SH^ tlTr\ 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. In September 2009, OUST
assembled experts in the field of vapor intrusion and petroleum releases from EPA, state regulatory agencies, private
consultants, and industry groups to provide technical and practitioner input for EPA to consider in developing the UST
PVI guidance. This information paper1 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 contaminated by leaking Subtitle I USTs or other
sources of petroleum hydrocarbons (PHCs).2
PHCs typically degrade biologically in groundwater as well as in unsaturated soil zones. In many cases, this
aerobic3 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.
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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 IUST 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:
1 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
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.
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essential behaviors characteristic of petroleum and chlorinated solvent contaminants that are often
observed under common site conditions.
The aerobic biodegradability of PHCs can generally limit the potential for subsurface migration of
petroleum vapors and PVI. Figure 1 depicts a typical UST release scenario and conceptually illustrates
how aerobic biodegradation can limit PVI into an overlying building. In contrast, because CHC
biodegradation is anaerobic and proceeds much more slowly, chlorinated vapor plumes (see Figure 2) are
often more extensive and generally more likely to result in VI. Although these generalized scenarios are
considered representative of most conditions, chlorinated solvent contamination does not always result in
VI. Similarly, subsurface biodegradation of PHCs does not always prevent PVI, and PHC vapors can
migrate into buildings under some circumstances. For example, PVI would more likely occur in a
structure located directly above shallow contaminated soil, such as the area near a buried tank as shown
on the left in Figure 1 or in cases where a building is so large that it limits movement of oxygen beneath
the center of the building (e.g., Patterson and Davis, 2009). See page 7, Conditions With Greater
Potential For PVI, for several specific site conditions that are more likely to lead to PVI impacts.
Unsaturated
Zone
Smear Zone
Flow
Figure 1. Typical petroleum hydrocarbon
transport conceptual scenario
Aerobic biodegradation of PHCs along the perimeter
of the vapor and dissolved plumes limits subsurface
contaminant spreading. Effective oxygen transport
(dashed arrows) maintains aerobic conditions in the
biodegradation zone. Petroleum LNAPL (light
nonaqueous phase liquid) collects at the groundwater
surface (the water table, blue triangle).
Figure 2. Typical chlorinated solvent
transport conceptual scenario
Biodegradation of CHCs is anaerobic and usually
slower than PHC biodegradation, so that the vapor
and dissolved plumes often migrate farther than
PHC plumes. CHC DNAPL (dense nonaqueous-
phase liquid), if present, can sink below the water
table, collecting in this case on a less penetrable
layer.
Effect Of Biodegradation
An aerobic biodegradation zone (see Figure 1) is typically present along the perimeter of the PHC plumes
in groundwater and soil gas. Within this bioactive zone, natural microbial activity can degrade many
PHCs into nontoxic end products like carbon dioxide and water (although some biodegradation pathways
can produce compounds like methane, as discussed later). Because soil microbes consume oxygen to
degrade PHCs, oxygen may become depleted where contaminant concentrations are elevated such as in
the interior of a groundwater or vapor plume. The aerobic biodegradation zone generally develops around
the perimeter of the contaminant plume, where oxygen transport from the atmosphere or oxygenated
groundwater (depicted as dashed arrows in Figure 1) can replenish the oxygen consumed from
degradation in this bioactive zone. Atmospheric oxygen migrates into the subsurface through diffusion
and advection (e.g., barometric pumping of soil gas into and out of the subsurface in response to changes
in barometric pressure), as well as dissolved in infiltrating rainwater.
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PHC plumes in the saturated (groundwater) and unsaturated (soil) zones can reach a relatively stable
condition, with oxygen replenished and contaminants biodegraded at the same rate chemicals are released
from a source through dissolution and volatilization. If the source is removed, this equilibrium is
disturbed and biodegradation can reduce the size of the plume, shrinking it back toward the original
source area to the point that the plume may dissipate completely over time (Wilson et al., 1986). As
documented in monitored natural attenuation guidance and literature (Wilson et al., 1986; EPA, 1999),
under favorable conditions biodegradation can provide an effective contaminant removal-and-control
mechanism for PHCs in groundwater, which effectively limits contaminant migration and reduces plume
extent overtime. Given that oxygen is usually more available in soil and unsaturated zones overlying
groundwater (where air is present in the soil pore space), it follows that similar processes effectively limit
vapor-phase PHC plumes in soil. See page 5, Fate And Transport Processes For Vapor-Phase
Contaminants, for more information about PHC biodegradation in the unsaturated zone, including typical
observed vertical concentration patterns in soil gas profiles.
