On February 23, 2023, California released its Final Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion (Supplemental VI Guidance), which provided updates to the Draft for Public Comment version published in February 2020.

After releasing the Draft Guidance in 2020, the California Environmental Protection Agency (CalEPA) Vapor Intrusion Workgroup (Workgroup), which is comprised of technical specialists from six agencies, received 620 public comments from more than 70 individual letters/emails. The Workgroup reviewed and incorporated the public feedback into the Supplemental VI Guidance.    

Notable highlights and changes in the Supplemental VI Guidance

Stakeholders are encouraged to review the Supplemental VI Guidance in full, but some notable aspects include:

• The guidance continues to recommend a default attenuation factor (AF) of 0.03, identified by the United States Environmental Protection Agency (EPA) in 2015, for sub-slab vapor and soil gas data. Using the AF of 0.03 has created controversy in the scientific, redevelopment, and legal communities as questions have arisen over the technical and/or legal footing of applying a one-size-fits-all AF of 0.03. Due to California’s relatively conservative screening levels, many sites are difficult to close and/or redevelop even when empirical indoor air data indicates risk to human health is below recommended thresholds.
• The 2020 Draft Guidance introduced an initiative of building a California-specific VI dataset in which practitioners upload data into California’s GeoTracker database. Early indications were that the GeoTracker database may be used to support a Cailfornia-specific AF more representative (and more likely to predict lower indoor air concentrations) of actual soil gas attenuation than the default value of 0.03. However, the new Supplemental VI Guidance only notes that California-specific AFs may be incorporated into future use scenarios and does not provide an update on database progress.
• In August 2022, the California Department of Toxic Substances Control (DTSC) released a Vapor Intrusion Advisory (Advisory) noting that site-specific AFs or a more realistic generic AF of 0.001 could be used in well-defined sites for the purpose of soil gas delineation (0.03 would still be used to evaluate potential indoor air VI risk). However, DTSC removed the August 2022 Advisory from its website, stating it has been superseded by the February 2023 Supplemental VI Guidance. As a result, many sites relying on the 2022 Advisory recommendations may need to be re-evaluated.

Additional changes in the Supplemental VI Guidance include:

• A recommendation that two soil gas sampling depths between 5 and 25 feet below ground surface can provide a reasonable understanding of the vertical soil gas distribution (although deeper sampling may be necessary for characterization).
• Recognition that while heating, ventilation, and air conditioning (HVAC) on and off sampling events are preferred, HVAC off events may not be safe or feasible.
• The addition of a post-screening approach to evaluate VI risk to future building occupants using a future soil gas to indoor air AF derived from site-specific subsurface data. Though, the feasibility of applying this technique at some sites remains uncertain.
• The addition of Attachment 1 discussing supplementary lines of evidence to provide a more comprehensive approach to evaluating the VI pathway.
• The inclusion of petroleum VI (PVI) specific considerations supporting the use of a bioattenuation factor (BAF) at PVI sites. The BAF is determined based on soil gas oxygen concentrations and can be used in combination with a default AF (e.g., 0.03), resulting in up to three orders of magnitude decrease in the applied AF.  
• The inclusion of a PVI specific considerations allowing vertical separation distances to determine if an indoor air investigation is needed at PVI sites.  If sufficient vertical separation from receptors to subsurface petroleum source is met, then the VI pathway may be considered incomplete without the need for further soil gas or indoor air investigation.

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