1 of 98 

### EPA 510 -R- 13

- xxx



# Guidance F or 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 201 3Page 2 of 98 

### Contents

P urpose

................................................................................................ ........................................... 7

Background

................................................................................................ ..................................... 7

Overview

................................................................................................ ......................................... 8

EPA Recommended Actions

............................................................................................................ 8

Supporting Technical Information

................................................................................................ 13

1. Petroleum Vapor Intrusion (PVI)

....................................................................................... 14

2. Typical PVI S cenarios

......................................................................................................... 31

3. Site Characterization A nd Conceptual Site Model (CSM)

................................................. 37

4. Lateral Inclusion Zone

....................................................................................................... 41

5. Vertical Separation Distance

............................................................................................. 44

6. Mobile And Residual Light Non -A queous Phase Liquid ( LNAPL )

...................................... 51

7. Groundwater Flow A nd Dissolved Contaminant Plumes

.................................................. 55

8. Soil Vapor P rofile

............................................................................................................... 59

9. Clean, Biologically Active Soil

............................................................................................ 65

10. Contaminants Other Than PHCs

........................................................................................ 70

11. Seasonal A nd Weather Effects

.......................................................................................... 76

12. Vapor Intrusion Attenuation Factor (α)

............................................................................. 80

13. Computer Model ing O f Petroleum Vapor Intrus ion

......................................................... 84

Glossary

................................................................................................ ......................................... 93Page 3 of 98 Tables

1. Components of A PVI Investigation

......................................................................................... 11

2

. Summary O f Characteristics O f Typical Scenarios O f Petroleum Vapor Sources A nd Potential Receptors

.................................................................................................................. 34

3

. Required Vertical Separation Distance Between Contamination A nd Building Foundation, Basement, or Slab

................................................................................................ 48Page 4 of 98 Figures

1. Flowchart O f the Receptor

- Specific PVI Assessment A nd 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) D issolved P hase

. .... 32

4. Typical Scenarios O f Potential PVI Sources A nd Potential Receptors

................................... 33

5. Lateral A nd Vertical Separation Distances Between Source O f PHC Contaminants And Hypothetical Receptor

........................................................................................................... 42

6. Vertical Separation Distances Between Source Of PHC Contaminants And Hypothetical Receptor: (a) Dissolved Source, (b) LNAPL S ource

................................................................ 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 Prof ile I n T he Unsaturated Zone F or PHCs, C arbon Dioxide , A nd O xygen

............................................................................................................. 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)

........................................................................................... 81Page 5 of 98 List Of Acronyms

1,2

- DCA 1,2

- Dichloroethane BTEX B enzene, toluene, ethylb enzene, 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 I ncreme ntal 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 Sto rage Tanks PHC Petroleum Hydrocarbon PVI Petroleum Vapor Intrusion RBC v R isk

- 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 CompoundPage 6 of 98 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 legall ybinding requirements on EPA, s tates, 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

.Page 7 of 98 P urpose This document provide s guidance to EPA, states , 1 and tribes 2 for investigating and assess ing petroleum contaminated 3 sites where vapor intrusion 4 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 -P0 042 )

. One of its recommendations is that EPA issue final PVI guidanc e on h ow to address risks from petroleum h ydrocarbon 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

re port, 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 p rogram, including the corrective action components, to tribal governments. Some tribal governments that carry out corrective actio n 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 i s 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 N on

- 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.Page 8 of 98 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 immediat e threats to safety (e.g. , fire/ explosi on 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 t hree classes of chemicals :

• PHCs found in gasoline, diesel, and jet fuel (e.g., benzene, trimethylbenzenes)

.
• V apor

- forming chemicals other than PHCs that may be found in petroleum (e.g., methyl tertiary

- butyl ether (MT BE ) and other fuel additives)

.
• M ethane , which can arise from anaerobic biodegradation of PHCs and other

c onstituents of petroleum fuels , especially ethanol

.

PHCs can biod eg rade aerobically and when biodegradation is complete, produce water and carbon dioxide

. Some petrole um 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 t o 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 requir es 6 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 vi sible 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 operat ors 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.Page 9 of 98 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 a ll information and data related to the site into a CSM

. This i nclud es assessment of the potential for biodegradation of the PHCs

. The CSM s hould then be used as the basis for planning the PVI investigation and making informed risk

mana gement decisions about the site and the threat posed by PVI to nearby receptor s

. To determine whether the vadose zone is comprised of clean soil and capable of supporting microbial populations th at facilitate aerobic biodegradation of PHCs, EPA recommends that t he site characterization include :

o A determination of the full exte nt and location of contamination

. o T he nature and characteristics of the contamination

. o T he hydrologic (e.g., soil moisture) and geological characteri stics underneath and

n ear 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 i s 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

. I f so, assess the potential for PVI

.
• Determine vertical separation distances for each building within the inclusi on zone D etermine whether contamination underlies a ny buildings within the lateral inclusion

zone

. If con taminated groundwater is within the vertical separation distance (6 feet) between the contamination and the foundation, slab, or basement of a ny building (s ), EPA recommends sub

- slab vapor sampling to assess the potential for PVI

. If light non

aqueous phase liquid ( LNAPL ) is within 15 feet of a n overlying building, EPA recommends sub

- slab vapor sampling

.
• Mitigate PVI , as appropriate S elect 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 th e maximum extent practicable

.

Table 1 and Figure 1 briefly outline the recommended actions above

. Note that this proc ess 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

.Page 10 of 98 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 sit es 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 resu lt in bette r- 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

.Page 11 of 98 Table 1

. Co mponents Of A PVI Investigation

Recommended Actions Purpos e Characteristics/Indicators Procedures

Assess and

mitigate immediate threats
to safety I dentifies potential threat of explosion or fire due to petroleum vapors or methane • LNAPL visible in building, possibly as sheen

• P etroleum odor,

headache, dizziness, or

nausea

• P otential disagreeable

taste of water

• M ethane is odorless and

cannot be detected on the basis of odor • I nvestigate all reports of petroleum odors and other indicators within buildings

• A ssess for presence of methane ; requires specialized detectors

• A lert first responders so that they

can evacuate building inhabitants until the potential for fire or explosion has been assessed and mitigated as necessary C onduct a site characterization

and develop a CSM D etermines spatial and temporal relationship between receptors

and sources of contamination • D etermines full extent and location of contamination and its nature and potential for biodegradation of PHCs

