On December 14, 2010, the Massachusetts DEP released its newly-revised vapor intrusion guidance document, seeking public review and comment by March 1, 2011. The new draft guidance is a continuation of MassDEP's pioneering efforts in this arena. In 1993, Massachusetts became the first state in the country to establish regulations and cleanup standards for the assessment and cleanup of sites where vapor intrusion was a pathway of concern. In April 2006, the DEP, because of vapor intrusion concerns, significantly lowered regulatory groundwater cleanup standards for chlorinated solvents associated with contaminated groundwater within 30 feet of a building and where the average depth to groundwater was 15 feet or less. In 2006, the DEP also undertook a PCE initiative that involved re-opening approximately 100 previously closed sites because of vapor intrusion concerns (with 68 of these sites requiring further action). In 2008 the DEP issued guidance identifying typical indoor air concentrations based on the detections of such contaminants in typical residential settings and the risks posed by these contaminants. The DEP also issued guidance describing how such values should be used in evaluating the vapor intrusion pathway.

The latest guidance document  is very negative on modeling. The DEP maintains that modeling based on subsurface contaminant levels (in groundwater, soil or soil gas) is not predictive of indoor air concentrations and therefore should not be used for site screening. Since the DEP does not accept modeling as a predictive tool, there would be no way to address the potential for vapor intrusion in future construction. As such, the DEP in its guidance proposes an engineering approach to protect future buildings.

The guidance document, including the main text and appendices, can be accessed on the DEP's Indoor Air Guidance Project blog (http://indoorairproject.wordpress.com). It is well worth reading.