What Is Vapor Intrusion?
What is vapor intrusion?
Vapor intrusion refers to chemical transport from the subsurface into building indoor air. Initially, human-health concerns related to vapor intrusion concerns were attributed to Radon, a naturally occurring radioactive gas that is considered carcinogenic. Beginning in the 1990s, vapor intrusion of VOCs resulting from environmental contamination began to receive significant attention.
VOCs are released to soil and groundwater from leaking underground and above-ground chemical storage tanks, infiltration from surface spills, and waste chemical disposal in leach ponds, dry wells, and leaking sanitary sewer lines. VOCs volatilize from contaminated soil and groundwater, and under certain conditions, will enter indoor air of buildings through cracks or seams in foundation slabs or basements.
VOCs of concern for vapor intrusion include aromatic petroleum hydrocarbons (e.g., benzene) and chlorinated hydrocarbons used as industrial degreasers and dry-cleaning solvents (e.g., tetrachloroethene [PCE], trichloroethene [TCE]); these chemicals are considered by the U.S. Environmental Protection Agency (EPA) to pose human health risks even at low concentrations in indoor air.
New Regulatory Guidance
New Regulatory Guidance Is Changing Environmental Risk Characterization
Vapor intrusion is under increasing focus by environmental regulators and often is a primary driver for remedial planning at contaminated sites. Regulatory decisions related to vapor intrusion are dictated by a patchwork of guidance documents released by state environmental agencies and the EPA. Recent changes to vapor intrusion regulatory guidance that may significantly impact the cost of vapor intrusion investigations and site remediation include the following:
U.S. EPA Region 9 Sets New “Short-Term Exposure Guidelines” for TCE
TCE is one of the most ubiquitous VOCs at contaminated sites; it has been found in at least 852 of the 1,430 National Priorities List (NPL) sites identified by the EPA. In a series of memorandums released from 2012 to 2014, the EPA established an “Accelerated Response Action Level” of 2 µg/m3 for TCE in residential indoor air at specific NPL sites Regions 9 and 10. Interim response measures if TCE is detected at or above 2 µg/m3 includes immediate relocation of residents.
The new Region 9 and 10 Accelerated Response Action Level is far below previous action levels or standards for TCE in indoor air resulting from vapor intrusion. For example, a guidance document published in 2008 by the California Regional Water Quality Control Board – San Francisco Bay Region had listed a requirement for immediate mitigation if TCE was detected at or above 120 µg/m3 in residential indoor air, or a level 60 times higher than the new standard. It is currently unclear if this new TCE standard will be implemented by the EPA nationwide.
U.S. EPA Recognizes Biodegradation Can Limit the Potential for Vapor Intrusion of Petroleum Hydrocarbons
In March 2012 guidance released by the U.S. EPA Office of Underground Storage Tanks, the EPA acknowledged that biodegradation of petroleum hydrocarbons (e.g., break-down of petroleum hydrocarbons by resident bacteria) can be active in the unsaturated zone below a building, limiting the potential for vapor intrusion of these chemicals. Previous EPA guidance had suggested that biodegradation be assumed to be negligible during site screening, and biodegradation has not been included in the EPA’s vapor intrusion models. In contrast to petroleum hydrocarbons, chlorinated solvents (e.g., PCE and TCE) are not expected to be subject to appreciable biodegradation in the unsaturated zone, and chlorinated solvents are, therefore, increasingly prioritized for vapor intrusion investigations.
U.S. EPA Suggests Earlier Adoption of Indoor Air Sampling and Moves Away from Mathematical Modeling
The EPA’s most recent official comprehensive guidance regarding vapor intrusion was published in draft form in 2002 and suggests a tiered approach for vapor intrusion investigations: (1) Primary Screening, (2) Secondary Screening, and (3) Site Specific Pathway Assessment.
The Secondary Screening step uses site-specific data to evaluate the potential for vapor intrusion based primarily on soil vapor data. It also relies on the EPA-published Johnson/Ettinger mathematical model to extrapolate VOC concentrations from soil vapor into indoor air. This tiered approach, including reliance on the Johnson/Ettinger model, has been replicated in many State-issued vapor intrusion guidance documents.
In April 2013, the EPA released an “external review draft” of the “final” vapor intrusion guidance. This draft eliminates the Secondary Screening and moves directly to a detailed site investigation based on the minimum criteria: (1) VOCs are present in the subsurface, and (2) there is “actual or potential future presence of buildings nearby.” Recommended detailed vapor intrusion investigation includes groundwater sampling, soil vapor sampling, building surveys, and indoor air sampling.
Indoor air sampling can be problematic because of the possibility of “false positives.” VOCs common to contaminated sites may also be present in indoor air due to off-gassing from consumer products (e.g., paints, solvents, aerosols). For this reason, soil vapor sampling and associated mathematical modeling has often been preferred to indoor air sampling during initial site evaluations and screening. The EPA’s suggested elimination of the secondary site-specific screening represents a significant shift away from reliance on soil vapor sampling and mathematical modeling and towards indoor air sampling. In guidance entitled, “Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air,” the EPA states:
Unless site-specific parameter values are obtained for input parameters and the mathematical model is calibrated to field data, use of default input parameter values will generate model results that lie at an unknown point within an uncertainty band of the model outcomes. Because the combined effect of parameter uncertainty is large, a one- or two-order of magnitude error might be made unknowingly.
In short, the EPA now lacks confidence in the results of mathematical modeling and, thus, has recommended moving towards reliance on actual field data rather than models.
Utility Corridors Receive Increased Attention As a Potential ‘Fast Pathway’ for Vapor Intrusion
Utility corridors are underground utility lines or pipes (e.g., sanitary sewer, storm, water, gas, electric, or telecommunications) and the excavated and back-filled trenches that contain them. In some cases the excavated trenches are back-filled with material that may more readily transmit vapors compared to native soils. Because these corridors are often specifically directed towards building foundations, there is concern that they may act as a fast pathway for vapor migration from contaminant sources towards or underneath buildings. A 2013 investigation of a home nearby a contaminated site in Massachusetts (Pennell et al., 2013 indicated that elevated PCE detected in the indoor air was caused by the presence of PCE in the sanitary sewer line and infiltration of sewer gas through a faulty toilet fixture. The 2013 EPA external review draft vapor intrusion guidance and several recent State guidance documents specifically call for evaluation of utility corridors during vapor intrusion investigations.
Innovative Field Techniques Recommended to Characterize Vapor Migration Pathways
Innovative field techniques are available to characterize the vapor migration pathway, reduce uncertainty regarding vapor intrusion rates, and plan for vapor intrusion mitigation when necessary. These innovative techniques include:
- Field soil vapor diffusivity testing
- Field soil vapor permeability testing
- Tracer studies for attenuation of vapor concentration from beneath the building slab to indoor air (including the use of Radon as a natural tracer)
- Tracer studies for air exchange rate within a building
- Building and sub-slab pressure measurements
The 2013 EPA external review draft vapor intrusion guidance and several recent State guidance documents recommend application of one or several of these techniques during vapor intrusion investigations.