• D efine s hydrologic and

geologic characteristics of the site

• I dentif ies potential

receptors • C ollect sufficient site data and information to construct CSM

• I dentify data gaps

• U pdate CSM as new data become available

Delineate a lateral

inclusion zone F ocuses investigation on potential receptors in close proximity to contamination • B ased on quality of CSM

• T he lower the uncertainty,

the smaller the lateral inclusion zone • C onstruct lateral inclusion zone based on distance between clean monitoring points

Identify preferential

transport pathways D etermines rate of movement of contaminants toward potential receptors • I nclude s both natural (i.e., geologic) and man

- made (i.e., underground utilities, excavations) features

• D ifficult to detect and

map potential preferential transport pathways • D etermine if presence is likely and locate extent as practicable

• E xercise caution if presence is known

or possible

• C onsider additional sampling to assess transport characteristics

Determine vertical separation distances N arrows investigation to potential receptors overlying contamination • T hickness of clean, biologically active soil separating contamination and poten tial receptors • C ollect 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 I nterrupt s the pathway between

the source of contamination and potential receptors • N umerous approaches depending on building characteristics • S elect a remedial design that is appropriate for building and site

• E stablish institutional controls to limit or prohibit access to affected areas of building , as necessary

• R emediate source of contamination
, including recovery of LNAPL (if present) to the maximum extent practicablePage 12 of 98 Figure 1

. Flowchart Of The Receptor

- Specific PVI Assessment A nd Mitigation ProcessPage 13 of 98 

### Supporting Technical Information

The foll owing sections provide technical information in support of EPA’s recommended action s 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 Ta nks (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 A nd Residual Light Non

- Aqueous Phase Liquid (LNAPL)

..................................... 51

7. Groundwater Flow A nd 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

........................................................ 84Page 14 of 98 

### 1

. Petroleum Vapor Intrusion (PVI)

Description Petroleum vapor i n trusion (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 indo or air concentrations that pose a risk to building occupants

. PVI may pose immediat e threats to safety (e.g.
, fire/explosion potential from petroleum vap ors or methane) and/or possible adverse health effects from inhalation of toxic chemicals (e.g.
, expos ure 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:

• G lobules of light non

- aqueous phase liquid (LNAPL) trapped within soil pore spaces (i.e., residual LNAPL)

. 8

• D issolved in soil moisture

.
• A dhered onto the surface of, or absorbed into
, soil solids

.
• V apors 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., contami nant 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 repre sent 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.Page 15 of 98 Figure 2

. Con ceptual 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, 201 2).

Vapor Migration Vapor migration results from two processes:

• D iffusion

• A dvection

Diffusion is t he process whereby net transport of vapo rs 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 int o 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, i ntact 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).Page 16 of 98 Advection refers to the movement of soil gas in response to pressure gradients

. Adv ection 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, w hen 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 b ar ometric pressure may also drive advective transport of oxygen into the subsurface beneath the b uilding (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 hydroca rbons (Gale, 1951; Ward, Singh, and Van Hamme, 2003; Prince, 2010)

. Alt hough 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 (Wan g 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, 201 2 ; Interstate Technology & Regulatory Council [ ITRC ], 2007)

. The end products of c omplete biodegradation (mineralization) of PHCs are water and carbon dioxide

. Mineralization of PHC s is almost always the consequence of microbial activity (Alexander, 1981).

G asoline 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 do cumented for many individual PHCs and classes of PHCs including :

• N- alkanes ( Bouchard
, et al., 2005; Prince, Parkerton, and Lee, 2007; B ail ey, Jo b son, and Rogers, 1973; Hult, 1989; Baedecker
, et al., 2011)

.
• B ranched alkanes (Prince and Douglas, 2010; Prince, Parke r ton, and Lee, 2007); cycloalkanes (Bouchard
, et al., 2005)

.Page 17 of 98 • A romatics (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)

.
• N aphthenes (Prince, Parketon, and Lee, 2007; Bailey, Jobson, and Rogers, 1973; Anderson
, et al., 1999; ZoBell, 1946)

.
• P henols (ZoBell, 1946)

.
• T rimethyl benzenes ( Chen
, et al., 2009)

.

Th ough 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 b y 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, 2 006; 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) , espe cially 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 docu menting 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 influen tial for aerobic biodegra dation of PHC vapors include :

• V apor source hydrocarbon concentration, flux, and composition (including methane)

.
• O xygen demand ( i.e. , the oxygen required to biodegrade the available hydrocarbons and any other organic matter present) and oxygen availability

.
• S oil type and properties (including texture and moisture content)

.
• A vailability of essential micronutrients

.
• T emperature

• pH

Additional factors cited by EPA (2012) as influencing t he potential for PVI include :

• S ize and characteristics of the building and adjacent land surface

.
• D istance 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

. AtPage 18 of 98 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 che mical 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 h ydrocarbons
, and relatively water

- soluble PHCs . Vapors emanating from LNAPL sources contain the same constituents plus a sizeable fraction of aliphatic and relatively insoluble hydrocarbons, especial ly if the source is large or un weathered ( Lahvis, et al. , 2012; EPA, 2012 )

. A nalyses of samples collected during site characterization (see Section 3 ) will provide information on specific contaminants t hat may warrant assessment for potential vapor intrusion.

Special Considerations Several factors ma y affect the vapor intrusion pathway

. They includ e: biodegradation, chemical transformation, sorption, contaminant source depletion, geologic heterogeneity (including preferential transport pathways) , soil properties (moisture content, permeability, organ ic 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
, the re 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 methan e. Large volume release s 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 additiv es in homogeneous soils

. Typically
, it is difficult to detect and map natural preferential transport pathways
, and contamination may present itself in unexpected locations

. L ocal government offices have maps of utility corridors that can provide information on the presence and location of man

- made preferential transport pathways.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 inve stigations and corrective action at leaking UST sites or where 40 CFR 280 requir es 9 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 indicat ors on neighboring properties

. I ndicators 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

. H owever, 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 l evels.