In contrast to PHCs, CHCs biodegrade much more slowly, often incompletely, and primarily under
anaerobic conditions in the subsurface. Although anaerobic biodegradation of chlorinated compounds can
effectively limit contaminant migration in the saturated zone in some cases (EPA, 1998), chlorinated
plumes (dissolved groundwater and vapor) often extend farther than typically observed petroleum
contaminant plumes (see Figures 1 and 2). Other than biodegradation, vapor transport mechanisms for
PHCs and chlorinated compounds are similar (see page 5).
Influence Of Density
Petroleum hydrocarbon liquids (e.g., gasoline) and chlorinated solvents are only moderately soluble in
water and often form separate phase liquids commonly referred to as NAPLs (nonaqueous phase liquids)
when released into the environment. On one hand, when NAPLs are lighter (i.e., less dense) than water,
as with PHCs, they are known as LNAPLs (light NAPLs) and can accumulate at the water table interface
and spread laterally, as shown in Figure 1. On the other hand, when they are denser (i.e., heavier) than
water, as with chlorinated solvents, they are known as DNAPLs (dense NAPLs) and can penetrate the
water table, sink in the groundwater, and collect as pools on less permeable interfaces (e.g., clay or
bedrock), as shown in Figure 2. When NAPLs of either kind move through soils and aquifer materials,
they leave behind immobile, discontinuous droplets of separate-phase liquid referred to as residual NAPL.
Residual NAPL (or residual) can be a long-lasting, immobile source of contamination for soil gas or
groundwater.
It is important to recognize that NAPL does not occupy the entire pore space; rather, water, NAPL, and
often gas/vapor phases are present together in a multiphase configuration controlled primarily by capillary
forces and gravity (buoyancy). The continuous NAPL zones may spread, depending on the available
volume of NAPL and the soil and liquid properties controlling NAPL mobility (e.g., multiphase
permeability and capillary relationships). When a release stops, NAPL zones will eventually reach a
dynamic equilibrium and thereafter remain relatively immobile. The formation of continuous-phase
NAPL depends on a sufficient release volume large enough to occupy the unsaturated pore space;
otherwise all of the separate-phase liquid may be trapped as immobile and discontinuous residual NAPL
in the unsaturated or saturated zones without collecting as a continuous-phase NAPL zone.
When water tables fluctuate because of seasonal changes, tidal influences, nearby pumping wells, or
rainfall events, a LNAPL layer, if present, will move up and down with the water table. These
fluctuations leave behind residual NAPL throughout the zone of water table fluctuation, resulting in a
smear zone above and below the water table (see Figure 1). Chemicals dissolve into groundwater from the
LNAPL source (the continuous LNAPL, as well as the discontinuous residual LNAPL smear zone),
forming a mobile dissolved plume that can migrate with flowing groundwater. Volatile contaminants can
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emanate from the LNAPL source, residual contamination in the smear zone, and the dissolved plume to
form vapors that can migrate through unsaturated soils and preferential pathways.
Fate And Transport Processes For Vapor-Phase Contaminants
Several fundamental fate and transport processes influence the behavior of subsurface vapor-phase
contaminants:
" Diffusion refers to the process whereby molecules move from an area of higher concentration to
an area of lower concentration. Diffusion will lead to chemical migration within unsaturated soils
away from the highest concentration source area (i.e., NAPL or a dissolved plume). Diffusion can
also lead to chemical migration into buildings directly through a dirt floor and crawlspace or
through cracks, pores, and other openings in the building slab and foundation such as passages for
utility lines and sumps. Diffusion is faster in the gaseous phase than in the aqueous phase, so that
a layer of clean recharge water above a contaminant plume can decrease the rate of volatilization
of contaminants from the plume. Although chemical-specific diffusion rates vary somewhat, in
general PHCs and CHCs behave similarly with respect to diffusion.
