverse Effect on Present and Future Land and Water Uses?
1. Is the groundwater plume stable or shrinking?
Plume trend analyses showing decreasing plume over time.
Yes ⊠
No ⧠
Section 3.3.1
2. Is there evidence of on-going natural attenuation
processes in the plume?
Analyses of natural attenuation processes and footprints of natural attenuation in the plume.
Yes ⊠
No ⧠
Section 3.3.2
3. Are conditions protective of potential and future
receptors?
Analyses showing all exposure pathways for receptors are incomplete or present acceptable
risks, and that future exposure will not occur.
Yes ⊠
No ⧠
Section 3.3.3
Yes ⊠
No ⧠
Section 3.3.4
4. Is there a near-term need for the impacted groundwater Evaluation of future needs for groundwater resource and associated overlying land uses.
resource or any impacted land uses?
KEY:
“Must Have” Data: Critical Line of evidence for low-risk site closure - necessary to demonstrate these criteria at almost all sites if applicable.
“Supporting” Data: Supporting line of evidence, with 0-4 of the supporting lines recommended for low-risk site closure.
MUST HAVE:
All Yes?
Yes ⊠ (Type A or B)
No ⧠ (Type C)
SUPPORTING:
How Many “Yes”?
Type A if 3-4 Yes ⊠
Type B if 0-2 Yes ⧠
WHAT IT MEANS
LoRSC Site Type A (Strongest case for low-risk closure or reduced monitoring) = All “Must Have” Questions = Yes AND 3 or 4 of the “Supporting” Questions = Yes
LoRSC Site Type B (Moderately good case for low-risk closure or reduced monitoring) = All “Must Have” Questions = Yes AND 0 to 2 of the “Supporting” Questions = Yes
LoRSC Site Type C (More difficult for low-risk closure or reduced monitoring) = Any “Must Have” Question = No
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CASE STUDY 1 – FAIRCHILD AFB
2.0
BACKGROUND
Located approximately 12 miles west of Spokane, Washington, Fairchild Air Force Base
occupies approximately 4300 acres. The Old Base Landfill Site (SW-1, Site) is situated
on the western part of the base, adjacent to the west end of Parallel Taxiway B (Figure
1). Old Base Landfill (Site) was the main disposal area for the base in the 1950s and
wastes disposed there may have included industrial wastes, plating sludges, solvents,
lubricating oils, cutting oils, shavings, dry cleaning filters and spent filtrates, paint
wastes, ash, and miscellaneous sanitary wastes (Battelle, 1989). The Site is
approximately 16 acres in size, capped with a 1 to 3 ft thick non-engineered soil cover,
and contains an estimated 10 to 20 ft of mounded landfill (CH2MHill, 2004). Currently,
the landfill is a topographic high elevation in an area of relatively flat natural topography
(CH2MHill, 2004).
A Record of Decision (ROD) was signed in July 1993 with trichloroethene (TCE)
identified as the primary constituent of interest in groundwater. In accordance with the
ROD, institutional controls were implemented to prevent potential exposure and
consumption of contaminated groundwater at the Site. A long-term monitoring (LTM)
program was initiated in 1994. In 1998, with TCE concentrations routinely below the
cleanup level in seven wells, the number of LTM wells was reduced from ten to three,
with approval of the Washington State Department of Ecology and U.S. Environmental
Protection Agency (USEPA).
Provision of point-of-use treatment or an alternate water supply for offsite residences
located in the vicinity of the Site has not been necessary (CH2MHill, 2009a). TCE
concentrations at the last monitoring event (September 2008) were below groundwater
protection standards (CH2MHill, 2009b).
3.0
SITE ANALYSIS METHODOLOGY
The LoRSC Manual methodology was applied to determine if the Old Base Landfill SW-1
site could be classified as a low-risk site. This information can be used to assist site
managers build effective exit strategies for closing the site and/or reducing long-term
monitoring intensity. The exit strategy for a given site can be effectively strengthened by
multiple lines of evidence; therefore, the LoRSC Manual provides weight-of-evidence
decision logic to build consensus between site stakeholders.
The LoRSC Manual decision logic is based on identifying and examining three main
categories of data: 1) a comprehensive Conceptual Site Model (CSM), 2) control of
sources, and 3) adverse effects of residual contamination. Evaluation of the decision
logic is presented below.
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CASE STUDY 1 – FAIRCHILD AFB
Figure 1. Site location (Battelle, 1989, Figure ES-3).
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CASE STUDY 1 – FAIRCHILD AFB
3.1
QUESTION I. DO YOU HAVE A COMPLETE CSM THAT REFLECTS KEY
LOW-RISK CLOSURE CONCEPTS?
3.1.1 Question I.1. Have all of the components of the CSM been evaluated
Criteria: CSM checklist is complete
The site conceptual model, summarized in Figure 2, is described in detail in the
references cited below.
1. Site Information
Former Use: Landfill SW-1 was the main disposal site for the base from about
1949 to 1958. Covering approximately 16 acres, the landfill is 10-20 ft in depth,
and was used for disposal of all base waste including industrial wastes, plating
sludges, solvents, lubricating oils, dry-cleaning filters, paint wastes, and
miscellaneous sanitary wastes (SAIC, 1990a).
Current Use: The site is currently capped with a 1-3 foot thick non-engineered
soil cover and is a topographically high elevation in an area of relatively flat
natural topography (CH2MHill, 2006). The site is restricted to both the public and
Base personnel. Off-base agricultural, residential, or open space areas exist
around SW-1 (HEC, 1992).
Future Use: None anticipated (HEC, 1992).
2. Site Investigations
Soil borings:
 1989 - Drilled 3 boreholes, collected soil samples (SAIC, 1990b).
 Sep 1991 - Drilled and installed 11 new monitoring wells (HEC, 1993a).
 Oct 1991 - Excavated 8 Test Pits and collected 18 subsurface soil
samples (HEC, 1993a).
 Oct 1991 - Collected 11 surface soil samples (HEC, 1993a).
 Sep-Dec 1991 - Performed Quantitative Soil Gas Survey (HEC, 1993a).
Geophysical investigations:
 1986 – Metal detector, electromagnetic conductivity, magnetometry, and
ground-penetrating radar surveys performed (SAIC, 1990b).
 1990 – Magnetometry and refraction seismic surveys performed (SAIC,
1990b).
Site geochemistry: 1986/1987 (Battelle, 1989); 1991 (HEC, 1993a); 2008
(CH2MHill, 2009a).
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CASE STUDY 1 – FAIRCHILD AFB
Figure 2. Conceptual site model (CSM).
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CASE STUDY 1 – FAIRCHILD AFB
Figure 3. Areal extent of SW-1 Site (CH2MHill, 2009b, Figure 2.2-1).
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CASE STUDY 1 – FAIRCHILD AFB
Presence of off-site affected groundwater: Several residential wells in the vicinity
of site SW-1 have been periodically sampled for volatile organic compounds
(VOC) since 1986. TCE was the only contaminant detected above the maximum
concentration limit (MCL) during the 1991/1992 sampling, in wells located north
and northeast of the landfill (HEC, 1993a; CH2MHill, 2009b).
Evidence of Non-Aqueous Phase Liquids (NAPL): No NAPL has been detected
at the site.
Most recent NAPL observation: None.
3. Source Characterization
Primary source location: The primary source of contamination is the landfilled
materials (HEC, 1992).
Secondary source locations: Secondary sources are the potentially contaminated
surface soils that act as a source of surface water contamination through
intermittent surface water runoff (HEC, 1992).
Release mechanisms: Infiltration of precipitation and percolation through the
landfill, as well as biodegradation of landfill wastes, generates leachate which
transports and releases contaminants to groundwater (HEC, 1992).
Size and boundary: The areal extent of the landfill is approximately 16 acres as
shown in Figure 3 (CH2MHill, 2009b).
Substance released: TCE (HEC, 1993a).
Date of release: 1949 – 1958 (HEC, 1993a).
Volume and mass of substance(s) released: The volume and mass of TCE
released is unknown. However, the 1993 estimated volume of contaminated
groundwater within the shallow bedrock aquifer is 20 million gallons, assuming
an average depth of contamination of 28 feet, and an average bedrock porosity
of 0.1 percent (Air Combat Command, 1993).
Source control measures taken: None – Institutional controls were maintained
(i.e., restrictions against on-base usage of TCE-contaminated groundwater). Onsite groundwater and off-site water supply wells were monitored. Provision of
point-of-use treatment or an alternate water supply for offsite residences located
in the vicinity of the Site has not been necessary (CH2MHill, 2009a).
4. Constituents of Concern
TCE is the main chemical of concern in groundwater (HEC, 1993a).
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5. Nature and Extent of Contamination
During the September 2008 sampling event, TCE was the only VOC detected in
groundwater with concentrations ranging from 0.94 μg/L (MW-131) to 1.4 μg/L
(MW-309) (CH2MHill, 2009a). Vertical migration of TCE appears to be limited to
the upper portion of Basalt A, which contains Low-Permeability layers between a
depth of 21 and 100 feet, thereby restricting vertical migration of TCE (HEC,
1993a).
6. Hydrogeology
Stratigraphy: Sediments overlying the basalt are 6 to 25 ft thick and consist of
layers of silt, clay, sand, gravelly silt, and clayey sand (SAIC, 1990a). Landfill
material may approach 20 ft thickness in the central area due to mounding of the
fill material (HEC, 1993a). Basalt A was estimated to be about 166 ft thick, with
relatively few fractures down to approximately 100 ft below ground surface (bgs).
Finally, the silty claystone of Interbed A is approximately 10 ft thick at SW-1
(HEC, 1993a). See Figures 4a through 4c for cross-sections (HEC, 1993a).
Vadose (unsaturated) and saturated zone types: Fill material, silt, clay, sand,
gravelly sand, and clayey sand (Figures 4a through 4c) (HEC, 1993a).
Aquifer properties: Average hydraulic conductivity ranges between 150 to 250
ft/day. Using an estimated effective porosity of 25 percent and average gradient
of 0.0045, the estimated range of groundwater velocity within the coarse-grained
overburden is 2.7 to 4.5 ft/day (HEC, 1993a).
Confining unit soil type: Groundwater in the overburden is under unconfined to
locally semi-confined conditions (HEC, 1993a)
Depth to top of aquifer: Groundwater at SW-1 is encountered within bedrock
(Figures 4a through 4c). The water table in the area around the landfill has not
been observed to extend up into the overburden (HEC, 1993a).
Depth to groundwater: Groundwater in the site vicinity is found in the overburden
overlying bedrock, and in the bedrock itself. Groundwater is encountered at
approximately 8-12 ft bgs (HEC, 1993a).
Direction of groundwater flow including preferential pathways: The potential for
contamination migration from shallow zones of contamination to the basal portion
of Basalt A is very limited but not absent, and the volume of water migrating
through this deeper flow system is low (HEC, 1993a). Generally, groundwater
flow is to the east (ICF, 1995), but the landfill exerts a local mounding effect on
the groundwater with a localized radial groundwater flow pattern away from the
landfill (HEC, 1993a).
Recharge: The aquifer is considered a leaky confined aquifer with recharge
provided by the overlying aquitard (HEC, 1993a).
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Figure 4a. Cross-section, SW-1 Site, Fairchild Air Force Base, Washington (HEC, 1993a,
Figure 3-13).
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CASE STUDY 1 – FAIRCHILD AFB
Figure 4b. Cross-section, SW-1 Site, Fairchild Air Force Base, Washington (HEC, 1993a, Figure 3-14).
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CASE STUDY 1 – FAIRCHILD AFB
Figure 4c. Cross-section, SW-1 Site, Fairchild Air Force Base, Washington (HEC, 1993a, Figure 3-15).
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Proximity to surface waters: Surface water is present as storm water, snowmelt
runoff, and industrial wastewater. It does not infiltrate into the ground and is
channeled into the wastewater lagoons via drainage ditches (HEC, 1993a).
Fairchild AFB is located approximately 7 miles south of the Spokane River. The
two main surface drainages near Fairchild AFB are Deep Creek (2 miles
northwest of the Base) and Marshall Creek (8 miles to the southeast), both of
which flow northwards into the Spokane River (HEC, 1993a).
Interaction between groundwater and surface water: Runoff at SW-1 infiltrates
into the ground, mainly via a low area southeast of the landfill which accumulated
ponded surface water during storm events and period of high snowmelt runoff
(HEC, 1993a).
7. Geochemistry
Limited groundwater geochemical data were collected. Concentrations of
oxygen, nitrate, sulfate, and iron are summarized below.
Dissolved Oxygen: Stated in CH2MHill (2009a), but data not reported.
Nitrate/Nitrite: 1986/1987: 1.3 mg/L (Battelle, 1989).
Sulfate: 1986/1987: 25.8 mg/L (Battelle, 1989).
8. Migration and Exposure Pathways
Groundwater: Groundwater could affect drinking water wells of offsite residents
through ingestion, inhalation, and dermal contact (HEC, 1992).
Surface water: Surface water could affect Base personnel through ingestion,
inhalation, and dermal contact. Surface water runoff flows radially away from the
site. Runoff flowing southeast is collected by the storm drainage system and is
discharged to a tributary of Deep Creek (HEC, 1992).
Soil: Infiltration and percolation through soils may impact groundwater (HEC,
1992).
Air: Dust and/or volatile emissions could be inhaled by Base personnel and
residents (HEC, 1992).
Sediment: Same as surface waters (HEC, 1992).
Biota: Terrestrial biota could be exposed through air, surface waters, and
sediments via ingestion, inhalation, and dermal contact (HEC, 1992).
Site access is restricted; therefore, Base residents or offsite residents would not
be directly exposed to potentially contaminated surface soils. Offsite residents
may be indirectly affected via particulates migrating in the wind (HEC, 1992).
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CASE STUDY 1 – FAIRCHILD AFB
9. Contaminant Attenuation Pathways
No information available. Contaminant concentrations show a decreasing trend,
indicating that all attenuation pathways are active at this site.
10. Receptors
a) Human receptors: Off-base residents who use domestic water supply wells
downgradient of the site. Additionally, Base personnel may be exposed to
contaminated surface soils during the performance of assigned duties in the
vicinity of SW-1. Site access is restricted, however, and Base residents or
offsite residents would not be directly exposed to potentially contaminated
surface soils (HEC, 1992).
b) Ecological receptors: Terrestrial biota in the vicinity of or migrating through the
landfill (HEC, 1992).
c) Sensitive receptors: None identified (HEC, 1992).
d) Current and future groundwater and surface water resources: The water
supply wells serving the Base are located in the Base-owned well field
located approximately 10 miles northeast of the Base. The water from these
wells is used for domestic, industrial, and fire protection purposes as well as
for irrigation of base grounds. At least two residential wells are located in the
vicinity of SW-1. Residents in these areas do not currently have the option of
tapping into a public water supply system. There are no current plans to
further develop the groundwater aquifer underlying the Base as a water
supply resource for any purpose (HEC, 1992).
Intermittent surface water may exist as a result of precipitation, but a
permanent surface water source does not exist in the vicinity of SW-1.
11. Soil Remediation
No soil remediation was conducted at this site because minimal contamination
was detected in the surface and subsurface soil samples collected from the
landfill. Although the soil gas results suggested the presence of hot spot areas of
TCE and tetrachloroethene (PCE), these compounds were not detected in the
surface or subsurface soil samples (HEC, 1993a).
12. Groundwater Remediation
No groundwater remediation was conducted at the site.
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13. 14 Compartment Model
The qualitative 14 Compartment Model (Figures 5a and 5b) for this site identifies
all phases/zones that could potentially contain the contaminants.
The 14 Compartment Model was completed using the conditions stated below to
determine if mass is likely to be present in low-permeability compartments:
 The release at the site occurred between 1949 and 1958. Therefore,
significant time has elapsed since contaminant was released.
 The cross-sections do not show significant low-permeability lenses in the
source or plume area. Overall, it is expected that low-permeability lenses
or strata exist in the source area or the affected aquifer due to
heterogeneity of aquifer.