In confined spaces, PHC vapors, including methane may pose a threat of fire or explosion and endanger building occupants

. Federal r egulations (40 CFR 280.61) require that immediate action be taken to prevent any further release of the regulated substance into the environment and that fir e, 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 respon ders, typi cally 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

ev acuated 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 characterization 10 and develop a conceptual site model (CSM)

Once the im mediate threats to safety have been mitigated (or it is determined that immediate threats do not exist), focus attention on dete rmining whether there is a long

- term threat to human health and the environment from intrusion of petroleum vapors. S ite characterization and CSM developm ent 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 pote ntial 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 r eports of activities and findings to the implementing agency.
10 The term site characterization is used throughout this document for consistency. The term site characterization is

o ften used interchangeably with site assessment, site evaluation, site invest igation, and sometimes site check as they all mean assembling and collecting information and data about a site.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 dissolv ed in groundwater, are considered within the lateral inclusion zone

. Buildings outside this zone generally may be excluded from further assessment for PVI unless:

• S ite conditions change (e.g., groundwater flow directions changes, contaminant plume migrat es beyond the lateral inclusion zone, development or redevelopment of nearby properties)

.
• P referential transport pathways (e.g., utility corridors, fractured rock, solution channels in karst) are present

.
• I mpermeable 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

.
• S oil 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 sampling 11 ) may be warranted to more fully evaluate the risk from PV I

. 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 pha se

. They include both natural and man

- made features such as:

11 Bulk s oil samples should be analyzed for T otal Petroleum Hydrocarbon (T PH ) and BTEX (plus any other potential contaminants) . Soil vapor sample s 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 u naccounted for as these gases should account for nearly 100 percent of the total present.Page 21 of 98 Natural Man

- made

• g ravel lenses and channels • u tility corridors

• s olution channels in karst terrain • t renches

• b edding planes and weathered surfaces • s umps and drainage pits

• f ractures, joints, and faults • o ther 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 recept or 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 separat ion 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 characterist ics 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 U nited S tates

. The findings of th e 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 L NAPL sources that are separated from overlying buildings by more than 15 feet of clean, biologically active soil, the potential threat of PVI is negligible and further investigation for PVI is generally
unnecessary

. These separation distances are believed to be sufficiently protective in most situations because they include a number of built

- in safety factors, which are discussed in more detail in Section 5

. Further Investigation Is Warranted

. To rule out the potential for PVI
, EPA recommends sub

slab sampling for PHCs, oxygen, carbon dioxide, methane, and any fuel additives when either condition exists :Page 22 of 98 • A building overlies LNAPL and the vertical separation distance is less than 15 feet

.
• A building overlie s 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 level s.

EPA’s recent empirical analysis (EPA, 2013 a ) 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:

• I nfluence of methanogenesis on oxygen demand (especially for ethanol blends of gasoline)

.
• E ffect of extensive high organic matter content soils ( e.g. , peat) with potentially high natural oxygen demand

.
• R educed oxygen flux caused by certain geologic conditions ( e.g., wet surface clay unde rlain by coarse

- grained soils)

.
• L imited knowledge of vapor attenua tion behavior in fractured rock

.
• L imited soil vapor data for non

- UST ( e.g. , petroleum refinery, fuel terminal) sites

.
• L imited data on vapor attenuation b ehavior of aliphatic compounds

.
• L ack 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 i n cases where sub

- slab vapor samples indicate the potential for indoor air concent rations 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 [ 201 3b ])

.

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, 201 3b ) provides health based screening levels for a variety of contaminants.Page 23 of 98 I nformation on historic concentrations of background 13 vapors is presented in Background Indoor Air Concentrations of Volatile Organic Compounds in North American Residences (1990 – 2005): A Compilation of Statistics for As sessing 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 recommend s distinguish ing between PHCs arisi ng from vapor intrusion versus background sources

. ITRC (2007) and EPA ( 201 3b) provide i nformation 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

.
• S ection 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, i f indoor air sampling indicates that PVI is occurring, then mitigation and/or remediation is recommended

. ITRC (2007) and EPA (201 3 b) provide i nformation 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, § 280.6 4 requires the recovery of LNAPL to the maximum extent practicable.

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- P. Vandecasteele

. 2003

. Gasoline and oil biodegradation

. Applied and Environmental Microbiology 86(6):1008

- 1 016.

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.Page 31 of 98 

### 2

. Typical PVI Scenarios D escri ption 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. S ource 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 biodegrada tion 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 frin ge 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 fring e 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., hi gh moisture saturation) and biodegradation in the capillary zone.

Importance A fe w c onfirmed occurrences of PVI at petroleum sites are reported in the literature (EPA, 201 3 ). Davis (2009) and McHugh
, et al. (2010) both observe that there are no report ed cases of vapor intrusion from dissolved

- phas e 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.

Asse ssment R ec ommended steps for investigatin g PVI are discussed in Section 1 . The screening criteria in the assessment and investigation allow for the determination of which building s are threaten ed 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.Page 32 of 98 Figure 3. Di fference In Potential For PVI Based On Type Of S ource : 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:

• M igration of contaminants, especially plumes in flowing groundwater

.
• P resence of preferential transport pathways, such as fractures and solution channels in karst

.
• E xtensive impermeable surface cover, and/or very large buildings, that may reduce the atmospheric oxygen flux to the subsurface

.
• S oils with high organic content (e.g., peat) that exert a high oxygen demand

.
• S oil conditions that are inhospitable to microorganisms such as insufficient soil moisture

.
• I nsufficient thickness of clean, biologically active soil.

Recommendatio n 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

.Page 33 of 98 Figure 4. Typical Scenarios O f Potential PVI Sources And Potential ReceptorsPage 34 of 98 T able 2

. Summary O f Characteristics Of Typical Scenarios Of Petroleum Vapor Sources A nd 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 NoPage 35 of 98 References Cited

Da vis, R.V. 2009. Bioattenuation of petroleum hydrocarbon vapors in the subsurface: Update on recent studies and proposed screening criteria for the vapor

- in trusion pathway. LUSTLine Bulletin 61:11

- 14. Ne w England Interstate Water Pollution Control Commission, Massachusetts.

EP A

. 201 3

. Evaluat ion Of Empirical Data T o Support Soil Vapor Intrusion Screening Criteria F or 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 tr ansport 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 V apor Intrusion Pathway: Studies of Natural Attenuation of Subsurface Petroleum Hydrocarbons & Recommended Screening Criteria. Association for Environmental Health and Sciences 21st Annual West Coas t 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.Page 36 of 98 Roggemans, S., C.L. Bruce, and P.C. Johnson. 2002. Vadose Zone Natural Attenuation of Hydrocarbon Vapors: An Empirical Assessment of Soil Gas Vertical Profile Data. API Technical Bulletin No. 15. American Petroleum Institute, Washington, D .C

. http://api

ep.api.org/environment.

Roggemans, S. 1998. Natural Attenuation of Hydrocarbon Vapors in the Vadose Zone. M.S. Thesis, Arizona State University.Page 37 of 98 

### 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 lin es of evidence into a three

dimensional picture of site conditions that illustrates contaminant distributions, release mechanisms, migration routes , exposure pathways and, potential receptors ( EPA 2012 ; ITRC, 2007 ). The CSM uses a combination of text and graphics to portray both known and hypothe tical 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 operati on.

