EPA/600/R-05/106
                                            September 2005
Review of Recent Research on Vapor
                   Intrusion
                        by
                   Fred D. Tillman
              National Research Council
         National Exposure Research Laboratory
                Athens, Georgia 30605

                  James W. Weaver
             Ecosystems Research Division
         National Exposure Research Laboratory
                Athens, Georgia 30605
         U. S. Environmental Protection Agency
           Office of Research and Development
               Washington, DC 20460

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Notice

The U.S. Environmental Protection Agency through its Office of Research and
Development funded and managed the research described here. It has been subjected to
the Agency's peer and administrative review and has been approved for publication as an
EPA document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.

Vapor intrusion is a complex problem where EPA is continuing to develop policies and
guidance.  This document presents the results of ORD-sponsored research and neither
states EPA policy nor requirements for assessment and clean up. The latest policies and
requirements should be obtained from the EPA Office of Solid Waste and Emergency
Response.
                                      11

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Abstract

       This paper reviews current and recent research in the area of vapor intrusion of
organic compounds into residential buildings.  We begin with a description of the
challenges in evaluating the sub surf ace-to-indoor air pathway. A discussion of the fate
and transport mechanisms affecting vapors along this pathway is then presented.
Following this discussion is a brief overview of current Federal regulations and proposed
guidance concerning vapor intrusion.  A review of site studies involving vapor intrusion
that have been published in scientific literature is then presented, with a focus on
evidence of the extent of the problem.  Published approaches to modeling vapor
intrusion are presented next, followed by conclusions and ideas about future research
needs.
                                        in

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Foreword

The National Exposure Research Laboratory's Ecosystems Research Division (ERD) in
Athens, Georgia, conducts research on organic and inorganic chemicals, greenhouse gas
biogeochemical cycles, and land use perturbations that create direct and indirect,
chemical and non-chemical stresses, exposures, and potential risks to humans and
ecosystems.  ERD develops, tests, applies and provides technical support for exposure
and ecosystem response models used for assessing and managing risks to humans and
ecosystems, within a watershed / regional context.

The Regulatory Support Branch (RSB) conducts problem-driven and applied research,
develops technology tools, and provides technical support to customer Program and
Regional Offices, States, Municipalities, and Tribes. Models are distributed and
supported via the EPA Center for Exposure Assessment Modeling (CEAM) and through
access to Internet tools (www.epa.gov/athens/onsite).

Intrusion of contaminated vapors into buildings ("vapor intrusion") may provide a
significant pathway for exposure to hazardous contaminants.  Assessment of this
problem is difficult, because of limitations of sampling methodologies, contamination in
ambient air, internal sources and sinks of contaminants and uncertainty in model
application.  The information in this  report is intended to provide a background for future
work that addresses the complexities  of this problem.
                                        Eric J. Weber, Ph. D.
                                        Director, Ecosystems Research Division
                                        Athens, Georgia
                                       IV

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Contents

Notice	ii
Abstract	iii
Foreword	iv
Contents	v
Acronyms and Abbreviations	vi
1   Introduction	- 1 -
2   Challenges in Evaluating the Vapor Intrusion Pathway	-2 -
3   Vapor Intrusion Fate and Transport Mechanisms	- 6 -
  3.1     Sources	- 6 -
  3.2     Transport	- 7 -
    3.2.1     Diffusion	-7-
    3.2.2     Advection	-9-
    3.2.3     Sorption	- 10-
    3.2.4     Biodegradation	- 11 -
  3.3    Building Effects	- 12-
4   Current Federal Regulations	- 14 -
  4.1     States	- 16-
5   Vapor Intrusion in the Field	- 17 -
6   Review of Vapor Intrusion Models	- 23 -
7   Conclusions and Future Work	-33-
Bibliography	-35-

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Acronyms and Abbreviations

ACH        Air changes per hour
ANAS       Alameda Naval Air Station
BTEX       Benzene, toluene, ethylbenzene and xylenes
EPA        Environmental Protection Agency
CDOT-MIL  Colorado Department of Transportation Materials Testing Laboratory
CERCLA    Comprehensive Environmental Response, Compensation, and Liability
            Act
GW         Ground water
IAQ         Indoor air quality
IMPACT    Integrated Moisture Plus Contaminant Transport Model
LUST       Leaking underground storage tank
MADEP     Massachusetts Department of Environmental Protection
NAPL       Nonaqueous phase liquid
OCHCA     Orange County Health Care Agency
OERR       Office of Emergency and Remedial Response
OSWER     Office of Solid Waste and  Emergency Response
RCRA       Resource Conservation and Recovery Act
THC        Total petroleum hydrocarbons
TCE        Trichloroethene
USEPA      United States Environmental Protection Agency
UST        Underground storage tank
VI          Vapor intrusion
VOC        Volatile organic compound
                                    VI

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1    Introduction





       The term "vapor intrusion" (VI) refers to the transport of vapors from volatile




organic compounds (VOCs) or other contaminants of interest from the subsurface into




buildings. As the average American spends over 21 hours per day indoor and roughly 18




hours indoors for every hour spent outdoors (Olson and Corsi 2002), the potential




presence of harmful vapors in buildings is of great importance. VOCs in living spaces




can serve as an immediate threat, for instance, explosion, or, more insidiously, as a long-




term source for exposures to potential carcinogenic or toxic compounds. The source of




organic vapors in the subsurface can come from accidental or intentional releases, leaking




landfills or leaking underground and above ground storage tanks.  Organic compounds of




concern in vapor intrusion are usually divided up into two broad categories: chlorinated




solvents and petroleum hydrocarbons.  Once organic compounds are introduced into the




subsurface, a complex series of fate and transport mechanisms act upon them, potentially




moving them away from the source area. Hydrocarbons may be transported beneath




residences as a separate phase (NAPL), dissolved in ground water or as a vapor in soil




gas.  Once these contaminants are present near or beneath buildings, they may move as a




vapor through soil gas and into the residence. Vapor intrusion is an area of active




research as engineers and scientists grapple with evaluating and predicting human




exposure to harmful vapors emanating from the subsurface.









       This report reviews current and recent research in the area of vapor intrusion of




organic compounds into residential buildings. We begin with a description of the




challenges in evaluating the sub surf ace-to-indoor air pathway.  A discussion of the fate
                                      - 1 -

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and transport mechanisms affecting vapors along this pathway is then presented.




Following this discussion is a brief overview of current Federal regulations and proposed




guidance concerning vapor intrusion. A review of site studies involving vapor intrusion




that have been published in scientific literature is then presented, with a focus on




evidence of the extent of the problem.  Published approaches to modeling vapor




intrusion are presented next, followed by conclusions and ideas about future research




needs.
2   Challenges in Evaluating the Vapor Intrusion Pathway




       Several challenging issues exist that make vapor intrusion a particularly difficult




pathway to assess.  First, while there have been reported cases where organic vapors have




been present above the odor threshold in homes, it is not known whether vapor intrusion




is a widespread problem, particularly for long-term exposure to low-level concentrations.