Source Zone
Zone/
Phases
Plume
Low
Permeability
Transmissive
Transmissive
Low
Permeability
Vapor
IP
IP
IP
IP
DNAPL
0
0
NA
NA
Aqueous
2
1
1
2
Sorbed
2
1
1
2
Legend:
Figure 5a. Depiction of SW-1 Site using the 14 Compartment Model. Higher numbers in
cells represent higher concentrations. Arrows represent mass transport between
compartments. (NA = Not Applicable; IP = Incomplete Pathway.)
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CASE STUDY 1 – FAIRCHILD AFB
Source Zone
Zone/
Phases
Plume
Low Permeability
Transmissive
Transmissive
Low Permeability
Vapor
Incomplete
Pathway
Incomplete
Pathway
Incomplete
Pathway
Incomplete
Pathway
DNAPL
Same as
Transmissive
Zone DNAPL
No NAPL observed
= “0”
NA
NA
Aqueous
Transmissive
zone “number” + 1
= “1”+1 = ”2”
Maximum observed
concentration in
source well = TCE
1.7 μg/L = “1”
Maximum
observed
concentration in
plume well = TCE
1.4 μg/L = “1”
Plume life >30 yrs,
therefore number =
“1”+1 = ”2”
Sorbed
Same as Source
Low-Permeability
Zone Aqueous
Same as Source
Transmissive Zone
Aqueous
Same as Plume
Transmissive
Zone Aqueous
Same as Plume
Low-Permeability
Zone Aqueous
Figure 5b. Methodology and decision logic used at this site on how to fill in the
concentrations for the 14-Compartment Model in Figure 5a. (NA = Not Applicable).
14. Stakeholders
U.S. Environmental Protection Agency (USEPA), AFCEE, Department of
Defense (DoD), and Washington State Department of Ecology.
Key Point Question I: The CSM includes all items 1-14 relevant to the Site, and
includes a qualitative 14 Compartment Model, therefore Question I.1 is
answered “YES”.
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CASE STUDY 1 – FAIRCHILD AFB
3.2
QUESTION II. ARE SOURCES CONTROLLED?
3.2.1. Question II.1. Are there any significantly mobile source materials?
Criteria: No mobile DNAPL. No Expanding LNAPL Zone.
There is no evidence of free phase product observation/measurement in the monitoring
wells or any other wells. Therefore, it is concluded that there are no significantly mobile
source materials.
Key Point: There are no significantly mobile NAPL in the source zone. Question
II.1 is answered “YES”.
3.2.2 Question II.2. Is the source zone free of any environmentally significant
quantity of NAPL?
Criteria: Little or no DNAPL observed in transmissive zones, and no significant LNAPL
accumulation based on specific volume calculations.
No NAPL has ever been detected beneath the site.
Key Point: There are no environmentally significant quantities of NAPL in the
source zone. Question II.2 is answered “YES”.
3.2.3. Question II.3. Is it possible that any further source zone cleanup will be
constrained by matrix diffusion processes?
Criteria: Qualitative evaluation of matrix diffusion processes based on geology, chemical
properties, timing of initial release, and remediation efforts.
The potential for matrix diffusion effects can be seen at virtually any site with
heterogeneity in the subsurface, NAPL, and/or where persistent groundwater
contaminant concentrations after source-zone remediation have been observed. At
SW-1, time of release and soil/groundwater remediation was evaluated as well as the
lithology and cross-sections. Key factors favoring matrix diffusion at this site include:


Release at the site occurred between 1949 and 1958. Therefore, significant time
has elapsed since contaminant was released.
While cross-sections do not show large-scale low-permeability lenses in the
source or plume area. Overall, it is expected that heterogeneity is typical in this
hydrogeologic setting, and that matrix diffusion effects would likely affect future
remediation at some point.
Key Point: It is possible that any further source zone cleanup will be
constrained by matrix diffusion processes. Question II.3 is answered “YES”.
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CASE STUDY 1 – FAIRCHILD AFB
3.2.4. Question II.4. Are sources relatively small?
Criteria: Plume is classified as a “Mag 4” Plume Magnitude Category or less based on
mass discharge estimates, OR maximum source concentrations are < 20x MCL.
In 2008, a maximum of 1.4 μg/L of TCE (MCL = 5 μg/L) was detected in the monitoring
wells located both in the source and plume areas. Since March 2005, TCE
concentrations at MW-90/MW-309 have been below 5 μg/L. Concentrations at MW-131
and MW-132 have been below 5 μg/L since 1998 and 1994, respectively. Monitoring at
the other wells was suspended because of historically low detections of TCE.
Key Point: Source is small. Question II.4 is answered “YES”.
3.2.5. Question II.5. Are source zone concentrations stable or decreasing?
Criterion: Representative source zone concentrations over time are shown to be stable,
decreasing, or probably decreasing.
In 1998, with the concurrence of Washington State Department of Ecology and the
USEPA, the number of wells sampled at SW-1 was reduced from ten to three (MW90/MW-309, MW-131, and MW-132), as detected concentrations of TCE in the other
seven monitoring wells were routinely below the MCL of 5 μg/L (CH2MHill, 2006). The
concentration vs. time plot for TCE in the currently monitored source well (MW-131) is
shown on Figure 6.
Trend analysis was performed using the non-parametric Mann-Kendall methodology (as
developed for AFCEE’s Monitoring and Remediation Optimization System (MAROS)
program). For this purpose, a value of half the detection limit was substituted for the nondetect results. Based on the trend analysis, the source zone well exhibits a “Decreasing”
trend.
Consequently, it is concluded that natural attenuation processes are active in the source
zone and the source zone concentrations are stable or decreasing.
Key Point: Source zone concentrations are decreasing.
answered “YES”.
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CASE STUDY 1 – FAIRCHILD AFB
GSI MANN-KENDALL TOOLKIT
for Constituent Trend Analysis
GSI Environmental Inc., Houston, Texas
Date Analyzed: 18-Jun-12
Facility: SW-01: Fairfield AFB
Well Identification:
Distance from Source (ft):
Sampling
Event
Constituent: Trichloroethene
Concentration Units: ug/L
MW-131
TRICHLOROETHENE CONCENTRATION (ug/L)
Date
1
Jun-91
2
Dec-91
3
Feb-94
4
May-94
5
Aug-94
6
Nov-94
7
Apr-95
8
Jun-95
9
Sep-95
10
Dec-95
11
Mar-96
12
Sep-96
13
Mar-97
14
Sep-97
15
Mar-98
16
Sep-98
17
Mar-99
18
Sep-99
19
Mar-00
20
Sep-00
21
Mar-01
22
Sep-01
23
Mar-02
24
Sep-02
25
Mar-03
26
Sep-03
27
Mar-04
28
Sep-04
29
Mar-05
30
Mar-06
31
Sep-06
32
Sep-07
33
Sep-08
Coefficient of Variation:
Mann-Kendall Statistic (S):
Confidence Factor:
Concentration Trend:
18
11
4.2
7.9
3.5
5.1
10
9.2
9
9
4
5.8
6
6
4
4
3.5
3.5
2
4.5
2.7
1.9
6.6
6.3
3.5
3.4
2.2
2.4
1.7
0.73
2.1
1.2
0.94
0.72
-333
>99.9%
Decreasing
Concentration (ug/L)
100
MW-131
10
1
0.1
05/07/90
01/31/93
10/28/95
07/24/98
04/19/01
01/14/04
10/10/06
07/06/09
04/01/12
Sampling Date
Notes:
1. At least four independent sampling events per well are required for calculating the trend. Methodology is only valid for 4 to 40 samples.
2. Confidence in Trend = Confidence (in percent) that constituent concentration is increasing (S>0) or decreasing (S<0).
≥ 90% = Probably Increasing or Decreasing; >95% = Increasing or Decreasing.
3. Methodology based on "MAROS: A Decision Support System for Optimizing Monitoring Plans", J.J. Aziz, M. Ling, H.S. Rifai, C.J. Newell,
and J.R. Gonzales, Ground Water , 41(3):355-367, 2003.
4. Values in bold represent detected values. Values in italics represent values below the detection limit and are shown as half the detection limit. No values
were below the detection limit for this monitoring location.
Figure 6. Source area concentration vs. time plots and Mann-Kendall analysis.
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CASE STUDY 1 – FAIRCHILD AFB
3.2.6. Question II.6. Is there evidence of on-going source attenuation processes?
Criteria: Footprints of source zone attenuation are seen (such as generation of daughter
products or consumption of electron acceptors).
There is evidence that natural attenuation of TCE is occurring to some degree, as
dissolved oxygen and oxidation-reduction potential field readings indicate that anaerobic
conditions exist at least seasonally, and cis-1,2-dichloroethene (cis-1,2-DCE, a
biodegradation daughter product of TCE) has been detected (CH2MHill, 2006, 2009a).
Key Point: Source zone attenuation is occurring as evidenced by the generation
of daughter product cis-DCE and presence of anaerobic conditions. Question
II.6 is answered “YES”.
3.2.7. Question II.7. Will future source remediation only marginally improve site
conditions?
Criteria: There is “Less Need For Source Treatment” based on weight of evidence from
the Qualitative Decision Chart (Figure 10 of LoRSC Manual).
Future source remediation will have very little to no effect on site conditions as
groundwater concentrations are already below MCL. Since March 2005, TCE
concentrations at MW-90/MW-309 have been below 5 μg/L. Concentrations at MW-131
and MW-132 have been below 5 μg/L since 1998 and 1994, respectively. Monitoring at
the other wells has been suspended due of historically low detections of TCE.
Based on the Qualitative Decision Chart, a weight of evidence of 8 was obtained,
therefore, there is “Less Need For Source Treatment” (Figure 7).
Key Point: Future source zone remediation will only marginally improve site
conditions. Question II.7 is answered “YES”.
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CASE STUDY 1 – FAIRCHILD AFB
Figure 7. Qualitative Decision Chart on the merits of source depletion (Sale et al., 2008;
Adapted from USEPA’s “The DNAPL Remediation Challenge: Is There a Case for Source
Depletion?” (Kavanaugh et al., 2003). (3c was selected due to the low concentrations and
evidence of on-going MNA; 5c was selected because remediation of the low
concentrations in the source zone will not significantly reduce time to reach MCLs; 6c was
selected assuming a 16 acre site and remediation cost of $3MM/acre (Sale et al., 2008)).
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CASE STUDY 1 – FAIRCHILD AFB
3.3.
QUESTION III. WILL RESIDUAL CONTAMINATION HAVE NO ADVERSE
EFFECT ON PRESENT AND FUTURE LAND AND WATER USES?
3.3.1. Question III.1. Is the groundwater plume stable, decreasing, or probably
decreasing?
Criterion: Plume trend analysis showing stable, decreasing, or probably decreasing
plume over time using method in Figure 8 of LoRSC Manual.
Trend analysis was performed using the non-parametric Mann-Kendall methodology (as
developed for AFCEE’s Monitoring and Remediation Optimization System (MAROS)
program). For this purpose, a detection limit of 50% was substituted for the non-detect
results. Based on the trend analysis, the plume zone wells (MW-90/MW-309 and MW132) both exhibit “Decreasing” trends (Figure 8).
Consequently, it is concluded that natural attenuation processes are active in the plume
and plume concentrations are stable or decreasing.
Key Point: Plume zone concentrations are decreasing.
answered “YES”.
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CASE STUDY 1 – FAIRCHILD AFB
GSI MANN-KENDALL TOOLKIT
for Constituent Trend Analysis
GSI Environmental Inc., Houston, Texas
Date Analyzed: 18-Jun-12
Facility: SW-01: Fairfield AFB
Well Identification:
Distance from Source (ft):
Sampling
Event
Constituent: Trichloroethene
Concentration Units: ug/L
MW-90/MW-309
MW-132
TRICHLOROETHENE CONCENTRATION (ug/L)
Date
1
Aug-90
2
Feb-91
3
Apr-91
4
Jun-91
5
Dec-91
6
Feb-94
7
May-94
8
Aug-94
9
Nov-94
10
Apr-95
11
Jun-95
12
Sep-95
13
Dec-95
14
Mar-96
15
Sep-96
16
Mar-97
17
Sep-97
18
Mar-98
19
Sep-98
20
Mar-99
21
Sep-99
22
Mar-00
23
Sep-00
24
Mar-01
25
Sep-01
26
Mar-02
27
Sep-02
28
Mar-03
29
Sep-03
30
Mar-04
31
Sep-04
32
Mar-05
33
Sep-05
34
Mar-06
35
Sep-06
36
Sep-07
37
Sep-08
Coefficient of Variation:
Mann-Kendall Statistic (S):
Confidence Factor:
Concentration Trend:
10
4
11
6
12
8
6.3
3.4
7.4
6.5
8.8
14
10
9.2
10
7.2
8.7
8
5
10
7.3
5.7
7.8
9.2
6.7
6.8
11
8.3
7.8
8.7
5.8
5.5
0.77
1.3
1.6
1.9
1.7
1.4
1
1.9
1.8
1
1
1
1
1
1
1
1
1
0.9
0.8
0.72
1
0.88
1.3
0.43
1.6
0.96
1.1
0.54
0.34
0.37
0.43
0.76
0.88
0.32
0.75
0.47
-211
99.8%
1.51
-265
>99.9%
Decreasing
Decreasing
100
MW-90/MW-309
Concentration (ug/L)
MW-132
10
1
0.1
08/11/87
05/07/90
01/31/93
10/28/95
07/24/98
04/19/01
01/14/04
10/10/06
07/06/09
04/01/12
Sampling Date
Notes:
1. At least four independent sampling events per well are required for calculating the trend. Methodology is only valid for 4 to 40 samples.
2. Confidence in Trend = Confidence (in percent) that constituent concentration is increasing (S>0) or decreasing (S<0).
≥ 90% = Probably Increasing or Decreasing; >95% = Increasing or Decreasing.
3. Methodology based on "MAROS: A Decision Support System for Optimizing Monitoring Plans", J.J. Aziz, M. Ling, H.S. Rifai, C.J. Newell,
and J.R. Gonzales, Ground Water , 41(3):355-367, 2003.
4. Values in bold represent detected values. Values in italics represent values below the detection limit and are shown as half the detection limit.
Figure 8. Plume area concentration vs. time plots and Mann-Kendall analysis.
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CASE STUDY 1 – FAIRCHILD AFB
3.3.2. Question III.2. Is there evidence of on-going natural attenuation processes
in the plume?
Criteria: Analysis of natural attenuation processes and footprints of natural attenuation
in the plume.
There is evidence that natural attenuation of TCE is occurring to some degree at MW132, as dissolved oxygen and oxidation-reduction potential field readings indicate that
anaerobic conditions exist at least seasonally, and cis-DCE, an anaerobic daughter
product of TCE was detected (CH2MHill, 2006, 2009a).
Key Point: Plume zone attenuation is occurring as evidenced by the generation
of daughter product cis-DCE and presence of anaerobic conditions. Question
III.2 is answered “YES”.
3.3.3. Question III.3. Are conditions protective of potential and future receptors?
Criteria: Analysis showing all exposure pathways for actual receptors are incomplete or
do not present excess risk, and that future exposure will not occur at levels above risk
criteria.
The majority of the site is restricted to the public and Base personnel. There is no
anticipated change in the future site use. Because contaminant concentrations at wells
are less than MCLs for TCE, conditions are protective of potential and future receptors.
Key Point: Conditions are protective of potential and future receptors. Question
III.3 is answered “YES”.
3.3.4. Question III.4. Is there a near-term need for the impacted groundwater
resource or any impacted land uses?
Criteria: Evaluation of future needs for groundwater resource and associated overlying
land uses.
There is no anticipated change in the future site use. The site is restricted to the public
and Base personnel. There are no plans to convert the property to civilian use.
Key Point: There is no anticipated near-term need for the impacted groundwater
resource or any impacted land uses. Question III.4 is answered “YES”.
4.0
SITE ASSESSMENT CONCLUSION
The exit strategy for a given site can be effectively strengthened by multiple lines of
evidence. The AFCEE LoRSC manual provides a weight-of-evidence decision logic to
evaluate such lines of evidence.