" 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. When
the pressure inside the building is lower than the pressure in the subsurface, vapors are drawn into
the building. 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.
Barometric pressure changes or wind may also drive advection of soil gas. For example, pressures
upwind from a building may exceed pressures downwind, leading to advection of soil gas beneath
the building (Lundegard et al., 2008). PHCs and CHCs behave similarly with respect to
advection.
" Biodegradation refers to the process by which chemical compounds are altered through the
biological activity of microorganisms in the subsurface. As discussed above, many PHCs degrade
readily in the subsurface under typical, aerobic environmental conditions. In contrast, CHCs
biodegrade much more slowly and under anaerobic conditions. In addition, CHC biodegradation
can produce toxic degradation products such as dichloroethylene and vinyl chloride, while
petroleum degradation usually produces carbon dioxide, water, and under certain conditions
methane or other simple hydrocarbons.
" Sorption refers to the partitioning of chemicals onto the solid phase. Both petroleum and
chlorinated solvent compounds tend to preferentially adsorb onto soil organic matter. Although
chemical-specific sorption characteristics (partitioning coefficients) vary somewhat, in general
PHCs and CHCs behave similarly with respect to sorption. Therefore, in soils with high organic
carbon content, movement of organic compounds is retarded.
" Mixing refers to the blending of intruding vapors with ambient indoor air, which if free of
volatile chemicals may dilute the concentration of intruding vapors. However, if the ambient
indoor (or outdoor) air itself contains volatile chemicals, for example resulting either from off-
gassing of building materials or products stored within the building, or if there are industrial
sources of contaminants in ambient outdoor air, you may find it difficult to distinguish between
VI and other sources of these other chemicals. This is especially true for chemicals that may
originate from several sources simultaneously. The most common chemicals associated with
PVI (e.g., benzene, toluene, TPH) and CVI (e.g., PCE, TCE) are components of products found in
many homes and attached garages; examples include glues, cleaners, solvents, dry-cleaned
clothes, and gasoline (EPA, 2011). Although EPA (2011) presents information on typical
background levels of both PHCs and CHCs in indoor air resulting from sources other than
vapor intrusion, this information should not substitute for site-specific data.
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The chemicals that pose the most significant potential inhalation risks (sometimes called risk drivers) in
PVI and CVI, such as benzene and PCE, respectively, generally have similar physical-chemical properties
controlling diffusion and sorption in the vapor phase; advection, in contrast, is unaffected by chemical
properties. Fundamentally though, these processes (diffusion, sorption, and advection) have a similar
influence on the subsurface distribution of vapor-phase contaminants for both PHC and CHCs. If
biodegradation is excluded, one generally would expect PHC and CHC vapors to behave similarly.
However, investigations from sites across the United States and other countries have shown that vapor
plumes of PHCs are typically less extensive than vapor plumes of chlorinated solvents, providing
empirical evidence that aerobic biodegradation can effectively limit the migration of petroleum vapors in
many situations (McHugh et al., 2010).
Biodegradation Of Petroleum Hydrocarbons In The Unsaturated Zone
Scientific research and site investigations going back decades have demonstrated conclusively that
microorganisms capable of aerobically degrading PHCs are present in nearly all subsurface soil
environments (Zobell, 1946; Atlas, 1981; Wilson et al., 1986; Leahy and Colwell, 1990; Bedient et al.,
1994; EPA, 1999). Effective aerobic biodegradation of PHCs depends on the 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 and generally much slower than aerobic
biodegradation.
Figure 1 depicts key processes for the biodegradation of PHCs in the unsaturated zone under common
conditions: downward oxygen transport from the atmosphere, upward hydrocarbon vapor migration from
the contaminant source, and aerobic biodegradation along the perimeter of the contamination zone where
PHCs are consumed by microbial activity. Important factors influencing aerobic biodegradation in the
vadose zone include source concentration, oxygen demand (the oxygen required to biodegrade the
available hydrocarbons and any ambient soil organic matter that is present), distance between the source
and the building, and soil type.
Aerobic biodegradation consumes oxygen and generates carbon dioxide and water. This leads to a
characteristic vertical concentration profile in the unsaturated zone in which oxygen concentrations
decrease with depth and VOCs (including PHCs and methane from anaerobic biodegradation) and carbon
dioxide concentrations increase with depth. Figure 3 depicts a characteristic vertical profile, which will
vary in shape depending on site-specific conditions (Roggemans et al., 2002).
