Draft EPA External Review Document. Do not cite or quote.
v>EPA
United States
Environmental Protection
Agency EPA 510-R-13-xxx
This document is an EPA External Review draft. This information is distributed
solely for the purpose of pre-dissemination public review. It does not represent
an interim or final Agency determination or policy. Do not cite or quote.
Guidance For Addressing
Petroleum Vapor Intrusion
At Leaking Underground Storage
Tank Sites
U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response
Office of Underground Storage Tanks
Washington, D.C.
April 2013
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Contents
Purpose 7
Background 7
Overview 8
EPA Recommended Actions 8
Supporting Technical Information 13
1. Petroleum Vapor Intrusion (PVI) 14
2. Typical PVI Scenarios 31
3. Site Characterization And Conceptual Site Model (CSM) 37
4. Lateral Inclusion Zone 41
5. Vertical Separation Distance 44
6. Mobile And Residual Light Non-Aqueous Phase Liquid (LNAPL) 51
7. Groundwater Flow And Dissolved Contaminant Plumes 55
8. Soil Vapor Profile 59
9. Clean, Biologically Active Soil 65
10. Contaminants Other Than PHCs 70
11. Seasonal And Weather Effects 76
12. Vapor Intrusion Attenuation Factor (a) 80
13. Computer Modeling Of Petroleum Vapor Intrusion 84
Glossary 93
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Tables
1. Components of A PVI Investigation 11
2. Summary Of Characteristics Of Typical Scenarios Of Petroleum Vapor Sources And
Potential Receptors 34
3. Required Vertical Separation Distance Between Contamination And Building
Foundation, Basement, or Slab 48
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Figures
1. Flowchart Of the Receptor-Specific PVI Assessment And Mitigation Process 12
2. Conceptual Model Of Typical Petroleum Hydrocarbon Release 15
3. Difference In Potential For PVI Based On Type Of Source: a) LNAPL b) Dissolved Phase 32
4. Typical Scenarios Of Potential PVI Sources And Potential Receptors 33
5. Lateral And Vertical Separation Distances Between Source Of PHC Contaminants And
Hypothetical Receptor 42
6. Vertical Separation Distances Between Source Of PHC Contaminants And Hypothetical
Receptor: (a) Dissolved Source, (b) LNAPL Source 45
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 53
8. Typical Vertical Concentration Profile In The Unsaturated Zone For PHCs, Carbon
Dioxide, And Oxygen 60
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) 81
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List Of Acronyms
1,2-DCA
1,2-Dichloroethane
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 1,1-dibromoethane)
ICLR
Incremental lifetime cancer risk
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
PVI
Petroleum Vapor Intrusion
RBCV
Risk-based soil vapor concentration
TAME
Tertiary-amyl methyl ether
TBA
Tertiary butyl alcohol
TEL
Tetraethyl lead
TML
Tetramethyl lead
TPH
Total Petroleum Hydrocarbons
UST
Underground Storage Tank
VI
Vapor Intrusion
VOC
Volatile Organic Compound
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Notice: This document provides guidance to EPA, state, and tribal staff. It also
provides guidance to the public and to the regulated community on how EPA
intends to exercise its discretion in implementing its regulations as a matter of
national policy. The document does not, however, substitute for EPA's statutes
or regulations, nor is it a regulation itself. Thus, it does not impose legally-
binding requirements on EPA, states, tribes, or the regulated community. This
document presents technical information on the phenomenon of petroleum
vapor intrusion. EPA may revise this document without public notice.
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Purpose
This document provides guidance to EPA, states,1 and tribes2 for investigating and assessing
petroleum contaminated3 sites where vapor intrusion4 by petroleum hydrocarbons (PHCs) may
occur. In this document, intrusion of PHC vapors is referred to as petroleum vapor intrusion
(PVI). This PVI guidance focuses on underground storage tanks (USTs), typically located at gas
stations and non-marketing facilities regulated under Subtitle I of the Solid Waste Disposal Act.5
Background
In November 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). This
draft guidance did not address PVI at UST sites regulated under Subtitle I.
In 2009, EPA's Office of Inspector General (OIG) released an evaluation report, Lack of Final
Guidance on Vapor Intrusion Impedes Efforts to Address Indoor Air Risks (Report No. 10-P-
0042). One of its recommendations is that EPA issue final PVI guidance on how to address risks
from petroleum hydrocarbon vapors. In response to OIG's recommendation, EPA is issuing this
document: Guidance For Addressing Petroleum Vapor Intrusion At Leaking Underground
Storage Tank Sites. In addition, in response to other recommendations in the OIG's evaluation
report, EPA is issuing OSWER Final Guidance For Assessing and Mitigating the Vapor Intrusion
Pathway From Subsurface Sources to Indoor Air.
1 States refers to states, territories, and the District of Columbia.
2 While this guidance is being provided to tribal governments, EPA by law cannot delegate implementation
authority for the UST program, including the corrective action components, to tribal governments. Some tribal
governments that carry out corrective action activities under their own regulations may find this guidance useful.
3 For the purposes of the federal UST program, as described under the definition of regulated substance in 40 CFR
280.12, the term petroleum includes "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." This
definition is subject to change if the regulations are revised in the future.
4 Vapor intrusion (VI) is the general term given to migration of hazardous vapors from any subsurface contaminant
source, such as contaminated soil or groundwater, through the vadose zone and into indoor air.
5 Non-Subtitle I releases of petroleum (e.g., oil/petroleum releases from terminals and refineries, above ground
storage tanks, pipelines), should be assessed and addressed under the OSWER Final Guidance for Assessing and
Mitigating the Vapor Intrusion Pathway From Subsurface Sources To Indoor Air. For more information on VI see
EPA's Vapor Intrusion website at htto://www.eDo.aov/oswer/vaDorintrusion/. The PVI guidance may, however, be
useful in informing decisions at petroleum-only brownfield sites that are similar to a typical Subtitle I release.
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Overview
PVI is a concern when PHCs emanate from petroleum-contaminated soil or groundwater and
migrate into buildings. This can result in indoor air concentrations that pose a risk to building
occupants. PVI may pose both immediate threats to safety (e.g., fire/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 classes
of chemicals:
• PHCs found in gasoline, diesel, and jet fuel (e.g., benzene, trimethylbenzenes).
• Vapor-forming chemicals other than PHCs that may be found in petroleum (e.g., methyl
tertiary-butyl ether (MTBE) and other fuel additives).
• Methane, which can arise from anaerobic biodegradation of PHCs and other
constituents of petroleum fuels, especially ethanol.
PHCs can biodegrade aerobically and when biodegradation is complete, produce water and
carbon dioxide. Some petroleum hydrocarbons may also degrade anaerobically, which is
slower and may produce methane, particularly if the source is from an ethanol-blended
gasoline.
The potential for human exposure from PVI may be limited because of the biodegradability of
PHCs. However, numerous factors may affect the vapor intrusion pathway and potentially
impact human health and the environment. These factors are identified and discussed in
greater detail in the Supporting Technical Information section of this guidance.
EPA Recommended Actions
Assessing the potential for PVI is an integral part of the normal response to a suspected or
confirmed release from a 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 and impact human
safety and health.
EPA's Office of Underground Storage Tanks (OUST) recommends the following actions for
situations in which EPA, states, and tribes are either undertaking PVI investigations and
corrective action at leaking UST sites or where 40 CFR 280 requires6 UST owners and operators
to undertake release investigation and corrective action activities:
• Assess and mitigate immediate threats to safety
Determine if residents of nearby buildings reported the presence of odors or visible
signs of PHC contamination. If so, alert first responders so that they can evacuate these
6 In the case of a suspected or confirmed release from a regulated UST system, Subparts E and F of 40 CFR 280
requires 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.
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buildings until the potential threat to human safety from fire or explosion due to PVI has
been assessed and mitigated as necessary.
• Conduct a site characterization and develop a conceptual site model (CSM)
Integrate all information and data related to the site into a CSM. This includes
assessment of the potential for biodegradation of the PHCs. The CSM should then 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.
To determine whether the vadose zone is comprised of clean soil and capable of
supporting microbial populations that facilitate aerobic biodegradation of PHCs,
EPA recommends that the site characterization include:
o A determination of the full extent and location of contamination,
o The nature and characteristics of the contamination.
o The hydrologic (e.g., soil moisture) and geological characteristics underneath and
near buildings in the lateral inclusion zone.
• Delineate a lateral inclusion zone
Assess the potential for PVI for each of the buildings within the lateral inclusion zone.
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.
• Identify preferential transport pathways within the inclusion zone
Assess whether preferential contaminant transport pathways are present and could
result in PVI into nearby buildings either inside or outside the lateral inclusion zone. If
so, assess the potential for PVI.
• Determine vertical separation distances for each building within the inclusion zone
Determine whether contamination underlies any buildings within the lateral inclusion
zone. If contaminated groundwater is within the vertical separation distance (6 feet)
between the contamination and the foundation, slab, or basement of any building(s),
EPA recommends sub-slab vapor sampling to assess the potential for PVI. If light non-
aqueous phase liquid (LNAPL) is within 15 feet of an overlying building, EPA
recommends sub-slab vapor sampling.
• Mitigate PVI, as appropriate
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.
Table 1 and Figure 1 briefly outline the recommended actions above. 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. Additional technical information is presented in the second
part of this guidance, Supporting Technical Information.
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Community Engagement
As you are conducting your assessment and follow-up actions, keep in mind the importance of
community engagement. EPA acknowledges there are various ways to engage the community
in cleanup decisions and it is not a one-size-fits-all approach. However, 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 OUST's
website:
• Community Engagement And The Underground Storage Tank Program.
• Guidelines For Tailoring Community Engagement Activities To Circumstances At Leaking
Underground Storage Tank Sites.
• Community Engagement Resources (Toolbox) For Underground Storage Tank Programs.
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Table 1. Components Of A PVI Investigation
Recommended
Actions
Purpose
Characteristics/Indicators
Procedures
Assess and
mitigate
immediate threats
to safety
Identifies potential
threat of explosion
or fire due to
petroleum vapors
or methane
LNAPL visible in building,
possibly as sheen
Petroleum odor,
headache, dizziness, or
nausea
Potential disagreeable
taste of water
Methane is odorless and
cannot be detected on the
basis of odor
Investigate all reports of petroleum
odors and other indicators within
buildings
Assess for presence of methane;
requires specialized detectors
Alert first responders so that they
can evacuate building inhabitants
until the potential for fire or
explosion has been assessed and
mitigated as necessary
Conduct a site
characterization
and develop a
CSM
Determines spatial
and temporal
relationship
between receptors
and sources of
contamination
Determines full extent and
location of contamination
and its nature and
potential for
biodegradation of PHCs
Defines hydrologic and
geologic characteristics of
the site
Identifies potential
receptors
Collect sufficient site data and
information to construct CSM
Identify data gaps
Update CSM as new data become
available
Delineate a lateral
inclusion zone
Focuses
investigation on
potential receptors
in close proximity
to contamination
• Based on quality of CSM
• The lower the uncertainty,
the smaller the lateral
inclusion zone
• Construct lateral inclusion zone
based on distance between clean
monitoring points
Identify
preferential
transport
pathways
Determines rate of
movement of
contaminants
toward potential
receptors
Includes both natural (i.e.,
geologic) and man-made
(i.e., underground utilities,
excavations) features
Difficult to detect and
map potential preferential
transport pathways
Determine if presence is likely and
locate extent as practicable
Exercise caution if presence is known
or possible
Consider additional sampling to
assess transport characteristics
Determine
vertical separation
distances
Narrows
investigation to
potential receptors
overlying
contamination
• Thickness of clean,
biologically active soil
separating contamination
and potential receptors
Collect sub-slab vapor samples if (a)
contaminated groundwater is within
6 feet of an overlying building or (b)
LNAPL is within 15 feet of an
overlying building
Mitigate PVI as
appropriate
Interrupts 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
Establish institutional controls to
limit or prohibit access to affected
areas of building, as necessary
Remediate source of contamination,
including recovery of LNAPL (if
present) to the maximum extent
practicable
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Figure 1. Flowchart Of The Receptor-Specific PVI Assessment And Mitigation Process
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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. Each section presents information in a standardized format, which is
easy to follow and allows for future revisions, if necessary.