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CASE STUDY 1 – FAIRCHILD AFB
The LoRSC Manual methodology was applied to the Old Base Landfill Site at Fairchild
Air Force Base, Washington. Three main categories of data were examined: 1) a
comprehensive CSM, 2) control of sources, and 3) adverse effects of residual
contamination on present and future land and water uses.
Based on an evaluation of existing data, the Old Base Landfill Site has:
 a comprehensive CSM,
 control of sources, and
 no potential for adverse effects of residual contamination on present and future
land and water uses.
Consequently, the site may be categorized as a LoRSC Site Type A, “Strongest case
for low-risk closure or reduced monitoring.”
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5.0
REFERENCES
Air Combat Command, 1993. Final Feasibility Study Report, On-Base Priority One
Operable Units Volume I of II, prepared by Air Combat Command, February 1993.
Battelle, 1989. Phase II Confirmation/Quantification Stage 1 Final Report Volume I,
prepared by Battelle Columbus Division, April 1989.
CH2MHill, 2004. Remedial Action Operations, 20034 Annual Report, Old Base Landfill,
Site SW-1 Fairchild Air Force Base, prepared by CH2MHill, October 2004.
CH2MHill, 2005. Remedial Action Operations, 2004 Annual Report, Old Base Landfill,
Site SW-1 Fairchild Air Force Base, prepared by CH2MHill, June 2005.
CH2MHill, 2006. Fourth Quarter and Annual 2005 Remedial Action Operations Report,
Priority One and Priority Two Sites Fairchild Air Force Base, Washington, prepared
by CH2MHill, May 2006.
CH2MHill, 2009a. Remedial Action Operations Report, Fourth Quarter and Annual 2008,
Priority One and Priority Two Sites Fairchild Air Force Base, Washington, prepared
by CH2MHill, July 2009.
CH2MHill, 2009b. Remedial Action Operations Report, First Quarter 2009, Priority One
and Priority Two Sites Fairchild Air Force Base, Washington, prepared by CH2MHill,
July 2009.
HEC, 1992. Installation Restoration Program – Conceptual Site Models for Priority One
Operable Units FT-1, IS-1, OU-1, SW-1 and WW-1, Fairchild Air Force Base,
Washington, prepared by Halliburton NUS Environmental Corporation, February
1992.
HEC, 1993a. Remedial Investigation Report Priority One Operable Units, Fairchild Air
Force Base, Washington, prepared by Halliburton NUS Environmental Corporation,
February 1993.
HEC, 1993b. Record of Decision On-Base Priority One Operable Units Final, prepared
by Halliburton NUS Environmental Corporation, June 1993.
ICF, 1995. Long Term Monitoring Report for Priority 1 Sites at Fairchild Air Force Base,
Washington, prepared by ICF Technology Inc., June 1995.
SAIC, 1990a. Remedial Investigation/Feasibility Study, Priority 1 Sites, Fairchild Air
Force Base, Washington, prepared by Science Applications International
Corporation, May1990.
SAIC, 1990b. Site Characterization Summary, Priority 1 Sites, Fairchild Air Force Base,
Washington, prepared by Science Applications International Corporation, December
1990.
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CASE STUDY 2 – OFFUTT AFB
CASE STUDY 2
SITE FT-03 – FIRE TRAINING AREA 3
OFFUTT AIR FORCE BASE
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CASE STUDY 2 – OFFUTT AFB
1.0
EXECUTIVE SUMMARY
The exit strategy for a given site can be effectively strengthened by multiple lines of
evidence. The AFCEE Low-Risk Site Closure (LoRSC) Manual provides a weight-ofevidence decision logic to evaluate such lines of evidence.
The LoRSC Manual methodology was applied to the Fire Training Area 3 Site at Offutt
Air Force Base, Nebraska. Three main categories of data were examined: 1) a
comprehensive Conceptual Site Model (CSM), 2) control of sources, and 3) adverse
effects of residual contamination on present and future land and water uses.
Based on an evaluation of existing data, the Fire Training Area 3 has:
 a comprehensive CSM,
 control of sources, and
 no potential for adverse effects of residual contamination on present and future
land and water uses.
Consequently, the site may be categorized as a LoRSC Site Type A, “Strongest case
for low-risk closure or reduced monitoring.”
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CASE STUDY 2 – OFFUTT AFB
Table ES.1
LoRSC Manual Decision Logic
Low-Risk Decision Questions
III.
Answers For “Must
Have” Questions”
Key Low-Risk Decision Criteria
Answers For
“Supporting” Questions
Reference
Do You Have A Complete Conceptual Site Model (CSM) That Reflects Key Low-Risk Closure Concepts?
Conceptual Site Model checklist.
Yes ⊠
No ⧠
Section 3.1.1
1. Are there any significantly mobile source materials?
DNAPL sites: no mobile DNAPL observed. LNAPL sites: no expanding LNAPL zone and zero
or low LNAPL transmissivity.
Yes ⊠
No⧠
Section 3.2.1
2. Is the source zone free of any environmentally
significant quantity of NAPL?
Little or no DNAPL observed in transmissive zones, and no significant LNAPL accumulation
based on specific volume calculations.
Yes ⊠
No ⧠
Section 3.2.2
3. Is it possible that any further source zone cleanup will
be constrained by matrix diffusion processes?
Qualitative evaluation of matrix diffusion processes based on geology, chemical properties,
timing of initial release, and remediation efforts.
Yes ⧠
No ⊠
Section 3.2.3
4. Are sources relatively small?
Plume is classified as a Mag 4 Plume Magnitude Category or less based on mass discharge
estimates, OR maximum source concentrations are < 20x Maximum Concentration Limit (MCL).
Yes ⊠
No ⧠
Section 3.2.4
5. Are source zone concentrations stable or decreasing?
Representative source zone concentrations over time are shown to be stable, decreasing, or
probably decreasing.
Yes ⊠
No ⧠
Section 3.2.5
6. Is there evidence of on-going natural attenuation
processes in the source zone?
Footprints of source zone attenuation are seen (such as generation of daughter products or
consumption of electron acceptors).
Yes ⊠
No ⧠
Section 3.2.6
7. Will future source remediation only marginally improve
site conditions?
There is “Less Need For Source Treatment” based on the Qualitative Decision Chart.
1. Have all of the components of the Conceptual Site
Model been evaluated?
IV. Are Sources Controlled?
III.
Yes ⊠
No ⧠
Section 3.2.7
Will Residual Contamination Have No Adverse Effect on Present and Future Land and Water Uses?
1. Is the groundwater plume stable or shrinking?
Plume trend analyses showing decreasing plume over time.
Yes ⊠
No ⧠
Section 3.3.1
2. Is there evidence of on-going natural attenuation
processes in the plume?
Analyses of natural attenuation processes and footprints of natural attenuation in the plume.
Yes ⊠
No ⧠
Section 3.3.2
3. Are conditions protective of potential and future
receptors?
Analyses showing all exposure pathways for receptors are incomplete or present acceptable
risks, and that future exposure will not occur.
Yes ⊠
No ⧠
Section 3.3.3
Yes ⊠
No ⧠
Section 3.3.4
4. Is there a near-term need for the impacted groundwater Evaluation of future needs for groundwater resource and associated overlying land uses.
resource or any impacted land uses?
KEY:
“Must Have” Data: Critical Line of evidence for low-risk site closure - necessary to demonstrate these criteria at almost all sites if applicable.
“Supporting” Data: Supporting line of evidence, with 0-4 of the supporting lines recommended for low-risk site closure.
MUST HAVE:
All Yes?
Yes ⊠ (Type A or B)
No ⧠ (Type C)
SUPPORTING:
How Many “Yes”?
Type A if 3-4 Yes ⊠
Type B if 0-2 Yes ⧠
WHAT IT MEANS
LoRSC Site Type A (Strongest case for low-risk closure or reduced monitoring) = All “Must Have” Questions = Yes AND 3 or 4 of the “Supporting” Questions = Yes
LoRSC Site Type B (Moderately good case for low-risk closure or reduced monitoring) = All “Must Have” Questions = Yes AND 0 to 2 of the “Supporting” Questions = Yes
LoRSC Site Type C (More difficult for low-risk closure or reduced monitoring) = Any “Must Have” Question = No
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2.0
BACKGROUND
Fire Training Area 3 (Site, FTA3) is located in the northeastern part of Offutt Air Force
Base, Nebraska (Figure 1). Open fields surround FTA3 on the north and west, Offutt
AFB air field on the south, and Base Lake on the east. FTA3 comprises of the former
fire training burn pit and the foundations of the former smokehouse. The fire training
area burn pit was at the center of the Site, a 3-ft deep depression approximately 200 ft in
diameter, where the Air Force completed fire protection training exercises from 1960
until the spring of 1990. A mock aircraft fuselage, removed in about 1995, was located at
the center of the pit. Building 654 (former smokehouse) was located west of the former
burn pit and removed prior to 1990. Wastewater from the main burn pit was discharged
to a shallow depression (discharge pond) north of the main burn pit via a drainpipe. The
discharge pond was excavated and removed in the fall of 2004 as part of the Hardfill 6
removal project (URS, 2011c).
Investigations completed in the 1990s indicated the presence of chlorinated solvent and
petroleum hydrocarbon soil and groundwater contamination due to the former
operations. A Statement of Basis was issued in November 2004 by the United States
Environmental Protection Agency (USEPA) specifying contaminated soil removal,
Monitored Natural Attenuation monitoring, and administrative land use controls (LUC)
(e.g., digging and drinking water restrictions) as the final remedy (URS, 2011c).
Source area soils were excavated in 2004, and in 2006, with USEPA’s approval, pulsed
bio-sparging was conducted to treat the residual groundwater contamination. Source
area pulsed bio-sparging was terminated in 2007 with a Response Complete approval
from USEPA. In 2008, the RCRA Permit was modified to include the old discharge pond
area and its associated groundwater monitoring. A distal area bio-sparging system was
installed in July 2008 and terminated in December 2009 when it was determined that
continued operation of the remedy was not warranted (URS, 2011c).
3.0
SITE ANALYSIS METHODOLOGY
The LoRSC Manual methodology was applied to determine if the Fire Training Area 3
site could be classified as a low-risk site. This information can be used to assist site
managers build effective exit strategies for closing the site and/or reducing long-term
monitoring intensity. The exit strategy for a given site can be effectively strengthened by
multiple lines of evidence; therefore, the LoRSC Manual provides weight-of-evidence
decision logic to build consensus between site stakeholders.
The LoRSC Manual decision logic is based on identifying and examining three main
categories of data: 1) a comprehensive Conceptual Site Model (CSM), 2) control of
sources, and 3) adverse effects of residual contamination. Evaluation of the low-risk site
decision logic is presented below.
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Figure 1. Site location map (URS, 2011c, Figure 1-2).
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3.1
QUESTION I. DO YOU HAVE A COMPLETE CSM THAT REFLECTS KEY
LOW-RISK CLOSURE CONCEPTS?
3.1.1 Question I.1. Have all of the components of the CSM been evaluated
Criteria: CSM checklist is complete
The site conceptual model, summarized in Figure 2, is described in detail in the
references cited below.
1. Site Information
Former Use: Fire Training Exercise Area (1960-1991). From 1960 to 1974,
training exercises were completed at the frequency of about once per week.
Waste fuels and solvents were used during the training exercises. After 1974,
exercises were conducted at a frequency of twice per calendar quarter (URS,
2006b).
Current Use: The Base runway and East Gate Drain to the east, open fields and
Landfill 5 are located to the west, the HF6 SWMU to north, and the Base Lake to
east (700 feet). The majority of the site is restricted to the public and Base
personnel (URS, 2006b).
Future Use: None anticipated due to proximity to runway (URS, 2006b).
2. Site Investigations
Site Investigations:
 1988-2011: Annual and Semi-annual groundwater monitoring of vinyl
chloride conducted (URS, 2011c; URS, 2006a).
 1988: “An investigation conducted in November 1988, identified floating
JP-4 in the ponds following a fire training exercise” (Parsons, 1999).
 1990s: “Several investigations completed in the 1990s” (URS, 2011c).
 1992: Quarterly groundwater monitoring conducted (Parsons, 1999).
 1994: Cone Penetrometer Technology (CPT) pushes (CPT-1 through 22)
were performed to characterize subsurface stratigraphy. Laser Induced
Fluorescent (LIF) performed simultaneously at these locations to evaluate
the presence of residual- and free-phase hydrocarbons in soil and
groundwater. (Parsons, 1999).
 1995: The mock aircraft fuselage was dismantled and removed (URS,
2011c).
 1999: Treatability study performed in support of intrinsic remediation
(Parsons, 1999).
 2004: Removal of soils with greater than 1 milligram per kilogram (mg/kg)
total BTEX or total chlorinated aliphatic hydrocarbons (CAH) (URS,
2006a; URS, 2011c).
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Figure 2. Conceptual site model (CSM).
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
2006-2009: Bio-sparging system installed and operated (URS, 2011c).
Soil borings:
 1988 to 2006: Various investigations and well installations (URS, 2006a).
Geophysical investigations: 1988-89, 1994 various investigations (Parsons,
1999).
Site geochemistry: 1994 (Parsons, 1999).
Presence of off-site affected groundwater: Presence of VC at and beyond the
operational fenceline (URS, 2011b; URS, 2011c).
Evidence of Non-Aqueous Phase Liquids (NAPL):
 1988: “An investigation conducted in November 1988, identified floating
JP-4 in the ponds following a fire training exercise” (Parsons, 1999).
 1988-1990: residual NAPL found in soil during site investigations
(Parsons, 1999).
 1994: residual NAPL found at the site; however, mobile NAPL was not
found at the site (Parsons, 1999).
Most recent NAPL observation: 1994 (Parsons, 1999).
3. Source Characterization
Primary source location: three sources of contamination have positively been
identified (Figure 3-1 in URS, 1999, Parsons, 1999):
 Release Area 1: Former Fire Training Area Burn Pit.
 Release Area 2: Former Building 654.
 Release Area 3: Former Wastewater Discharge Pond (Two ponds).
Secondary source locations: No mobile NAPL is found at the site, however
residual NAPL was found during CPT and LIF investigations and collection of soil
samples (Parsons, 1999).
Release mechanisms: No liners or other devices were used within the burn pit to
prevent the fuels and waste solvents used during fire training exercises from
percolating into the soils to groundwater.
Size and boundary: See Figures 2 and 3 (Figure 3-1 in URS, 1999 and Figure
4.1 in Parsons, 1999).
Substance released: fuel hydrocarbons derived from JP-4 and chlorinated
solvents (TCE) (Parsons, 1999).
Date of release: 1960-1990
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Figure 3. Site plan (URS, 2011c, Figure 4-1).
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Volume and mass of substance(s) released:
 1960-1974: 2,000 gallons of waste fuels and solvents per training day at a
frequency of once a week (Parsons, 1999).
 1974-1990: 300 gallons of jet fuel (JP-4) at a frequency of approximately
twice per calendar quarter (Parsons, 1999).
Source control measures taken: excavation and pulsed bio-sparging (Parsons,
2011c).
4. Constituents of Concern
On Base, the plume consists of BTEX, trichloroethene (TCE); 1,1-dichloroethane
(1,1-DCA); trans-1,2-dichloroethene (trans-1,2-DCE); cis-1,2-dichloroethene (cis1,2-DCE); and vinyl chloride (VC), with benzene and VC being the primary
constituents of concern. Off Base, the plume consists exclusively of VC (URS,
2006a).
5. Nature and Extent of Contamination
Plan view of the plume:
- Figures 4-3, 4-4, and 4-5 of URS, 2011c
- Figure 2-1 of URS, 2011b
- Figure 4-1, 4-2, and 4-3 of URS, 1999
- Figure 6-2 of URS, 2006b
- Figure 5-2, 5-3, 5-4, 5-6, and 5-7 of URS, 2008b
Cross-sections:
- Figure 3-2 of URS, 1999
- Figure 6-3 of URS, 2006b
- Figure 5-5 of URS, 2008b
6. Hydrogeology
Stratigraphy: Shallow sediments underlying the Site comprise of 1.5 to 9 feet of a
silt, clay, and sand mixture in shades of olive, gray, and brown (Figure 4). The
shallow sediments are underlain by a poorly-graded sand which frequently
contains a trace or more of silt, clay, or gravel. Depth to bedrock has not been
determined (Parsons, 1999).