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Depth
-- Surface
Aerobic
Biodegradation
Zone
Source Zone
f
I (anaerobic)
Concentration
Figure 3. Typical vertical concentration profile in the unsaturated zone for VOCs, CO2, and O2
With aerobic biodegradation in unsaturated soils, VOCs (red) degrade, carbon dioxide
(green) is produced, and oxygen (blue) is consumed. The aerobic biodegradation zone
extends over the area of active biodegradation. The source zone, which is anaerobic, is
characterized by the maximum VOC concentrations and little biodegradation.
PHC vapor concentrations will almost always be much greater adjacent to a LNAPL hydrocarbon 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. When oxygen is limited, anaerobic
biodegradation of LNAPL or other organic sources can produce methane. Significant anaerobic
biodegradation and methane generation can occur in some situations within anoxic zones of the plume
interior and adjacent to the LNAPL source. Methane readily biodegrades under aerobic conditions and,
when present, will create an additional oxygen demand. Importantly, methane also creates an explosion
hazard if it accumulates within confined spaces (e.g., utility vaults and passages, basements, garages) in
sufficiently high concentrations (and there is sufficient oxygen present and a source of ignition).
A number of well-characterized field studies demonstrate extensive aerobic biodegradation of PHC
vapors in unsaturated soils (Ostendorf and Kampbell, 1991; Ririe and Sweeney, 1995; Ririe et al., 1998;
Ostendorf et al., 2000; Hers et al., 2000; Roggemans et al., 2002; Sanders and Hers, 2006; Davis et al.,
2009; Patterson and Davis, 2009). Several of these studies document vapor concentrations at least two to
three orders of magnitude lower than would be predicted, through modeling of simple diffusion, in the
absence of biodegradation.
Conditions With Greater Potential For Petroleum Vapor Intrusion
Although aerobic biodegradation of PHC vapors can reduce the potential for PVI, certain site conditions
can reduce the effectiveness of biodegradation and increase the potential for PVI. The potential for PVI
depends on a number of factors including the characteristics of the source of PHCs and the volume of the
release. In general, there is a greater potential for PVI when the source of the contamination is more
volatile. For example, a release of gasoline, which has a high percentage of VOCs, has a greater
potential for PVI than a release of a heavier, less volatile source material such as #2 fuel oil. Likewise,
there is a greater potential for PVI when the volume of the VOC source is larger. Additional factors
include proximity of the receptor to the contaminant source, presence of preferential transport pathways,
and insufficient oxygen to support aerobic biodegradation.
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Insufficient Separation Distance. In order for biodegradation to limit the potential for PVI, a
sufficiently thick layer of biologically active soil5 is needed between the building foundation and
the contamination to allow biodegradation to occur. Conditions leading to an insufficient
separation distance may include high PHC concentrations, shallow or fluctuating water tables, or
nearby petroleum sources in the unsaturated zone. Note that when the water table fluctuates,
seasonal variability can bring contaminated groundwater or LNAPL close to or in contact with
the building foundation during the wetter parts of the year.
Direct Building Contact. Direct contact between a contaminant source (groundwater or LNAPL)
and a building foundation may result from shallow water tables or perched zones (less-permeable
materials above which water or LNAPL may collect). This direct contact may lead to
contaminant vapor migration through the foundation or actual penetration of contaminated water
or LNAPL into the building. For example, a basement dewatering system with an associated
sump (see Figure 4) may draw contaminated groundwater into the building, resulting in PHC
vapor impacts to indoor air. Many documented cases of PVI involve actual contact of petroleum
NAPL or petroleum-contaminated water with the building foundation (McHugh et al., 2010).
Figure 4. Vapor intrusion from direct building contact
A very shallow or perched water table can bring contaminants (LNAPL or contaminated
groundwater) into direct contact with a building foundation. Foundation cracks or basement
drainage systems (e.g., a sump) can bring source materials into the interior space. Volatilization
from these sources likely results in PVI.