Additional sources of information may be found in the Petroleum Vapor Intrusion Compendium
(http://www.epa.eov/oust/cat/pvi/). located on the Office of Underground Storage Tanks
(OUST) website (http://www.epa.eov/oust).
Page
1. Petroleum Vapor Intrusion (PVI) 14
2. Typical PVI Scenarios 31
3. Site Characterization And Conceptual Site Model (CSM) 37
4. Lateral Inclusion Zone 41
5. Vertical Separation Distance 44
6. Mobile And Residual Light Non-Aqueous Phase Liquid (LNAPL) 51
7. Groundwater Flow And Dissolved Contaminant Plumes 55
8. Soil Vapor Profile 59
9. Clean, Biologically Active Soil 65
10. Contaminants Other Than PHCs 70
11. Seasonal And Weather Effects 76
12. Vapor Intrusion Attenuation Factor (a) 80
13. Computer Modeling Of Petroleum Vapor Intrusion 84
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1. Petroleum Vapor Intrusion (PVI)
Description
Petroleum vapor intrusion (PVI) occurs when vapors from petroleum hydrocarbons (PHCs)
migrate into buildings. Fuels such as gasoline, diesel, aviation gasoline, and jet fuel are
comprised primarily of PHCs with some non-petroleum based additives.7 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/explosion potential from
petroleum vapors or methane) and/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, and eventually at some distance the
concentrations become negligible.
When petroleum fuels are released into the subsurface from a leaking underground storage
tank (UST), PHCs may partition into several phases:
• Globules of light non-aqueous phase liquid (LNAPL) trapped within soil pore spaces (i.e.,
residual LNAPL).8
• Dissolved in soil moisture.
• Adhered onto the surface of, or absorbed into, soil solids.
• Vapors in soil.
Phase Partitioning
If the volume of a fuel release is sufficient, the fuel may accumulate on the capillary fringe as
mobile LNAPL that may accumulate in monitoring wells. The mobile LNAPL may spread laterally
in the direction of groundwater flow. 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. Some of the LNAPL dissolves into the groundwater and is
transported down gradient by flowing groundwater as an aqueous phase. 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 for a more detailed discussion of typical PVI
scenarios.
7 PHCs present in gasoline belong to one of four major groups: paraffins, olefins, naphthenes, or aromatics. The
aromatic PHCs benzene, toluene, ethylbenzene, and the three isomers of xylene are collectively referred to as
BTEX. Although BTEX represent the group of PHCs that receive the most attention at typical leaking UST sites, they
are not the only compounds that may pose a risk to human health. Additives such as methyl tertiary-butyl ether
(MTBE) and tertiary-butyl alcohol (TBA), plus other PHCs (e.g., naphthalene), may be risk-drivers instead of, or in
addition to, BTEX and/or other PHCs.
8 Mobile LNAPL is often referred to as "free product", especially in older documents including 40CFR280.
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LNAPL {including
smear zone/Residual
k and Free-Phase)
VaporPhase
Dissolved-Phase
VadoseZone
MDxygen^
Transport
Oxygen Transport
Aerobic i,
~
-Biodegradation
W Zone
/ Water ^
Table F3
Saturated Zone
GROUNDWATER FLOW
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
• 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 and sumps. Also,
intact concrete has appreciable permeability to 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).
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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 into or out of
(beneath) a building. Heating and cooling systems can create differential pressures inside the
building. On the one hand, when the pressure inside the building is lower than the pressure in
the subsurface, vapors are drawn into the building. On the other hand, when the pressure
inside the building is greater than the pressure in the subsurface, air within the building may be
forced into the subsurface causing some degree of reoxygenation. 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; and Robinson, Sextro, and Riley, 1997;
Luo, et al., 2009; and Hong, Holton, and Johnson, 2012).
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). Microorganisms that degrade PHCs are widely
distributed in the environment and most are recognized as having some ability to metabolize
hydrocarbons (Gale, 1951; Ward, Singh, and Van Hamme, 2003; Prince, 2010). Although most
microbes only degrade a narrow range of organic compounds, they typically exist as a mixed
consortium that collectively can biodegrade a wide range of organic compounds (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). Biodegradation progresses through stages with different microbes being
predominant until environmental conditions (e.g., availability of specific hydrocarbons,
micronutrients, electron acceptors) become unfavorable for them and different microbes then
take over. Thus, aerobic and anaerobic microbes may coexist with one class essentially
dormant while the other is active. The biodegradability of PHCs often reduces the potential for
human exposure from PVI (McHugh, et al., 2010; EPA, 2012; Interstate Technology &
Regulatory Council [ITRC], 2007). 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).
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 is somewhat less biodegradable (Marchal, et al., 2003). 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).
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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).
• Trimethyl benzenes (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 (for example, 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
anaerobes grow slower than their aerobic counterparts (Widdel, Knittel, and Galushko, 2010).
Instead of water and carbon dioxide, anaerobic biodegradation of PHCs can produce methane
(Zengler, et al., 1999), especially with a release of an ethanol-blended gasoline (Jewell and
Wilson, 2011; Ma, et al., 2012). 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.
• Temperature
• pH
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.
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 regulated UST site. At
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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. Any other phenomena (e.g., climatological conditions)
that may influence the transport of contaminants and potentially impact the safety and health
of nearby residents should also be investigated.
Vapors emanating from dissolved-phase sources are primarily benzene, toluene, ethylbenzene,
xylenes (BTEX) and other aromatic hydrocarbons, and relatively water-soluble PHCs. Vapors
emanating from LNAPL sources contain the same constituents plus a sizeable fraction of
aliphatic and relatively insoluble hydrocarbons, especially if the source is large or unweathered
(Lahvis, et al., 2012; EPA, 2012). Analyses of samples collected during site characterization (see
Section 3) will provide information on specific contaminants that may warrant assessment for
potential vapor intrusion.
Special Considerations
Several factors may affect the vapor intrusion pathway. They include: biodegradation, chemical
transformation, sorption, contaminant source depletion, geologic heterogeneity (including
preferential transport pathways), soil properties (moisture content, permeability, organic
carbon content), building properties (basements, sumps, cracked slabs), meteorological
conditions, and building ventilation rates (Hers, et al., 2003). In particular, the age and volume
of release should be determined or estimated. For large volume releases and more recent
releases, there is greater potential for PVI because the source is less weathered and therefore
contains a higher percentage of more volatile compounds. Newer releases of ethanol-blended
gasoline may result in generation of methane. Large volume releases may require a greater
separation distance for biodegradation to be effective.
Other factors, such as exceptionally dry soils, areas of extensive impervious paving, and the
presence of preferential transport pathways, may reduce the potential for biodegradation of
PHC vapors and may warrant additional investigative steps (e.g., collection of soil vapor
samples).
Preferential transport pathways may be either natural (e.g., fractures in rock, solution channels
in karst terrain) or man-made (e.g., utility corridors). Because they increase the speed at which
the contaminants move through the subsurface, they 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.
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Recommended Steps For Investigating The Potential For PVI
OUST recommends these actions for situations in which EPA, states, and tribes are either
undertaking PVI investigations and corrective action 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
Many 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 and/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 respective health-based concentrations. PHC odors are a nuisance and may trigger the
need for abatement and/or mitigation even if the concentration in indoor air is below acute or
chronic health-based levels.
In confined spaces, PHC vapors, including 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 in basement sumps or floors. Building occupants may need to be
evacuated until the threat from fire or explosion has been mitigated. Since methane is
odorless, EPA recommends that monitoring devices be used if methane is suspected.
Conduct a site characterization10 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), focus attention on determining whether there is a long-term threat to
human health and the environment from intrusion of petroleum vapors. Site characterization
and CSM development provides 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;
9 In the case of a suspected or confirmed release from a regulated UST system, Subparts E and F of 40 CFR 280
requires owners and operators to investigate, report, and perform corrective action (including recovery of LNAPL
to the maximum extent practicable) if contamination is present, and submit timely reports of activities and
findings to the implementing agency.
iaThe term site characterization is used throughout this document for consistency. The term 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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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. See Section 3 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 directions changes, contaminant plume
migrates beyond the lateral inclusion zone, development or redevelopment of nearby
properties).
• Preferential transport pathways (e.g., utility corridors, fractured rock, solution channels
in karst) are present.
• 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 — less than 2 percent
soil moisture — in arid areas) such that biodegradation is insufficient to mitigate the
threat of PVI.
In such instances, additional investigation (e.g., soil vapor sampling11) may be warranted to
more fully evaluate the risk from PVI. See Section 4 for more information on delineating a
lateral inclusion zone.
S Identify preferential transport pathways
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:
11 Bulk soil samples should be analyzed for Total Petroleum Hydrocarbon (TPH) and BTEX (plus any other potential
contaminants). Soil vapor samples should be analyzed for oxygen, carbon dioxide, PHCs, 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.
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Natural
Man-made
• gravel lenses and channels
• solution channels in karst terrain
• bedding planes and weathered surfaces
• fractures, joints, and faults
• utility corridors
• trenches
• sumps and drainage pits
• other types of excavations
Preferential transport pathways increase the speed at which contamination moves through the
subsurface such that contaminants are not biodegraded by the time they reach receptors. They
can also allow atypical movement, which in some cases may be opposite groundwater flow
(ITRC, 2007).
S Determine vertical separation distances
Some buildings within the lateral inclusion zone will overlie 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 is the depth of
clean, biologically active soil between a contaminant mass and the lowest point of an overlying
receptor (building basement, foundation, slab, or crawl space). 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 be
adequately mapped in the subsurface, and the nature and characteristics of the contamination
should be determined.
Determine Whether Further Investigation Is Unnecessary. EPA (2013a) analyzed petroleum
vapor source data and soil gas data from a number of leaking UST sites across the United
States. The findings of the report indicate that 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.
Further Investigation Is Warranted. To rule out the potential for PVI, EPA recommends sub-
slab sampling for PHCs, oxygen, carbon dioxide, methane, and any fuel additives when either
condition exists:
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• A building overlies LNAPL and the vertical separation distance is less than 15 feet.
• A building overlies dissolved PHC contamination and the vertical separation distance is
less than 6 feet.
The purpose of the sampling is to demonstrate that biodegradation of PHCs and volatile organic
compounds (VOCs) has attenuated vapor concentrations to acceptable levels.
EPA's recent empirical analysis (EPA, 2013a) of a database containing soil vapor and source
concentration data recognizes that there are a number of sources of uncertainty that may
justify a greater vertical separation distance in some cases. These sources of uncertainty
include:
• Influence of methanogenesis on oxygen demand (especially for 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., wet surface clay
underlain by coarse-grained soils).