Importance At any leaking UST site, it is important to have a thorough understanding of the full extent and location of contam ination, 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 m anager in evaluating the interactio n 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 manag ement 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:

• I dentific ation of potential contaminants

.
• I dentification and characterization o f the source(s) of contaminants

.
• D elineation of potential migration pathways through environmental media, such as groundwater, sur face water, soils, sediment, biota, and air

.
• E stablishment of background levels of contaminants and areas of contaminati on for each contaminated medium

.
• I dentification and characterizat ion of potential receptors

.Page 38 of 98 • D etermination of the limits of the study area or system boundaries.

EPA recommends that t he CSM include a diagrammatic or schematic presentation that relates the source of contamination to receptors and identifies all potential sources of contamination,
t he 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 a s well as the e fforts 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, t he CSM should continually be refined ( EPA, 1993 ; ITRC, 2007 ). ITRC (2007) and EPA (2013) provide a dditional guidance for develop ing 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 describ e 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 s tored 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 prese nce and source of the release.”

• §280.62(a)(5) Initial Abatement Measures and Site C hecks “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 a ssessment 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 thePage 39 of 98 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 potentiall y 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, qual ity 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

su bpart.”

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 manag ement decisions about the site and the threat posed by PVI to nearby receptor s.

References Cited

ASTM. 2008

. Standard Guide for Developing Conceptual Site Models for Contaminated Sites. E1689

- 95. ASTM International, West Conshohocken, P ennsylvania

.

EPA. 1993. Guidance for Evaluating the Technical Impracticability of the Gro und

- 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).Page 40 of 98 EPA. 2011. Environmental Cleanup Best Management Practices: Effective Use of the Project Life Cycle Conce ptual Site Model. Quick Reference Fact Sheet (EPA 542 -F- 11

- 011).

EPA

. 201 3

. 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).Page 41 of 98 

### 4. Lateral Inclusion Zone

Description The lateral inclusion zone is the area surrounding a contaminant source through which vapor

phase contamination may travel and intrude into buildings

. Determining the lateral distance beyond which buildings and other structures may not be threatened by potential PVI is site

specific. In general, as the level of confidence in the site characterization and CSM increases, the distance the lateral inclusion zone extends outward from sources of PHC vapors decreases

. All buildings within the lateral inclusion zone should be further assessed to determine if they are separated from vapor sources by an adequate vertical separation distance (see Section 5 ). F urther assessment may be un necessary for those buildings outside the inclusion zone. If contaminated groundwater is the s ource of vapors , m igration of the contaminant plume ( in the longitudinal, transverse, and vertical directions ) should be assess ed when evaluating the potential for future risks.

Importance The lateral inclusion zone is a screening criteri on 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 contaminati on, 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 characteri zation in order to construct an accurate CSM (see Section 3 )

. Th rough 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 determin ed. Site characterization gener ally 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 u ntil 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 reg arding the full extent and location of contamination.

G roundwater 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 verti cal 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 LNA PL 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 ne eded , possibly over more than one annual cycle. S ee Section 7 for addition al information on groundwater flow and dissolved contaminant plumes.Page 42 of 98 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, 201 3a ; 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
gene rally warranted ( EPA, 201 3a)

.

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 d istance 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 transpo rt 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 H owever, 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. Acce s sible at http://www.api.org/ehs/groundwater/NAPL/soilgas.cfm

.Page 43 of 98 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. T he 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 (A merican P etroleum I nstitute (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 gr oundwater usage, and additional releases of contaminants.

Recommendation O ne approach to address the uncertainty is to delineate a lateral inclusion zone that is based on a n 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 Fue l 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. 201 3a. Evaluation O f Emp irical Data To Support Soil Vapor Intrusion Screening Criteria F or 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.Page 44 of 98 

### 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 A erobic biodegradation will degra de vapor

- phase contamination before it intrudes into buildings if the thickness of clean, biologically active soil is sufficient and oxygen is present

. EPA (201 3 ) 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) usin g somewhat different data sets. There is a high degree of consistency among these studies. This consistency enables determin ation 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 sepa ration 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 disso l ved 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 are as 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 fe et for LNAPL sources. T hese 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 Fin al 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://w ww.epa.gov/oust/pvi/index.htm

.Page 45 of 98 (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 Distance s Between Source Of PHC Contaminants And Hypothetical Receptor : (a) Dissolved Source, (b) LNAPL sourcePage 46 of 98 Assessment As part of a normal leaking UST investigation, the potential cont aminants of concern should be identified. Although b enzene, 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 a dditives 16 such as MTBE and tertiary

- butyl alcohol ( TBA )

. Vapors emanating from gasoline sources (either dissolved

- phase or LNAPL) will contain many of these compounds ; any or all of which may be risk

- drivers. Dissolved sources will be comprised primarily of more soluble compounds while LNAPL sources will contain a sizeable fraction of aliphatic and relatively insoluble hydrocarbons (e.g., naphthalene) , especially if the source is large or un

weathered (Lahvis, et al., 2012; EPA, 201 3 ). 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 bu lk 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 G roundwater contamination is less than or equal to 30 m g/L TPH (gasoline) or benzene is less than or equal to 5 mg/L ; or

o S oil contamination is less than or equal to 250 mg/ k g TPH (gasoline) or benzene less than or equal to 10 mg/ k g, and

o T he vertical separation distance between contamination and the lowest point of a building foundation, basement, or slab is 6 feet or mo re

.

• For high levels of soil or groundwater contamination (i.e., LNAPL is present):

o G roundwater contamination is greater than 30 m g/L TPH (gasoline) or benzene is greater than 5 mg/L; or

o S oil contamination is greater than 250 mg/kg TPH (gasoline) or grea ter 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

mo re information about lead scavengers, see Appendix F in Evaluation Of Empirical Data T o Support Soil Vapo r Intrusion Screening Criteria For Petroleum Hydrocarbon Compounds ( EPA, 2013 ).Page 47 of 98 o T he vertical separation distance between contamination and the lowest point of a building foundation, basement, or slab is greater than 15 feet

.
Special Considerations T he presence of preferential transport pathways may circumvent the protectiveness that a sufficiently thick layer of clean, biologically active soil would otherwise provide. Preferential transport pathways such as utility conduits typically enter buildings through holes in the foundation or slab and can facilitate the entry of PHC vapors into the building

. Consideration should be given to field instrument screening at utility access point (s) as an initial step to determine if the utility is acting as a conduit for vapors. If the transport of vapors from the source area to the building could occur along utility conduits, then vapor sampling inside the utility conduits, manholes, or sumps should be considered in addition to vadose zone and sub

slab soil v apor sampling. Any utility sampling program should include safety precautions to protect personnel (e.g., oxygen and combustible gas monitoring, c onfined

- space entry requirements) and to avoid damage to utilities

. Specific guidance for utility sampling i s 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.Page 48 of 98 Table 3

. Re commend ed 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 ≤ 3 0 6

> 5 (LNAPL) > 3 0 (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 info rmation, see API (2000).