While caution would require the evaluation of the soil-to-indoor air pathway for all




subsurface contamination, there are, in fact, not many cases of proven vapor intrusion




documented in the scientific literature.  This is particularly true for organic vapors subject




to aerobic biodegradation, such as gasoline compounds (petroleum hydrocarbons).




Secondly, determining the impact of organic vapor intrusion in residential buildings is




not necessarily a straight forward exercise. Indoor air sampling and analysis is a fairly




routine procedure, yet the interpretation of the results is often difficult. Many household




building supplies and products such as furniture, carpets, textiles, household cleaners,




sealants, gules, adhesives, paints, waxes, lubricants, heating systems (i.e. fuels), cooking




vapors, and personal care products contain organic compounds identical to common







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contaminants in soil or ground water (Hers et al. 2001). The quality of the air outside the




home may also be important because some of these contaminants may be present from




this source.  Therefore, detection of VOCs in indoor air that are also present in the




subsurface does not conclusively link the two systems.  Additionally, there are household




materials that serve as a sink for VOCs from indoor air including wallboard, ceiling tile,




carpet, and upholstery. During high-concentration periods, adsorption of VOCs by these




materials can reduce peak concentrations. The adsorbed VOCs may become an




additional indoor source during periods of reduced indoor air concentration or as a result




of changes in temperature or other environmental factors (Hers et al. 2001).









       Due to the difficulty in conclusively identifying the soil-to-indoor air pathway via




indoor air sampling, researchers have suggested moving the focus of VI investigations




outside the home.  Several published studies have sampled soil, soil gas and ground water




from the subsurface near and beneath potentially impacted homes.  These measurements




are often used with "generic" attenuation factors to estimate indoor air concentrations and




subsequent risk to occupants. However, subsurface sampling is subject to spatial and




temporal variability.  For example, it is not known if moisture content measurements




taken outside the building footprint are representative of moisture content directly




beneath the building.  This uncertainty may have a great impact on vapor intrusion




predictions as vapors travel in air-filled pore space.  Studies have also shown that soil gas




concentrations may vary at different locations beneath buildings (Laubacher et al. 1997).




Vapors may flow along more permeable routes associated with utility conduits, untrapped




drains or naturally  existing macropores. An important component of the potential

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biodegradation of gasoline vapors is adequate oxygen supply in the subsurface. While




one modeling study showed little impact of building foundation on oxygen




concentrations in the shallow subsurface beneath a structure (Hers et al. 2000), another




field study indicated anoxic locations directly beneath a slab (Laubacher et al. 1997).




Temporal factors affecting subsurface measurements include seasonal changes in




building depressurization due to the use of fireplaces, heaters, open windows, air




conditioners or wind; the movement of subsurface soil gas from barometric pumping




caused by both diurnal and longer-term atmospheric pressure changes; and temperature




effects on contaminant partitioning. Moisture content will also change over time with




climatic conditions controlling precipitation and evapo/transpiration. Precipitation may




cause the water table to rise and with it the contaminant source zone. However, there is




evidence that some gasoline components may become trapped beneath the infiltrating




recharge, greatly reducing their ability to volatilize into  soil gas - a phenomenon known




as "plume diving" (Weaver and Wilson, 2000).  The influence of water table fluctuation




on NAPLs can result in repeated trapping and exposing  of NAPL to soil gas. Drought




conditions may lower the water table and expose previously-trapped NAPL product,




greatly increasing the NAPL partitioning into soil gas.  These and other conditions may




confound synoptic field data and need to be addressed in order to provide practitioners




with guidance as to under what conditions sampling should occur in order to provide a




conservative, "worst-case" sampling event.









       Finally, some researches have suggested using models with site-specific data in




order to evaluate the vapor intrusion pathway.  Site-specific data suffers from the
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uncertainty described above. Models have their own additional challenges, often being




either simplified and not accounting for all fate and transport processes or complex and




containing unmeasured (or unmeasurable) parameters. The purpose of model usage is




very important in determining the required level of detail in field data or assessment. A




common use of vapor intrusion models is to screen out sites, or individual homes at sites,




that are deemed to require no further investigation. For screening-level purposes, a




simplified model may be appropriate if it can be shown to produce a "worst-case"




prediction of current and future exposure in all cases.  An example of the screening-level




model is the widely used Johnson and Ettinger vapor intrusion model. One-dimensional




diffusion through the unsaturated zone and advection and diffusion through the building




slab are incorporated into the model, but biodegradation of organic vapors is not




included. While this may be considered "conservative" with respect to occupant




exposure to vapors, ignoring biodegradation of organic vapors may greatly over-estimate




the potential exposure to aerobically degradable petroleum hydrocarbons. On the other




end of the complexity spectrum are models that are used for detailed predictive analyses




of current and future vapor exposures that are dependent upon site-specific parameters.




These models might include multi-dimensional, multi-species vapor transport through the




unsaturated zone with sorption to the soil moisture phase. Biodegradation could occur




stoichiometrically (with oxygen concentration) in the soil-moisture phase with the rate




being temperature dependent.  While the additional level of complexity may help account




for other fate and transport properties of the organic vapor, detailed multidimensional




data for defining parameters and calibrating such a model are not routinely collected and




suffer from the same spatial and temporal variability described previously.  While several
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vapor intrusion models have been published in the scientific literature, there has been

little evaluation of the false-negative (type II) error produced by the models at field sites,

with the possible exception of the widely-studied Johnson and Ettinger model

(Fitzpatrick and Fitzgerald 2002, Johnson et al. 2002, Hers et al. 2003).
3   Vapor Intrusion Fate and Transport Mechanisms

3.1  Sources
   Contaminants of concern in vapor transport in the unsaturated zone are typically

volatile organic compounds (VOCs), although vapors emanating from inorganic sources

such as mercury vapor may be of concern as well. A chemical is considered to be

volatile if its Henry's Law Constant is 1 x xlO"5 atm-mVmol or greater (Environmental

Quality Management 2003).  Examples of VOCs that are important in impacted

environmental systems include chlorinated solvents such as carbon tetrachloride,

tetrachloroethylene, and trichloroethylene (TCE), and their degradation compounds), fuel

hydrocarbons such as benzene, toluene, ethylbenzene and o,m,p-xy\enes as well as

volatile pesticides such as chlordane, aldrin and lindane. The U.S. Environmental

Protection Agency lists 107 compounds whose toxicity and volatility produce a

potentially unacceptable inhalation risk to receptors (Environmental Quality Management

2003).  These VOCs can be released into the subsurface environment from leaking

landfill liners, improper disposal, accidental spillage, or leaking underground storage

tanks (LUSTs). Once in the subsurface, these compounds can become bound to the soil

matrix, dissolved in groundwater (or soil water) and/or exist as a separate, residual phase

known as a non-aqueous phase liquid (NAPL). Soil, aqueous, and NAPL-phase organics
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may all be sources of organic vapors in the subsurface. Therefore, organic vapor

transport in the unsaturated zone requires understanding of interphase mass-transfer

processes as the contaminant can be distributed between soil gas, water, soil, and NAPL

phases.
3.2  Transport
    Organic vapors emanating from contaminated soil or groundwater or from a residual

phase such as gasoline floating on the water table may move through unsaturated zone

soil gas by diffusion or soil-gas advection due to pressure or density gradients or a

combination of these processes.