• Limited knowledge of vapor attenuation behavior in fractured rock.
• Limited soil vapor 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 lead scavengers, ethylene dibromide (EDB) and 1,2-
dichloroethane (1,2-DCA).
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. If the potential for PVI cannot be
ruled out based on analysis of exterior soil vapor and bulk soil samples, then EPA recommends
sub-slab vapor sampling to demonstrate that PVI is not of potential concern.
EPA recommends indoor and outdoor ambient air sampling in cases where sub-slab vapor
samples indicate the potential for indoor air concentrations from PVI to exceed applicable
human health thresholds.12 Guidance for collecting and analyzing sub-slab vapor samples and
ambient indoor and outdoor air samples is beyond the scope of this policy document, but is
provided in other documents (ITRC [2007] and EPA [2013b]).
12 The federal UST program does not prescribe human health values for any contaminants. However, EPA provides
such information that may be applicable in some instances. For example, the OSWER Final Guidance for Assessing
and Mitigating the Vapor Intrusion Pathway From Subsurface Sources To Indoor Air (EPA, 2013b) provides health
based screening levels for a variety of contaminants.
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Information on historic concentrations of background13 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, 2011). The objective of
this compilation is to illustrate the ranges and variability of VOC concentrations in indoor air
during the study period (1990-2005), resulting from sources other than vapor intrusion. To
determine if subsurface sources are responsible for indoor air contamination, EPA recommends
distinguishing between PHCs arising from vapor intrusion versus background sources. ITRC
(2007) and EPA (2013b) provide information on background sources, techniques, and methods
to account for background contributions to indoor air concentrations.
See the following sections for more information on the factors discussed in the paragraphs
above:
• Section 4 on the lateral inclusion zone.
• Section 5 on the vertical separation distance.
• Section 6 on LNAPL.
• Section 7 on groundwater flow and dissolved contaminant plumes.
• Section 8 on soil vapor profile.
• Section 9 on clean, biologically active soil.
S Mitigate petroleum vapor intrusion, as appropriate
If contaminant concentrations represent a potential threat of fire or explosion, EPA
recommends that active mitigation measures be immediately initiated. Likewise, if indoor air
sampling indicates that PVI is occurring, then mitigation and/or remediation is recommended.
ITRC (2007) and EPA (2013b) provide information on mitigation and remediation of vapor
intrusion. In addition, the source of contamination should be remediated following Subpart F
of the Federal Regulations (§280.60 through 280.67) (Federal Register, 1988). In particular,
§280.64 requires the recovery of LNAPL to the maximum extent practicable.
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Intrusion into a Building with a Combined Crawl Space and Basement. Environmental
Science and Technology 41(14):4993-5001.
Reinhard, M., G.D. Hopkins, E. Steinle-Darling, and C.A. LeBron. 2005. In situ biotransformation
of BTEX compounds under methanogenic conditions. Ground Water Monitoring and
Remediation 25(4):50-59.
Siddique, T., P.M. Fedorak, and J.M. Foght. 2006. Biodegradation of short-chain n-alkanes in oil
sands tailings under methanogenic conditions. Environmental Science and Technology
40(17) :5459-5464.
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.
Solano-Serena, F., R. Marchal, M. Ropars, J.-M. Lebeault, and J.-P. Vandecasteele. 2003.
Gasoline and oil biodegradation. Applied and Environmental Microbiology 86(6):1008-
1016.
Uhler, A., K. McCarthy, S. Emsbo-Mattingly, S. Stout, and G. Douglas. 2010. Predicting Chemical
Fingerprints of Vadose Zone Soil Gas and Indoor Air from Non-Aqueous Phase Liquid
Composition. Environmental Forensics ll(4):342-354.
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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 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).
LNAPL sources will tend to be distributed above 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 they exist below the
capillary fringe which serves as a barrier to vapor transport (Golder 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
A few confirmed occurrences of PVI at petroleum sites are reported in the literature (EPA,
2013). Davis (2009) and McHugh, et al. (2010) both observe that there are no reported cases of
vapor intrusion from dissolved-phase petroleum hydrocarbon sources vertically separated from
building foundations in the literature. 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.
Assessment
Recommended steps for investigating PVI are discussed in Section 1. The screening criteria in
the assessment and investigation 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
permutations 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.
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a) LNAPL SOURCE
UNSATURATEDZONE
* * high mass ^ *
lV l""
T £ CAPILLARY ZONE (
SATURATED ZONE
r"PHCs\
S.
sharp
reaction
front
constituent
distributions
b) DISSOLVED-PHASE SOURCE
UNSATURATEDZONE
limited mass
flux
CAPILLARYZONE
SATURATEDZONE
PHCs 1
Sa-
LEGEND
LNAPL (free- or
resicfual-phase)
Dissolved phase
~
Water Table
\
PHC Vapors
sharp
• reaction
front
constituent
distributions
Figure 3. Difference In Potential For PVI Based On Type Of Source:
a) LNAPL b) Dissolved Phase (source: Lahvis, et al., 2012)
Special Considerations
While biodegradation may reduce the potential for human exposure to petroleum vapors, its
effectiveness in mitigating PVI may be limited by:
• Migration of contaminants, especially plumes in flowing groundwater.
• Presence of preferential transport pathways, such as fractures and solution channels in
karst,
• Extensive impermeable surface cover, and/or very large buildings, that may reduce the
atmospheric oxygen flux to the subsurface.
• 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.
Recommendation
EPA recommends conducting an adequate PVI investigation and following the steps described
in Section 1 to determine which buildings may be at risk for PVI.
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r ¦ - J
Figure 4. Typical Scenarios Of Potential PVI Sources And Potential Receptors
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Table 2. Summary Of Characteristics Of Typical Scenarios Of
Petroleum Vapor Sources And Potential Receptors.
Scenario
(as illustrated
in Figure 4)
Contamination
Beneath Building?
(building is within
lateral inclusion zone)
Potential
For PVI
Sub-Slab Vapor
Sampling
Recommended?
A
Yes; residual LNAPL
High
Yes, if vertical separation
distance is less than 15
feet, otherwise No
B
Yes; residual including
smear zone, LNAPL,
dissolved in
groundwater
High
Yes, if vertical separation
distance is less than 15
feet, otherwise No
C
Yes; smear zone,
LNAPL, dissolved in
groundwater
Medium
Yes, if vertical separation
distance is less than 15
feet, otherwise No
D
Yes; dissolved in
groundwater
Low
Yes, if vertical separation
distance is less than 6
feet, otherwise No
E
Maybe; plume may be
diving beneath water
table
Low -
None
Yes, if vertical separation
distance is less than 6
feet, otherwise No
F
No
None
No
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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.
EPA . 2013. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
Golder 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.
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. 2012 (in press). Vapor Intrusion
Screening Criteria for Application at Petroleum UST Release Sites. Groundwater
Monitoring and Remediation.
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.
Additional Information
Davis, R.V. 2009. Update on recent studies and proposed screening criteria for vapor-intrusion
pathway. LUSTLine Bulletin 61:11-14.
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.
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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://api-
ep.api.org/environment.
Roggemans, S. 1998. Natural Attenuation of Hydrocarbon Vapors in the Vadose Zone. 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 lines of evidence into a three-
dimensional picture of site conditions that illustrates contaminant distributions, release
mechanisms, migration routes, exposure pathways and, potential receptors (EPA 2012; ITRC,
2007). The CSM uses a combination of text and graphics to portray both known and
hypothetical information (EPA, 2011). The CSM documents current site conditions and is
supported by maps, cross-sections, and site diagrams that illustrate human and environmental
exposure through contaminant release and migration to potential receptors (EPA, 1996a).
Frequently, a CSM may be presented as a site map and/or developed as a flow diagram which
describes potential migration of contaminants to site receptors (EPA, 1995). The CSM
synthesizes data acquired from historical research, site characterization, and remediation
system operation.
Importance
At any leaking UST site, it is important to have a thorough understanding of the full extent and
location of contamination, the characteristics of the site that influence contaminant migration,
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. In
addition, remedial action costs are influenced by the quality of the CSM (EPA, 1996b).
Assessment
A primary objective of site characterization is delineation of the aerial and vertical extent of
contamination in the subsurface. This includes changes in plume boundaries, geochemical
parameters that affect biodegradation, and contaminant mass and/or concentration.
All information and data about the site should be integrated into a CSM. EPA's guidance on
conceptual site models lists six basic activities associated with developing a CSM:
• Identification of potential contaminants.
• Identification and characterization of the source(s) of contaminants.
• Delineation of potential migration pathways through environmental media, such as
groundwater, surface water, soils, sediment, biota, and air.
• Establishment of background levels of contaminants and areas of contamination for
each contaminated medium.
• Identification and characterization of potential receptors.
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• Determination of the limits of the study area or system boundaries.
EPA recommends that the CSM include a diagrammatic or schematic presentation that relates
the source of contamination to receptors and identifies all potential sources of contamination,
the potentially contaminated media, and exposure pathways. 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). Another important use of the CSM is
to identify data gaps and locations from which additional information and data should be
gathered. As new information and data become available, the CSM should continually be
refined (EPA, 1993; ITRC, 2007). ITRC (2007) and EPA (2013) provide additional guidance for
developing CSMs for vapor intrusion.
Special Considerations
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
An adequate site characterization is the key factor in making informed decision. Subparts E and
F in 40 CFR 280.50 through 280.67 establish the regulatory foundation for, and describe the
fundamental components of an adequate site characterization. Specifically:
• §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
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
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following: (1) Data on the nature and estimated quantity of release; (2) Data from available
sources and/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. . ."
• §280.65(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
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."
When the information and data described above have been collected, EPA recommends that
they be integrated into a CSM and used as the basis for making informed risk management
decisions about the site and the threat posed by PVI to nearby receptors.
References Cited
ASTM. 2008. Standard Guide for Developing Conceptual Site Models for Contaminated Sites.
E1689-95. ASTM International, West Conshohocken, Pennsylvania.
EPA. 1993. Guidance for Evaluating the Technical Impracticability of the Ground-Water
Restoration. Publication 9234.2-25
EPA. 1995. Superfund Program Representative Sampling Guidance. OSWER Directive 9360.4-
10 (EPA 540-R-95-141).
EPA. 1996a. Soil Screening Guidance: User's Guide. Publication 9355.4-23 (EPA 540-R-96-018).
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).
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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).
EPA. 2013. OSWER Final Guidance for Assessing and Mitigating the Vapor Intrusion Pathway
From Subsurface Sources To Indoor Air (EPA xxx-xx-xx-xxx).
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.
ITRC. 2007. Vapor Intrusion Pathway: A Practical Guideline. Interstate Technology and
Regulatory Council, Vapor Intrusion Team, Washington, D.C. (January).
Additional Information
EPA. 2003. Improving Decision Quality: Making The Case For Adopting Next-Generation Site
Characterization Practices (EPA 542-F-03-012).
EPA. 2004. Improving Sampling, Analysis, and Data Management for Site Investigation and
Cleanup (EPA 542-F-04-001a).
EPA. 2008. Triad Issue Paper: Using Geophysical Tools to Develop the Conceptual Site Model
(EPA 542-F-08-007).
EPA. 2010. Innovations in Site Characterization, Streamlining Cleanup at Vapor Intrusion and
Product Removal Sites Using the Triad Approach: Hartford Plume Site, Hartford, Illinois
(EPA 542-R-10-006).