*The v ertical 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 sampl ing be conduct ed to evaluate the risk of PVI whenever contamination above the specified threshold is present in any sample and the distance between the contamination and an overlying building is less than these vertical distances. If the potential for PVI cannot be ruled out based on sub

slab vapor sampling, then EPA recommends indoor air sampling.

References Cited

API. 2000. Non

- Aqueous Phase Liquid (NAPL) Mobility Limits in Soil

. Soil and Groundwater Research Bulletin No. 9.

API. 2005. A Practical Strategy for Assessing the Subsurface Vapor

- to

- 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/vapor

intrusion/vi

- publications/assessing

- vapor

- intrusion.aspx.Page 49 of 98 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, O hio

. 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 V apor 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

. 201 3

. Evaluation Of Empirical Data T o Support Soil Vapor Intrusion Screening Criteria F or 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. 201 1

. Empirical data supporting groundwater benzene concentration exclusion criteria for PVI investigations. Proceedings of Battelle 8th International Symposium on Bior emediation 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 4 th International Contaminated Site Remediation Conference, September 11

- 15 , Adelaide, Australia.

Wright, J. 2012. Evaluation of the Australian Petroleum Hydrocarbon VI Database: Exclusion Criteria. Presented at: Recent Advances to VI Application & Implementation

— A State

of

- the

- Science Update. Association for Environmental Health and Sciences West C oast Conference
, San Diego, C alifornia . March 19

- 22

.Page 50 of 98 https://iavi.rti.org/WorkshopsAndConferences.cfm?PageID=documentDetails&AttachID =549

.
Additional In formation

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 Groundwa ter 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 (i n 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 Re search 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. Co lwell. 1990. Microbial degradation of hydrocarbons in the environment. Microbiological Reviews 54(3):305

– 315.Page 51 of 98 

### 6. Mobile And Residual Light Non

- Aqueous Phase Liquid (LNAPL )

Description L NAPLs 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

- volati le. N ewer 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 PHC s. Vapors emanating from LNAPL sources contain the same constituents in addition to a sizeable fraction of aliphatic and rela tively insoluble hydrocarbons, especially if the source is large or un

- weathered (Lahvis, et al., 2012; EPA, 201 3 ).

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 t o 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 migrat e

. 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 subsurfac e and occur w hen 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 volum es of petroleum vapors as well as dissolved

- phase contaminants.

Assessment T he distinction between the LNAPL and dissolved phases is important, though the precise threshold is difficult to pinpoint. 17 As a somewhat conservative estimate, EPA (201 3 ) 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 approxim ate 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 evid ence of the presenc e of LNAPL. However, b ecause 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 -A queous Phase Liquid (NAPL) Mobility Limits in Soil (API, 2000).Page 52 of 98 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 threshold s 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 measur a ble LNAPL i n 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 b ulk s oil samples collected in the source area for TPH and specific petroleum constituents (e.g., BTEX and other volatile and semi

- volatile organic chemicals , an d fuel additives )

. I t 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 t he 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.Page 53 of 98 Figure 7. Conceptual Model Illustrating The Potential For Vapor Intrusion F or 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

. 201 3

. 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

.Page 54 of 98 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 thic kness in a uniform sand aquifer. Ground Wat er 32(5): 708

- 718.

Cohen, R.M., A.P. Bryd a, S.T. Shaw, and C.P. Spalding. 1992. Evaluation of visual methods t o 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 nonaqueou s phase liquids in porous media. Ground Water 28(1):48

- 56.

K emblowski, M.W. and C.Y. Chiang. 1990. Hydrocarbon thickness f luctuations 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 ( EPA

540 -5- 95

- 500 ).Page 55 of 98 

### 7. Groundwater Flow A nd 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 dire ctions 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 i n direct co ntact with a building or other structure.

Assessment Contaminant plumes generally necessita te three

- dimensional monitoring to assess the transient behavior of groundwater flow and the movement of contaminant plumes (EPA, 2004 a, 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: T his should not delay additional investigatio n activities and measures to mitigate or remediate threats to safety and health

. ) As the plume migrates, appropriate adjustments to the samplin g 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 parti cularly 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 fro m 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 PHC s leaked to the water table. Groundwater flow directions changed nearly 90 degrees in less than four months in response to changing flow conditions i n a nearby intermittent stream.Page 56 of 98 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
cap acity 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 migrat e beneath building s 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 pet roleum 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 va por 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 t he de pth to contaminated groundwater in relation to overlying buildings and the concentration of contaminants

. Due to the transient nature of groundwater migration, pe riodic 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 3 0 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 groundwa ter directly below a building is less than 6 feet, EPA recommend s 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 determinePage 57 of 98 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

. 2004 a

. 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, T exas

.
Wilson, J. T. 2003. Fate and transport of MTBE and oth er gasoline components. In: MTBE Remediation Handbook, Amherst, M assachusetts : 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.Page 58 of 98 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.Page 59 of 98 

### 8. Soil Vapor Profile

Description Shallow soil ga s 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 f rom mobile LNAPL released into the sub surface, 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 , e t 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, 201 2 ). PHC vapor concentrations will generally be much greater adjacent to a LNAPL sour ce 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., etha nol) 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 m ethane , 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).Page 60 of 98 Figure 8. Typical Vertical Concentration Profile In The Unsaturated Zone For PHCs, Carbon Dioxide And Oxygen

Importance Aerobic biodegradation of PHC vapors occurs in m any subsurface environments (Lahvis, Baker, and Behr, 1998; McHugh, et al . 2010, Roggemans, 1998 ; Roggemans, Bruce, and Johnson, 200 2 ; Z oBell, 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 indicat e 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, v olatile 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. A t a relatively short distance from the source,
concentra tions 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 i f the vapor source is LNAPL.Page 61 of 98 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 methanotrop h ic 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 equippe d 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 (s ee 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 need ed 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 th ose 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 (201 3 ) indicates that for a n oxygen shadow 20 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.Page 62 of 98 of vapors must be highly concentrated and in relatively close proximity to the bott om of the building.