Vadose (unsaturated) and saturated zone types: Comprised of fill material, clay,
silty sand, silty clay, and silt (based on cross Section A-Aˊ, Figure 5-5 in URS,
2008b) (see Figure 4).
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GSI Job No. G-3587-104
Issued: 10 May 2012
Page 77 of 130
PRELIMINARY
Figure 4a. Cross-section A-Aˊ (URS, 2008b, Figure 5-1).
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Issued: 10 May 2012
Page 78 of 130
PRELIMINARY
Figure 4b. Cross-section A-Aˊ (URS, 2008b, Figure 5-5). As noted in URS 2008b, the green shaded area represents the 2007 vinyl chloride groundwater plume.
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Aquifer properties: General groundwater flow is to the east-southeast with an
average gradient of 0.0004 ft/ft. Historically, groundwater gradients are steeper
in the spring with a gradient of 0.0006 (value used in groundwater models)
(Parsons, 1999).
Hydraulic conductivities ranging from 7.7 to 9.5 ft/day were reported for wells
screened across the water table within the silty sand at FTA3. Hydraulic
conductivities were estimated from rising head slug tests performed on site wells.
An average hydraulic conductivity of 3.5 ft/day was estimated from rising head
slug tests at wells FTA3-MW6 and HF6-MW4. A conductivity of ~6.1 ft/day,
based on the average of the historic and current K measurements, was used in
groundwater models (Parsons, 1999).
An effective porosity value of 0.2 was assumed for fine sand (Parsons, 1999).
Confining unit soil type: The water bearing unit is not confined (Parsons, 1999).
Depth to top of aquifer: Depth to groundwater is approximately 8 to 10 feet below
ground surface (bgs) across the majority of the site (Parsons, 1999).
Depth to groundwater: Depth to groundwater is approximately 8 to 10 feet bgs
across the majority of the site (Parsons, 1999).
Direction of groundwater flow including preferential pathways: General
groundwater flow is to the east-southeast. Historically, groundwater gradients
are steeper in the spring (Parsons, 1999).
Recharge: No information on groundwater recharge could be obtained from the
documents examined.
Proximity to surface waters: The site is adjacent to the Base Lake. The edge of
the pit that was excavated is approximately 350 feet from the lake (Figure 4-3 in
URS, 2011c).
Interaction between groundwater and surface water: Groundwater is connected
to surface water in the lake, but based on cross-sections, the plume dips into the
deeper water bearing unit and goes underneath the lake (Figure 2-3 in URS,
2008a).
7. Geochemistry
Groundwater geochemical data were collected between 1994 and 2011. The
geochemical data included dissolved oxygen (DO), nitrate + nitrite (NO2+NO3),
ammonia, soluble manganese, ferrous iron, sulfate, methane, redox potential,
alkalinity, free carbon dioxide, pH, temperature, conductivity, and chloride (Table
4-5 in Parsons, 1999).
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Dissolved Oxygen: Measured DO concentrations ranged from
 1994: 0.0 mg/L to 3.3 mg/L
 1998: 0.17 mg/L to 3.69 mg/L
 2007: 0.76 mg/L to 5.57 mg/L
 2008: 0.21 mg/L to 0.7 mg/L
 2009: 0.27 mg/L to 3.39 mg/L
 2010: 0.04 mg/L to 1.03 mg/L
 2011: 0.16 mg/L to 0.43 mg/L
Nitrate/Nitrite:
 1994: Nitrate/nitrite (as N) concentrations were not detected above 0.05
mg/L except at two sampling locations, including background locations.
The highest nitrate/nitrite (as N) concentration was measured in the
groundwater sample collected from well FTA3-MW4 at 1.5 mg/L. This well
is located within 100 feet of the inlet to Base Lake, and it is suspected
that this concentration may be the result of surface water/groundwater
interaction.
Sulfate: Measured sulfate concentrations ranged from
 1994: <0.5 mg/L to 391 mg/L
 2007: 1.6 mg/L to 47 mg/L
 2008: Non-Detect to 1,500 mg/L
 2009: 16 mg/L to 1,800 mg/L
 2010: 8.8 mg/L to 1,400 mg/L
 2011: 13 mg/L to 920 mg/L
Ferrous Iron (Fe+2): Measured ferrous iron concentrations ranged from
 1994: <0.05 mg/L to 26.3 mg/L
 1998: 0.1 mg/L to 44.6 mg/L
 2007: 0.2 mg/L to 3.2 mg/L
 2008: 0 mg/L to 2.35 mg/L
 2009: Non-Detect to 3.2 mg/L
 2010: 0.12 mg/L to 9.35 mg/L
 2011: <0.1 mg/L to 5.52 mg/L
8. Migration and Exposure Pathways
The primary source of releases at the Site are waste oil, solvents, and fuels that
were used during fire training exercises. The primary release mechanism is
penetration of residual phase and dissolved phase contaminants in source areas
to the underlying vadose zone soil upper portion of the saturation zone (via
infiltration and percolation). Dissolution of the residual phase resulted in
development of the dissolved phase plumes. Following source zone removal
action in 2004, it is assumed the residual phase contamination is no longer
present.
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A chlorinated aliphatic hydrocarbon (CAH) and BTEX groundwater plume
emanates from the former soil source. On Base, the plume consists primarily of
BTEX; 1,1-DCA; cis-1,2-DCE; and VC. Off Base, the plume consists exclusively
of VC. VC has not been detected in surface water samples collected from Base
Lake, the closest point of potential exposure. Natural attenuation processes are
reducing contaminant mass within the dissolved plume.
There is limited potential for exposure since the contaminant source (i.e., soils
within the former training area) was removed in 2004 and administrative LUCs
(i.e., digging and drinking well restrictions, and Base fence) are in place. Fuelcontaminated soils remaining outside of the former source do not pose a risk to
human health. The general public and Base personnel are restricted from direct
access to the site.
There are no existing complete exposure pathways associated with the
groundwater plume since there are no domestic or other supply wells within the
plume, or at the current estimated distal end of the plume near Base Lake (URS,
2006b).
9. Contaminant Attenuation Pathways
Natural attenuation processes are reducing contaminant mass within the
dissolved plume (URS, 2006b).
10. Receptors
There is a lake (Base Lake) adjacent to the site at approximately 650 feet from
the excavated area and downgradient of the source area. However, according to
the cross-sections (for example, see Figure 5-5 in URS, 2008b or Figure 4-4 in
URS, 2011a) the plume dives underneath the surface water. An investigation
conducted in November 1988, identified floating JP-4 fuel in the ponds following
a fire training exercise (Parsons, 1999). The CY11 results from monitoring wells
HF6-MW4S, HF6-MW5S, and HF6-MW6S located downgradient of the former
FTA3 old pond area, remain below all MCLs and Regional Screening Levels
(RSLs).
a) Human receptors: Currently, the source area is limited to hypothetical
construction workers excavating the area. Routes of exposure to these
receptors may be through contact with subsurface soil via inhalation of dust,
ingestion, and dermal contact (USEPA, Statement of Basis).
b) Ecological receptors: There are no current risks to ecological receptors
(USEPA, Statement of Basis).
c) Sensitive receptors: There are no current risks to sensitive receptors. The
source areas within the fenced property boundary are off-limits to the general
public and vast majority of the Base personnel. Since the source areas are
also within the lateral clearance zone of the main operational runway,
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buildings cannot be constructed in the lateral clear zone. Currently, there are
no plans to close or transfer portions of the Base, therefore, there are no
anticipated future residential or industrial receptors in the source areas
(USEPA, Statement of Basis).
d) Current and future groundwater and surface water resources: The shallow
aquifer has the potential to be a domestic water source (USEPA, Statement
of Basis).
11. Soil Remediation
Soil Excavation: The discharge pond was excavated as part of the Hardfill 6
rubble pile removal project in fall 2004. Approximately 612,500 ft3 of bulk soil
was excavated (i.e., 49,000 ft2 × 12.5 ft) (Figure 6-2 and 6-3 in URS, 2006b).
12. Groundwater Remediation
Biosparging:
 The source area biosparging system was operated from June 2006 to March
2007, after which it was shutdown to assess contaminant concentration
stability. Groundwater samples were collected at the same seven monitoring
wells in April, July, and October 2007 to assess concentration stability. The
USEPA approved a Response Complete and termination of the FTA3 source
area biosparging in a letter dated December 19, 2007.
 Offutt AFB’s RCRA Permit was modified in 2008 to include FTA3’s old
discharge pond area and its associated groundwater monitoring as part of the
FTA3 solid waste management unit (SWMU) (previously within the HF5
SWMU footprint).
 The distal area biosparging system was installed in July 2008, began
operating in August 2008, and continued through December 17, 2009, when
the system was shut down due to freezing conditions. The purpose of biosparging in the distal portion of the groundwater plume was to accelerate the
cleanup of contaminants remaining following the soil source remedial action
and source biosparging.
 In 2010, it was determined that operation of the biosparging system was no
longer warranted.
13. 14 Compartment Model
The qualitative 14 Compartment Model (Figures 5a and 5b) for this site identifies
all phases/zones that could potentially contain the contaminants.
The 14 Compartment Model was completed using the conditions stated below to
determine if mass is likely to be present in low-permeability compartments:

The release at the site occurred between 1960 and 1991. Therefore,
significant elapsed time since contaminant was released.
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


Large amounts of contaminants were released.
High concentrations of contaminants released were observed in the
monitoring wells.
The cross-sections do not show significant low-permeability lenses in the
source or plume area. Overall, it is expected that low-permeability lenses or
strata exist in the source area or the affected aquifer due to heterogeneity of
aquifer.
Source Zone
Zone/
Phases
Plume
Low
Permeability
Transmissive
Transmissive
Low
Permeability
Vapor
IP
IP
IP
IP
DNAPL
2
2
NA
NA
Aqueous
3
2
1
2
Sorbed
3
2
1
2
Legend:
Figure 5a. Depiction of FTA3 using the 14 Compartment Model. Larger numbers
represent higher concentrations. Arrows represent mass transport between
compartments. (NA = Not Applicable; IP = Incomplete Pathway).
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Source Zone
Zone/
Phases
Plume
LowPermeability
Transmissive
Vapor
Incomplete
Pathway
Incomplete
Pathway
Incomplete
Pathway
Incomplete
Pathway
DNAPL
Same as
Transmissive
Zone DNAPL
Residual NAPL
observed = “2”
NA
NA
Aqueous
Transmissive
zone “number” +
1 = “2”+1 = ”3”
Maximum
observed
concentration in
source well = VC
13 μg/L = “2”
Maximum
observed
concentration in
plume well = VC
2.3 μg/L = “1”
Plume life >30 yrs,
therefore number =
“1”+1 = ”2”
Sorbed
Same as LowPermeability
Zone Aqueous
Same as
Transmissive
Zone Aqueous
Same as
Transmissive
Zone Aqueous
Same as LowPermeability Zone
Aqueous
Transmissive
Low-Permeability
Figure 5b. Methodology and decision logic used at this site on how to fill in the
concentrations for the 14 Compartment Model in Figure 5a. (NA = Not Applicable).
14. Stakeholders
U.S. Environmental Protection Agency (USEPA), AFCEE, and Department of
Defense (DoD).
Key Point Question I: The CSM includes all items 1-14 relevant to the Site, and
includes a qualitative 14 Compartment model, therefore Question I.1 is
answered “YES”.
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3.2
QUESTION II. ARE SOURCES CONTROLLED?
3.2.1. Question II.1. Are there any significantly mobile source materials?
Criteria: No mobile DNAPL. No Expanding LNAPL Zone.
There is no evidence of free phase product observation/measurement in the monitoring
wells or any other wells, for both chlorinated solvents and jet fuel. Therefore, it is
concluded that there are no significantly mobile source materials.
Key Point: There are no significantly mobile NAPL in the source zone. Question
II.1 is answered “YES”.
3.2.2 Question II.2. Is the source zone free of any environmentally significant
quantity of NAPL?
Criteria: Little or no DNAPL observed in transmissive zones, and no significant LNAPL
accumulation based on specific volume calculations.
Chlorinated Solvent NAPL: Since DNAPL has never been observed in core samples and
the source zone area soil was excavated, there is no environmentally significant quantity
of DNAPL in the source zone.
Jet Fuel NAPL: Since LNAPL has never observed in core samples and the source zone
area soil was excavated, there is no environmentally significant quantity of LNAPL in the
source zone.
Key Point: There are no environmentally significant quantities of NAPL in the
source zone. Question II.2 is answered “YES”.
3.2.3. Question II.3. Is it possible that any further source zone cleanup will be
constrained by matrix diffusion processes?
Criteria: Qualitative evaluation of matrix diffusion processes based on geology, chemical
properties, timing of initial release, and remediation efforts.
Time of release and soil/groundwater remediation was evaluated as well as the lithology
and cross-sections. The potential for matrix diffusion effects can be seen at virtually any
site with heterogeneity in the subsurface, NAPL, and/or where persistent groundwater
contaminant concentrations after source-zone remediation have been observed. Key
factors favoring matrix diffusion at this site include:


The release at the site occurred between 1960 and 1991. Therefore, significant
time has elapsed since contaminant was released.
Large amounts of contaminants were released.
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

High concentrations of contaminants released were observed in the monitoring
wells.
The cross-sections do not show significant low-permeability lenses in the source
or plume area. Overall, it is expected that low-permeability lenses or strata exist
in the source area or the affected aquifer due to heterogeneity of aquifer.
Key Point: It is possible that any further source zone cleanup will be
constrained by matrix diffusion processes. Question II.3 is answered “YES”.
3.2.4. Question II.4. Are sources relatively small?
Criteria: Plume is classified as a “Mag 4” Plume Magnitude Category or less based on
mass discharge estimates, OR maximum source concentrations are < 20x MCL.
The table below summarizes maximum concentrations of key groundwater constituents
in all of the monitoring wells located in both source and plume areas. Since 2007, the
maximum detected concentrations have been less than 20x MCL. Because the
maximum concentration of key groundwater constituents is less than 20x their MCLs, the
source is defined as a small source.
Analytical data show significant decreases in the petroleum hydrocarbons and CAH
concentrations as a result of the source removal and bio-sparging remedies. This mass
reduction, in addition to the biosparging system installed in the distal portion of the
plume in 2008, will facilitate achievement of MCLs by natural attenuation in the distal
portions of the groundwater plume.
Year
2007
2008
2009
2010
2011
MCL (µg/L)
Vinyl Chloride
Maximum
(µg/L)
18
32
5.8
11
13
2
Benzene
Maximum
(µg/L)
23
2.1
4.5
3.0
2.3
5
TCE
Maximum
(µg/L)
2.2
1.7
3.4
1.1
2.0
5
1,2-DCA
Maximum
(µg/L)
23
23
9.2
8.4
10
5
Key Point: Source is small. Question II.4 is answered “YES”.
3.2.5. Question II.5. Are source zone concentrations stable or decreasing?
Criterion: Representative source zone concentrations over time are shown to be stable,
decreasing, or probably decreasing.
Source zone trends were determined at monitoring wells that contained at least five
years of temporal data in the source area, to ensure enough time to determine source
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zone trends. Trend analysis was performed using the non-parametric Mann-Kendall
methodology (as developed for AFCEE’s Monitoring and Remediation Optimization
System (MAROS) program). For this purpose, half the detection limit was used for the
non-detect samples.
Mann-Kendall analysis was performed for benzene, VC, and TCE at four source zone
monitoring wells, MW-13S, MW-13SI, MW-15S, and MW-22S (Figures 6a through 6c).
The trends in these source zone wells (using the method employed in MAROS) are
either “Probably Decreasing”, “Decreasing”, or “Stable”. “No Trend” results were
obtained for benzene in MW-22S, which has been non-detect since January of 2007,
and TCE in MW-13S.
