Development of a Default Vapor Intrusion Attenuation Factor for Industrial Buildings

An investigation at a major industrial facility in the Midwestern United States provides insights regarding the amount of attenuation of sub‐surface vapors occurring at industrial buildings. The buildings at the facility were ranked in terms of vapor intrusion potential and testing began in October 2016 and is ongoing. Results have been evaluated for data collected at 718 unique locations across 77 buildings. A total of 1646 sample pairs (sub‐slab and indoor air) have been collected and analyzed for 65 analytes, resulting in a total of 106,990 data pairs. As many as 49 sample pairs were collected within a given building during a single sampling event and up to 11 rounds of seasonal testing have been performed at selected buildings. Seasonal variability in sub‐slab soil‐gas concentrations was found to be negligible. Data analysis was performed to look for data trends across the entire data set and identify inter‐building comparisons. This data evaluation focused on individual volatile organic compounds (e.g., tetrachloroethylene, trichloroethylene) present in the sub‐slab soil gas at concentrations exceeding 1000 μg/m3. A total of 157 building‐specific attenuation coefficients (α) were evaluated. This evaluation demonstrated that large industrial buildings have a much greater attenuation than that assumed for single‐family residential buildings. All attenuation coefficient values were lower than 0.03, which is the standard regulatory default for non‐residential buildings. The median value was 9.3E‐05 and the 95% upper confidence limit was 2.7E‐04. There is some evidence of lower attenuation under wintertime conditions. The data suggests that the default attenuation factor of 0.03 over‐predicts indoor air impacts at this industrial facility by at least two orders of magnitude.