&ER&
United States
Environmental Protection
Agency EPA 510-R-15-001
Technical Guide For Addressing
Petroleum Vapor Intrusion
At Leaking Underground Storage
Tank Sites
U.S. Environmental Protection Agency
Office of Underground Storage Tanks
Washington, D.C.
June 2015
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Contents
Recommendations 1
Background 1
Overview Of Petroleum Vapor Intrusion (PVI) 2
Scope And Applicability 3
Recommended Actions For Addressing PVI 3
Supporting Technical Information 10
1. Petroleum Vapor Intrusion (PVI) 11
2. Typical PVI Scenarios 33
3. Site Characterization And Conceptual Site Model (CSM) 39
4. Lateral Inclusion Zone 44
5. Vertical Separation Distance 48
6. Mobile And Residual Light Non-Aqueous Phase Liquid (LNAPL) 57
7. Ground water Flow And Dissolved Contaminant Plumes 61
8. Soil Gas Profile 66
9. Clean, Biologically Active Soil 75
10. Non-PHC Fuel Additives 81
11. Seasonal And Weather Effects 96
12. Vapor Intrusion Attenuation Factor (a) 100
13. Computer Modeling Of Petroleum Vapor Intrusion 106
Glossary 117
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Tables
1. Recommended Actions For Addressing PVI At Leaking Underground Storage Tank Sites 7
2. Summary Of Characteristics Of Typical Scenarios Of Petroleum Vapor Sources And
Potential Receptors 36
3. Recommended Vertical Separation Distance Between Contamination And Building Floor
Foundation, Or Crawlspace Surface 52
4. Equations For Target Indoor Air Screening Concentrations For Volatile Chemicals 85
5. Example Target Residential Indoor Air Concentrations For EDB And 1,2-DCA 85
6. Equations For Groundwater And Soil Gas Screening Levels Based On Target Indoor Air
Screening Levels 86
7. Example Screening Concentrations For EDB And 1,2-DCA In Groundwater And Soil Gas 88
8. Comparison Of Risk Levels And Achievable Analytical Detection Limits For The Lead
Scavengers EDB And 1,2-DCA In Indoor Air 89
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Figures
1. Flowchart For Addressing PVI At Leaking Underground Storage Tank Sites 9
2. Conceptual Model Of Typical Petroleum Hydrocarbon Release 13
3. Difference In Potential For PVI Based On Type Of Source: a) LNAPL b) Dissolved Phase 33
4. Typical Scenarios Of Potential PVI Sources And Potential Receptors 35
5. Lateral Separation Distance Between Source Of PHC Contamination And Hypothetical
Receptor 45
6. Vertical Separation Distances Between Source Of PHC Contaminants And Hypothetical
Receptor: (a) Dissolved Source, (b) LNAPL Source 49
7. Conceptual Model Illustrating The Potential For Vapor Intrusion For a) Free-Phase
LNAPL Source, b) Residual-Phase LNAPL Source, And c) Dissolved-Phase Source 58
8. Typical Vertical Concentration Profile In The Unsaturated Zone For PHCs (Plus
Methane), Carbon Dioxide, And Oxygen 66
9. Relationship Between Source Vapor Concentration And Vapor Intrusion Attenuation
Factor (a) As A Function Of Vertical Separation Depth Between Contaminant Source
And Base Of Building (Receptor) 101
10. Rescaled Figure 9 That Expresses Source Vapor Concentration In Conventional Units 103
IV
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Acronyms
1,2-DCA 1,2-Dichloroethane (also known as ethylene dichloride or EDC)
ATSDR Agency for Toxic Substances and Disease Registry
BTEX Benzene, Toluene, Ethylbenzene, Xylenes (there are three isomers of xylene)
CFR Code of Federal Regulations
CSM Conceptual Site Model
EDB Ethylene Dibromide (also known as 1,2-dibromoethane)
IUR Inhalation Unit Risk
ILCR Incremental Lifetime Cancer Risk
IRIS Integrated Risk Information System
ITRC Interstate Technology & Regulatory Council
JEM Johnson-Ettinger Model
LNAPL Light Non-Aqueous Phase Liquid
MTBE Methyl Tertiary-Butyl Ether
NAPL Non-Aqueous Phase Liquid
OIG Office of Inspector General
OSWER Office of Solid Waste and Emergency Response
OUST Office of Underground Storage Tanks
PHC Petroleum Hydrocarbon
PPRTV Provisional Peer Reviewed Toxicity Value
PVI Petroleum Vapor Intrusion
RBCV Risk-based Concentration, Soil Vapor
RfC Reference Concentration (inhalation)
RSL Regional Screening Level (for Chemical Contaminants at Superfund Sites)
SIM Selective Ion Monitoring
TAME Tertiary-Amyl Methyl Ether
TBA Tertiary-Butyl Alcohol
TEL Tetraethyl Lead
TMB Trimethylbenzene
TML Tetramethyl Lead
TPH Total Petroleum Hydrocarbons
UST Underground Storage Tank
VI Vapor Intrusion
VISL Vapor Intrusion Screening Level
VOC Volatile Organic Compound
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Disclaimer
This document presents current technical recommendations of the U.S.
Environmental Protection Agency (EPA) based on our current understanding of
petroleum vapor intrusion (PVI) into indoor air from subsurface sources. This
document provides technical information to EPA, state, tribal, and local agencies.
It also informs the public and the regulated community on how EPA intends to
implement its regulations. This guidance document does not impose any
requirements or obligations on the EPA, the states, or local or tribal
governments, or the regulated community. Rather, the sources of authority and
requirements for addressing subsurface vapor intrusion are the relevant statutes
and regulations. Decisions regarding a particular situation should be made based
upon statutory and regulatory authority. Decision-makers retain the discretion
to adopt or approve approaches on a case-by-case basis that differ from this
document. Contact information for your state's UST-implementing agency may
be found at http://www.epa.Qov/oust/states/statconl.htm. EPA may revise this
document in the future, as appropriate.
VI
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Recommendations
This document provides technical information to regulatory personnel from the U.S.
Environmental Protection Agency (EPA) and state1, tribal, and local agencies for investigating
and assessing petroleum vapor intrusion (PVI) at sites where petroleum hydrocarbons (PHCs)
have been released from underground storage tanks (USTs). This document is comprised of
two parts: Recommendations, which provides a description of EPA's recommended approach
for addressing PVI, and Supporting Technical Information, which provides detailed technical
information supporting the recommendations.
Background
In 2002, EPA's Office of Solid Waste and Emergency Response (OSWER) issued draft vapor
intrusion guidance, OSWER Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air
Pathway from Groundwater and Soils (Draft Vapor Intrusion Guidance) (EPA, 2002).2 This draft
guidance explicitly states that it is not recommended for addressing petroleum vapor intrusion
(PVI) at UST sites regulated under Subtitle I of the Solid Waste Disposal Act through the
Hazardous and Solid Waste Amendments of 1984.
In 2009, EPA's Office of Underground Storage Tanks (OUST), at the request of partners and
stakeholders, initiated a collaborative effort to develop a technical guide for petroleum vapor
intrusion. Further highlighting the need for information on PVI, EPA's Office of Inspector
General later that year released an evaluation report, Lack of Final Guidance on Vapor
Intrusion Impedes Efforts to Address Indoor Air Risks (Report No. 10-P-0042).3 The report
included recommendations, one of which was for EPA to issue final vapor intrusion guidance
that incorporates information on how risks from petroleum hydrocarbon vapors should be
addressed. In response to stakeholder requests, EPA's Office of Underground Storage Tanks
developed this technical guide, which is a companion to OSWER's more general vapor intrusion
guide.4 Together, these two documents replace the 2002 Draft Vapor Intrusion Guide.
EPA developed the two guides to address different scenarios and meet the needs of different
audiences. The UST program regulates a very large universe of sites, typically gas stations,
which share many similar characteristics, including small release volumes (compared to
pipelines and tank farms, for example) and the potential for aerobic biodegradation of
petroleum vapors. Based on these facts, and to meet the request of UST regulators and
practitioners, EPA developed a guide specifically focused on petroleum UST releases. This PVI
guide provides screening criteria based on physical separation distances between vapor sources
and potential receptors. EPA OUST derived the screening criteria from an analysis of a large
1 The term state refers to regulatory agencies of states, territories, and the District of Columbia.
2 The OSWER draft guidance is accessible at
http://www.epa.ciov/osw/hazard/correctiveaction/eis/vapor/complete.pdf
3 The DIG report is accessible at http://www.epa.ctov/oict/reports/2010/20091214-10-P-0042.pdf
4 OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway From Subsurface Sources To
Indoor Air (OSWER Publication 9200.2-154), accessible at http://www.epa. ctov/oswer/vapohntrusion/.
Page 1 of 123
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data set of samples from leaking UST sites.5 In contrast, the OSWER Vapor Intrusion Guide
addresses a wide variety of sites and a broader range of contaminants.
Overview Of Petroleum Vapor Intrusion
Vapor intrusion is the general term given to migration of volatile organic chemicals (VOCs) from
any subsurface contaminant source, such as contaminated soil or groundwater, through the soil
and into an overlying building. There are two general classes of VOCsthat account for a large
number of soil and groundwater contamination sites in the United States:
• Petroleum hydrocarbons (PHCs) and non-PHC fuel additives
• Chlorinated solvents (e.g., the dry cleaning chemical tetrachloroethylene, also known as
perchloroethylene, (PCE), and the degreasing solvents trichloroethylene (TCE), 1,1,1-
trichloroethane (TCA)).
In this guide, petroleum vapor intrusion (PVI) is defined as the intrusion of vapors from
subsurface PHCs and non-PHC fuel additives into overlying or nearby buildings or structures.
Vapors emanating from petroleum-contaminated soil or groundwater that enter buildings may
result in indoor air concentrations that pose a risk to building occupants. PVI may pose both
immediate threats to safety (e.g., fire or explosion potential from petroleum vapors or
methane) and possible adverse health effects from inhalation of toxic chemicals (e.g., exposure
to benzene from gasoline). PVI may be associated with three groups of volatile chemicals:
• PHCs found in gasoline, diesel, and jet fuel (e.g., benzene, trimethylbenzenes (TMBs),
naphthalene)
• Volatile chemicals other than PHCs that may be found in petroleum fuels, such as
ethers, alcohols, and other fuel additives (e.g., methyl tertiary-butyl ether (MTBE),
tertiary-butyl alcohol (TBA), ethylene dibromide (EDB), and 1,2-dichloroethane (1,2-
DCA))
• Methane, which is generated from anaerobic biodegradation of PHCs and other
constituents of petroleum fuels (especially ethanol), and organic matter in soil
In contrast to chlorinated solvents, PHCs generally biodegrade rapidly under aerobic conditions
and if biodegradation is complete, produce only water and carbon dioxide. If biodegradation is
incomplete a variety of intermediate degradation products may be formed, but these are
usually less toxic than the parent PHCs.6 If chlorinated solvents biodegrade it is usually under
5 Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For Petroleum Hydrocarbon
Compounds (EPA 510-R-13-001), accessible at http://www.epa.qov/oust/cat/pvi/PVI Database Report.pdf
6 Some petroleum hydrocarbons may also degrade anaerobically and may produce methane, particularly if the
source is from an ethanol-blended gasoline. A recent modeling study cautions that for releases of high ethanol
fuel blends (i.e., greater than E-20) advective methane transport may result in methane buildup inside buildings
and pose a risk of explosion (Ma, et al., 2014, Numerical Model Investigation for Potential Methane Explosion and
Benzene Vapor Intrusion Associated with High-Ethanol Blend Releases, Environmental Science and Technology
48(1):474-481).
Page 2 of 123
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anaerobic conditions, which is slower and may produce intermediate degradation products that
are more toxic than the parent compounds.
The aerobic biodegradability of PHCs typically reduces the potential for PVI and justifies a
different approach for addressing PVI than for vapor intrusion from chlorinated solvents and
other non-aerobically biodegradable VOCs.7
Scope And Applicability
This PVI guide focuses on releases of petroleum-based fuels (e.g., gasoline, diesel), including
both PHCs and non-PHC fuel additives, from underground storage tanks (USTs) regulated under
Subtitle I of the Solid Waste Disposal Act of 1984, which are typically located at gas stations.8
This guide applies to new and existing releases of PHCs and non-PHC fuel additives from leaking
USTs and to previously closed sites where the implementing agency has reason to suspect that
there may be a potential for PVI. Although EPA developed the PVI guide based on data from
typical UST sites, this technical guide may also be helpful when addressing petroleum
contamination at comparable non-UST sites. Petroleum contamination at sites that are not
comparable to UST sites (such as refineries, petrochemical plants, terminals, aboveground
storage tank farms, pipelines, and large scale fueling and storage operations at federal
facilities), or sites with releases of non-petroleum chemicals including comingled plumes of
petroleum and chlorinated solvents regardless of the source, should be addressed under
OSWER's more general vapor intrusion guide.
This PVI guide does not impose legally binding requirements on implementing agencies or the
regulated community. Decision-makers retain the discretion to adopt or approve approaches
on a case-by-case basis that differ from this technical guide.
Recommended Actions For Addressing PVI
Addressing the potential for PVI is an integral part of the normal response to a suspected or
confirmed release from any Subtitle I regulated UST system. At any leaking UST site, it is
important to have a thorough understanding of the release (i.e., source, composition, and
magnitude) and other factors that may influence the distribution and transport of contaminants
that impact human safety and health. Until it is clear that human health and the environment
are adequately protected from adverse impacts caused by the release, appropriate site
characterization, risk assessment, and corrective action activities should continue.
7 For more information on the differences between PHCs and chlorinated solvents, see Petroleum Hydrocarbons
And Chlorinated Solvents Differ In Their Potential For Vapor Intrusion (http://www.epa.aov/oust/cat/pvi/pvicvi.pdf)
8 EPA's UST regulations are contained in 40 CFR Parts 280, 281, and 282.50-282.105 (see
http://www.epa.aov/oust/fedlaws/cfr.htm). Definitions of key terms such as UST and petroleum are found in 40
CFR 280. These definitions may change if the regulations are revised in the future.
Page 3 of 123
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EPA recommends the following actions for situations in which EPA, state, tribal, and local
agencies are investigating releases of petroleum-based fuels (including addressing potential
risks due to PVI) at leaking UST sites or where 40 CFR 280 requires9 UST owners and operators
to undertake release investigation and corrective action activities:
S Assess and mitigate immediate threats to safety - see Section 1 (p.11)
Identify whether there is a potential threat of explosion or fire due to the presence of
flammable PHCs and non-PHC fuel additive vapors or methane10. A threat could be
indicated by reports of the presence of odors, disagreeable taste of water, or visible
signs of PHC contamination by building occupants. If so, alert first responders so they
can, if necessary, evacuate these buildings until the potential threat to human safety
from fire or explosion due to PVI has been assessed and mitigated as needed.
S Conduct a site characterization and develop a conceptual site model (CSM) - see
Section 3 (p.39)
Site characterization data should be integrated into a conceptual site model (CSM). This
includes characterization of the physical, biological, and chemical systems at the site,
with emphasis on determining the spatial and temporal relationships between receptors
and sources of contamination. The CSM should be used as the basis for planning the PVI
investigation and making informed risk management decisions about the site and the
threat posed by PVI to nearby receptors. EPA recommends that the site
characterization include:
• Determining the full extent and location of contamination and its nature
• Assessing the potential for biodegradation of PHCs (and non-PHC fuel additives)
• Defining the hydrologic and geologic characteristics of the site
• Identifying potential receptors in the vicinity
• Determining whether preferential transport pathways are present and connect
PHC vapor sources with potential receptors11
• Considering whether there are any other factors that may preclude the use of
screening criteria
9 In the case of a suspected or confirmed release from a regulated UST system, Subparts E and F of 40 CFR 280
require owners and operators to investigate, report, and perform corrective action (including recovery of light non-
aqueous phase liquid (LNAPL) to the maximum extent practicable) if contamination is present, and submit timely
reports of activities and findings to the implementing agency.
10 Note that methane cannot be detected based on odor, taste or visible signs. Methane-detecting devices must
be used. For additional information on evaluating the presence of methane and potential hazards, see ASTM's
"New Practice for Evaluating Potential Hazard Due to Methane in the Vadose Zone", which is accessible at
http://www.astm.om/DATABASE.CART/WORKITEMS/WK32621.htm.
11 Preferential transport pathways can short-circuit the protectiveness provided by the extent of the lateral
inclusion zone and the vertical separation distances described in this guide. If preferential transport pathways
connect a vapor source directly to a building, indoor air sampling paired with sub-slab vapor sampling is
recommended.
Page 4 of 123
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Delineate a lateral inclusion zone - see Section 4 (p.44)
Delineate a lateral inclusion zone to focus the investigation on buildings located within
these boundaries. The lateral inclusion zone is based on the spacing between clean
monitoring points; the closer the spacing of the clean monitoring points, the less
extensive the lateral inclusion zone.
Determine vertical separation distances for each building within the lateral inclusion
zone - see Section 5 (p.48)
Further narrow the investigation to potential receptors (e.g., buildings) within the lateral
inclusion zone and directly overlie contamination. For such buildings, determine the
vertical distance between the contamination and the building basement, foundation, or
slab. This distance is determined by collecting soil gas, soil, and groundwater samples as
necessary. The thickness of clean soil separating contamination from the deepest point
of the building basement, foundation, or slab is the vertical separation distance.
Additional investigation is generally unnecessary if the vertical separation distance is
greater than 6 feet for dissolved contamination beneath buildings of any size, or 15 feet
for light non-aqueous phase liquid (LNAPL) if the overlying building has at least one side
shorter than 66 feet in length. If the distance to contamination is less than the
appropriate vertical separation distance (i.e., 6 feet or 15 feet; see Section 5), then
additional investigation is recommended.
Evaluate vapor source and attenuation of PHC vapors - see Section 5 (p.48), Section 8
(p.66), Section 9 (p.75), Section 10 (p.81), Section 12 (p.100), and Section 13 (p.106)
If contamination (either dissolved, or LNAPL whether mobile or residual) is in direct
contact with a building EPA recommends indoor air sampling. In the case of direct
contact, sub-slab samples cannot be collected because there is no subsurface soil
between the contamination and the building. Where contamination is not in direct
contact with an overlying building, then choose one of two options: (1) collect near-slab
(exterior) shallow soil gas samples paired with deep (source) soil gas samples, or (2)
collect indoor air samples paired with sub-slab soil gas samples. If the potential for PVI
cannot be ruled out based on near-slab and deep soil gas sampling, then EPA
recommends indoor air sampling paired with sub-slab vapor sampling. If the
attenuation factor calculated from results of analysis of the chosen pair or vapor
samples indicates that there may be a potential for PVI above applicable exposure
limits, EPA recommends gathering additional information and data to determine
whether mitigation is appropriate.
Page 5 of 123
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S Mitigate PVI, as appropriate - see Section 1 (p.11)
Mitigation involves interruption of the transport pathway for vapors between the
source of contamination and potential receptors. Select a remedial design that is
appropriate for the building and site. As necessary, establish institutional controls to
limit or prohibit access to affected areas. Remediate the source of the contamination,
including recovery of LNAPL (if present) to the maximum extent practicable.
Community Engagement
When conducting PVI assessments and follow-up actions, it is important to consider proactive
community engagement. EPA acknowledges there is no single correct approach to engage the
potentially impacted community in cleanup decisions. Community engagement can occur at
any step in the process and may occur more than once. It is generally recognized that earlier
and more frequent communication yields positive results, particularly for sites that pose a
threat to human health or the environment, or when the public expresses an elevated level of
concern or interest in the site. Depending on site circumstances, obtaining meaningful
community input is a sound approach that may result in better-informed decisions. EPA
developed several community engagement resources, which are available on the Office of
Underground Storage Tanks website: Community Engagement And The Underground Storage
Tank Program.12 Some of the resources include:
• Guidelines For Tailoring Community Engagement Activities To Circumstances At Leaking
Underground Storage Tank Sites.
• Community Engagement Resources (Toolbox) For Underground Storage Tank Programs.
Table 1 and Figure 1 briefly outline the Recommended Actions for Addressing PVI. Note that
this process is not necessarily linear and some of these activities may occur in a different order
or recur throughout the PVI investigation; this is especially true for community engagement.
Additional technical information is presented in the second part of this guide, Supporting
Technical Information.
12 The URL for this web site is http://www.epa.cjov/oust/communitvencjacjement/index.htm.
Page 6 of 123
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Table 1. Recommended Actions For Addressing PVI At Leaking Underground Storage Tank
Sites
Recommended
Actions
Purpose And Objectives
Procedures
Assess and
mitigate
immediate
threats to safety
(see Section 1,
p.ll)
Identify potential threat of explosion or fire due
to petroleum vapors or methane. Threat may be
indicated by:
• LNAPL visible in building, possibly as sheen in
sump
• Noticeable petroleum odor; headache,
dizziness, or nausea
• Atypical, unusual, or disagreeable taste or smell
in the water supply
NOTE: Methane cannot be detected on the basis
of odor, taste, or visible signs
Investigate all reports of petroleum
odors and other indicators within
buildings
Detection of the presence of methane;
requires specialized devices
Alert first responders so that they can,
if necessary, evacuate building
occupants as necessary until the
potential for fire or explosion has been
assessed and mitigated as needed
Conduct a site
characterization
and develop a
conceptual site
model (CSM)
(see Section 3,
p.39)
Characterize the physical, biological and chemical
systems at the site, with emphasis on
determining the spatial and temporal relationship
between receptors and sources of contamination
by:
• Determining the full extent and location of
contamination and its nature
• Assessing the potential for biodegradation of
PHCs
• Defining the hydrologic and geologic
characteristics of the site
• Identifying potential receptors in the vicinity
• Identifying whether preferential transport
pathways are present and connect PHC vapor
sources with potential receptors. Preferential
transport pathways Include both natural (i.e.,
geologic) and man-made (i.e., underground
utilities, excavations) features.
Collect sufficient site data and
information to construct CSM
Identify data gaps
Update CSM as new data become
available
Where preferential transport pathways
connect PHC vapor sources to
receptors (e.g., buildings), indoor air
sampling paired with sub-slab vapor
sampling is recommended
Delineate a
lateral inclusion
zone
(see Section 4,
p.44)
Screen out buildings that are not likely to be
impacted by PVI to narrow the investigation to
only those buildings that have a greater potential
for PVI and for which further investigation should
be conducted.
The lateral inclusion zone is site-specific and:
• Based on the extent of contamination and
distance between clean monitoring points
• Decreases in extent as additional data are
collected to reduce uncertainty in the CSM
• Construct lateral inclusion zone based
on distance between clean monitoring
points (includes consideration of the
presence of preferential transport
pathways)
Determine
vertical
separation
distances
(see Section 5,
p.48)
Further screen out buildings that are not likely to
be impacted by PVI to focus the investigation on
potential receptors that overlie contamination in
the dissolved, vapor, and/or LNAPL phase. The
vertical separation distance is:
• The thickness of clean, biologically-active soil
For each building within the lateral
inclusion zone, collect additional soil
gas, soil, and groundwater samples as
necessary to determine the vertical
separation distance. Additional
investigation is generally unnecessary
Page 7 of 123
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Recommended
Actions
Purpose And Objectives
Procedures
(see Section 9, p.75) separating
contamination from overlying buildings or
other potential receptors
• Determined by site-specific sampling to
determine the depth at which contamination is
present
If the distance to contamination is
greater than:
• 6 feet for dissolved
contamination beneath
buildings of any size, or
• ISfeetforLNAPLifthe
overlying building has at least
one side shorter than 66 feet in
length
If the distance to contamination is
less than those indicated above, then
additional investigation is
recommended.
Evaluate vapor
source and
attenuation of
PHC vapors
(see Section 5,
p.48, Section 8,
p.66, Section 9,
p.75, Section 10,
p.81, Section 12,
p.100, and
Section 13,
p.106)
Carefully evaluate the potential for PVI into those
buildings identified as being the most likely to be
impacted by PVI. This is a building-by-building
evaluation based on sampling conducted within
close proximity to the building or inside the
building as necessary.
If contamination is in direct contact with
building basement, foundation, or slab,
then collect indoor air samples.
Otherwise choose either option (1) or (2)
below:
1. Collect near-slab soil gas samples
coupled with deep (source) soil gas
samples. If a potential threat of PVI
is indicated, then proceed to option
2. If not, PVI is not likely to be a
concern.
2. Collect indoor air samples paired
with sub-slab soil gas samples. If
these results indicate a potential
threat of PVI, mitigate PVI as
appropriate.
Mitigate PVI as
appropriate
(see Section 1,
p.ll)
Interrupt the pathway between the source of
contamination and potential receptors.
• Numerous approaches depending on building
characteristics
Select a remedial design that is
appropriate for building and site
Remediate source of contamination,
including recovery of LNAPL (if
present) to the maximum extent
practicable
Establish institutional controls to limit
or prohibit access to affected areas of
building, as necessary
Page 8 of 123
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For any confirmed or
suspected release,
START HERE:
Conduct an adequate
site characterization
and construct a
Conceptual Site
Model (CSM)
(including all factors
that may affect the
vapor intrusion
pathway—see Special
Considerations,
Section 1)
(a) Alert first
responders &
assess potential
threat of fire
and/or explosion
(b) Mitigate threats as
appropriate
present, do
preferential
pathways connect
vapor source and
building?
Evaluate vapor
source(s) and
mitigate PVI as
appropriate
Community Engagement
Federal regulations under 40 CFR 280.67
require implementing agencies to provide
notice to those members of the public who
are directly affected by a release from a UST
and the planned corrective action if such a
release requires a corrective action plan.
Implementing agencies are advised to tailor
community engagement activities based on
site-specific circumstances. Such activities
may occur at any point(s) in the assessment
and mitigation process. It is recognized that
earlier and more frequent communication
yields positive results.
YES
Delineate a
Lateral
Inclusion Zone
(including all
factors that
may affect the
vapor intrusion
pathway)
Are any
existing or planne
buildings within the
lateral inclusion
zone?
YES
Evaluate vapor source* and
attenuation of PHC vapors by
either:
(1) Measuring PHCs in near-slab
and deep (near source) soil
gas, or
Collecting indoor air samples
paired with sub-slab soil gas
samples
* If contamination is in direct
contact with a building, EPA
recommends indoor air sampling.
(2)
Determine Vertical
Separation Distances for
each building (including all
factors that may affect the
vapor intrusion pathway)
Is the
thickness of
clean, biologically active
soil greater than the
minimum vertical
separation
distance?
Is any
tential threat
of PVI indicated by
near-slab & deep
(source) soil gas
sampling?
Is any
otential threa
of PVI indicated by
indoor air & sub-
slab soil gas
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Supporting Technical Information
The following sections provide technical information in support of EPA's recommended actions
as outlined in Table 1 (p.7) and depicted in Figure 1 (p.9). Each section presents information in
a standardized format, which is easy to follow and allows for future revisions, as necessary.
Additional sources of information may be found in the Petroleum Vapor Intrusion Compendium
(http://www.epa.gov/oust/cat/pvi/), located on the Office of Underground Storage Tanks
(OUST) website (http://www.epa.qov/oust/}.
Page
1. Petroleum Vapor Intrusion (PVI) 11
2. Typical PVI Scenarios 33
3. Site Characterization And Conceptual Site Model (CSM) 39
4. Lateral Inclusion Zone 44
5. Vertical Separation Distance 48
6. Mobile And Residual Light Non-Aqueous Phase Liquid (LNAPL) 57
7. Groundwater Flow And Dissolved Contaminant Plumes 61
8. Soil Gas Profile 66
9. Clean, Biologically Active Soil 75
10. Non-PHC Fuel Additives 81
11. Seasonal And Weather Effects 96
12. Vapor Intrusion Attenuation Factor (a) 100
13. Computer Modeling Of Petroleum Vapor Intrusion 106
Page 10 of 123
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1. Petroleum Vapor Intrusion (PVI)
Description
Petroleum vapor intrusion (PVI) occurs when vapors from petroleum hydrocarbons (PHCs)
migrate through the subsurface into overlying or nearby buildings. Fuels such as gasoline,
diesel, aviation gasoline, and jet fuel are comprised primarily of PHCs with some non-petroleum
based additives. Under certain circumstances, PVI may result in indoor air concentrations that
pose a risk to building occupants. PVI may pose immediate threats to safety (e.g., fire or
explosion potential from petroleum vapors or methane) or possible adverse health effects from
inhalation of toxic chemicals (e.g., exposure to benzene from gasoline). Vapor concentrations
generally decrease with increasing distance from a subsurface vapor source due to aerobic
biodegradation, and eventually at some distance the concentrations become negligible.
Composition Of Petroleum Fuels
Petroleum fuels are comprised of hundreds of individual compounds. PHCs present in
petroleum fuels generally belong to one of two major groups: aromatics and aliphatics. The
aromatic PHCs are characterized as having one or more benzene rings. Benzene, toluene,
ethylbenzene, and the three isomers of xylene are collectively referred to as BTEX. The
aliphatics are non-aromatic PHCs consisting of straight-chains, branched chains, or non-
aromatic rings. Although BTEX represent the group of PHCs that receive the most attention at
typical leaking underground storage tank (UST) sites, they are not the only compounds that may
pose a risk to human health.13 Petroleum fuels may also contain a variety of non-PHC volatile
organic chemicals (VOCs) as additives to enhance performance. Fuel oxygenates such as methyl
The federal UST program does not prescribe human health values for contaminants; implementing authorities
should use exposure values and attenuation factors appropriate for the contaminants present and the
characteristics of exposure (e.g., residential vs industrial). Although there is a lack of toxicological data for many
PHCs, EPA provides some information that may be applicable. For example, EPA provides vapor intrusion
screening levels (VISLs) for a variety of volatile chemicals known to pose a potential cancer risk or noncancer
hazard through the inhalation pathway. These VISLs, which are calculated by the VISL Calculator (EPA, 2014b), are
generally recommended, medium-specific, risk-based screening-level concentrations intended for use in
identifying areas or buildings that may warrant further investigation and mitigation of vapor intrusion, as
appropriate. VISLs are calculated for concentrations of volatile chemicals in groundwater, soil gas (exterior to
buildings and sub-slab), and indoor air for default target risk levels and exposure scenarios. The VISL Calculator
does not account for biodegradation so attenuation factors may need to be adjusted for biodegradable chemicals.
The VISL Calculator draws on toxicity values from Regional Screening Levels (RSLs) for Chemical Contaminants at
Superfund Sites, accessible at http://www.epa.cjov/recj3hwmd/risk/human/rb-concentration table/index.htm.
Both the VISL Calculator User's Guide (EPA, 2014a) and VISL Calculator (EPA, 2014b) may be downloaded from
EPA's web site: http://www.epa.Qov/oswer/vaporintrusion/Quidance.html. RSLs are drawn from a variety of
sources according to EPA's three-tiered hierarchy of toxicity data (see "Human Health Toxicity Values in Superfund
Risk Assessments" OSWER Directive 9285.7-53, 2003}. Tier 1 (highest quality data) is EPA's Integrated Risk
Information System (IRIS), accessible at http://www.epa.ciov/iris/. Tier 2 are Provisional Peer Reviewed Toxicity
Values (PPRTVs), accessible at http://hhpprtv.ornl.ciov/(also see EPA, 2009). Tier 3 include toxicity values from
other sources such as the Agency for Toxic Substances and Disease Registry (ATSDR). Several states have also
developed toxicity values, including California (CA DTSC, 2009), Hawai'i (HI DOH, 2008, 2012), Massachusetts (MA
DEP, 2003), New Jersey (NJ DEP, 2013), and Washington (WA DEC, 2006). Links to these sources are provided
under References Cited at the end of this section.
