“Exposure” is at the heart of vapor intrusion (VI) work, in two senses of the word. This work primarily involves remediation to remove or mitigate potential exposure to toxic chemicals. Legal exposure is another factor driving property owners with potential contamination to address potential hazards.

The regulatory guidance shaping VI responses is the subject of changes being explored by California environmental authorities. Those changes have potentially significant ramifications for landowners and developers with environmental liabilities there.

Although VI is a nationwide issue, regulatory circumstances in California are prompting a fresh look at guidance standards. In essence, the California Department of Toxic Substances Control (DTSC) has argued that the current status of conflicting guidance is untenable. There is disagreement between the guidance issued by the U.S. Environmental Protection Agency (EPA), the DTSC, the state Water Board and the San Francisco Bay Water Board. Those differences can result in substantially different outcomes for sites, depending on which agency is overseeing evaluation and cleanup.

To remedy this situation, the California agencies have been developing a supplemental guidance document on screening and evaluating VI to better align results. Although expected to be released to the public in winter 2018-19, the interim document had not been published as of Feb. 4, 2020, and continues to be subject to a lengthy public comment period before it is finalized.

The new guidance document is expected to address several points of difference:

  • It will recommend adopting the EPA’s empirically derived default attenuation factors of 0.03 for soil gas/subslab gas and 0.001 for groundwater.
  • It will recommend a preference — but not necessarily a requirement — for indoor air sampling.
  • It should provide specificity in the standard number of samples required per unit area of a building, and include recommendations for sampling frequency focused on understanding spatial and temporal variability.
  • It may require sampling at two depths — near the source and halfway between source and the ground surface — among other sampling location and collocation recommendations.
  • It may require evaluation and sampling of sewers as preferential pathways for VI.
  • Detailed information on the risk management framework and a California-specific VI database are also expected.

The practical upshot of the expected revisions is a shift toward more conservative standards as conflicting regulations are aligned along stricter lines. These changes could have a significant impact on utility companies, the military, and owners of commercial and industrial facilities.

As new sites undergo VI remediation, they will likely be held to the newer default attenuation factors. For sites that are already subject to a five-year review process with either the state or the EPA, it is quite possible that review could trigger new or revised evaluations for VI. It remains to be seen how the state will handle sites that have already achieved closure under earlier, less strict guidance.

Landowners and developers will be keeping a keen eye on these developments in California. We continue to monitor the issues and consider strategies for navigating regulatory expectations.


Working with regulators to try adaptive remediation approaches can pay dividends, as it did on the award-winning cleanup of a contaminated fertilizer plant site.

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Aaron Christensen is a senior geological engineer and department manager at Burns & McDonnell. He specializes in vapor intrusion evaluations, investigation and remediation of former manufactured gas plants (MGPs), and environmental compliance for construction projects and industrial facilities.