The use of soil gas concentrations, particularly sub-slab soil gas, is a primary tool of practitioners and regulators in decision-making for the vapor intrusion pathway. Soil gas concentrations typically are utilized by some conversion factor based on the premise that chemical vapors attenuate across a foundation and result in measurable indoor air concentrations.
This conversion is represented in a variety of terms commonly used to describe the relationship, such as sub-slab to indoor air attenuation factors, soil gas target levels, or soil gas screening levels. In general, these generic soil gas screening tools are based on a combination of expected results obtained through modeling, attenuation factors based on empirical datasets, and estimation of tolerable risk based on actual soil gas concentrations. The lure of a generic soil gas screening tool is apparent. It provides a simplistic, single value that may be applied to any chemical at any building; eliminates the confounding presence of background sources; and minimizes the data required to make risk-based decisions. However, the desire to have a simplistic policy tool based on soil gas concentrations has resulted in widespread acceptance of a technique that provides little control for the variability inherent in soil gas data, little control for the variability in flux across a slab, and places too much risk-management emphasis on a data point that does not represent the point of exposure. Alternate risk-management approaches are presented that rely on improved lines of evidence with increased weight on empirical data for indoor air, the actual point of exposure and the only data point that integrates the entire physical and chemical processes involved in vapor intrusion.
- Geosyntec Authors: Ben Martich
- Title: A Perspective: Less Emphasis on Sub-Slab Data for the Vapor Intrusion Pathway
- Event or Publication: AEHS Annual International Conference on Soil, Sediments, Water, and Energy: Amherst, MA
- Practice Areas: Groundwater Assessment and Remediation, Vapor Intrusion
- Date: 2013
- Location: Amherst, Massachusetts