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tertiary-butyl ether (MTBE) and tertiary-butyl alcohol (TBA), and the lead scavengers14 ethylene
dibromide (EDB), and 1,2-dichloroethane (1,2-DCA), plus other PHCs (e.g., naphthalene), may
also pose a risk to human health. If present, their vapor intrusion potential should be assessed
(see Section 10, p.81). The presence of biodegradable VOCs other than benzene may result in
depletion of oxygen that is necessary for aerobic biodegradation of benzene, potentially
resulting in farther migration of benzene vapors.
Phase Partitioning
When petroleum fuels are released into the subsurface from a leaking UST, PHCs may partition
into several phases:
• Globules of light non-aqueous phase liquid (LNAPL) trapped within soil pore spaces
(i.e., residual LNAPL)
• Dissolved in soil moisture
• Adhered onto the surface of, or absorbed into, soil solids
• Vapors in soil gas
• Accumulations of mobile LNAPL on and in the capillary fringe15
• Dissolved in groundwater
Low volume releases may result in contamination of only soil (including soil gas and soil
moisture) and remain in the vadose zone. If the volume of a fuel release is sufficient, the fuel
may accumulate on and in the capillary fringe and become mobile LNAPL. The mobile LNAPL
generally spreads in the direction of groundwater flow, and may accumulate in monitoring
wells. Temporal fluctuations in the elevation of the water table typically create a vertical smear
zone of residual LNAPL contamination both above and below the average water table elevation.
The more soluble components of the LNAPL mass dissolve into groundwater and are
transported down gradient by the flowing groundwater as an aqueous phase. The remaining
LNAPL mass will contain a sizeable fraction of aliphatic and relatively insoluble PHCs (e.g.,
naphthalene), especially if the source is large or unweathered (Lahvis, et al., 2013; EPA, 2013a).
PHCs in the residual phase (both above and below the water table), the mobile phase (i.e., free
product, LNAPL plume), and the dissolved phase (i.e., contaminant plume) all can serve as
sources of PHC vapors. Figure 2 illustrates the typical distribution of petroleum fuels in the
subsurface resulting from a leaking UST. See Section 2 (p.33) for a more detailed discussion of
typical PVI scenarios.
14 Older sites, where leaded gasoline was released to the subsurface, should be assessed for EDB and 1,2-DCA as
they may represent a potential source of vapors (see Section 10). For more information about lead scavengers,
see Appendix F in Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For Petroleum
Hydrocarbon Compounds (EPA, 2013) and EPA's Lead Scavengers web site at
http://www.epa.gov/oust/cat/leadscav.htm.
15 Mobile LNAPL is often referred to as free product, especially in older documents and 40 CFR 280.
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Aerobic
Biodegradation
Zone
Figure 2. Conceptual Model Of Typical Petroleum Hydrocarbon Release
Aerobic biodegradation of PHCs along the perimeter of the vapor and dissolved plumes may
limit the spread of subsurface contamination. Effective oxygen transport (dashed arrows)
maintains aerobic conditions in the biodegradation zone. Petroleum LNAPL collects at the
capillary fringe between the saturated and unsaturated zones (EPA, 2012).
Vapor Migration
Vapor migration results from two processes: diffusion and advection. Diffusion is the process
whereby net transport of vapors from a source area of higher concentration (e.g., LNAPL,
residual LNAPL, or dissolved plume) to an area of lower concentration occurs as a result of
random molecular motion. Diffusion can also lead to chemical migration into buildings directly
through a dirt floor or crawlspace, or through openings in the building slab and foundation such
as passages for utility lines, sumps, and elevator pits. Also, intact concrete has appreciable
permeability to diffusive gas movement (Kobayashi and Shuttoh, 1991; Sanjuan and Munoz-
Martialay, 1996; and Tittarelli, 2009) and the permeability increases substantially when cracks
are present (Daoud and Renken, 1999; EPA, 1995).
Advection refers to the movement of soil gas in response to pressure gradients. Advection can
be an important mechanism for drawing soil gas and contaminant vapors through cracks in the
basement floor or foundation into the building or back into the soil beneath the building.
Heating and cooling systems can create differential pressures inside the building. When the
pressure inside the building is lower than the pressure in the subsurface, vapors are drawn into
the building. Conversely, when the pressure inside the building is greater than the pressure in
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the subsurface, air within the building may be forced into the subsurface causing some degree
of reoxygenation (Lundegard, Johnson, and Dahlen, 2008). Wind or changes in barometric
pressure may also drive advective transport of oxygen into the subsurface beneath the building
(Lundegard, Johnson, and Dahlen, 2008; Patterson and Davis, 2009; McHugh, DeBlanc, and
Pokluda, 2006; Luo and Johnson, 2011; Robinson, Sextro, and Riley, 1997; Luo, et al., 2009; and
Hong, Holton, and Johnson, 2012) (see Section 11).
Biodegradation Of PHCs
Biodegradation of PHCs is recognized as one of the primary mechanisms by which petroleum
and other hydrocarbon pollutants are removed from the environment (Baedecker, Cozzarelli,
and Hoppel, 1987; Leahy and Colwell, 1990). The biodegradability of PHCs often reduces the
potential for human exposure from PVI (McHugh, et al., 2010; EPA, 2012; Interstate Technology
& Regulatory Council [ITRC], 2014). Microorganisms are widely distributed in the environment
and most are recognized as having some ability to metabolize PHCs (Gale, 1951; Ward, Singh,
and Van Hamme, 2003; Prince, 2010). Although most microbes degrade a narrow range of
organic compounds, they typically exist as a mixed consortium that collectively can biodegrade
a wide range of organic compounds. Biodegradation progresses through stages with certain
microbes being predominant until environmental conditions (e.g., availability of specific
hydrocarbons, micronutrients, electron acceptors) become unfavorable for them at which time
different microbes then become dominant (Wang and Deshusses, 2006; Suflita and Mormile,
1993; Corseuil, et al., 1998; Moyer, et al., 1996; Boopathy, 2004; Alexander, 1980; Prince,
Parkerton, and Lee, 2007; Prince, 2010; and Bekins, et al., 2001). Thus, aerobic and anaerobic
microbes may coexist with one class essentially dormant while the other is active.
Gasoline and diesel fuel (including biodiesel) may be completely biodegraded under aerobic
conditions (Hult, 1987; Prince and Douglas, 2010; Prince, Parkerton, and Lee, 2007; Marchal, et
al., 2003), though diesel fuel is more difficult and slower to biodegrade (Marchal, et al., 2003).16
The end products of complete biodegradation (mineralization) of PHCs are water and carbon
dioxide. Mineralization of PHCs is almost always the consequence of microbial activity
(Alexander, 1981). If aerobic biodegradation of PHCs is incomplete, a variety of intermediate
degradation products may be formed, but none of these are more toxic than the parent PHCs.
Aerobic biodegradation is well documented for many individual PHCs and classes of PHCs
including:
• N-alkanes (Bouchard, et al., 2005; Prince, Parkerton, and Lee, 2007; Bailey, Jobson, and
Rogers, 1973; Hult, 1989; Baedecker, et al., 2011)
• Branched alkanes (Prince and Douglas, 2010; Prince, Parkerton, and Lee, 2007);
cycloalkanes (Bouchard, et al., 2005)
16 The rate of aerobic biodegradation slows down with decreasing concentration of oxygen. Many aerobic
microorganisms continue to function at concentrations as low as 0.1 mg/L of available oxygen, which is equivalent
to an air concentration of 0.2%. For more information see research by Alagappan and Cowan (2004), Miralles-
Wilhelm, Gelhar, and Kapoor (1997), and Mohamed, Saleh, and Sherif (2010).
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• Aromatics (Wang and Deshusses, 2007; Phelps and Young, 1999; Landmeyer and
Bradley, 2003; Lahvis, Baker, and Baehr, 2004; Lahvis, Baehr, and Baker, 1999; Bailey,
Jobson, and Rogers, 1973; ZoBell, 1946; Corseuil, et al., 1998; Richnow et al., 2003)
• Naphthenes (Prince, Parketon, and Lee, 2007; Bailey, Jobson, and Rogers, 1973;
Anderson, et al., 1999; ZoBell, 1946)
• Phenols (ZoBell, 1946; Basha, Rajendran, and Thangavelu, 2010)
• Trimethylbenzenes (Chen, et al., 2009)
Though aerobic biodegradation has been studied for over a century, anaerobic biodegradation
of PHCs has been recognized only within the past three decades (Widdel, Boetius, and Rabus,
2006; Spormann and Widdel, 2000; and Townsend, et al., 2003). Anaerobic microorganisms
degrade PHCs by using an electron acceptor other than oxygen (e.g., sulfate, nitrate, ferrous
iron, or carbon dioxide). Anaerobic biodegradation is a slower process than aerobic
biodegradation (Widdel, Boetius, and Rabus, 2006; Bailey, Jobson, and Rogers, 1973; and
Lanham et al., 2013) and anaerobes grow slower than their aerobic counterparts (Widdel,
Knittel, and Galushko, 2010). Instead of water and carbon dioxide, complete anaerobic
biodegradation of PHCs (and naturally-occurring organic matter in soil, such as peat) can
produce methane (Zengler, et al., 1999), especially with a release of an ethanol-blended
gasoline (Jewell and Wilson, 2011; Ma, et al., 2012; Ma, et al., 2014). Incomplete anaerobic
biodegradation of PHCs can produce compounds of higher toxicity, but these vapors are readily
biodegraded in the vadose zone under aerobic conditions, and thus should not present a threat
of vapor intrusion. Anaerobic biodegradation is typically the predominant mechanism of
biodegradation in the source zone (Anderson and Lovley, 1997). Additional references
documenting anaerobic biodegradation of PHCs are listed under Additional Information at the
end of this section.
Importance
Important factors cited by Lahvis and Baehr (1996) and Suarez and Rifai (1999) as being
influential for aerobic biodegradation of PHC vapors include:
• Vapor source hydrocarbon concentration, flux, and composition (including
methane)
• Oxygen demand (i.e., the oxygen required to biodegrade the available hydrocarbons
and any other organic matter present) and oxygen availability
• Soil type and properties (including texture and moisture content)
• Availability of essential micronutrients
• Ambient temperature in the subsurface
• The pH of the soil and groundwater
Additional factors cited by EPA (2012) as influencing the potential for PVI include:
• Size and characteristics of the building and adjacent land surface
• Distance between the vapor source and the building
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Assessment
An assessment of the potential for PVI is not an isolated activity, but rather an integral part of
the normal response to a suspected or confirmed release of PHCs from a leaking UST. At any
leaking UST site (including abandoned sites or those that will be redeveloped for other uses), it
is important to have a thorough understanding of the nature and magnitude of the release; the
physical, biological, and chemical characteristics of the subsurface environment; an
understanding of the preferential pathways for contaminant transport; and locations of
receptors in the vicinity of the release. This is determined through collection and analysis of
samples of soil, soil gas, groundwater, and sometimes LNAPL. Any other conditions (e.g.,
seasonal, weather-related; see Section 11, p.96) that may influence the transport of
contaminants and potentially impact the safety and health of nearby building occupants should
also be investigated.
Vapors emanating from dissolved-phase sources are primarily water soluble compounds, the
more soluble aromatic hydrocarbons (e.g., BTEX) and other volatile and semi-volatile
hydrocarbons and fuel additives (Lahvis, et al, 2013; EPA, 2013a). Vapors emanating from
LNAPL sources contain a significantly larger fraction of aliphatic compounds and relatively
insoluble hydrocarbons, especially if the source is large or unweathered (Lahvis, et al., 2013;
EPA, 2012). Analyses of samples of soil, soil gas, groundwater, and LNAPL collected during site
characterization (see Section 3, p.39) will provide information on specific contaminants that
may warrant assessment for potential vapor intrusion.
Special Considerations
Several factors may preclude the effectiveness of aerobic biodegradation to mitigate the threat
of vapor intrusion. They include:
• Source volume and composition (including PHCs and non-PHC fuel additives)
• Soil properties (moisture content, permeability, high organic carbon content,
especially peat)
• Large building size
• Extensive impermeable surface covering (e.g., asphalt, concrete)
• Preferential transport pathways (including both natural and man-made)
If present, these factors may reduce the potential for biodegradation of PHC vapors and
warrant additional investigative steps (e.g., collection of soil gas samples—see Section 8, p.66)
to determine if the use of screening criteria (e.g., vertical separation distance) is appropriate.
The age and volume of release should be determined or estimated. When the release is
relatively recent or if the volume of the release is relatively large17, there is greater potent
PVI than for smaller or older releases, which may be more weathered. Large volume PHC
17 The adjective large refers to either the total volume of the release or the areal extent (footprint) of the LNAPL
mass in the subsurface.
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releases may require a greater separation distance for biodegradation to be effective due to
increased oxygen demand (EPA, 2013a).
Biodegradation of recent releases of high ethanol blended gasoline (i.e., E-20 or greater) may
consume oxygen that would otherwise be available for biodegradation of PHCs resulting in an
increased potential for PVI (Ma et al., 2014). In addition, the biodegradation of ethanol may
result in the advective transport of methane and a potential risk of explosion. Thus, larger
separation distances may be necessary to mitigate the threat of explosion or PVI at sites where
high ethanol blended fuel has been released into the subsurface (Ma et al., 2014).
Preferential transport pathways may be either natural (e.g., fractures in rock, solution channels
in karst terrain, bedding planes, joints, high permeability layers) or man-made (e.g., utility
corridors including sewer lines themselves, trenches, excavations). Because they increase the
speed at which the contaminants move through the subsurface, preferential transport
pathways can potentially short circuit protectiveness that would otherwise be provided by
biodegradation of PHCs and other fuel additives in homogeneous soils. Typically, it is difficult
to detect and map natural preferential transport pathways, and contamination may present
itself in unexpected locations. Local government offices have maps of utility corridors that can
provide information on the presence and location of man-made preferential transport
pathways.18
Recommended Steps For Addressing The Potential Risk From PVI
EPA recommends the following actions for situations in which EPA, state, tribal, and local
agencies are investigating releases of petroleum-based fuels (including addressing potential
risks due to PVI) at leaking UST sites or where 40 CFR 280 requires19 UST owners and operators
to undertake release investigation and corrective action activities:
S Assess and mitigate immediate threats to safety
Some releases from UST systems are discovered through noticeable sensory indicators on
neighboring properties. Indicators may include sight, smell, taste, or physiological effects (e.g.,
dizziness, headache, nausea, vomiting, and confusion). The presence of odors does not
necessarily correspond to adverse health or safety impacts from PVI, as the odors could be the
result of indoor vapor sources. However, it is generally prudent to investigate any reports of
odors in close proximity to UST systems as the odor threshold for some chemicals exceeds their
acceptable health-based concentrations. PHC odors are a nuisance and may trigger the need
18 A federally mandated national call center was established to ensure that utility lines are marked before digging
or boring. Dial 811 to have the locations of utilities marked before conducting site work that involves digging or
boring. For more information, visit http://www.callSl 1.com/default.aspx
19 In the case of a suspected or confirmed release from a regulated UST system, Subparts E and F of 40 CFR 280
require owners and operators to investigate, report, and perform corrective action (including recovery of LNAPLto
the maximum extent practicable) if contamination is present, and submit timely reports of activities and findings
to the implementing agency.
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for abatement or mitigation even if the concentration in indoor air is below acute or chronic
health-based levels.
In confined spaces, the presence of flammable PHC vapors and non-PHCfuel additive vapors or
methane may pose a threat of fire or explosion and endanger building occupants. Federal
regulations (40 CFR 280.61) require that immediate action be taken to prevent any further
release of the regulated substance into the environment and that fire, explosion, and vapor
hazards be identified and mitigated (Federal Register, 1988). Section 280.64 requires that free
product (mobile LNAPL) be recovered to the maximum extent practicable and that records be
kept of the volumes recovered. First responders, typically fire department personnel, should be
notified if there are reports of either odor from petroleum or the presence of an oily sheen on
basement floors or in sumps, drains, or elevator pits. It may be necessary to evacuate building
occupants until the threat from fire or explosion has been mitigated. Since methane is odorless
and colorless, monitoring devices are required if methane is suspected.
^ Conduct a site characterization and develop a conceptual site model (CSM)
Once the immediate threats to safety have been mitigated (or it is determined that immediate
threats do not exist), determine whether there is a long-term threat to human health and the
environment from intrusion of petroleum vapors. Site characterization20 and CSM
development provide information about the full extent and location of the contamination; the
nature and characteristics of the contamination; the characteristics of the site that influence
contaminant migration, including the potential for biodegradation of PHCs; and the locations of
receptors. Information derived from the CSM helps ensure that sources, pathways, and
receptors throughout the site are considered; this knowledge can lead to selection of the most
appropriate sampling locations and techniques. A systematic soil gas sampling program may
also aid in defining the full extent and location of contamination, detecting the presence of
preferential transport pathways, and locating pockets of PHC vapors. Preferential transport
pathways are avenues of least resistance to the migration of contaminants whether in the
dissolved phase, LNAPL phase, or vapor phase. They include both natural and man-made
features such as:
Natural
gravel lenses and channels
solution channels in karst terrain
bedding planes
fractures, joints, and faults in
consolidated rock
Man-made
utility corridors (including
sewer lines themselves) and
trenches
elevator pits
sumps and drainage pits
other types of excavations
20The term site characterization is used throughout this document for consistency. Site characterization is often
used interchangeably with site assessment, site evaluation, site investigation, and sometimes site check as they all
mean assembling and collecting information and data about a site.
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Preferential transport pathways increase the speed at which contamination moves through the
subsurface such that contaminants may not biodegrade by the time they reach receptors. They
can also allow atypical movement, which in some cases may be opposite groundwater flow
(ITRC, 2014). Because preferential transport pathways can short-circuit the protectiveness
provided by lateral and vertical separation distances described in this PVI guide, indoor air
sampling is recommended in situations where they connect vapor sources and receptors. See
Section 3 (p.39) for more information about site characterization and CSMs.
S Delineate a lateral inclusion zone
Based on the CSM, delineate a lateral inclusion zone. The lateral inclusion zone is the area
surrounding a contaminant mass through which petroleum vapors may travel, intrude into
buildings, and potentially pose a threat to human health and the environment. Buildings
directly above contamination sources, whether as mobile LNAPL, residual LNAPL, or PHCs
dissolved in groundwater, are considered within the lateral inclusion zone. Buildings outside
this zone generally may be excluded from further assessment for PVI unless:
• Site conditions change (e.g., groundwater flow direction changes, contaminant plume
migrates beyond the lateral inclusion zone, development or redevelopment of nearby
properties)
• Preferential transport pathways are present
In such instances, additional investigation may be warranted to more fully evaluate the risk
from PVI. See Section 4 (p.44) for more information on delineating a lateral inclusion zone.
-S Determine vertical separation distances
The vertical separation distance is the thickness of clean, biologically active soil (see Section 9,
p.75) between a contaminant mass and the lowest point of an overlying receptor (e.g., building
basement floor, foundation, or crawl space surface). Consolidated rock is not soil and should
not be included in the vertical separation distance. For example, for a situation in which there
is 3 feet of soil above fractured rock and the depth to contaminated groundwater is 7 feet, the
vertical separation distance is 3 feet, not 7 feet. Some buildings within the lateral inclusion
zone will overlie PHC contamination that exists as either a mobile LNAPL mass, residual soil
contamination (including the smear zone), or dissolved in a groundwater plume. However, not
all of these buildings will be threatened by PVI due to aerobic biodegradation of PHCs provided
there is sufficient vertical separation distance between the receptor and the vapor source. The
vertical separation distance between contamination and overlying buildings is determined as
part of the normal site characterization process. The full extent and location of contaminant
sources should have been adequately mapped in the subsurface and the nature and
characteristics of the contamination should have been determined during site characterization
and conceptual site model development (see Section 3, p.39).
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EPA (2013a) presents analysis of petroleum vapor source data and soil gas data from a number
of leaking UST sites across the United States. The report findings support screening criteria for
dissolved and LNAPL PHC releases from leaking USTs. For dissolved PHC sources that are
separated from overlying buildings by more than 6 feet of clean, biologically active soil, the
potential threat of PVI is negligible and further investigation for PVI is generally unnecessary.
For LNAPL sources that are separated from overlying buildings by more than 15 feet of clean,
biologically active soil, the potential threat of PVI is negligible and further investigation for PVI
is generally unnecessary. These separation distances are believed to be sufficiently protective
in most situations because they include a number of built-in safety factors, which are discussed
in more detail in Section 5 (p.48). If the distance separating the source of PHC vapors and
overlying buildings is less than 6 feet for dissolved sources and 15 feet for LNAPL sources,
additional investigation is recommended.
EPA (2013a) recognizes that there are a number of precluding factors that may justify a greater
vertical separation distance in some cases. These factors include:
• Influence of methanogenesis on oxygen demand (especially for higher ethanol blends of
gasoline)
• Effect of extensive high organic matter content soils (e.g., peat) with potentially high
natural oxygen demand
• Reduced oxygen flux caused by certain geologic conditions (e.g., low permeability
surface layer overlying coarse-grained soils, soil moisture from precipitation (Luo et al.,
2009))
• Limited knowledge of vapor attenuation behavior in fractured rock
• Limited soil gas data for non-UST (e.g., petroleum refinery, fuel terminal) sites
• Limited data on vapor attenuation behavior of aliphatic compounds
• Lack of soil vapor data for the lead scavengers ethylene dibromide (EDB) and 1,2-
dichloroethane (1,2-DCA) (see Section 10, p.81, for more information on these
contaminants)
Other site characteristics that may warrant additional investigation include exceptionally dry
soils (<2 percent soil moisture), areas covered by extensive impervious paving or large
buildings, and presence of preferential transport pathways (see Section 3, p.39). Also, soil gas
movement may vary seasonally in response to differential pressures created by heating and
cooling of overlying buildings (see Section 11, p.96).
S Evaluate vapor source and attenuation of PHC vapors
Where contamination is not in direct contact with an overlying building, EPA recommends one
of two options: (1) collection of near-slab (exterior) shallow soil gas samples paired with deep
(near source) soil gas samples, or (2) collection of indoor air samples paired with sub-slab soil
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gas samples to evaluate attenuation of PHC vapors and the potential for PVI.21 When collecting
soil gas samples22, use option 1 under the following conditions (EPA, 2013b):
• A building, with the shortest side no longer than 66 feet, overlies LNAPL and the vertical
separation distance is less than 15 feet, but not in direct contact with the building
basement floor, foundation, or crawl space surface.
• A building, of any dimension, overlies dissolved PHC contamination and the vertical
separation distance is less than 6 feet, but not in direct contact with the building
basement floor, foundation, or crawl space surface.
Use option 2 for buildings larger than 66 feet on a side or if near-slab soil gas samples from
around smaller buildings do not clearly demonstrate that biodegradation is sufficient to
mitigate the threat of PVI by reducing PHC concentrations to below applicable human health
thresholds (see Footnote #13).
The purpose of collecting paired samples is to enable determination of a building-specific vapor
intrusion attenuation factor. Generic attenuation factors that do not account for
biodegradation of PHCs are conservative and, likely overestimate the transfer of contaminants
from soil gas to indoor air in most buildings. Attenuation factors (see Section 12, p.100) that
account for biodegradation can be derived from models such as BioVapor or PVIScreen (see
Section 13, p.106). Additional information may be found in Wilson etal. (2014).
If contamination (either dissolved, or LNAPL whether mobile or residual) is in direct contact
with a building basement floor, foundation, or crawlspace surface, EPA recommends indoor air
sampling (these samples cannot be paired with subsurface soil gas samples because there is no
clean, biologically active soil between the contamination and the building). Information on
collecting and analyzing sub-slab vapor samples and indoor air samples is beyond the scope of
this document, but is provided in other documents, for example ITRC (2014) and EPA (2015).
Indoor air in many buildings will contain detectable levels of a number of vapor-forming
compounds whether or not the building overlies a subsurface source of vapors, because indoor
air can be impacted by a variety of indoor and outdoor sources. The composition of outdoor air
surrounding a building is referred to as ambient air throughout this document. The combined
contribution of indoor and outdoor sources of vapors to indoor air concentrations is referred to
as background throughout this document. To differentiate and quantify the relative
contribution of contaminants from PVI versus background sources, indoor air samples must be
collected in conjunction with sub-slab (or near-slab, as appropriate) soil gas samples. ITRC
21 Implementing authorities may opt for sub-slab soil gas and indoor air sampling in any situation they deem
necessary to protect the safety and health of building occupants.
22 Soil gas samples should be analyzed for oxygen, carbon dioxide, PHCs (and any other fuel constituents likely to
be present including fuel additives), and methane. As a quality assurance/quality control check, nitrogen can be
added to the analyte list at a nominal cost. This will enable determination of whether significant concentrations of
other gases are unaccounted for as these gases should account for nearly 100 percent of the total present. (See
Section 8).
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(2014) and EPA (2015) provide information on background sources, techniques, and methods to
account for background contributions to indoor air concentrations.
Information on historic concentrations of background vapors is presented in Background Indoor
Air Concentrations of Volatile Organic Compounds in North American Residences (1990 - 2005):
A Compilation of Statistics for Assessing Vapor Intrusion (EPA, 2011b). In addition, the Montana
Department of Environmental Quality (MT DEQ, 2012) conducted an extensive investigation of
the indoor air quality of typical uncontaminated buildings in Montana. The objective of these
studies is to illustrate the ranges and variability of VOC concentrations in indoor air resulting
from sources other than vapor intrusion. While these studies provide expected ranges of
indoor air contaminants, EPA recommends building-specific sampling (rather than using
literature values) to characterize actual contaminant levels. If measured indoor air
concentrations are found to greatly exceed the historic range of background levels, there is a
greater likelihood that the indoor air concentrations are the result of vapor intrusion. Studies
such as EPA (2011) and MT DEQ (2012) can be employed to determine whether measured
indoor air concentrations exceed the historic range of background concentrations.
If the attenuation factor calculated from results of analysis of the chosen pair or vapor samples
indicates that there may be a potential for PVI above applicable exposure limits, EPA
recommends additional investigation to determine whether mitigation is appropriate.
^ Mitigate petroleum vapor intrusion, as appropriate
If contaminant concentrations represent a potential threat of fire or explosion (i.e., vapor
concentrations are more than 10% of the lower explosive limit), or indoor air sampling indicates
that PVI is occurring, EPA recommends that active mitigation measures be immediately
initiated. ITRC (2014) and EPA (2015) provide information on mitigation and remediation of
vapor intrusion. In addition, the source of contamination should be remediated perSubpart F
of the Federal Regulations (40 CFR 280.60 through 280.67) (Federal Register, 1988). In
particular, 40 CFR 280.64 requires the recovery of LNAPL to the "maximum extent practicable".
See the following sections for more information on the factors discussed in the paragraphs
above:
• Section 3 (p.39) Site Characterization and Conceptual Site Model (CSM)
• Section 4 (p.44) Lateral Inclusion Zone
• Section 5 (p.48) Vertical Separation Distance
• Section 6 (p.57) Mobile and Residual Light Non-Aqueous Phase Liquid (LNAPL)
• Section 7 (p.61) Groundwater Flow and Dissolved Contaminant Plumes.
• Section 8 (p.66) Soil Gas Profile
• Section 9 (p.75) Clean, Biologically Active Soil
Page 22 of 123
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2. Typical PVI Scenarios
Description
The potential for PVI is primarily a function of the location of the contamination source relative
to a potential receptor, source volume and strength, and the source mass distribution in the
subsurface. Source concentrations are typically much higher for LNAPL sources than for
dissolved-phase sources. Higher source concentrations will generate higher rates of mass
diffusion (flux). The higher mass flux will also be more sustained over time because LNAPL
sources will contain significantly more mass compared to dissolved-phase sources. Oxygen
demand and the potential for encountering anaerobic conditions are also uniquely different
between LNAPL and dissolved-phase sources. For both dissolved and LNAPL sources, the
biodegradation reaction front is relatively narrow, but it occurs higher in the unsaturated zone
(closer to land surface) over an LNAPL source than it does over a dissolved-phase source (Figure
3).
a) LNAPL SOURCE
UNSATURATED ZONE
* k high mass ^ *
V? """ U.<
T ,> CAPILLARY ZONE ( ~
SATURATED ZONE
CO
dis
02
X
/PHCs\
V
sharp
/ reaction
front
nstituent
tributions
b) DISSOLVED- PHASE SOURCE
UNSATURATEDZONE
limited mass
flux
._* A _
V C CAPILLARY ZONE (
SATURATEDZONE
CO
dis
02
PHCs
vj
nstitue
tributi
sharp
— reaction
front
;nt
ons
LEGEND
LNAPL (free- or
residual-phase)
Dissolved phase
^^ Water Table
\ PI 1C Vapors
Figure 3. Difference In Potential For PVI Based On Type Of Source: a) LNAPL, b) Dissolved Phase
(Source: Lahvis, et al., 2013. Reprinted from Groundwater Monitoring & Remediation with
permission of the National Ground Water Association. Copyright 2013.)
Page 33 of 123
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LNAPL sources may be distributed both above and below the capillary fringe as a result of
smearing from water-table fluctuations. This phenomenon will tend to enhance mass flux to
the unsaturated zone because of direct partitioning between LNAPL (residual) and vapor
phases. Conversely, the mass flux will be more limited for dissolved-phase sources because
vapor transport through water is reduced relative to diffusion in soil gas (Colder Associates,
2006; Lahvis and Baehr, 1996). Vapor diffusion is limited by low effective air-phase porosity
(i.e., high moisture saturation) and biodegradation in the capillary zone.
Importance
Relatively few confirmed occurrences of PVI at petroleum sites are reported in the literature
(EPA, 2013, Section 2.6, p.9). The most likely scenarios for PVI to occur are shallow PHC sources
directly beneath buildings and mobile LNAPL or groundwater plumes with high concentrations
of PHCs that are in direct contact with buildings (Davis, 2009; and McHugh, et al., 2010). A
study by Peargin and Kolhatkar (2011) suggests that a dissolved source with benzene greater
than 1 mg/L may behave like a LNAPL source in terms of vapor-generating capability.
Assessment
Recommended steps for investigating PVI are discussed in Section 1 (p.11). Application of the
screening criteria allow for the determination of which buildings are threatened by PVI. Using
this approach, resources can be appropriately focused on those buildings potentially impacted
by PVI.
Figure 4 presents typical scenarios of the spatial relationship between PHC sources and
potential receptors. However, it is not intended to be a comprehensive depiction of all possible
configurations of such a relationship.
Table 2 summarizes the characteristics of these six scenarios relative to lateral and vertical
distances from contamination and necessary investigation activities.
Page 34 of 123
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-a
0»
00
NJ
OJ
A
B
D
Land
Surface
Residual
LNAPL
vr f
J j VVVVV
vvvvvvvvvvvvvv
High Water Table
Range of Water
Table Fluctuation
Low Water Table
vvvvvvvvvvvvvvv
LNAPL
V V V V^^Q^^V^T V V~^V V
vvvv vvvvvvvvvvv
V V V V V
Smear zone j
(Residual LNAPL}
GROUNDWATERFLOW
Smear zone
(Residual LNAPL)
Dissolved-Phase
Figure 4. Typical Scenarios Of Potential PVI Sources And Potential Receptors
-------
Table 2. Summary Of Characteristics Of Typical Scenarios Of
Petroleum Vapor Sources And Potential Receptors.
