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MPCA Announces Interim Changes to Indoor Air Screening Levels

What are ISV’s?

An important environmental driver on many property investigations and redevelopment projects is the presence of contaminated vapors in the air and in the pore spaces of soil, also known as soil gas. These vapors come from contaminated soil or groundwater and may travel hundreds of feet from the original source of contamination. State and federal regulators have determined that if high concentrations of contaminated vapors are present near or below a building, those contaminated vapors may enter the building through cracks or small openings in the building floor slab, and mix with the indoor air.

As a result, the Minnesota Pollution Control Agency (MPCA) has released screening values for acceptable levels of common air contaminants; officially called Intrusion Screening Values (ISVs). The MPCA continually reviews the ISVs in light of studies and calculations provided by the U.S. Environmental Protection Agency. Multiple ISVs are useful in determining if contaminants in soil gas provide a risk of intrusion.

What has changed?

The MPCA recently announced interim changes to several ISVs. In particular, the ISV for ethylbenzene – a common constituent of gasoline, became more stringent. The ISVs for a handful of other volatile organic compounds (VOCs) increased slightly, or became less strict. The VOCs with higher ISVs included benzene – another common constituent of gasoline; tetrachloroethylene – which was a common dry cleaner solvent; and trichloroethylene – which was a common industrial degreaser. Although the changes affect a small number of VOCs, these VOCs are frequently encountered in soil gas on contaminated sites.

Changes in ISVs can affect the necessary response, and expense, for a property owner or developer. The MPCA has increasingly recommended the installation of vapor intrusion mitigation systems in both new and existing buildings with observed concentrations of contaminants in the soil gas. Those mitigation systems range from vapor barriers beneath new buildings to electrically-powered fan systems that produce a vacuum below the floor slab intended to collect and exhaust vapors to the atmosphere above the building. The selection of the mitigation approach depends on the concentrations of contaminants, the regulatory requirements, and the owner’s risk tolerance.

So, what’s next?

The MPCA plans to release updated guidelines for responding to elevated concentrations of contaminants in soil vapor in the year. We believe those guidelines will further clarify the required actions for property owners and developers at sites with VOCs in soil gas.

Braun Intertec works with dozens of clients to address the presence of contaminated soil gas vapors on properties with new and existing buildings. We constantly interact with MPCA officials to understand their guidelines and develop best approaches to threats of vapor intrusion. Contact Bruce Schaepe for more information.

Bruce Schaepe Senior Engineer

P: 952.995.2453