At sites with a new release or unweathered LNAPL source, the oxygen demand will be high . I t is important to determine whether temporal variations in oxygen flux into the vadose zone will limit the effectiveness of aerobic biodegrada tion, 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 b e 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 recommend s that sub

- slab soil gas samples be collected for buildings within the lateral i nclusion 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 sampl ing

. 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 disso lved 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, N ew Y ork , pp.289

- 350.Page 63 of 98 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

. 201 2

. Petroleum Hydrocarbons And Chlorinated Hydrocarbons Differ In Their Potential For Vapor Intrusion

. March. http://www.epa.gov/oust/cat/pvi/pvicvi.pdf

.

EPA

. 201 3 (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 S low 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 Volatili zation 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. Env ironmental 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.Page 64 of 98 Patterson, B.M., and G.B. Davis. 2009. Quantification of vapor intrusion pathways into a slab

on

- ground building under varying environmental conditions. Environmental Science and Technology 43(3):650

- 656.

Prince, R.C. and G.S. Douglas. 2010. Remediation of petrol and diesel in subsurface from petrol station leaks, i n 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 11 th , sponsored by the Groundwater Resources Association of California, Sacramento , California

.

Zo B ell, 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 Gaso line 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 Bui lding Overlying Shallow Petroleum Hydrocarbon

Impacted Soils. Ground Water Monitoring and Remediation 29 (1) :81

- 91.Page 65 of 98 

### 9. Clean, Biologically Active Soil

Description For a PVI investigation, c lean 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 activ ity of the soil is not diminished and the subsurface environment will support sufficient populatio ns 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 vapo rs to migrate from the contaminant source in to 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 so ils 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 (201 3 ) notes that soil moisture content greater than 2 percent is ad equate to support biodegradation activity (Leeson and Hinc hee, 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 indigen ous vegetation (Riser

- Roberts, 1992)

.

Certain geologic materials do not qualify as biologically active soil

. The se geologic materials include:

• C oarse sand and gravel with a low content of silt, clay
, or organic matter

.
• F ractured consolidated rock

.
• C onsolidated rock with solution channels (i.e., karst)

.
Importance Eff ective 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 microorganis ms. 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 biodegrad ation (Widdel, Boetius, and R abus, 2006; Bailey, Jobson, and Rogers, 1973; and Bruce, Kolhatkar, Cuthbertson
, 2010 ).Page 66 of 98 Assessment Scientific research and site characteriz ations have demonstrated that microorganisms capable of aerobically degrading many PHCs are present in nearly all subsurface soil environments (Zo B ell, 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 U nited Sates These results, which

ar e 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 disso l ved

sources , and 13. 5 feet for LNAPL sources

. These distances are believed to be conservative in most environmental settings.

Special Considerat ions Coarse sand and gravel with a low content of silt, clay
, organic matter, fractured consolidated rock, or consolidated rock with solution channels
, may not ha ve 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 pre sent below buildings in the lateral inclusion

zone.

In addition, b eneath 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 m icroorganisms 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, m ost soils contain indigenous microorganisms
, sufficient oxygen, and adequate soil moisture necessary for degrading PHC vapors

. T hus, 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

recommen ds that appropriate samples be collected and analyzed to verify conditions at the site.Page 67 of 98 The vertical separation distances described in this guidance (see Section 5 ) should not be used at si tes 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, N ew J ersey

.
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

. 201 3

. Evaluation Of Empir ical Data T o Support Soil Vapor Intrusion Screening Criteria F or 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 bioreact or. Journal of the Air Pollution Control Association 37:1236

- 1240.Page 68 of 98 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 o f 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 ki netics determined in soil core, microcosm, and tubing cluster studies. Groundwater Monitoring and Remediation 16(1):141

- 153.

Ostendorf, D.W., E.S. Hinlein, A.J. Lutenegger, and S.P. Kelley. 2000. Soil gas transport above a jet fuel/solvent spill at Plattsburgh Air Force Base. Water Resources Research 36(9): 2531

- 2547.

Ostendorf, D.W., and D.H. Kampbell. 1991. Biodegradation of hydrocarbon vapors in the unsaturated zone. Water Resources Research 27(4):453

- 462.

Patterson, B.M., and G.B. Davis. 2009. Quantification of vapor intrusion pathways into a slab

on

- ground building under varying environmental conditions. Environmental Science and Technology 43(3):650

- 656.

Phelps, C.D., and L.Y . Young. 1999. Anaerobic biodegradation of BTEX and gasoline in various aquatic sediments. Biodegradation 10(1):15

- 25.

Ririe, T., and R. Sweeney. 1995. Fate and transport of volatile hydrocarbons in the vadose zone. In: Proceedings of the 1995 Petrol eum Hydrocarbon and Organic Chemicals in Groundwater Conference
, American Petroleum Institute and the National Ground Water Association, Houston, T exas , 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, T exas

.Page 69 of 98 Riser

- Roberts, E

. 199 2

. Bioremediati on 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):6 3- 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.

Zo B ell, C.E. 1946. Action of microorganisms on hydrocarbons. Bacteriological Reviews 10(1

- 2): 1- 49.

Zwick, T.C., A. Leeson, R.E. Hinchee, L. Hoeppel, and L. Bowling. 1995. Soil Moisture Effects During Bioventing in Fuel

- Contaminated Arid Soils. Third International In

- Situ and On

Site Bioreclamation Symposium. In

- Situ Aeration, v. 3, Bat t elle Press, San Diego, California

.Page 70 of 98 

### 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 . C ontaminants other than PHCs may be present at a site as the result of releases of petroleum fuel s 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 PHC s 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, a erobic biodegradation has been observed for several classes of compounds that are, or have been, constituents of petroleum fuels , including :

• A lcohols , in particular :

o E thanol (P owers , et al., 2001; Corseuil et. al, 1998)

. o TBA (Wang and Deshusses, 2007; Landmeyer
, et al., 2010)

. o M ethanol (Powers
, et al., 2001)

.
• E thers , 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 T ertiary

- amyl methyl ether (TAME ) (Landmeyer
, et al., 2010).

• O rganic lead compounds (Prince and Douglas, 2010; Gallert and Winter, 2004)

.
• T he lead scavenger s 23 , in particular :

o EDB ( Prince and Douglas, 2010; Pignatello, 1986)

. o 1,2

- DCA (Falta, 2004)

.