Scenario
(as illustrated
in Figure 4)
A
B
C
D
E
F
Contamination
Beneath Building?
(building is within
lateral inclusion zone)
Yes; shallow residual
LNAPL in the vadose
zone
Yes; residual including
smear zone, LNAPL,
dissolved in
groundwater
Yes; smear zone,
LNAPL, dissolved in
groundwater
Yes; dissolved in
groundwater
Maybe; plume may be
diving beneath water
table
No
Potential
For PVI
High
High
Medium
Low
Low-
None
None
Near-Slab* Soil Gas
Sampling
Recommended?
Yes, if vertical separation
distance is less than 15
feet from the top of
residual LNAPL,
otherwise No
Yes, if vertical separation
distance is less than 15
feet from the top of the
smear zone, otherwise
No
Yes, if vertical separation
distance is less than 15
feet from the top of the
smear zone, otherwise
No
Yes, if vertical separation
distance is less than 6
feet from the historical
high water table
elevation, otherwise No
Yes, if vertical separation
distance is less than 6
feet from the historical
high water table
elevation, otherwise No
No
*Near-slab soil gas samples should be collected from each side of the
potentially impacted building and as close to the building as possible. These
samples should be paired with deep (near source) soil gas samples. If these
samples do not clearly demonstrate that biodegradation is sufficient to
mitigate the threat of PVI into the building, EPA recommends collection of
indoor air samples paired with sub-slab soil gas samples.
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Special Considerations
While biodegradation may reduce the potential for human exposure to petroleum vapors, its
effectiveness in mitigating PVI may be limited by precluding factors such as:
• Migration of contaminants, especially plumes in flowing groundwater
• Presence of non-PHC chemicals that biodegrade too slowly (or the rate is not known
with certainty)
• Presence of preferential transport pathways
• Extensive impermeable surface cover, or very large buildings
• Presence of higher blends of ethanol in gasoline that consumes oxygen that would
otherwise be available for aerobic biodegradation of PHCs
• Generation of methane from higher blends of ethanol in gasoline that exerts high
oxygen demand and presents a vapor intrusion threat itself
• Soils with high organic content (e.g., peat) that exert a high oxygen demand
• Soil conditions that are inhospitable to microorganisms such as insufficient soil moisture
• Insufficient thickness of clean, biologically active soil
• LNAPL source is relatively unweathered and rich in volatile PHCs
Recommendation
EPA recommends conducting an adequate PVI investigation and following the steps described
in Section 1 (p.11) to determine which buildings may be at risk for PVI.
References Cited
Davis, R.V. 2009. Bioattenuation of Petroleum Hydrocarbon Vapors in the Subsurface: Update
on Recent Studies and Proposed Screening Criteria for the Vapor-Intrusion Pathway.
LUSTLine Bulletin 61:11-14. New England Interstate Water Pollution Control
Commission, Massachusetts.
http://www.neiwpcc.orQ/lustline/lustline pdf/lustline 61.pdf
EPA. 2012. Petroleum Hydrocarbons And Chlorinated Solvents Differ In Their Potential For Vapor
Intrusion. March.
http://www.epa.Qov/oust/cat/pvi/pvicvi.pdf
EPA. 2013. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
http://www.epa.gov/oust/cat/pvi/PVI Database Report.pdf
Colder Associates. 2006. NJDEP-Golder Subsurface Vapor Intrusion Research Project: Report
On: Investigation of Indoor Air Quality in Structures Located above VOC-Contaminated
Groundwater, Year Two, Part 1: Evaluation of Soil Vapor Intrusion at Mount Holly Site,
New Jersey, 22 pp.
Page 37 of 123
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Lahvis, M.A., and A.L. Baehr. 1996. Estimating Rates of Aerobic Hydrocarbon Biodegradation by
Simulation of Gas Transport in the Unsaturated Zone. Water Resources Research 32(7):
2231-2249.
Lahvis, M.A., I. Hers, R.V. Davis, J. Wright, and G.E. DeVaull. 2013. Vapor Intrusion Screening at
Petroleum UST Release Sites. Groundwater Monitoring and Remediation 33(2):53-67.
McHugh, T., R. Davis, G. DeVaull, H. Hopkins, J. Menatti, and T. Peargin. 2010. Evaluation of
Vapor Attenuation at Petroleum Hydrocarbon Sites. Soil and Sediment Contamination:
An International Journal 19(6):725-745.
Peargin, T. and R. Kolhatkar. 2011. Empirical Data Supporting Groundwater Benzene
Concentration Exclusion Criteria for PVI Investigations. Proceedings of Battelle 8th
International Symposium on Bioremediation and Sustainable Environmental
Technologies, Reno, Nevada, June 27-30.
Additional Information
Davis, R.V. 2011a. Evaluating the Petroleum Vapor Intrusion Pathway: Studies of Natural
Attenuation of Subsurface Petroleum Hydrocarbons & Recommended Screening Criteria.
Association for Environmental Health and Sciences 21st Annual West Coast International
Conference on Soil, Sediment, Water & Energy, Mission Valley, San Diego, California,
March 15.
Davis, R.V. 2011b. Attenuation of Subsurface Petroleum Hydrocarbon Vapors: Spatial and
Temporal Observations. Association for Environmental Health and Sciences 21st Annual
West Coast International Conference on Soil, Sediment, Water & Energy, Mission Valley,
San Diego, California, March 16.
Roggemans, S., C.L. Bruce, and P.C. Johnson. 2002. VadoseZone Natural Attenuation of
Hydrocarbon Vapors: An Empirical Assessment of Soil Gas Vertical Profile Data. API
Technical Bulletin No. 15. American Petroleum Institute, Washington, D.C.
http://www.api.org/environment-health-and-safetv/clean-water/ground-water/vapor-
intrusion/vi-publications/attenuation-hvdrocarbon-vapors
Roggemans, S. 1998. Natural Attenuation of Hydrocarbon Vapors in the VadoseZone. M.S.
Thesis, Arizona State University.
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3. Site Characterization And Conceptual Site Model (CSM)
Description
Site characterization is the process by which site-specific information and data are gathered
from a variety of sources to characterize the physical, biological, and chemical systems at a
contaminated site. A conceptual site model (CSM) integrates all data and information into a
three-dimensional picture of site conditions that illustrates contaminant distributions, release
mechanisms, migration routes, exposure pathways, and potential receptors (EPA, 2012; ITRC,
2014). The CSM uses a combination of text and graphics to portray both known and
hypothetical information (EPA, 2011). The CSM documents current conditions at the site and is
supported by maps, cross-sections, and site diagrams. The CSM illustrates potential human and
environmental exposure through contaminant release and migration toward receptors (EPA,
1995, 1996a). The CSM should be refined as new data are collected.
Importance
At any leaking UST site, it is important to have a thorough understanding of the full extent and
location of contamination (including both PHCs and non-PHC fuel additives), the characteristics
of the site that influence contaminant migration (especially the presence of preferential
transport pathways), and the locations of potential receptors. A CSM helps ensure that
sources, pathways, and receptors throughout the site have been considered; this knowledge
can lead to selection of the most appropriate sampling locations and techniques. The CSM
assists the site manager in evaluating the interaction of different site features. Risk assessors
use conceptual models to help plan for risk assessment activities (EPA, 1995). The CSM is the
basis for making informed risk management decisions about the site and the threat posed by
PVI to nearby buildings and their occupants. In addition, remedial action costs are influenced
by the quality of the CSM (EPA, 1996b).
Assessment
An investigation for PVI potential is not separate from the normal response to a confirmed UST
release; an adequate site characterization is essential in order to construct an accurate CSM. A
primary objective of site characterization is delineation of the aerial and vertical extent of
contamination in the subsurface (per 40 CFR 280.65(a)) so that lateral and vertical separation
distances can be accurately determined.23 It is also important to determine whether
preferential transport pathways are present and, if so, delineate them to determine if they
connect vapor sources directly to potential receptors. Site characterization generally proceeds
in a systematic manner, often beginning in or near the source area and working outward and in
the downgradient direction in which groundwater flows.
The separation distances described in this document (Section 5, Table 3, p.52) and the protectiveness provided
by clean, biologically active soil against vapor intrusion by PHCs may be insufficient to protect against vapor
intrusion by non-PHC fuel additives. Additional investigation should be conducted where certain additives are
present (see Section 10, p.81).
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All information and data about the site should be integrated into a CSM, which is continually
updated and refined to account for changing conditions and new information. Basic activities
associated with developing a CSM include:
• Identification of potential contaminants24
• Identification and characterization of the source of contaminants
• Characterization of the geochemical parameters that affect biodegradation
• Characterization of the geologic and hydrogeologic characteristics of the subsurface
• Delineation of potential migration pathways, including preferential transport pathways,
through environmental media
• Establishment of background levels of contaminants and areas of contamination for
each contaminated medium
• Identification and characterization of potential receptors
• Determination of the limits of the study area or system boundaries
Tracking contaminant migration from sources to receptors is one of the most important uses of
the CSM (ASTM, 2008). Uncertainties associated with the CSM should also be identified as well
as the efforts taken to reduce uncertainties to acceptable levels (ASTM, 2008). As new
information and data become available, the CSM should continually be refined (EPA, 1993;
ITRC, 2007). ITRC (2014), EPA (2013, 2015), and Wilson et al., (2014) provide additional
information about developing CSMs.
Special Considerations
The separation distances described in this document (Section 5, Table 3, p.52) and the
protectiveness provided by clean, biologically active soil against vapor intrusion by PHCs may be
insufficient to protect against vapor intrusion by non-PHC fuel additives. Additional
investigation should be conducted where certain additives are present (see Section 10, p.81).
The presence and locations of preferential transport pathways should be identified and
incorporated into the CSM. All new information and data about a site, including potential
future land uses, should also be identified to refine the CSM.
Recommendation
Per Subparts E and F in 40 CFR 280.50 through 280.67 (see
h ftp ://www. epa.QO v/o us t/fed la ws/cfr. htm). EPA recommends that an adequate site
characterization considers the following:
• §280.52(b) Release Investigation and Confirmation Steps:
"Owners and operators must measure for the presence of a release where contamination is
most likely to be present at the UST site. In selecting sample types, sample locations, and
measurement methods, owners and operators must consider the nature of the stored
24 The list of potential contaminants should include BTEX and other PHCs as well as non-PHC fuel additives likely to
have been present in the fuel stored at the site. See Sectionl (p.11) and Section 10 (p.81) for more information.
Page 40 of 123
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substance, the type of initial alarm or cause for suspicion, the type of backfill, the depth of
ground water, and other factors appropriate for identifying the presence and source of the
release."
• §280.62(a)(5) Initial Abatement Measures and Site Checks
"Measure for the presence of a release where contamination is most likely to be present at
the UST site, unless the presence and source of the release have been confirmed in
accordance with the site check required by §280.52(b) or the closure site assessment of
§280.72(a). In selecting sample types, sample locations, and measurement methods, the
owner and operator must consider the nature of the stored substance, the type of backfill,
depth to ground water and other factors as appropriate for identifying the presence and
source of the release. . ."
• §280.63(a)(l-4) Initial Site Characterization
". . .owners and operators must assemble information about the site and nature of the
release, including information gained while confirming the release or completing the initial
abatement measures. .. This information must include, but is not necessarily limited to the
following: (1) Data on the nature and estimated quantity of release; (2) Data from available
sources or site investigations concerning the following factors: surrounding populations,
water quality, use and approximate locations of wells potentially affected by the release,
subsurface soil conditions, locations of subsurface sewers, climatological conditions, and
land use; (3) Results of the site check required under §280.62(a)(5); and (4) Results of the
free product investigations. . ."
• §280.64 Free product removal
"At sites where investigations under §280.62(a)(6) indicate the presence of free product,
owners and operators must remove free product to the maximum extent practicable as
determined by the implementing agency while continuing, as necessary, any actions
initiated under §§280.61 through 280.63, or preparing for actions required under §§280.65
through 280.66. In meeting the requirements of this section, owners and operators must:. .
. (d) Unless directed to do otherwise by the implementing agency, prepare and submit to
the implementing agency, within 45 days after confirming a release, a free product removal
report that provides at least the following information: ... (2) The estimated quantity, type,
and thickness of free product observed or measured in wells, boreholes, and excavations;"
• §280.65(a) Investigation for soil and ground water cleanup
"In order to determine the full extent and location of soils contaminated by the release and
the presence and concentrations of dissolved product contamination in the groundwater,
owners and operators must conduct investigations of the release, the release site, and the
surrounding area possibly affected by the release. . ."
• §280.66(b)(l-6) Corrective Action Plan
"In making this determination, the implementing agency should consider the following
factors as appropriate: (1) The physical and chemical characteristics of the regulated
Page 41 of 123
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substance, including its toxicity, persistence, and potential for migration; (2) The
hydrogeologic characteristics of the facility and the surrounding area; (3) The proximity,
quality and current and future uses of nearby surface water and ground water; (4) The
potential effects of residual contamination on nearby surface water and ground water; (5)
An exposure assessment; and (6) Any information assembled in compliance with this
subpart."
References Cited
ASTM International (ASTM). 2008. Standard Guide for Developing Conceptual Site Models for
Contaminated Sites. E1689-95. West Conshohocken, Pennsylvania.
http://www. astm. org/Standards/El 689.htm
EPA. 1993. Guidance for Evaluating the Technical Impracticability of the Ground-Water
Restoration. OSWER Directive 9234.2-25.
http://www. epa. aov/superfund/health/conmedia/awdocs/techimp. htm
EPA. 1995. Superfund Program Representative Sampling Guidance. Volume 4: Waste. Interim
Final. OSWER Directive 9360.4-10 (EPA 540-R-95-141).
http://www.epa.Qov/tio/download/char/sf rep samp quid waste.pdf
EPA. 1996a. Soil Screening Guidance: User's Guide. Publication 9355.4-23 (EPA 540-R-96-018).
http://www. epa. gov/superfund/health/conmedia/soil/
EPA. 1996b. The Role of Cost in the Superfund Remedy Selection Process. Quick Reference Fact
Sheet. Publication 9200.3-23FS (EPA 540-F-96-018).
http://www.epa.gov/superfund/policy/remedy/pdfs/cost dir.pdf
EPA. 2011. Environmental Cleanup Best Management Practices: Effective Use of the Project Life
Cycle Conceptual Site Model. Quick Reference Fact Sheet (EPA 542-F-11-011).
http://www.epa.gov/tio/download/remed/csm-life-cycle-fact-sheet-final.pdf
EPA. 2013. Introduction to In Situ Bioremediation ofGroundwater. (EPA 542-R-13-018).
http://www. clu-in. org/download/remed/
introductiontoinsitubioremediationofgroundwater dec2013.pdf
EPA. 2015. OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway
From Subsurface Sources To Indoor Air (OSWER Publication 9200.2-154). Office of Solid
Waste and Emergency Response.
http://www. epa. gov/oswer/vaporintrusion/
Page 42 of 123
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Federal Register. 1988. 40 CFR Parts 280 and 281: Underground storage tanks; technical
requirements and state program approval; final rules. September 23,1988. 53(185):
37082-38344.
http://www.epa.gov/oust/fedlaws/cfr.htm
Interstate Technology & Regulatory Council (ITRC). 2014. Petroleum Vapor Intrusion:
Fundamentals of Screening, Investigation, and Management. Interstate Technology and
Regulatory Council, Vapor Intrusion Team, Washington, D.C. October.
http://itrcweb.org/PetroleumVI-Guidance
Wilson, J.T., K. Jewell, C. Adair, C. Paul, C. Ruybal, G. DeVaull, and J. Weaver. 2014. An Approach
that Uses the Concentrations of Hydrocarbon Compounds in Soil Gas at the Source of
Contamination to Evaluate the Potential for Intrusion of Petroleum Vapors into Buildings
(PVI). (EPA/600/R-14/318). ORD Issue Paper, U.S. Environmental Protection Agency,
Washington, DC.
http://cfpub.epa.gov/si/si public record Report.cfm?dirEntryld=305910
Additional Information
EPA. 2003. Improving Decision Quality: Making The Case For Adopting Next-Generation Site
Characterization Practices (EPA 542-F-03-012).
http://nepis. epa. Qov/Exe/ZyPURL ccii?Dockev=100046CK. TXT
EPA. 2004. Improving Sampling, Analysis, and Data Management for Site Investigation and
Cleanup (EPA 542-F-04-001a).
http://www.epa.gov/tio/download/char/2004triadfactsheeta.pdf
EPA. 2008. Triad Issue Paper: Using Geophysical Tools to Develop the Conceptual Site Model
(EPA542-F-08-007).
http://www.epa.gov/tio/download/char/Geophysics-lssue-Paper.pdf
EPA. 2010. Innovations in Site Characterization: Streamlining Cleanup at Vapor Intrusion and
Product Removal Sites Using the Triad Approach: Hartford Plume Site, Hartford, Illinois
(EPA542-R-10-006).
http://nepis. epa. cjov/Exe/ZyPURL ccji?Dockey=P100GGl G. TXT
EPA. (undated). Key Groundwater Guidance and Reports.
http://www. epa. gov/superfund/health/conmedia/gwdocs/
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4. Lateral Inclusion Zone
Description
The lateral inclusion zone is the area surrounding a contaminant source through which vapor-
phase contamination may travel and intrude into buildings. Determination of the lateral
distance within which buildings and other structures may be threatened by PVI is site-specific.
In general, with increasing confidence in the site characterization and the CSM, there can be a
corresponding decrease in the distance the lateral inclusion zone extends from clean
monitoring points.25 All buildings within the lateral inclusion zone should be further assessed
to determine if they are separated from vapor sources by an adequate vertical separation
distance (see Section 5, p.48). Further assessment may be unnecessary for those buildings
outside the lateral inclusion zone unless preferential transport pathways are present. If
contaminated groundwater is the source of vapors, migration of the contaminant plume (in the
longitudinal, transverse, and vertical directions) should be assessed when evaluating the
potential for future risks.
Importance
The lateral inclusion zone is a screening criterion to help determine which sites should
definitely be assessed further for PVI; which sites might need additional site characterization
and assessment for PVI; and which sites can reliably be excluded from consideration for further
evaluation of PVI. All buildings that overlie, or are reasonably expected to overlie,
contamination, whether LNAPL or the dissolved phase, are considered to be within the lateral
inclusion zone.
Assessment
Lateral separation distance is schematically depicted in Figure 5. Though in theory the length of
the lateral separation distance may be on the same scale as the vertical separation distance
(EPA, 2013a; ITRC, 2014), a greater lateral distance is generally warranted in the down gradient
direction (Lahvis, et a I, 2013; EPA, 2013a). This is because the lateral boundaries of a migrating
plume are more difficult to accurately delineate, as they are not stationary. Groundwater
elevations fluctuate which may result in changes in the direction and velocity of groundwater
flow. The lateral and down gradient investigation should continue until the full extent and
location of contamination is determined. This is typically achieved by surrounding the
dissolved-phase plume with clean monitoring points.
Both mobile LNAPL and dissolved contaminant plumes are dynamic and may move from one
monitoring event to the next. As discussed in Section 7 (p.61), periodic monitoring of
groundwater flow directions and plume migration should be conducted, possibly over more
than one annual cycle.
25 A monitoring point is defined as a sampling point at which soil and groundwater samples are collected (typically
from monitoring wells, though not exclusively) and which define the full extent and location of contamination. A
clean monitoring point is defined by dissolved benzene concentration less than 5 u.g/L and soil TPH concentration
less than 20 mg/Kg.
Page 44 of 123
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Lateral Separation
Clean Monitoring Distance
\.
Point
Vadose Zone
1=1
Dissolved-Phase
U
Water V
Table ^
Saturated Zone
Figure 5. Lateral Separation Distance Between Source Of PHC Contamination And
Hypothetical Receptor
Special Considerations
It can be difficult to accurately determine the exact location of contamination relative to
potential receptors. This is in part due to the dynamic nature of contaminant plumes (both
LNAPL and dissolved PHCs); the presence of heterogeneities and preferential transport
pathways in geologic material; and the distance between monitoring points, such as soil borings
and monitoring wells. It may be necessary to assess some nearby buildings for PVI before all
site characterization activities have been completed.
It is important to consider whether, and what type of preferential transport pathways are
present and could facilitate the migration of petroleum vapors. The presence of preferential
transport pathways may circumvent the protectiveness that a sufficiently thick layer of clean,
biologically active soil would otherwise provide. Preferential transport pathways such as utility
conduits typically enter buildings through holes in the foundation or slab and can facilitate the
entry of PHC vapors into the building. For example, if the transport of vapors from the source
Page 45 of 123
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area to the building could occur along utility conduits, then vapor sampling inside those utility
conduits (e.g., sewers) should be considered. Field instrument screening at utility access points
may help determine if the utility is acting as a conduit for vapors. Although specific guidance
for utility sampling is beyond the scope of this document, EPA recommends that any utility
sampling program include safety precautions to protect personnel (e.g., oxygen and
combustible gas monitoring, confined-space entry requirements) and to avoid damage to
utilities. For guidance on utility sampling, see A Practical Strategy for Assessing the Subsurface
Vapor-to-indoor Air Migration Pathway at Petroleum Hydrocarbon Sites (API, 2005).
Lateral separation distances that are usually protective against PVI may not be sufficiently
protective in situations where methane is produced in large quantity, such as sites where high-
ethanol blends of gasoline (i.e., E-20 or greater) have been released (Ma et al., 2014), and at
sites where non-PHCfuel additives are present (see Section 10, p.81). In both of these cases,
additional investigation should be conducted to assess the potential for vapor intrusion.
Another consideration is changing site conditions. Factors to consider in deciding whether to
include sites for further evaluation of PVI may include future land use—that is, whether: future
new buildings will be constructed within the lateral inclusion zone, utility trenches will be
excavated through or near previous contamination, groundwater usage will potentially be
increased, and additional releases of contaminants may occur.
Recommendation
Delineation of a lateral inclusion zone is site-specific. EPA recently published An Approach for
Developing Site-Specific Lateral and Vertical Inclusion Zones within which Structures Should be
Evaluated for Petroleum Vapor Intrusion due to Releases of Motor Fuel from Underground
Storage Tanks (EPA, 2013b). This Issue Paper describes a procedure for constructing a lateral
inclusion zone that decision makers may find useful. EPA recommends that all buildings within
the lateral inclusion zone be further assessed to determine if they are separated from vapor
sources by an adequate vertical separation distance. Further assessment may be unnecessary
for those buildings outside the lateral inclusion zone unless:
• Preferential transport pathways are present that connect PHC vapor sources to
receptors
• Impermeable surface cover (e.g., concrete, asphalt, ice, very large buildings) is so
extensive that there is concern whether there is sufficient oxygen in the subsurface to
support biodegradation
• Soil conditions are inhospitable to microorganisms (e.g., dry soils with less than 2
percent soil moisture by dry weight) such that biodegradation is insufficient to mitigate
the threat of PVI
References Cited
API. 2005. A Practical Strategy for Assessing the Subsurface Vapor-to-indoor Air Migration
Pathway at Petroleum Hydrocarbon Sites. API Publication 4741.
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EPA. 2013a. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510 R-13-001).
http://www.epa.gov/oust/cat/pvi/PVI Database Report.pdf
EPA. 2013b. An Approach for Developing Site-Specific Lateral and Vertical Inclusion Zones within
which Structures Should be Evaluated for Petroleum Vapor Intrusion due to Releases of
Motor Fuel from Underground Storage Tanks (EPA 600/R-13/047).
http://www.epa.Qov/oust/cat/pvi/epa600rl3047.pdf
Interstate Technology & Regulatory Council (ITRC). 2014. Petroleum Vapor Intrusion:
Fundamentals of Screening, Investigation, and Management. Interstate Technology and
Regulatory Council, Vapor Intrusion Team, Washington, D.C. October.
http://itrcweb.ora/PetroleumVI-Guidance
Ma, J., H. Luo, G.E. DeVaull, W.G. Rixey, and P.J. J. Alvarez. 2014. Numerical Model Investigation
for Potential Methane Explosion and Benzene Vapor Intrusion Associated with High-
Ethanol Blend Releases. Environmental Science & 7ec/7no/ogy48(l):474-481.
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5. Vertical Separation Distance
Description
The vertical separation distance is the thickness of clean, biologically active soil (see Section 9,
p.75) between the highest vertical extent of a contaminant source and the lowest point of an
overlying building. This lowest point could be a building basement floor, foundation, or
crawlspace surface.
Importance
If the thickness of clean, biologically active soil is sufficient and oxygen and soil moisture are
present, aerobic biodegradation will usually degrade vapor-phase PHCs before they can intrude
into buildings. EPA (2013a) presents a compilation and analysis of soil gas data from a large
number of sites that represent many different hydrogeologic settings where gasoline was
released from USTs.26 This analysis builds on the work of Davis (2009, 2010, 2011a, and 2011b).
In addition, EPA (2013a) summarizes the results of a number of parallel efforts (Lahvis, et al.,
2013; Peargin and Kolhatkar, 2011; Wright, 2011, 2012). Although these studies used
somewhat different data sets, there is a high degree of consistency among them. This
consistency supports the establishment of vertical screening distances based on whether PHC
contamination is present as LNAPL or dissolved PHCs; that is, the thickness required to
aerobically biodegrade PHCs is directly related to the strength of the source. Because LNAPL
sources are capable of producing higher concentrations of vapors compared to dissolved
sources, the necessary separation distance between receptors and LNAPL is greater than the
necessary separation distance between dissolved sources and receptors. At sites where non-
PHC fuel additives are present (see Section 10, p.81), the vertical separation distance may not
be sufficient to protect against vapor intrusion. In this case, additional investigation should be
conducted to assess the potential for vapor intrusion.
Assessment
The vertical separation distance is measured from the lowest point of the overlying building
basement floor, foundation, slab, or crawlspace surface and the highest vertical extent of
contamination. For dissolved sources this is the historic high water table elevation; for LNAPL
sources this is the top of the smear zone or residual LNAPL in the source area. Vertical
separation distances for dissolved plumes and LNAPL sources are schematically depicted in
Figure 6a and 6b, respectively. Both mobile LNAPL and dissolved contaminant plumes are
dynamic and may move from one monitoring event to the next. As discussed in Section 7
(p.61), periodic monitoring of groundwater flow directions and plume migration are
recommended, possibly over more than one annual cycle.
The presence of LNAPL may be determined from direct or indirect evidence. Direct evidence
includes measureable accumulations of free product in monitoring wells, an oily sheen or
floating globules on the water table, and petroleum hydrocarbon-saturated bulk soil samples.
26 The final report (EPA, 2013a) addresses the peer review comments received. The report, database, and peer
review record are accessible on EPA's PVI Compendium Web page: http://www.epa.cjov/oust/cat/pvi/index.htm.
Page 48 of 123
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Vadose Zone
Water
Table
a) Vertical separation distance for dissolved-phase source of PHCs.
Vadose Zone
Saturated Zone
(b) Vertical separation distance for LNAPL (residual or mobile phase) source of PHCs.
Figure 6. Vertical Separation Distances Between Source Of PHC Contaminants And
Hypothetical Receptor: (a) Dissolved Source, (b) LNAPL Source.
Page 49 of 123
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Lahvis et al. (2013) caution that the presence or absence of free-phase LNAPL in monitoring
wells may not be a reliable indicator of the presence of residual-phase LNAPL. The absence of
LNAPL can only be determined through analysis of core samples. This is important to recognize
because free-phase and residual LNAPL have a greater vapor-generating capability than
dissolved sources. Indirect evidence includes high concentrations of benzene and other PHCs,
often measured as TPH.27 There is considerable variation and uncertainty in LNAPL thresholds
determined from indirect evidence and Lahvis et al. (2013) suggest that multiple indicators of
the presence of LNAPL be evaluated. EPA (2013a) selected a benzene concentration of 5 mg/L
to differentiate between dissolved and LNAPL sources. A study by Peargin and Kolhatkar (2011)
suggests that a dissolved source with benzene greater than 1 mg/L may behave like a LNAPL
source in terms of vapor-generating capability. For more information on indicators of LNAPL,
see Section 6 (p.57).
Special Considerations
Preferential transport pathways such as utility conduits typically enter buildings through holes
in the foundation or slab and can facilitate the entry of PHC vapors into the building.
Consideration should be given to field instrument screening at utility access point(s) as an initial
step to determine if the utility is acting as a conduit for vapors. If the transport of vapors from
the source area to the building could occur along utility conduits, then vapor sampling inside
the utility conduits, manholes, or sumps should be considered in addition to vadose zone and
sub-slab soil gas sampling. Any utility sampling program should include safety precautions to
protect personnel (e.g., oxygen and combustible gas monitoring, confined-space entry
requirements) and to avoid damage to utilities. Specific guidance for utility sampling is
available in A Practical Strategy for Assessing the Subsurface Vapor-to-indoor Air Migration
Pathway at Petroleum Hydrocarbon Sites (API, 2005).
Vertical separation distances that are usually protective against PVI may not be sufficiently
protective in situations where methane is produced in large quantity, such as sites where high-
ethanol blends of gasoline (i.e., E-20 or greater) have been released (Ma et al., 2014), or
beneath very large buildings, or where the ground surface is covered by extensive impermeable
material (e.g., pavement) (EPA, 2013c).
In addition, consideration should be given to whether future new buildings will be constructed
within the lateral inclusion zone and whether they may be impacted by PVI.
27 Toxicological data for TPH fractions may be found in EPA (2009, 2013b), ATSDR (1999), Tveit et al. (1999), and HI
DOH (2012). More recently, Brewer et al. (2013) have developed a quantitative method for risk-based evaluation
of TPH in PVI investigations.
Page 50 of 123
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Recommendation
EPA recommends using the criteria presented in Table 3 to determine the necessary vertical
separation distance between PHC contamination from leaking USTs and an overlying building
foundation, basement, or slab. These distances are 6 feet for dissolved vapor sources (beneath
buildings of any size) and 15 feet for LNAPL sources (beneath buildings up to 66 feet on the
shortest side).28 Where the respective vertical separation distance is met or exceeded,
generally no further investigation for PVI is necessary if there are no precluding factors present
(e.g., preferential transport pathways) and the PCH source is not a high-ethanol blend (i.e., E-20
or greater) of gasoline. If the applicable separation distance is not met and where
contamination is not in direct contact with an overlying building, then choose one of two
options: (1) collect near-slab (exterior) shallow soil gas samples paired with deep (source) soil
gas samples, or (2) collect indoor air samples paired with sub-slab soil gas samples. If the
potential for PVI cannot be ruled out based on near-slab and deep soil gas sampling, then EPA
recommends indoor air sampling paired with sub-slab vapor sampling. If the attenuation factor
calculated from results of analysis of the chosen pair or vapor samples indicates that there may
be a potential for PVI above applicable exposure limits, EPA recommends gathering additional
information and data to determine whether mitigation is appropriate.
Although biodegradation is known to occur for many individual non-PHC fuel additives and
classes of additives, the rate of biodegradation in soil gas has not necessarily been rigorously
quantified; this is especially true for the lead scavengers EDB and 1,2-DCA. Therefore, for these
two chemicals in particular, vertical separation distances recommended in this guide may not
be sufficient for petroleum fuel releases that contain EDB and 1,2-DCA and additional
investigation may be necessary to assess their potential for vapor intrusion (See Section 10,
p.81 for more information).
