Office of Underground Storage Tanks, Washington, D.C. 20460 September 2011
www.epa.gov/oust
Petroleum Hydrocarbons And Chlorinated Hydrocarbons
Differ In Their Potential For Vapor Intrusion
Contents
Page
Background And Purpose 1
Introduction 2
Differences Under Common Subsurface Scenarios 2
Effect Of Biodegradation 3
Influence Of Density 4
Fate And Transport Processes For Vapor-Phase Contaminants 5
Biodegradation Of Petroleum Hydrocarbons In The Unsaturated Zone 6
Conditions With Greater Potential For Petroleum Vapor Intrusion 7
Considerations For Petroleum Site Investigation And Screening 10
Site Investigation Considerations 10
Site Screening Considerations 11
References 11
Background And Purpose
In November 2002, the U.S. Environmental Protection Agency (EPA), Office of Solid Waste and Emergency
Response issued draft vapor intrusion guidance (EPA, 2002), which specifically states that it is not recommended for
Subtitle I underground storage tank (UST) sites. EPA’s Office of Underground Storage Tanks (OUST) is thus
currently developing guidance to address petroleum vapor intrusion (PVI) at UST sites. OUST has consulted on
several occasions with experts in the field of vapor intrusion and petroleum releases from EPA, state regulatory
agencies, private consultants, and industry groups to obtain their individual input on technical and practitioner issues
1
for EPA to consider in developing the UST PVI guidance. This information paper describes how petroleum
compounds behave differently in the subsurface from other volatile chemicals, in particular chlorinated hydrocarbons
(CHCs), and how these behaviors can be considered when evaluating the potential for vapor intrusion at sites
2
contaminated by leaking Subtitle I USTs or other sources of petroleum hydrocarbons (PHCs).
PHCs typically degrade biologically in groundwater as well as in unsaturated soil zones. In many cases, this
3
aerobic biodegradation is substantial and can limit the potential for PVI. In contrast, biodegradation of CHCs is
anaerobic (under anoxic conditions), which is generally slower than aerobic biodegradation of PHCs. This limited
biodegradability is to some degree responsible for the greater observed prevalence of chlorinated solvent vapor
intrusion (CVI) when compared with PVI.
During the 1980s and 1990s, a better understanding of PHC biodegradation in groundwater led to the
development of monitored natural attenuation, a remediation approach that involves no external inputs and has now
been used successfully to address groundwater contamination at many leaking UST sites (Wilson et al.,1986;
Bedient et al., 1994). Based on a review of current literature (e.g., Sanders and Hers, 2006; Davis et al., 2009;
McHugh et al., 2010), EPA recognizes that analogous aerobic biodegradation processes are active in the unsaturated
zone and that these processes can limit the potential for PVI.
1
This information paper is intended to communicate the overall concepts of petroleum vapor intrusion. It is not intended to be
interpreted as either a technical guidance document or statement of regulatory policy.
2
Petroleum hydrocarbons are chemical compounds made up of hydrogen and carbon that are constituents of petroleum and
various refined products of petroleum, including automotive gasoline, diesel fuel, lubricating oils, and the like.
3
Aerobic means that the process requires oxygen. In contrast, anaerobic means the process does not require oxygen. Anoxic
refers to the absence of oxygen.
1
, Introduction
This paper discusses the impact on the inhalation exposure pathway from vapor intrusion (VI) of volatile
organic chemicals (VOCs).4 VI occurs when vapor-phase contaminants migrate from subsurface sources
into buildings. The primary concerns regarding VI are immediate threats to safety (e.g., explosive
concentrations of petroleum vapors or methane) and possible adverse health effects from inhalation
exposure to toxic chemicals. This paper focuses primarily on the latter concern, although the reader
should recognize that in high enough concentrations, petroleum compounds and methane (a
biodegradation product) can collect in buildings, leading to imminent explosive hazards. The information
in this paper focuses on small-scale Subtitle I UST sites as opposed to sites with large sources (e.g.,
refineries and tank farms); however, you can use this information to inform decisions at non-Subtitle I
petroleum releases. In addition, this paper does not address sites with comingled plumes such as mixed
chlorinated and petroleum hydrocarbon contamination.