3.2.1  Diffusion

    Molecular diffusion is the spreading out of compounds from random collisions

resulting from thermal motion of atoms. These collisions may be between molecules

themselves or between molecules and their surroundings. Under most environmental

conditions, molecular diffusion in natural systems proceeds from locations of higher

concentration towards locations of lower concentrations. In a typical scenario, organic

vapors above a contaminated water table (high concentration) diffuse towards land

surface (lower concentration). The well-known relation describing the diffusion of a

compound across a unit of cross-sectional area is Pick's First Low:
           dx
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where Fx is the mass flux [ML"2!"1], Z/^is the effective diffusion coefficient of the




compound in the gas phase [L2!"1], and dC/dx is the concentration gradient of the




compound in one-dimension [ML"3!/1]. From this equation, it is apparent that the rate of




molecular diffusion in the gas phase depends upon the concentration gradient and the




effective diffusion coefficient of the compound of interest.  Several relations exist that




relate the effective diffusion coefficient to the free-air diffusion coefficient of the




compound and the total and air-filled porosities of the diffusing media (Buckingham




1904, Penman 1940, Van Bavel 1952, Rust et al. 1957, Dye and Dallavale  1958,




Millington 1959, Currie 1970, Nilson et al. 1991, Bartelt-Hunt and Smith 2002, Rolston




and Moldrup 2002). An increase in diffusive flux is seen in soils with greater air




passageways (i.e. greater porosities and air-filled porosities).  Therefore, in a layered




unsaturated zone, vapor diffusion from depth to land surface will be limited by the




wettest, least porous soil layer. As free-air diffusion coefficients are compound




dependent, ranging from 2.5><10"3 cmV1 for hexachloroethane to 2.71X10"1 cmV1 for




chloroethane for the 107 volatile compounds of concern of USEPA (Environmental




Quality Management 2003), different chemicals will diffuse at different rates under the




same concentration gradients. Also, increased temperature produces an increased  free-air




diffusion coefficient, leading to a greater rate of diffusion relative to the same system at




lower temperatures. Situations where Pick's First Law may not sufficiently describe




vapor diffusion include systems where pore  sizes are very small (Knudsen diffusion) and




when volatile species constitute a substantial fraction of the total soil gas concentration




(non-equimolar diffusion) (Thorstenson and Pollock 1989, Baehr and Bruell 1990,

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Partridge et al. 2002).  In these situations, it may be necessary to employ the more




rigorous Stefan-Maxwell equation.






3.2.2  Advection





       The flow of soil gas (advection) in the subsurface may be caused by gas-pressure




gradients or, in certain cases, gas density gradients. Pressure-driven advection is




produced when differences in soil-gas pressure form, causing soil gas to flow and




carrying any vapors present with it. Air pressure gradients in the subsurface of natural




systems may result from several phenomena.  As diurnal or weather related atmospheric




pressure cycles occur at land surface, pressure waves are transmitted into the unsaturated




zone and air may flow in response - a process known as "barometric pumping".




Barometric pumping may cause  soil gas to flow either towards land surface carrying soil




vapor or away from land surface bringing in fresh atmospheric air (Sleep and Sykes




1989, Thorstenson and Pollock 1989, Nilson et al. 1991, Massmann and Farrier 1992,




Auer et al. 1996, Elberling et al. 1998,  Tillman et al. 2001, Choi et al. 2002, Neeper 2002,




Rossabi and Falta 2002,  Tillman and Smith 2005).  The underpressurization of an




overlying building will produce  gas pressure differences in subsurface soils.  This




underpressurization may be caused by thermal differences between indoor and outdoor




air (stack effects), wind loading  on the building superstructure, and imbalanced building




ventilation (Nazaroff et al. 1987, Garbesi and Sextro 1989). Soil gas pressure gradients




may also be produced by a rapidly rising or falling water table, as in coastal zones (Li et




al. 2002), or through the buildup of gas pressure from decomposing organic matter inside




a landfill (Little et al. 1992).  Finally, natural temperature differences between warmer




deep and cooler shallow soil gas will cause soil  gas to rise (Gustin et al.  1997), (Krylov
                                       -9-

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and Ferguson 1998).  Density-driven flow of organic vapors may occur in the vicinity of




residual-phase organic compounds whose saturation gas densities are greater than that of




air. As organic liquids with high vapor pressures and molecular weights volatilize, the




density of the soil gas surrounding the liquid changes.  In almost all cases, organic liquids




have molecular weights which are greater than air so the resulting density-driven flow




will be in a downward direction and be proportional to soil permeability and density




differences between the vapor and air (Falta et al.  1989, Mendoza and Frind 1990a,




Mendoza and Frind 1990b). Organic  compounds for which density-driven advection may




be significant include methylene chloride, 1,2-dichloroethylene, 1,1,1-trichloroethane,




carbon tetrachloride and 1,1-dichloroethane, among others (Falta et al. 1989, Mendoza




and Frind 1990a, Mendoza and Frind  1990b).






3.2.3  Sorption







       As organic vapors move through the unsaturated zone by diffusion and advection,




they come in contact  with soil moisture, infiltrating rainwater and the soil matrix itself.




Each of these interactions may affect the concentration of the contaminant in the soil gas.




Depending on the compound,  organic vapors may adsorb to soil grain surfaces or




partition into soil organic matter (Goss 1994b, a, Goss and Eisenreich 1996, Popovicova




and Brusseau 1998, Ruiz et al. 1998, Goss et al. 2004).  Adsorption of relatively non-




polar organic vapors  is suppressed by the presence of high humidity in the subsurface, as




polar water molecules can effectively out-compete organic vapors for mineral-surface




adsorption sites (Chiou and Shoup 1985, Smith et al. 1990).  For these high-humidity




conditions, sorption may be limited to organic vapor partition into soil organic matter
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(Chiou and Shoup 1985, Smith et al. 1990).  Soil moisture trapped in unsaturated-zone




pore space or infiltrating rain water may also sorb organic vapors to varying degrees (Cho




and Jaffe 1990, Cho et al. 1993).  Finally, gas-phase organic compounds may adsorb to




the air-water interface in unsaturated zones (Kim et al. 1997, Kim et al. 1998).  Each of




these sorption processes may act as both a source and a sink of organic vapors in the




unsaturated zone.






3.2.4  Biodegradation







       Under favorable conditions, organic vapors in the unsaturated zone that partition




into soil moisture may be biodegraded in oxidation/reduction reactions by indigenous




bacteria.  The aerobic biodegradation of petroleum hydrocarbons requires an abundant




oxygen supply as well as necessary nutrients of nitrogen and phosphorus  (Ostendorf and




Kampbell 1991, Norris et al. 1994, Lahvis and Baehr 1996, Lahvis et al. 1999, Hers et al.




2000).  When oxygen is depleted, other possible electron acceptors for biodegradation of




petroleum hydrocarbons include nitrate (N(V), iron oxides (e.g. Fe(OH)3), sulfate (SC>42")




and carbon dioxide (CO2) (Norris et al. 1994).  Lightly chlorinated compounds (e.g.




chlorobenzene, dichlorobenzene) may be biodegraded under aerobic conditions. The




more highly chlorinated hydrocarbons are  recalcitrant to aerobic biodegradation but may




undergo direct or cometabolic anaerobic reductive dechlorination.









