Vapor Intrusion (VI)

Vapor intrusion (VI) is a term that describes the migration of sufficiently volatile compounds from subsurface media (such as soil or groundwater) into the indoor air of buildings. VI may result in unacceptable chronic or even acute exposures to occupants (e.g., residents or employees) of buildings near or overlying volatile compound contaminated sites. VI is assessed as a part of investigation and cleanup activities performed under environmental restoration programs, but characterization can be difficult because several consumer products also contain volatile organic compounds (VOCs). VI is typically addressed using a combination of subsurface source remediation and VI mitigation of exposure in buildings. Because of the rapid biodegradation of petroleum hydrocarbon vapors, fewer petroleum hydrocarbon sites have a complete vapor intrusion pathway than sites containing chlorinated solvents.

CONTRIBUTOR(S): Chris Lutes, Dr. Loren Lund with John Lowe

Key Resource(s):

• OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air
• A Quantitative Decision Framework for Assessing Navy Vapor Intrusion Sites

Introduction

VI is the migration of chemical vapors from a subsurface vapor source in soil or groundwater to indoor air. VI is one example of an exposure pathway - a means by which hazardous substances can move through the environment and come into contact with people. Other examples of exposure pathways include groundwater ingestion and direct contact with contaminated soils. Volatile chemicals, including chlorinated compounds such as trichloroethene (TCE) and tetrachloroethene (PCE), are often considered constituents of concern for VI. Petroleum hydrocarbons such as benzene, toluene, ethylbenzene, and xylenes can also cause VI problems, but this occurs less frequently because these compounds readily biodegrade in soil. Some inorganic substances such as mercury also might volatilize from the subsurface into indoor air.

Vapor Sources in a Conceptual Site Model

A conceptual site model (CSM) is a simple description of the key processes at a site and is used to support characterizations, risk assessments, remediation planning, and implementation. In general, the CSM for VI consists of vapor sources, subsurface vapor migration potential, driving forces from the subsurface into buildings, and mixing of chemicals in indoor air. A simplified VI CSM is as follows (Fig. 1):

Vapor sources: Two classes of vapor sources may be included in a CSM:

• Primary releases are located immediately adjacent to former chemical release locations such as underground storage tanks, landfills, or spill sites. Primary releases that are adjacent to or within buildings, such as spills from a vapor degreaser in a shop, have a higher potential for creating a significant VI pathway.
• Secondary vapor sources are located at a distance from chemical release locations. For example, a volatile chemical plume that has migrated through soil vapor or groundwater away from a spill site (such as a vapor degreaser). The potential for a significant VI pathway from a secondary vapor source depends on the concentrations and depth in soil vapor or groundwater, the properties of the soil underlying the building, and the locations of atypical preferential pathways for vapor migration (e.g., sewer utilities, McHugh, 2016).

Vapor Migration and Driving Forces

Contaminants in groundwater can volatilize into the overlying soil. The tendency for contaminants to volatilize from groundwater to soil depends on their Henry's Law constant, which is the partitioning coefficient between water and air. Volatile compounds migrate through the air-filled porosity in soil primarily via diffusion from areas with higher vapor concentrations (such as near contaminant sources in the subsurface) to lower vapor concentrations (such as inside a building). The tendency for volatile compounds to diffuse through soil depends on their chemical and physical properties (diffusion coefficients in air and water), soil porosity, and the soil moisture content.

As volatile compounds in soil gas migrate toward the surface, they encounter the building’s zone of influence where advective air movement becomes more dominant. Advective transport occurs when there is a difference in pressure between the subsurface and indoor spaces. This pressure differential is related to factors such as the stack effect (the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, resulting from air buoyancy); operation of the heating, ventilation, and air conditioning (HVAC) system; and wind pressure on the building exterior. Even slight depressurization of a building relative to the subsurface can increase the potential for VI. Vapor entry may occur through openings in floors and walls including cracks, mortar joints, foundations gaps, plumbing penetrations, etc. Water, sewer and other utility lines can act as vapor conduits, enhancing vapor transport into buildings.

Key references that describe the vertical distribution and temporal variation of VOCs include McHugh et al., 2007; USEPA, 2009; Schumacher et al., 2010; USEPA, 2012b, and the Case Studies presented below.

Investigation and Assessment

VI guidance generally recommends that buildings within 100 feet of locations known to have chlorinated volatile contaminants above soil and groundwater screening levels undergo a VI evaluation). For petroleum hydrocarbons, which more readily biodegraded in soil, this distance is reduced to 30 feet.

Regulatory guidance commonly recommends that multiple lines of evidence be evaluated to determine if VI pathways into buildings are complete and significant. Important lines of evidence may include groundwater, soil gas, sub-slab and/or indoor air sampling data, building survey and chemical use information, and data from controlled building depressurization testing or other advanced techniques if available. In recent years, there has been increased acceptance of longer (14 day) time-weighted sorbent based samplers as an alternative to short-term (1 day) vapor sampling using summa canisters. However, some regulators require use of 1-day samples to evaluate TCE due to the controversial concerns regarding potential birth defects associated with short-term exposure.

Guidance for assessing the VI pathway varies from national to state levels; however, most guidance documents follow investigation approaches similar to those recommended by the EPA, 2015; ITRC, 2007; and DoD, 2009. Most VI regulatory guidance recommends assessing VI by applying a tiered investigation approach:

An initial tier of VI investigation (screening assessment) involves collecting groundwater and soil gas samples to trace a pathway for volatile compounds from the contaminant sources to nearby buildings. These results are compared with conservative (highly protective) screening levels, to determine if further investigation of surrounding buildings is warranted.