Preferential Transport Pathways. If preferential transport pathways connect sources of volatile
chemicals with buildings, the associated chemical transport may be faster and extend farther than
transport through the surrounding soils. Preferential pathways may be geologic features, such as
fractures or coarse-grained channels, solution channels in carbonate rock (karst), or engineered
features, such as utility lines, drains, and sumps. The resulting transport patterns can be complex.
For example, petroleum vapors (or LNAPL) may migrate along the permeable fill surrounding a
5 Biologically active soil in the context of a PVI investigation means the subsurface soil environment will support populations of
microorganisms that are present in sufficient quantities to aerobically biodegrade PHC vapors before they intrude into a confined
space. Effective aerobic biodegradation of PHCs depends on the soil having sufficient oxygen and enough soil water 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 when they are exposed to the PHC vapors and when they
begin to biodegrade (metabolize) the vapors. This is why it is critical to demonstrate, through collection of on-site samples, that
biodegradation of PHC vapors is occurring.
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main utility line (see Figure 5). Gasoline stations often present a special preferential pathway. As
the tanks are backfilled with clean, well-sorted pea gravel, vapors emanating from the tank pit can
migrate rapidly and without significant biodegradation due to the absence of microorganisms.
The pavement is typically on a layer of gravel, which if continuous over the site, could allow
relatively unimpeded vapor migration and intrusion into buildings on site.
Iti?^ N
..-.•-... • - • -.. L r' ;
• • - _
Permeable
Backfill
Vapor
Plume
Figure 5. Preferential transport through a utility trench
PHC vapors migrate preferentially within the permeable backfill of a utility trench that intersects
contamination. Vapors may migrate preferentially through the more permeable backfill (arrows);
however, oxygen may also migrate more readily through these materials, allowing aerobic
biodegradation to counter the preferential vapor migration.
Anaerobic Conditions. As described on page 6, Biodegradation Of Petroleum Hydrocarbons In
The Unsaturated Zone, aerobic biodegradation requires sufficient oxygen to be an effective
contaminant-removal mechanism. Some site conditions are relatively less conducive to oxygen
transport from the atmosphere. For example, concrete foundations and pavement adjacent to
buildings is relatively less pervious to oxygen than an open soil surface. Available data from a
few sites suggest these surfaces may not reduce oxygen levels under buildings enough to inhibit
biodegradation significantly (Lundegard et al., 2008). However, data from other sites (e.g.,
Patterson and Davis, 2009) suggest oxygen may be limited if the building footprint is very large,
the building foundation is relatively impervious to vapor flow, and high hydrocarbon source
vapor concentrations are in close proximity to the building foundation. Natural conditions can
also limit oxygen availability, as evidenced by low oxygen concentrations found in the presence
of some highly organic soils (e.g., peat). Thus, the presence of highly organic soils can be a
marker for limited oxygen availability and potential limits to petroleum biodegradation.
Production Of Methane. Methane may be produced through anaerobic degradation of PHCs in
zones with relatively large release volumes and high levels of contamination (e.g., in the presence
of LNAPL). Methane occurrence might 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 volume and pressures and result in advective
soil gas flow toward receptors. PHC vapors may also migrate with the methane. In addition,
aerobic biodegradation of methane may deplete oxygen that otherwise could be used for
biodegradation of the PHC contaminants. Moreover, in situations where intrusion of methane into
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confined spaces results in the accumulation of very high concentrations, there can be a risk of
explosion.
Considerations For Petroleum Site Investigation And Screening
The preceding discussion highlights how aerobic biodegradation can limit the migration of PHC vapors in
many cases. Thus, you should consider biodegradation for investigating and screening sites with PHC
contamination. This section discusses some key considerations for site investigation and screening, which
regulators and other practitioners may take into account as they address potential PVI at sites with
subsurface petroleum contamination.
Site Investigation Considerations
A key consideration in a PVI site investigation is whether sufficient oxygen is available and whether there
is a sufficiently thick biologically active soil layer between the source and the receptor for aerobic
organisms to biodegrade the PHC vapors before they could conceivably reach indoor air. With sufficient
oxygen, soil moisture, and an acclimated population of microorganisms the soil column will act as a
natural biofilter within which PHC vapors are degraded at sufficiently fast rates, effectively eliminating
the potential for PVI.
