Five Steps for a Vapor Intrusion Investigation

Should you care about the implications of vapor intrusion at your facility or on your property? With the U.S. Environmental Protection Agency (EPA) moving away from enforcement, maybe you can let these concerns go for now—or not. Your state may be stepping up to the plate to tackle problems associated with vapor intrusion. We will explore one state’s suggestions for embarking on a vapor intrusion investigation and deciding whether to proceed with mitigation.

vapor / smoke

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Who Cares?

Vapor intrusion (VI) is of particular concern to property owners, municipalities, and real estate developers, especially those involved in revitalization of a perceived brownfield. Liability implications are important considerations for future development and for past closure actions.

What Is Vapor Intrusion?

Vapor intrusion occurs when contaminants spilled on the ground migrate to the subsurface soils and/or groundwater, move around in the subsurface environment, and eventually enter buildings as a gas or vapor through cracks in the basement or foundation and sewer lines.  If the vapors accumulate, residents or workers in the buildings can be at risk.

In a 2015 technical guide, the EPA has described subsurface contaminants that have the greatest potential to pose a health concern through vapor intrusion, based on their volatility and potential hazards. These are known as vapor-forming chemicals.

EPA’s guide for vapor intrusion provides information on determining if a vapor intrusion pathway is complete, meaning there is an opportunity for human exposure and if a response action should be taken. If health risks are unacceptable, the guide says that a response action would typically include a combination of:

  • Remediation to reduce or eliminate subsurface vapor sources
  • Engineered exposure controls for specific buildings to reduce VI or reduce concentrations of vapor-forming chemicals that have already entered the building
  • Monitoring to assess and verify the performance and effectiveness of the remediation systems and engineered exposure controls
  • Institutional controls to restrict land use and/or to alert parties (e.g., prospective developers, owners, and municipalities) of the presence of subsurface sources of vapor-forming chemicals and to foster operation, maintenance, and monitoring of the remediation systems and engineered exposure controls

Five Steps in Investigating VI

The Minnesota Pollution Control Agency (MPCA) offers a five-step process for conducting vapor investigations and a mitigation decision framework for determining if mitigation is necessary. This process can help anyone, anywhere, in identifying and considering key factors related to VI assessments, risk management decisions, and mitigation.

Step 1: Evaluate and identify

In this initial step, you should review available site information for known and/or potential release sources of vapor-forming chemicals. Next, identify buildings within 100 feet (ft) of the potential releases that could be affected by vapor intrusion from the release source(s). Make sure to identify the building’s use (e.g., residential, commercial, industrial). Include both existing structures and proposed new buildings that are designed for human occupancy, including workplaces.

Step 2: Collect samples

Collect soil gas and/or sub-slab samples within 100 ft of known or suspected vapor sources in order to define the extent and magnitude of soil gas impacts. From this, develop a VI area of concern (AOC). Next, collect soil gas and/or sub-slab samples within a 100 ft buffer of known or suspected vapor sources (i.e., groundwater plumes, known volatile organic compounds (VOC) release sources). If soil gas and/or sub-slab concentrations are detected above 33 times the intrusion screening value (33X ISV) for the facility and type of chemical being monitored, a vapor source with potential VI risk is present. The intrusion screening value is determined based on the type of facility, the manner in which people contact the indoor air at the facility, how the chemical in question impacts health, and how the chemical behaves.

Note. According to the EPA, sub-slab soil gas concentrations can be expected to typically exceed indoor air concentrations by 33 times or more in buildings that are impacted by vapor intrusion.

You should conduct multiple sampling events, depending on your geographic location, because of temporal and seasonal variability. According to the EPA, seasonal variable conditions such as soil moisture levels or groundwater depth can lead to variable concentrations and distributions of vapors.

Check tomorrow’s Advisor for the next steps if the first two steps show that there is a VI risk in any of your buildings.