       Reductive dechlorination has been  observed to be most effective under sulfate-




reducing and  methanogenic conditions (U.S. Environmental Protection Agency 2000). In




direct reductive dechlorination, the chlorinated hydrocarbon is used as an electron
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acceptor and the bacteria gain energy and grow as a result of the reaction (McCarty




1997). In cometabolic reductive dechlorination, enzymes produced during microbial




metabolism of another hydrocarbon fortuitously reduce and dehalogenate the chlorinated




contaminant.  For either reductive dechlorination process to be successful, a primary




substrate (electron donor) such as soil organic matter, lactate, acetate, methanol, or




glucose is necessary (Vogel et al. 1987).
3.3 Building Effects







       The effects of overlying buildings play a very important role in the sub surf ace-to-




indoor-air pathway. Different building construction techniques may have different




impacts on the ability of vapors to enter indoor air space.  Buildings with basements may




have more surface area through which vapors can move inside, as well as be closer to




subsurface sources than slab-on-grade buildings. A single-pour cement foundation may




not have the "perimeter-crack" often associated with foundations whose footers and floor




are poured separately, but may still become cracked along stress lines. Foundations and




subsurface walls constructed from cement blocks may contain cracks around mortar that




can allow subsurface gas to enter the building. Homes build over a crawl space may




benefit from the dilution of soil gas by ventilated crawlspace air, but do not have the




impedance to vapors that concretes slabs provide. A properly installed and sealed vapor




barrier will provide resistance to vapor intrusion into crawlspaces.  Building




underpressurization relative to soil gas pressure can be caused by temperature differences




between indoor and outdoor air (i.e. stack effects), imbalanced air handling systems,
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wind, or barometric pressure cycles. Typical values for building underpressurization




range from 2 to 10 Pa, but may be as high as 15 Pa during the heating season (Hers et al.




2001). The underpressurization of buildings relative to subsurface pressure may cause




contaminated soil gas to flow into indoor air spaces, increasing exposure over diffusive




transport alone. However, scientists are still working to define the zone of influence




surrounding the building within which soil gas vapors are likely to flow into a building,




and the effect of construction type and soil moisture on the breadth and depth of this




zone. This soil gas flow can occur through untrapped drains, sumps, perimeter cracks,




expansion/settling cracks or utility conduits.  Conversely, a positive building pressure




may greatly reduce the intrusion of subsurface vapors by causing air to flow out these




same cracks and penetrations of the building envelope.









       Once volatile contaminants enter a building, several processes come into play that




have an effect on potential human exposure.  Building ventilation may serve to reduce




the indoor air concentration of vapors that emanate from the subsurface.  Natural




ventilation may occur through open windows, openings between windows or doors and




walls, or through cracks in walls, foundations and floors. Mechanical ventilation  may be




provided by attic fans or, in the  case of large buildings such as office buildings, with




heating or cooling systems that utilize outside air (Olson and Corsi 2002).  Ventilation is




usually described in terms of air exchanges (or changes) per hour (ACH) and values for




residential air exchange  rates are usually on the order of ~ 0.1 to 1.0 ACH (Hers et al.




2001). If ventilation is occurring with uncontaminated outdoor air then indoor air




concentrations will become diluted.  Conversely, if outdoor air is contaminated, then
                                       - 13 -

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ventilation may become an additional source of vapors entering the home.  In an indoor




air study by (Brown 2002), it was estimated that 70% of the indoor VOC concentration




resulted from unknown sources in the established dwellings that were sampled.




Additionally, VOC pollution was one to two orders of magnitude higher in new and




renovated buildings than in established dwellings, owing to building materials and




furnishings.  Dominant indoor sources of VOCs include latex paints,  carpets, and tobacco




smoke for benzene, with additional sources of wood burning, paint removers,




adhesives/tape and solvents for toluene (Hers et al. 2001).  The adsorption of VOCs by




indoor materials will reduce peak concentrations, with desorption serving to prolong the




presence of an indoor air contaminant (Meininghaus and Uhde 2002). In a laboratory




study of sorptive interactions between VOCs and indoor materials  by (Won et al. 2001),




the authors identified carpet as the most significant sorptive sink for non-polar VOCs of




the materials investigated (carpet, gypsum board, upholstery, vinyl and wood flooring,




acoustic tiles, and fruit). Virgin gypsum board was observed to be a significant sink for




highly polar VOCs.  There are also significant seasonal variations in indoor air




concentrations of VOCs, as discussed by (Rehwagen et al. 2003), who found the VOC




load in indoor air approximately three times higher in the winter months than in summer




in a 7-year study of indoor air in Germany.







4  Current Federal  Regulations





      US Environmental Protection Agency (USEPA) has promulgated a "Draft




Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater




and Soils (Subsurface Vapor Intrusion Guidance)" in the Federal Register dated




November 29, 2002 (Volume 67, Number 230 - see http://www.epa.gov/fedrgstr/EPA-







                                     - 14-

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AIR/2002/November/Day-29/a30261.htm for a description).  The guidance in this




document is recommended for use at RCRA Corrective Action sites, CERCLA (National




Priorities List and Superfund Alternative) sites, and Brownfields sites, but not




recommended for use at Subtitle I Underground Storage Tank (UST) sites (U.S.




Environmental Protection Agency 2002). As  such, the contaminants for which this draft




guidance will be used are mainly  chlorinated solvents and will not routinely involve




petroleum hydrocarbons (a common fuel in underground storage tanks). This guidance




document does not impose any requirements or obligations on EPA, states, or the




regulated community, and the sources of authority and requirements for addressing




subsurface vapor intrusion currently remain the relevant statutes and regulations (e.g.,




RCRA, CERCLA and the NCP).









      Procedures in the draft guidance for evaluating vapor intrusion include a tiered




screening system. Primary screening (Tier 1) involves obtaining knowledge of the




chemicals present at the site, determining if they are sufficiently volatile and toxic to pose




a potential threat, and determining if inhabited buildings are located (or will be




constructed) above or in close proximity to the subsurface contamination. If primary




screening does not rule  out the vapor-to-indoor air pathway, then Secondary screening




(Tier 2) is recommended.  Secondary screening involves comparing measured or




"reasonably estimated"  concentrations of contaminants in either ground water, soil gas or




indoor air, to generic attenuation factors for a particular risk level.  If unacceptable




exposure cannot be ruled out from the generic attenuation factor, measured




concentrations are compared to attenuation factors based on soil type and depth to
                                      - 15-

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contamination.  These screening level concentrations were derived using the (Johnson




and Ettinger 1991) simplified model. If Tier 2 screening cannot rule out vapor intrusion,




then a Site-Specific Pathway Assessment (Tier 3) is recommended. This tier requires




direct measurement of foundation air and/or indoor air concentrations from a subset of




potentially effected buildings, and complementary site-specific modeling as appropriate.




