California Releases Long-Awaited Vapor Intrusion Guidance

On February 14, 2020, California released its long-awaited Draft Supplemental Guidance for Vapor Intrusion for public comment. Trihydro will be providing comments, which are due to the agency by April 30, 2020. Prior to providing this most recent guidance, the San Francisco Bay Regional Water Quality Control Board (SF Bay Regional Water Board) had released two vapor intrusion (VI) publications in July 2019 that provided interim direction. While California’s previous VI guidance (released in 2011) established a framework for assessing and evaluating VI in California, it had some cascading effects as other stakeholders across the country adopted certain protocols in absence of their own state-specific guidance. As seen with the 2011 framework, the updated California guidance will likely initiate changes for practitioners, regulators, and other stakeholders nationwide.

A few highlights of the new California draft VI guidance

Similar to the VI publications released last July, the new draft VI guidance recommends a 0.03 default attenuation factor (AF) (sub-slab/exterior soil gas to indoor air) for initial screening and minimizes the use of Johnson and Ettinger Modeling (J&E Model).  The new 0.03 AF results in a substantial increase in the number of buildings subject to indoor air assessment.

Stakeholders are encouraged to review the Draft Supplemental Guidance for Vapor Intrusion in full, but a few notable aspects include:

  • The guidance is presented as a supplement to existing information. It is generally prescriptive for a preliminary step-wise VI evaluation and approach, and provides guidance on the number and type of samples for indoor assessments. It does not, however, establish specific protocol for sampling technique, which generally includes conventional (SUMMA® canisters), passive longer term, and real time monitoring. Additionally, the new guidance provides a sampling framework for large buildings, parking garage/podium buildings, and buildings with crawl spaces. An illustration of the 4-step evaluation and approach is provided below.
  • The guidance stresses that petroleum releases from underground storage tanks (USTs) should continue to be evaluated for VI using the State Water Resources Control Board’s Low-Threat Underground Storage Tank (UST) Case Closure Policy (LTCP).
  • The guidance provides for continued use of urgent or accelerated response actions for trichloroethene (TCE) short-term inhalation hazard. This hazard provides unique challenges for assessment and mitigating issues for short duration human health exposure. For information about short-term response actions for TCE, see EPA, DTSC’s "Human Health Risk Assessment [HHRA] Note Number 5,”or the SF Bay Regional Water Board’s “Vapor Intrusion Framework.”
  • The document provides direction and guidance for preferential pathway assessment, specifically with respect to sampling of sewers data-sf-ec-immutable="" mains and cleanouts. Other preferential pathways are also addressed. The treatment of preferential pathways presents more specifics than most previous VI guidance documents.
  • The guidance describes ongoing efforts to develop California-specific AFs. The Department of Toxic Substances Control (DTSC) is requesting that VI assessment data be uploaded into the state’s Geotracker system in order to develop a California-specific AF database potentially used in the future to establish regional AFs.
The Draft Supplemental Guidance for Vapor Intrusion also provides a full flowchart for VI evaluation. An abbreviated version of the 4-step evaluation and approach is provided below. Draft California vapor intrusion guidance flowchart    

Your input is requested by the end of April

The State Water Resources Control Board and DTSC request stakeholder feedback on the draft guidance by April 30, 2020. Throughout April, the State Water Board and DTSC will host public meetings both in-person and online. Specific schedule and location details can be found here and interested stakeholders are encouraged to attend.

Need more details?

Contact a Trihydro subject matter expert to address your VI concerns.

Matt Jones
Lead Project Geologist/Hydrogeologist
(360) 312-9109

Nyree Melancon
Risk Assessor
(925) 270-0036

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Matt Jones, PG
Lead Project Geologist/Hydrogeologist, Ferndale, WA

Matt has over 18 years of experience as a geologist, project manager, and project director in the environmental industry. His expertise includes coordinating vapor intrusion investigations, technical vapor intrusion data evaluation/recommendations, and vapor intrusion mitigation/remediation and performance using techniques like sub-slab depressurization and soil vapor extraction.
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