ASSESSMENT OF VAPOR INTRUSION RISK AT PETROLEUM RELEASE SITES IN MINNESOTA

In May 2005, the Minnesota Pollution Control Agency (MPCA) released guidance for addressing vapor intrusion in to building structures located at or near petroleum release sites in the state. Vapor intrusion, while accepted as being a significant human health and safety concern, is an emerging issue nationwide with related methods and techniques still evolving. Therefore prior to implementing any statewide policy, pilot studies at petroleum release sites were necessary in order to derive the most appropriate and practical methods of investigating and interpreting vapor intrusion.

Phase 1 of the pilot study was completed in 2003, focusing on the basics of soil gas definition and pathway evaluation. Three (3) petroleum release sites were studied, and methodologies for collecting and analyzing soil gas, sub-slab gas, and indoor air samples were tested. From the Phase I work it was evident that petroleum vapors can migrate significantly beyond the extent of the soil and ground water contamination.

Phase 2 was subsequently conducted in 2004 to develop a better understanding of soil gas migration. This phase consisted of soil gas investigations at twenty three (23) additional petroleum release sites. The work at each of these sites consisted of vertical soil gas profiling and collection of soil for geotechnical parameters. Data collected is also being used to develop a potential vapor migration screening model.

Phase 3 is being conducted in 2005 at selected Phase 2 sites. The objective will be to re-sample soil gas in order to verify migration as predicted by the draft screening model. Phase 4 will be conducted during the 2005-2006 winter to analyze the impact of winter conditions – particularly the capping effect of frozen ground – on soil gas migration and therefore on vapor intrusion risk.

In the interim, the results of Phase 1 and Phase 2 were used to develop the new vapor intrusion guidance document, MPCA Guidance Document 4.1a – ‘Soil Vapor Intrusion Screening Assessments conducted during Site Investigations', with mandatory implementation from May 15, 2005. The impact of this new policy and guidance will be evaluated over the 2005 field season. This experience, along with the results of Phase 3 and Phase 4 of the pilot study will be used to make revisions and release a final guidance prior to the commencement of the 2006 field season.