USEPA's Office of Emergency and Remedial Response (OERR) publishes a "User's




Guide for Evaluating Subsurface Vapor Intrusion into Buildings" (Environmental Quality




Management 2003) to be used as a companion for the draft guidance document.  This




user's guide includes detailed information on using OERR-distributed spreadsheets that




run the Johnson and Ettinger model (U.S. Environmental Protection Agency 1999) which




are available at




http://www.epa.gOv/superfund/programs/risk/airmodel/johnson_ettinger.htm. If a




significant risk for vapor intrusion cannot be ruled out then remedial action may be




required, as determined by the site manager.







4.1 States





       Under the Leaking Underground Storage Tanks Program (LUST), individual




states are required to address accidental petroleum discharges within their borders. As




there is no federally-mandated requirement to evaluate the vapor intrusion pathway,




states' responses to vapor intrusion from petroleum leaks vary greatly and are evolving as




science and policy in this field progress. Several state regulations include the evaluation




of vapor intrusion as a potential exposure pathway of organic contaminants. Links to




many of these states' documents can be found at http ://www.envirogroup.com/links.htm




or http://www.geosyntec.com/vi_links.asp. Additionally, the Interstate Technology  &







                                      - 16-

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Regulatory Council (ITRC) publishes a background document for vapor intrusion at




Brownfield sites (http://www.itrcweb.org/BRNFLD-l .pdf) and is currently at work on a




document discussing vapor intrusion issues at petroleum-contaminated sites.







5  Vapor Intrusion  in the  Field





       Few peer-reviewed articles exist in the scientific literature that present indoor air




and subsurface vapor data from sites impacted by vapor intrusion of VOCs. However,




several newspaper articles receiving national attention were published in the Denver Post




in early 2002 by journalist Mark Obmascik about human exposure to 1,1-Dichloroethene




(DCE) vapors from the contamination at Denver's Redfield rifle scope factory (Obmascik




2002e, a, c, d, b). This series of articles were highly critical of EPA's use of the




simplified Johnson & Ettinger model to screen sites for potential vapor intrusion.









       In evaluating the Johnson and Ettinger model, Hers et al. (2003) present a review




of previously published data from several field sites with contamination from both




chlorinated and BTEX compounds.  Sources ranged from 0.5 m to 10.7 m below




foundation and included both ground water and soil gas sources.  For petroleum




hydrocarbon sites, measured vapor attenuation factors ranged from ~10"7 to 10"5.  For




chlorinated solvent sites, ground water attenuation factors were on the order of 10"5 to 10"




4 for the most reliable data sets. The authors conclude that, for almost all cases, the best




estimate Johnson and Ettinger model-predicted attenuation factors were one to two orders




of magnitude more conservative than 50th percentile or median measured values.
                                      - 17-

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       One of the most thoroughly studied vapor-intrusion sites in the United States is




the Colorado Department of Transportation Materials Testing Laboratory (CDOT-MTL)




located in Denver, CO. More than 1,000 groundwater, soil gas, and indoor air samples




were analyzed (cost estimated >$ 1,000,000) in studying the potential impacts of




chlorinated solvent contamination in soil and groundwater (Johnson et al. 2002). The




authors of this study note that assessing the significance of the sub surf ace-to-indoor air




pathway via direct measurement is likely to be impracticable at many sites and




recommend using limited site data (depth to groundwater, qualitative soil boring data,




and approximate building characteristics) with a screening level model such as Johnson




& Ettinger. Empirically-derived vapor attenuation factors for the site fall in the range of




10"6 to 10"4, with an overall average of 3 x 10"5.









       In Fitzpatrick and Fitzgerald (2002), the authors present a review of




Massachusetts VOC-contaminated field site data in order to determine field attenuation




coefficients and evaluate the J&E transport model in predicting indoor air concentrations.




To identify sites where ground water VOC contamination impacted indoor air, a database




search was conducted on over 6,000 files maintained by the Massachusetts Department of




Environmental Protection (MADEP) in its Northeast Regional Office, servicing 95 cities




and towns in the greater Boston metro area representing a population of 3 million people




(roughly half the state's population).  An initial search for impacted indoor  air resulted in




a list of 165  sites, which were investigated to determine if annual average depth to ground




water is <5m below ground surface and ground water contamination exists within 10m of




an occupied building.  Of the 165 sites, 68 had relevant ground water, soil gas and/or
                                      - 18-

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indoor air data.  The list was narrowed to include sites with high-quality data and to




eliminate sources in soil and 47 sites and sub-sites were selected from these criteria. Of




the 47 sites identified, 26 (55%) had been impacted by  chlorinated VOCs, while 21




(45%) were associated with gasoline releases. No site had been impacted by both classes




of compounds.  Of the impacted buildings, 24 (52%) were residential homes, 1 (2%) was




a school, and 21 (46%) were commercial buildings. Attenuation coefficients were




calculated for 22 sites with available indoor air and soil gas data with values ranging




from 2x 10"6 to 1 x 10"1. Attenuation at TCE sites (11) ranged from 9x 10"5 to 9.7x 10"2




while benzene sites (3) ranged from l.SxlO"5 to 4xlO"5. Groundwater concentrations of




VOCs protective of indoor-air exposure, codified by MADEP as "GW-2  Standards",




were calculated by the agency using the Johnson and Ettinger model. Out of the 35




relevant study sites with available GW and IAQ data, three had VOC levels in ground




water below acceptable GW-2 standards and also had acceptable VOC measurements in




indoor air.  Unacceptable indoor air concentrations were measured at 15 sites which




exceeded the GW-2 standards.  Of these 15 sites,  14 were associated with chlorinated




VOCs and 1 was associated with a gasoline release. 13 sites with ground water




concentrations above GW-2 standards exhibited acceptable levels of indoor air VOCs.




Of these sites, 10 were related to gasoline releases with the other 3 above a TCE plume.




Lastly, 4 sites had unacceptable indoor  air concentrations but GW concentrations less




than GW-2 standards. Based on a review of the data, authors conclude that attenuation




for chlorinated VOCs appear to be about 2 orders of magnitude higher than the 5 x 10"4




value used in developing the MADEP GW-2 standards and attenuation values for




chlorinated VOCs appear to be significantly higher than values for nonchlorinated VOCs.
                                      - 19-

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It appears that the GW-2 standards for chlorinated sites may not be protective of indoor




air exposures, while standards derived for nonchlorinated sites are adequate.  It is




difficult to draw conclusions regarding the extent of vapor intrusion of petroleum




hydrocarbons from this study due to only three sites with benzene contamination having




data quality sufficient to calculate an attenuation coefficient. However, a state-wide




review of field-collected data such as this that is subject to data-quality and peer-review




has potential to be very useful in establishing actual expected ranges of exposure to




subsurface vapors in a wide range of sites and climatic conditions.









       Fischer et al. (1996) present a report of a field-study of soil-gas transport of VOCs




into a building at a former gasoline station at the Alameda Naval Air Station  (ANAS) in




California.  High VOC concentrations  (30-60 g m"3) were measured in soil gas 0.7-m




below the building.  Indoor air concentrations  had attenuated by ~106 due to a sharp




gradient in soil-gas concentrations between 0.1 and 0.7 m (attenuation of ~103) and the




dilution of soil gas entering the building by wind-driven building ventilation  (an




attenuation factor of ~103).