Therefore, it can be concluded that natural attenuation processes are active in the
source zone and the source zone concentrations are stable or decreasing.
Key Point: Source zone concentrations are decreasing.
answered “YES”.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
Question II.5 is
M A N U A L
87
CASE STUDY 2 – OFFUTT AFB
GSI MANN-KENDALL TOOLKIT
for Constituent Trend Analysis
GSI Environmental Inc., Houston, Texas
Date Analyzed: 8-May-12
Facility: FTA3 - Offutt AFB, Nebraska
Well Identification:
Distance from Source (ft):
Sampling
Event
Constituent: Benzene
Concentration Units: ug/L
FT3-MW13S
FT3-MW13SI
FT3-MW15S
FT3-MW22S
0
0
0
0
BENZENE CONCENTRATION (ug/L)
Date
1
Apr-00
2
Oct-00
3
Apr-01
4
Sep-01
5
Oct-01
6
Jul-02
7
Jul-03
8
Nov-04
9
Jul-05
10
10-Jan-07
11
13-Apr-07
12
17-Apr-07
13
12-Jul-07
14
13-Jul-07
15
16-Jul-07
16
17-Jul-07
17
22-Oct-07
18
15-Apr-08
19
16-Apr-08
20
14-Oct-08
21
18-Nov-08
22
17-Mar-09
23
10-Apr-09
24
14-Apr-09
25
15-Apr-09
26
15-Dec-09
27
22-Mar-10
28
07-Apr-10
29
08-Apr-10
30
09-Jul-10
31
22-Oct-10
32
06-Dec-10
33
06-Apr-11
34
07-Apr-11
Coefficient of Variation:
Mann-Kendall Statistic (S):
Confidence Factor:
Concentration Trend:
62
93
9.6
14
290
58
68
44
5.7
1.3
350
200
0.2
450
220
230
230
0.21
0.2
0.18
0.2
1
1.6
0.2
0.2
0.2
0.46
0.2
0.2
1.5
1.1
0.2
0.2
0.2
0.2
0.51
0.72
0.2
0.98
0.94
1.2
0.74
0.2
1.61
-41
99.8%
0.93
-8
95.8%
1.49
-45
96.5%
0.2
1.15
-15
89.2%
Decreasing
Decreasing
Decreasing
No Trend
1000
FT3-MW13S
Concentration (ug/L)
FT3-MW13SI
100
FT3-MW15S
FT3-MW22S
10
1
0.1
12/06/99
04/19/01
09/01/02
01/14/04
05/28/05
10/10/06
02/22/08
07/06/09
11/18/10
04/01/12
Sampling Date
Notes:
1. At least four independent sampling events per well are required for calculating the trend. Methodology is only valid for 4 to 40 samples.
2. Confidence in Trend = Confidence (in percent) that constituent concentration is increasing (S>0) or decreasing (S<0).
≥ 90% = Probably Increasing or Decreasing; >95% = Increasing or Decreasing.
3. Methodology based on "MAROS: A Decision Support System for Optimizing Monitoring Plans", J.J. Aziz, M. Ling, H.S. Rifai, C.J. Newell,
and J.R. Gonzales, Ground Water , 41(3):355-367, 2003.
4. Values in bold represent detected values. Values in italics represent values below the detection limit and are shown as half the detection limit.
Figure 6a. Benzene source area concentration vs. time plots and Mann-Kendall analysis.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
88
CASE STUDY 2 – OFFUTT AFB
GSI MANN-KENDALL TOOLKIT
for Constituent Trend Analysis
GSI Environmental Inc., Houston, Texas
Date Analyzed: 8-May-12
Facility: FTA3 - Offutt AFB, Nebraska
Well Identification:
Distance from Source (ft):
Sampling
Event
Constituent: Vinyl Chloride
Concentration Units: ug/L
FT3-MW13S
FT3-MW13SI
FT3-MW15S
FT3-MW22S
0
0
0
0
VINYL CHLORIDE CONCENTRATION (ug/L)
Date
1
Apr-00
2
Oct-00
3
Apr-01
4
Sep-01
5
Oct-01
6
Jul-02
7
Jul-03
8
Nov-04
9
Jul-05
10
10-Jan-07
11
13-Apr-07
12
17-Apr-07
13
12-Jul-07
14
13-Jul-07
15
16-Jul-07
16
17-Jul-07
17
22-Oct-07
18
15-Apr-08
19
16-Apr-08
20
14-Oct-08
21
18-Nov-08
22
17-Mar-09
23
10-Apr-09
24
14-Apr-09
25
15-Apr-09
26
15-Dec-09
27
22-Mar-10
28
07-Apr-10
29
08-Apr-10
30
09-Jul-10
31
22-Oct-10
32
06-Dec-10
33
06-Apr-11
34
07-Apr-11
Coefficient of Variation:
Mann-Kendall Statistic (S):
Confidence Factor:
Concentration Trend:
740
520
1300
380
730
850
760
540
260
170
530
100
250
150
52
78
0.5
0.5
0.5
0.5
1.1
8.3
7.6
0.5
0.5
0.5
0.23
0.5
1.8
0.68
1.2
0.5
0.5
0.5
0.5
1.7
5.5
0.5
11
6.2
13
5.2
0.5
1.19
-34
99.0%
0.83
-10
99.2%
1.53
-37
93.0%
0.5
1.58
-17
92.2%
Decreasing
Decreasing
Prob. Decreasing
Prob. Decreasing
10000
FT3-MW13S
FT3-MW13SI
Concentration (ug/L)
1000
FT3-MW15S
FT3-MW22S
100
10
1
0.1
12/06/99
04/19/01
09/01/02
01/14/04
05/28/05
10/10/06
02/22/08
07/06/09
11/18/10
04/01/12
Sampling Date
Notes:
1. At least four independent sampling events per well are required for calculating the trend. Methodology is only valid for 4 to 40 samples.
2. Confidence in Trend = Confidence (in percent) that constituent concentration is increasing (S>0) or decreasing (S<0).
≥ 90% = Probably Increasing or Decreasing; >95% = Increasing or Decreasing.
3. Methodology based on "MAROS: A Decision Support System for Optimizing Monitoring Plans", J.J. Aziz, M. Ling, H.S. Rifai, C.J. Newell,
and J.R. Gonzales, Ground Water , 41(3):355-367, 2003.
4. Values in bold represent detected values. Values in italics represent values below the detection limit and are shown as half the detection limit.
Figure 6b. VC source area concentration vs. time plots and Mann-Kendall analysis.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
89
CASE STUDY 2 – OFFUTT AFB
GSI MANN-KENDALL TOOLKIT
for Constituent Trend Analysis
GSI Environmental Inc., Houston, Texas
Date Analyzed: 8-May-12
Facility: FTA3 - Offutt AFB, Nebraska
Well Identification:
Distance from Source (ft):
Sampling
Event
Constituent: TCE
Concentration Units: ug/L
FT3-MW13S
FT3-MW13SI
FT3-MW15S
FT3-MW22S
0
0
0
0
TCE CONCENTRATION (ug/L)
Date
1
Apr-00
2
Oct-00
3
Apr-01
4
Sep-01
5
Oct-01
6
Jul-02
7
Jul-03
8
Nov-04
9
Jul-05
10
10-Jan-07
11
13-Apr-07
12
17-Apr-07
13
12-Jul-07
14
13-Jul-07
15
16-Jul-07
16
17-Jul-07
17
22-Oct-07
18
15-Apr-08
19
16-Apr-08
20
14-Oct-08
21
18-Nov-08
22
17-Mar-09
23
10-Apr-09
24
14-Apr-09
25
15-Apr-09
26
15-Dec-09
27
22-Mar-10
28
07-Apr-10
29
08-Apr-10
30
09-Jul-10
31
22-Oct-10
32
06-Dec-10
33
06-Apr-11
34
07-Apr-11
Coefficient of Variation:
Mann-Kendall Statistic (S):
Confidence Factor:
Concentration Trend:
3.8
0.43
2.4
2.3
2.5
2.5
6.6
3.9
0.3
2.5
2.5
2.5
1.3
0.2
2.5
0.89
2.5
0.5
1.1
39
63
2.2
2.8
0.5
1.1
2.2
0.36
0.5
1.1
0.5
0.5
3.4
2.4
1.7
0.86
0.5
0.5
1.1
0.5
0.2
0.21
2
0.97
0.68
2
52.7%
0.75
-2
59.2%
0.5
0.94
-64
99.6%
1.85
-32
99.9%
No Trend
Stable
Decreasing
Decreasing
100
FT3-MW13S
Concentration (ug/L)
FT3-MW13SI
FT3-MW15S
FT3-MW22S
10
1
0.1
12/06/99
04/19/01
09/01/02
01/14/04
05/28/05
10/10/06
02/22/08
07/06/09
11/18/10
04/01/12
Sampling Date
Notes:
1. At least four independent sampling events per well are required for calculating the trend. Methodology is only valid for 4 to 40 samples.
2. Confidence in Trend = Confidence (in percent) that constituent concentration is increasing (S>0) or decreasing (S<0).
≥ 90% = Probably Increasing or Decreasing; >95% = Increasing or Decreasing.
3. Methodology based on "MAROS: A Decision Support System for Optimizing Monitoring Plans", J.J. Aziz, M. Ling, H.S. Rifai, C.J. Newell,
and J.R. Gonzales, Ground Water , 41(3):355-367, 2003.
4. Values in bold represent detected values. Values in italics represent values below the detection limit and are shown as half the detection limit.
Figure 6c. TCE source area concentration vs. time plots and Mann-Kendall analysis.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
90
CASE STUDY 2 – OFFUTT AFB
3.2.6. Question II.6. Is there evidence of on-going source attenuation processes?
Criteria: Footprints of source zone attenuation are seen (such as generation of daughter
products or consumption of electron acceptors).
The source has been removed during soil excavation. There is evidence of on-going
anoxic attenuation in the former source area:



Generation of daughter products: As of April 2011 daughter products cis-1,2DCE, trans-1,2-DCE, VC, ethane, ethane, and methane have been detected in
monitoring wells located in or near the former source area.
Depletion of oxygen, nitrate and sulfate: Dissolved oxygen has been depleted
over the entire period DO was measured (1994-2011). Maximum DO in the
monitoring wells located in the source area was reduced from 5.57 mg/L in MW13SI in 2007 to 0.38 mg/L in MW-13S in 2011. Sulfate concentrations have also
been depleted. See geochemistry section of Question I.1 for details on nitrate
and sulfate concentrations.
Low oxygen, nitrate, and sulfate concentrations: Average measured DO
concentrations are below ~0.5 mg/L, AND plume doesn’t meet all of the
anaerobic indicators (i.e., sulfate concentrations are not less than 50 mg/L). See
geochemistry section of Question I.1.
Key Point: Source zone attenuation is occurring as evidenced by the generation
of daughter product cis-DCE, consumption of electron acceptors, and presence
of anoxic conditions. Question II.6 is answered “YES”.
3.2.7. Question II.7. Will future source remediation only marginally improve site
conditions?
Criteria: There is “Less Need For Source Treatment” based on weight of evidence from
the Qualitative Decision Chart (Figure 10 of LoRSC Manual).
The source has been removed during soil excavation. The Qualitative Decision Chart
was completed to evaluate the need for additional source treatment for residual NAPL
that may have remained in the soil after excavation. A weight of evidence of 8 was
obtained, therefore, there is “Less Need For Source Treatment” (Figure 7).
Key Point: Future source zone remediation will only marginally improve site
conditions. Question II.7 is answered “YES”.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
91
CASE STUDY 2 – OFFUTT AFB
Figure 7. Qualitative Decision Chart on the merits of source depletion (Sale et al., 2008;
Adapted from USEPA’s “The DNAPL Remediation Challenge: Is There a Case for Source
Depletion?” (Kavanaugh et al., 2003). (3c was selected due to the relatively low
concentrations and evidence of on-going MNA; 5c was selected because remediation of
the relatively low concentrations in the source zone will not significantly reduce time to
reach MCLs; 6c was selected assuming a 4.4 acre site and remediation cost of $3MM/acre
(Sale et al., 2008))
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
92
CASE STUDY 2 – OFFUTT AFB
3.3.
QUESTION III. WILL RESIDUAL CONTAMINATION HAVE NO ADVERSE
EFFECT ON PRESENT AND FUTURE LAND AND WATER USES?
3.3.1. Question III.1. Is the groundwater plume stable, decreasing, or probably
decreasing?
Criterion: Plume trend analysis showing stable, decreasing, or probably decreasing
plume over time using method in Figure 8.
Plume zone trends were determined at monitoring wells that contained at least five years
of temporal data, to ensure enough time to determine trends. Trend analysis was
performed using the non-parametric Mann-Kendall methodology (as developed for
AFCEE’s Monitoring and Remediation Optimization System (MAROS) program). For this
purpose, half the detection limit was substituted for the non-detect results.
Mann-Kendall analysis was performed for benzene, VC, and TCE at six plume area
monitoring wells: MW-6A, MW-8SI, MW-9SI, MW-10SI, MW-11SI, and MW-16SI. The
trends in these wells (using the method employed in MAROS) are either “Probably
Decreasing”, “Decreasing”, or “Stable” (Figures 8a through 8c). “No Trend” results were
obtained for benzene at MW-16SI and vinyl chloride at MW-9SI and MW-11SI.
Therefore, it can be concluded that natural attenuation processes are active in the plume
area and the plume concentrations are stable or decreasing.
Key Point: Plume zone concentrations are decreasing.
answered “YES”.
Question III.1 is
3.3.2. Question III.2. Is there evidence of on-going natural attenuation processes
in the plume?
Criteria: Analysis of natural attenuation processes and footprints of natural attenuation
in the plume.
There is evidence of on-going anoxic attenuation in and near the plume:


Generation of daughter products: As of April 2011 daughter products cis-1,2DCE, trans-1,2-DCE, VC, ethane, ethane, and methane have been detected in
the monitoring wells located in and near the plume.
Depletion of oxygen, nitrate and sulfate: Dissolved oxygen has been depleted
over the period DO was measured (1994-2011). Maximum DO in the monitoring
wells located in or near the plume has reduced from 1.84 mg/L in MW-11SI in
2007 to 0.2 mg/L in MW-9SI in 2011. Sulfate concentrations have also been
depleted. See geochemistry section of Question I.1 for details on nitrate and
sulfate concentrations.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
93
CASE STUDY 2 – OFFUTT AFB
GSI MANN-KENDALL TOOLKIT
for Constituent Trend Analysis
GSI Environmental Inc., Houston, Texas
Date Analyzed: 8-May-12
Facility: FTA3 - Offutt AFB, Nebraska
Well Identification:
Distance from Source (ft):
Sampling
Event
Constituent: Benzene
Concentration Units: ug/L
FT3-MW6A
FT3-MW8SI
FT3-MW9SI
FT3-MW10SI
FT3-MW11SI
FT3-MW16SI
150
250
420
550
600
420
BENZENE CONCENTRATION (ug/L)
Date
Nov-89
1
Mar-92
2
May-92
3
Aug-92
4
Nov-94
5
Jul-96
6
7
Jun-98
Jul-98
8
9
Dec-98
10
Jun-99
11
Apr-00
Oct-00
12
13
Apr-01
Oct-01
14
Jul-02
15
Jul-03
16
Nov-04
17
Jul-05
18
16-Jul-07
19
17-Jul-07
20
16-Apr-08
21
14-Oct-08
22
23
14-Apr-09
24
15-Apr-09
25
07-Apr-10
08-Apr-10
26
27
22-Oct-10
28
06-Apr-11
07-Apr-11
29
Coefficient of Variation:
Mann-Kendall Statistic (S):
Confidence Factor:
Concentration Trend:
2.5
5.6
6.6
10
5.4
3
2.5
0.935
4.03
4.7
6.4
7.5
12
7.7
9
12
2.5
0.31
0.5
0.45
1
1.1
1
1
0.41
23
2.5
2.5
2.5
2.5
0.49
1
0.75
1.8
0.14
0.88
0.57
2.5
2.5
2.5
2.5
2.5
0.2
2.5
2.5
2.5
2.5
2.5
2.5
0.2
0.5
1
2.1
0.43
0.2
0.2
0.2
1.3
1.3
2.4
0.2
0.2
0.2
0.2
0.2
0.2
0.27
0.2
0.2
2.5
2.5
3
2.3
2.3
0.2
0.88
-6
55.9%
0.2
0.86
-18
90.5%
0.84
-32
99.9%
0.78
-39
99.7%
0.89
-36
99.3%
0.52
6
66.8%
Stable
Prob. Decreasing
Decreasing
Decreasing
Decreasing
No Trend
FT3-MW6A
100
FT3-MW8SI
Concentration (ug/L)
FT3-MW9SI
FT3-MW10SI
FT3-MW11SI
10
FT3-MW16SI
1
0.1
12/23/88
09/19/91
06/15/94
03/11/97
12/06/99
09/01/02
05/28/05
02/22/08
11/18/10
Sampling Date
Notes:
1. At least four independent sampling events per well are required for calculating the trend. Methodology is only valid for 4 to 40 samples.