28 See 3-D Modeling of Aerobic Biodegradation of Petroleum Vapors: Effect of Building Area Size on Oxygen
Concentration Below the Slab (EPA 510-R-13-002)(EPA, 2013c).
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Table 3. Recommended Vertical Separation Distance Between Contamination And Building
Basement Floor, Foundation, Or Crawlspace Surface.
Soil
(mg/Kg)
Groundwater
(mg/L)
< 100 (unweathered gasoline), or
< 250 (weathered gasoline, diesel)
>10 (LNAPL) > 10° (unweatnered gasoline)
>250 (weathered gasoline, diesel)
<5 <30
>5 (LNAPL) >30 (LNAPL)
6
15
6
15
Vertical
Media Benzene TPH Separation
Distance (feet)*
The thresholds for LNAPL indicated in this table are indirect evidence of the presence of LNAPL.
These thresholds may vary depending on site-specific conditions (e.g., soil type, LNAPL source). The
value of 5 mg/L benzene is from EPA (2013a, p.31). A study by Peargin and Kolhatkar (2011) suggests
that a dissolved source with benzene greater than 1 mg/L may behave like a LNAPL source in terms of
vapor-generating capability. Decision-makers may have different experiences with LNAPL indicators
and may use them as appropriate. For more information, see Section 6 (p.57) and Figure 7 in
particular.
Bulk soil samples should be analyzed for Total Petroleum Hydrocarbon (TPH) and BTEX (plus any
other potential contaminants). The objective of measuring TPH is to quantify the total vapor phase
concentration of PHCs. TPH may be analyzed by methods appropriate for the type of fuel released.
These methods may be designated as TPH-gasoline (or sometimes gasoline range organics or GRO),
TPH-diesel (or sometimes diesel range organics or DRO). Method TO-15 (see
http://www.epa.Qov/ttn/amtic/files/ambient/airtox/to-15r.pdi] by itself only measures a small
fraction of PHCs that may be present in the vapor-phase. TO-15 analyses require a correction factor
to estimate bulk TPH. An extended TO-15 analysis can provide such an estimate. For more
information on TPH in vapor intrusion studies, see Brewer et al. (2013).
*The vertical separation distance represents the thickness of clean, biologically active soil between
the source of PHC vapors (LNAPL, residual LNAPL, or dissolved PHCs) and the lowest (deepest) point
of a receptor (building basement floor, foundation, or crawlspace surface).
Page 52 of 123
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References Cited
American Petroleum Institute (API). 2005. A Practical Strategy for Assessing the Subsurface
Vapor-to-indoor Air Migration Pathway at Petroleum Hydrocarbon Sites. API Publication
4741.
http://www.api.org/environment-health-and-safety/clean-water/ground-water/vapor-
intrusion/vi-publications/assessing-vapor-intrusion.aspx
Agency for Toxic Substances and Disease Registry (ATSDR). 1999. Toxicological Profile for Total
Petroleum Hydrocarbons. U.S. Department of Health and Human Services, Atlanta, GA.
http://www. atsdr. cdc. gov/toxprofiles/tpl 23. pdf
Brewer, R., J. Nagashima, M. Kelley, M. Heskett, and M. Rigby. 2013. Risk-based evaluation of
total petroleum hydrocarbons in vapor intrusion studies. International Journal of
Environmental Research and Public Health. 10: 2441-2467.
http://www.mdpi.eom/1660-4601/10/6/2441/pdf
Bruce, L, T. Miller, and B. Hockman. 1991. Solubility Versus Equilibrium Saturation of Gasoline
Compounds: A Method to Estimate Fuel/Water Partition Coefficient Using Solubility or
Koc. In: Proceedings of the NWWA/API Conference on Petroleum Hydrocarbons in
Ground Water, p. 571-582, by National Water Well Association, Dublin, Ohio.
http://info.nQwa. orcj/cjwol/pdf/910155295. PDF
California Department of Toxic Substances Control (CA DTSC). 2009. Evaluating Human Health
Risks from Total Petroleum Hydrocarbons. Human and Ecological Risk Division,
Sacramento, CA.
http://www.oehha.ca.gov/risk/chemicalDB/index.asp
Davis, R.V. 2009. Bioattenuation of Petroleum Hydrocarbon Vapors in the Subsurface: Update
on Recent Studies and Proposed Screening Criteria for the Vapor-Intrusion Pathway.
LUSTLine Bulletin 61:11-14. New England Interstate Water Pollution Control
Commission, Massachusetts.
http://www.neiwpcc.org/lustline/lustline pdf/lustline 61.pdf
Davis, R.V. 2011a. Evaluating the Petroleum Vapor Intrusion Pathway: Studies of Natural
Attenuation of Subsurface Petroleum Hydrocarbons & Recommended Screening Criteria.
Association for Environmental Health and Sciences 21st Annual West Coast International
Conference on Soil, Sediment, Water & Energy, Mission Valley, San Diego, California,
March 15.
Davis, R.V. 2011b. Attenuation of Subsurface Petroleum Hydrocarbon Vapors: Spatial and
Temporal Observations. AEHS 21st Annual West Coast International Conference on Soil,
Sediment, Water & Energy, Mission Valley, San Diego, California, March 16.
Page 53 of 123
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EPA. 2009. Provisional Peer-Reviewed Toxicity Values for Complex Mixtures of Aliphatic and
Aromatic Hydrocarbons (CASRN Various) FINAL 9-30-2009. Superfund Health Risk
Technical Support Center, National Center for Environmental Assessment, Office of
Research and Development.
http://hhpprtv.ornl.gov/issue papers/ComplexMixturesofAliphaticandAromaticHydrocar
bons.pdf
EPA. 2013a. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
http://www.epa.aov/oust/cat/pvi/PVI Database Report.pdf
EPA. 2013b. Regional Screening Levels, Generic Tables, Residential Air and Industrial Air.
November.
http://www.epa.aov/rea3hwmd/risk/human/rb-concentration table/index.htm
EPA. 2013c. 3-D Modeling of Aerobic Biodegradation of Petroleum Vapors: Effect of Building
Area Size on Oxygen Concentration Below the Slab (EPA 510-R-13-002).
http://www.epa.aov/oust/cat/pvi/buildina-size-modelina.pdf
Hawai'i Department of Health (HI DOH). 2012. Field Investigation of the Chemistry and Toxicity
ofTPH in Petroleum Vapors: Implications for Potential Vapor Intrusion Hazards. Office of
Hazard Evaluation and Emergency Response: Honolulu, HI.
http://www.hawaiidoh.org/tgm.aspx
Lahvis, M.A., I. Hers, R.V. Davis, J. Wright, and G.E. DeVaull. 2013. Vapor Intrusion Screening at
Petroleum UST Release Sites. Groundwater Monitoring and Remediation 33(2):53-67.
Ma, J., H. Luo, G.E. DeVaull, W.G. Rixey, and P.J. J. Alvarez. 2014. Numerical Model Investigation
for Potential Methane Explosion and Benzene Vapor Intrusion Associated with High-
Ethanol Blend Releases. Environmental Science & 7ec/7no/ogy48(l):474-481.
Massachusetts Department of Environmental Protection (MA DEP). 2003. Updated Petroleum
Hydrocarbon Fraction Toxicity Values for the VPH/EPH/EPH Methodology. Bureau of
Waste Site Cleanup, Boston, MA.
McHugh, T., R. Davis, G. DeVaull, H. Hopkins, J. Menatti, and T. Peargin. 2010. Evaluation of
Vapor Attenuation at Petroleum Hydrocarbon Sites. Soil and Sediment Contamination:
An International Journal 19(6):725-745.
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Peargin, T. and R. Kolhatkar. 2011. Empirical Data Supporting Groundwater Benzene
Concentration Exclusion Criteria for PVI Investigations. Proceedings of Battelle 8th
International Symposium on Bioremediation and Sustainable Environmental
Technologies, Reno, Nevada, June 27-30.
Tveit, A., L.A. Hayes, S.H. Youngren, D.V. Nakles. 1997. Total Petroleum Hydrocarbon Working
Group Series, Volume 4: Development of Fraction Specific Reference Doses (RfDs) and
Reference Concentrations (RfCs)for Total Petroleum Hydrocarbons. Association for
Environmental Health and Sciences: Amherst, MA.
Washington Department of Ecology (WA DEC). 2006. Cleanup Levels and Risk Calculations Focus
Sheets: Reference Doses for Petroleum Mixtures. Lacey, WA.
https -.//fortress. wa.gov/ecv/clarc/FocusSheets/petroToxParameters.pdf
Wright, J. 2011. Establishing Exclusion Criteria from Empirical Data for Assessing Petroleum
Hydrocarbon Vapour Intrusion. CleanUp 2011: Proceedings of the 4th International
Contaminated Site Remediation Conference, September 11-15, Adelaide, Australia.
Wright, J. 2012. Evaluation of the Australian Petroleum Hydrocarbon VI Database: Exclusion
Criteria. Presented at: Recent Advances to VI Application & Implementation—A State-
of-the-Science Update. Association for Environmental Health and Sciences West Coast
Conference, San Diego, California. March 19-22.
https://iavi.rti.org/WorkshopsAndConferences.cfm?PagelD=documentDetails&AttachlD
=549
Additional Information
DeVaull, G. 2007. Indoor Vapor Intrusion with Oxygen-Limited Biodegradation fora Subsurface
Gasoline Source. Environmental Science and Technology 41(9):3241-3248.
Hers, I., J. Atwater, L. Li, and R. Zapf-Gilje. 2000. Evaluation of Vadose Zone Biodegradation of
BTX Vapours. Journal of Contaminant Hydrology 46:233-264.
Hers, I., D. Evans, R. Zapf-Gilje, and L. Li. 2002. Comparison, Validation and Use of Models for
Predicting Indoor Air Quality from Soil and Groundwater Contamination. Journal of Soil
and Sediment Contamination ll(4):491-527.
Hers, I., P. Jourabchi, M. Lahvis, P. Dahlen, E.H. Luo, P. Johnson, and U. Mayer. 2012 (in
preparation). Cold Climate Study of Soil Vapor Intrusion at a Residential House above a
Petroleum Hydrocarbon Plume.
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Interstate Technology & Regulatory Council (ITRC). 2014. Petroleum Vapor Intrusion:
Fundamentals of Screening, Investigation, and Management. Interstate Technology and
Regulatory Council, Vapor Intrusion Team, Washington, D.C. October.
http://itrcweb.org/PetroleumVI-Guidance
Lahvis, M.A., A.L. Baehr, and R.J. Baker. 1999. Quantification of Aerobic Biodegradation and
Volatilization Rates of Gasoline Hydrocarbons Near the Water Table under Natural
Attenuation Conditions. Water Resources Research 35(3):753-765.
Lahvis, M.A., A.L. Baehr, and R.J. Baker. 1996. Estimation of Rates of Aerobic Hydrocarbon
Biodegradation by Simulation of Gas Transport in the Unsaturated Zone. Water
Resources Research 32(7):2231-2249.
Leahy, J. G., and R.R. Colwell. 1990. Microbial Degradation of Hydrocarbons in the Environment.
Microbiological Reviews 54(3):305-315.
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6. Mobile And Residual Light Non-Aqueous Phase Liquid (LNAPL)
Description
LNAPLs released from petroleum USTs are typically fuel products such as gasoline and diesel
fuel. Fuel products are comprised of a large number of volatile and semi-volatile PHCs and
synthetic additives. Among these compounds are some that are volatile and some that are
semi-volatile. Newer, unweathered releases typically contain a higher proportion of more
volatile PHCs than do older releases that may be more weathered and depleted in the more
volatile PHCs. Similarly, gasoline contains a higher proportion of more volatile PHCs than does
diesel fuel and other middle distillates such as heating oil and kerosene. Vapors emanating
from dissolved-phase sources are primarily BTEX and other aromatic hydrocarbons, and
relatively water-soluble PHCs. Vapors emanating from LNAPL sources contain the same
constituents in addition to a sizeable fraction of aliphatic and relatively insoluble hydrocarbons
(e.g., naphthalene), especially if the source is large or unweathered (Lahvis, et al., 2013; EPA,
2013).
Importance
Depending upon the volume of the release and the characteristics of the soil, PHC vapors from
LNAPL sources can reach concentrations high enough to deplete oxygen needed by
microorganisms to biodegrade them. Compared to a dissolved plume, a LNAPL plume from a
leaking UST does not typically migrate far from the site of release (e.g., the leaking UST or
connected piping). However, the larger the mass of the release the greater the potential for
the LNAPL plume to migrate. When LNAPL underlies a receptor or comes into direct contact
with a basement, foundation, or slab, there is increased potential for explosive levels of vapors
to accumulate within the building or other structure.
Residual PHCs are non-mobile in the subsurface and occur when the release stops prior to the
accumulation of a sufficient amount of LNAPL for flow to occur, or when a fluctuating water
table smears the LNAPL across the water table and reduces the LNAPL saturation of the soil.
This smearing inhibits the lateral migration of LNAPL. Although residual contamination is not
free flowing, residual sources represent a large mass of contaminants that can persist for long
periods of time and generate considerable volumes of PHC vapors as well as dissolved-phase
contaminants.
Monitoring wells with residual LNAPL may not have a measurable accumulation of LNAPL so
they look exactly like monitoring wells with only dissolved contamination (that is, there is no
measurable LNAPL in the monitoring well). However, due to the presence of residual LNAPL,
the vapor source area acts like a free-phase LNAPL source in terms of vapor-generating
character (Lahvis, et al., 2013). A study by Peargin and Kolhatkar (2011) suggests that a
dissolved source with benzene concentration greater than 1 mg/L may have the same vapor-
generating capacity as a LNAPL source. This situation is depicted in Figure 7.
Page 57 of 123
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ACT THE SAME
LOOK THE SAME
MW
MW
MW
UNSATURATED
ZONE
? (CAPILLARY ZONE(
SATURATED
ZONE
a) free-phase
LNAPL source
UNSATURATED
ZONE
• (CAPILLARYZONE(
SATURATED
ZONE
—
b) residual -phase
LNAPL source
UNSATURATED
ZONE
A 1.
j/CAPILlARYZONE/
SATURATED
ZONE
—
c) dissolved- phase
source
LEGEND
LNAPL (free- or
residual-phase)
Dissolved
phase
Water
Table
\PHC
Vapors
MW Monitoring
Well
Figure 7. Conceptual Model Illustrating The Potential For Vapor Intrusion For a) Free-Phase
LNAPL Source, b) Residual-Phase LNAPL Source, And c) Dissolved-Phase Source (Source:
Lahvis, et al., 2013. Reprinted from Groundwater Monitoring & Remediation with permission of
the National Ground Water Association. Copyright 2013.)
Assessment
The distinction between petroleum contamination present as LNAPL and contamination
present purely as a dissolved phase is important. Unfortunately, there is no precise
concentration threshold between dissolved phase PHCs and PHCs present in a mixed phase that
includes LNAPL.29 EPA (2013) used a threshold for the benzene groundwater concentration
equal to 5 mg/L and a total petroleum hydrocarbon (TPH) threshold groundwater concentration
of 30 mg/L for identification of LNAPL sites. The TPH threshold adopted by EPA (2013) is based
on the calculated approximate average ratio of the concentration of benzene to TPH in
groundwater at UST sites. A site with a LNAPL source was identified on the basis of either the
benzene or TPH groundwater concentration exceeding the threshold. The thresholds adopted
for identifying LNAPL sites based on soil concentrations are 10 mg/Kg benzene, 100 mg/Kg TPH
for unweathered gasoline, and 250 mg/Kg TPH for diesel or weathered gasoline.
Table 4 in EPA (2013) presents a variety of direct and indirect indicators of LNAPL. For example, Bruce, et al.
(1991) suggest groundwater concentrations greater than one-fifth (0.2) of the effective solubility of LNAPL as
indirect evidence of the presence of LNAPL However, because the effective solubility depends on characteristics
of the LNAPL mass (e.g., composition, weathering); there is uncertainty in the threshold. Additional discussions of
screening concentrations for LNAPL are presented in Evaluating Hydrocarbon Removal from Source Zones and its
Effect on Dissolved Plume Longevity and Concentration (API, 2002), and ITRC (2014).
Page 58 of 123
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Special Considerations
Direct means for detecting the presence of LNAPL include measurable accumulations of free
product in monitoring wells, an oily sheen on the water, and saturation of bulk soil samples.30
The presence of residual LNAPL may not be recognizable from monitoring well data. This is
because the soil is not sufficiently saturated with LNAPL to allow it to flow into wells. Lahvis et
al. (2013) suggest that multiple indicators (both direct and indirect) be evaluated to determine
whether or not LNAPL is present.
Recommendation
EPA recommends subsurface sampling to determine the full extent and location of LNAPL (both
mobile LNAPL and residual). LNAPL may be present even when there is no measureable
accumulation of free product in a monitoring well. In addition, federal regulations (40 CFR
280.64) require that when free product is present, it must be "removed to the maximum
extent practicable as determined by the implementing agency." Effective source removal will
mitigate a long term source of PHC vapors as well as mitigate dissolved and residual LNAPL
contamination.
EPA recommends analyzing bulk soil samples collected in the source area for TPH (e.g., gasoline
or diesel depending on which fuels were stored on site) and specific petroleum constituents
(e.g., BTEX and other volatile and semi-volatile organic chemicals, and fuel additives). EPA also
recommends analysis of LNAPL samples (if present) to determine the degree of weathering and
potential for vapor generation.
References Cited
American Petroleum Institute (API). 2002. Evaluating Hydrocarbon Removal from Source Zones
and its Effect on Dissolved Plume Longevity and Concentration. API Publication 4715.
http://www.api.org/environment-health-and-safety/clean-water/ground-
water/lnapl/hydrocarbon-removal
EPA. 1996. How To Effectively Recover Free Product At Leaking Underground Storage Tank
Sites: A Guide For State Regulators. OUST (EPA 510-R-96-001).
http://www.epa.gov/swerustl/pubs/fprg.htm
EPA. 2013. Evaluation Of Empirical Data Studies To Support Soil Vapor Intrusion Screening
Criteria For Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
http://www.epa.gov/oust/cat/pvi/PVI Database Report.pdf
30 Consistent with the findings in EPA (2013) EPA recommends that these same thresholds also be applied for PVI
investigations conducted using this guide. These thresholds are presented in Table 3 in Section 5 (p.52).
Page 59 of 123
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Federal Register. 1988. 40 CFR Parts 280 and 281: Underground Storage Tanks; Technical
Requirements and State Program Approval; Final Rules. September 23, 1988. 53(185):
37082-38344.
h ftp ://www. epa.go v/o us t/fed la ws/cfr. h tm
Interstate Technology & Regulatory Council (ITRC). 2014. Petroleum Vapor Intrusion:
Fundamentals of Screening, Investigation, and Management. Interstate Technology and
Regulatory Council, Vapor Intrusion Team, Washington, D.C. October.
http://itrcweb.ora/PetroleumVI-Guidance
Lahvis, M.A., I. Hers, R.V. Davis, J. Wright, and G.E. DeVaull. 2013. Vapor Intrusion Screening at
Petroleum UST Release Sites. Groundwater Monitoring and Remediation 33(2):53-67.
Peargin, T. and R. Kolhatkar. 2011. Empirical Data Supporting Groundwater Benzene
Concentration Exclusion Criteria for PVI Investigations. Proceedings of Battelle 8th
International Symposium on Bioremediation and Sustainable Environmental
Technologies, Reno, Nevada, June 27-30.
Additional Information
Abdul, A.S., S.F. Kia, and T.L. Gibson. 1989. Limitations of Monitoring Wells for the Detection
and Quantification of Petroleum Products in Soils and Aquifers. Ground Water
Monitoring Review 9(2):90-99.
Ballestero, T.P., F.R. Fiedler and N.E. Kinner. 1994. An Investigation of the Relationship Between
Actual and Apparent Gasoline Thickness in a Uniform Sand Aquifer. Ground Water 32(5):
708-718.
Cohen, R.M., A.P. Bryda, ST. Shaw, and C.P. Spalding. 1992. Evaluation of Visual Methods to
Detect NAPL in Soil and Water. Ground Water Monitoring Review 12(4):132-141.
Farr, A.M., R.J. Houghtalen and D.B. McWhorter. 1990. Volume Estimation of Light Nonaqueous
Phase Liquids in Porous Media. Ground Water 28(l):48-56.
Kemblowski, M.W. and C.Y. Chiang. 1990. Hydrocarbon Thickness Fluctuations in Monitoring
Wells. Ground Water 28(2):244-252.
Lenhard, R.J. and J.C. Parker. 1990. Estimation of Free Hydrocarbon Volume from Fluid Levels in
Monitoring Wells. Ground Water 28(l):57-67.
Newell, C.J., S.D. Acree, R.R. Ross, and S.G. Huling. 1995. Light Nonaqueous Phase Liquids.
USEPA/ORD Robert S. Kerr Environmental Research Laboratory, Ada, Oklahoma (EPA-
540-5-95-500).
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7. Groundwater Flow And Dissolved Contaminant Plumes
Description
Contaminant plumes are dynamic, three-dimensional distributions of contaminants in
groundwater. Contaminants dissolved in groundwater can migrate with flowing groundwater
thereby spreading contamination. In some aquifers, where the direction and speed of
groundwater flow are stable, the plumes are usually long and narrow. Other plumes appear to
spread in both the transverse as well as the longitudinal direction. This apparent transverse
dispersion may be the direct result of changes in the direction of groundwater flow. What may
appear to be transverse dispersion is actually longitudinal dispersion occurring in different
directions as the direction of flow changes (EPA, 2005; Wilson, 2003).
Importance
The potential for PVI from dissolved PHC contaminant plumes is typically limited to sites where
there are high concentrations of dissolved contaminants or the plume is in direct contact with a
building foundation, basement, or slab. A study by Peargin and Kolhatkar (2011) suggests that a
dissolved source with benzene greater than 1 mg/L may have the same vapor-generating
capacity as a LNAPL source.
Assessment
Contaminant plumes generally necessitate three-dimensional monitoring to assess the
transient behavior of groundwater flow and the movement of contaminant plumes (EPA,
2004a, b). Contaminant plumes migrate with flowing groundwater, which can exhibit seasonal
variations as well as responses to pumping, tides, or stage of a nearby river.31 Groundwater
flow directions can and often change over time, and may necessitate periodic monitoring over
more than one annual cycle to understand the groundwater flow regime at a given site.
(Note: This monitoring need not delay additional investigation activities and measures to
mitigate or remediate threats to safety and health.) As the plume migrates, appropriate
adjustments to the sampling plan should be made to ensure that potential receptors are
Groundwater flow directions can change frequently and relatively quickly. Changes in groundwater flow
directions may be more prevalent than is realized, because the variation in the direction of groundwater flow is
rarely evaluated in any formal way (EPA, 2005). Wilson (2003) studied data from a site in North Carolina where
groundwater flow was influenced by the stage of a nearby river. Over the course of one year of monthly
monitoring, groundwater flow directions fluctuated by 120 degrees. Wilson et al. (2005) also studied data from a
gas station site in New Jersey. Over a six-year period groundwater monitoring data were collected on 23
occasions; the predominant flow direction was 90-degrees from the presumed direction on which the conceptual
model was constructed, and the direction of flow fluctuated by nearly 180 degrees. Mace et al. (1997) studied the
variation in groundwater flow directions at 132 gas stations in Texas. Fluctuations in flow directions occurred over
a range of 120 degrees. Goode and Konikow (1990) characterized a site where PHCs leaked to the water table.
Groundwater flow directions changed nearly 90 degrees in less than four months in response to changing flow
conditions in a nearby intermittent stream.
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protected. If new PHC releases to groundwater occur, then appropriate mitigation steps can be
implemented.
Plume monitoring networks should be able to detect changes in plume boundaries as well as
fluctuations in the concentrations of geochemical parameters and contaminant concentrations.
Collection of samples from the contaminant plume is needed to determine the extent of
contamination and provide information that can be used to estimate the vapor generation
capacity of the dissolved contamination. The contaminant plume should be surrounded by
sampling points that are free of contamination (i.e., clean monitoring points).
Conventional monitoring wells may provide an incomplete picture of the true distribution of
contaminants in groundwater. If the length of the screen in a monitoring well is long compared
to the thickness of the plume of contamination, the sample obtained will be diluted by the
inflow of clean(er) groundwater from above or below the plume. Also, plumes may dive below
the screened interval of the wells leading to the false impression that the plume is shorter than
it actually is (EPA, 2005).
Special Considerations
Dissolved plumes are dynamic and contamination may migrate beneath buildings over time.
This is best evaluated by determining the range of fluctuation in groundwater flow direction
and water table elevations beneath buildings over at least one annual cycle. However, in the
interim, the remaining PVI-related activities should continue. Preferential transport pathways,
if present, may facilitate the intrusion of petroleum vapors into buildings.32 When contaminant
plumes intersect preferential transport pathways, the spread of contamination can be very
rapid compared to the velocity of groundwater flow through the soil.
Volatilization of contaminants from the plume into soil gas is greatly reduced when a plume
dives beneath the water table surface. Volatile contaminants diffuse more slowly through the
water column than through soil gas.
Recommendation
EPA recommends groundwater monitoring and sampling to determine the depth to
contaminated groundwater and the vertical distribution of contaminants in the water column
beneath overlying buildings33. Due to the transient nature of groundwater migration, EPA
recommends periodic monitoring and sampling over more than one annual cycle to fully
32 Dissolved petroleum contaminants may threaten building inhabitant's health through their water supply rather
than through vapor intrusion. Exposure may occur from wells drawing from a contaminated plume, or
contamination permeating the water supply piping. Though fuel constituents generally impart a disagreeable odor
and taste, building occupants may still be exposed to potentially harmful levels of contaminants. Such exposure
may occur when PHCs volatilize from the dissolved phase during showering or washing clothes and dishes, or
through ingesting contaminated water. Identifying the mechanism of exposure is important because methods for
remediation/mitigation of PVI will be different than treatment or remediation of contaminated groundwater.
33 If groundwater samples contain greater than 30 mg/L TPH (or greater than 5 mg/L benzene), it is possible that
residual LNAPL is present (see Section 6, p.57).
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understand the groundwater flow regime at a given site. Where the respective vertical
separation distance (see Section 5) is met or exceeded, generally no further investigation for
PVI is necessary if there are no precluding factors present (e.g., preferential transport
pathways) and the PCH source is not a high-ethanol blend (i.e., E-20 or greater) of gasoline. If
the applicable separation distance is not met and where contamination is not in direct contact
with an overlying building, then choose one of two options: (1) collect near-slab (exterior)
shallow soil gas samples paired with deep (source) soil gas samples, or (2) collect indoor air
samples paired with sub-slab soil gas samples. If the potential for PVI cannot be ruled out
based on near-slab and deep soil gas sampling, then EPA recommends indoor air sampling
paired with sub-slab vapor sampling. If the attenuation factor calculated from results of
analysis of the chosen pair or vapor samples indicates that there may be a potential for PVI
above applicable exposure limits, EPA recommends gathering additional information and data
to determine whether mitigation is appropriate.
Even in cases where there is no threat of PVI from contaminated groundwater, EPA
recommends that the plume be assessed to determine if remediation is necessary to prevent
ingestion of contaminated drinking water and protect and restore actual or potential sources of
drinking water.
References Cited
EPA. 2004a. Performance Monitoring of MNA Remedies for VOCs in Ground Water. Office of
Research and Development, National Risk Management Research Laboratory, Ada,
Oklahoma. April (EPA/600/R-04/027).
http://www.epa.Qov/nrmrl/pubs/600r04027.html
EPA. 2004b. Monitored Natural Attenuation. Chapter IX in How To Evaluate Alternative Cleanup
Technologies For Underground Storage Tank Sites: A Guide For Corrective Action Plan
Reviewers (EPA 510-B-94-003; EPA 510-B-95-007; and EPA 510-R-04-002).
http://www. epa. gov/swerustl/pubs/tums. htm
EPA. 2005. Monitored Natural Attenuation ofMTBE as a Risk Management Option at Leaking
Underground Storage Tank Sites (EPA/600/R-04/1790).
http://nepis. epa. gov/Exe/ZyPURL cgi?Dockey=20017I6R. txt
Goode, D.J. and L.F. Konikow. 1990. Apparent Dispersion in Transient Groundwater Flow.
Water Resources Research 26(10):2339-2351.
Mace, R. E., R. S. Fisher, D. M. Welch, and S. P. Parra. 1997. Extent, Mass, and Duration of
Hydrocarbon Plumes from Leaking Petroleum Storage Tank Sites in Texas. Geological
Circular 97-1, Bureau of Economic Geology, University of Texas, Austin, Texas.
Page 63 of 123
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Peargin, T. and R. Kolhatkar. 2011. Empirical Data Supporting Groundwater Benzene
Concentration Exclusion Criteria for PVI Investigations. Proceedings of Battelle 8th
International Symposium on Bioremediation and Sustainable Environmental
Technologies, Reno, Nevada, June 27-30.
Wilson, J. T. 2003. Fate and Transport of MTBE and Other Gasoline Components. In: MTBE
Remediation Handbook, Amherst, Massachusetts: Amherst Scientific Publishers, pp.19-
61.
Wilson, J.T., C. Adair, P.M. Kaiser, and R. Kolhatkar. 2005. Anaerobic Biodegradation of MTBE at
a Gasoline Spill Site. Ground Water Monitoring and Remediation 25(3):103-115.
Additional Information
Bredehoeft, J.D. and G.F. Pinder. 1973. Mass Transport in Flowing Groundwater. Water
Resources Research 9(1):194-210.
EPA. (undated). Key Groundwater Guidance and Reports.
http://www. epa. gov/superfund/health/conmedia/gwdocs/
Gelhar, L.W., C. Welty and K.R. Rehfeldt. 1992. A Critical Review of Data on Field-Scale
Dispersion in Aquifers. Water Resources Research 28(7):1955-1974.
Greenkorn, R.A. and D.P. Kessler. 1969. Dispersion in Heterogeneous Nonuniform Anisotropic
Porous Media. Industrial and Engineering Chemistry 61(9):14-32.
Pickens, J.F. and G.E. Grisak. 1981. Scale-Dependent Dispersion in a Stratified Granular Aquifer.
Water Resources Research 17(4):1191-1211.
Scheidegger, A.E. 1954. Statistical Hydrodynamics in Porous Media. Journal of Applied Physics
25(8):994-1001.
Scheidegger, A.E. 1961. General Theory of Dispersion in Porous Media. Journal of Geophysical
Research 66(10):3273-3278.
Schwartz, F.W. 1977. Macroscopic Dispersion in Porous Media: The Controlling Factors. Water
Resources Research 13(4):743-752.
U.S. Geological Survey. 2013. Factors Affecting Public-Supply Well Vulnerability to
Contamination: Understanding Observed Water Quality and Anticipating Future Water
Quality. Circular 1385. U.S. Department of the Interior, National Water-Quality
Assessment Program.
http://pubs.usQS.Qov/circ/1385/
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Wilkin, R.T., M.S. McNeil, S.J. Adair, and J.T. Wilson. 2001. Field Measurement of Dissolved
Oxygen: A Comparison of Methods. Ground Water Monitoring and Remediation
21(4):124-132.
Zheng, C. and S.M. Gorelick. 2003. Analysis of Solute Transport in Flow Fields Influenced by
Preferential Flowpaths at the Decimeter Scale. Ground Water 41(2)-.142-155.
