The Indiana Department of Environmental Management (IDEM) has approved the use of the vapor intrusion attenuation factors set forth in the US Environmental Protection Agency’s Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air and Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites.  The EPA guides were published in June 2015, and IDEM approved the use of the new attenuation factors on October 20, 2015.

The new attenuation factors are less restrictive than the old IDEM attenuation factors.  Under the old attenuation factors, a preferential pathway for vapor intrusion was presumed to exist if the sub-slab vapor contaminant concentrations exceed the indoor air vapor contaminant concentrations by a factor of 10.  Under the new protocol, a preferential pathway for vapor intrusion will be presumed to exist if the sub-slab vapor contaminant concentrations exceed the indoor air vapor contaminant concentrations by a factor of 33.  A table is provided below comparing the old and new attenuation factors for some common contaminants.

As a result of IDEM’s adoption of the new EPA vapor intrusion attenuation factors, property owners with subsurface contamination should be able to more easily demonstrate that impacted soil or groundwater does not warrant additional investigation to eliminate the pathway for vapor intrusion.

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