There are two classes of VOCs that together account for a large number of soil and groundwater
contamination sites in the United States:
Petroleum hydrocarbons (PHCs) such as gasoline, diesel, and jet fuel
Chlorinated solvents such as the dry cleaning chemical tetrachloroethylene (perchloroethylene,
or PCE) and the degreasing solvents trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and
PCE
This information paper discusses and compares petroleum vapor intrusion (PVI) and chlorinated solvent
vapor intrusion (CVI) with respect to processes that influence whether and how vapors can migrate
through vadose zone materials into buildings and other confined spaces as well as some implications for
addressing PVI.
The foremost difference between PHC and chlorinated hydrocarbon (CHC) vapors in the subsurface is
that PHCs biodegrade readily under aerobic (oxygenated) environmental conditions, whereas CHCs
typically biodegrade much more slowly and under anaerobic conditions (Howard, 1991). Because PHC
biodegradation is relatively rapid when oxygen is present, aerobic biodegradation can typically limit the
concentration and subsurface migration of petroleum vapors in unsaturated soils. In addition, CHC
biodegradation can produce toxic degradation products, such as dichloroethylene and vinyl chloride,
while petroleum degradation usually produces carbon dioxide, water, and sometimes methane or other
simple hydrocarbons. A second primary difference is density: PHC liquids (e.g., gasoline, diesel fuel) are
lighter (less dense) than water and when released from a leaking UST, can float on the groundwater
surface (water table), whereas chlorinated solvents (e.g., TCE, PCE) are heavier than water and sink
through the groundwater column to the bottom of the aquifer. These key differences (biodegradability and
density) lead to very different subsurface behavior that often reduce the potential for human exposure.
Differences Under Common Subsurface Scenarios
Figures 1 and 2 illustrate differences in subsurface transport behavior for PHC and CHC chemicals under
commonly observed subsurface conditions. The conceptual scenarios in these figures are simplified and
do not represent the complexity of actual subsurface environments, such as variations in contaminant
distribution due to subsurface heterogeneities. Rather, they are intended to illustrate and contrast several
essential behaviors characteristic of petroleum and chlorinated solvent contaminants that are often
observed under common site conditions.
4
Vapor intrusion may also occur with inorganic contaminants. One example with well-known public health impacts is radon, an
inorganic and volatile radioactive gas that can emanate from some natural soil and rocks.
2
www.epa.gov/oust
Petroleum Hydrocarbons And Chlorinated Hydrocarbons
Differ In Their Potential For Vapor Intrusion
Contents
Page
Background And Purpose 1
Introduction 2
Differences Under Common Subsurface Scenarios 2
Effect Of Biodegradation 3
Influence Of Density 4
Fate And Transport Processes For Vapor-Phase Contaminants 5
Biodegradation Of Petroleum Hydrocarbons In The Unsaturated Zone 6
Conditions With Greater Potential For Petroleum Vapor Intrusion 7
Considerations For Petroleum Site Investigation And Screening 10
Site Investigation Considerations 10
Site Screening Considerations 11
References 11
Background And Purpose
In November 2002, the U.S. Environmental Protection Agency (EPA), Office of Solid Waste and Emergency
Response issued draft vapor intrusion guidance (EPA, 2002), which specifically states that it is not recommended for
Subtitle I underground storage tank (UST) sites. EPA’s Office of Underground Storage Tanks (OUST) is thus
currently developing guidance to address petroleum vapor intrusion (PVI) at UST sites. OUST has consulted on
several occasions with experts in the field of vapor intrusion and petroleum releases from EPA, state regulatory
agencies, private consultants, and industry groups to obtain their individual input on technical and practitioner issues
1
for EPA to consider in developing the UST PVI guidance. This information paper describes how petroleum
compounds behave differently in the subsurface from other volatile chemicals, in particular chlorinated hydrocarbons
(CHCs), and how these behaviors can be considered when evaluating the potential for vapor intrusion at sites
2
contaminated by leaking Subtitle I USTs or other sources of petroleum hydrocarbons (PHCs).
PHCs typically degrade biologically in groundwater as well as in unsaturated soil zones. In many cases, this
3
aerobic biodegradation is substantial and can limit the potential for PVI. In contrast, biodegradation of CHCs is
anaerobic (under anoxic conditions), which is generally slower than aerobic biodegradation of PHCs. This limited
biodegradability is to some degree responsible for the greater observed prevalence of chlorinated solvent vapor
intrusion (CVI) when compared with PVI.
