MassDEP Releases Its Draft Vapor Intrusion Guidance

VI GraphicMassDEP has recently released for public comment draft Guidance on Vapor Intrusion.   The proposed guidance would replace MassDEP’s December 2011 Interim Final Vapor Intrusion Guidance, which saw minor revisions in the spring of 2013.  The guidance has undergone a substantial revision, largely to make changes that correspond to the recent regulatory reforms to the Massachusetts Contingency Plan.

The guidance provides MassDEP’s recommended technical and regulatory approaches to addressing the vapor intrusion pathway at sites contaminated with releases of oil and/or hazardous materials regulated under the MCP. The issue of vapor intrusion arises when contaminants in soil and groundwater volatilize and resultant vapor-phase contaminants enter the indoor air in occupied buildings.

The guidance highlights the importance of considering the disposal site as a whole when developing a plan to address vapor intrusion in specific buildings. New emphasis has been placed on the identification and delineation of sources, the incorporation of data collection into the conceptual site model, and eventual source elimination and control.

In addition to the revisions designed to address the MCP’s new regulatory framework, new topics addressed in the guidance include the use of remote telemetry on active mitigation systems, the collection of exterior soil gas samples, and the application of remedial additives to address sources.

MassDEP is also actively soliciting feedback on two specific sections of the draft guidance:

  • 2.2.3 – Special Considerations for the Assessment of Petroleum Vapor Intrusion from Discrete, Well-defined and Stable Petroleum Sources. MassDEP has provided the EPA’s inclusion distance approach for screening out petroleum disposal sites and asked for comment on how this approach might be used in a vapor intrusion pathway assessment.
  • – Exposure Point Concentrations – Existing Buildings (Future Use). MassDEP has called attention to the problem posed when estimating exposure point concentrations for future uses in existing buildings. As buildings change over time due to aging, repairs, and renovations, the potential for vapor intrusion could increase. One problematic scenario occurs when an existing building’s indoor air scenario poses no significant risk to occupants, but measured sub-slab soil gas concentrations exceed screening levels. MassDEP is seeking comment on three different proposals for how to address this scenario and enable closure for the site, including one option that would require further amendments to the MCP.

MassDEP will hold a pre-comment discussion meeting on the guidance on December 11, 2014, MassDEP, One Winter Street, Boston, 2nd floor, from 9:30 am to noon.  Written comments are due February 2, 2015.

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