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8. Soil Gas Profile
Description
Aerobic biodegradation of PHC vapors occurs in many subsurface environments (Lahvis, Baker,
and Behr, 1998; McHugh, et al., 2010; Roggemans, 1998; Roggemans, Bruce, and Johnson,
2002; ZoBell, 1946; Atlas, 1981; Leahy and Colwell, 1990; DeVaull, 2007). The soil gas profile
can provide confirmation that aerobic biodegradation is occurring in the subsurface.
Decreasing oxygen concentration and increasing carbon dioxide and methane concentrations
indicate biodegradation of PHCs (Hult and Grabbe, 1988). Shallow soil gas typically contains
water vapor and fixed gases: nitrogen, oxygen, carbon dioxide, carbon monoxide, and argon.
These gases infiltrate into the soil from the atmosphere. Vapor phase PHC contamination may
be the result of volatilization from mobile LNAPL released into the subsurface, residual soil
contamination (including the smear zone), and dissolved phase contamination. The lower
proportion of volatile hydrocarbon compounds in diesel fuel will lead to a comparatively
smaller vapor plume in comparison to the release of a similar volume of gasoline (Prince and
Douglas, 2010; Marchal, et al., 2003). In addition to PHCs, soil gas may also contain
degradation products from the breakdown of PHCs and naturally occurring organic matter. The
principal gases resulting from the biodegradation of PHCs are carbon dioxide (under aerobic
conditions) or methane (under anaerobic conditions).
Figure 8 presents a characteristic vertical concentration profile in the unsaturated zone; oxygen
concentrations decrease with depth and PHCs (including methane) and carbon dioxide
Land Surface
A
Oxygenated
Oxygen
Flux
PHC + CH4
Flux
Impacted
Soil
Increasing Concentration
Figure 8. Typical Vertical Concentration Profile In The Unsaturated Zone For PHCs (Plus
Methane), Carbon Dioxide And Oxygen
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concentrations increase with depth toward the source of contamination. This typical vertical
profile may vary somewhat in shape depending on site-specific conditions (Roggemans, Bruce,
and Johnson, 2002). During aerobic biodegradation in unsaturated soils, PHCs degrade, oxygen
is consumed, and carbon dioxide is produced. The aerobic biodegradation zone is within
oxygenated soil (generally greater than 1 percent oxygen in soil gas). In the oxygenated soil
zone (where aerobic biodegradation occurs between land surface and the depth of impacted
soil) the decrease in PHC concentrations is typically quite rapid and occurs over a narrow
interval (the reaction zone in Figure 8) (Abreu, Ettinger, and McAlary, 2009).
The impacted soil zone, which is anaerobic, is characterized by the maximum PHC
concentrations (and often LNAPL) and biodegradation is slow (EPA, 2012a). Generally, PHC
vapor concentrations will be much greater adjacent to a LNAPL source than adjacent to a
dissolved hydrocarbon plume. If PHC concentrations are high enough, available oxygen may be
depleted, which in turn limits aerobic biodegradation. The core of any PHC contaminant mass
is typically depleted with respect to oxygen, thus anaerobic biodegradation of LNAPL or other
organic sources (e.g., ethanol) can produce significant amounts of methane (Anderson and
Lovley, 1997; Wiedemeier, et al., 1999; Koenigsberg and Norris, 1999; Ma et al., 2014).
Methane readily biodegrades under aerobic conditions and, when present, will create an
additional oxygen demand (Jewell and Wilson, 2011; Ma, et al, 2012). High concentrations of
methane, oxygen, and a source of ignition can create a fire or explosion hazard in confined
spaces (e.g., utility vaults and passages, basements, or garages) (Ma et al., 2014). For additional
information on evaluating the presence of methane and potential hazards, see ASTM's "New
Practice for Evaluating Potential Hazard Due to Methane in the Vadose Zone".34
Differences between near-slab soil gas profiles and sub-slab soil gas profiles are reported in two
EPA modeling studies: the conceptual model scenarios report EPA (2012b) and the building size
modeling report for PVI (EPA, 2013a). However, EPA (2012b) assumed that building
foundations, basements, and slabs were impermeable and, thus, oxygen transport was not
simulated through the foundation, basement, or slab into the subsurface beneath the building.
Simulations presented in EPA (2013a) allowed for oxygen transport using reasonably expected
oxygen permeability values for concrete (Fischer et al., 1996; McHugh, DeBlanc, and Pokluda,
2006; Lundegard, Johnson, and Dahlen, 2008; Patterson and Davis, 2009; Tittarelli, 2009).
When oxygen transport is accounted for, the differences in soil gas profiles were less
pronounced between near-slab and sub-slab samples very close to the building basement and
slab. Thus, near-slab soil gas samples can be substituted for sub-slab samples in situations
where dissolved contamination is present within 6 feet of (but not in contact with) a building
basement floor, foundation, or crawlspace surface, and where LNAPL is present within 15 feet
of (but not in contact with) a building basement floor, foundation, or crawlspace surface. For
dissolved sources this holds for buildings of any size, and for LNAPL sources it applies to
buildings with the shortest side being no longer than 66 feet (EPA, 2013a). Deep soil gas
samples are needed to determine the depth to contaminated soil and the thickness of clean,
34 The new ASTM methane guide is accessible at
http://www. astm.om/DA TABASE. CART/WORKITEMS/WK32621.htm
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biologically active soil necessary to attenuate PHC vapors. Shallow soil gas samples should be
paired with deep (near source) soil gas samples to evaluate the strength of the PHC vapor
source and the attenuation of PHC vapors.
Importance
The potentialforPVI is a function of the oxygen demand exerted by all biodegradable vapors,
not just the keychemicalsof potential concern (Jewell and Wilson, 2011; Ma, et al., 2012).
When present, volatile PHCsand methane also exert an oxygen demand that may limit aerobic
biodegradation of benzene (Abreu, Ettinger, and McAlary, 2009; Wilson, 2011). An estimate of
the biodegradation rate can be determined from the stoichiometric relationship between the
flux of oxygen, carbon dioxide, and methane (Lahvisand Baehr, 1996). PHC vapor
concentrations gene rally decrease with increasing distance from a subsurface vapor source. At
a relatively shortdistance from the source, concentrationsof PHCs in soil gas will typicallyfall
be low potentially significant levels of concern provided that oxygen replenish mentis adequate
to ensure complete aerobic biodegradation. Lahvis, Baehr, and Baker (1999) observed that PHC
vapors from a dissolved plume were almost complete lydegraded within lmeter(3.3feet)
above the watertable and that significant transport of PHC vapors may only be significant if the
vapor source isLNAPL. This is consistent with the find ings of EPA's(2013b) PVI database
analysisreportand Lahvisetal. (2013).
Assessment
Soil gas samples provide information on the distribution of contamination nearthe source area,
whether biodegradation is occurring, and how effective it is in reducing the potentialforPVI.
When there isan impermeable surface coveradjacentto a building, soil gas probesshould be
installed beneath the surface in order for the soil gas profile to adequately characterize
conditions belowthe surface. Forvery large buildings, orwhere there isextensive
impermeable surf ace cove rand the vapor source is relatively shallow, additional investigation is
recommended to verify that biodegradation isoccurring beneath the building.35 Vaporsamples
should be analyzed forPHCs (and non-PHCfuel additives), methane, oxygen, and carbon
dioxide (Lahvis, Baehr, and Baker, 1999).
35 EPA (2013a) presents modeling results for a variety of soil types, building sizes, vapor source strengths, and
vertical separation distances. These results, while not exhaustive, indicate that for dissolved sources and very
large buildings, an oxygen shadow does not form, thus the subsurface stays sufficiently oxygenated to support
aerobic biodegradation and preclude the potential for PVI. For LNAPL sources, an oxygen shadow was not
observed to form beneath buildings up to 66 feet on the shortest side. This length represents the threshold below
which oxygen replenishment is sufficient to support aerobic biodegradation; above this length oxygen
replenishment may be impeded and there may be in sufficient oxygen present to support aerobic biodegradation.
For larger buildings underlain by LNAPLwithin 15 feet of the foundation, basement, or slab, sub-slab soil gas
sampling paired with indoor air sampling is necessary to assess whether PVI isoccurring. Another potential
concern for large build ings and extensive, impermeable surface cover (e.g., asphalt, concrete) is formation of a
moi stu re shadow, which represents soil moisture content too low to support micro bial biodegradation (see
Section 9, p.75, for more i nformation).
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An estimate of the total oxygen demand can be determined in two ways: sample for methane
and petroleum hydrocarbons (PHCs); or sample and measure the oxygen demand for all the
organic compounds in the soil gas at the source.36 If methane and all the PHCs in soil gas are
measured, these concentrations should be converted to an equivalent concentration of
benzene and summed. The total oxygen demand of the aggregate of methane and the PHCs
(expressed as an equivalent concentration of benzene) can be used to determine an
attenuation factor (a) that can be used along with the actual concentration of benzene in soil
gas at the source to determine whether aerobic biodegradation is capable of degrading the PHC
vapors to acceptable concentrations (see Section 12, p.106, Figure 9, p.101, and Figure 10,
p.103).
In some cases, relatively shallow soil gas samples (less than five feet below ground surface) will
be needed to characterize active biodegradation zones in the shallow soil (e.g., in the presence
of shallow contamination sources). Some state regulatory programs do not allow soil gas
sampling at depths less than 5 feet based on the concern that accurate sampling may not be
possible at shallow depths because air from the surface may leak into the sample. However,
recent research has shown that the collection of accurate shallow-soil gas samples is possible at
depths as shallow as 2 feet below ground surface using appropriate field methods (e.g., leak
testing), such as those documented in Temporal Variation ofVOCs in Soils from Groundwater to
the Surface (EPA, 2010). It is also possible under certain conditions to collect representative soil
gas samples using previously installed groundwater monitoring wells (see Wilson et al., 2014).
Special Considerations
There are several factors that can limit replenishment of oxygen to deep soils. These include
presence of low permeability layers, concrete or asphalt covering at the surface, high soil
moisture from a recent rainfall event or from irrigation, and buildings that are so large that
oxygen is depleted beneath the center of the building (Patterson and Davis, 2009). However, a
recent study by EPA (2013a) indicates that for an oxygen shadow37 to form beneath a building,
the PHC vapor source must be shallow LNAPL and the building must be greater than 66 feet in
length on the shortest side. For simulations with dissolved sources, no oxygen shadow formed
even under a square building with sides that were 2,073 feet in length.
At sites with a new release or unweathered LNAPL source, the oxygen demand will be high. It is
important to determine whether temporal variations in oxygen flux into the vadose zone will
limit the effectiveness of aerobic biodegradation, potentially resulting in intermittent vapor
intrusion impacts. For such sites, more than one round of soil gas monitoring may be needed
to confirm that aerobic biodegradation consistently prevents PVI impacts at the site.
36 EPA recommends using modified Method TO-15 (see http://www.epa.Qov/ttn/amtic/files/ambient/airtox/to-
15r.pdf) for organic compounds. The concentration of methane measured as a fixed gas can then be added to the
results of TO-15 to give an approximation of TPH.
37 For the purposes of this modeling study, an oxygen shadow is defined as less than 1 percent oxygen.
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Recommendation
EPA recommends that soil gas samples be analyzed for PHCs, non-PHC fuel additives, methane,
oxygen, and carbon dioxide. Sampling for nitrogen (and other fixed gases) in soil gas can
provide a check on the quality of the analyses since the sum of these gases should be 100
percent. If they are substantially less than 100 percent, then some constituents are
unaccounted for and the analyses should be interpreted with caution.
For buildings of any size within the lateral inclusion zone if the vertical separation distance
between the building basement, foundation, or slab and dissolved contamination is less than 6
feet, but not in contact with the building EPA recommends that near-slab soil gas samples
paired with deep (near source) soil gas be collected. For buildings up to 66 feet on the shortest
side that directly overlie LNAPL masses, and the vertical separation distance is less than 15 feet
(but the building is not in direct contact with LNAPL), EPA recommends collection of near-slab
soil gas samples paired with deep (near source) soil gas samples. Near-slab soil gas samples
should be collected from each side of the building and as close to the building as practicable. If
the attenuation factor calculated from results of analysis of the chosen pair or vapor samples
indicates that there may be a potential for PVI above applicable exposure limits, EPA
recommends gathering additional information and data to determine whether mitigation is
appropriate. If contamination is in direct contact with an overlying building (and thus, collection
of shallow soil gas samples is not possible), indoor air sampling is recommended.
In addition, for very large buildings or where there is extensive impermeable surface covering,
EPA recommends that near-slab or sub-slab soil gas samples be collected if there is concern
that these conditions may impede the flux of oxygen to the subsurface and create an oxygen or
soil moisture shadow.
References Cited
Abreu, L.D.V, R. Ettinger, and T. McAlary. 2009. Simulated Soil Vapor Intrusion Attenuation
Factors Including Biodegradation for Petroleum Hydrocarbons. Ground Water
Monitoring and Remediation 29(1):105-117.
Anderson, R.T., and D.R. Lovley. 1997. Ecology and Biogeochemistry of In Situ Groundwater
Bioremediation, in Jones, J.G. (ed) Advances in Microbial Ecology, Volume 15, Plenum
Press, New York, New York, pp.289-350.
ASTM International (ASTM). 2014. New Practice for Evaluating the Potential Hazard Due to
Methane in the Vadose Zone. ASTM WK3221. West Conshohocken, Pennsylvania.
http://www.astm.orQ/DATABASE.CART/WORKITEMS/WK32621.htm
Atlas, R.M. 1981. Petroleum Microbiology. Microbiological Reviews 45(1):180-209.
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DeVaull, G.E. 2007a. Indoor Vapor Intrusion with Oxygen-Limited Biodegradation for a
Subsurface Gasoline Source. Environmental Science & Technology 41(9):3241-3248.
EPA. 2010. Temporal Variation of VOCs in Soils from Groundwater to the Surface/Subslab
(EPA/600/R-10/118). http://www.epa.aov/esd/cmb/pdf/EPA600R-l0-118.pdf
EPA. 2012a. Petroleum Hydrocarbons And Chlorinated Solvents Differ In Their Potential For
Vapor Intrusion. March, http://www.epa.cjov/oust/cat/pvi/pvicvi.pdf
EPA. 2012b. Conceptual Model Scenarios for the Vapor Intrusion Pathway. (EPA 530-R-10-003)
http://www.epa.aov/oswer/vaporintrusion/documents/vi-cms-vllfinal-2-24-2012.pdf
EPA. 2013a. 3-D Modeling of Aerobic Biodegradation of Petroleum Vapors: Effect of Building
Area Size on Oxygen Concentration Below the Slab (EPA 510-R-13-002).
http://www.epa.aov/oust/cat/pvi/buildina-size-modelina.pdf
EPA. 2013b. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510 R-13-001).
http://www.epa.gov/oust/cat/pvi/PVI Database Report.pdf
Fischer, M.L, Bentley, A.J., Dunkin, K.A., Hodgson, AT., Nazaroff, W.W., Sextro, R.G., and J.M.
Daisey. 1996. Factors affecting indoor air concentrations of volatile organic compounds at
a site of subsurface gasoline contamination. Environmental Science & Technology 30:
2948-2957.
Jewell, K.P. and J.T. Wilson. 2011. A New Screening Method for Methane in Soil Gas Using
Existing Groundwater Monitoring Wells. Ground Water Monitoring and Remediation
31(3):82-94.
Koenigsberg, S.S. and R.D. Norris (eds). 1999. Accelerated Bioremediation Using Slow Release
Compounds: Selected Battelle Conference Papers 1993-1999. California: Regenesis
Bioremediation Products.
Lahvis, M.A., and A.L. Baehr. 1996. Estimation of Rates of Aerobic Hydrocarbon Biodegradation
by Simulation of Gas Transport in the Unsaturated Zone. Water Resources Research
32(7):2231-2249.
Lahvis, M., A. Baehr, and R. Baker. 1999. Quantification of Aerobic Biodegradation and
Volatilization Rates of Gasoline Hydrocarbons near the Water Table under Natural
Attenuation Conditions. Water Resources Research 35(3):753-765.
Lahvis, M.A., I. Hers, R.V. Davis, J. Wright, and G.E. DeVaull. 2013. Vapor Intrusion Screening at
Petroleum UST Release Sites. Groundwater Monitoring and Remediation 33(2):53-67.
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Leahy, J.G., and R.R. Colwell. 1990. Microbial Degradation of Hydrocarbons in the Environment.
Microbiological Reviews 54(3):305-315.
Lundegard, P.D., P.C. Johnson, and P. Dahlen. 2008. Oxygen transport from the atmosphere to
soil gas beneath a slab-on-grade foundation overlying petroleum-impacted soil.
Environmental Science and Technology 42(15):5534-5540.
Ma, J. W.G. Rixey, G.E. DeVaull, B.P. Stafford, and P.J. J. Alvarez. 2012. Methane Bioattenuation
and Implications for Explosion Risk Reduction along the Groundwater to Soil Surface
Pathway above a Plume of Dissolved Ethanol. Environmental Science & Technology
46(11):6013-6019.
Ma, J., H. Luo, G.E. DeVaull, W.G. Rixey, and P.J. J. Alvarez. 2014. Numerical Model Investigation
for Potential Methane Explosion and Benzene Vapor Intrusion Associated with High-
Ethanol Blend Releases. Environmental Science & 7ec/7no/ogy48(l):474-481.
Marchal, R., S. Penet, F. Solano-Serena, and J.-P. Vandecasteele. 2003. Gasoline and Oil
Biodegradation. Oil and Gas Science and Technology 58(4):441-448.
McHugh, T. E., P.C. DeBlanc, and R.J.Pokluda. 2006. Indoor Air as a Source of VOC
Contamination in Shallow Soils Below Buildings. Soil & Sediment Contamination 15(1):
103-122.
McHugh, T., R. Davis, G. DeVaull, H. Hopkins, J. Menatti, and T. Peargin. 2010. Evaluation of
vapor attenuation at petroleum hydrocarbon sites. Soil and Sediment Contamination: An
International Journal 19(6):725-745.
Patterson, B.M., and G.B. Davis. 2009. Quantification of Vapor Intrusion Pathways into a Slab-
on-Ground Building under Varying Environmental Conditions. Environmental Science
and Technology 43(3):650-656.
Prince, R.C. and G.S. Douglas. 2010. Remediation of Petrol and Diesel in Subsurface from Petrol
Station Leaks, in Timmis, K.N. (ed) Handbook of Hydrocarbon and Lipid Microbiology:
Part 14, Springer-Verlag, Berlin.
Roggemans, S. 1998. Natural Attenuation of Hydrocarbon Vapors in the Vadose Zone. M.S.
Thesis, Arizona State University.
Roggemans, S., C.L. Bruce, and P.C. Johnson. 2002. Vadose Zone Natural Attenuation of
Hydrocarbon Vapors: An Empirical Assessment of Soil Gas Vertical Profile Data. API
Technical Bulletin No. 15. American Petroleum Institute, Washington, D.C.
http://www.api.org/environment-health-and-safetv/clean-water/ground-water/vapor-
intrusion/vi-publications/attenuation-hydrocarbon-vapors
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Tittarelli, F. 2009. Oxygen Diffusion Through Hydrophobic Cement-Based Materials. Cement and
Concrete Research 39(10):924-928.
Wiedemeier, T.H., H.S. Rifai, C.J. Newell, and J.T. Wilson. 1999. Natural Attenuation of Fuels and
Chlorinated Solvents. New York: John Wiley & Sons.
Wilkin, R.T., M.S. McNeil, S.J. Adair, and J.T. Wilson. 2001. Field Measurement of Dissolved
Oxygen: A Comparison of Methods. Ground Water Monitoring and Remediation
21(4):124-132.
Wilson, J.T. 2011. Impact of Methane at Gasoline Spill Sites on the Potential for Vapor Intrusion.
Webinar on January 11th, sponsore
California, Sacramento, California.
Webinar on January 11th, sponsored by the Groundwater Resources Association of
Wilson, J.T., K. Jewell, C. Adair, C. Paul, C. Ruybal, G. DeVaull, and J. Weaver. 2014. An Approach
that Uses the Concentrations of Hydrocarbon Compounds in Soil Gas at the Source of
Contamination to Evaluate the Potential for Intrusion of Petroleum Vapors into Buildings
(PVI). (EPA/600/R-14/318). ORD Issue Paper, U.S. Environmental Protection Agency,
Washington, DC.
http://cfpub.epa.gov/si/si public record Report.cfm?dirEntryld=305910
ZoBell, C.E. 1946. Action of Microorganisms on Hydrocarbons. Bacteriological Reviews
Additional Information
Amos, R.T., K.U. Mayer, B.A. Bekins, G.N. Delin, and R.L. Williams. 2005. Use of Dissolved and
Vapor-Phase Gases to Investigate Methanogenic Degradation of Petroleum
Hydrocarbon Contamination in the Subsurface. Water Resources Research
41(2):W02001.
Brenner, D. 2010. Results of a Long-Term Study of Vapor Intrusion at Large Buildings at the
NASA Ames Research Center. Journal of the Air and Waste Management Association
60(6):747-758.
Hawai'i Department of Health (HI DOH). 2011. Evaluation of Environmental Hazards at Sites
with Contaminated Soil and Groundwater. Office of Hazard Evaluation and Emergency
Response: Honolulu, HI.
Hawai'i Department of Health (HI DOH). 2012. Field Investigation of the Chemistry and Toxicity
ofTPH in Petroleum Vapors: Implications for Potential Vapor Intrusion Hazards. Office of
Hazard Evaluation and Emergency Response: Honolulu, HI.
http://www.hawaiidoh.org/tgm.aspx
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Kristensen, A., T. Poulsen, L. Mortensen, and P. Moldrup. 2010. Variability of Soil Potential for
Biodegradation of Petroleum Hydrocarbons in a Heterogeneous Subsurface. Journal of
Hazardous Materials 179(l-3):573-580.
Lahvis, M., A. Baehr, and R. Baker. 1999. Quantification of Aerobic Biodegradation and
Volatilization Rates of Gasoline Hydrocarbons near the Water Table under Natural
Attenuation Conditions. Water Resources Research 35(3):753-765.
Luo, H., P. Dahlen, P. Johnson, T. Peargin, and T. Creamer. 2009. Spatial Variability of Soil-Gas
Concentrations near and beneath a Building Overlying Shallow Petroleum Hydrocarbon-
Impacted Soils. Ground Water Monitoring and Remediation 29(1):81-91.
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9. Clean, Biologically Active Soil
Description
For a PVI investigation, clean soil does not necessarily mean that it is contaminant-free, but
rather that the level of any contamination present is low enough so that the biological activity
of the soil is not diminished and the subsurface environment will support sufficient populations
of microorganisms to aerobically biodegrade PHC vapors. This means that LNAPL is not
present. The oxygen demand of all of the contamination present in the soil should not deplete
the available supply of oxygen to such an extent that the rate of biodegradation is reduced.
Effective aerobic biodegradation of PHCs depends on the soil having sufficient oxygen and
enough moisture to provide a habitat for adequate populations of active microorganisms.
Although most soils contain indigenous microorganisms capable of degrading PHC vapors,
typically there is an acclimation period between the time they are exposed to the PHC vapors
and the time they begin to biodegrade the vapors. Prior exposure to PHCs has been observed
to both increase the number of microbes and the microbial mass available for biodegradation
of the PHCs and consequently speed up the degradation rate (ZoBell, 1946; Moyer, et al., 1996;
Phelps and Young, 1999; and Siddique, et al., 2007).
The habitat of soil bacteria is the thin film of water held to the surface of soil particles by
capillary attraction. EPA (2013a) notes that soil moisture content greater than 2 percent is
adequate to support biodegradation activity (Leeson and Hinchee, 1996), although
biodegradation is limited when the moisture content is at or below the permanent wilting point
(Zwick, et al., 1995; Holden, Halverson, and Firestone, 1997). Adequate soil moisture is also
indicated if the landscape supports the growth of non-irrigated vegetation (Riser-Roberts,
1992).
Certain geologic materials do not qualify as biologically active soil and should not be included in
the vertical separation distance (see Section 5, p.48). These geologic materials include:
• Coarse sand and gravel with a low content of silt, clay, and organic matter, and low
moisture content that is less than 2 percent dry weight
• Fractured, faulted, or jointed consolidated rock
• Consolidated rock with solution channels (i.e., karst)
Importance
Effective aerobic biodegradation of PHCs depends on a thick layer of soil having sufficient
oxygen and enough soil water to provide a habitat for adequate populations of active
microorganisms. If oxygen is present, these organisms will generally consume available PHCs.
Furthermore, aerobic biodegradation of petroleum compounds can occur relatively quickly,
with degradation half-lives as short as hours or days under some conditions (DeVaull, 2007).
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Some petroleum compounds can also biodegrade under anaerobic conditions; however, above
the water table, where oxygen is usually available in the soil zone, this process is less important
because it is generally much slower than aerobic biodegradation (Widdel, Boetius, and Rabus,
2006; Bailey, Jobson, and Rogers, 1973; and Bruce, Kolhatkar, Cuthbertson, 2010).
Assessment
Scientific research and site characterizations have demonstrated that microorganisms capable
of aerobically degrading many PHCs are present in nearly all subsurface soil environments
(ZoBell, 1946; Atlas, 1981; Wilson, et al., 1986; Leahy and Colwell, 1990; Bedient, Rifai, and
Newell, 1994; EPA, 1999). A number of well-characterized field studies demonstrate extensive
aerobic biodegradation of PHC vapors in unsaturated soils (Kampbell, et al., 1987; Ostendorf
and Kampbell, 1991; Ririe and Sweeney, 1995; Ririe, et al., 1998; Ostendorf, et al., 2000; Hers,
et al., 2000; Roggemans, Bruce, and Johnson, 2002; Sanders and Hers, 2006; Davis, Patterson,
and Trefry, 2009; Patterson and Davis, 2009; Lahvis, Baehr, and Baker, 1999; and Lavhis and
Baehr, 1996). Several of these studies document vapor concentrations at least two to three
orders of magnitude lower than would be predicted to occur merely by simple diffusion in the
absence of biodegradation.
EPA (2013a) presents findings of an analysis of a large number of vapor samples from leaking
UST sites. These results, which are consistent with several recent analyses of different PVI
databases (and which are summarized in the report), indicate that in most settings, PHC vapors
are biodegraded over relatively short distances in clean, biologically active soil.
Special Considerations
Preferential transport pathways are avenues of least resistance to the migration of
contaminants whether in the dissolved phase, LNAPL phase, or vapor phase. The presence of
preferential transport pathways can increase the speed at which contamination moves through
the subsurface such that contaminants may not biodegrade by the time they reach receptors.
Preferential transport pathways include both natural and man-made features (e.g., solution
channels, gravel layers, utility corridors and excavations). Natural geologic materials such as
coarse sand and gravel with a low content of silt and clay; fractured consolidated rock; or
consolidated rock with solution channels, may not have enough soil moisture in contact with
soil gas to support adequate densities of biologically active microorganisms. Particularly in
cases with shallow contamination, site investigations should evaluate whether a sufficiently
thick layer of clean, biologically active soil is present below buildings in the lateral inclusion
zone.
In addition, beneath very large buildings or under areas of extensive impermeable surface
cover, soil moisture content may be lower than optimal38 to support an adequate population of
biologically active microorganisms necessary to degrade PHC vapors and prevent PVI (see
Tillman and Weaver, 2007; EPA, 2013b).
38 Such reduced soil moisture beneath large buildings is referred to as a so/7 moisture shadow.
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Recommendation
Based on EPA (2013a), clean, biologically active soil does not contain LNAPL, EPA recommends
LNAPL thresholds of 100 mg/Kg TPH (fresh gasoline) and 250 mg/Kg TPH (weathered gasoline
and diesel). Except for the geological materials identified in Special Considerations, most soils
contain indigenous microorganisms, sufficient oxygen, and adequate soil moisture necessary
for degrading PHC vapors. Thus, it is typically not necessary to run microcosm studies or plate
counts to test for microbial presence. However, if the conditions at the site are uncertain for
supporting aerobic biodegradation, EPA recommends that appropriate samples be collected
and analyzed to verify conditions at the site.
Due in part to difficulties in measuring this level of accuracy in the field, EPA recommends
vertical separation distances of 6 feet for purely dissolved sources of PHCs and 15 feet for
LNAPL sources. These distances are believed to be conservative in most environmental
settings. The vertical separation distances described in this guidance (see Table 3 in Section 5,
p.52) should not be used at sites where the geologic materials may not have enough soil
moisture in direct contact with soil gas. EPA recommends collection and analysis of adequate
soil samples for soil moisture, which should be greater than 2 percent by dry weight. In
situations where densities of biologically active microorganisms may not be adequate to
biodegrade PHCs then soil gas samples should be collected following the recommendations in
Section 8. If the attenuation factor calculated from results of analysis of the chosen pair or
vapor samples indicates that there may be a potential for PVI above applicable exposure limits,
EPA recommends gathering additional information and data to determine whether mitigation
is appropriate.
References Cited
Atlas, R.M. 1981. Microbial Degradation of Petroleum Hydrocarbons: An Environmental
Perspective. Microbiological Reviews 45(1):180-209.
Bailey, N.J.L., A.M. Jobson, and M.A. Rogers. 1973. Bacterial Degradation of Crude Oil:
Comparison of Field and Experimental Data. Chemical Geology 11(3):203-221.
Bedient, P.B., H.S. Rifai, and C.J. Newell. 1994. Ground Water Contamination: Transport and
Remediation. PTR Prentice-Hall, Inc., Englewood Cliffs, New Jersey.
Bruce, L., A. Kolhatkar, and J. Cuthbertson. 2010. Comparison of BTEX Attenuation Rates Under
Anaerobic Conditions. International Journal of Soil, Sediment and Water 3(2): Article 11.
DeVaull, G. 2007. Indoor Vapor Intrusion with Oxygen-Limited Biodegradation for a Subsurface
Gasoline Source. Environmental Science and Technology 41(9):3241-3248.
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EPA. 1999. Monitored Natural Attenuation of Petroleum Hydrocarbons. Remedial Technology
Fact Sheet (EPA/600/F-98/021).
http://nepis. epa. gov/Exe/ZyPURL cgi?Dockey=30002379. txt
EPA. 2013a. Evaluation Of Empirical Data Jo Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
http://www.epa.gov/oust/cat/pvi/PVI Database Report.pdf
EPA. 2013b. 3-D Modeling of Aerobic Biodegradation of Petroleum Vapors: Effect of Building
Area Size on Oxygen Concentration Below the Slab (EPA 510-R-13-002).
http://www.epa.gov/oust/cat/pvi/buildina-size-modeHna.pdf
Hers, I., J. Atwater, L. Li, and R. Zapf-Gilje. 2000. Evaluation of Vadose Zone Biodegradation of
BTX Vapours. Journal of Contaminant Hydrology 46(3-4):233-264.
Holden, P.A., L.J. Halverson, and M.K. Firestone. 1997. Water Stress Effects on Toluene
Biodegradation by Pseudomonas putida. Biodegradation 8(3):143-151.
Kampbell, D.H., J.T. Wilson, H.W. Read, and T.T. Stocksdale. 1987. Removal of Volatile Aliphatic
Hydrocarbons in a Soil Bioreactor. Journal of the Air Pollution Control Association
37:1236-1240.