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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. Determining the lateral distance
beyond which buildings and other structures may not be threatened by potential PVI is site-
specific. In general, as the level of confidence in the site characterization and CSM increases,
the distance the lateral inclusion zone extends outward from sources of PHC vapors decreases.
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).
Further assessment may be unnecessary for those buildings outside the inclusion zone. 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 can reliably be
excluded from consideration for further evaluation of PVI potential; which sites might need
additional site characterization; and which sites should definitely be assessed further for PVI.
All buildings directly over the contamination, whether LNAPL or the dissolved phase, are
considered to be within the lateral inclusion zone.
Assessment
An investigation for PVI potential is not separate from the normal response to a confirmed UST
release, which requires an adequate site characterization in order to construct an accurate CSM
(see Section 3). Through this investigation, the full extent and location of contamination must
be determined (per 40 CFR 280.65(a)) so that lateral and vertical separation distances can be
accurately determined. 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 outward and downgradient investigation should continue until
the full extent and location of contamination is determined. Because this process may progress
in phases, it may be necessary to assess nearby buildings for PVI before site characterization is
complete, when there is still significant uncertainty regarding the full extent and location of
contamination.
Groundwater elevations fluctuate which may result in changes in the direction and velocity of
groundwater flow and changes in the thickness of the vadose zone. These fluctuations may
change the vertical separation distance between a potential receptor and source of PHC vapors.
The vertical separation distance is measured from the lowest point of the overlying foundation,
basement, or slab, and the historic high water table elevation. Both mobile LNAPL and
dissolved contaminant plumes are dynamic and may move from one monitoring event to the
next. As discussed in Section 2, periodic monitoring of groundwater flow directions and plume
migration are needed, possibly over more than one annual cycle. See Section 7 for additional
information on groundwater flow and dissolved contaminant plumes.
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Lateral separation distance is schematically depicted in Figure 5. In general, the length of the
lateral separation distance is on the same scale as the vertical separation distance (EPA, 2013a;
ITRC, 2007). However, the lateral boundaries of the plume are more difficult to accurately
delineate, in part because plumes migrate down gradient, thus a greater lateral distance is
generally warranted (EPA, 2013a).
Lateral Separation
Distance
{ust)
VadoseZone
Vapors
Residual- or Free-Phase LNAPL
Dissolved-Phase
Water V
Table
Saturated Zone
Figure 5. Lateral Separation Distance Between Source
Of PHC Contaminants 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.
As discussed previously, it is important to consider whether preferential transport pathways are
present and could facilitate the migration of petroleum vapors. If the transport of vapors from
the source area to the building could occur along preferential transport pathways, such as
utility conduits, then vapor sampling inside the utility conduits, including manholes and sumps,
should be considered in addition to vadose zone and sub-slab soil vapor sampling. Specific
guidance for utility sampling is beyond the scope of this document. However, consideration
For additional guidance, see A Practical Strategy for Assessing the Subsurface Vapor-to-Indoor Air Migration
Pathway at Petroleum Hydrocarbon Sites. API Publication 4741, November 2005. Accessible at
http://www.api.org/ehs/groundwater/NAPL/soilgas.cfm.
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should be given to field instrument screening at utility access points as an initial step to
determine if the utility is acting as a conduit for vapors. The CSM should establish whether it
appears that vapor migration is taking place along the utility backfill or if there is actual vapor
transport inside the utility itself. 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 (American Petroleum
Institute (API), 2005).
Another consideration is changing site conditions. Factors to consider in deciding whether to
exclude sites from further evaluation of PVI may include future land use, construction of utility
trenches through or near previous contamination, increased groundwater usage, and additional
releases of contaminants.
Recommendation
One approach to address the uncertainty is to delineate a lateral inclusion zone that is
based on an adequate site investigation and CSM. EPA's Office of Research and Development
recently published an Issue Paper describing one such approach: 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). If it is determined that inhabited buildings and/or future buildings are not
located in the lateral inclusion zone, the vapor intrusion pathway may be considered
incomplete and no further consideration of the pathway should be necessary.
References Cited
API. 2005. A Practical Strategy for Assessing the Subsurface Vapor-to-lndoor Air Migration
Pathway at Petroleum Hydrocarbon Sites. API Publication 4741.
EPA. 2013a. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510 R-13-001).
EPA. 2013b (in press). 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-12-xxx).
Interstate Technology & Regulatory Council (ITRC). 2007. Vapor Intrusion Pathway: A Practical
Guideline. Interstate Technology and Regulatory Council, Vapor Intrusion Team,
Washington, D.C. January. http://www.itrcweb.org/Documents/VI-l.pdf.
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5. Vertical Separation Distance
Description
The vertical separation distance is the thickness of clean, biologically active soil (see Section 9)
between the highest vertical extent of a contaminant source and the lowest point of an
overlying building. This lowest point could be a basement, foundation, or slab.
Importance
Aerobic biodegradation will degrade vapor-phase contamination before it intrudes into
buildings if the thickness of clean, biologically active soil is sufficient and oxygen is present. EPA
(2013) presents a compilation and analysis of soil vapor data from a large number of sites that
represent many different hydrogeologic settings where gasoline was released from USTs.15 This
analysis builds on the work of Davis (2009, 2010, 2011a, and 2011b). In addition, EPA (2013)
summarizes the results of a number of parallel efforts (Lahvis, et al., 2012; Peargin and
Kolhatkar, 2011; Wright, 2011, 2012) using somewhat different data sets. There is a high
degree of consistency among these studies. This consistency enables determination of a
vertical separation distance based on whether 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, the necessary separation distance between receptors and LNAPL is greater than the
necessary separation distance between dissolved sources and receptors.
In most situations, EPA (2013) finds the vertical separation distance of 5.4 feet from dissolved
sources and 13.5 feet for LNAPL sources adequate to eliminate the potential for PVI. Although
these distances are believed to be protective in most environmental settings, there are some
site-specific factors (e.g., preferential transport pathways, low soil moisture, large areas of
impervious paving) that may necessitate further consideration. Because of the difficulty in
accurately measuring precise distances to contamination under field conditions, EPA
recommends vertical separation distances of 6 feet for dissolved and 15 feet for LNAPL sources.
These separation distances are schematically depicted in Figure 6a and 6b, respectively.
15 EPA contracted for a thorough QA/QC evaluation of the data in this database and to then analyze the data and
prepare a report on the findings. Subsequently, EPA contracted for an independent, external peer review of the
report in accordance with OMB's Final Information Quality Bulletin for Peer Review/EPA's Peer Review Handbook.
The final report (EPA, 2013) addresses the peer review comments received. The report, database, and peer review
record are accessible on EPA's PVI Compendium Web page at http://www.epa.gov/oust/pvi/index.htm.
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1
~ S
VadoseZone
p o rs
t
>6 ft
Water \7
*
^Residual- or Free-Phase LNAP^^^
k *
Saturated Zone
Table
(a) Vertical separation distance for dissolved-phase source of PHCs.
VadoseZone
>15 ft
Vapors
Residual-or Free-
Phase LNAPL
Water
Table S
Dissolved-Phase
Saturated Zone
(b) Vertical separation distance for gasoline- LNAPL (residual or liquid phase) source of PHCs.
Figure 6. Vertical Separation Distances Between Source Of PHC Contaminants
And Hypothetical Receptor: (a) Dissolved Source, (b) LNAPL source
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Assessment
As part of a normal leaking UST investigation, the potential contaminants of concern should be
identified. Although benzene, toluene, ethylbenzene, and the three isomers of xylene (BTEX)
represent the group of aromatic hydrocarbons that receive the most attention at typical leaking
UST sites, they are not the only compounds that may be a threat to human health. There are
three other classes of PHCs in gasoline: paraffins, olefins, and naphthenes. Some of the PHCs in
these classes may also present a potential risk of PVI. In addition, gasoline also contains
synthetic additives16 such as MTBE and tertiary-butyl alcohol (TBA). Vapors emanating from
gasoline sources (either dissolved-phase or LNAPL) will contain many of these compounds; any
or all of which may be risk-drivers. Dissolved sources will be comprised primarily of more
soluble compounds while LNAPL sources will contain a sizeable fraction of aliphatic and
relatively insoluble hydrocarbons (e.g., naphthalene), especially if the source is large or un-
weathered (Lahvis, et al., 2012; EPA, 2013). The presence of 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.
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.
Indirect evidence includes high concentrations of benzene and other PHCs, often measured as
TPH. For more information on indirect evidence for LNAPL, see Section 6.
In many situations further investigation for PVI may be unnecessary:
• For low levels of soil contamination (clean soil; i.e., LNAPL is not present as mobile or
residual material) or groundwater contamination:
o Groundwater contamination is less than or equal to 30 mg/L TPH (gasoline) or
benzene is less than or equal to 5 mg/L; or
o Soil contamination is less than or equal to 250 mg/kg TPH (gasoline) or benzene less
than or equal to 10 mg/kg, and
o The vertical separation distance between contamination and the lowest point of a
building foundation, basement, or slab is 6 feet or more.
• For high levels of soil or groundwater contamination (i.e., LNAPL is present):
o Groundwater contamination is greater than 30 mg/L TPH (gasoline) or benzene is
greater than 5 mg/L; or
o Soil contamination is greater than 250 mg/kg TPH (gasoline) or greater than 10
mg/kg benzene; and
16 At older sites, where leaded gasoline was released to the subsurface, the lead scavengers ethylene dibromide
(EDB) and 1,2-DCA 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).
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o The vertical separation distance between contamination and the lowest point of a
building foundation, basement, or slab is greater than 15 feet.
Special Considerations
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. 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 vapor 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 beyond
the scope of this document, but more information is available in A Practical Strategy for
Assessing the Subsurface Vapor-to-lndoor Air Migration Pathway at Petroleum Hydrocarbon
Sites (API, 2005).
Recommendation
EPA recommends using the criteria in Table 3 to determine the necessary vertical separation
distance between contamination and an overlying building foundation, basement, or slab.
These vertical separation distances are consistent with the findings presented in EPA (2013) and
consider the practicalities of determining precise depths below ground surface and collecting
samples during the course of conducting field work.
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Table 3. Recommended Vertical Separation Distance Between Contamination And Building
Foundation, Basement, Or Slab.
Media
Benzene
TPH
Vertical
Separation
Distance (feet)*
Soil
(mg/kg)
<10
>10 (LNAPL)
<250
>250 (LNAPL)
6
15
Groundwater
(mg/L)
<5
>5 (LNAPL)
<30
>30 (LNAPL)
6
15
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). Investigators
may have different experiences with LNAPL indicators and may use them as appropriate. Direct
indicators of LNAPL also apply; these include measurable accumulations of free product, oily sheens,
and saturated bulk soil samples. For more information, see API (2000).
*The vertical separation distance represents the thickness of clean (TPH < 100 mg/kg), biologically active
soil between the source of PHC vapors (LNAPL, residual LNAPL, or dissolved PHCs) and the lowest
(deepest) point of a receptor (building foundation, basement, or slab). EPA recommends that sub-slab
sampling be conducted to evaluate the risk of PVI whenever contamination above the specified
threshold is present in any sample and the distance between the contamination and an overlying
building is less than these vertical distances. If the potential for PVI cannot be ruled out based on sub-
slab vapor sampling, then EPA recommends indoor air sampling.