2. Confidence in Trend = Confidence (in percent) that constituent concentration is increasing (S>0) or decreasing (S<0).
≥ 90% = Probably Increasing or Decreasing; >95% = Increasing or Decreasing.
3. Methodology based on "MAROS: A Decision Support System for Optimizing Monitoring Plans", J.J. Aziz, M. Ling, H.S. Rifai, C.J. Newell,
and J.R. Gonzales, Ground Water , 41(3):355-367, 2003.
4. Values in bold represent detected values. Values in italics represent values below the detection limit and are shown as half the detection limit.
Figure 8a. Benzene plume area concentration vs. time plots and Mann-Kendall analysis.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
94
CASE STUDY 2 – OFFUTT AFB
GSI MANN-KENDALL TOOLKIT
for Constituent Trend Analysis
GSI Environmental Inc., Houston, Texas
Date Analyzed: 8-May-12
Facility: FTA3 - Offutt AFB, Nebraska
Well Identification:
Distance from Source (ft):
Sampling
Event
Constituent: Vinyl Chloride
Concentration Units: ug/L
FT3-MW6A
FT3-MW8SI
FT3-MW9SI
FT3-MW10SI
FT3-MW11SI
FT3-MW16SI
150
250
420
550
600
420
VINYL CHLORIDE CONCENTRATION (ug/L)
Date
1
Nov-89
Mar-92
2
May-92
3
4
Aug-92
Nov-94
5
Jul-96
6
7
Jun-98
Jul-98
8
Dec-98
9
10
Jun-99
11
Apr-00
Oct-00
12
13
Apr-01
14
Oct-01
Jul-02
15
16
Jul-03
17
Nov-04
Jul-05
18
19
16-Jul-07
20
17-Jul-07
16-Apr-08
21
22
14-Oct-08
23
14-Apr-09
15-Apr-09
24
25
07-Apr-10
26
08-Apr-10
22-Oct-10
27
28
06-Apr-11
07-Apr-11
29
Coefficient of Variation:
Mann-Kendall Statistic (S):
Confidence Factor:
Concentration Trend:
1
1
1
1
6.2
1.6
1
1
1.77
2.2
3.6
3.6
3.7
2.5
2
4.4
13
13
13
12
18
17
15
8.9
0.9
2.8
0.5
4.1
0.45
2
3
3.2
3.9
7
7.6
5
5.4
0.9
0.5
0.9
3.6
5.1
4
4.7
3.3
3.1
18
32
8
3.6
6.1
0.3
5
6.1
5.9
2.6
2.2
3.9
1
0.33
0.5
0.36
0.13
0.19
0.5
0.5
0.5
0.5
0.5
0.28
3.8
2.8
0.5
2.3
0.5
0.77
-1
50.0%
0.5
0.65
-26
97.5%
1.50
-4
60.3%
0.76
-23
93.3%
0.97
4
58.0%
0.25
-21
96.4%
Stable
Decreasing
No Trend
Prob. Decreasing
No Trend
Decreasing
FT3-MW6A
100
FT3-MW8SI
Concentration (ug/L)
FT3-MW9SI
FT3-MW10SI
FT3-MW11SI
10
FT3-MW16SI
1
0.1
12/06/99
04/19/01
09/01/02
01/14/04
05/28/05
10/10/06
02/22/08
07/06/09
11/18/10
04/01/12
Sampling Date
Notes:
1. At least four independent sampling events per well are required for calculating the trend. Methodology is only valid for 4 to 40 samples.
2. Confidence in Trend = Confidence (in percent) that constituent concentration is increasing (S>0) or decreasing (S<0).
≥ 90% = Probably Increasing or Decreasing; >95% = Increasing or Decreasing.
3. Methodology based on "MAROS: A Decision Support System for Optimizing Monitoring Plans", J.J. Aziz, M. Ling, H.S. Rifai, C.J. Newell,
and J.R. Gonzales, Ground Water , 41(3):355-367, 2003.
4. Values in bold represent detected values. Values in italics represent values below the detection limit and are shown as half the detection limit.
Figure 8b. VC plume area concentration vs. time plots and Mann-Kendall analysis.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
95
CASE STUDY 2 – OFFUTT AFB
GSI MANN-KENDALL TOOLKIT
for Constituent Trend Analysis
GSI Environmental Inc., Houston, Texas
Date Analyzed: 8-May-12
Facility: FTA3 - Offutt AFB, Nebraska
Well Identification:
Distance from Source (ft):
Sampling
Event
Constituent: TCE
Concentration Units: ug/L
FT3-MW6A
FT3-MW8SI
FT3-MW9SI
FT3-MW10SI
FT3-MW11SI
FT3-MW16SI
150
250
420
550
600
420
TCE CONCENTRATION (ug/L)
Date
Nov-89
1
Mar-92
2
May-92
3
Aug-92
4
Nov-94
5
Jul-96
6
Jun-98
7
Jul-98
8
Dec-98
9
Jun-99
10
Apr-00
11
Oct-00
12
Apr-01
13
Oct-01
14
Jul-02
15
Jul-03
16
Nov-04
17
Jul-05
18
16-Jul-07
19
17-Jul-07
20
16-Apr-08
21
14-Oct-08
22
14-Apr-09
23
15-Apr-09
24
07-Apr-10
25
08-Apr-10
26
22-Oct-10
27
06-Apr-11
28
07-Apr-11
29
Coefficient of Variation:
Mann-Kendall Statistic (S):
Confidence Factor:
Concentration Trend:
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0.5
6.3
2.5
2.5
2.5
2.5
0.5
0.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.12
0.5
0.5
0.5
0.5
0.12
0.5
0.5
0.5
0.5
0.5
0.5
0.58
-73
98.6%
0.5
0.48
-24
96.4%
0.85
-27
99.2%
0.70
-36
99.3%
0.74
-39
99.7%
0.5
0.79
-24
98.2%
Decreasing
Decreasing
Decreasing
Decreasing
Decreasing
Decreasing
0.5
10
FT3-MW6A
FT3-MW8SI
Concentration (ug/L)
FT3-MW9SI
FT3-MW10SI
FT3-MW11SI
FT3-MW16SI
1
0.1
12/06/99
04/19/01
09/01/02
01/14/04
05/28/05
10/10/06
02/22/08
07/06/09
11/18/10
04/01/12
Sampling Date
Notes:
1. At least four independent sampling events per well are required for calculating the trend. Methodology is only valid for 4 to 40 samples.
2. Confidence in Trend = Confidence (in percent) that constituent concentration is increasing (S>0) or decreasing (S<0).
≥ 90% = Probably Increasing or Decreasing; >95% = Increasing or Decreasing.
3. Methodology based on "MAROS: A Decision Support System for Optimizing Monitoring Plans", J.J. Aziz, M. Ling, H.S. Rifai, C.J. Newell,
and J.R. Gonzales, Ground Water , 41(3):355-367, 2003.
4. Values in bold represent detected values. Values in italics represent values below the detection limit and are shown as half the detection limit.
Figure 8c. TCE plume area concentration vs. time plots and Mann-Kendall analysis.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
96
CASE STUDY 2 – OFFUTT AFB

Low oxygen, nitrate, and sulfate concentrations: Average measured
dissolved oxygen concentrations are below ~2 mg/L, AND plume doesn’t meet all
of the anaerobic indicators (i.e., sulfate concentrations are not less than 50
mg/L). See geochemistry section of Question I.1.
Key Point: Plume zone attenuation is occurring as evidenced by the generation
of daughter product cis-DCE and VC and consumption of electron acceptors.
Question III.2 is answered “YES”.
3.3.3. Question III.3. Are conditions protective of potential and future receptors?
Criteria: Analysis showing all exposure pathways for actual receptors are incomplete or
do not present excess risk, and that future exposure will not occur at levels above risk
criteria.
The site at its current use is bounded on the south by the Base runway and East Gate
Drain, open fields and Landfill 5 to the west, HF6 SWMU to the north, and Base Lake to
the east (700 feet). The majority of the site is restricted to the public as well as the
majority of Base personnel. There is no anticipated change in the future site use due to
proximity to runway.
The only potential receptor is Base Lake located approximately 700 feet downgradient
from the excavated source area. To date, vinyl chloride has not been detected in surface
water samples collected from the lake. According to the cross-sections (for example, see
Figure 4b) the plume goes underneath the surface water. Additionally, natural
attenuation processes are reducing contaminant mass within the dissolved plume.
CY11 results from monitoring wells HF6-MW4S, HF6-MW5S, and HF6-MW6S located
downgradient of the former FTA3 old pond area, remain below all MCLs and Regional
Screening Levels.
There is limited potential for exposure since the contaminant source (i.e., soils within the
former training area) was removed in 2004 and administrative LUCs (i.e., digging,
drinking well restrictions, and Base fence) are in place. Fuel-contaminated soils
remaining outside of the former source do not pose a risk to human health. The general
public and Base personnel are restricted from direct access to the SWMU.
There are no existing complete exposure pathways associated with the groundwater
plume since there are no domestic or other supply wells within the plume or at the
current estimated distal end of the plume near Base Lake (URS, 2006b).
Key Point: Conditions are protective of potential and future receptors. Question
III.3 is answered “YES”.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
97
CASE STUDY 2 – OFFUTT AFB
3.3.4. Question III.4. Is there a near-term need for the impacted groundwater
resource or any impacted land uses?
Criteria: Evaluation of future needs for groundwater resource and associated overlying
land uses.
There is no anticipated change in the future site use due to proximity to runway. The
majority of the site is restricted to the public and Base personnel. There are no plans for
exploitation of groundwater resources and LUCs are in place.
Key Point: There is no anticipated near-term need for the impacted groundwater
resource or any impacted land uses. Question III.4 is answered “YES”.
4.0
SITE ASSESSMENT CONCLUSION
The exit strategy for a given site can be effectively strengthened by multiple lines of
evidence. The AFCEE LoRSC Manual provides a weight-of-evidence decision logic to
evaluate such lines of evidence.
The LoRSC Manual methodology was applied to the Fire Training Area 3 Site at Offutt
Air Force Base, Nebraska. Three main categories of data were examined: 1) a
comprehensive Conceptual Site Model, 2) control of sources, and 3) adverse effects of
residual contamination on present and future land and water uses.
Based on an evaluation of existing data, Fire Training Area 3 Site has:



a comprehensive CSM,
control of sources, and
no potential for adverse effects of residual contamination on present and future
land and water uses.
Consequently, the site may be categorized as a LoRSC Site Type A, “Strongest case
for low-risk closure or reduced monitoring.”
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
98
CASE STUDY 2 – OFFUTT AFB
5.0
REFERENCES
Parsons, 1999. Treatability Study in Support of Intrinsic Remediation for Fire Protection
Training Area 3, Offutt Air Force Base, Omaha, Nebraska, prepared by Parson
Engineering Science, Inc., 30 November 1999.
USEPA, Statement of Basis, no date.
URS, 1999. FY98 Long Term Monitoring Fire Protection Training Area 3 (FT-003), Offutt
Air Force Base, Nebraska, prepared by URS, May 7, 1999.
URS, 2006a. Fire Training Area 3 Biosparging Corrective Measures Implementation
Work Plan, ACC 4-BASE PBC, Offutt Air Force Base, Nebraska, prepared by URS,
May 15, 2006.
URS, 2006b. Environmental Restoration Program Basewide Work Plan, ACC 4-BASE
PBC, Offutt Air Force Base, Nebraska, prepared by URS, June 27, 2006.
URS, 2008a. Fire Training Area 3 Biosparging Corrective Measures Implementation
Work Plan – 2008 Addendum, ACC 4-BASE PBC, Offutt Air Force Base, Nebraska,
prepared by URS, July 1, 2008.
URS, 2008b. Basewide CY07 Annual Report and Remedy in Place Documentation ACC
4-BASE PBC, Offutt Air Force Base, Nebraska, prepared by URS, August 12, 2008.
URS, 2009. Basewide CY08 Annual Report ACC 4-BASE PBC, Offutt Air Force Base,
Nebraska, prepared by URS, June 30, 2009.
URS, 2010. Basewide CY09 Annual Report ACC 4-BASE PBC, Offutt Air Force Base,
Nebraska, prepared by URS, August 23, 2010.
URS, 2011a. Basewide CY10 Annual Report ACC 4-BASE PBC, Offutt Air Force Base,
Nebraska, prepared by URS, September 14, 2011.
URS, 2011b. 2011 Groundwater Sampling Work Plan Addendum, ACC 4-BASE PBC,
Offutt Air Force Base, Nebraska, prepared by URS, September 14, 2011.
URS, 2011c. Basewide CY11 Annual Report ACC 4-BASE PBC, Offutt Air Force Base,
Nebraska, prepared by URS, November 17, 2011.
WCC, 1993. Installation Restoration Program – Remedial Investigation, Offutt Air Force
Base, Nebraska, prepared by Woodward-Clyde Consultants, November 1993.
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
99
APPENDICES
APPENDICES
Appendix A.
Summary of State Programs for Site Exit/Closure
Appendix B.
Low-Risk Site Quick Reference Checklist
Appendix C.
Conceptual Site Model
Appendix D.
14 Compartment Model Template
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
100
APPENDIX A
APPENDIX A
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
L O W - R I S K
S I T E
C L O S U R E G U I D A N C E
▼ AFCEE ▼
M A N U A L
101
Page 1 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
Alabama
Regulatory Agency
Acronym
Web Site
Groundwater Site
Exit/Closure
Criteria Defined?
Dept. of Env.
Management, Water
Division, Groundwater,
UST Corrective Action
Information
ADEM
www.adem.state.al.us
Yes
Dept. of Env.
Conservation
ADEC
www.dec.alaska.gov
Dept. of Env. Quality
ADEQ
http://www.azdeq.gov
Dept. of Env. Quality
ADEQ
www.adeq.state.ar.us
Guidance for Site
Exit/Closure
Other State References
Yes
18 AAC 75.380 (non-LUST); 18
AAC 78.276 (LUST); and
Streamlined Cleanup Program
Guidance.
Proposed Environmental Site
Closeout Concepts, Criteria,
and Definitions; and, Site
Closure Memoradum.
Yes
Arizona Department of
Environmental Quality UST
Program Release Reporting &
Corrective Action Guidance
Alaska
Arizona
Arkansas
Groundwater Site
Closure with
Contaminants in
Place?
Page 2 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
Regulatory Agency
Acronym
Web Site
Groundwater Site
Exit/Closure
Criteria Defined?
California
Environmental
Protection Agency,
State Water
Resources Control
Board, and 9 Regional
Water Quality Control
Boards.
Cal/EPA
www.swrcb.ca.gov
Yes
Groundwater Site
Closure with
Guidance for Site
Contaminants in
Exit/Closure
Place?