Lahvis, M.A., A.L. Baehr, and R.J. Baker. 1999. Quantification of Aerobic Biodegradation and
Volatilization Rates of Gasoline Hydrocarbons near the Water Table under Natural
Attenuation Conditions. Water Resources Research 35(3):753-765.
Lahvis, M.A., A.L. Baehr, and R.J. Baker. 1996. Estimation of Rates of Aerobic Hydrocarbon
Biodegradation by Simulation of Gas Transport in the Unsaturated Zone. Water
Resources Research 32(7):2231-2249.
Leahy, J. G., and R.R. Colwell. 1990. Microbial Degradation of Hydrocarbons in the Environment.
Microbiological Reviews 54(3):305-315.
Leeson, A., and R.E. Hinchee. 1996. Principles and Practices of Bioventing. Volume 1: Bioventing
Principles and Volume 2: Bioventing Design. Battelle Memorial Institute. September.
Moyer, E.E., D.W. Ostendorf, R.J. Richards, and S. Goodwin. 1996. Petroleum Hydrocarbon
Bioventing Kinetics Determined in Soil Core, Microcosm, and Tubing Cluster Studies.
Groundwater Monitoring and Remediation 16(1):141-153.
Ostendorf, D.W., E.S. Hinlein, A.J. Lutenegger, and S.P. Kelley. 2000. Soil Gas Transport Above a
Jet Fuel/Solvent Spill at Plattsburgh Air Force Base. Water Resources Research 36(9):
2531-2547.
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Ostendorf, D.W., and D.H. Kampbell. 1991. Biodegradation of Hydrocarbon Vapors in the
Unsaturated Zone. Water Resources Research 27(4):453-462.
Patterson, B.M., and G.B. Davis. 2009. Quantification of Vapor Intrusion Pathways into a Slab-
on-Ground Building under Varying Environmental Conditions. Environmental Science
and Technology 43(3):650-656.
Phelps, CD., and L.Y. Young. 1999. Anaerobic Biodegradation of BTEX and Gasoline in Various
Aquatic Sediments. Biodegradation 10(l):15-25.
Ririe, T., and R. Sweeney. 1995. Fate and Transport of Volatile Hydrocarbons in the Vadose
Zone. In: Proceedings of the 1995 Petroleum Hydrocarbon and Organic Chemicals in
Groundwater Conference, American Petroleum Institute and the National Ground Water
Association, Houston, Texas, pp. 529-542.
Ririe, T., R. Sweeney, S. Daughery, and P. Peuron. 1998. A Vapor Transport Model that is
Consistent with Field and Laboratory Data. In: Proceedings of the 1998 Petroleum
Hydrocarbon and Organic Chemicals in Groundwater Conference, American Petroleum
Institute and the National Ground Water Association, Houston, Texas.
Riser-Roberts, E. 1992. Bioremediation of Petroleum Contaminated Sites. Florida: CRC Press,
Inc.
Roggemans, S., C.L. Bruce, and P.C. Johnson. 2002. Vadose Zone Natural Attenuation of
Hydrocarbon Vapors: An Empirical Assessment of Soil Gas Vertical Profile Data. API
Technical Bulletin No. 15. American Petroleum Institute, Washington, D.C.
http://www.api.org/environment-health-and-safetv/clean-water/ground-water/vapor-
intrusion/vi-publications/attenuation-hydrocarbon-vapors
Sanders, P.F., and I. Hers. 2006. Vapor Intrusion into Homes over Gasoline-Contaminated
Ground Water in Stafford, New Jersey. Groundwater Monitoring and Remediation
26(l):63-72.
Siddique, T., P.M. Fedorak, M.D. Mackinnon, and J.M. Foght. 2007. Metabolism of BTEX and
Naphtha Compounds to Methane in Oil Sands Tailings. Environmental Science and
Technology 41(7):2350-2356.
Tillman, F. and J. Weaver. 2007. Temporal Moisture Content Variability Beneath and External to
a Building and the Potential Effects on Vapor Intrusion Risk Assessment. Science of the
Total Environment 379:1-15.
Wang, X. and M.A. Deshusses. 2007. Biotreatment of Groundwater Contaminated with MTBE:
Interaction of Common Environmental Co-Contaminants. Biodegradation 18(1):37-50.
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Widdel, F., A. Boetius, and R. Rabus. 2006. Anaerobic Biodegradation of Hydrocarbons Including
Methane. Prokaryotes 2:1028-1049.
Wilson, J.T., I.E. Leach, M. Henson, and J.N. Jones. 1986. In Situ Biorestoration as a Ground
Water Remediation Technique. Ground Water Monitoring Review 6(4):56-64.
ZoBell, C.E. 1946. Action of Microorganisms on Hydrocarbons. Bacteriological Reviews 10(1-2):
1-49.
Zwick, T.C., A. Leeson, R.E. Hinchee, L. Hoeppel, and L. Bowling. 1995. Soil Moisture Effects
During Bioventing in Fuel-Contaminated Arid Soils. Third International In-Situ and On-
Site Bioreclamation Symposium. In-Situ Aeration, v. 3, Battelle Press, San Diego,
California.
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10. Non-PHC Fuel Additives
Description
Petroleum fuels are comprised of hundreds of compounds; both natural components of
petroleum as well as a number of synthetic (non-PHC) additives intended to improve certain
performance properties of the fuel. Contaminants other than PHCs may be present at a site as
the result of releases of petroleum fuels that contain additives, including alcohols (e.g., ethanol
and tertiary-butyl alcohol [TBA]), ethers (e.g., MTBE), organic lead (e.g., the tetraalkyl lead
compounds: tetraethyl lead [TEL], and tetramethyl lead [TML]), and lead scavengers (e.g.,
ethylene dibromide [EDB] and 1,2-dichloroethane [1,2-DCA]).39 Non-petroleum contaminants
may also be from releases of substances (e.g., chlorinated solvents) other than petroleum fuels.
Their presence may be from prior uses of the site or as the result of migration from an off-site
source (e.g., dry cleaner, chemical plant, landfill).40
Importance
When assessing the potential threat of vapor intrusion, the presence of non-PHC fuel additives
may pose a variety of additional challenges. Depending on the class of additive, the challenges
include:
• Uncertainty regarding the aerobic biodegradation rates of some additives as well as
some that do not biodegrade aerobically (or do not biodegrade quickly enough) in the
shallow subsurface
• Biodegradation of an additive such as ethanol that consumes oxygen that would
otherwise be available for biodegradation of other PHCs and produces a VOC (methane),
which may migrate into buildings and hasten the spread of PHC vapors
• Toxicity levels of some additives are below the detection limit of conventional analytical
methods
39 Although leaded gasoline, which also contains the lead scavengers EDB and 1,2-DCA, is no longer used for
automotive fuel, it is still used for certain off-road applications such as automobile racing and in aviation fuel
(Avgas). At these and older automotive fuel sites where leaded gasoline was released to the subsurface, lead
scavengers may be present and could represent a potential source of vapors that should be assessed. For more
information about lead scavengers, see Appendix F in Evaluation Of Empirical Data To Support Soil Vapor Intrusion
Screening Criteria For Petroleum Hydrocarbon Compounds (EPA, 2013).
40 While these substances are not the primary focus of a petroleum LIST release investigation (including site
characterization and subsequent cleanup, if necessary), there is the possibility that their presence may be detected
through the use of certain analytical methods for identification of contaminants in groundwater, soil, and vapor
samples. In particular, both EPA methods 8260B (EPA, 1996a) and 8021B (EPA, 1996b) can detect a number of
volatile chlorinated solvents that are not associated with petroleum fuels or typically stored in USTs. The Leaking
Underground Storage Tank Trust Fund cannot be used to assess or cleanup contamination from non-UST and non-
petroleum sources. Volatile chlorinated solvents (e.g., PCE, TCE, TCA, Carbon tetrachloride, Chloroform) also do
not biodegrade under aerobic conditions, therefore their potential for vapor intrusion should instead be assessed
using the OSWER Final Guidance For Assessing and Mitigating the Vapor Intrusion Pathway From Subsurface
Sources to Indoor Air (OSWER Publication 9200.2-154)(EPA, 2015). Should any contaminants from non-UST
sources be discovered at a leaking UST site, contact the appropriate state or federal implementing agency.
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The separation distances described in this document (Section 5, Table 3, p.52) and the
protectiveness provided by clean, biologically active soil against vapor intrusion by PHCs may be
insufficient to protect against vapor intrusion by non-PHC fuel additives. Additional
investigation should be conducted where certain additives are present. The following narrative
provides additional information on several additives and introduces the Vapor Intrusion
Screening Levels (VISL) Calculator, which may be particularly useful when investigating vapor
intrusion from non-PHC fuel additives.
Although biodegradation is known to occur for many individual additives and classes of
additives,41 the rate of biodegradation in soil gas has not necessarily been rigorously quantified;
this is especially true for the lead scavengers EDB and 1,2-DCA. Therefore, for these two
chemicals in particular, vertical separation distances recommended in this guide may not be
sufficient for petroleum fuel releases that contain EDB and 1,2-DCA and additional investigation
may be necessary to assess their potential for vapor intrusion. Note: Though the use of ethers
and lead scavengers in gasoline has been reduced or eliminated in recent years, these
compounds may still be present at some older petroleum release sites (Weaver, et al., 2005,
2008, 2009; EPA, 2008).
Alternative fuels, especially those that contain higher percentages of ethanol present a
challenge because ethanol readily biodegrades to create methane (Jewell and Wilson, 2011;
Ma, et al., 2012 and 2014; Freitas, et al., 2010). The use of ethanol in motor fuels is increasing.
Methane generation may be more significant at sites where large volumes of ethanol-blended
gasoline (and higher ethanol content fuels, greater than E-20) have been released into the
subsurface (Ma et al., 2014). Methane production can increase soil gas pressures and may
result in advective soil gas flow toward receptors. In such situations, intrusion of methane into
confined spaces may result in the accumulation of very high concentrations creating a risk of
fire and explosion. In addition, methane also biodegrades under aerobic conditions and
depletes oxygen that otherwise could be available for the biodegradation of PHCs.
The depletion of oxygen may result in PHC vapors being transported farther than they
otherwise would be, possibly increasing the threat of PVI.
41 Aerobic biodegradation has been observed for the lead scavengers EDB (Prince and Douglas, 2010; Pignatello,
1986), and 1,2-DCA (Falta, 2004); the ethers MTBE (Prince and Douglas, 2010; Wang and Deshusses, 2007; Phelps
and Young, 1999; Landmeyer and Bradley, 2003; Landmeyer, et al., 2010; Bradley and Landmeyer, 2006; Kuder,
2005; Lesser, et al., 2008; Baehr, Charles, and Baker, 2001) and Tertiary-amyl methyl ether (TAME) (Landmeyer, et
al., 2010); the alcohols Ethanol (Powers, et al., 2001; Corseuil et. al, 1998), TBA (Wang and Deshusses, 2007;
Landmeyer, et al., 2010), and Methanol (Powers, et al., 2001); and some organic lead compounds (Prince and
Douglas, 2010; Gallert and Winter, 2004). Although anaerobic biodegradation is slower than aerobic
biodegradation, anaerobic biodegradation may be a significant mechanism for destruction of non-PHCs fuel
additives (and PHCs, especially in source areas.) Selected references on anaerobic biodegradation of various non-
PHC fuel additives are listed under Additional Information at the end of this section.
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Thus, separation distances that are usually protective against PVI may not be sufficiently
protective in situations where methane is produced in large quantity (Ma et al., 2014). For
additional information on sampling for the presence of methane, assessing potential risks, and
how to manage the risks, see ASTM's "New Practice for Evaluating Potential Hazard Due to
Methane in the VadoseZone".42
Finally, in addition to the uncertainties regarding the rates of biodegradation of the lead
scavengers EDB and l,2-DCA,existing analytical methods are not able to detect them at very
low concentrations representative of a cancer risk level of 1E-06. However, EPA (2013)
suggests that although there are no soil gas data for lead scavengers in the PVI database, "a
screening approach is feasible where groundwater concentrations are measured to determine
the potential for vapor intrusion risks from EDB and 1,2-DCA." (see Section F.6). This approach
is illustrated in the Assessment subsection through a sequence of equations and detailed
discussion of the results for EDB and 1,2-DCA.
Assessment
Federal UST regulations (40 CFR 280.52(b)) stipulate that when conducting an investigation of a
release from a regulated UST, investigators "must measure for the presence of a release where
contamination is most likely to be present. In selecting sample types, sample locations, and
measurement methods investigators must consider the nature of the stored substance, the
type of initial alarm or cause for suspicion, the type of backfill, the depth of groundwater, and
other factors appropriate for identifying the presence and source of release". Results of this
sampling should also identify which contaminants should be assessed for potential vapor
intrusion.
Once the candidate contaminants have been identified, the next step is to determine the target
indoor air screening level for each of them. While the federal UST program does not prescribe
human health values for contaminants, implementing authorities should use exposure values
appropriate for the contaminants present and the characteristics of exposure (e.g., residential
vs industrial). EPA provides a source of such exposure values in the Vapor Intrusion Screening
Levels (VISL) Calculator. 43 VISLs for human health protection are generally recommended,
medium-specific, risk-based screening-level concentrations intended for use in identifying areas
or buildings that may warrant further investigation and mitigation as appropriate.
These VISLs are calculated and documented in the VISL Calculator and are based on:
42 The new ASTM methane guide is accessible at
http://www. astm.ora/DA TABASE. CART/WORKITEMS/WK32621.htm
43The VISL Calculator provides recommended, but not mandatory, screening levels for use in evaluating the vapor
intrusion pathway at Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) and
Resource Conservation and Recovery Act (RCRA) sites. The user's guide for the VISL Calculator provides additional
information about derivation of the indoor air and subsurface screening levels (EPA, 2014a). Both the VISL
Calculator (EPA, 2014b) and user's guide may be downloaded from EPA's website:
http://www.epa.cjov/oswer/vaporintrusion/cjuidance.html.
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Current toxicity values selected considering OSWER's hierarchy of sources for toxicity values
(EPA, 2003)44
• Physical-chemical parameters for vapor forming chemicals
• EPA recommended approaches for human health risk assessment
The VISLs include target indoor air screening levels for long-term (i.e., chronic) exposures that
consider the potential for cancer and non-cancer effects of vapor-forming chemicals. The VISLs
also include subsurface screening levels for comparison to sampling results for sub-slab soil gas,
"near-source" soil gas, and groundwater. These subsurface screening levels are back-calculated
from the target indoor air screening levels for chronic exposures using medium-specific, generic
attenuation factors that reflect generally reasonable worst-case conditions (EPA, 2015,
Appendix B). VISLs are not automatically response action levels, although EPA recommends
that similar calculation algorithms be employed to derive cleanup levels (see EPA, 2015, Section
7.6 for more information).
The VISL Calculator allows users to specify an exposure scenario, target risk for carcinogens
(TCR) and target hazard for non-carcinogens (THQ), and the average groundwater temperature
at a site, and calculates screening levels for the target indoor air concentration, sub slab and
exterior soil gas concentrations, and ground water concentration.
In the VISL Calculator, target indoor air concentrations are calculated using the equations
presented in Table 4. For carcinogens, the inhalation unit risk (IUR) is the appropriate toxicity
value. For non-carcinogens, the reference concentration (RfC) is the appropriate toxicity value.
Each of these toxicity values is weighted by the appropriate exposure factors to determine the
target indoor air screening concentrations. The smaller value (between Cia,c and Cia,nc) is used as
the target indoor air screening value.
Example calculations using the equations in Table 4 are presented in Table 5 for EDB and 1,2-
DCA in indoor air under a residential exposure scenario. Note that the cancer screening levels
(Cia,c) are consistently lower than the non-cancer screening levels (Cia, nc), thus the cancer
screening levels would generally be used to assess risk to receptors for a given chemical.
OSWER's toxicity data hierarchy is three-tiered. Tier 1 are values from EPA's Integrated Risk Information System
(IRIS). The IRIS database is web accessible at http://www.epa.goy/iris/. Tier 2 are Provisional Peer Reviewed
Toxicity Values (PPRTVs), which are accessible at http://hhpprtv.ornl.qov/. Tier 3 are "Other"sources, such as the
Agency for Toxic Substances and Disease Registry (ATSDR), and various states (e.g., California (CA DTSC, 2009),
Hawai'i (HI DOH, 2011, 2012), Massachusetts (MA DEP, 2009), New Jersey (NJ DEP, 2013), Washington (WA DEC,
2006). Links to these sources are provided under References Cited at the end of this section.) EPA's Regional
Screening Levels (RSLs) for Superfund Sites compiles available toxicity information based on this hierarchy. The
VISL Calculator draws on these RSL tables for toxicity values that are used to calculate VISLs. RSLs are accessible at
http://www.epa.cjov/recj3hwmd/risk/human/rb-concentration table/index.htm.
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Table 4. Equations For Target Indoor Air Screening Concentrations For Volatile Chemicals
Cancer
TCR-ATc-365 (days/year}-24(hours/day)
EF-ED-ET-WR
Non-Cancer
THQ-RfC-ATnc-365-24-WOO(ug/mg)
EF-ED-ET
Cia,c is the indoor air concentration for cancer risk, and Cia,nc is the indoor air concentration for
non-cancer risk; the smaller value is used as the indoor air screening value. ATc and ATnc are
the averaging times for cancer and non-cancer, respectively, and EF, ED and ET are exposure
parameters (exposure frequency, duration, and time). The exposure factors should be
consistent with those in Human Health Evaluation Manual, Supplemental Guidance: Update
of Standard Default Exposure Factors OSWER Directive 9200.1-120
http://www.epa.gov/oswer/riskassessment/pdf/superfund-hh-exposure/OSWER-Directive-
9200-1 -120-ExposureFactors.pdf
Table 5. Example Target Residential Indoor Air Concentrations For EDB And 1,2-DCA
Chemical
Cia,c (ug/m3)
Qa,nc(ug/m3)
1,2-Dibromoethane (EDB)
4.7E-03
9.4
1,2-Dichloroethane (1,2-DCA)
1.1E-1
7.3
Values (and units) of other variables used in these example residential calculations
(equations in Table 4) are:
Variable
IUR(c)orRfC(nc)(EDB)
IUR(c) or RfC(nc) (1,2-DCA)
TCR(c) orTHQ(nc) (unitless)
ATc or ATnc (years)
ED (years)
EF (days/year)
ET (hours/day)
Cancer (c)
6.0E-04 (ug/m3)"1
2.6E-05 (ug/m3)"1
10"6
70
26
350
24
Non-Cancer (nc)
9 (mg/m3)
7 (mg/m3)
1.0
26
26
350
24
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After target indoor air screening levels (Cia) have been established, the next step is to determine
vapor source concentrations and assess whether these are high enough to potentially pose a
threat of vapor intrusion. The equations in Table 6 are used by the VISL Calculator to calculate
groundwater and soil gas screening levels based on target indoor air screening levels. These
groundwater and soil gas screening levels can then be compared to actual field measurements
of groundwater and/or soil gas concentrations. If the measured concentrations are greater
than the screening levels, then there is a potential for vapor intrusion, otherwise not.45
Table 6. Equations For Groundwater And Soil Gas Screening Levels
Based On Target Indoor Air Screening Levels.
Ground Water
Concentration
Cgw
agw • 1000 • HLC
Soil Gas
Concentration
Cia is the target indoor air screening level concentration (u.g/m3).
Cgw is the screening concentration in groundwater (u.g/L).
Csg is the screening concentration in soil gas (u.g/m3).
HLC is the unitless Henry's Law constant.
(Xgw and (Xsg are the groundwater and soil gas vapor intrusion attenuation
factors, respectively (both unitless).
1,000 is the number of liters per m3 (to convert from units of u.g/m3 to
ug/L).
An individual subsurface sampling result that exceeds the respective, chronic screening level does not establish
that vapor intrusion will pose an unacceptable human health risk to building occupants. Conversely, these generic,
single-chemical VISLs do not account for the cumulative effect of all vapor-forming chemicals that may be present.
Thus, if multiple chemicals that have a common, non-cancer toxic effect are present, a significant health threat
may exist at a specific building or site even if none of the individual substances exceeds its VISL (see discussion of
non-cancer hazard index in EPA (2015) Section 7.4.1).
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These equations also may be rearranged to calculate a potential upper-bound indoor air
screening concentration based on actual field measurements of groundwater and/or soil gas
concentrations. For groundwater, the equation is rearranged like this:
(upper bound}Cia = Cgw • agw • 1000 • HLC
Note that in order to calculate a screening level concentration in groundwater, a value for
Henry's Law constant (HLC) is necessary.46
For soil gas the equation is rearranged like this:
(upper bound}Cia = Csg • asg
Appropriate values for the field measured concentration in groundwater or soil gas, and for the
respective attenuation factors are plugged into the equation to yield an upper bound indoor air
screening level for Cia. The respective upper-bound indoor air screening level (derived from
either groundwater or soil gas sampling data) is then compared to the target indoor air
screening concentration (Cia) from the VISL Calculator. If the upper-bound value is greater than
the target value, then there is a potential for vapor intrusion, otherwise not (see Footnote #45).
In both cases, if there is a potential for vapor intrusion and where contamination is not in direct
contact with an overlying building, then paired vapor samples should be collected to assess
vapor attenuation. These paired samples should either be (a) near-slab (exterior) shallow soil
gas samples paired with deep (source) soil gas samples, or (b) indoor air samples paired with
sub-slab soil gas samples.
Table 7 presents example screening concentrations of EDB and 1,2-DCA in groundwater and soil
gas using the equations in Table 6. These values represent the upper-bound concentrations
according to Henry's Law that could be present in groundwater and soil gas, respectively, and
not result in indoor air concentrations in excess of the target screening levels (i.e., VISLs)
presented in Table 5. If concentrations measured in groundwater (Cgw) exceed these
thresholds, it is possible that the target indoor air concentration will also be exceeded and
mitigation may be necessary.
46 The VISL Calculator is one source of Henry's Law constants. Because these constants are temperature
dependent, the VISL Calculator automatically calculates the correct constant based on a temperature that is
selected by the user.
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Table 7. Example Screening Concentrations For EDB And 1,2-DCA In Groundwater And
Soil Gas
Chemical
-gw
-sg
1,2-Dibromoethane (EDB)
0.18
.16
1,2-Dichloroethane (1,2-DCA)
2.2
3.7
Target residential indoor air concentrations (Cia,c) are from Table 5.
Selected attenuation factors (a) are 0.001 for groundwater and 0.03 for soil gas. These are
taken from the VISL Calculator and do not account for biodegradation.
Dimensionless Henry's Law constants (HLC) for groundwater at 25°C are 0.0266 for EDB and
0.048 for 1,-DCA. These values are also taken from the current version of the VISL Calculator.
The lead scavengers present an additional challenge in that existing analytical methodology is
not able to detect them at very low concentrations representative of a cancer risk level of 1E-06
for either EDB or 1,2-DCA. As shown in Table 8, Selective Ion Monitoring (SIM) is able to
achieve a detection limit representative of the l.OE-04 risk level for EDB and l.OE-05 for 1,2-
DCA. Commercial low level is able to achieve a detection limit representative of the l.OE-05
risk level for 1,2-DCA. However, an analytical detection limit does not impact the risk level for a
certain chemical. The chemical may be present at a concentration greater than the appropriate
risk level concentration, but below the limit of detection, which may result in undetected risk to
potential receptors. Approaches to compensate for such analytical limitations include using
available modeling data and professional judgment to evaluate whether the chemical may be
present and having samples reanalyzed by special analytical services. For the screening level
assessment, the chemical should be carried through assuming that it is present at the
concentration equivalent to the quantitation limit. This allows the risk at the quantitation limit
to be compared to the risks associated with other chemicals at the site. At minimum, the
chemical should be addressed qualitatively. These topics are beyond the scope of this PVI
guide; additional information may found in EPA's Risk Assessment Guidance for Superfund
(RAGS).47
The Risk Assessment Guidance for Superfund (RAGS) document is accessible at
http://www.epa.ciov/oswer/riskassessment/racisa/index.htm. In particular see Section 5.3 in Part A, and Part F:
Supplemental Guidance for Inhalation Risk Assessment.
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Table 8. Comparison Of Risk Levels And Achievable Analytical Detection Limits For The
Lead Scavengers EDB And 1,2-DCA In Indoor Air.
Compound
EDB
1,2-DCA
Target Indoor Air Concentration
Risk Level
l.OE-04
0.47
11
Risk Level
l.OE-05
0.047
1.1
Risk Level
l.OE-06
0.0047
0.11
Analytical Method
Commercial
conventional
3.8
2.0
Commercial
low level
0.77
0.40
Commercial
SIM
0.23
0.12
NOTE: all values in u.g/m3
l.OE-04 = increased lifetime cancer risk of 1 per 10,000
l.OE-05 = increased lifetime cancer risk of 1 per 100,000
l.OE-06 = increased lifetime cancer risk of 1 per 1,000,000
Commercial conventional = EPA Method TO-15 (see
http://www.epa.gov/ttn/amtic/files/ambient/airtox/to-15r.pdf)
Commercial low level = EPA Method TO-15 (modified)
SIM = Selective Ion Monitoring
The achievable detection limits in this table are representative of the general state of the technology
as of the present date. Some laboratories may be able to achieve lower detection limits using
modified techniques. Future technological improvements may also result in lower detection limits.
I I Achievable
I | Not Achievable
Special Considerations
VISL Calculator screening levels do not include the effects of biodegradation on the
concentrations of vapors in soil that could potentially intrude into indoor air. The generic
attenuation factors used in calculating VISLs (i.e., 0.001 for groundwater, 0.03 for soil gas) are
conservative, and may overestimate the transfer of some contaminants (e.g., those that
biodegrade aerobically) from soil gas to indoor air in some buildings. As a result these
screening levels will usually overestimate the true indoor air concentrations of aerobically-
biodegradable volatile contaminants (e.g., PHCs). Decision-makers may choose to use alternate
approaches (e.g., attenuation factors that account for biodegradation) that may be more
appropriate for specific sites where circumstances do not match the underlying assumptions
used in calculating the VISLs.
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When Information is available on the separation distance between the source of contamination
and the receptor, on the total concentration of biodegradable compounds in soil gas, and on
the rate constant for degradation of contaminant vapors in soil, it is possible to refine the
estimate of the attenuation factor (a) between soil gas and indoor air for some VOCs and PHCs.
Approaches to refine the estimate of the attenuation factor (a) for PHCs are discussed in
Section 12 (p.100) and by Wilson et al. (2014). Also see Section 13 (p.106) for information on
the use of models to estimate attenuation factors. However, until more is known about the
rates of biodegradation of EDB and 1,2-DCA in soil gas, the separation distances for PHCs shown
in Section 5 and the approaches described in Section 12 or Section 13 for determining
attenuation factors are not recommended for these two contaminants.
In addition to concerns discussed earlier, other potential concerns with increasing ethanol
content are in relation to (re)mobilization of LNAPL (McDowell, et al., 2003; Yu, et al., 2009)
and increased solubility of PHCs (Powers, et al., 2001). The implications of these impacts may
extend beyond vapor intrusion; see Section 7 (p.61) for information about LNAPL and Section 6
(p.57) for information about dissolved contaminant plumes.
Recommendation
EPA recommends that groundwater samples be analyzed for PHCs and non-PHC fuel additives
(e.g., alcohols, ethers, organic lead, lead scavengers) typically found in petroleum-based fuels,
when appropriate. At the present level of knowledge, the groundwater and soil gas screening
levels in Table 7 are the best values to use to determine whether indoor air target levels will be
exceeded for EDB and 1,2-DCA. If measured concentrations of EDB and 1,2-DCA in
groundwater exceed the screening levels in Table 7, EPA recommends gathering additional
information and data to determine whether mitigation is appropriate. The methodology
illustrated in the Assessment subsection above can be applied to any non-PHC (or any VOC for
that matter) for which an appropriate toxicity value and attenuation factor are available.
References Cited
Agency for Toxic Substances and Disease Registry (ATSDR). 1999. Toxicological Profile for Total
Petroleum Hydrocarbons. U.S. Department of Health and Human Services, Atlanta, GA.
http://www. atsdr. cdc. gov/toxprofiles/tpl 23. pdf
ASTM International (ASTM). 2014. New Practice for Evaluating the Potential Hazard Due to
Methane in the Vadose Zone. ASTM WK3221. West Conshohocken, Pennsylvania.
http://www. astm. org/DA TABASE. CART/WORKITEMS/WK32621. htm
Baehr, A.L., E.G. Charles, and R.J. Baker. 2001. Methyl Tert-Butyl Ether Degradation in the
Unsaturated Zone and the Relation between MTBE in the Atmosphere and Shallow
Groundwater. Water Resources Research 37(2):223-233.
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Bradley, P.M., and J.E. Landmeyer. 2006. Low-Temperature MTBE Biodegradation in Aquifer
Sediments with a History of Low, Seasonal Ground Water Temperatures. Ground Water
Monitoring and Remediation 26(1):101-105.
California Department of Toxic Substances Control (CA DTSC). 2009. Evaluating Human Health
Risks from Total Petroleum Hydrocarbons. Human and Ecological Risk Division,
Sacramento, CA.
http://www.oehha.ca.Qov/risk/chemicalDB/index.asp
Corseuil, H.X., C. Hunt, R. dos Santos Ferreira, and P.J.J. Alvarez. 1998. The Influence of the
Gasoline Oxygenate Ethanol on Aerobic and Anaerobic BTX Biodegradation. Water
Research 32(7):2065-2072.
EPA. 1996a. Method 8021B: Aromatic and Halogenated Volatiles by Gas Chromatography using
Photoionization and/or Electrolytic Conductivity Detectors. Test Methods for Evaluating
Solid Waste, Physical/Chemical Methods. EPA publication SW-846.
http://www.epa.aov/wastes/hazard/testmethods/sw846/pdfs/8021b.pdf
EPA. 1996b. Method 8260B: Volatile Organic Compounds by Gas Chromatography/Mass
Spectrometry (GC/MS). Test Methods for Evaluating Solid Waste, Physical/Chemical
Methods. EPA publication SW-846.
http://www.epa.gov/wastes/hazard/testmethods/sw846/pdfs/8260b.pdf
EPA. 2003. Human Health Toxicity Values in Superfund Risk Assessments. OSWER Directive
9285.7-53. Office of Solid Waste and Emergency Response.
http://www. epa. gov/oswer/riskassessment/pdf/hhmemo.pdf
EPA. 2006. Lead Scavengers Compendium: Overview Of Properties, Occurrence, And Remedial
Technologies, http://www.epa.gov/oust/cat/pbcompnd.htm
EPA. 2008. Natural Attenuation of the Lead Scavengers 1,2-Dibromoethane (EDB) and 1,2-
Dichloroethane (1,2-DCA) at Motor Fuel Release Sites and Implications for Risk
Management (EPA 600/R-08/107).
http://nepis.epa.cjov/Exe/ZyPDF.ccii?Dockev=P1002UTI.PDF
EPA. 2009. Provisional Peer-Reviewed Toxicity Values for Complex Mixtures of Aliphatic and
Aromatic Hydrocarbons (CASRN Various) FINAL 9-30-2009. Superfund Health Risk
Technical Support Center, National Center for Environmental Assessment, Office of
Research and Development.
http://hhpprtv.ornl.gov/issue papers/ComplexMixturesofAliphaticandAromaticHydrocar
bons.pdf
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EPA. 2011. Exposure Factors Handbook: 2011 Edition. Office of Research and Development.