References Cited
API. 2000. Non-Aqueous Phase Liquid (NAPL) Mobility Limits in Soil. Soil and Groundwater
Research Bulletin No. 9.
API. 2005. A Practical Strategy for Assessing the Subsurface Vapor-to-lndoor Air Migration
Pathway at Petroleum Hydrocarbon Sites. API Publication 4741. Accessible at:
http://www.api.org/environment-health-and-safety/clean-water/ground-water/vapor-
intrusion/vi-publications/assessing-vapor-intrusion.aspx.
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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 .ngwa. org/gwol/pdf/910155295. PDF.
Davis, R.V. 2009. Update on recent studies and proposed screening criteria for vapor-intrusion
pathway. LUSTLine Bulletin 61:11-14.
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.
EPA. 2013. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
Lahvis, M.A., I. Hers, R.V. Davis, J. Wright, and G.E. DeVaull. 2012 (in press). Vapor Intrusion
Screening Criteria for Application at Petroleum UST Release Sites. Groundwater
Monitoring and Remediation.
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.
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.
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https://iavi. rti.org/WorkshopsAndCo nferences.cfm?PagelD=documentDetails&Attach ID
=549.
Additional Information
DeVaull, G. 2007. Indoor vapor intrusion with oxygen-limited biodegradation for a 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.
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 PHCs and synthetic additives. Among
these compounds are some that are volatile and some that are semi-volatile. Newer releases
(especially gasoline), which have not been subject to weathering for any appreciable length of
time, will generate more vapors than older releases since they will contain a higher proportion
of more volatile PHCs. 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, especially if the source is large or un-weathered (Lahvis, et
al., 2012; EPA, 2013).
Importance
Depending upon the volume of the release and the characteristics of the soil, hydrocarbon
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 itself 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 hydrocarbons 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, coupled with water-filled porosity, 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 can generate considerable volumes
of petroleum vapors as well as dissolved-phase contaminants.
Assessment
The distinction between the LNAPL and dissolved phases is important, though the precise
threshold is difficult to pinpoint.17 As a somewhat conservative estimate, 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 was adopted based on the calculated approximate average ratio of
17 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 Non-Aqueous Phase Liquid (NAPL) Mobility Limits in Soil (API,
2000).
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the concentration of benzene to TPH in groundwater at UST sites in the PVI database. 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 and 250 mg/kg TPH (gasoline). Direct
means for detecting the presence of LNAPL also apply. These would include measurable
accumulations of free product in monitoring wells, an oily sheen on the water, and saturation
of bulk soil samples.18
Special Considerations
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.
Monitoring wells with residual LNAPL (above 30 mg/L TPH and/or 5 mg/L benzene) 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., 2012). This situation is
depicted in Figure 7.
Recommendation
As part of site characterization and CSM development, EPA recommends analyzing bulk soil
samples collected in the source area for TPH and specific petroleum constituents (e.g., BTEX
and other volatile and semi-volatile organic chemicals, and fuel additives). It may be prudent to
collect and analyze samples of LNAPL. This information, in addition to providing useful
information for assessing PVI potential (such as determining whether the LNAPL has been
degraded), can also inform decision making related to subsurface source remediation and risk
management.
EPA recommends that the full extent and location of LNAPL (both mobile LNAPL and residual)
be determined through subsurface sampling as part of site characterization and CSM
development. LNAPL may be present even when there is no measureable accumulation of free
product in a monitoring well. In addition, federal regulations (40 CFR280.64) 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
contaminant vapors as well as dissolved and residual LNAPL contamination.
18 Consistent with the findings in EPA (2013) EPA recommends that these same thresholds also be applied for PVI
investigations conducted using this guidance.
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ACTTHESAME
LOOK THE SAME
MW
UNSATURATED
ZONE
te.iv.
¦(
PILLARY ZONEf
SATURATED
ZONE
q) free-phase
LNWPl source
MW
UNSATURATED
ZONE
I (CAPILLARY Z0NE(
SATURATED
ZONE
b) residual-phase
LNAPL source
MW
UNSATURATED
ZONE
*
CAPILLARY ZONE
i
SATURATED
ZONE
c) dissolved-phase
source
LEGEND
LNAPL (free- or
residual-phase)
Dissolved
phase
V
Water
Table
\
PHC
I
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., 2012)
References Cited
API. 2000. Non-Aqueous Phase Liquid (NAPL) Mobility Limits in Soil. Soil arid Groundwater
Research Bulletin No. 9.
EPA. 1996. How To Effectively Recover Free Product At Leaking Underground Storage Tank
Sites: A Guide For State Regulators. OUST (EPA 510-R-96-0Q1).
EPA. 2013. Evaluation Of Empirical Data Studies To Support Soil Vapor Intrusion Screening
Criteria For Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
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.
Lahvis, M.A., I. Hers, R.V. Davis, J. Wright, and G.E. DeVaull. 2012 (in press). Vapor Intrusion
Screening Criteria for Application at Petroleum UST Release Sites. Groundwater
Monitoring and Remediation.
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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, S.T. 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 cases where
there are high concentrations of dissolved contaminants and/or the plume is in direct contact
with a building or other structure.
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 river stage.19 Groundwater flow directions
can and often change over time, and may require periodic sampling over more than one annual
cycle to understand the groundwater flow regime at a given site. (Note: This should 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 protected. If new impacts 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.
19
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, and was particularly strong over a range of 90
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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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.
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 overtime.
This is best evaluated by determining the range of groundwater fluctuations present at the site
over at least one annual cycle. However, in the interim, the remaining PVI-related activities
should continue. Preferential pathways, if present, may facilitate the intrusion of petroleum
vapors into the building. The spread of contamination can be very rapid compared to the
velocity of groundwater flow through the soil when preferential transport pathways intersect
contaminant plumes.
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, residents may still be exposed
to potentially harmful levels of contaminants. Exposure occurs when PHCs volatilize from the
dissolved phase during showering, washing, or ingesting contaminated water. Identifying the
mechanism of exposure is important because methods for remediation/mitigation of PVI will be
different than treatment/remediation of contaminated groundwater.
Volatilization of contaminants from the plume into soil vapor is greatly reduced when a plume
dives beneath the water table surface. Volatile contaminants are slower to diffuse through the
water column than through soil gas.
Recommendation
EPA recommends groundwater monitoring and sampling to determine the depth to
contaminated groundwater in relation to overlying buildings and the concentration of
contaminants. Due to the transient nature of groundwater migration, periodic monitoring and
sampling over more than one annual cycle is generally needed to fully understand the
groundwater flow regime at a given site. 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). If the depth to contaminated groundwater directly below a building is less than 6 feet, EPA
recommends collecting sub-slab soil vapor samples. If the potential for PVI cannot be ruled out
based on sub-slab soil vapor sampling, then EPA recommends indoor air sampling to determine
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whether PVI is a health threat. Where there is no threat of PVI from contaminated
groundwater, EPA recommends that the plume be assessed to determine if remediation is
necessary to protect 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).
EPA. 2004b. Monitored Natural Attentuation. 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).
EPA. 2005. Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking
Underground Storage Tank Sites (EPA/600/R-04/1790).
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.
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 & 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.
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.
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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.
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 Vapor Profile
Description
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. In contrast to gasoline sources, the composition of diesel fuel leads to relatively
minimal vapor phase contamination (Prince and Douglas, 2010; Marchal, et al., 2003). In
addition to PHCs, soil vapor may also contain degradation products from the breakdown of
naturally occurring organic matter and contaminants. 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
concentrations increase with depth toward the source of contamination. With 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). The impacted (source) zone, which is anaerobic, is characterized by the
maximum PHC concentrations (and often LNAPL) and little biodegradation (EPA, 2012). PHC
vapor concentrations will generally 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. In the oxygenated soil zone
(where aerobic biodegradation occurs) the decrease in PHC concentrations is typically quite
rapid and occurs over a narrow interval (Abreu, Ettinger, and McAlary, 2009). T his profile may
vary somewhat in shape depending on site-specific conditions (Roggemans, Bruce, and
Johnson, 2002).
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). 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 (such as utility vaults and passages, basements, or
garages).
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Land Surface
A
/
Oxygenated
Soil
/
/
Impacted
Soil
PHCs + CH
I
Oxygen
Flux
PHC + CH4
Flux
Increasing Concentration
Figure 8. Typical Vertical Concentration Profile In The Unsaturated Zone For PHCs, Carbon
Dioxide And Oxygen
Importance
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 vapor profile can
provide confirmation that aerobic biodegradation is occurring in the subsurface. Decreasing
oxygen concentration and increasing carbon dioxide and methane concentrations indicate
biodegredation of PHCs (Hult and Grabbe, 1988). The potential for PVI is a function of the
oxygen demand exerted by all biodegradable vapors, not just the key chemicals of potential
concern (Jewell and Wilson, 2011; Ma, et al., 2012). When present, volatile PHCs and methane
also exert an oxygen demand that may limit the biodegradation of benzene (Abreu, Ettinger,
and McAlary, 2009; Wilson, 2011). Biodegradation rates are stoichiometrically related to the
flux of oxygen, carbon dioxide, and methane, which allows for estimation of the biodegradation
rate (Lahvis and Baehr, 1996). Vapor concentrations generally decrease with increasing
distance from a subsurface vapor source. At a relatively short distance from the source,
concentrations of PHCs become negligible primarily due to aerobic biodegradation. Lahvis,
Baehr, and Baker (1999) observed that PHCs vapors from a dissolved plume were almost
completely degraded within 1 meter above the water table and that significant transport of
PHC vapors may only be significant if the vapor source is LNAPL.
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Assessment
Soil gas samples provide information on the distribution of contamination near the source area,
whether biodegradation is occurring, and how effective it is in reducing the potential for PVI.
When there is an impermeable surface cover adjacent to a building, soil gas probes should be
installed beneath the surface in order for the soil vapor profile to adequately characterize
conditions below the surface. For very large buildings, and/or where there is extensive
impermeable surface cover, and the vapor source is relatively shallow, sub-slab vapor sampling
is recommended to verify that biodegradation is occurring beneath the building. Vapor
samples should be analyzed for PHCs, methane, oxygen, and carbon dioxide to assess
biodegradation of PHCs (Lahvis, Baehr, and Baker, 1999). Relative depletion and enrichment in
argon and nitrogen are indicators of methanogenic and methanotrophic zones (Amos, et al.,
2005).
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. If sampling for all the organic compounds, a
simple explosimeter calibrated to methane or a field methane meter equipped with an
electrochemical cell can be used. If methane and all the PHCs in soil gas are measured, these
concentrations should be converted to an equivalent concentration of benzene and summed
(see Section 12 and Figure 9). 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 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.
In some cases, relatively shallow soil gas samples (less than five feet) 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 misimpression 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).
Special Considerations
There are several factors that can limit replenishment of oxygen to deep soils. These include
presence of low permeable layers, concrete or asphalt covering at the surface, high soil
moisture from 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 (2013) indicates that for an oxygen shadow20 to form beneath a building,
the building must be very large (including the surrounding impermeable cover), and the source
20 For the purposes of this modeling study, an oxygen shadow is defined as less than 1% oxygen.
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of vapors must be highly concentrated and in relatively close proximity to the bottom of the
building.
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 vapor monitoring may be needed
to confirm that aerobic biodegradation consistently prevents PVI impacts at the site.
