In July 2019, the San Francisco Bay Regional Water Quality Control Board (SF Bay Regional Water Board) released two vapor intrusion (VI) publications to provide guidance prior to the release of the Draft Supplemental Guidance: Screening and Evaluation Vapor Intrusion, which is currently under development by the SF Bay Regional Water Board, California Department of Toxic Substances Control (DTSC), and California State Water Resources Control Board (State Water Board). A release date has not yet been announced for this supplementary guidance.
The newly released publications include:
The publications have garnered attention because the content likely represents a substantial shift in the way VI has been approached to date.
Why is This Important?
The California Environmental Protection Agency (CalEPA) now recommends using United States Environmental Protection Agency’s (USEPA) conservative default attenuation factor (AF) of 0.03 for soil vapor and sub-slab data when screening a building, as presented in SF Bay Regional Water Board January 2019 Environmental Screening Levels and DTSC’s April 2019 Human Health Risk Assessment (HHRA) Note 3 – DTSC Modified Screening Levels (DTSC-SLs). This change leads to a higher number of buildings for consideration during VI assessments, including both existing buildings and new construction.
Additionally, the VI pathway for new construction and redevelopment efforts has traditionally been managed through passive VI mitigation systems (VIMS). However, the SF Bay Regional Water Board cites increasing concern over the long-term effectiveness of passive VIMS and recommends active sub-slab depressurization systems (SSDS) for slab on grade design. The SF Bay Regional Water Board is also encouraging active cleanup at VI properties in order to remove sources and reduce or eliminate the need for VIMS. Additionally, the board is requiring documentation of financial assurance for costs associated with VIMS.
Are Other VI Regulations Changing?
Previously, practitioners were allowed to develop site-specific AFs using the Johnson and Ettinger Model (J&E Model). However, in 2017, DTSC removed the DTSC Screening-Level Model spreadsheets that used the J&E Model from their website. Concurrently, the Regional Water Boards and DTSC started to recommend that the J&E Model not be used in evaluating the attenuation into indoor air from the soil vapor. Although the USEPA updated the J&E Model in September 2017, various state agencies within CalEPA either do not accept the model or require additional information prior to accepting the use of the model. If a project site uses the J&E Model, the Regional Water Board has published a checklist that must be included with all VI models.
What are the Expectations of the Regional Water Board’s Checklist?
Stakeholders are encouraged to read the checklist in full. Notable checklist recommendations include:
- One model run is required for each future or existing building.
- Sub-slab to indoor air AFs in the model should equal 0.03.This accounts for building aging and modifications over time at existing or future buildings.
- A detailed conceptual site model should be prepared, providing both a written description and geological cross-section of the geological and hydrogeological characteristics of the project site with a detailed focus of the soil types and layer continuity immediately beneath the buildings.
- Exposure point concentrations should be based on sampling locations representative of conditions below individual buildings, as opposed to averaged across an entire site, and on multiple rounds of sampling.
- For grain-size analyses, three or more soil samples for each modeled soil layer should be provided, and soil samples should not be impacted by moisture.
- Calibration of models should use measured indoor air concentrations for current risk assessment at existing buildings.
- Calibration of the source to slab component of models should be performed using multi-depth soil gas sampling (including sub-slab samples).
- The method used to account for preferential pathways in the model needs to be discussed in written reports and illustrated on maps.
What Does This Mean for Me?
The changes announced in the SF Bay Regional Water Board’s fact sheet have implications for existing commercial/industrial sites, developers, cities, homeowners, homeowners associations, and the general public. Although currently California-specific, these VI updates could cascade to other states in the future. For many stakeholders, the updated expectations appear to lead to an increase in the number of applicable buildings as well as an increase in the level of effort required to ensure compliance. Further, if using the J&E Model, the additional needs of the newly published checklist will increase the time associated with modeling, leading to increased costs.
At this point, it’s unknown whether previously investigated sites will be grandfathered. If not, thousands of sites could be affected. As mentioned, the Draft Supplemental Guidance: Screening and Evaluation Vapor Intrusion, is currently under development and Trihydro is actively tracking developments.
Need More Details?
Contact a Trihydro subject matter expert to address your VI concerns:
Lead Project Geologist/Hydrogeologist