National Groundwater Association Publishes Peer-Revied DTSC Attenuation Factor Study that Should Replace Outdated USEPA 2015 Study

The internationally respected National Groundwater Association (NGWA) has just published a peer reviewed DTSC study concluding that the appropriate soil vapor attenuation factor (AF) for VOCs is 0.0009. This value implies a PCE soil vapor screening concentration of 2,233 ug/m3 instead of the 67 ug/m3 (0.03 AF) from USEPA’s 2015 study. Given the California-specific and recent data, why should USEPA’s 0.03 AF even remain a viable consideration? Why does SWRCB and factions within DTSC insist on avoiding best science?  Would Meredith Williams, Todd Sax, Cheryl Prowell, Nicole Fry, and Jonathan Bishop kindly help the regulated community understand why the DTSC 0.0009 screening AF is not fully embraced? Please convince this brownfield investor to stop directing resources out of state. Here is the NGWA study abstract:

The California Department of Toxic Substances Control (DTSC) gathered empirical data from sites contaminated with chlorinated volatile organic compounds (VOCs) and generated a vapor intrusion database. The database includes 52 sites across California with 213 buildings, of which, 53% are residential, and 47% are commercial / industrial (non-residential). DTSC's objective is to improve its knowledge and understanding of vapor intrusion and derive empirical attenuation factors that are representative of the climatic conditions and types of buildings commonly found in the State.

​Filtering was applied to remove data of suspect quality that were potentially affected by background sources. After filtering to 600 pairs from 32 sites across California, a subslab attenuation factor (AF) was calculated yielding a 95th percentile of 0.005. After filtering to 2926 paired measurements from 39 sites across California, a soil vapor AF was calculated yielding a 95th percentile of 0.0009. The groundwater data was not analyzed due to the small size of the dataset. The AFs from this study are similar to AFs in a California-specific study by Lahvis and Ettinger (2021). Accordingly, converging lines of evidence suggest that vapor attenuation in California is different from what is observed nationwide by the United States Environmental Protection Agency (USEPA), where an AF of 0.03 should be used for the initial screening of buildings for potential vapor intrusion exposure (USEPA, 2015).

And here is the NGA study abstract link: https://ngwa.onlinelibrary.wiley.com/doi/abs/10.1111/gwmr.12559.