Subsequent tiers of a VI assessment may involve investigating nearby buildings. Building investigations typically involve collecting sub-slab soil gas samples , which are collected through the building floors, and collecting indoor and outdoor air samples. Building surveys are performed to:

• Evaluate the characteristics and condition of the building envelope
• Determine activities and uses within the building that may contribute to observed volatile organic compound (VOC) concentrations
• Assess operation of the HVAC system and other factors that may dilute VOCs
• Identify the location of subsurface utilities or other building features that might represent atypical preferential pathways for vapor migration.

Several consumer products including such glues, gun cleaners, Christmas ornaments, and spot cleaners can contain VOCs. Several diagnostic methods are available to determine if VOCs found in a building originated from the subsurface or from indoor sources including:

• Real-time monitoring of VOCs during controlled building depressurization using a fan or blower-door;
• Use of portable analyzers ( gas chromatograph/mass spectrometer) to identify sources in buildings.

The results of a VI investigation are then assessed to determine if there is a complete VI pathway into a building and if potential exposures to VOCs in indoor air represent a significant health risk to building occupants.

Remediation and Mitigation of VI Pathways

If an exposure and risk assessment of a particular building finds that VI exposures exceed regulatory criteria, then the VI pathway is generally addressed through a combination of:

• Remediation focused on the environmental media/source (such as a groundwater plume underneath a building) from which the contaminants originate to reduce their concentrations
• Mitigation, with the objective to reduce or eliminate exposures in the building impacted by VI. Mitigation is generally considered a short to medium term remedy.

In general, mitigation technologies used to control VI are based on the practices originally developed for radon mitigation. Mitigation is most often considered an interim measure to control exposures in VI-impacted buildings during remediation of volatile chemicals in soil and groundwater.

There are two types of building mitigation methods:

• Passive mitigation methods prevent the entry of chemical vapors into the building by sealing openings (cracks, perforations) or installing sub-slab liners (passive membranes, vapor barriers, and passive venting techniques). These methods are applicable to both existing and future residential and commercial buildings and may be appropriate when residual volatiles in soil gas are unlikely to contribute to indoor air concentrations above target levels. These methods tend to be cheaper but may be less effective and thus require more monitoring. Passive methods are more likely to be successful and economical in new construction, when the floor system is readily accessible and they can be integrated with building features intended to provide moisture protection.
• Active mitigation methods include mechanical systems that can be implemented in both existing and new construction, and are designed to change the pressure difference between the sub-slab and the inside of the building to keep vapors out. These mitigation methods include sub-slab depressurization and building over-pressurization techniques, sub-slab venting and indoor air treatment. These control methods are typically more expensive and require long-term operation and maintenance, but more effective than passive systems.

Passive and active mitigation technologies are further described in Engineering Issue: Indoor Air Vapor Intrusion Mitigation Approaches.

Causing VI Inadvertently via Remediation

Precautions are sometimes required when designing remediation systems to avoid causing or exacerbating a VI situation. In theory, technologies such as air sparging, enhanced anaerobic bioremediation, chemical oxidation, or in situ thermal methods can generate additional volatiles in the vadose zone, or change how those volatiles move near or into structures and cause VI problems. To control VI risk from remediation, the following measures can be employed:

• Engineering controls, such as soil vapor extraction (SVE) systems between targeted remediation zones and structures
• Judicious selection of technologies to be used in proximity to buildings
• Understanding the characteristics of buildings near remediation systems that may provide preferential pathways for gas migration
• Monitoring and if necessary, VI mitigation

Despite the theoretical potential, as of 2016, we are not aware of many well-documented cases where remediation has caused or worsened actual VI problems at a site, suggesting that remediation-caused VI problems may be rare or that existing industry precautions have generally been successful in managing the risks.

Research and Technology Development Efforts

The conceptual and mechanistic understandings of VI pathways and tools for assessment and mitigation of VI are rapidly evolving.

In addition to primary literature studies, a variety of general guidance documents are listed as key resources at the start of this article. Key SERDP/ESTCP VI projects can be found here: Vapor Intrusion

Case Studies

Some of the most important knowledge about VI has been obtained from detailed studies of buildings. Two of the most extensive data sets from individual residential buildings are:

• The Sun Devil Manor VI Research House, where large variations in VI behavior was observed with distinct summer vs. winter patterns.

• The Indiana Duplex House where seasonal variations and different VI characterization methods were evaluated .

Several multi-building database studies have been conducted of differing populations of buildings that seek to derive general descriptions of vapor intrusion behavior. In particular these databases often compare buildings using attenuation factors which are defined as “The ratio of the indoor air concentration arising from vapor intrusion to the soil gas concentration at the source or a depth of interest in the vapor migration route”. Multi-building analyses are also contained in Song ; USEPA, 2012a; Johnston and Gibson, 2013; Yao et al., 2013; Brewer et al., 2014; and DON, 2015.

See Also

• VI mitigation in new buildings
• VI mitigation in existing buildings
• Chlorinated Solvents

Sites With Large Bibliographies or Link Collections

• Vapor Intrusion Overview
• Vapor Intrusion Workshops and Conferences
• Vapor Intrusion Cleanup-Initiatives