National Center for Environmental Assessment.
http://www. epa. gov/ncea/efh/pdfs/efh-complete.pdf)
EPA. 2013. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
http://www.epa.gov/oust/cat/pvi/PVI Database Report.pdf
EPA. 2014a. Vapor Intrusion Screening Level (VISL) Calculator: User's Guide. Office of Solid
Waste and Emergency Response.
http://www.epa.gov/oswer/vaporintrusion/documents/VISL-UsersGuide.pdf
EPA. 2014b. Vapor Intrusion Screening Level (VISL) Calculator. Office of Solid Waste and
Emergency Response.
http://www.epa.gov/oswer/vaporintrusion/documents/VISL-Calculator.xlsm
EPA. 2015. OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway
From Subsurface Sources To Indoor Air (OSWER Publication 9200.2-154). Office of Solid
Waste and Emergency Response.
http://www. epa. gov/oswer/vaporintrusion/
Falta, R.W. 2004. The Potential for Ground Water Contamination by the Gasoline Lead
Scavengers Ethylene Dibromide and 1,2-Dichloroethane. Ground Water Monitoring and
Remediation 24(3):76-87.
Freitas, J.G., B. Fletcher, R. Aravena, and J.F. Barker. 2010. Methane Production and Isotopic
Fingerprinting in Ethanol Fuel Contaminated Sites. Ground Water 48(6):844-857.
Gallert, C. and J. Winter. 2004. Degradation of Alkyllead Compounds to Inorganic Lead in
Contaminated Soil. Water Research 38(19):4204-4212.
Hawai'i Department of Health (HI DOH). 2008. Screening for Environmental Hazards at Sites
with Contaminated Soil and Groundwater. Office of Hazard Evaluation and Emergency
Response: Honolulu, HI.
http://eha-web. doh. hawaii. gov/eha-cma/Leaders/HEER/environmental-hazard-
evaluation-and-environmental-action-levels
Hawai'i Department of Health (HI DOH). 2012. Field Investigation of the Chemistry and Toxicity
ofTPH in Petroleum Vapors: Implications for Potential Vapor Intrusion Hazards. Office of
Hazard Evaluation and Emergency Response: Honolulu, HI.
http://www.hawaiidoh.org/tgm.aspx
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Jewell, K.P. and J.T. Wilson. 2011. A New Screening Method for Methane in Soil Gas Using
Existing Groundwater Monitoring Wells. Ground Water Monitoring and Remediation
31(3):82-94.
Kuder, T., J.T. Wilson, P. Kaiser, R. Kolhatkar, P. Philp, and J. Allen. 2005. Enrichment of Stable
Carbon and Hydrogen Isotopes during Anaerobic Biodegradation of MTBE: Microcosm
and Field Evidence. Environmental Science and Technology 39(1):213-220.
Landmeyer, J.E., and P.M. Bradley. 2003. Effect of Hydrologic and Geochemical Conditions on
Oxygen-Enhanced Bioremediation in a Gasoline-Contaminated Aquifer. Bioremediation
7ourno/7(3-4):165-177.
Landmeyer, J.E., P.M. Bradley, D.A. Trego, K.G. Hale, and J.E. Haas, II. 2010. MTBE, TBA, and
TAME Attenuation in Diverse Hyporheic Zones. Ground I/Voter 48(1) :30-41.
Lesser, L. E., P.C. Johnson, R. Aravena, G.E. Spinnler, C.L. Bruce, and J.P. Salanitro. 2008. An
Evaluation of Compound-Specific Isotope Analyses for Assessing the Biodegradation of
MTBE at Port Hueneme, California. Environmental Science and Technology
42(17):6637-6643.
Ma, J. W.G. Rixey, G.E. DeVaull, B.P. Stafford, and P.J. J. Alvarez. 2012. Methane Bioattenuation
and Implications for Explosion Risk Reduction along the Groundwater to Soil Surface
Pathway above a Plume of Dissolved Ethanol. Environmental Science & Technology
46(11):6013-6019.
Ma, J., H. Luo, G.E. DeVaull, W.G. Rixey, and P.J. J. Alvarez. 2014. Numerical Model Investigation
for Potential Methane Explosion and Benzene Vapor Intrusion Associated with High-
Ethanol Blend Releases. Environmental Science & 7ec/7no/ogy48(l):474-481.
Massachusetts Department of Environmental Protection (MA DEP). 2003. Updated Petroleum
Hydrocarbon Fraction Toxicity Values for the VPH/EPH/EPH Methodology. Bureau of
Waste Site Cleanup, Boston, MA.
http://www.mass.aov/eea/docs/dep/cleanup/laws/tphtox03.pdf
McDowell, C.J., T. Buscheck, and S.E. Powers. 2003. Behaviour of Gasoline Pools Following a
Denatured Ethanol Spill. Ground Water 41(6):746-757.
New Jersey Department of Environmental Protection (NJ DEP). 2013. Vapor Intrusion Technical
Guidance. Version 3.1. Site Remediation Division. March.
http://www.ni.aov/dep/srp/auidance/vaporintrusion/vig main.pdf
Phelps, C.D., and L.Y. Young. 1999. Anaerobic Biodegradation of BTEX and Gasoline in Various
Aquatic Sediments. Biodegradation 10(l):15-25.
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Pignatello, J.J. 1986. Ethylene Dibromide Mineralization in Soils under Aerobic Conditions.
Applied and Environmental Microbiology 51(3):588-592.
Powers, S.E., C.S. Hunt, S.E. Heermann, H.X. Corseuil, D. Rice, and P.J.J. Alvarez. 2001. The
Transport and Fate of Ethanol and BTEX in Groundwater Contaminated by Gasohol.
Critical Reviews in Environmental Science and Technology 31(1):79-123.
Prince, R.C. and G.S. Douglas. 2010. Remediation of Petrol and Diesel in Subsurface from Petrol
Station Leaks, in Timmis, K.N. (ed) Handbook of Hydrocarbon and Lipid Microbiology:
Part 14, Springer-Verlag, Berlin.
Wang, X. and M.A. Deshusses. 2007. Biotreatment of Groundwater Contaminated with MTBE:
Interaction of Common Environmental Co-Contaminants. Biodegradation 18(1):37-50.
Washington Department of Ecology (WA DEC). 2006. Cleanup Levels and Risk Calculations Focus
Sheets: Reference Doses for Petroleum Mixtures. Lacey, WA.
https://fortress.wa.Qov/ecv/clarc/FocusSheets/petroToxParameters.pdf
Weaver, J.W., L.Jordan and D.B. Hall. 2005. Predicted Ground Water, Soil and Soil Gas Impacts
from US Gasolines, 2004: First Analysis of the Autumnal Data (EPA/600/R-05/032).
United States Environmental Protection Agency, Washington, D.C.
http://www.epa.aov/athens/publications/reports/Weaver600R05032PredictedGroundW
ater.pdf
Weaver, J.W., L.R. Exum, L.M. Prieto. 2008. Gasoline Composition Regulations Affecting LUST
Sites (EPA/600/R-10/001). United States Environmental Protection Agency, Washington,
D.C.
http://www.epa.gov/athens/publications/reports/Weaver EPA600R10001 Gasoline Co
mposition Regulations Affecting LUST' Sites^final.pdf
Weaver, J.W., S. A. Skaggs, D.L. Spidle, and G.C. Stone. 2009. Composition and Behavior of Fuel
ft/7ono/(EPA/600/R-09/037). United States Environmental Protection Agency,
Washington, D.C.
http://www.epa.gov/athens/publications/reports/Weaver EPA600R09037 Composition
Fuel Ethanol.pdf
Wilson, J.T., K. Jewell, C. Adair, C. Paul, C. Ruybal, G. DeVaull, and J. Weaver. 2014. An Approach
that Uses the Concentrations of Hydrocarbon Compounds in Soil Gas at the Source of
Contamination to Evaluate the Potential for Intrusion of Petroleum Vapors into Buildings
(PVI). (EPA/600/R-14/318). ORD Issue Paper, U.S. Environmental Protection Agency,
Washington, DC.
http://cfpub.epa.gov/si/si public record Report.cfm?dirEntryld=305910
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Yu, S., J.G. Freitas, A.J.A. linger, J.F. Barker, and J. Chatzis. 2009. Simulating the Evolution of an
Ethanol and Gasoline Source Zone within the Capillary Fringe. Journal of Contaminant
Hydrology 10S(1-2):1-17.
Additional Information
Donaldson, C.B., J.F. Barker, and I. Chatzis. 1994. Subsurface Fate and Transport of a
Methanol/Gasoline Blend (M85). Report prepared for the American Petroleum Institute,
Washington D.C., Publication number 4569.
Eichler, B. and B. Schink. 1984. Oxidation of Primary Aliphatic Alcohols by Acetobacterium
carbinolicum sp. nov., a Homoacetogenic Anaerobe. Archives of Microbiology 140(1-
2):147-152.
Henderson, J. K., D.L. Freedman, R.W. Falta, T. Kuder, and J.T. Wilson. 2008. Anaerobic
Biodegradation of Ethylene Dibromide and 1,2-Dichloroethane in the Presence of Fuel
Hydrocarbons. Environmental Science and Technology 42(3):864-870.
Mormile, M.R., S. Liu, and J. Suflita. 1994. Anaerobic Biodegradation of Gasoline Oxygenates:
Extrapolation of Information to Multiple Sites and Redox Conditions. Environmental
Science and Technology 28(9):1727-1732.
Peargin, T. and R. Kolhatkar. 2011. Empirical Data Supporting Groundwater Benzene
Concentration Exclusion Criteria for Petroleum Vapor Intrusion Investigations.
Proceedings of Battelle 8th International Symposium on Bioremediation and Sustainable
Environmental Technologies, Reno, Nevada, June 27-30.
Suflita, J.M. and M.R. Mormile. 1993. Anaerobic Biodegradation of Known and Potential
Gasoline Oxygenates in the Terrestrial Subsurface. Environmental Science and
Technology 27(6):976-978.
Vogel, T. M. and M. Reinhard. 1986. Reaction-Products and Rates of Disappearance of Simple
Bromoalkanes, 1,2-Dibromopropane, and 1,2-Dibromoethane in Water. Environmental
Science and Technology 20(10):992-997.
Widdel, F. 1986. Growth of Methanogenic Bacteria in Pure Culture with 2-Propanol and other
Alcohols as Hydrogen Donors. Applied Environmental Microbiology 51(5):1056-1062.
Page 95 of 123
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11. Seasonal And Weather Effects
Description
The generation and movement of petroleum vapors are subject to seasonal effects such as
temperature trends and fluctuations; and weather effects such as precipitation, barometric
pressure changes, and wind (Lundegard, Johnson, and Dahlen, 2008). Cycling of heating and
cooling systems inside buildings in response to seasonal and weather effects may also influence
vapor intrusion.
Importance
Biological processes slow down with decreasing temperatures, though microorganisms
continue to biodegrade PHCs at environmentally significant rates even when temperatures are
near freezing (Bradley and Chapelle, 1995; Bradley, Richmond, and Chapelle, 2005; Hers, et al.,
2011). Bradley and Landmeyer (2006) documented microbial degradation of MTBE in the
wintertime when groundwater temperatures were below 5°C.
There is conflicting evidence as to whether, and under what additional conditions, frozen or ice-
covered soil reduces the movement of oxygen into the subsurface. Hers, et al. (2011) studied a
residential site in Canada where subsurface oxygen readings taken throughout the winter did
not indicate a decrease in oxygen content of soil gas and there was evidence that
biodegradation was occurring. However, the residence was above a crawl space and the soil
below the house was never covered by ice or snow. It is known that the air permeability of a
snow layer is a complex function of pore size, grain size, ice fraction, and density (Armstrong,
2008; Bender, 1957; Conway and Abrahamson, 1984). Rike (2003) observed ongoing
biodegradation in frozen arctic soils. In that study, a lengthy period of subfreezing soil
temperatures at a petroleum contaminated site did not result in decreasing oxygen
concentrations. In contrast, Freyman (1967) and Yanaia (2010) report that oxygen depletion
has been observed in other studies of soils under ice sheets and snow cover. More study is
needed to resolve this issue.
Precipitation events can impact biodegradation of petroleum vapors. A certain amount of soil
moisture is necessary for microorganisms to live; not enough and they are not actively
degrading PHC vapors; too much and reoxygenation is impeded, possibly leading to anaerobic
conditions at greater depths (Silver, 1999; Ludemann, 2000; Pezeschki, 2001). Wind and
barometric pressure changes can produce pressure gradients inside buildings. Negative
pressure inside buildings can result in enhanced intrusion of PHC vapors. Positive pressure
inside buildings can both prevent intrusion of PHC vapors into buildings and facilitate oxygen
transport through cracks in the foundation into the subsurface. This can result in
reoxygenation of the soil beneath the building that would otherwise be depleted of oxygen
(Lundegard, Johnson, and Dahlen, 2008).
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Heating systems in buildings, which operate most frequently during winter months, can create
a chimney effect, whereby PHC vapors are pulled into buildings at much higher rates than they
would ordinarily. Cooling systems, which operate only during summer months, can have the
opposite effect, creating positive pressure gradients in basements that both prevent intrusion
of PHC vapors into buildings and allow oxygen to enter the soil (Lundegard, Johnson, and
Dahlen, 2008).
Assessment
Seasonal and weather conditions can influence the characteristics of PHC vapor migration over
time. Data on temporal changes in temperature, barometric pressure, wind speed and
direction, relative humidity, and precipitation can aid in correctly identifying trends and result
in a more accurate CSM.
In addition, site characteristics that may indicate susceptibility to the effects of seasonal and
weather factors should be assessed. These include:
• Poor drainage around the building indicated by flooded soils
• Area subject to permafrost/long lasting snow cover (based on altitude or latitude)
• Shallow and highly variable water table
Special Considerations
Seasonal effects may also influence the formation and migration of dissolved plumes and
LNAPL. Changes in water table elevation can create a smear zone of residual LNAPL
contamination. LNAPL in the smear zone can act as a long-term source of dissolved
contamination during periods of high water table elevation and as a source of petroleum
vapors during periods of low water table elevation when contaminants reemerge from a
previously submerged condition. Groundwater levels in the vicinity of USTs may also be subject
to the influence of water within the tank pit. After rainfall events (and potentially snowmelt)
water levels within tank pits are typically above the level of ambient groundwater;
consequently a groundwater recharge mound may form. This mound disrupts the local
groundwater flow field and contaminants can migrate away from the tank excavation,
potentially in all directions.
Recommendation
During site characterization activities, weather conditions such as temperature, barometric
pressure, and wind speed/direction should be recorded to aid in recognizing the cause of trends
or anomalies in the PVI data and not merely attributed to unknown factors. This information
may be obtained from the National Oceanic and Atmospheric Administration (NOAA) (see
http://www.noaa.Qov/wx.html) or a nearby airport where weather data are recorded hourly.
Page 97 of 123
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References Cited
Armstrong, R.L, and E. Brun. 2008. Snow and Climate: Physical Processes, Surface Energy
Exchange and Modeling. Cambridge University Press.
Bender, J.A. 1957. Air Permeability of Snow. Res. Rep. 37 Snow, Ice and Permafrost Research
Establishment (US Army Corps of Engineers) Wilmette, Illinois p 46-62.
Bradley, P. M., and F. H. Chapelle. 1995. Rapid Toluene Mineralization by Aquifer Microorganisms
at Adak, Alaska: Implications for Intrinsic Bioremediation in Cold Environments.
Environmental Science and Technology 29(11) :2778-2781.
Bradley, P.M., and J.E. Landmeyer. 2006. Low-Temperature MTBE Biodegradation in Aquifer
Sediments with a History of Low, Seasonal Ground Water Temperatures. Ground Water
Monitoring and Remediation 26(1):101-105.
Bradley, P. M., S. Richmond and F. H. Chapelle. 2005. Chloroethene Biodegradation in
Sediments at 4°C. Applied Environmental Microbiology 71(10):6414-6417.
Conway, H. and J. Abrahamson. 1985. Air Permeability as a Textural Indicator of Snow. Journal of
Glaciology 30(106):328-333.
Freyman, S. 1967. The Nature of Ice Sheet Injury to Forage Plants. Ph.D. Thesis, University of
British Columbia.
https://circle. ubc. ca/handle/2429/37752
Hers, I., Lahvis, M., Dahlen, P., Luo, E.H., DeVaull, G., and P. Johnson. 2011. Cold Climate Vapor
Intrusion Research Study - Results of Seasonal Monitoring of House at North Battleford,
Saskatchewan, in Proceedings 21st Annual International Conference on Soil, Water,
Energy and Air and AEHS [Association for Environmental Health and Sciences]
Foundation Annual Meeting, March 14-17, 2011, San Diego, California.
Liidemann, H. I. Arth, and W. Liesack. 2000. Spatial Changes in the Bacterial Community
Structure along a Vertical Oxygen Gradient in Flooded Paddy Soil Cores. Applied
Environmental Microbiology 66(2):754-762.
Lundegard, P.D., P.C. Johnson, and P. Dahlen. 2008. Oxygen Transport from the Atmosphere to
Soil Gas Beneath a Slab-on-Grade Foundation Overlying Petroleum-Impacted Soil.
Environmental Science and Technology 42(15):5534-5540.
Luo, H., P. Dahlen, P.C. Johnson, T. Peargin, and T. Creamer. 2009. Spatial Variability of Soil-Gas
Concentrations Near and beneath a Building Overlying Shallow Petroleum Hydrocarbon-
Impacted Soils. Ground Water Monitoring and Remediation 29(1):81-91.
Page 98 of 123
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Pezeshki, S.R. 2001. Wetland Plant Responses to Soil Flooding. Environmental and Experimental
Botany 46(3):299-312.
Rike, A.G., K.B. Haugen, M. B0rresen, B. Engenec, P. Kolstad. 2003. In Situ Biodegradation of
Petroleum Hydrocarbons in Frozen Arctic Soils. Cold Regions Science and Technology
37(2):97-120.
Silver, W.L, A.E. Lugo, and M.Keller. 1999. Soil Oxygen Availability and Biogeochemistry Along
Rainfall and Topographic Gradients in Upland Wet Tropical Forest Soils.
Biogeochemistry 44(3):301-328.
Yanaia,Y., T. Hirotab, Y. Iwataa, M. Nemotob, 0. Nagatac, N. Kogaa. 2011. Accumulation of
Nitrous Oxide and Depletion of Oxygen in Seasonally Frozen Soils in Northern Japan -
Snow Cover Manipulation Experiments. Soil Biology and Biochemistry 43(9):1779-1786.
Additional Information
Hintenlang, E.E., and K.K AI-Ahmady. 1992. Pressure Differentials for Radon Entry Coupled to
Periodic Atmospheric Pressure Differentials. Indoor Air 2:208-215.
Luo, H., and P.C. Johnson. 2011. Incorporating Barometric Pressure and Wind Effects into Vapor
Intrusion Simulations. Association for Environmental Health and Sciences Conference,
Petroleum Hydrocarbon Vapor Intrusion Session, March 16, San Diego, California.
Tillman, F. and J. Weaver. 2007. Temporal Moisture Content Variability Beneath and External to
a Building and the Potential Effects on Vapor Intrusion Risk Assessment. Science of the
Total Environment 379:1-15.
U.S. Geological Survey. 1998. Assessment of the Potential for Biodegradation of Petroleum
Hydrocarbons in the Railroad Industrial Area, Fairbanks, Alaska, 1993-1994. Open-File
Report 98-287. Fairbanks, Alaska. lOp.
http://pubs.usQS.qov/of/1998/0287/report.pdf
Web Resources
U.S. Geological Survey's Soil Surveys
http://websoilsurvev.nrcs.usda.Qov/app/HomePaae.htm
U.S. Department of Homeland Security Federal Emergency Management Agency's flood plain
maps https://msc.fema.gov/portal
2012 U.S. Department of Agriculture Plant Hardiness Zone Map
http://planthardiness. ars. usda. gov/PHZMWeb/
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12. Vapor Intrusion Attenuation Factor (a)
Description
When Johnson and Ettinger (1991) published their vapor intrusion model they introduced a
parameter to relate the vapor concentration of a volatile chemical inside a building to its vapor
concentration at the subsurface source. The parameter, designated alpha (a), is also called the
vapor intrusion attenuation factor. It is defined mathematically as the concentration in indoor
air divided by the concentration in soil gas at the source (with concentrations in the same
units). The source is defined as the region of highest vapor concentration in the vadose zone.
Large values of a (i.e., values approaching one) indicate that little attenuation is taking place,
whereas small values of a (i.e., values much smaller than one) indicate that significant
attenuation is taking place.
Importance
As part of a risk evaluation, the concentrations of a chemical in indoor air can actually be
measured, or they can be predicted. The attenuation factor is used as a part of a risk
evaluation to predict or estimate the concentration of a chemical in indoor air from the
concentration measured in soil gas below or near a building. To predict the indoor air
concentration, the measured concentration in soil gas is multiplied by the suitable attenuation
factor.
Assessment
U.S. EPA (2013, Table 6-1) provides recommended vapor attenuation factors for risk-based
screening of the vapor intrusion pathway for residential buildings. For example, the generic
values of a in EPA (2015) are l.OE-03 (0.001) for groundwater, 3.0E-02 (0.03) for sub-slab soil
gas, and 3.0E-02 (0.03) for deep (near-source) soil gas. These values of a are derived from
measurements made during case studies of the vapor intrusion of chlorinated solvents such as
trichloroethylene (TCE), which are not biologically degraded in aerobic unsaturated soil or
sediment. Likewise, values for concentrations in indoor air that are derived from the model of
Johnson and Ettinger (1991) also do not include any consideration of biodegradation. As a
result, the generic values of a in U.S. EPA (2015) and values for indoor air that are calculated
using the Johnson and Ettinger model (JEM) overestimate the indoor air concentrations of
PHCs. Thus, these values of a are not applicable to PVI from leaking USTs.48 For additional
information on estimation of sub-slab attenuation factors, see Brewer et al. (2014).
To provide estimates of a that are more suitable for PHCs, Abreu, Ettinger, and McAlary (2009)
developed a three-dimensional computer model to predict the effects of biodegradation in the
unsaturated zone below a building on the concentrations of chemicals in the indoor air of the
building. They performed a series of model simulations to estimate semi-generic values of a
from site-specific information on the vertical separation distance between the receptor building
48 Attenuation factors that account for biodegradation can be derived from models such as BioVapor or PVIScreen
(see Section 13, p.106 for more information). Additional guidance may be found in Wilson et al. (2014).
Page 100 of 123
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and the source, and the total concentration of biodegradable compounds in soil gas at the
source of the hydrocarbons. Figure 9 compiles the computer simulations conducted by Abreu,
Ettinger, and McAlary (2009) of the attenuation factor during vapor intrusion into a building
with a basement. This figure presents the concentration of biologically degradable
hydrocarbon in an unconventional unit (mg/L as benzene).
l.E-02
l.E-03 i
No Biodegradation; L = 1 m
No Biodegr adation; L = 10 m
l.E-10 -
0.1
1 f 10 1 100
Source Vapor Concentration {mg/L)
A= 0.79 h'1
1000
Figure 9. Relationship Between Source Vapor Concentration And Vapor Intrusion Attenuation
Factor (a) As A Function Of Vertical Separation Depth Between Contaminant Source And Base
Of Building (Receptor) (Source: modified from Abreu, Ettinger, and McAlary, 2009, Figure 7,
page 114. Reprinted from Groundwater Monitoring & Remediation with permission of the
National Ground Water Association. Copyright 2009.)
To generate Figure 9, Abreu, Ettinger, and McAlary (2009) used conservative assumptions for
the rate of biodegradation. In this particular set of simulations, the first order rate constant for
biodegradation (A.) was set at 0.79 h"1, a reasonable average rate based on the range of rates
published in the literature (DeVaull 2007). Model simulations assume the building has a
basement and that it is surrounded by homogeneous, uniform sandy soil that is directly
exposed to the atmosphere and that preferential pathways for vapor migration into the
building or through the vadose zone are not present. As a result, the concentration of oxygen
Page 101 of 123
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in the soil gas in the topmost layer of exposed soil is the concentration of oxygen in the
atmosphere. Compared to silty or clayey soils, sandy soils have more air filled porosity and as a
result, vapors diffuse more rapidly through them (and they also allow more oxygen to diffuse
from the atmosphere). The simulations assumed that the square building was 10 meters (33
feet) on each side.
Figure 9 can be used to estimate the value of a for situations where the total concentration of
vapors at the source and the vertical separation distance between the contaminant source and
the bottom of the building are known and all of the other input parameters match site
conditions. For example, for a source vapor concentration of 10 mg/L and a vertical separation
distance (L) of 2 meters (6.6 feet), the estimated value of a would be approximately l.OE-07.
To complete the exposure assessment, the measured concentration of benzene in soil gas at
the source of contamination is multiplied by the value of a, to predict the indoor air
concentration in a building.
Figure 10 is a redraft of Figure 9, where the source concentration of vapors is expressed in
more conventional units for vapors in soil gas (u.g/m3). The oxygen demand of all the
hydrocarbons that might be in soil gas is expressed as the concentration of TPH (gasoline) plus
the concentration of methane. The concentration of TPH (gasoline) can be determined by
modified EPA Method TO-15 (see http://www.epa.gov/ttn/amtic/files/ambient/airtox/to-
15r.pdf) referenced to heptane. The concentration of methane can be determined as a fixed
gas or by EPA Method 3C. The concentration of methane is multiplied by 1.136 to correct for
the differences between the theoretical oxygen demand of methane and heptane.
Special Considerations
Figure 9 or Figure 10 should only be used for UST sites with the same characteristics that were
simulated by Abreu, Ettinger, and McAlary (2009). These conditions were relatively
conservative. Figure 9 and Figure 10 are not appropriate for use at sites where the oxygen flux
from the surface is impeded.
Some documents define the vapor intrusion attenuation factor differently than defined by
Johnson and Ettinger (1991) and discussed in this section. The JEM (see Section 13, p.106)
ignores background sources when estimating the indoor air concentration arising from vapor
intrusion. When used in this PVI guide, the Greek letter alpha (a) refers strictly to attenuation
during vapor intrusion, which might be observable if there were no background (ambient)
vapor sources. In contrast, some empirical attenuation factors (sometimes designated AF) are
based on indoor air concentrations that include both background sources and vapor
concentrations that intrude into the building from a subsurface vapor source. Thus, when there
is a measurable contribution from an ambient source, an attenuation factor such as AF (which
includes the contribution of ambient sources) would be somewhat greater than the Johnson &
Ettinger alpha (a), which would indicate less attenuation than is actually occurring.
Page 102 of 123
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Source Vapor Concentration as TPH plus Methane (ug/m3)
Figure 10. Rescaled Figure 9 That Expresses The Source Vapor Concentration In Conventional
Units.
Recommendation
When evaluating the vapor source and attenuation of PHC vapors, paired vapor samples are
required to measure the actual attenuation that occurs due to aerobic biodegradation. Where
contamination is not in direct contact with an overlying building, choose one of two options: (1)
collect near-slab (exterior) shallow soil gas samples paired with deep (source) soil gas samples,
or (2) collect indoor air samples paired with sub-slab soil gas samples. Note that for option 2 if
the measured concentration of vapor in indoor air is below the applicable allowable indoor
concentration there is no need to measure sub-slab vapor concentration. If contamination is in
direct contact with a building basement, foundation, or slab, it is necessary to collect indoor air
samples as it will not be feasible to collect sub-slab vapor samples. If a generic vapor intrusion
attenuation factor and the measured concentration of a PHC in shallow soil gas predict an
acceptable concentration in indoor air, that prediction may be adequate to support a screening
decision. However, generic attenuation factors may not be appropriately representative of
conditions at a particular site.
Models may provide better estimates of a, but only if the actual conditions at a specific site
match the assumptions of a particular model. For biodegradable PHCs it would be better to
implement a transport and fate model that is designed to simulate the contribution of
biodegradation. The three-dimensional models of Abreu, Ettinger, and McAlary (2009) and
Verginelli and Baciocchi (2014) are potential options. BioVapor, a model developed by the
American Petroleum Institute, is another option. U.S. EPA is developing a model called
PVIScreen that is intended for this purpose. See Section 13 (p.106) for more discussion of the
appropriate use of computer models for PVI investigations.
Page 103 of 123
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References Cited
Abreu, L.D.V, R. Ettinger, and T. McAlary. 2009. Simulated Soil Vapor Intrusion Attenuation
Factors Including Biodegradation for Petroleum Hydrocarbons. Ground Water
Monitoring and Remediation 29(1): 105-117.
Brewer, R., J. Nagashima, M. Rigby, M. Schmidt, and H. O'Neill. (2014). Estimation of Generic
Subslab Attenuation Factors for Vapor Intrusion Investigations. Groundwater Monitoring
& Remediation 34(4):79-92.
DeVaull, G. 2007. Indoor Vapor Intrusion with Oxygen-Limited Biodegradation fora Subsurface
Gasoline Source. Environmental Science and Technology 41(9):3241-3248.
EPA. 2015. OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway
From Subsurface Sources To Indoor Air (OSWER Publication 9200.2-154). Office of Solid
Waste and Emergency Response.
http://www. epa. Qov/oswer/vaporintrusion/
Johnson, P.C. and R.J. Ettinger. 1991. Heuristic Model for Predicting the Intrusion Rate of
Contaminant Vapors into Buildings. Environmental Science and Technology 25(8):1445-
1452.
Lahvis, M.A., Baehr, A.L., and R.J. Baker. 1999. Quantification of Aerobic-Biodegradation and
Volatilization Rates of Gasoline Hydrocarbons Near the Water Table During Natural-
Attenuation Conditions. Water Resources Research 35(3):753-765.
Verginelli, I. and R. Baciocchi. (2014). Vapor Intrusion Screening Model for the Evaluation of
Risk-Based Vertical Exclusion Distances at Petroleum Contaminated Sites. Environmental
Science & Technology 48:13263-13272.
Wilson, J.T., K. Jewell, C. Adair, C. Paul, C. Ruybal, G. DeVaull, and J. Weaver. 2014. An Approach
that Uses the Concentrations of Hydrocarbon Compounds in Soil Gas at the Source of
Contamination to Evaluate the Potential for Intrusion of Petroleum Vapors into Buildings
(PVI). (EPA/600/R-14/318). ORD Issue Paper, U.S. Environmental Protection Agency,
Washington, DC.
http://cfpub.epa.Qov/si/si public record Report.cfm?dirEntryld=305910
Additional Information
Abreu, L.D., and P.C. Johnson. 2005. Effect of Vapor Source, Building Separation and Building
Construction on Soil Vapor Intrusion as Studied with a Three-Dimensional Numerical
Model. Environmental Science & Technology 3(12):4550-4561.
Page 104 of 123
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Abreu, L.D., and P.C. Johnson. 2006. Simulating the Effect of Aerobic Biodegradation on Soil
Vapor Intrusion into Buildings: Influence of Degradation Rate, Source Concentrations.
Environmental Science & Technology 40(7):2304-2315.
