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 EPA 510-R-13-xxx

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 (α)............................................................................. 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 (α) 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 legallybinding 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-P0042). 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 http://www.epa.gov/oswer/vaporintrusion/. 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 nonaqueous 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.gov/oust/cat/pvi/), located on the Office of Underground Storage Tanks 

(OUST) website (http://www.epa.gov/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 (α) ........................................................................... 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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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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Page 19 of 98

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:

 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.

10The 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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Page 20 of 98

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. 

 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.

 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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Page 21 of 98

Natural Man-made

• gravel lenses and channels • utility corridors

• solution channels in karst terrain • trenches

• bedding planes and weathered surfaces • sumps and drainage pits

• fractures, joints, and faults • 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). 

 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 subslab sampling for PHCs, oxygen, carbon dioxide, methane, and any fuel additives when either 

condition exists: 

*** Draft EPA External Review Document. Do not cite or quote.  
Page 22 of 98

• 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. 

** * Draft EPA External Review Document. Do not cite or quote. ****
Page 23 of 98

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.

 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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13Indoor air in many buildings will contain detectable levels of a number of vapor-forming compounds whether or 

not the building overlies a subsurface source of vapors, because indoor air can be impacted by a variety of indoor 

and outdoor sources. The composition of outdoor air surrounding a building is referred to as ambient air 

throughout this document. The combined contribution of indoor and outdoor sources of vapors to indoor air 

concentrations is referred to as background throughout this document. 

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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 11(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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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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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://apiep.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 threedimensional 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)(1-4) 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)(1-6) 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 vaporphase 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 sitespecific. 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). 

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.

14 However, consideration 

 

14 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-Indoor 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-1.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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(a) Vertical separation distance for dissolved-phase source of PHCs.

(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 unweathered (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 subslab 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-Indoor 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 ≤250 6

>10 (LNAPL) >250 (LNAPL) 15

Groundwater

(mg/L)

≤ 5 ≤30 6

>5 (LNAPL) >30 (LNAPL) 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 subslab 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-Indoor Air Migration 

Pathway at Petroleum Hydrocarbon Sites. API Publication 4741. Accessible at: 

http://www.api.org/environment-health-and-safety/clean-water/ground-water/vaporintrusion/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 Stateof-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/WorkshopsAndConferences.cfm?PageID=documentDetails&AttachID

=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 11(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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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 and 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-96-001).

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(1):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(1):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 (EPA540-5-95-500). 

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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 over time. 

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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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 (α) 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 of VOCs 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(1):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 slabon-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(1-2):1-49.

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(1-3):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 HydrocarbonImpacted 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(1):180–209. 

Bailey, N.J.L., A.M. Jobson, and M.A. Rogers. 1973. Bacterial degradation of crude oil: 

Comparison of field and experimental data. Chemical Geology 11(3):203-221.

Bedient, P.B., H.S. Rifai, and C.J. Newell. 1994. Ground Water Contamination: Transport and 

Remediation. PTR Prentice-Hall, Inc., Englewood Cliffs, New Jersey. 

Bruce, L., A. Kolhatkar, and J. Cuthbertson. 2010. Comparison of BTEX Attenuation Rates 

Under Anaerobic Conditions. International Journal of Soil, Sediment and Water

3(2):Article 11. 

DeVaull, G. 2007. Indoor vapor intrusion with oxygen-limited biodegradation for a subsurface 

gasoline source. Environmental Science and Technology 41(9):3241-3248.

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 slabon-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(1):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(1):63-72. 

Siddique, T., P.M. Fedorak, M.D. Mackinnon, and J.M. Foght. 2007. Metabolism of BTEX and 

naphtha compounds to methane in oil sands tailings. Environmental Science and 

Technology 41(7):2350-2356.

Tillman, F. and J. Weaver. 2007. Temporal Moisture Content Variability Beneath and External 

to a Building and the Potential Effects on Vapor Intrusion Risk Assessment. Science of 

the Total Environment 379:1-15.

Wang, X. and M.A. Deshusses. 2007. Biotreatment of groundwater contaminated with MTBE: 

Interaction of common environmental co-contaminants. Biodegradation 18(1):37-50.

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 OnSite 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, et al., 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 scavengers

23, 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 alcoholsespecially 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(1):213-220.

Landmeyer, J.E., and P.M. Bradley. 2003. Effect of hydrologic and geochemical conditions on 

oxygen-enhanced bioremediation in a gasoline-contaminated aquifer. Bioremediation 

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(1):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(1):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/R10/001). 

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/R09/037). 

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. 

Lüdemann, 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 SoilGas 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.Børresen, 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, O. 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. 10p.

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?storeId=10001&cat

alogId=10001&langId=-1. 

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 (α)

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 (α), 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, α values are always less than one when vapors are attenuated even if only by 

a small amount. Where there is no attenuation, the α 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 α values (i.e., values approaching one) indicate that little attenuation is 

taking place, whereas small α 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 α is straightforward. If the indoor air 

concentration is not known, but concentration at the source is known and a suitable value for α 

can be estimated, the indoor air concentration may be estimated by multiplying the measured 

concentration at the source with the estimated value of α. 

Abreu, Ettinger, and McAlary (2009) used a series of computer simulations to estimate semigeneric values of α 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 (λ) 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 α 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 α 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 (λ) 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 α would be 1 x 10-7. 

Figure 9. Relationship Between Source Vapor Concentration And 

Vapor Intrusion Attenuation Factor (α) 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 (α) 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 (α), 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 naturalattenuation 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 chemicalspecific 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 11(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(1):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 Slabon-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 11(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-toindoor-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, O., 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 11(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(1):63-78. 

Park, H. 1999. A Method For Assessing Soil Vapor Intrusion From Petroleum Release Sites: 

Multi-Phase/Multi-Fraction Partitioning. Global Nest 1(3):195-204. 

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Pennell K.G., O. Bozkurt, E.M. Suuberg. 2009. Development and Application of a ThreeDimensional 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-Indoor Air Exposure Models for Volatile Organic 

Compounds: The Effect of Soil Moisture. Environmental Toxicology and Chemistry

16(12):2597-2604. 

Schreüder 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 nonaqueous 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 manmade (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 (α): 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” (α), 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, α values are always less than one. The vapor intrusion 

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attenuation factor is an inverse measurement of the attenuation: α values decrease with increasing 

attenuation and α values increase with decreasing attenuation. In other words, α values represent the 

fraction of soil gas contaminant that reaches indoor air. Large α 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 α 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 over time.

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.

*** Draft EPA External Review Document. Do not cite or quote. ****