This past October I attended the Soils Conference at the University of Massachusetts and was fortunate enough to sit in on a number of excellent presentations. Despite this positive experience, I was planning to leave early on the last day of the conference, but as I thought about my cluttered desk and pile of unanswered messages back at the office, I decided to stay a little longer and hear three presentations on vapor intrusion assessment. This proved to be a very good decision as each of them was outstanding.
New VI Assessment Method
The first of these presentations was given by Ms. Lila Beckley of GSI Environmental in Austin Texas; her title was “Vapor Intrusion (or Not): Sniffing out the Source of VOCs in Indoor Air”. She described a new method for assessing vapor intrusion (new to me anyway) and gave an eye opening description of their techniques. Her firm has developed a vapor intrusion assessment method that uses real time sampling and analysis of indoor air while the pressure within the building is adjusted from positive to negative. Based on the pattern of change observed in VOC air concentrations as the pressure is reduced, it was possible to determine whether the VOCs were truly from vapor intrusion or whether they actually originated from a source within the building.
This technique represents a significant improvement over the methods previously used to assess vapor intrusion.
A Specialized Application of the Same Technique
Following a short break, there was a presentation on vapor intrusion assessment by Mr. David Shea of Sanborn, Head & Associates that used the same general technique described by Beckley, but with the additional objective of identifying specifically how and where vapor intrusion was taking place in a large building located over a known groundwater contaminant plume. The example used in Shea’s presentation nicely complemented and expanded on the methods illustrated in the previous talk.
In summary, both presenters described a highly credible method that appears to quickly and accurately assess vapor intrusion. The method requires only a 1-2 day assessment, but does need to be conducted with sensitive specialized field instruments. The method seems capable of differentiating between indoor sources of VOCs in air and those originating from a sub-slab source.
How Good is the Correlation between Sub-Slab VOC Concentrations and Vapor Intrusion?
The last of these three talks was given by Mr. Ben Matrich of Geosyntec Consultants’ Anchorage Alaska office. His title was “The Case for Less Emphasis on Sub-Slab Data in Decision Making for the Vapor Intrusion Pathway”. The thrust of his presentation was that investigators have come to place too much emphasis on sub-slab soil gas measurements without adequately understanding their limitations. The scientific basis for his talk comes from the comprehensive vapor intrusion research the USEPA has sponsored at Sun Devil Manor, a house purchased for this purpose in Layton, Utah. Investigators are using the Sun Devil Manor (which is located over a contaminated aquifer) to study the detailed mechanisms of how vapor intrusion takes place; the results have upset much of the old thinking about vapor intrusion.
Details of the findings go beyond what I have space for here, but here are some examples. EPA found that at Sun Devil Manor sub-slab soil gas concentrations can vary more than expected over time, as can the indoor air concentrations caused by vapor intrusion. Temperature, barometric pressure and wind speed can all strongly affect vapor intrusion rates as well as the resulting indoor air VOC concentrations. Of course these findings all come from a single location and may not be generally applicable.
The simple vapor intrusion models used to develop the early regulatory efforts to limit the vapor intrusion pathway are being revised based on new studies and assessment techniques. It is reasonable to expect that the approach to vapor intrusion will continue to evolve as additional research is published.