21 While the se 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 groun dwater, 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 fr om 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 biodeg radation 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).Page 71 of 98 Alternative fuels, especially those that contain higher percentages of non

- petroleum constituents ( et hanol 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 h and, alcohols have a greater tendency than ethers to remain in the dissolved phase, and they readily biodegrade to create methane. M ethane generation m ay 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 recepto rs. 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 t hat 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 locati ons, 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 CSM s.

Special Considerati ons The literature is relatively sparse in regard to ethanol releases at leaking UST sites in the U nited 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 a nd 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 c oncerns wi th increasing ethanol content are in relation to (re) mobiliz ation of LNAPL (McDowell
, et al.
, 2003; Yu , et al.
, 2009) and increased solubility of PHCs (Powers
, et al., 2001).

Tho ugh 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 H enry’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)

.Page 72 of 98 Recommendation EPA recommends that s oil and groundwater samples be analyzed for PHCs typically found in petroleum

- based fuels, in addition to other volatile additives (e.g. ethers and alcohols

e specially ethanol) that are constituents of petroleum

- based fuels

. Soil vapor samples should be c ollected 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 PH Cs, 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 Remed iation 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 S cavengers 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

. 201 3

. Evaluation Of Empir ical Data T o Support Soil Vapo r Intrusion Screening Criteria F or 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.Page 73 of 98 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. Envir onmental 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, C alifornia

. 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. Powe rs. 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. 2 012

. 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 sedimen ts. 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.Page 74 of 98 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 s ubsurface 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 envi ronmental 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 Af fecting LUST Sites, United States Environmental Protection Agency, Washington, D.C. ( EPA/600/R

10/001 ).

Weaver, J

. W., S

. A. Skaggs, D

. L. Spidle, and G

. C. Stone. 2009. Composition and Behavior of Fuel Ethanol, United States Environmental Protection Agenc y, Washington, D.C. ( EPA/600/R

09/037 ).

Yu, S., J.G. Freitas, A.J.A. Unger, J.F. Barker, and J. Chatzis. 2009. Simulating the evolution of an ethanol and gasoline source zone within the capillary fringe. Journal of Contaminant Hydrology 105 (1-2) :1

- 17.

Additional Information

Donaldson, C.B., J.F. Barker, and I. Chatzis. 1994. Subsurface Fate and Transport of a Methanol/Gasoline Blend (M85). Report prepared for the American Petroleum Institute, Washington D .C. , Publication number 4569.

Eichler , B. and B. Schink. 1984. Oxidation of primary aliphatic alcohols by Acetobacterium carbinolicum sp. nov., a homoacetogenic anaerobe. Archives of Microbiology 140( 1- 2 ): 147

- 152

.
Henderson, J. K., D.L. Freedman, R.W. Falta, T. Kuder, and J.T. Wilson. 20 08. 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 oxyge nates: extrapolation of information to multiple sites and redox conditions. Environmental Science and Technology 28(9):1727

- 1732

.Page 75 of 98 Peargin, T. and R. Kolhatkar. 201 1

. Empirical data supporting groundwater benzene concentration exclusion criteria for petroleum vapor intrusion investigations. Proceedings of Battelle 8th International Symposium on Bior emediation 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 bromoalka nes, 1,2

- dibromopropane, and 1,2

- dibromoethane in water. Environmental Science and Technology 20(10):992

- 997.

Widdel, F. 1986

. Growth of methanogenic bacteria in pure culture with 2

- propanol and other alcohols as hydrogen donors. Applied Environmental Microbiology 51( 5 ): 1056

- 1062

.Page 76 of 98 

### 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 sign ificant rates even when temperatures are near freezing ( Bradley and Chapelle, 1995; Bradley, Richmond, and Chapelle, 2005; Hers, et al., 2011). B radley and Landmeyer (2006) documented microbial degradation of MTBE in the wintertime when groundwater temperatures we re be l ow 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 hou se 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. H owever , 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, o xygen 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 depth s ( Silver, 1999; Ludemann, 2000; Pezeschki, 2001)

. Changes in barometric pressure can result in enhanced intrusion of PHC vapors into building s 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 month s, can create a chimney effect whereby PHC vapors are pulled into buildings at much higher rates than they

wo uld 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

.Page 77 of 98 Assessme nt 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, s ite characteristics that may indicate susceptibility to the effects of seasonal and weather factors should be assessed. These include :

• P oor drainage around the building indicated by flooded soils

.
• A rea subject to permafrost/long lasting snow cover (based on altitude or latitude)

.
• S hallow 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 potent ially snowmelt) water levels within
t ank pit s 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 t he 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

.Page 78 of 98 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. Abraham son

. 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., DeV aull, 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 Microbiol ogy 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. J ohnson, T. Peargin, and T. Creamer. 2009. Spatial Variability of Soil

Gas Concentrations Near and beneath a Building Overlying Shallow Petroleum Hydrocarbon

– Impacted Soils. Ground Water Monitoring & Remediation 29 (1) :81

- 91.

Pezeshki
, S.R. 2001. Wetland plant responses to soil flooding

. Environmental and Experimental Botany 46(3 ): 299

- 312.

Rike, A.G., K.B. Haugen, M

. 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

.Page 79 of 98 S ilver, W.L., A.E. L ugo, 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
p eriodic 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, Petrol eum 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

- 19 94. Open

- File Report 98

- 287. Fairbanks, Alaska . 10p.

Web

- based Resources:

U.S

. G eological S urvey ’s soil su rveys 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 Ha rdiness Zone Map http://planthardiness.ars.usda.gov/PHZMWeb/.Page 80 of 98 

### 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 sour ce. Th e 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 co n centration in soil gas at the source (with concentration s in the same units), and thus it is a ratio. 24 The source is defined as the region of highest vapor concentration in the v adose

zone. T herefore, α values are al ways less than one when vapors are att enuated even if only by a small amount

. W here 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., value s approaching one ) indicate that little attenuation is taking place, whereas small α values ( i.e ., values much smaller than one ) indicate that significant attenuat ion is taking place

.

Assessment The vapor intrusion attenuation factor can be either measured, when background sources do not contribute significant ly 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 m ultiplying 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 semi

generic 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

s oil 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 . A s 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 clay ey 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 s ide.

24 α can be understood as the concentration in indoor air normalized to the soil gas source concentration.Page 81 of 98 One set of simulations assumed t hat the building had a basement, while a nother 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) generate d 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 A nd Vapor In trusion Attenuation Factor (α) As A Function O f Vertical

Separation Depth Between Contaminant Source And Base O f

Building (Receptor) (s ource: m odified from Abreu, Ettinger, and McAlary, 2009, Figure 7 , page 114)Page 82 of 98 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 betw een 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
, i t m ight 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 J ohnson

- 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 th at 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 A n estimated vapor intrusion attenuation factor may help support screening decisions, although EPA recommend s 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 C ited

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.Page 83 of 98 DeVaull, G. 2007. Indoor vapor intrusion with oxygen

- limited biodegradation for a subsurface gasoline source. Environmental Science and Technology 41(9):3241

- 3248.

