by Maggie Strom, QEP, CHMM - Founder and president
The science of environmental assessment, remediation and risk assessment is continually evolving as the understanding of contaminant behavior and pathways for human exposure are better defined. Vapor intrusion, the migration of potentially hazardous vapors from groundwater or soil contamination into indoor spaces, has recently become a major focus for the Tennessee Department of Environment and Conservation as it pertains to risk assessment for human exposure.
Volatile contaminants, such as chlorinated volatile organic chemicals, petroleum hydrocarbons and other vapor forming chemicals, are generally soluble in water and can migrate considerable distances from the source of a release and create a vapor contamination issue at properties where there is no reason to believe an environmental risk would be present. Contaminated vapors can accumulate beneath the slab of a building and migrate into the ambient indoor air through preferential pathways, such as cracks in the foundation and utility trenches, and expose people in the building to health hazards. In Tennessee, successful completion of a vapor intrusion assessment, under the direction of TDEC’s Division of Remediation, is often a requirement for a property in its Voluntary Clean-up, Oversight, and Assistance Program. Vapor intrusion assessments are also performed in response to complaints, spills, or as a component of other regulatory programs.
Initial U.S. Environmental Protection Agency guidance on vapor intrusion was released in 2002 and was adopted by many state environmental agencies. Two final policy technical guides were published by the EPA in 2015, one an update of the 2002 release and the other which specifically addressed petroleum vapor at leaking underground storage tank sites. Tennessee does not have state-specific vapor intrusion regulations but defers to the New Jersey Department of Environmental Protection Vapor Intrusion Technical Guidance for conducting environmental vapor intrusion assessments. Tennessee has also prepared a process flowchart to help investigators through the various steps of an investigation.
In the state of Tennessee, once it has been confirmed that a release has occurred and an existing building of concern has been identified, the first step is a sub-slab vapor assessment to determine if a risk is present by identifying whether an accumulation of hazardous sub-slab vapors exists. If there is no sub-slab vapor, there is no risk for vapor intrusion into indoor ambient air. A sub-slab vapor assessment is performed by drilling through the slab of a building and installing a temporary or permanent sub-slab vapor monitoring point. Using this monitoring point, vapor samples are collected from beneath the slab of the building and analyzed for volatiles using EPA method TO-15. During the collection of sub-slab samples, a prescribed methodology for sample collection is used to ensure there are no leaks in the sample train and that ambient indoor air is not infiltrating the sampling system.
If a plume of sub-slab vapor is identified, the next step is to determine if a complete pathway between sub-slab vapor and indoor ambient is present. If sub-slab vapor exists without a complete pathway into indoor air, then there is no risk for human exposure. To determine if a complete pathway is present, a second round of sub-slab samples is collected along with concurrent indoor ambient air samples. While sub-slab vapor samples are, by regulation, not allowed to be collected for greater than 30 minutes, indoor ambient air samples are generally collected over a period of eight hours in a commercial setting. The sample time mimics the amount of exposure a worker would receive during a typical work shift. In residential settings, sample collection can run up to 24 hours.
If a complete pathway is identified, more sampling is required to determine if unacceptable vapor intrusion risks may exist to occupants or potential occupants of a site. This sampling can include collection of exterior soil gas near the foundation, sub-slab soil gas and indoor air sampling. Vapor intrusion conditions can vary seasonally due to rainfall, temperature and multiple other factors. For this reason, TDEC often requires sampling at multiple points during a year to develop a full picture of the potential for exposure.
Tennessee requires the use of the EPA Vapor Intrusion Screening Level calculator to calculate screening levels for volatile constituents. These screening levels are compared with analytical results of the vapor intrusion investigation to determine the risk to human health. The concentration of vapor that is viewed as potentially hazardous to human health varies from state to state as state-mandated inputs to this calculator can be different and some states have developed their own guidelines. For example, the target indoor air concentration for trichloroethylene at a commercial facility (using specific parameters for exposure) may be 1.8 ug/m3 for long-term exposure in Tennessee compared with 8.8 ug/m3 in Indiana. While this may not seem like much, a study by Johnson et al in 2003 identified that an exposure of just 2 ug/m3 could cause hazardous effects given a long-term dosage. This example illustrates that a very small amount of contaminated vapor can have lasting, perhaps deadly effects. Also, as more is learned about chemical exposure, new limits are being implemented. For example, a few states have begun to evaluate trichloroethylene for short-term exposure.
Identification of a complete pathway with concentrations that are potentially hazardous will most likely require some sort of action such as monitoring, establishment of institutional controls or remedial actions, including sub-slab depressurization, increased ventilation, installation of vapor barriers (for new construction), building pressurization and more.
If you are not familiar with vapor intrusion risks and environmental practices at the moment, you will be in the future. In predominately urban areas, it is the most common method for the public to be exposed to environmental hazards. But with proper assessment, identification and remediation, the risk to human health can be minimized.