       Moseley and Meyer (1992) published an air, soil-gas and groundwater monitoring




study investigating the source and extent of petroleum contamination in an elementary




school located adjacent to a gasoline station and a petroleum tank farm in the Midwest.




Ground water and soil-gas data indicated a contaminant plume had formed between a




tank known to have leaked 20,000 gallons of gasoline and the school building.  After




odor complaints by staff and students at the school, the local fire department  measured
                                       -20-

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levels of airborne vapors up to 40% of the lower explosive limit, and the school was




evacuated.  Subsequent investigations involving air samples taken inside the school




revealed benzene concentrations that were slightly elevated over outdoor concentrations.




However, indoor total hydrocarbon concentration (THC) levels were up to 40 times




outdoor levels. Authors noted many possible indoor sources of THC including paint, ink,




clothing, gas and oil heating systems, and cleaning agents.  Concentrations found in the




school below floor level were significantly higher than both indoor and outdoor




measurements. Crawlspace vapors were 2600 parts per billion (ppb) benzene, compared




with no-detect (ND)-5 ppb in the classroom.  Crawlspace THC concentrations were




120,000 ppb, compared with 530-2600 ppb in classrooms.









       Hodgson  et al. (1992) present a study at a single-family residence located




approximately 70m from a landfill perimeter in Stanislaus County, CA. The house was a




single-story structure build over a basement and a garage. Twenty six VOCs were




identified in soil gas samples, mainly halogenated hydrocarbons and oxidized




compounds. Thirteen compounds were also detected in indoor air, although at very low




concentrations (6 ppb or less).  Authors conclude that the existence of soil-gas




contamination alone is not sufficient to result in significantly elevated indoor exposures.









       A line-leak at a petroleum distribution terminal that  had been operating for about




70 years produced a dissolved-phase gasoline  plume in the groundwater that migrated




beneath a residential neighborhood, described in a study by Laubacher et al. (1997).  No




NAPL-phase contamination was noted and dissolved HC concentrations in two
                                      -21 -

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monitoring wells have varied from 12,000 to 39,000 ppb total BTEX. TPH, CO2 and O2




data from outside the building slab indicate that aerobic bioactivity may be occurring




down to a depth of approximately 10 feel below ground surface, and anaerobic activity




from 10 ft to the water table.  Vapor profiles from beneath the building indicate that the




vadose zone beneath the basement is completely anaerobic. Authors state that this




suggests that the hydrocarbon-vapor plume has accumulated beneath the basement slab




because it is less permeable to diffusion.  Indoor air was sampled on "a number of




occasions"  and never produced readings greater than control homes (1-2.2 ppb benzene).




This study is significant in that it presents data indicating an anaerobic zone may occur




beneath a building due to oxygen replenishment limitations. Even though this anaerobic




region would eliminate aerobic biodegradation of petroleum vapors, there is no evidence




in this study of increased vapor concentration inside the home.









       Hers and Zapf-Gilje (1998) and Hers et al. (2000) present data from the




Chatterton field site  located near Delta, BC (near Vancouver).  This former petrochemical




plant has BTX residual NAPL distributed over a 1-m interval at the water table. Regular




soil gas monitoring was used to assess the effect of seasonal changes on soil gas fate and




transport with vadose zone.  A small  greenhouse was built on a 6.1  x 9.3-m at-grade




concrete slab to investigate vapor intrusion into buildings. Indoor air, outdoor air, and




flux chamber measurements were conducted in and around the greenhouse under both




static and dynamic (mechanical ventilation) conditions.  Results indicate that BTX




concentrations were  similar at vapor probes located near the north,  east and south edges




of the slab.  In contrast, BTX concentrations directly below the west edge of the slab
                                      -22-

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were slightly higher, and BTX concentrations at the probe directly below the slab were




significantly higher (> 2 orders of magnitude). Statistical hypothesis testing indicated




that, in all but one case, the mean indoor and outdoor concentrations were significantly




different for the dynamic case (using a two-tailed test and significance level of 0.10).




Differences were not statistically significant for the static case.









    Although not a peer-reviewed scientific journal, a recent publication by the petroleum




industry group "American Petroleum Institute" by Roggemans et al. (2001) presents an




empirical assessment of soil gas profile data from previously published, unpublished and




two new petroleum hydrocarbon release sites. The objective of the study was to assess




whether or not the  soil gas  data was consistent with the occurrence of aerobic




biodegradation. While evidence of biodegradation of organic vapors was seen for several




of the sites, the authors were unable to correlate the lack of signs of biodegradation with




the  presence of surface features such as pavement, buildings or with very wet surface




soils.  Of the 28 soil-gas profiles presented, 7 (25%) were able to be fit by a model




considering diffusion-only  and no biodegradation. Additionally, the authors compared




flux predicted using the deepest available soil gas concentration with flux computed




using the concentration measured closest to land surface. While 6 of the data sets




presented indicate vapor fluxes attenuated by two to four orders-of-magnitude, in 5 of the




15 sets (33%) the effect of aerobic biodegradation was seen to be insignificant.
6  Review of Vapor Intrusion Models
                                      -23 -

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       The following is a brief review of several vapor intrusion models that have




appeared in the scientific literature or in conference proceedings. The intent here is not to




present an exhaustive description of each model, but to introduce the approach and




features that make each of them unique.  While much of the current understanding of




vapor intrusion of organics into buildings stems from research on radon intrusion into




buildings, only models dealing specifically with organic vapors are presented here.




Readers are directed to Clements and Wilkening (1974), Schery et al. (1984), Schery et




al. (1988b, a), Nazaroff (1992), and Holford et al. (1993) for further information on radon




modeling.









       Paul Johnson and Robert Ettinger (1991) present the first major modeling effort of




vapor intrusion of VOCs into indoor air.  Beginning with the general, transient transport




equation that includes advection, diffusion, and formation in 4 phases (vapor, sorbed, free




phase and soil  moisture), the authors develop a "heuristic" (i.e., for the purposes of




education or problem solving) equation for predicting indoor air concentration from soil




data.  The free-phase is assumed to be small enough to ignore, and the sorbed phase is not




included in further model development, leaving only the aqueous and vapor phases. The




vapor phase is related to the aqueous phase through Henry's law and diffusive transport is




assumed significant only in the vapor and soil moisture phases.  The Millington (1959)




approximation is used for estimating the  effective diffusion coefficient through the




unsaturated zone from porosity and moisture content information.  Vapor flow in




response to building depressurization is described by Darcy's law. Next, chemical and




biological transformations are ignored and a steady-state solution is assumed. Diffusive
                                      -24-

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transport from the source to a region near the structure is approximated by discretized




Pick's law, incorporating an overall effective diffusion coefficient for subsurface layers.