Johnson, P.C. and R.J. Ettinger. 1991. Heuristic Model for Predicting the Intrusion Rate of Contaminant Vapors into Buildings. Environmental Science and Technology 25(8):1445

- 1452.

Lahvis, M.A., Baehr, A.L., and R.J. Baker. 1999

. Quantification of aerobic

- biodegradation and volatilization rates of gasoline hydrocarbons near the water table during natural

attenuation conditions

. Water Resources Research 35 (3) : 753

- 765.

Additional Information

Abreu, L.D., and P.C. Johnson. 2005. E ffect 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 Ef fect 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 N o. 16.Page 84 of 98 

### 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 t he JEM include:

• A steady or transient sour ce of subsurface vapors from groundwater or residual chemicals

.
• G aseous

- phase diffusive vapor flow through a layer of soil

.
• V apor transport through a slab

- on

- grade or basement foundation

.
• B uilding air exchange.

The original JEM does not include biodegradati on, 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, 2007 a ; 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 an d Fitzgerald, 2002; Sinke, 2001; Ririe , Sweeny, and Daugherty , 2002; Hers , et al., 2003; Davis
, 2006; Golder Associates
, 2008; Davis 2009). The potentia l 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 cm 2 /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 significant ly

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.htmPage 85 of 98 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, 2007 a ; API, 2010) uses a conceptual model similar to the JEM
, including the following features :

• A steady su bsurface petroleum vapor source

.
• G aseous

- phase diffusive vap or flow through a layer of soil

.
• V apor transport through a slab

- on

- grade or basement foundation

.
• B uilding air exchange.

In contrast to JEM, BioVapor accounts for oxygen

- limited, a erobic biodegradation

. Aerobic biodegradation is included as a coupled reaction between petroleum vapors and oxygen. Oxygen availability in the subsurface is dictated by transport through and around the building foundation, and by diffusion into the soil. The BioVapor model requires estimates of chemical

specific aerobic degradation rates for vadose zone soils. DeVaull (2007 a,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 ( 201 1 ); Wright ( 2011 ); and McHugh
, et al. (2010). Site assessment and field data including t he depth to contamination
, source strength , and type (LNAPL or dissolved) are key parameters for determin ing these exclusion distance criteria.

Estimates using t he BioVapor model indicate that for moderate or we ak sources (especially dissolved plumes) , biodegradation effectively eliminates the potential for PVI. Conversely,

w here vapor sources are both high in concentration and in close proximity to the bottom of a foundation , t he 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, 2 009; 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 biode gradation, source depth, and source strength dominatePage 86 of 98 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 model s 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
, later al inclusion zone, and Section 5 , vertical separation distance).

Assessment When selecting a n 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 represent ative of the actual physical, chemical
, and biological pro perties of the site. A common limitation with computer models is that f ield measurements of all the input parameters (e.g ., biodegradation rates , soil moisture content beneath buildings, air exchange rates ) are typicall y 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 l ead s to uncertainty as to whethe r parameter values are truly represent ative of the site conditions

. Therefore , EPA recommends that an uncertainty analysis be conducted to provide error bounds on predictions of the computer model

. In a ddition, EPA recommends that pr edi cted indoor air concentrations be verified with field data in making a determin ation as to w hether 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 A n appropriate framework for t he 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, t h is can include using models to improve a site

- specific sampling str ategy, 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.Page 87 of 98 Ameri can Petroleum Institute (API) . 2 00 9. 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

.

Da vis, R.V. 2009. Bioattenuation of petroleum hydrocarbon vapors in the subsurface: Update on recent studies and proposed screening criteria for the vapor

- in trusion pathway. LUSTLine Bulletin 61: 11

- 14. Ne w England Interstate Water Pollution Control Commission, Massachusetts.

Dav is, 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, Mas sachusetts.

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 Bior emediation and Sustainable Environmental Technologies , Reno, Nevada, June 27

- 30.

Energy Institut e. 2009. Screening the potential for hydrocarbon vapour intrusion risks, Petroleum Review August 2009, Energy Institute, London, U nited K ingdom , 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

.Page 88 of 98 Haghigha t, 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 Ai r 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 Variatio ns

. 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: Sit e- specific alternatives to generic estimates , Publication No. 4674, API , Washington, D .C. , December, 44p.

Jo hnson, 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 material s. 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.

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- 2235.Page 93 of 98 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, a lthough 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 o f 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 deg rade 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).Page 94 of 98 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 L NAPL; the oxygen concent ration 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
nat ural 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 fu ture 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 liq uid or residual phase L NAPL; 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 moti on). 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.

d own gradient: in the direction of decreasing static head (potential).

first responder: ref ers 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: refer s 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).Page 95 of 98 flux: the rate of movement of mass through a unit cross

- sectional area per unit time in response to a concentration gradient or some advective force.

free product: a petroleum hydrocarbon in the liquid ("free" or non

- aqueous) phase (see also light non

aqueous phase liquid, L NAPL).

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: t he 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 contam inant 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 acq uired 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.Page 96 of 98 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 com ponents of petroleum (crude oil), including the various products that result from distillation of crude oil.

porosity: the volume fraction of a rock or unconsolidated sediment not occupied by solid material but usually occupied by water and/or air (gas).

preferential transport pathways: pathways through which contaminants may be transported at a higher rate than through surrounding materials. Preferential transport pathways are heterogeneities within geologic media and include features that are natural (such as facies changes, sand or gravel stringers, solution channels in karst, bedding planes and weathered surfaces, fractures, and joints) as well as man

made (such as utility corridors, trenches, other types of excavations).

regulated substance: (a) An y 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 substan ces 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 use d 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) t he 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 horizo ntal

— 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) T he product (e.g., CSM , report ) resulting from the site characterization process. ( NOTE: S ite assessment, site investigation, site evaluation, and site check are all synonyms of site characterization. )

site check: see site characterization

.Page 97 of 98 site evaluation: see site characte rization

.
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 ha zardous 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. Con taminated 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 measuri ng 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 India n tribe pursuant to the f ederally 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 intrusionPage 98 of 98 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 c ontaminant 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 v apor 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 p rocess 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.