Technical Q&A: New Evaluation Approach

What are some key changes to expect in the upcoming EPA vapor intrusion guidance?

The U.S. Environmental Protection Agency (EPA) is expected to issue new guidance documents later this year. The most obvious change is that the guidance will be separated into two documents by chemical class. The EPA's Office of Underground Storage Tanks (OUST) will provide guidance on petroleum hydrocarbons, and the Office of Solid Waste and Emergency Response (OSWER) will address essentially everything else, primarily chlorinated solvents. There will be different mechanisms for evaluating vapor intrusion for the two chemical classes.

Chlorinated hydrocarbons differ from petroleum compounds because they are much less likely to biodegrade. They migrate easily and are highly toxic. Under the OSWER document, most vapor intrusion evaluations likely will be required to include some sampling of the building's interior, either indoor air or subslab soil gas.

In contrast, under the OUST guidance, petroleum hydrocarbon investigations can look at chemical concentrations in soil and groundwater and the distance between the contamination and the building as an initial screening step. If the distance is sufficient, then potential risks from vapor intrusion can be ruled out and interior sampling can be avoided. That's a significant change.

OSWER also is introducing longer-term indoor sampling. Residential sampling historically has involved taking a 24-hour sample, but the EPA is showing an inclination toward seven- or 30-day samples to better compensate for variations in indoor air concentrations.

One other unusual development: The guidance documents are expected to be issued as final, with no public review period. Technical papers laying out many of the issues were published for public review and comment a year ago.

For more information, contact Diana Marquez, 816-822-3453.

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