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Overview of the Vapor Intrusion Pathway
Vapor intrusion is a pathway of potential human health concern at sites where volatile organic chemicals (VOCs) or other potentially toxic volatile substances are present in soil and/or groundwater, and where buildings (homes, workplaces) currently exist or might exist in the future.
Vapor intrusion is a multi-step transport process that may be thought of as occurring in a series of steps, as follows:
- A VOC that is either dissolved in groundwater or adsorbed to sub-surface soil is volatilized from the liquid or solid phase to the vapor phase and is released into the pore space that exists between soil particles.
- The vapor phase VOC diffuses upward through the pores in the soil, usually undergoing repeated cycles of adsorption to and release from the soil particles and/or the water associated with the soil particles.
- As the VOC approaches the foundation of a building, it may move from the soil gas phase into the building by one or both of two pathways: a) by diffusion from the soil gas through cracks in the foundation into the indoor space of the building, or b) by bulk air flow from the soil gas into the building. The latter occurs because most buildings have a lower indoor air pressure than the outdoor (soil gas) pressure, and gasses will flow from areas of higher pressure towards areas of lower pressure.
There are a large number of factors that influence the level of VOCs that may build up in indoor air as a result of the vapor intrusion pathway, including:
- The concentration of VOC in the source medium (groundwater or subsurface soil).
- The attributes of the VOC, including its inherent volatility, solubility in water, molecular weight, chemical stability, etc.
- The attributes of the soil, including particle size, permeability, organic carbon content, water content, temperature, etc.
- The vertical distance between the top of the contaminated source medium and the bottom of the building foundation.
- The pressure difference between the building and the soil gas.
- The size and ventilation rate of the building.
EPA Guidance and Resources on Vapor Intrusion
EPA National Guidance
OSWER's 2002 draft guidance provides current technical and policy recommendations on determining if the vapor intrusion pathway poses an unacceptable risk to human health at RCRA, CERCLA and Brownfields sites.
The draft guidance suggests following a tiered approach as follows:
- Perform a primary screening to identify the potential for exposure from the vapor intrusion pathway and to determine if time critical actions are necessary,
- Perform a secondary screening to determine if concentrations of volatile organic compounds (VOCs) detected at the site are high enough to warrant further evaluation, given site conditions, and
- Perform a site-specific pathway assessment to characterize the nature and extent of VOC contamination and to determine if the vapor intrusion pathway is complete.
More recently, EPA has been working to update the guidance for evaluation of the vapor intrusion pathway. EPA made available external review drafts of the guidance documents for public comment. EPA is working to issue final subsurface vapor intrusion guidance. When released, this will supercede all previous guidance documents on vapor intrusion. Further information on EPA’s efforts and additional vapor intrusion resources can be found here.
EPA Vapor Intrusion Technical Documents and Tools Database
EPA has prepared several documents and tools to support development of its Final Subsurface Vapor Intrusion Guidance. Several of these are described in brief below and links to these documents and tools can be found here.
Vapor attenuation occurs as a result of the processes that control vapor transport in soil (e.g., diffusion, advection, sorption, transformation reactions) coupled with the dilution that occurs when the vapors enter a building and mix with indoor air. The process by which vapors migrate to and into buildings is complex and depends on site-specific conditions which can vary over time and space. EPA has developed a Vapor Intrusion database to store and analyze data on the relationship between concentrations of volatile chemicals in the source media (Csource) and concentrations in indoor air in the home (Cindoor). This ratio (Cindoor/Csource) is referred to as the attenuation factor (AF). Attenuation factors offer a fast and easy way to estimate indoor air based on measures of Csource: (Cindoor = Csource × AF)
EPA has developed a spreadsheet calculator that provides vapor intrusion screening levels (VISLs) for groundwater, soil gas and indoor air.
Sampling and Analysis Guidance
Characterizing the vapor intrusion pathway generally requires sampling and analysis of indoor air, soil gas, groundwater and/or subsurface soil. This page provides further guidance on the sampling and analysis of VOCs in the environment.
EPA Vapor Intrusion Models
EPA has developed a series of spreadsheets based on the mathematical model of Johnson and Ettinger (1991) for volatile contaminant partitioning and subsurface vapor transport from sub-surface soil, groundwater, and/or soil gas into buildings. The spreadsheets allow a user to input site-specific parameters including soil properties and building parameters. Default model input parameters represent mean or typical values that should provide a reasonably (but not overly) conservative estimate of the vapor intrusion AF for a site. User's Guide for Evaluating Subsurface Vapor Intrusion Into Buildings (2002) (PDF) (133 pp, 1.2 MB, About PDF).
A link to the most recent versions of these spreadsheets is provided below, along with links to resources describing the assumptions, limitations and uncertainties associated with fate and transport modeling to characterize the vapor intrusion pathway.
- Johnson and Ettinger (1991) Model for Subsurface Vapor Intrusion into Buildings
This page includes a link (under "3-Phase System Models and Soil Gas Models") to a zip file containing the Johnson and Ettinger spreadsheet tools for modeling vapor intrusion from soil, groundwater and soil gas.
- Uncertainty and the Johnson-Ettinger Model (PDF) (43 pp, 645 K, About PDF) (EPA/600/R-05/110, September 2005)
This document addresses uncertainty in the selection of parameter values.
- Appendix G of OSWER’s 2002 Draft Guidance (PDF) (27 pp, 481 K, About PDF)
Describes the assumptions and limitations of the Johnson and Ettinger Model (JEM) and provides guidance for using the JEM as a site-specific tool to estimate indoor air impacts resulting from vapor intrusion.
Vapor Intrusion Mitigation
Remediation of vapor intrusion impacts may be required when the results of a vapor intrusion assessment indicate that the pathway is complete and that risks are above a level of concern. This page provides more information on vapor intrusion mitigation.
Vapor Intrusion Case Studies in Region 8
- Redfield Site Exit, Denver, Colo.
Information on the investigation and remediation efforts for 1,1-dichloroethene and trichloroethene contaminated groundwater and indoor air in homes near the Redfield Site.
- Lockwood Solvent Ground Water Plume Site, Billings, Mont.
Summary of the investigation and proposed remedial action of tetrachloroethene, trichloroethene, dichloroethene, vinyl chloride, and carbon tetrachloride contaminated groundwater underlying 580 acres in Billings, Montana.