The diffusion length is taken to be the distance between the source and the foundation -




assuming that convection, when significant, is only dominant in a region very near the




foundation.  Uniform convective velocity is approximated by dividing soil gas velocity




by crack area.  The mass transport rate by diffusion between source and foundation is set




equal to the mass entry rate of contaminant into the building and solved for soil gas




contaminant concentration. This result is substituted into the steady-state, 1-d solution to




the transport equation to obtain the rate of contaminant entry into the building.  The




relationship for the rate of contaminant entry is incorporated into a steady-state mass




balance for a basement or building to produce an explicit expression for indoor air




concentration. This indoor air mass balance assumes no other sources or sinks and a




well-mixed building.  An attenuation coefficient (Cbuiiding/CSOurce) is produced containing




three  dimensionless groups: one the equivalent Peclet number, one the attenuation




coefficient for diffusion-dominated transport to a bare-dirt floor, and one the attenuation




coefficient for convective transport from a source located adjacent to the building (i.e. no




diffusion length). Three limiting situations are examined: convection as the dominant




mechanism through floor and walls, diffusion the dominant mechanism, and no building




ventilation.  The solution is extended to accommodate diffusion through permeable




below-grade walls rather than foundation cracks. Finally, expressions are presented for




evaluating if a transient solution is appropriate  and a transient solution for depleting




sources is derived. A sensitivity analysis is presented for several parameters including




crack-factor and air permeability. A separate review of the Johnson and Ettinger model
                                       -25-

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with a detailed sensitivity analysis is also presented in (Johnson 2002) and (Johnson




2005).









       Little et al. (1992) present a first-order estimate of the elevation in indoor VOCs




from subsurface contamination analogous to radon transport. Authors state that VOC




attenuation coefficients would be smaller because radon emanates from soil right next to




the building and VOCs must be transported over some distance. Model assumptions




include a constant source, isotropic and homogenous medium, and that VOCs are




immediately swept into the building when they arrive at the zone of influence.  A one-




dimensional, diffusion-only model utilizing the Millington and Quirk relation for




effective diffusion coefficient and retardation (assuming linear sorption to soil moisture)




is presented. The resulting indoor air concentration is estimated as the rate of VOC mass




entering the building divided by the volumetric flow rate of air through the building.  A




transient solution for an attenuation coefficient is presented for a planar source  at depth L




diffusing through originally uncontaminated soil. A second transient attenuation




coefficient solution is presented for a uniform source of contamination surrounding the




building. Finally, a one-dimension advection/sorption model is presented incorporating a




Darcy velocity for vapor transport from a landfill to a building.  A steady-state  solution is




given along with estimated time for the contaminant to travel from the landfill to the




building.









       Sanders and Stern (1994) adapt two previously published time-varying,




deterministic models to predict indoor air concentrations and dose. The Little et al.
                                       -26-

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(1992) vapor-intrusion model was modified to allow first-order decay of the contaminant




source.  The Jury et al. (1990) vapor transport model was multiplied by the area of




influence of the depressurized building and divided by the building air flow rate to obtain




indoor air concentration.  Expressions for dose (integration of the product of




concentration and inhalation rate) for both models are also presented.









       Ferguson et al. (1995) present an equilibrium, analytical, 3-box model for indoor




air concentration. Linear partitioning is assumed between soil, soil gas and soil water.




Fickian molecular diffusion is allowed between the following three compartments:  soil,




living space, and outdoor space (including attic).  Diffusion between each compartment is




presented as a series of diffusion through layers comprising the boundaries of the




compartment (e.g. through concrete, insulation, decking, etc.).  Suction flow is  allowed




and determined using Darcy's law, with pressure difference measured between  indoors




and soil-gas pressure (taken as  equal to atmospheric pressure) and pressure flow length




(characteristic path length) assumed to be 1 meter. This length is the path length of the




flux beneath outside walls. Ventilation is computed from air exchanges per hour. A time




averaged production term [ng/hr] is introduced to allow for indoor sources of




contamination (e.g cigarettes, paints, oils glues, cleaning fluids, etc.). All of these terms




are combined in a mass balance equation to determine indoor air concentration. Five




limiting cases are derived from the general  mass balance: no indoor contamination




sources; unpolluted soil and polluted outdoor air; unpolluted outdoor air; unpolluted soil;




unpolluted outdoor air and unpolluted soil (i.e. indoor sources only). Finally, a first-order
                                       -27-

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decay process of benzene in bulk soil, due to volatilization and chemical/biological




degradation, is incorporated.









       Waitz et al. (1996) present a spreadsheet-based risk assessment model




incorporating human exposure to VOCs in indoor air that is unique in its applicability to




homes on crawl spaces.  Sources for the model include well-mixed groundwater




contamination both beneath the structure and intercepting the crawl space, a floating soil-




contaminant layer both beneath the structure and intercepting the crawl space, a trapped




NAPL phase in the unsaturated zone, and a NAPL phase beneath the water table.




Contaminant fluxes are calculated assuming linear, equilibrium partitioning between soil-




water and soil-gas (Henry's Law) and linear partitioning between soil and soil-water




(linear sorption isotherm). Both diffusion and pressure-driven flow are allowed in the




unsaturated zone. Air flux from the crawl space through the floor into indoor air is




driven by advection.  An indoor air concentration is computed from this flux and




ventilation and air-exchange rates









       Jeng et al. (1996) begin their model development with a one-dimensional




unsteady-state mass balance partial differential equation containing vapor, water and




solid phases and first-order degradation.  Simplifying assumptions include no advective




flux, both vapor and liquid phase diffusion are described by Pick's  law (with Millington-




Quirk effective diffusion coefficient), degradation occurs only in the liquid phase and the




rate constant is based only on the dissolved concentration. Linear equilibrium




relationships are assumed between water/air phases (Henry's Law) and soil/water phases
                                       -28-

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(KD). Boundary conditions of an exponentially depleting source and no change in




concentration at infinite distance are used along with an initial condition of zero




concentration to develop an analytical solution to the transport equation (taken from van




Genuchten and Alves (1982)).  Vapor flux is calculated from concentration data.




Concentration in a building is obtained from the product of area, volume, air exchanges




and flux. Several cases are presented for comparison including a constant source with no




retardation or attenuation; a constant source with retardation but no degradation; a




constant source with water and soil partitioning and including degradation; and a




depleting source with all attenuation mechanisms.  A sensitivity analysis is performed on




water content, organic carbon content and degradation/source depletion.









       Sanders and Talimcioglu (1997) compare the previously published modifications




to the Jury et al. (1990) model (Sanders and Stern 1994) with a second model named the




Integrated Moisture Plus Contaminant Transport (IMPACT) model.  IMPACT is a 2-




dimensional model designed for the calculation of soil cleanup criteria for hazardous




waste sites as controlled by the soil-to-groundwater pathway, but also includes the




volatilization pathway. The model incorporates equilibrium partitioning, diffusive,




degradation, and mass-balance processes equivalent to the Jury model, but also includes




hydrodynamic dispersion.  It can also simulate soil moisture contents and moisture




transport (using Richard's equation and Darcy's Law).  An expression for hydraulic




conductivity is used, and soil moisture retention and soil diffusivities are calculated by




the Clapp and Hornberger (1978) empirical equation. The contaminant transport and




moisture flow equations for IMPACT are solved using  numerical techniques. The
                                      -29-

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modified Jury model and the IMPACT model are compared under hypothetical and actual




moisture content data.