Recommendation
EPA recommends that soil vapor samples be analyzed for volatile PHCs, methane, oxygen, and
carbon dioxide. Sampling for nitrogen (and other fixed gases) in soil vapor 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.
EPA recommends that sub-slab soil gas samples be collected for buildings within the lateral
inclusion zone if the vertical separation distance between the building basement, foundation,
or slab is less than 6 feet for dissolved contamination. For buildings that directly overlie LNAPL
masses, and the vertical separation distance is less than 15 feet, EPA recommends sub-slab soil
vapor sampling. If the potential for PVI cannot be ruled out based on these sub-slab vapor
samples, then indoor air sampling is recommended to determine whether PVI poses a threat to
building inhabitants.
In addition, for very large buildings and/or where there is extensive impermeable surface
covering, EPA recommends that soil vapor samples be collected if there is concern that these
conditions may impede the flux of oxygen to the subsurface and create an oxygen shadow. The
oxygen content should be greater than 1 percent throughout the thickness of clean, biologically
active soil necessary for aerobic biodegradation of PHC vapors emanating from the source.
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.
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.
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.
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Atlas, R.M. 1981. Petroleum Microbiology. Microbiological Reviews 45(l):180-209.
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).
EPA. 2012. Petroleum Hydrocarbons And Chlorinated Hydrocarbons Differ In Their Potential For
Vapor Intrusion. March, http://www.epa.gov/oust/cat/pvi/pvicvi.pdf.
EPA. 2013 (manuscript in preparation). Vapor Transport Modeling Simulations to Assess the
Impact of Building Footprint on Underlying Oxygen Shadow (EPA 510-R-13-xxx).
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.
Leahy, J.G., and R.R. Colwell. 1990. Microbial Degradation of Hydrocarbons in the
Environment. Microbiological Reviews 54(3):305-315.
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.
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., 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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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.
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.
Wilson, J.T. 2011. Impact of Methane at Gasoline Spill Sites on the Potential for Vapor
Intrusion. Webinar on January 11th, sponsored by the Groundwater Resources
Association of California, Sacramento, California.
ZoBell, C.E. 1946. Action of Microorganisms on Hydrocarbons. Bacteriological Reviews
10(l-2):l-49.
Additional Information
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.
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. The oxygen demand of the
contamination present in the soil must be low enough so that aerobic microorganisms are not
inhibited from biodegrading PHC vapors in less time than it would take for vapors to migrate
from the contaminant source into a building.
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 actual habitat of soil bacteria is the thin film of water held to the surface of soil particles by
capillary attraction. EPA (2013) notes that soil moisture content greater than 2 percent is
adequate to support biodegradation activity (Leeson and Hinchee, 1996), although it 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 indigenous vegetation (Riser-Roberts, 1992).
Certain geologic materials do not qualify as biologically active soil. These geologic materials
include:
• Coarse sand and gravel with a low content of silt, clay, or organic matter.
• Fractured 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).
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).
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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.
As previous discussed in Section 5, EPA (2013) presents findings of an analysis of a large
number of vapor samples from leaking UST sites across the United Sates 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. The vertical separation distances
adequate to eliminate the potential for PVI identified in EPA (2013) are 5.4 feet for dissolved
sources, and 13. 5 feet for LNAPL sources. These distances are believed to be conservative in
most environmental settings.
Special Considerations
Coarse sand and gravel with a low content of silt, clay, organic matter, 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 optimal to support an adequate population of
biologically active microorganisms necessary to degrade PHC vapors and prevent PVI (see
Tillman and Weaver, 2007).
Recommendation
EPA (2013) recommends a threshold concentration 100 mg/kg TPH for clean soil. Except for
the geological materials identified above, 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.
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.
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The vertical separation distances described in this guidance (see Section 5) should not be used
at sites where the geologic materials listed above occur because they may not have enough soil
moisture in intimate contact with soil gas to support adequate densities of biologically active
microorganisms necessary to biodegrade PHCs.
References Cited
Atlas, R.M. 1981. Microbial degradation of petroleum hydrocarbons: an environmental
perspective. Microbiological Reviews 45(l):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.
EPA. 1999. Monitored Natural Attenuation of Petroleum Hydrocarbons. Remedial Technology
Fact Sheet (EPA/600/F-98/021).
EPA. 2002. Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from
Groundwater and Soils (EPA 530-D-02-004).
EPA. 2013. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
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.
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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.
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, C.D., 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.
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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.
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(l):37-50.
Widdel, F., A. Boetius, and R. Rabus. 2006. Anaerobic biodegradation of hydrocarbons
including methane. Prokaryotes 2:1028-1049.
Wilson, J.T., L.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. Contaminants Other Than PHCs
Description
Petroleum fuels are comprised of hundreds of compounds, both natural components of
petroleum as well as a number of synthetic organic 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., EDB
and 1,2-DCA). Non-petroleum contaminants may be from releases of substances other than
petroleum fuels (e.g., solvents). Their presence may be from prior uses of the site or as the
result of migration from off-site sources (e.g., dry cleaner, chemical plant, landfill).21
Importance
Biodegradation of many PHCs and some fuel additives (such as alcohols, ethers, organic lead,
and lead scavengers) is well recognized, and occurs under both aerobic and anaerobic
conditions.22 Specifically, aerobic biodegradation has been observed for several classes of
compounds that are, or have been, constituents of petroleum fuels, including:
• Alcohols, in particular:
o Ethanol (Powers, et al., 2001; Corseuil et. al, 1998).
o TBA (Wang and Deshusses, 2007; Landmeyer, et al., 2010).
o Methanol (Powers, et al., 2001).
• Ethers, in particular:
o MTBE (Prince and Douglas, 2010; Wang and Deshusses, 2007; Phelps and
Young, 1999; Landmeyer and Bradley, 2003; Landmeyer, etal., 2010; Bradley
and Landmeyer, 2006; Kuder, 2005; Lesser, et al., 2008; Baehr, Charles, and
Baker, 2001).
o Tertiary-amyl methyl ether (TAME) (Landmeyer, et al., 2010).
• Organic lead compounds (Prince and Douglas, 2010; Gallert and Winter, 2004).
• The lead scavengers23, in particular:
o EDB (Prince and Douglas, 2010; Pignatello, 1986).
o 1,2-DCA (Falta, 2004).
21 While these substances are not the primary focus of a petroleum UST 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. As federal
monies from the Leaking Underground Storage Tank Trust Fund cannot be used to assess or cleanup
contamination from non-UST and non-petroleum sources, should any contaminants from non-UST sources be
discovered at a site, the appropriate state and/or federal cleanup agency should be notified.
22 Although anaerobic biodegradation is slower than aerobic biodegradation, anaerobic biodegradation may be a
significant mechanism for destruction of PHCs, especially in source areas. Some selected references on anaerobic
biodegradation of various non-petroleum compounds are listed under "Additional Information" at the end of this
section.
23 For more information about EDB and 1,2-DCA see EPA (2006) and Appendix F in EPA (2013).
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Alternative fuels, especially those that contain higher percentages of non-petroleum
constituents (ethanol in particular), present a different type of vapor intrusion problem (Jewell
and Wilson, 2011; Ma, et al., 2012; Freitas, et al., 2010). On the one hand, alcohols have a
greater tendency than ethers to remain in the dissolved phase, and they readily biodegrade to
create methane. Methane generation may be more significant at sites where large volumes of
ethanol-blended gasoline (and higher ethanol content fuels) have been released into the
subsurface. As the ethanol content increases, so does the potential for creating larger volumes
of methane. 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.
On the other hand, methane also biodegrades under aerobic conditions and consumes oxygen
that otherwise could be available for the biodegradation of the PHC contaminants. The
depletion of oxygen may result in PHC vapors being transported farther than they otherwise
would be, possibly increasing the threat of PVI.
Assessment
Federal UST regulations 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" (40 CFR 280.52(b)). Results of
this sampling should also indicate which contaminants should be assessed for potential vapor
intrusion. See Section 3 for a more detailed discussion of site characterization and CSMs.
Special Considerations
The literature is relatively sparse in regard to ethanol releases at leaking UST sites in the United
Sates. Most of the releases that have been studied have been E-10 (10 percent ethanol and 90
percent gasoline), though some E-85 (85 percent ethanol and 15 percent gasoline) releases in
the Midwest have also been studied. It is anticipated that as the proportion of ethanol
increases in gasoline, the methane generation potential will also increase, though specifics are
as yet unknown. 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).
Though the use of lead scavengers and ethers (e.g., MTBE) in gasoline have 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). Ethers tend to remain in the dissolved phase as
indicated by their relatively low Henry's Law constants. Therefore, though they may be a
problem from the perspective of groundwater-pollution, they are not likely to be a common
vapor intrusion problem at leaking UST sites (McHugh, et al., 2012).
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Recommendation
EPA recommends that soil and groundwater samples be analyzed for PHCs typically found in
petroleum-based fuels, in addition to other volatile additives (e.g. ethers and alcohols-
especially ethanol) that are constituents of petroleum-based fuels. Soil vapor samples should
be collected and analyzed for PHCs, VOCs, methane, oxygen, carbon dioxide (and optionally
nitrogen), and any fuel additives. From these data, assess whether the non-PHC constituents
exert an oxygen demand that could result in less aerobic biodegradation of PHCs, and/or
present a potential vapor intrusion threat themselves.
References Cited
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.
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.
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.
EPA. 1996b. Method 8260B: Volatile Organic Compounds by Gas Chromatography/Mass
Spectrometry (GC/MS).
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.gov/Exe/ZyPDF.cgi?Dockey=P1002UTI.PDF.
EPA. 2013. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For
Petroleum Hydrocarbon Compounds (EPA 510-R-13-001).
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.
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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.
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(l):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
Journal 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 Water 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.
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.
McHugh, T.E., R. Kamath, P.R. Kilkarni, C.J. Newell, J.A. Connor, and S. Garg. 2012. Remediation
progress at California LUFT sites: insights from the Geotracker database. Soil and
Groundwater Research Bulletin No. 25; API: Washington, D.C.
Phelps, C.D., and L.Y. Young. 1999. Anaerobic biodegradation of BTEX and gasoline in various
aquatic sediments. Biodegradation 10(l):15-25.
Pignatello, J.J. 1986. Ethylene dibromide mineralization in soils under aerobic conditions.
Applied and Environmental Microbiology 51(3):588-592.
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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(l):37-50.
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, United States
Environmental Protection Agency, Washington, D.C. (EPA/600/R-05/032).
Weaver, J.W., L.R. Exum, L.M. Prieto. 2008. Gasoline Composition Regulations Affecting LUST
Sites, United States Environmental Protection Agency, Washington, D.C. (EPA/600/R-
10/001).
Weaver, J.W., S. A. Skaggs, D.L. Spidle, and G.C. Stone. 2009. Composition and Behavior of Fuel
Ethanol, United States Environmental Protection Agency, Washington, D.C. (EPA/600/R-
09/037).
Yu, S., J.G. Freitas, A.J.A. Unger, 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 105(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.
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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.
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11. Seasonal And Weather Effects
Description
The generation and movement of petroleum vapors is 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).
