Vapor Intrusion – Historic Perspectives, Current Considerations

As we all know, one of the hottest topics in the environmental industry right now is vapor intrusion.  In Massachusetts, vapor intrusion considerations have been around for more than 20 years, since the beginning of the Massachusetts Contingency Plan (MCP) program.  This is no surprise, as Massachusetts has been a technical and regulatory leader in the environmental field since the early days.  However, Massachusetts has reached a critical crossroads in the regulation of the vapor intrusion pathway, and stakeholders, especially those who are involved in brownfields redevelopment projects, are hoping that they choose a wise pathway moving forward.

As a rookie regulator at the start of my professional career in 1992, I first became aware of vapor intrusion as an exposure pathway when a colleague of mine was working on a project in Needham.  Apparently, an industrial facility was discharging wastewater containing chlorinated solvents into corroded subsurface drain lines, resulting in groundwater contamination.  The impacted groundwater migrated downgradient to a school and a residential subdivision.  An indoor air testing program detected the solvents in indoor air.  From that event, vapor intrusion considerations were propelled forward, and the concept of MassDEP’s GW-2 standards based on the Johnson & Ettinger Model was born.

MassDEP’s GW-2 standards were based on an early version of this model that had been around since the early 1990s.  However, not long after implementation in late 1993, MassDEP technical staff began questioning whether the model was portraying an accurate picture of what was being observed in the real world.  In the late 1990s, MassDEP began to see a growing body of data collected at sites that indicated the Johnson & Ettinger Model was not providing a consistent prediction of what was being observed.  MassDEP lowered GW-2 standards for many common chlorinated volatile organic compounds and continued its evaluation of the data collected during site assessment and clean-ups.

Fast-forward to where we are today:  MassDEP has issued updated Vapor Intrusion Guidance, currently in draft form dated December, 2010.  While MassDEP’s updated guidance may be the state of the science available at this time, it is a substantial departure from how vapor intrusion has been regulated in Commonwealth over the past 20 years.  As it is currently issued by MassDEP, the updated guidance appears to bring a substantial amount of additional uncertainty to the redevelopment of brownfields sites.  Many brownfields projects are undertaken due to the availability of transferrable tax credits which make these projects economically feasible.  Without the availability of these credits, many projects would fail to receive financing.  However, the availability (and the amount) of the tax credits is dependent on these brownfields projects reaching some type of Permanent Solution.  The updated guidance, as currently proposed, reduces the likelihood of Permanent Solutions at vapor intrusion sites.

While all of us who work on vapor intrusion sites appreciate the hard work that MassDEP has put into the updated guidance, we also hope that the final version will not make it more difficult to achieve Permanent Solutions, especially at brownfields projects.  While we can not predict what the final guidance will include, there are actions that can be done now to reduce the likelihood of regulatory risk on projects now in the pipeline.  These actions include designing projects to include vapor intrusion barriers and properly engineered subslab depressurization system piping in new construction and rehab work.  First and foremost, consideration of potential vapor intrusion issues, and appropriate early due diligence and evaluation, should be at the top of every brownfields project to-do list.