       Krylov and Ferguson (1998) present a model incorporating diffusive and




advective transport between different model compartments (similar to Ferguson et al.




(1995)).  Diffusive transport is allowed from soil to crawl space, from crawlspace to




outdoor air, from crawlspace to living space, and from living space through walls and




ceiling. Advective transport may occur from soil gas to crawlspace (by wind-induced




pressure gradients, from crawlspace to living space (from stack and ventilation effects),




from crawlspace to outdoor air (by ventilation of crawlspace), from living space to




outdoor air (by ventilation of living space).  These diffusive and advective fluxes are used




in flux balance equations to derive indoor air concentration. Effective diffusion




coefficients between compartments are computed from individual layers by Millington




relation and inverse summed for total layer diffusivity.  Soil gas flow is assumed to a




depth of 1 m. A relation for pressure gradients based on wind speed is incorporated.




Resulting solution is a steady-state, analytical relationship between indoor air




concentration and compartmental diffusions, air flow rates, surface areas, initial soil




concentration and partition coefficient (including air, soil and water partitioning). A first-




order decay process for total soil  concentration is added to account for volatilization and




biodegradation effects on source  concentration.  Permissible bulk soil concentrations for




benzene indoor air concentration of 5 ppb are determined for differing wind loadings and




building systems.
                                       -30-

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       Ririe et al. (1998) modify a mathematical approach given in an unpublished




presentation by J. Gustafson in 1997 that was originally used for laboratory studies of




diffusion and biodegradation.  This equation relates a normalized concentration profile




(c/c0) to exponential terms that include a first-order degradation constant and an effective




diffusion coefficient. An expression for attenuation is also given, defined here as the




ratio of surface flux with and without biodegradation. Authors also describe the Orange




County Health Care Agency (OCHCA) Vapor Transport Model that includes source




partitioning from water (Henry's Law) or liquid phase (Raoult's Law) and transport by




diffusion. Attenuation through a slab is determined by a "slab factor". Flux is




instantaneously mixed in the building and is in dynamic steady-state with the exchange




rate (ventilation) of building air with outside air.  Flux may also occur from the




subsurface to outside air, and then enter the building by air exchange.  Authors propose




plotting field data with Gustafson relationship to curve-fit a bioattenuation factor. This




factor can then be used in the OCHCA model to determine flux.









       Johnson et al. (1998, 1999) present refinements to the Johnson and Ettinger




(1991) J&E model. A vapor source expression is presented that assumes a single-




component, linear-partitioning relationship and three phase equilibrium (vapor, sorbed,




dissolved phases) source. A relation is given for estimating time to steady state, derived




from solution to transient diffusion with step-change boundary condition at time zero.




An additional relation is presented for a concentration profile with first-order




biodegradation.  This first-order relation is then incorporated into the J&E attenuation




equation.  A family of type-curves is presented to facilitate determining biodegradation
                                       -31 -

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rate constant from field data. Equations for describing a "dominant-layer" of degradation




are presented and incorporated into attenuation expression.









       Olson and Corsi (2001) present an exposure model based on mass balance




equations for two compartments: basement and ground floor.  Processes include air flow




from ambient (outdoor) to basement, air flow from basement to ground floor and ground




floor to basement, and a term for the pollution emission rate from soil to the basement.




Simplifications allow for CSTR-type equations to be integrated, producing a time-




varying solution for pollutant concentrations in the basement and first floor.  Steady-state




solutions are also provided.  Mass intrusion was evaluated from a measured SFe emission




rate, Darcy's Law and a steady-state mass balance.









       Parker (2003) developed a model to assess human exposure and health risk




associated with VOC emissions to indoor air. This model considers a finite source mass;




vapor transport due to advection, diffusion,  and barometric pumping; oxygen-limited




biodecay; and building underflow.  A relationship for vapor source above a dissolved




contaminant in groundwater (that incorporates changes in aqueous concentration with




time) involving Henry's Law is presented.  An average source concentration over a




defined time period is also presented.  Vapors above a NAPL  source are computed using




Raoult's law, with relationships for the decrease in NAPL thickness due to volatilization




(leaching is ignored) and time to deplete the COC from the entire NAPL  source also




presented.  For transport, a solution to quasi-steady-state diffusion is presented with the




effective diffusion coefficient including both Millington tortuosity and dispersion from
                                      -32-

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barometric pumping. Relations for mean Darcy velocity at ground surface as well as the




depth-averaged velocity are given. The depth of barometric pressure propagation is taken




as the lesser of the depth to ground water (capillary fringe), or max depth limited by




permeability (a relationship is provided to determine this depth). Biodecay rate per soil




volume is the lesser of (1) the maximum, non-oxygen-limiting "intrinsic" rate, (2) the




rate limited by diffusion oxygen transport from the soil surface or soil-building interface,




or (3) the rate limited by oxygen from air flowing from soil under the building (by wind).




Relationships are given for each of these cases.  A relationship is also given for the




increased volatilization that will result from biodegradation of vapors. For mixtures of




hydrocarbons, the total potential for decay is distributed among multiple species; a




relative biodecay rate (of dissolved-phase concentrations) is computed and included in




each of the rates mentioned above. The Parker model uses the Johnson and Ettinger




(1991) model for advective and diffusive transport into a building while adding terms




accounting for loss due to airflow under the building and biodecay.









7  Conclusions and Future Work







       The intrusion of organic vapors from the subsurface into indoor air spaces may be




a human health concern from either carcinogenic or toxic exposures.  While much




attention has been focused on vapor intrusion in recent years, many challenging issues




remain. It is still unclear, based on published studies, whether or not the problem is




widespread in nature or just confined to a few sites where climatic, hydrogeologic and




other conditions serve to link the subsurface-to-indoor air pathway. Difficulty in




evaluating whether or not VI is occurring stems from the temporal and spatial variability







                                      -33 -

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in soil gas and sub-slab measurements, unknown indoor sources confounding indoor air




sampling, and a lack of information on the accuracy of models. While the Johnson and




Ettinger (1991) model is widely used and has become the de-facto model of choice for




screening sites for vapor intrusion, the model is still not routinely used with a thorough




uncertainty analysis to obtain a range of reasonable outputs.









       Continued research in the field of vapor intrusion will focus on providing answers




and guidance towards overcoming these current limitations.  A national database of data




from vapor intrusion sites currently being identified will help to identify the




extensiveness of the problem. Improvements in field data collection including




identifying and removing indoor VOC sources before performing indoor air sampling




could greatly aid in establishing the vapor intrusion pathway. Additional research in




measuring flux at land surface outside the footprint of a building may provide researchers




with information on seasonal variability in VOC flux in the subsurface. Investigations




into the critical parameters of subsurface vapor transport, particularly moisture movement




beneath buildings, should be performed. Finally, users who rely on models for vapor




intrusion screening and risk assessment should be provided with models that




automatically incorporate uncertainty of input parameters and provide a range of outputs.




This would allow users to gain confidence in model predictions or allow users to better




focus their efforts in obtaining field data to improve model uncertainty.
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