During the 1980s and 1990s, a better understanding of PHC biodegradation in groundwater led to the
development of monitored natural attenuation, a remediation approach that involves no external inputs and has now
been used successfully to address groundwater contamination at many leaking UST sites (Wilson et al.,1986;
Bedient et al., 1994). Based on a review of current literature (e.g., Sanders and Hers, 2006; Davis et al., 2009;
McHugh et al., 2010), EPA recognizes that analogous aerobic biodegradation processes are active in the unsaturated
zone and that these processes can limit the potential for PVI.
1
This information paper is intended to communicate the overall concepts of petroleum vapor intrusion. It is not intended to be
interpreted as either a technical guidance document or statement of regulatory policy.
2
Petroleum hydrocarbons are chemical compounds made up of hydrogen and carbon that are constituents of petroleum and
various refined products of petroleum, including automotive gasoline, diesel fuel, lubricating oils, and the like.
3
Aerobic means that the process requires oxygen. In contrast, anaerobic means the process does not require oxygen. Anoxic
refers to the absence of oxygen.
1
, Introduction
This paper discusses the impact on the inhalation exposure pathway from vapor intrusion (VI) of volatile
organic chemicals (VOCs).4 VI occurs when vapor-phase contaminants migrate from subsurface sources
into buildings. The primary concerns regarding VI are immediate threats to safety (e.g., explosive
concentrations of petroleum vapors or methane) and possible adverse health effects from inhalation
exposure to toxic chemicals. This paper focuses primarily on the latter concern, although the reader
should recognize that in high enough concentrations, petroleum compounds and methane (a
biodegradation product) can collect in buildings, leading to imminent explosive hazards. The information
in this paper focuses on small-scale Subtitle I UST sites as opposed to sites with large sources (e.g.,
refineries and tank farms); however, you can use this information to inform decisions at non-Subtitle I
petroleum releases. In addition, this paper does not address sites with comingled plumes such as mixed
chlorinated and petroleum hydrocarbon contamination.
There are two classes of VOCs that together account for a large number of soil and groundwater
contamination sites in the United States:
Petroleum hydrocarbons (PHCs) such as gasoline, diesel, and jet fuel
Chlorinated solvents such as the dry cleaning chemical tetrachloroethylene (perchloroethylene,
or PCE) and the degreasing solvents trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and
PCE
This information paper discusses and compares petroleum vapor intrusion (PVI) and chlorinated solvent
vapor intrusion (CVI) with respect to processes that influence whether and how vapors can migrate
through vadose zone materials into buildings and other confined spaces as well as some implications for
addressing PVI.
The foremost difference between PHC and chlorinated hydrocarbon (CHC) vapors in the subsurface is
that PHCs biodegrade readily under aerobic (oxygenated) environmental conditions, whereas CHCs
typically biodegrade much more slowly and under anaerobic conditions (Howard, 1991). Because PHC
biodegradation is relatively rapid when oxygen is present, aerobic biodegradation can typically limit the
concentration and subsurface migration of petroleum vapors in unsaturated soils. In addition, CHC
biodegradation can produce toxic degradation products, such as dichloroethylene and vinyl chloride,
while petroleum degradation usually produces carbon dioxide, water, and sometimes methane or other
simple hydrocarbons. A second primary difference is density: PHC liquids (e.g., gasoline, diesel fuel) are
lighter (less dense) than water and when released from a leaking UST, can float on the groundwater
surface (water table), whereas chlorinated solvents (e.g., TCE, PCE) are heavier than water and sink
through the groundwater column to the bottom of the aquifer. These key differences (biodegradability and
density) lead to very different subsurface behavior that often reduce the potential for human exposure.
Differences Under Common Subsurface Scenarios
Figures 1 and 2 illustrate differences in subsurface transport behavior for PHC and CHC chemicals under
commonly observed subsurface conditions. The conceptual scenarios in these figures are simplified and
do not represent the complexity of actual subsurface environments, such as variations in contaminant
distribution due to subsurface heterogeneities. Rather, they are intended to illustrate and contrast several
essential behaviors characteristic of petroleum and chlorinated solvent contaminants that are often
observed under common site conditions.
4
Vapor intrusion may also occur with inorganic contaminants. One example with well-known public health impacts is radon, an
inorganic and volatile radioactive gas that can emanate from some natural soil and rocks.
2