When evaluating the potential for aerobic biodegradation, it is important to consider how readily oxygen
can move through shallow soils around and under buildings to replace oxygen consumed during
biodegradation. Oxygen replenishment beneath a building may be influenced by: size of the building;
construction type and integrity of the building foundation; type of surface cover beside the building (e.g.,
pavement versus landscaping); soil properties (e.g., air permeability); weather; and other site-specific
factors. By observing and measuring these factors, along with soil gas oxygen, carbon dioxide, and PHC
vapor profiles, you can determine and confirm when subsurface oxygen levels may be too low to support
an adequate rate of aerobic biodegradation.
As with any investigation of a leaking UST site, you must fully characterize the site in three dimensions
to develop a conceptual site model (CSM) based on all available data.6 The CSM is continually updated
as new data become available. Because vapors migrate more easily than NAPL or dissolved
contamination, determining soil transport properties and preferential migration pathways is especially
important. It is especially critical to assess whether unsaturated soils under a building have elevated PHC
concentrations. In some cases where PHC vapors appear to be migrating from a deeper source (e.g.,
LNAPL at the water table or a groundwater plume), the shallow soil may be contaminated as well and
may be contributing to PHC vapors entering the building. In addition, biodegradation of PHCs consumes
oxygen and can limit biodegradation processes at sites where soil concentrations of hydrocarbons are
elevated.
When biodegradation is active in the unsaturated zone, soil gas concentrations (PHCs, methane, carbon
dioxide, and oxygen) usually vary with depth in characteristic patterns (see Figure 3). Accordingly, soil
gas samples collected at different depths (vertical profiles) can provide evidence of whether or not aerobic
6 Federal regulations (40CFR280) require determination of the full extent of the contamination from a leaking UST release.
Section 280.62 paragraph (a)(5) directs owners and operators to "measure for the presence of a release where contamination is
most likely to be present..." It goes on to say that sample types, locations, and methods must be appropriate for identifying the
presence and source of the release. There are no limits placed on the extent of the investigation; if contamination is likely to be
present somewhere, then its presence should be determined. This applies to NAPL, residual, vapor, and dissolved phases. State
action levels would come into play to determine whether a contaminant level was actionable. Section 280.65, paragraph (a)
requires owners and operators 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", and further that the investigation must extend to "the
surrounding area possibly affected by the release." "Full extent" implies delineation of contamination in three-dimensions
outward from the source area to non-detect levels, even when this means going "off-site".
10
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biodegradation at a site is occurring. Developing vertical profiles requires a series of soil gas samples at
different depths. In some cases, you may need relatively shallow soil gas samples (less than five feet) to
characterize active biodegradation zones in the shallow soil (e.g., in the presence of shallow
contamination sources). Some state-level regulatory programs do not allow soil gas sampling at depths
less than five feet based on the belief that accurate sampling may not be possible at shallow depths
because air from the surface may leak into the sample. However, research has shown that accurate
shallow-soil gas samples are possible at depths of up to two feet below ground surface using appropriate
field methods (e.g., leak testing), such as those documented in EPA (2010). Particularly in cases with
shallow contamination, site investigation should evaluate whether an active biodegradation zone is
present below the building.
With respect to indoor air investigations at PVI sites, background hydrocarbon concentrations from
indoor and outdoor sources unrelated to PVI can make indoor air samples difficult to interpret. In some
situations indoor air sampling can provide valuable exposure information, although you may find it
difficult to distinguish whether contaminants derive from PVI or an unrelated indoor or outdoor source.
Site Screening Considerations
As highlighted in the discussions above, aerobic biodegradation can limit the potential for PVI under
many conditions. We attribute most of the known, documented cases of PVI to one or more of the site
conditions discussed on pages 7-9; for example, see the published sites compiled by McHugh et al.
(2010). Therefore, site-screening criteria may effectively identify sites where PVI is unlikely to occur
(i.e., deep sources with sufficiently low concentrations; sufficiently thick, aerobic soil zones) as well as
when petroleum sources are strong enough and in such close proximity to a building that PVI might
occur. By quickly identifying cases where biodegradation is unlikely to result in significant petroleum
vapor attenuation, you can use financial resources (e.g., state trust funds) most effectively to protect
public health by addressing sites where PVI is more likely.
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