Yes
Assessment Tool for Closure of
Low-Threat Chlorinated
Solvent Sites (California
Regional Water Quality Control
Board, San Francisco Bay
Region, 2009);
Supplemental Instructions to
State Water Board December
8, 1995, Interim Guidance on
Required Cleanup at Low Risk
Fuel Sites (California Regional
Water Quality Control Board,
San Francisco Bay Region,
1996);
California
Other State References
Cleanup Programs - Status
Report Including Case
Closures;
Fact Sheet Underground
Storage Tank Program Site
Closure Process;
Fact Sheet Petroleum
Hydrocarbon Cleanup
Approach for Soils;
Draft California Independent
UST Case Closure Study;
and
Other low risk documents for
Underground Storage Tank
Program Site Closure Process fuels and chlorinated volatile
organic compounds.
(California Regional Water
Quality Control Board, North
Coast Region, 2009);
Low-Threat Underground
Storage Tank Case Closure
Policy (State Water Resources
Control Board, 2012).
Dept. of Env. Health's
Hazardous Materials
Compliance Division
HMCD
www.sccgov.org
Recommended Minimum
Verification Analyses for
Underground Storage Tank
Leaks
Page 3 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
California
(Cont'd)
Regulatory Agency
Acronym
Web Site
Dept. of Env. Health's
Dertified Unified
Program Agency (The
County of Fresno)
CUPA
www.co.fresno.ca.us
County of San Mateo
Health System
California Department
of Toxic Substances
Control
California Regional
Water Quality Control
Board: North Coast
Region
California Regional
Water Quality Control
Board: San Francisco
Bay Region
Groundwater Site
Exit/Closure
Criteria Defined?
Groundwater Site
Closure with
Contaminants in
Place?
Guidance for Site
Exit/Closure
Underground Storage Tank
Closure Guidelines
www.co.sanmateo.ca.us
DTSC
Case Closure
www.dtsc.ca.gov/
www.waterboards.ca.go
v/northcoast/
Underground Storage Tank
Program Site Closure Process
(2009)
www.waterboards.ca.go
v/sanfranciscobay/
Assessment Tool for Closure of
Low-Threat Chlorinated
Solvent Sites (2009);
Supplemental Instructions to
State Water Board December
8, 1995, Interim Guidance on
Required Cleanup at Low Risk
Fuel Sites (1996).
California Regional
Water Quality Control
Board: Central Coast
Region
www.waterboards.ca.go
v/centralcoast/
California Regional
Water Quality Control
Board: Los Angeles
Region
www.waterboards.ca.go
v/losangeles/
Other State References
Page 4 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
California
(Cont'd)
Colorado
Connecticut
Delaware
District of
Columbia
Regulatory Agency
Acronym
Web Site
California Regional
Water Quality Control
Board:Central Valley
Region
www.waterboards.ca.go
v/centralvalley/
California Regional
Water Quality Control
Board: Lahontan
Region
www.waterboards.ca.go
v/lahontan/
California Regional
Water Quality Control
Board: Colorado River
Basin Region
www.waterboards.ca.go
v/coloradoriver/
California Regional
Water Quality Control
Board: Santa Ana
Region
California Regional
Water Quality Control
Board: San Diego
Region
Dept. of Labor and
Employment, Division
of Oil and Public
Safety, Remediation
Section
Dept. of Env.
Protection
Dept. of Natural
Resources and Env.
Control
District Dept. of the
Ennvironment
www.waterboards.ca.go
v/santaana/
Groundwater Site
Exit/Closure
Criteria Defined?
Groundwater Site
Closure with
Contaminants in
Place?
Guidance for Site
Exit/Closure
Fact Sheet (2/1/99)
Underground Storage Tank
(UST) Program: Site
Investigation and
Remediation.
www.waterboards.ca.go
v/sandiego/
CDPHE
www.cdphe.state.co.us
CT DEP
www.ct.gov/dep/site/def
ault.asp
www.dnrec.delaware.go
v
DNREC
DDOE
www.ddoe.dc.gov
Other State References
Yes
DRAFT Guidance for the
Closure of Low-Threat Sites
with Residual Ground Water
Contamination
Yes
Closure Plan Guidline
Document
Page 5 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
Acronym
Web Site
Groundwater Site
Exit/Closure
Criteria Defined?
Dept. of Env.
Protection
DEP
www.dep.state.fl.us
Yes
Dept. of Natural
Resources
Dept. of Health, Env.
Mgmt. Div., Solid and
Haz. Waste Branch
DNR
www.dnr.state.ga.us
HDOH
www.hawaii.gov/health/
environmental/waste/ust
/index.html
Dept. of Env. Quality
DEQ
www.deq.idaho.gov
Regulatory Agency
Florida
Georgia
Hawaii
Idaho
Yes
Groundwater Site
Closure with
Guidance for Site
Other State References
Contaminants in
Exit/Closure
Place?
Yes
"No Further Action" Status-62- Storage Tank System
77.680
Closure Assessment
Requirements; RCRA
Generator Closure Actions;
and, Guidance for Evaluation
of Low Yield/Poor Quality
Criteria; A Comprehensive
Study of the Relative
Success of Site Cleanups
Under Preapproval and Pay
for Performance Contracting.
Technical Guidance Manual for
Underground Storage Tank
Closure and Release
Response
UST Closure and Change-inService; Evaluation of
Environmental Hazard at
sites with Contaminated Soil
and Groundwater, Volume 1:
User's Guide; Assessment
and Cleanup of
Contaminated Sites; Fast
Track Cleanup (Section 15
of the Hazard Evaluation and
Emergency Response
Technical Guidance
Manual); and, Voluntary
Response Program.
Page 6 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
Acronym
Web Site
Env. Protection
Agency
EPA
www.epa.state.il.us
Yes
Dept. of Env. Mgmt.
IDEM
www.in.gov/idem
Dept. of Natural
Resources
Dept. of Health and
Env.
Div. of Waste Mgmt.
Dept. of Env. Quality
Dept. of Env.
Protection
Dept. of the Env.
Dept. of Env.
Protection
DNR
www.iowadnr.com
KDHE
www.kdheks.gov
DWM
DEQ
MEDEP
www.waste.ky.gov
www.deq.louisiana.gov
www.state.me.us/dep
Yes
Closure for Landfills (38 §1310- Municipal Landfill
C.) and USTs
Remediation Program
MDE
MassDEP
www.mde.state.md.us
www.mass.gov
Yes
Commonwealth of
Massachusetts Underground
Storage Tank Closure
Assessment Manual
DEQ
MPCA
www.michigan.gov/deq
www.pca.state.mn.us
MDEQ
DNR
www.deq.state.ms.us
www.dnr.mo.gov/
DEQ
NDEQ
www.deq.mt.gov
www.deq.state.ne.us
Regulatory Agency
Illinois
Yes
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Groundwater Site
Closure with
Contaminants in
Place?
Groundwater Site
Exit/Closure
Criteria Defined?
Dept. of Env. Quality
Pollution Control
Agency
Dept. of Env. Quality
Dept. of Natural
Resources
Dept. of Env. Quality
Dept of Env. Quality
Guidance for Site
Exit/Closure
Other State References
RCRA Closure Plan - 35III.
Adm. Code Part 724 and 725;
Guidance for Preparing RCRA
Closure Plans; and Appendix F
to LPC-PA2: Instructions for
Closure Plan and Post-Closure
Care Plans for Putrescible and
Chemical Waste Landfills.
Independent Closure Process Voluntary Remediation
(ICP) Guidance
Program; RISC Technical
Resource Guidance
Document; and RISC User's
Guide.
Page 7 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
Nevada
New Hampshire
Groundwater Site
Exit/Closure
Criteria Defined?
Groundwater Site
Closure with
Contaminants in
Place?
Acronym
Web Site
Div. of Env. Protection
NDEP
www.ndep.nv.gov
Dept. of Env. Services
NHDES
www.des.state.nh.us
Yes
Contaminated Site Closure: A
Property Owner's Guide
Dept. of Env.
Protection
NJDEP
www.state.nj.us./dep
Yes
Environment Dept.
NMED
www.nmenv.state.nm.us
Yes
Closure information in
regulations; and, Municipal
Landfill Site Closure,
Remediation and
Redevelopment Act.
Clusure for Pits (DRAFT - OCD October 2009 Closure Plan
Pit Rule Guidance V.1.0)
Amendment as Changed
DEC
www.dec.ny.gov
www.ncdenr.gov
New Jersey
New Mexico
New York
Dept. of Env.
Conservation
Dept. of Env. and
Natural Resources
NCDENR
North Carolina
Dept. of Health
Dept. of Commerce,
Bureau of
Underground Storage
Tanks Regulations
NDDoH
www.ndhealth.gov/
DOC
www.com.ohio.gov/fire/b
(BUSTR)
ustmain.aspx
North Dakota
Ohio
Dept of Env. Quality
ODEQ
www.deq.state.ok.us
Dept. of Env. Quality
DEQ
www.oregon.gov/DEQ
Oklahoma
Oregon
Guidance for Site
Exit/Closure
Regulatory Agency
Other State References
Closure for Landfills (Solid
Waste Financial Assurance
Program Report)
Yes
Yes
Yes
Guidelines for Assessment and
Corrective Action for UST
Releases
Closure forLandfills (Solid
Waste Financial Assurance
Program Report Chap.1 and 5;
252.616-13-1; and 252.621;
252.606)
Draft: VCP & Brownfields
Site Characterization
Template; Directions for Use
of Suitable Portions of the
Solid Waste Stream for Land
Restoration/Reclamation
Projects.
Page 8 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
Pennsylvania
Rhode Island
Regulatory Agency
Dept. of Env.
Protection
Acronym
Web Site
DEP
Groundwater Site
Exit/Closure
Criteria Defined?
Dept. of Env. Mgmt.
Dept. of Health and
Env. Control
DEM
DHEC
www.depweb.state.pa.u
s/portal/server.pt/comm
unity/dep home/5968
www.dem.ri.gov
www.scdhec.gov
Dept. of Env. and
Natural Resources
Dept. of Env. and
Conservation
Commission on Env.
Quality
SD DENR
www.denr.sd.gov
TDEC
www.state.tn.us/environ
ment
TCEQ
www.tceq.state.tx.us
Yes
Dept. of Env. Quality
UDEQ
www.deq.utah.gov
Yes
Dept. of Env.
Conservation
Dept. of Env. Quality
DEC
www.anr.state.vt.us/dec
VDEQ
www.deq.state.va.us
WDOE
WV DEP
www.ecy.wa.gov
www.dep.wv.gov
Groundwater Site
Closure with
Contaminants in
Place?
Guidance for Site
Exit/Closure
Soil and Groundwater
Closure Projects Manual:
Evaluation of Source
Materials at SRS Waste
Units
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Dept. of Ecology
Dept. of Env.
Protection
Other State References
Process for Expedited Closure
Evaluation for Priority 4.1
Petroleum Hydrocarbon LPST
Sites
Preparation of Solid Waste
Facility Closure and PostClosure Plans Guidance; and,
Leaking Underground Storage
Tank (LUST) Subsurface
Investigation Report Guide.
TRRP: Compatibility with
Resource Conservation and
Recovery Act (RCRA)
UST Branch: Closure Plan
Requirements; RCRA
Corrective Action; USEPA
Superfund; and, State
Voluntary Cleanup Program.
Draft Guidance Manual for
Closure and and PostClosure Plans for Hazardous
Waste Management
Facilities
Page 9 of 9
TABLE A.1
SUMMARY OF STATE PROGRAMS FOR SITE EXIT/CLOSURE
Low-Risk Site Closure Guidance Manual
Air Force Center for Engineering and the Environment
State
Regulatory Agency
Acronym
Web Site
Groundwater Site
Exit/Closure
Criteria Defined?
DNR
www.dnr.wi.gov
Yes
WDEQ
www.deq.state.wy.us
Yes
Dept. of Natural
Resources
Wisconsin
Dept. of Env. Quality
Wyoming
Definitions:
LUST = Leaking Underground Storage Tank
OCD = Oil Conservation Division
RCRA = Resource Conservation and Recovery Act
RISC = Risk Integrated System of Closure
SRS = Savannah River Site
TRRP = Texas Risk Reduction Program
UST = Underground storage tank
VCP = Voluntary Cleanup Program
Groundwater Site
Closure with
Guidance for Site
Contaminants in
Exit/Closure
Place?
Yes
Risk Screening and Closure
Criteria for Petroleum Product
Contaminated Sites, and
Agancy Roles and
Responsibilities
Solid and Hazardous Waste
Division Storage Tank Program
Guidance Document #1
(Subject: Site Closure)
Other State References
APPENDIX B
APPENDIX B
LOW-RISK SITE QUICK REFERENCE CHECKLIST AND BLANK
FORMS
⧠
⧠
⧠
⧠
⧠
⧠
⧠
⧠
⧠
⧠
⧠
⧠
LoRSC Decision Logic? (blank form attached to Appendix B)
Comprehensive conceptual site model developed? (blank CSM attached to
Appendix B)
⧠ Site information
⧠ Site investigations
⧠ Source characterization
⧠ Constituents of concern
⧠ Nature and extent of contamination
⧠ Hydrogeology
⧠ Geochemistry
⧠ Migration and exposure pathways
⧠ Contaminant attenuation pathways
⧠ Receptors
⧠ Soil remediation
⧠ Groundwater remediation
⧠ Stakeholders
⧠ 14 Compartment Model
Information available about presence of NAPLs?
Qualitative evaluation of matrix diffusion processes at the site? (blank 14
Compartment Model attached to Appendix B)
Plume Magnitude Category based on mass discharge estimate? (blank Plume
Magnitude Category table attached to Appendix B)
Multiple years of concentration vs. time records available for wells in the source
zone?
Observation of footprints of natural attenuation in the source zone?
Evaluation of the potential performance of remediation technologies? (blank
Qualitative Decision Chart attached to Appendix B)
Analysis of exposure pathways for actual receptors
Multiple years of concentration vs. time records available for wells in the plume?
Observation of footprints of natural attenuation in the plume?
Evaluation of future resource needs for affected groundwater?
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Low-Risk Decision Questions
III.
Do You Have A Complete Conceptual Site Model (CSM) That Reflects Key Low-Risk Closure Concepts?
2.
Have all of the components of the Conceptual Site
Model been evaluated?
Answers For
“Supporting”
Questions
Answers For “Must
Have” Questions
Key Low-Risk Decision Criteria
Manual Reference
Conceptual Site Model checklist.
Yes ⧠
No ⧠
Section 3.1.1
8. Are there any significantly mobile source materials?
DNAPL sites: no mobile DNAPL observed. LNAPL sites: no expanding LNAPL zone and zero
or low LNAPL transmissivity.
Yes ⧠
No ⧠
Section 3.2.1
9. Is the source zone free of any environmentally
significant quantity of NAPL?
Little or no DNAPL observed in transmissive zones, and no significant LNAPL accumulation
based on specific volume calculations.
Yes ⧠
No ⧠
Section 3.2.2
10. Is it possible that any further source zone cleanup will
be constrained by matrix diffusion processes?
Qualitative evaluation of matrix diffusion processes based on geology, chemical properties,
timing of initial release, and remediation efforts.
Yes ⧠
No ⧠
Section 3.2.3
11. Are sources relatively small?
Plume is classified as a Mag 4 Plume Magnitude Category or less based on mass discharge
estimates, OR maximum source concentrations are < 20x Maximum Contaminant Level (MCL).
Yes ⧠
No ⧠
Section 3.2.4
12. Are source zone concentrations stable or decreasing?
Representative source zone concentrations over time are shown to be stable, decreasing, or
probably decreasing.
Yes ⧠
No ⧠
Section 3.2.5
13. Is there evidence of on-going natural attenuation
processes in the source zone?
Footprints of source zone attenuation are seen (such as generation of daughter products or
consumption of electron acceptors).
Yes ⧠
No ⧠
Section 3.2.6
14. Will future source remediation only marginally improve
site conditions?
There is “Less Need For Source Treatment” based on the Qualitative Decision Chart.
IV. Are Sources Controlled?
III.
Yes ⧠
No ⧠
Section 3.2.7
Will Residual Contamination Have No Adverse Effect on Present and Future Land and Water Uses?
5. Is the groundwater plume stable or shrinking?
Plume trend analyses showing decreasing plume over time.