EPA. 2013. 3-D Modeling of Aerobic Biodegradation of Petroleum Vapors: Effect of Building Area
Size on Oxygen Concentration Below the Slab (EPA 510-R-13-002).
http://www.epa.aov/oust/cat/pvi/buildina-size-modelina.pdf
Johnson, P., R.A. Ettinger, J. Kurtz, R. Bryan, and J.E. Kester. 2002. Migration of Soil Gas Vapors
to Indoor Air: Determining Vapor Attenuation Factors Using a Screening-Level Model
and Field Data from the CDOT-MTL Denver, Colorado Site. API Soil and Groundwater
Research Bulletin No. 16.
http://www.api.ora/environment-health-and-safetv/clean-water/ground-water/vapor-
intrusion/vi-publications/determinina-vapor-attenuation-factors
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13. Computer Modeling Of Petroleum Vapor Intrusion
Description
A number of models have been developed and applied in estimating transport of volatile
chemicals from subsurface soil and groundwater to indoor air (Bekele, et al. 2013). Lahvis
(2011) presents a summary of 35 different analytical screening-level models, including a
discussion of features, and assumptions. Models generally used for simulation of PVI are either
the Johnson-Ettinger model (JEM) or BioVapor.
Johnson-Ettinger Model (JEM)
Johnson and Ettinger introduced one of the first vapor intrusion models in 1991. This model is
referred to as the Johnson-Ettinger Model or JEM. Features of the JEM include:
• A steady or transient source of subsurface vapors from groundwater or residual
chemicals
• Gaseous-phase diffusive vapor flow through a layer of soil
• Vapor transport through a slab-on-grade or basement foundation
• Building air exchange
The JEM presumes that the concrete foundation is impermeable and vapor movement occurs
only through cracks and other openings. However, concrete is permeable to vapors and gases.
Effective diffusion rates for intact air-dry concrete have been measured for hydrocarbons,
oxygen, methane, and radon with an overall measured range from 1.08 to 15.6 cm2/hr
(Haghighat, et al., 2002; Patterson and Davis, 2009; Kobayashi and Shuttoh, 1991; Tittarelli,
2009; Yu, et al., 1993). Thus, diffusive vapor flow for typical foundation areas and thicknesses
can be significant (McHugh, de Blanc, and Pokluda, 2006; Luo, et al., 2012). Actual
measurement of differential pressure across varied building foundations show a significantly
variable component over time (Nazaroff, et al., 1985; Hintenlang and AI-Ahmady, 1992;
Robinson, et al., 1997a,b; McHugh , de Blanc, and Pokluda, 2006; Lundegard, Johnson, and
Dahlen, 2008; Patterson and Davis, 2009; Luo and Johnson, 2011).
For sites where PHCs are present and aerobic biodegradation of PHCs occurs in the vadose
zone, comparisons to JEM consistently show the model to over-predict indoor air
concentrations by at least several orders of magnitude (Fitzpatrick and Fitzgerald, 2002; Sinke,
2001; Ririe, Sweeny, and Daugherty, 2002; Hers, et al., 2003; Davis, 2006; Colder Associates,
2008; Davis 2009). The potential for over-prediction is greatest for sites with low
concentrations of PHCs in soil and groundwater (API, 2009; Davis, 2009; Energy Institute, 2009).
The original model has been revised numerous times to attempt to account for biodegradation,
which was not included in the original JEM (see Johnson, Kemblowski, and Johnson, 1998; Ririe,
et al., 1998; Johnson, Hermes, and Roggemans, 2000; Spence and Walden, 2010; Parker, 2003;
Environmental Systems and Technologies, 2004; DeVaull, 2007a; Mills, et al., 2007;
Turczynowicz and Robinson, 2007; API, 2010; Lahvis, 2011). EPA also revised the original JEM.
Page 106 of 123
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The most current information on EPA's revised model may be found on EPA's website at
http://www.epa.gov/oswer/riskassessment/airmodel/johnson ettinger.htm.
BioVapor
The BioVapor model (DeVaull, 2007a; API, 2010) is a Microsoft Excel© macro that uses a
conceptual model similar to the JEM. BioVapor includes the following features:
• A steady subsurface petroleum vapor source.
• Gaseous-phase diffusive vapor flow through a layer of soil.
• Vapor transport through a slab-on-grade or basement foundation.
• Building air exchange.
In contrast to JEM, BioVapor accounts for oxygen-limited, aerobic biodegradation. Aerobic
biodegradation is included as a coupled reaction between petroleum vapors and oxygen.
Oxygen availability in the subsurface is dictated by transport through and around the building
foundation, and by diffusion into the soil. The BioVapor model requires estimates of chemical-
specific aerobic degradation rates for vadose zone soils. DeVaull (2007a,b) provides default
values based on measured data. DeVaull (2011) provides improved estimates of both median
values and observed ranges for an expanded set of specific chemicals.
PVIScreen
PVIScreen (Weaver, 2015) is based on the equations of BioVapor but is coded in Java to
improve computational efficiency and allow for implementation of algorithms to automate
uncertainty analysis. Most computer models must be run multiple times with varying input
parameters in order to conduct a typical sensitivity analysis. PVIScreen automates this function
by treating input variables as ranges and then conducting a Monte Carlo analysis. The results,
which are provided in a matter of seconds, are presented as the probability that the indoor air
concentration is less than a risk-based level. This is in contrast to most models that provide
single values for various output parameters. PVIScreen also allows for flexible unit choices and
presents results in an automatically-generated report.
Importance
Vapor intrusion models that include oxygen limited biodegradation support development of
petroleum-specific exclusion distance criteria (i.e., lateral inclusion zone—see Section 4, p.44;
vertical separation distance—see Section 5, p.48). Model results are consistent with empirical
exclusion distance values derived from several PVI field investigations. These include Lahvis, et
al. (2013); Davis (2009); Peargin and Kolhatkar (2011); Wright (2011); McHugh, et al. (2010),
and Verginelli and Baciocchi (2014). Site assessment and field data including the depth to
contamination, source strength, and type (LNAPL or dissolved) are key parameters for
determining these exclusion distance criteria.
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Estimates using the BioVapor model indicate that for moderate or weak sources (especially
dissolved plumes), biodegradation effectively eliminates the potential for PVI. Conversely,
where vapor sources are both high in concentration and in close proximity to the bottom of a
foundation, the BioVapor model predicts significant potential for PVI.49 Notably, in these cases
the BioVapor model predicts significantly higher potential for PVI below a foundation, where
oxygen availability is more limited, than adjacent to the foundation where the soil surface is
open to air and oxygen availability is greater. This prediction is consistent with measured
vertical profiles of hydrocarbons and oxygen for high concentration vapor sources taken both
below a foundation and beside a foundation (Patterson and Davis, 2009; Laubacher, et al.,
1997).
Weaver (2012) presents results of a sensitivity analysis that indicates when biodegradation
occurs, it dominates the other processes included in the BioVapor model. In these cases, the
parameters representing aerobic biodegradation, source depth, and source strength dominate
the model results. In the other cases where biodegradation is insignificant, building parameters
become more important, as they are in the JEM (Tillman and Weaver, 2007).
More complex numerical models including oxygen-limited biodegradation have been developed
and applied. Abreu and Johnson (2006) present results for a three-dimensional model. With
matched model parameters, there is reasonable agreement between the three dimensional
results and those predicted with the BioVapor model (DeVaull, 2007b) and PVIScreen (Weaver,
2015). Each of these model results show similar sensitivities to changes in model parameters,
and support the use of exclusion distances such as those recommended in this document (see
Section 4, p.44, lateral inclusion zone, and Section 5, p.48, vertical separation distance).
Assessment
When selecting an appropriate computer model, the mathematical formulation needs to be
consistent with conditions at the site and the CSM. If the computer model is not matched to
conditions at the site, then error is likely introduced into the computer model results. This
means that input parameters for the computer model should be representative of the actual
physical, chemical, and biological properties of the site. Typically all factors influencing vapor
intrusion are not included in currently available models. Even with more advanced modeling,
resource limitations would prevent the detailed characterization necessary to determine
representative values for some of the input parameters. Some of these factors include
subsurface heterogeneity, variation in building operation, subsurface moisture content,
variations in weather and others. For most other types of environmental models, limitations in
characterization are mitigated by calibration to known endpoints, typically concentration
distributions. Though calibration results may not be unique (that is the same results could
potentially be obtained using different values for the same suite of input parameters), when
these results match field conditions, a model is deemed to be useful for predictive simulations.
49 Moderate or weak sources are associated with dissolved plumes. Strong sources are associated with LNAPL
Table 3 on page 52 presents concentration thresholds associated with dissolved plumes and LNAPL
Page 108 of 123
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Special Considerations
Most of the parameters describing model processes will not be known with certainty. A
common limitation is that field measurements of all the input parameters (e.g., biodegradation
rates, soil moisture content beneath buildings, air exchange rates) are typically not available,
and those that are (e.g., source concentration) may be spatially or temporally variable.
Literature values are typically substituted for site-specific data. This leads to uncertainty as to
whether parameter values are truly representative of site conditions. Model results will
likewise inevitably lack certainty (Bekele, et a I, 2013). The importance of these (and other)
parameters is determined through an uncertainty analysis. By determining the impact of
parameter variability on the model results, the uncertainty analysis adds confidence to the
conclusions drawn from the model.
Recommendation
An appropriate framework for the use of a mathematical model and understanding of model
characteristics is critical when using the results of mathematical models for regulatory purposes
(Hers, et al., 2003). The appropriate role for a model in a PVI investigation is as a means to
explain observed behavior. EPA recommends the use of a model that considers aerobic
biodegradation when assessing the potential for PVI. Regardless of which model is used to
simulate PVI, EPA recommends that an uncertainty analysis be conducted to provide error
bounds on predictions of the computer model.
Model results obtained by using site-specific inputs can provide results that inform decision-
making. In particular, model results can be used to demonstrate that: sufficient oxygen exists
to degrade petroleum contaminants, contaminant vapor distributions are plausible given
conditions at the site, estimates of the vapor attenuation anticipated in the subsurface due to
biodegradation are reasonable. Models may also be used for purposes such as improving a
site-specific sampling strategy, validation (or refutation) of the CSM by comparing a model to
measured soil gas data, and in estimating the effect of varied or changed site conditions (e.g.,
including construction of a new building on a brownfields site).
Model results can thus be used as one line of evidence that a building is not likely to be
impacted by PHC vapors. At the present time and state of knowledge, EPA cautions that model
results should not be used as the sole rationale for determining that a building is not
threatened or impacted by PVI.
References Cited
Abreu, L. D. V. and P. C. Johnson. 2006. Simulating the Effect of Aerobic Biodegradation on Soil
Vapor Intrusion into Buildings: Influence of Degradation Rate, Source Concentration,
and Depth. Environmental Science and Technology 40(7):2304-2315.
American Petroleum Institute (API). 2009. Simulating the Effect of Aerobic Biodegradation on
Soil Vapor Intrusion into Buildings—Evaluation of Low Strength Sources Associated with
Page 109 of 123
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Dissolved Gasoline Plumes. Publication No. 4775; American Petroleum Institute:
Washington, D.C.
http://www.api.org/environment-health-and-safety/clean-water/ground-water/vapor-
intrusion/vi-publications/simulating-aerobic-biodegredation
American Petroleum Institute (API). 2010. BioVapor Indoor Vapor Intrusion Model.
http://www.api.org/environment-health-and-safetv/clean-water/ground-water/vapor-
intrusion/biovapor-form
Bekele, D.N., R. Naidu, M. Bowman, and S. Chadalavada. 2013. Vapor Intrusion Models for
Petroleum and Chlorinated Volatile Organic Compounds: Opportunities for Future
Improvements. Vadose Zone Journal 12(2).
Davis, R. 2006. Vapor Attenuation in the Subsurface from Petroleum Hydrocarbon Sources: An
Update and Discussion on the Ramifications of the Vapor-Intrusion Risk Pathway.
LUSTLine Bulletin 52:22-25. New England Interstate Water Pollution Control
Commission, Massachusetts.
http://www.neiwpcc.org/lustline/lustline pdf/lustline52.pdf
Davis, R.V. 2009. Bioattenuation of Petroleum Hydrocarbon Vapors in the Subsurface: Update
on Recent Studies and Proposed Screening Criteria for the Vapor-Intrusion Pathway.
LUSTLine Bulletin 61:11-14. New England Interstate Water Pollution Control
Commission, Massachusetts.
http://www.neiwpcc.org/lustline/lustline pdf/lustline 61.pdf
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DeVaull, G. E. 2011. Biodegradation Rates for Petroleum Hydrocarbons in Aerobic soils: A
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Haghighat, F., C.S. Lee, and W.S. Ghaly. 2002. Measurement of Diffusion Coefficients of VOCs
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Hers, I., R. Zapf-Gilje, P.C. Johnson, and L. Li. 2003. Evaluation of the Johnson and Ettinger
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Johnson, P.C. and R.J. Ettinger. 1991. Heuristic Model for Predicting the Intrusion Rate of
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Johnson, P.C., M.W. Kemblowski, R.L. Johnson. 1998. Assessing the Significance of Subsurface
Contaminant Vapor Migration to Enclosed Spaces: Site-Specific Alternatives to Generic
Estimates, Publication No. 4674, API, Washington, D.C., December, 44p.
Johnson, P.C., V.A. Hermes, S. Roggemans. 2000. An Oxygen-Limited Hydrocarbon Vapor
Migration Attenuation Screening Model, written communication, Paul C. Johnson,
Department of Civil and Environmental Engineering, Arizona State University, Tempe,
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Kobayashi, K. and K. Shuttoh. 1991. Oxygen Diffusivity of Various Cementitious Materials.
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Lundegard, P.D., P.C. Johnson, and P. Dahlen. 2008. Oxygen Transport from the Atmosphere to
Soil Gas Beneath a Slab-on-Grade Foundation Overlying Petroleum-Impacted Soil.
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Luo, H. and P.C. Johnson. 2011. Incorporating Barometric Pressure and Wind Effects into Vapor
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Source Effects on Subslab Soil Gas Concentrations. 22nd Annual International
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McHugh, T.E., P.C. de Blanc, and R.J. Pokluda. 2006. Indoor Air as a Source of VOC
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McHugh, T.E., R. Davis, G. DeVaull, H. Hopkins, J. Menatti, and T. Peargin. 2010. Evaluation of
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Mills, W.B., S. Liu, M.C. Rigby, and D. Brenner. 2007. Time-Variable Simulation of Soil Vapor
Intrusion into a Building with a Combined Crawl Space and Basement. Environmental
Science and 7ec/7no/ogy41(14):4993-5001.
Nazaroff, W. W., H. Feustel, A.V. Nero, K.L. Revzan, D.T. Grimsrud, M.A. Essling, and R.E.
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Parker, J.C. 2003. Modeling Volatile Chemical Transport, Biodecay, and Emission to Indoor Air.
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Patterson, B. M. and G.B. Davis. 2009. Quantification of Vapor Intrusion Pathways into a Slab-
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Glossary
Note: Most of these definitions are from EPA's on-line glossaries (see
http://iaspub.epa.gov/sor internet/registrv/termreg/searchandretrieve/termsandacronyms/search.do).
Absorption: the penetration of atoms, ions, or molecules into the bulk mass of a substance. In contrast,
adsorption is the retention of atoms, ions, or molecules onto the surface of another substance
Advection: the process of transfer of fluids (vapors or liquid) through a geologic formation in response
to a pressure gradient that may be caused by changes in barometric pressure, water table levels, wind
fluctuations, or infiltration
Aerobic: able to live, grow, or take place only when free oxygen is present
Anaerobic: able to live, grow, or take place where free oxygen is not present
Analyte: the element, ion, or compound that an analysis seeks to identify; the element of interest
Attenuation: the reduction or lessening in amount (e.g., a reduction in the amount of contaminants in a
plume as it migrates away from the source)
Biodegradability (or biodegradation potential): the relative ease with which organic chemicals will
degrade as the result of biological metabolism. With respect to petroleum hydrocarbons, although
virtually all petroleum hydrocarbons are biodegradable, biodegradability is highly variable and
dependent somewhat on the specific type of hydrocarbon. In general, biodegradability increases with
increasing solubility; solubility is inversely proportional to molecular weight.
Biodegradation: a process by which microbial organisms transform or alter (through metabolic or
enzymatic action) the structure of chemicals introduced into the environment
Biologically active soil: in the context of a PVI investigation means that the subsurface soil environment
will support populations of microorganisms that are present in sufficient quantities to aerobically
degrade PHC vapors before they intrude into a receptor. Effective aerobic biodegradation of petroleum
hydrocarbons depends on the soil having sufficient oxygen and enough soil water to provide a habitat
for adequate populations of active microorganisms. The presence of sufficient oxygen must be
determined by the collection and analysis of soil gas. Soil that is too dry will not support microbial life.
The soil generally will not be too dry for bacteria if the depth to the water table is less than 300 feet, or
if the soil around the receptor supports the growth of plants characteristic of temperate climates.
(NOTE that in hot, arid climates lack of soil moisture may inhibit biodegradation of PHCs)
Concentrations of carbon dioxide which are ten-fold higher than concentrations in the atmosphere are
an acceptable indication that conditions support microbial respiration. The actual habitat of soil
bacteria is the thin film of water held to the surface of soil particles by capillary attraction. Coarse sand
and gravel with a low content of silt or clay or organic matter, or fractured consolidated rock, or
consolidated rock with solution channels, may not have enough soil water in intimate contact with soil
gas to support adequate densities of biologically active microorganisms. These geological materials do
not qualify as "biologically active soil."
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BTEX: acronym for the aromatic hydrocarbons benzene, toluene, ethylbenzene, and xylenes (three
isomers)
Capillary fringe: the zone of a porous medium above the water table within which the porous medium
is saturated by water under pressure that is less than atmospheric pressure. See also vadose zone and
unsaturated zone.
Clean monitoring point: Concentration thresholds for "clean" monitoring points are: the BTEX
concentration in groundwater is equal to or less than the respective maximum contaminant level (e.g., 5
u.g/L for benzene); the TPH concentration in soil is less than 20 mg/Kg; there is no potential presence of
liquid or residual phase LNAPL; the oxygen concentration is greater than 0.2 percent; and the
combustible gas concentration in soil gas is less than 100 ppm (v/v).
Clean soil: In the context of a PVI investigation, clean soil does not necessarily mean that the soil is free
from all contamination, but rather that any contamination present is at concentrations low enough that
the biological activity of the soil is sufficient to biodegrade PHC vapors before they reach a receptor.
Computer model: a mathematical representation of a physical process or system. Computer models
are based upon sound conceptual site models to provide meaningful information. As the complexity of
computer models increases, so does the amount of data required, and the quality of the output from
computer models is directly related to the quality of the input data. Because of the complexity of
natural systems, models necessarily rely on simplifying assumptions that may or may not accurately
represent the dynamics of the natural system. Calibration and sensitivity analyses are important steps
in the appropriate use of models.
Conceptual site model (CSM): a three-dimensional representation that conveys what is known or
suspected about potential contamination sources, release mechanisms, and the transport and fate of
those contaminants. The conceptual model provides the basis for assessing potential remedial
technologies at the site. "Conceptual site model" is not synonymous with "computer model"; however,
a computer model may be helpful for understanding and visualizing current site conditions or for
predictive simulations of potential future conditions.
Contamination: in the context of a PVI investigation, contamination means that: the BTEX
concentration in groundwater is greater than the respective MCL; or the TPH concentration in soil is
greater than 100 mg/Kg; or there is potential presence of liquid or residual phase LNAPL; or the
combustible gas concentration in soil gas is greater than 100 ppm (v/v).
Diffusion: the process by which molecules in a single phase equilibrate to a zero concentration gradient
by random molecular motion (Brownian motion). The flux of molecules is from regions of high
concentration to low concentration and is governed by Pick's Second Law.
Dispersion: the process by which a substance or chemical spreads and dilutes in flowing groundwater
or soil gas
Downgradient: in the direction of decreasing potentiometric head; the general direction of
groundwater flow
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First responder: refers to those individuals who in the early stages of an incident are responsible for the
protection and preservation of life, property, evidence, and the environment. Typically these are police,
firefighters, or emergency medical personnel.
Fixed gases: refers to the gases nitrogen, oxygen, argon, carbon dioxide, and carbon monoxide. The
volume of these gases together accounts for virtually 100 percent of the composition of the earth's
atmosphere. Presence and concentration of these gases are determined using gas chromatography
(GC).
Flux: the rate of movement of mass through a unit cross-sectional area per unit time in response to a
concentration gradient or some advective force
Free product: a petroleum hydrocarbon in the liquid ("free" or non-aqueous) phase (see also light non-
aqueous phase liquid, LNAPL)
Gradient: the rate of change in value of a physical or chemical parameter per unit change in position
For example, hydraulic gradient is equal to the difference in head measured at two points (usually wells)
divided by the distance separating the two points. The dimensions of head and distance are both
lengths, therefore the gradient is expressed as a dimensionless ratio (L/L).
Groundwater: the water contained in the pore spaces of saturated geologic media
Henry's law constant: the ratio of the concentration of a compound in air (or vapor) to the
concentration of the compound in water under equilibrium conditions
Henry's law: the relationship between the partial pressure of a compound and the equilibrium
concentration in the liquid through a proportionality constant known as the Henry's law constant
Heterogeneous: varying in structure or composition at different locations in space
Homogeneous: uniform in structure or composition at all locations in space
Hydraulic gradient: the change in total potentiometric (or piezometric) head between two points
divided by the horizontal distance separating the two points
Hydrocarbon: chemical compounds composed only of carbon and hydrogen
Inclusion zone: the area surrounding a contaminant mass through which vapor-phase contamination
may travel and intrude into buildings and potentially result in adverse health effects to inhabitants
Indian country: (1) All land within limits of any Indian reservation under the jurisdiction of the United
States government, notwithstanding the issuance of any patent, and, including rights-of-way running
through the reservation; (2) All dependent Indian communities within the borders of the United States
whether within the original or subsequently acquired territory thereof, and whether within or without
the limits of a state; and (3) All Indian allotments, the Indian titles to which have not been extinguished,
including rights-of-way running through the same.
Indigenous: living or occurring naturally in a specific area or environment; native
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Isotropic: the condition in which hydraulic properties of an aquifer are equal when measured in any
direction
Lateral inclusion zone: the area surrounding a contaminant mass and for which all buildings within its
boundaries should be assessed for potential PVI. By definition, all buildings that overlie contamination
in any phase are within the lateral inclusion zone.
Light non-aqueous phase liquid (LNAPL): contaminants that remain as the original bulk liquid with a
density less than that of water (see also free product)
Microorganisms: microscopic organisms including bacteria, protozoans, yeast, fungi, mold, viruses, and
algae
Permeability: a qualitative description of the relative ease with which rock, soil, or sediment will
transmit a fluid (liquid or gas). Often used as a synonym for hydraulic conductivity or coefficient of
permeability.
Petroleum: 40CFR280.12 defines the term petroleum to include crude oil or any fraction thereof
that is liquid at standard conditions of temperature and pressure (60 degrees Fahrenheit and
14.7 pounds per square inch absolute).
Petroleum hydrocarbons (PHCs): hydrocarbons (i.e., compounds comprised of combinations of
hydrogen and carbon atoms) that are components of petroleum (crude oil), including the various
products that result from distillation of crude oil
Petroleum vapor intrusion (PVI): intrusion of petroleum hydrocarbon vapors into buildings or other
structures
Porosity: the volume fraction of a rock or unconsolidated sediment not occupied by solid material but
usually occupied by water or air (gas)
Preferential transport pathways: pathways through which contaminants may be transported at a
higher rate than through surrounding materials. Preferential transport pathways are heterogeneities
within geologic media and include features that are natural (such as facies changes, sand or gravel
stringers, solution channels in karst, bedding planes and weathered surfaces, fractures, and joints) as
well as man-made (such as utility corridors, trenches, other types of excavations).
Regulated substance: (a) Any substance defined in section 101(14) of the Comprehensive Environmental
Response, Compensation and Liability Act (CERCLA) of 1980 (but not including any substance regulated
as a hazardous waste under subtitle C), and (b) Petroleum, including crude oil or any fraction thereof
that is liquid at standard conditions of temperature and pressure (60 degrees Fahrenheit and 14.7
pounds per square inch absolute). The term regulated substance includes but is not limited to
petroleum and petroleum-based substances comprised of a complex blend of hydrocarbons derived
from crude oil through processes of separation, conversion, upgrading, and finishing, such as motor
fuels, jet fuels, distillate fuel oils, residual fuel oils, lubricants, petroleum solvents, and used oils.
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RfC (reference concentration, inhalation): an estimate (with uncertainty spanning perhaps an order of
magnitude) of a continuous inhalation exposure of a chemical to the human population through
inhalation (including sensitive subpopulations), that is likely to be without risk of deleterious noncancer
effects during a lifetime
Selective ion monitoring (SIM): a mass spectrometry scanning mode in which only a limited mass-to-
charge ratio range is transmitted or detected by the instrument, as opposed to the full spectrum range
Semi-volatile organic compound (SVOC): an organic compound which has a boiling point higher than
water and which may vaporize when exposed to temperatures above room temperature. Semi-volatile
organic compounds include phenols and polynuclear aromatic hydrocarbons (PAHs).
Site assessment: see site characterization
Site characterization: (verb) the process by which site-specific information and data are gathered from
a variety of sources to characterize the physical, biological, and chemical systems at a contaminated site.
A primary objective of site characterization is delineation of the areal (both horizontal—longitudinal
and lateral—transverse) and vertical extent of contamination. This includes changes in plume
boundaries, changes in geochemical parameters that affect biodegradation, and contaminant mass (or
concentration) increases or decreases, (noun) The product (e.g., CSM, report) resulting from the site
characterization process. (Note: Site assessment, site investigation, site evaluation, and site check are
all synonyms of site characterization.)
Site check: see site characterization
Site evaluation: see site characterization
Site investigation: see site characterization
Soil moisture: the water contained in the pore spaces in the unsaturated zone
Solubility: the amount of mass of a compound that will dissolve in a unit volume of solution
Sorption: a general term used to encompass the processes of absorption, adsorption, ion exchange, and
chemisorption
Source material: material that includes or contains contaminants that act as a reservoir (either
stationary or mobile) for migration of contamination to the ground water, to surface water, to air, (or
other environmental media), or acts as a source for direct exposure. Contaminated ground water
generally is not considered to be a source material although non-aqueous phase liquids (NAPLS
[occurring either as residual- or free-phase]) may be viewed as source materials.
Source zone: the impacted area immediately surrounding the source of a release of regulated
substances comprising source materials
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Total petroleum hydrocarbons (TPH): a measure of the concentration or mass of petroleum
hydrocarbon constituents present in a given amount of air, soil, or water. (Note: The term total is a
misnomer, in that few, if any, of the procedures for quantifying hydrocarbons are capable of measuring
all fractions of petroleum hydrocarbons present in the sample. Volatile hydrocarbons are usually lost in
the process and not quantified. Additionally, some non-petroleum hydrocarbons may be included in the
analysis.)
Travel time: the time it takes a contaminant to travel from the source to a particular point
downgradient
Tribe: Indian tribe or tribe means an Indian or Alaska Native tribe, band, nation, pueblo, village, or
community that the Secretary of the Interior acknowledges to exist as an Indian tribe pursuant to the
federally Recognized Indian Tribe List Act of 1944, 25 U.S.C. 479a.
Underground storage tank (UST): 40CFR280.12 defines an underground storage tank as any one or
combination of tanks (including underground pipes connected thereto) that is used to contain an
accumulation of regulated substances, and the volume of which (including the volume of underground
pipes connected thereto) is 10 percent or more beneath the surface of the ground. This term does not
include any: (a) Farm or residential tank of 1,100 gallons or less capacity used for storing motor fuel for
noncommercial purposes; (b) Tank used for storing heating oil for consumptive use on the premises
where stored; (c) Septic tank; (d) Pipeline facility (including gathering lines) regulated under: (1) The
Natural Gas Pipeline Safety Act of 1968 (49 U.S.C. App. 1671, etseq. ), or (2) The Hazardous Liquid
Pipeline Safety Act of 1979 (49 U.S.C. App. 2001, etseq.), or (3) Which is an intrastate pipeline facility
regulated under state laws comparable to the provisions of the law referred to in paragraph (d)(l) or
(d)(2) of this definition; (e) Surface impoundment, pit, pond, or lagoon; (f) Storm-water or wastewater
collection system; (g) Flow-through process tank; (h) Liquid trap or associated gathering lines directly
related to oil or gas production and gathering operations; or (i) Storage tank situated in an underground
area (such as a basement, cellar, mine working, drift, pit, or tunnel) if the storage tank is situated upon
or above the surface of the floor. The term underground storage tank or UST does not include any pipes
connected to any tank which is described in paragraphs (a) through (i) of this definition.
Unsaturated zone: the zone between land surface and the capillary fringe within which the moisture
content is less than saturation and pressure is less than atmospheric. Soil pore spaces also typically
contain air or other gases. The capillary fringe is not included in the unsaturated zone.
Vadose zone: the zone between land surface and the water table within which the moisture content is
less than saturation (except in the capillary fringe) and pressure is less than atmospheric. Soil pore
spaces also typically contain air or other gases. The capillary fringe is included in the vadose zone.
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Vapor intrusion attenuation factor (a): a parameter defined by Johnson and Ettinger (1991) to relate
the vapor concentration of a volatile chemical inside the building to its vapor concentration at the
subsurface source. This parameter, designated alpha (a), is defined mathematically as the vapor
concentration in indoor air divided by the vapor concentration in soil gas at the source (with
concentration being in the same units), and thus it is a ratio. The source is defined as the region of
highest vapor concentration. Therefore, a values are always less than one. The vapor intrusion
attenuation factor is an inverse measurement of the attenuation: a values decrease with increasing
attenuation and a values increase with decreasing attenuation. In other words, a values represent the
fraction of soil gas contaminant that reaches indoor air. Large a values (i.e., values approaching 1)
indicate that a large fraction of the soil gas contaminant has reached the indoor air; therefore, little
attenuation is taking place, whereas small a values indicate that a small fraction of the soil gas
contaminant has reached the indoor air; therefore, significant attenuation is taking place.
Vapor pressure: the force per unit area exerted by a vapor in an equilibrium state with its pure solid,
liquid, or solution at a given temperature. Vapor pressure is a measure of a substance's propensity to
evaporate. Vapor pressure increases exponentially with an increase in temperature.
Vertical separation distance: the thickness of clean, biologically active soil that separates the source of
contamination from a building basement, foundation, or slab
Volatile: is a tendency of a substance to vaporize or the speed at which it vaporizes. Volatility is
indicated by a substance's vapor pressure. Substances with a higher vapor pressure will vaporize more
readily at a given temperature than substances with a lower vapor pressure. A volatile organic
compound is an organic compound which has a boiling point below that of water and which can easily
vaporize or volatilize.
Volatile organic compound (VOC): organic compound that at room temperature and normal
atmospheric pressure produces vapors that escape easily from volatile liquid chemicals. Volatile organic
compounds include a variety of chemicals such as gasoline, benzene, toluene, xylene, formaldehyde,
tetrachloroethylene, and perchloroethylene.
Volatilization: the process of transfer of a chemical from the aqueous or liquid phase to the gas phase.
Solubility, molecular weight, and vapor pressure of the liquid and the nature of the gas-liquid interface
affect the rate of volatilization.
Water table: the water surface in an unconfined aquifer at which the fluid pressure in the pore spaces
is at atmospheric pressure
Weathering: the process during which a complex compound is reduced to its simpler component parts,
transported via physical processes, or biodegraded overtime
Wilting point: the minimal point of soil moisture the plant requires not to wilt. Wilting point values
under field conditions are not constant for any given soil, but are determined by the integrated effects
of plant, soil and atmospheric conditions.
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