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 frozen soil 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 vapor and there was evidence that biodegradation was occurring
throughout the winter. However, the house was above a crawl space and the soil below the
house was never covered by ice or snow. In addition, 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. However, 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). In contrast, oxygen depletion has been observed in other studies of soils
under ice sheets and snow cover (Freyman, 1967; Yanaia 2010). 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 re-oxygenation is impeded, possibly leading to anaerobic
conditions at greater depths (Silver, 1999; Ludemann, 2000; Pezeschki, 2001). Changes in
barometric pressure can result in enhanced intrusion of PHC vapors into buildings and other
structures. Similarly, wind can create differential pressures that can accelerate intrusion of PHC
vapors into buildings. Wind and barometric pressure changes can also 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 through cracks, allowing re-oxygenation of the
soil beneath the building that would otherwise be depleted in oxygen.
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.
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Assessment
During site characterization activities, weather conditions such as temperature, barometric
pressure, and wind speed/direction should be recorded so trends or anomalies in the PVI data
may be identified and not attributed to unknown factors. This information may be obtained
from the National Oceanic and Atmospheric Administration (NOAA) or a nearby airport where
weather data are recorded hourly.
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. Groundwater levels in the vicinity of USTs are normally 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 forms beneath them. This mound disrupts the local groundwater flow field
and contaminants can migrate away from the tank excavation, potentially in all directions.
Seasonal changes in water table elevation can also create a smear zone of residual LNAPL
contamination that acts as a long-term source of dissolved contamination during periods of
high water (as in spring and fall rainy seasons) and as a source of petroleum vapors during
periods of low water (typically in the summer) when contaminants reemerge from a previously
submerged condition.
Recommendation
Seasonal and weather conditions are transient, and although typically short-lived, can influence
the characteristics of PHC vapor migration over time. Data on temporal changes in these
conditions can aid in correctly identifying the cause of trends and result in a more accurate
CSM.
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.
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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.
Ludemann, 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 & Remediation 29(1):81-91.
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.
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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 Al-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.
Web-based Resources:
U.S. Geological Survey's soil surveys http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm..
U.S. Department of Homeland Security Federal Emergency Management Agency's flood plain
maps
https://msc.fema.gov/webapp/wcs/stores/servlet/FemaWelcomeView?storeld=10001&cat
alogld=10001&langld=-l.
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
Johnson and Ettinger (1991) introduced their vapor intrusion model and 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), and thus it is
a ratio.24 The source is defined as the region of highest vapor concentration in the vadose
zone. Therefore, a values are always less than one when vapors are attenuated even if only by
a small amount. Where there is no attenuation, the a value would be equal to one.
Importance
Attenuation is the reduction in the amount of contaminants in a plume as it migrates away
from the source. The vapor intrusion attenuation factor is an inverse measurement of the
attenuation. Large a values (i.e., values approaching one) indicate that little attenuation is
taking place, whereas small a values (i.e., values much smaller than one) indicate that
significant attenuation is taking place.
Assessment
The vapor intrusion attenuation factor can be either measured, when background sources do
not contribute significantly to indoor air concentrations, or estimated, using a mathematical
model. On a building-specific basis, if both the source vapor concentration and the indoor air
concentration arising from PVI are known, calculation of a is straightforward. If the indoor air
concentration is not known, but concentration at the source is known and a suitable value for a
can be estimated, the indoor air concentration may be estimated by multiplying the measured
concentration at the source with the estimated value of a.
Abreu, Ettinger, and McAlary (2009) used a series of computer simulations to estimate semi-
generic values of a from site-specific information on the vertical separation between the
receptor building and the source, and the total concentration of biodegradable compounds in
soil gas. The simulations assume the building 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 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). In the simulations, the first order rate constant for
biodegradation of vapors (A.) was set at 0.0 h 1 (no biodegradation), or 0.079 h"1, or 0.79 h"1, or 2
h"1. The simulations assumed that the square building was 10 meters (33 feet) on each side.
24 a can be understood as the concentration in indoor air normalized to the soil gas source concentration.
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One set of simulations assumed that the building had a basement, while another set assumed
the building had a slab on grade,
From the computer simulations of a vapor migration beneath a building with a basement,
Abreu, Ettinger, and McAlary (2009) generated what is effectively a nomograph (Figure 9) from
which a can be estimated if the total concentration of vapors and the vertical separation
distance between the contaminant source and the building are known. In this particular set of
simulations, the first order rate constant (A) was set at 0.79 h"1, a reasonable average rate
based on the range of rates published in the literature (DeVaull 2007). In the example below,
for a source vapor concentration of 10 mg/L and a vertical separation distance of 2 meters (6.6
feet), the estimated value of a would be 1 x 10 ' .
l.E-02
No Biodegradation; L = 1 m
l.E-03
No Biodegradation; L = 10 m
l.E-04
L = 1 ni
l.E-05
L = 2 ill
l.E-08
l.E-09
L = 10 ill
L = 3 in
l.E-10
1 f 10) 100
Source Vapor Coffee ritration (mg/L)
A= 0.79 tr1
1000
0.1
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)
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DeVaull (2007) conducted a modeled sensitivity analysis using a broad range of specified
scenario parameters (variation of more than nine orders of magnitude) for a specified
attenuation factor of 1 x 10~8 while the corresponding vertical separation distance between the
vapor source and the building foundation varied within a factor of only three. He concluded
that the attenuation factor is much more variable due to the insensitivity of the model input
parameters and thus less robust as a screening criterion than is the vertical separation distance.
Lahvis, et al. (1999) reached a similar conclusion.
Special Considerations
The nomograph (Figure 9) should only be used for UST sites with the same conditions that were
simulated by Abreu, Ettinger, and McAlary (2009). If this method is to be used accurately on a
site-specific basis, a similar nomograph can be constructed for the soil type(s) encountered at
the site(s) in question. Also, while the assumed bioattenuation rate coefficient (i.e., 0.79 h"1)
may be appropriate for many situations, it might be optimistically high for areas of low soil
moisture, where extensive impermeable surface cover restricts oxygen flux to the subsurface,
and where the thickness of the clean, biologically active soil is thin.
Some documents define the vapor intrusion attenuation factor differently than described in this
section (and Section 13), which is the same as used by Johnson and Ettinger (1991). When used
in this PVI guidance, the Greek letter alpha (a) refers strictly to attenuation during vapor
intrusion, which might be observable if there were no background (ambient) vapor sources.
The Johnson-Ettinger model (JEM) (see Section 13) ignores background sources when
estimating the indoor air concentration arising from vapor intrusion. In contrast, some
empirical attenuation factors (sometimes designated AF) are based on indoor air
concentrations that include a contribution from background sources in addition to 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.
Recommendation
An estimated vapor intrusion attenuation factor may help support screening decisions,
although EPA recommends that it not be the sole basis for excluding sites from consideration of
potential PVI. Also, it should be noted that generation of site-specific nomographs may be
prohibitively complex and expensive and, thus, limit the usefulness of this approach.
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.
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DeVaull, G. 2007. Indoor vapor intrusion with oxygen-limited biodegradation for a subsurface
gasoline source. Environmental Science and Technology 41(9):3241-3248.
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.
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.
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. 2002. Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from
Groundwater and Soils. Office of Solid Waste and Emergency Response (EPA 530-D-02-
004).
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.
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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. 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)
The JEM was introduced in a publication by Johnson and Ettinger (1991).25 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 original JEM does not include biodegradation, although later versions have incorporated
certain aspects of biodegradation (Johnson, Kemblowski, and Johnson, 1998; Ririe, et al., 1998;
Johnson, Hermes, and Roggemans, 2000; Spence and Walden, 2001; Parker, 2003;
Environmental Systems and Technologies, 2004; DeVaull, 2007a; Mills, et al., 2007;
Turczynowicz and Robinson, 2007; API, 2010; Lahvis, 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; Golder 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 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
25 EPA has revised the original model by Johnson and Ettinger (1991) a number of times since it was first published.
The most current information on EPA's revised model may be found on EPA's web site at
http://www.epa.gov/oswer/riskassessment/airmodel/johnson_ettinger.htm
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variable component over time (Nazaroff, et al., 1985; Hintenlang and Al-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).
BioVapor
The BioVapor model (DeVaull, 2007a; API, 2010) uses a conceptual model similar to the JEM,
including 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.
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, vertical
separation distance—see Section 5). Model results are consistent with empirical exclusion
distance values derived from several PVI field investigations. These include Lahvis, et al. (2012);
Davis (2009); Peargin and Kolhatkar (2011); Wright (2011); and McHugh, et al. (2010). 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.
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. 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
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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, agreement between the three dimensional results and those
predicted with the BioVapor model (DeVaull, 2007b) are favorable. Both sets of model results
show similar sensitivities to changes in model parameters, and both support the use of
exclusion distances such as those recommended in this guidance document (see Section 4,
lateral inclusion zone, and Section 5, 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. A common limitation with computer
models 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 the site conditions. Therefore, EPA recommends
that an uncertainty analysis be conducted to provide error bounds on predictions of the
computer model. In addition, EPA recommends that predicted indoor air concentrations be
verified with field data in making a determination as to whether buildings are impacted by PVI.
Special Considerations
When evaluating the potential for PVI, consider background sources of PHCs in indoor air,
which cannot be attributed to subsurface vapor sources.
Recommendation
An appropriate framework for the use of a mathematical model and understanding of model
characteristics is needed when using the results of mathematical models for regulatory
purposes (Hers, et al., 2003). For PVI, this can include using models to improve a site-specific
sampling strategy, validation (or refutation) of concept by comparing a model to measured soil
vapor data, and in estimating the effect of varied or changed site conditions (e.g., including
construction of a new building on a brownfields site).
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.
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American Petroleum Institute (API). 2009. Simulating the Effect of Aerobic Biodegradation on
Soil Vapor Intrusion into Buildings—Evaluation of Low Strength Sources Associated with
Dissolved Gasoline Plumes. Publication No. 4775; American Petroleum Institute:
Washington, D.C.
American Petroleum Institute (API). 2010. BioVapor Indoor Vapor Intrusion Model.
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.
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.
DeVaull, G.E. 2007a. Indoor Vapor Intrusion with Oxygen-Limited Biodegradation for a
Subsurface Gasoline Source. Environmental Science and Technology 41(9):3241-3248.
DeVaull, G. 2007b. Indoor Air Vapor Intrusion: Predictive Estimates for Biodegrading
Petroleum Chemicals. Presentation at: Air and Waste Management Association
(A&WMA) Specialty Conference: Vapor Intrusion: Learning from the Challenges,
Providence, Rhode Island. September 26-28.
DeVaull, G. E. 2011. Biodegradation rates for petroleum hydrocarbons in aerobic soils: A
summary of measured data. Proceedings of Battelle 8th International Symposium on
Bioremediation and Sustainable Environmental Technologies, Reno, Nevada, June 27-30.
Energy Institute. 2009. Screening the potential for hydrocarbon vapour intrusion risks,
Petroleum Review August 2009, Energy Institute, London, United Kingdom, p. 40-42.
Environmental Systems and Technologies, Inc. 2004. VAPEX4. 3708 South Main Street, Suite D,
Blacksburg, Virginia.
Fitzpatrick, N.A. and J.J. Fitzgerald. 2002. An Evaluation of Vapor Intrusion Into Buildings
through a Study of Field Data. Soil and Sediment Contamination ll(4):603-623.
Golder Associates. 2008. Report on evaluation of vadose zone biodegradation of petroleum
hydrocarbons: Implications for vapour intrusion guidance. Research study for Health
Canada and the Canadian Petroleum Products Institute. Golder Associates Ltd.,
Burnaby, British Columbia, July.
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Haghighat, F., C.S. Lee, and W.S. Ghaly. 2002. Measurement of diffusion coefficients of VOCs
for building materials: review and development of a calculation procedure. Indoor Air
12(2): 81-91.
