What You Need to Know – California’s Long-Awaited draft Supplemental Vapor Intrusion Guidance

The Cal/EPA Vapor Intrusion Workgroup published the draft Supplemental Vapor Intrusion Guidance for public comment last Friday (February 14, 2020). GSI’s Senior Risk Assessor and Vapor Intrusion Expert, Pete Scaramella, has summarized primary points of the Cal/EPA draft Supplemental Vapor Intrusion Guidance and will be participating in the comment period to provide input toward a protective yet practical framework for California’s regulated community. Read Pete’s summary below.
The objective of the Soil Vapor Intrusion Guidance (SVIG) is to supplement the existing guidance documents and promote state-wide standard practices for the evaluation of the vapor intrusion (VI) pathway. DTSC and Water Board will be accepting written comments to this draft document until noon on April 30, 2020. GSI will attend the informal public meetings held by DTSC and Water Board as well as submit written comments to DTSC. 

The focus of the SVIG is the assessment of VI at existing buildings, but some of the recommendations are applicable at future buildings as well. The primary implications of the SVIG, if published as currently drafted, include:

  • More sites will be screened in for VI evaluations. The SVIG would establish more conservative screening levels for the evaluation of soil gas data by revising the Cal/EPA default attenuation factors for subslab soil gas, external soil gas, and groundwater to match the USEPA recommendations.
  • More sampling events will be required, with more samples recommended per event, than previously required in California. An increased emphasis will be placed on the collection and evaluation of indoor air and subslab soil gas samples, versus the current approach of relying primarily on deeper soil gas (i.e., 5 and 15 feet below ground surface) and groundwater data for VI evaluations. And, since VI evaluations will require additional steps and considerations, more comprehensive reporting likely will be required to demonstrate VI evaluations are consistent with the detailed process outlined in the SVIG. 
  • Establishment of a California VI Database to determine if there is sufficient justification to develop California-specific VI attenuation factors. The USEPA recommended attenuation factors are based on a database of indoor air and soil gas data that were collected in colder climates (Northeast and Midwest), with only limited data collected in California. The data compiled via a GeoTracker upload portal will be applied to determine if empirical attenuation factors can be estimated for California.  The compilation of California-specific data may support different, and in some cases less conservative, attenuation factors in the future. 

The SVIG can be downloaded at the following website: 

For additional information regarding the draft SVIG, contact Pete Scaramella – This email address is being protected from spambots. You need JavaScript enabled to view it. or 949.679.1070