Yes ⧠
No ⧠
Section 3.3.1
6. Is there evidence of on-going natural attenuation
processes in the plume?
Analyses of natural attenuation processes and footprints of natural attenuation in the plume.
Yes ⧠
No ⧠
Section 3.3.2
7. Are conditions protective of potential and future
receptors?
Analyses showing all exposure pathways for receptors are incomplete or present acceptable
risks, and that future exposure will not occur.
Yes ⧠
No ⧠
Section 3.3.3
Yes ⧠
No ⧠
Section 3.3.4
8. Is there a near-term need for the impacted groundwater Evaluation of future needs for groundwater resource and associated overlying land uses.
resource or any impacted land uses?
KEY:
“Must Have” Data: Critical Line of evidence for low-risk site closure - necessary to demonstrate these criteria at almost all sites if applicable.
“Supporting” Data: Supporting line of evidence, with 0-4 of the supporting lines recommended for low-risk site closure.
MUST HAVE:
All Yes?
Yes ⧠ (Type A or B)
No ⧠ (Type C)
SUPPORTING:
How Many “Yes”?
Type A if 3-4 Yes ⧠
Type B if 0-2 Yes ⧠
WHAT IT MEANS
LoRSC Site Type A (Strongest case for low-risk closure or reduced monitoring) = All “Must Have” Questions = Yes AND 3 or 4 of the “Supporting” Questions = Yes
LoRSC Site Type B (Moderately good case for low-risk closure or reduced monitoring) = All “Must Have” Questions = Yes AND 0 to 2 of the “Supporting” Questions = Yes
LoRSC Site Type C (More difficult for low-risk closure or reduced monitoring) = Any “Must Have” Questions = No
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Conceptual Site Model
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14 Compartment Model
Source Zone
Zone/
Phases
Low
Permeability
Transmissive
Plume
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
Sorbed
Legend:
Figure. 14 Compartment Model. Arrows show mass potential transfer links between compartments.
Dashed arrows indicate irreversible fluxes (Sale and Newell, 2011). NA = Not Applicable.
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Application of Plume Magnitude Classification System
Mass
Discharge
(g/day)
< 0.001
Plume
Classification
Mag 1
Low-Risk Plume?
YES
⧠
Impact*
Limited impact
Could impact a domestic well,
pumping at 150 gallons per day (gpd)
or less
Could impact a well pumping at 1
0.01 to 0.1
Mag 3
YES
⧠ gallons per minute (gpm) or less
Could impact a well pumping at 10
0.1 to 1
Mag 4
YES
⧠ gallons per minute (gpm) or less
Could impact a well pumping at 100
1 to 10
Mag 5
MAYBE
⧠ gpm or less
Could impact a stream with a mixing
10 to 100
Mag 6
MAYBE
⧠ zone base flow of 1 cubic feet per
second (cfs) or less
Could impact a stream with a mixing
100 to
Mag 7
LIKELY NOT
⧠ zone base flow of 10 cfs or less
1,000
Could impact a stream with a mixing
1,000 to
Mag 8
LIKELY NOT
⧠ zone base flow of 100 cfs or less
10,000
Could impact a stream with a mixing
10,000 to
Mag 9
⧠ zone base flow of 1,000 cfs or less
LIKELY NOT
100,000
Could impact a stream with a mixing
>100,000
Mag 10
LIKELY NOT
⧠ zone base flow of >10,000 cfs
* Impact based on a drinking water standard in pumped water or mixing zone of 5 μg/L.
0.001 to
0.01
Mag 2
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Qualitative Decision Chart
Figure 10. Qualitative Decision Chart on the merits of source depletion (Sale et al., 2008;
Adapted from USEPA’s “The DNAPL Remediation Challenge: Is There a Case for Source
Depletion?” (Kavanaugh et al., 2003).
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APPENDIX C
APPENDIX C
CONCEPTUAL SITE MODEL
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APPENDIX D
APPENDIX D
14 COMPARTMENT MODEL STEP-BY-STEP GUIDE AND TEMPLATE
Chlorinated solvents in source zones can be present in the vapor, DNAPL, aqueous, or
sorbed phase. In plumes, solvents can be present in the vapor, aqueous, or sorbed
phase. The 14 Compartment Model is a graphic tool (Figure D.1) that identifies the
phases, in both low-permeability and transmissive zones, in which contaminants occur
as sources and plumes. The 14 Compartment Model is used to determine: 1) what stage
the site is at, 2) if the CSM is complete, and 3) what’s going to happen if remediation is
performed.
As discussed in Sale and Newell (2011):
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Source: Sale and Newell, 2011.
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Source: Sale and Newell, 2011.
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The 14 Compartment Model template can be completed using the conditions stated
below to determine if mass is likely to be present in low-permeability compartments:
 Presence of low-permeability lenses or strata in an affected aquifer in contact
with transmissive zones containing plumes.
 High concentrations of contaminants.
 Older release sites (i.e., significant elapsed time since contaminant release).
 Geologic settings where transmissive zones are only a small fraction of the total
volume of the aquifer.
 Aquifers with relatively slow groundwater flow rates.
 Sediments with high fraction organic carbon content.
 Presence of contaminants that exhibit stability in their physical setting.
 Release of large amounts of contaminants.
Source Zone
Zone/
Phases
Low
Permeability
Transmissive
Plume
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
Sorbed
Figure D.1. 14 Compartment Model. Arrows show mass potential transfer links between
compartments. Dashed arrows indicate irreversible fluxes (Sale and Newell, 2011). NA = Not
Applicable.
Step-by-Step Guide to Completing the 14 Compartment Model Before Remediation
Both mass and concentration measurements can be used to complete the 14
Compartment Model (14-CM). However, due to the relative ease of availability of
groundwater concentration data, this guide focuses on the concentration approach. For
illustration purposes, this step-by-step guide utilizes the Offutt Air Force Base Fire
Training Area 3 case study (i.e., Case Study 2).
Transmissive zone aqueous groundwater concentration data for monitoring wells located
in the source zone and plume areas are readily available at any site. Therefore, Step 1
(shown in red in the figure below) of this guide starts with the analysis of such Source
Zone Transmissive Zone Aqueous Phase data:
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Source Zone
Plume
Low
Permeability
Transmissive
Transmissive
Low
Permeability
Vapor
Step 12
Step 11
Step 13
Step 14
DNAPL
Step 10
Step 9
NA
NA
Aqueous
Step 3
Step 1
Step 5
Step 7
Sorbed
Step 4
Step 2
Step 6
Step 8
Zone/
Phases
Step 1 (Source Transmissive Zone Aqueous Phase):
a. Determine the current (not historic) maximum observed groundwater
concentration in the source zone area monitoring wells. For example, for FTA3,
the maximum observed concentration = 13 μg/L for vinyl chloride.
b. Determine the 14-CM table number and cell color using the observed
concentration and the legend below. Note that in the legend, 1) concentrations
are classified on an order of magnitude scale, and 2) the numbers and colors
represent the numbers and color shading entered into the 14 Compartment
Model.
c. For the FTA3 Case Study, source zone Cmax = 13 μg/L. Therefore, the 14-CM
number = 2 (for 10s of μg/L in water) and color = cream.
d. Enter “2” in the cell for Step 1 (Source Zone Transmissive Zone Aqueous Phase)
and color the cell cream:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
2
Sorbed
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Step 2 (Source Transmissive Zone Sorbed Phase):
a. Transmissive Zone Sorbed Phase = Transmissive Zone Aqueous Phase
concentration.
For pre-remediation conditions, the sorbed phase in the
transmissive zone is probably in equilibrium with the aqueous phase, so these
two compartments would have the same value. (If filling the 14 Compartment
Model using mass, however, you would take the number in the aqueous phase
and multiply by the Retardation Factor minus 1).
b. For the FTA3 case study, the 14-CM number = 2 and color = cream:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
2
Sorbed
2
Step 3 (Source Low-Permeability Zone Aqueous Phase):
a. The Low-Permeability Zone Aqueous Phase concentration = Transmissive Zone
Aqueous Phase Concentration plus 1 (based on experience at several sites, lowpermeability compartment concentrations are typically 10 times higher than
transmissive zone concentrations), therefore in the absence of any data from the
low-permeability compartment take the Step 1 14-TC number +1.
b. For the FTA3 case study, Low-Permeability Zone Aqueous Phase number = 2+1
= 3 and color = orange:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
3
2
Sorbed
2
Step 4 (Source Low-Permeability Zone Sorbed Phase):
a. As with Step 2, it is assumed that the Low-Permeability Zone Sorbed Phase is in
equilibrium with the Low-Permeability Zone Aqueous Phase concentration.
b. For the FTA3 case study, Low-Permeability Zone Sorbed Phase = 3 and color =
orange:
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Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
3
2
Sorbed
3
2
Step 5 (Plume Transmissive Zone Aqueous Phase):
a. Determine the current maximum (not historical) observed groundwater
concentration in the middle of the plume area monitoring wells (middle being
about halfway from source to downgradient edge of the plume). (Note Step 1 was
for source zone wells).
b. For the FTA3 case study, the maximum observed concentration = 2.3 μg/L for
vinyl chloride. Therefore, the Low-Permeability Zone Sorbed Phase = 1 and
color = green:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
3
2
Sorbed
3
2
1
Step 6 (Plume Transmissive Zone Sorbed Phase):
a. As with Step 2, the Transmissive Zone Sorbed Phase = Transmissive Zone
Aqueous Phase concentration because these two compartments are likely in
equilibrium.
b. For the FTA3 case study, the 14-CM number = 1 and color = green:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
3
2
1
Sorbed
3
2
1
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Step 7 (Plume Low-Permeability Zone Aqueous Phase):
a. If the plume age is <10 years, then Plume Low-Permeability Aqueous Phase
number = [Step 5 number (Transmissive Zone Aqueous Phase) – 1]. This is
because it is unlikely that high concentrations from earlier decades have
penetrated the low-permeability strata.
b. If the plume age is 10 - 30 years, then Plume Low-Permeability Aqueous Phase
number = Transmissive Zone Aqueous Phase.
c. If the plume age is >30 years, then Plume Low-Permeability Aqueous Phase
number = [Step 5 number (Transmissive Zone Aqueous Phase) + 1]. This is
based on experience at research sites where higher concentrations from
decades ago had penetrated the low-permeability strata.
d. For the FTA3 case study, the release occurred between 1960 and 1990 (i.e.,
2011-1960 = 51 years and 2011-1990 = 21 years). Assuming an average of 36
years, the 14-CM number = 1+1 = 2 and color = cream:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
NA
NA
2
Vapor
DNAPL
Aqueous
3
2
1
Sorbed
3
2
1
Step 8 (Plume Low-Permeability Zone Sorbed Phase):
a. As with Step 2 and Step 6, the Low-Permeability Sorbed Phase = LowPermeability Aqueous Phase concentration because these two compartments
are likely in equilibrium.
b. For the FTA3 case study, Low-Permeability Zone Sorbed Phase = 2 and color =
cream:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
NA
NA
Vapor
DNAPL
Aqueous
3
2
1
2
Sorbed
3
2
1
2
Step 9 (Source Zone Transmissive Zone DNAPL):
a. Although very subjective, the following rules can be applied if there are no
detailed data about NAPL presence. If no NAPL has ever been directly observed
in wells or properly evaluated cores (such as coring + dye tests), then Source
Transmissive Zone DNAPL Phase number = 0 and color = clear.
b. If the presence of residual NAPL is observed then the 14-CM number = 2 and
color = cream.
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c. If the presence of mobile NAPL is detected then depending on the quantity of
observed NAPL, the 14-CM number = 1 to 4, where 1 would be a small amount
of NAPL observed in a few cores, and 4 would be used for sites where significant
NAPL pools have been observed.
d. For the FTA3 case study, residual NAPL has been observed in soil borings in
1988-1994, therefore, the 14-CM number = 2 and color = cream:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Transmissive
Low
Permeability
2
NA
NA
Vapor
DNAPL
Aqueous
3
2
1
2
Sorbed
3
2
1
2
Step 10 (Source Low-Permeability DNAPL):
a. This is difficult to assess. In some cases it can be assumed there is likely no
NAPL in the Low-Permeability Compartment if large fractures are absent. If large
fractures are present, then a simple assumption is that the Low-Permeability
Zone DNAPL Phase = Transmissive Zone DNAPL Phase.
b. For the FTA3 case study, the 14-CM number = 2 (site is underlain by limestones
and shales) and color = cream:
Source Zone
Zone/
Phases
Plume
Low
Permeability
Transmissive
Transmissive
Low
Permeability
DNAPL
2
2
NA
NA
Aqueous
3
2
1
2
Sorbed
3
2
1
2
Vapor
Step 11 (Source Transmissive Zone Vapor):
a. If the subsurface vapor-to-receptor pathway is active, then one can use vapor
measurements taken at the point of exposure (e.g., inside a building with
receptors), then apply Henry’s law, and enter an equivalent aqueous phase
concentration assuming complete equilibrium.
b. Alternatively,
i.
If no NAPL or aqueous-phase contaminants are present at the top of the
water table, then the 14-CM number = 0.
ii.
If NAPL is present at the top of the water table, then the 14-CM number =
Step 9 number - 1.
iii.
If aqueous-phase contaminants are present at the top of the water table,
then the 14-CM number = Step 1 number - 1.
c. If this pathway is incomplete and therefore not an important part of the
Conceptual Site Model, then enter “IP” for “Incomplete Pathway”.
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d. For the FTA3 case study, the vapor pathway is incomplete, therefore the Source
Zone Transmissive Zone Vapor Phase number = “IP” (Incomplete Pathway) and
color = clear:
Source Zone
Zone/
Phases
Low
Permeability
Plume
Transmissive
Vapor
Transmissive
Low
Permeability
IP
DNAPL
2
2
NA
NA
Aqueous
3
2
1
2
Sorbed
3
2
1
2
Step 12 (Source Low-Permeability Zone Vapor):
a. For the Source Low-Permeability Zone Vapor, the 14-CM number = Step 11
number - 1.
b. For the FTA3 case study, the vapor pathway is incomplete, therefore the14-CM
number = “IP” (Incomplete Pathway) and color = clear:
Source Zone
Zone/
Phases
Plume
Low
Permeability
Transmissive
Transmissive
Low
Permeability
Vapor
IP
IP
DNAPL
2
2
NA
NA
Aqueous
3
2
1
2
Sorbed
3
2
1
2
Step 13 (Plume Transmissive Zone Vapor):
a. Like Step 11, if the subsurface vapor-to-receptor pathway is active, then one can
use vapor measurements taken at the point of exposure (e.g., inside a building
with receptors), then apply Henry’s law, and enter an equivalent aqueous phase
concentration assuming complete equilibrium.
b. Alternatively,
a. If no aqueous-phase contaminants are present at the top of the water
table, then the 14-CM number = 0.
b. If aqueous-phase contaminants are present at the top of the water table,
then the 14-CM number = Step 5 number - 1.
c. If this pathway is incomplete and therefore not an important part of the
Conceptual Site Model, then enter “IP” for “Incomplete Pathway”.
d. For the FTA3 case study, the vapor pathway is incomplete, therefore the14-CM
number = “IP” (Incomplete Pathway) and color = clear:
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Source Zone
Zone/
Phases
Plume
Low
Permeability
Transmissive
Transmissive
Low
Permeability
Vapor
IP
IP
IP
DNAPL
2
2
NA
NA
Aqueous
3
2
1
2
Sorbed
3
2
1
2
Step 14 (Plume Low-Permeability Zone Vapor):
a. For the Plume Low-Permeability Zone Vapor, the 14-CM number = Step 13
number - 1.
b. For the FTA3 case study, the vapor pathway is incomplete, therefore the 14-CM
number = “IP” (Incomplete Pathway) and color = clear:
Source Zone
Zone/
Phases
Plume
Low
Permeability
Transmissive
Transmissive
Low
Permeability
Vapor
IP
IP
IP
IP
DNAPL
2
2
NA
NA
Aqueous
3
2
1
2
Sorbed
3
2
1
2
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