Assessing the significance of subsurface contaminant vapor migration to enclosed spaces: Site-specific alternatives to generic estimates

The move toward more structured risk-based corrective action (RBCA) approaches has led to an increased awareness and interest in better understanding vapor migration to enclosed spaces. The significance of this pathway is currently the subject of intense debate, with many believing that existing non-site-specific (aka "Tier 1", or "generic") risk-based screening levels are too conservative. However, little data are available to justify alternate approaches, and so this pathway must be addressed further on a more site-specific basis at many sites. Unfortunately, little guidance exists for doing this. In answer to this need, options for addressing this pathway on a more site-specific basis are presented here. These include a more refined use of the existing screening algorithms for layered geologic settings, as well as use of recently updated algorithms that consider biodegradation. Each option has associated with it site-specific data collection and interpretation requirements, and these can vary in complexity of analysis and intensity of data requirements. Also, a vision for a much simpler pathway screening analysis is presented and accompanied by a discussion of the developments necessary to progress toward that goal. This improved approach would determine, based on site characteristics assessed during an initial site assessment (specifically moisture content or measured effective diffuison coefficient and source zone vapor concentration), whether the vapor intrusion pathway needs be addressed further at the site.