Hers, I., R. Zapf-Gilje, P.C. Johnson, and L. Li. 2003. Evaluation of the Johnson and Ettinger
Model for Prediction of Indoor Air Quality. Ground Water Monitoring and Remediation
23(1): 62-76.
Hintenlang, D. E. and K. K. Al-Ahmady. 1992. Pressure Differentials for Radon Entry Coupled to
Periodic Atmospheric Pressure Variations. Indoor Air 2(4):208-215.
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.
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,
Arizona.
Kobayashi, K. and K. Shuttoh. 1991. Oxygen diffusivity of various cementitious materials.
Cement and Concrete Research 21(2-3):273-284.
Lahvis, M.A.; I. Hers; R.V. Davis, J. Wright, G.E. DeVaull. 2012. Screening Criteria for Application
at Petroleum UST Release Sites, Groundwater Monitoring and Remediation [submitted,
2012],
Lahvis, M. 2011. Vapour Transport from Soil and Groundwater to Indoor Air: Analytical
Modeling Approach, Chapter 5., in Vapor Emissions to Outdoor Air and Enclosed Spaces
for Human Health Risk Assessment: Site Characterization, Monitoring, and Modeling. S.
Saponaro, E. Sezenna, L. Bonomo, Eds., Nova Science Publishers, Inc., New York. pp. 91-
112.
Laubacher, R.C., Bartholomae, P., Velasco, P., Reisinger, H.J. 1997. An evaluation of the vapour
profile in the vadose zone above a gasoline plume. Proceedings of 1997 Petroleum
Hydrocarbon and Organic Chemicals in Ground Water, Houston, Texas, November, pp.
396-409.
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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.
Environmental Science and Technology 42(15):5534-5540.
Luo, H. and P.C. Johnson. 2011. Incorporating Barometric Pressure and Wind Effects into
Vapor Intrusion Simulations, Association for Environmental Health and Sciences Conference
Session, March 16, San Diego - Petroleum Hydrocarbon Vapor Intrusion.
Luo, E.H., C. Holton, Y. Guo, and P.C. Johnson. 2012. Field and Modeling Studies of Indoor Air
Source Effects on Subslab Soil Gas Concentrations. 22nd Annual International
Conference on Soil, Water, Energy, and Air. March 19-22, 2012, San Diego, California.
McHugh, T.E., P.C. de Blanc, and R.J. Pokluda. 2006. Indoor Air as a Source of VOC
Contamination in Shallow Soils Below Buildings. Soil and Sediment Contamination
15(1):103-122.
McHugh, T.E., R. Davis, G. DeVaull, H. Hopkins, J. Menatti, and T. Peargin. 2010. Evaluation of
vapor attenuation at petroleum hydrocarbon sites: Considerations for site screening and
investigation. Soil and Sediment Contamination 19(1):1-21.
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 Technology 41(14):4993-5001.
Nazaroff, W. W., H. Feustel, A.V. Nero, K.L. Revzan, D.T. Grimsrud, M.A. Essling, and R.E.
Toohey. 1985. Radon Transport into a Detached One-Story House with a Basement.
Atmospheric Environment 19(l):31-46.
Parker, J.C. 2003. Modeling volatile chemical transport, biodecay, and emission to indoor air.
Ground Water Monitoring and Remediation 23(1):107-120.
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.
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.
Ririe, G.T., R.E. Sweeney, S.J. Daugherty, and P.M. Peuron. 1998. A vapor transport model that
is consistent with field and laboratory data, in, Petroleum Hydrocarbons and Organic
Chemicals in Ground Water: Prevention, Detection, and Remediation Conference,
Ground Water Association Publishing, Houston, Texas, pp.299-308.
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Ririe, G.T., R.E. Sweeney, and S.J. Daugherty. 2002. A comparison of hydrocarbon vapor
attenuation in the field with predictions from vapor diffusion models. Soil and Sediment
Contamination ll(4):529-544.
Robinson, A. L., R.G. Sextro, and W.J. Fisk. 1997a. Soil-Gas Entry into an Experimental
Basement Driven by Atmospheric Pressure Fluctuations - Measurements, Spectral
Analysis, and Model Comparison. Atmospheric Environment 31(10):1477-1485.
Robinson, A. L., R.G. Sextro, and W.J. Fisk. 1997b. Soil-Gas Entry into Houses Driven by
Atmospheric Pressure Fluctuations-The Influence of Soil Properties. Atmospheric
Environment 31(10):1487-1495.
Sinke, A.J.C. 2001. Risk reduction of volatile compounds by degradation in the unsaturated
zone, Project no. 96-030, Dutch Research Programme on Biotechnological In-Situ
Remediation, (NOBIS).
Spence, L.R. and T. Walden. 2001. RISC4 - Risk Integrated Software for Cleanups - Version 4.0,
GroundwaterSoftware.com, Groton, Massachusetts.
Tillman, F.D. and J.W. Weaver. 2007. Parameter sets for upper and lower bounds on soil-to-
indoor-air contaminant attenuation predicted by the Johnson and Ettinger vapor
intrusion model. Atmospheric Environment 41(27):5797-5806.
Tillman, F.D. and J.W. Weaver. 2006. Uncertainty from Synergistic Effects of Multiple
Parameters in the Johnson and Ettinger (1991) Vapor Intrusion Model. Atmospheric
Environment 40(22):4098-4112.
Tillman, F.D., and J.W. Weaver. 2005. Review of recent research on vapor intrusion., U. S.
Environmental Protection Agency Office of Research and Development, Washington,
D.C., September (EPA/600/R-05/106).
Tittarelli, F. 2009. Oxygen diffusion through hydrophobic cement-based materials. Cement
and Concrete Research 39(10):924-928.
Turczynowicz, L. and N. I. Robinson. 2007. Exposure assessment modeling for volatiles-
towards an Australian indoor vapor intrusion model. Journal of Toxicology and
Environmental Health Part A 70(19):1619-1634.
Weaver, J. 2012. BioVapor Model Evaluation in Bio-Vapor Hands-On Workshop. 23rd National
Tanks Conference Pre-Conference Workshop, St. Louis, Missouri, March 18.
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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.
Yu, C., C. Loureiro, J.-J. Cheng, L.G. Jones, Y.Y. Wang, Y.P. Chia, and E. Faillace. 1993. Data
Collection Handbook to Support Modeling Impacts of Radioactive Material in Soil,
Environmental Assessment and Information Sciences Division, Argonne National
Laboratory, Argonne, Illinois.
Additional Information
Abreu, L. D. V., 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 and Technology 39(12):4550-4561.
Bozkurt, 0., K. Pennell, and E. Suuberg. 2009. Simulation of the Vapor Intrusion Process for
Nonhomogeneous Soils Using a Three-Dimensional Numerical Model. Ground Water
Monitoring and Remediation 29(1):92-104.
Davis, G.B., M.G. Trefry, and B.M. Patterson. 2009. Petroleum vapour model comparison, CRC
for Contamination Assessment and Remediation of the Environment, Technical Report
Number 9, 24p.
EPA. 2005. Uncertainty and the Johnson-Ettinger Model for Vapor Intrusion Calculations
(EPA/600/R-05/110).
Hers, I., R. Zapf-Gilje, D. Evans, and L. Li. 2002. Comparison, Validation, and Use of Models for
Predicting Indoor Air Quality from Soil and Groundwater Contamination^ Soil and
Sediment Contamination ll(4):491-527.
Interstate Technology & Regulatory Council (ITRC). 2007. Vapor intrusion: A practical guideline.
Interstate Technology & Regulatory Council, Washington, D.C., January. 74p.
Johnson, P. 2005. Identification of Application-Specific Critical Inputs for the 1991 Johnson and
Ettinger Vapor Intrusion Algorithm. Ground Water Monitoring and Remediation
25(l):63-78.
Park, H. 1999. A Method For Assessing Soil Vapor Intrusion From Petroleum Release Sites:
Multi-Phase/Multi-Fraction Partitioning^ Global Nest l(3):195-204.
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Pennell K.G., 0. Bozkurt, E.M. Suuberg. 2009. Development and Application of a Three-
Dimensional Finite Element Vapor Intrusion Model. Journal of Air and Waste
Management 59(4):447-460.
Provoost, J., A. Bosman, L. Reijnders, J. Bronders, K. Touchant, and F. Swartjes. 2009. Vapour
Intrusion from the Vadose Zone - Seven Algorithms Compared. Journal of Soils and
Sediments 10(3):473-483.
Sanders, P. and N. Talimcioglu. 1997. Soil-to-lndoor Air Exposure Models for Volatile Organic
Compounds: The Effect of Soil Moisture. Environmental Toxicology and Chemistry
16(12):2597-2604.
Schreuder W. 2006. Uncertainty approach to the Johnson and Ettinger vapor intrusion model.
Proceedings of the 4th Annual National Ground Water Association Ground Water And
Environmental Law Conference, July 6-7, Chicago, Illinois, pp. 164-173.
Tillman, F.D. and J.W. Weaver. 2006. Uncertainty from Synergistic Effects of Multiple
Parameters in the Johnson and Ettinger (1991) Vapor Intrusion Model. Atmospheric
Environment 40(22):4098-4112.
Van Wijnen, H.J. and J.P.A. Lijzen. 2006. Validation of the VOLASOIL model using air
measurements from Dutch contaminated sites: Concentrations of four chlorinated
compounds RIVM Report 711701041/2006, Rijksinstituut Voor Volksgezondheid en
Milieu, Bilthoven, 68 pp.
Yao, Y., R. Shen, K. Pennell, and E. Suuberg. 2011. Comparison of the Johnson-Ettinger Vapor
Intrusion Screening Model Predictions with Full Three-Dimensional Model Results.
Environmental Science and Technology 45(6):2227-2235.
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GLOSSARY
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."
BTEX: Acronym for the aromatic hydrocarbons benzene, toluene, ethylbenzene, and xylenes (three
isomers).
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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.
clean: in the context of a PVI investigation 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.
Concentration thresholds for "clean" are: the BTEX concentration in groundwater is equal to or less
than the respective maximum contaminant level; the TPH concentration in soil is less than 100 mg/kg;
there is no potential presence of liquid or residual phase LNAPL; the oxygen concentration is greater
than 1 percent; and the combustible gas concentration in soil gas is less than 100 ppm (v/v).
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 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 Fick's Second Law.
dispersion: the process by which a substance or chemical spreads and dilutes in flowing groundwater or
soil gas.
down gradient: in the direction of decreasing static head (potential).
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, carbon monoxide. The volume
of these gases together accounts for virtually 100 percent of the composition of the atmosphere.
Presence and concentration of these gases are determined using gas chromatography (GC).
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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.
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.
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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 hydrocarbons: hydrocarbons that are components of petroleum (crude oil), including the
various products that result from distillation of crude oil.
porosity: the volume fraction of a rock or unconsolidated sediment not occupied by solid material but
usually occupied by water and/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.
semi-volatile: a semi-volatile organic compound is 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
(and/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.
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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 hazardous substances, pollutants or 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.
(United States Environmental Protection Agency. 1991b. A guide to principal threat and low level threat
wastes, Superfund Publication 9380.3-06FS, Office of Emergency Remedial Response. Washington, D.C.).
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.
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. T he 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.
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. T herefore, a values are always less than one. The vapor intrusion
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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.
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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