By Catherine W. Johnson
In February 2020, California EPA (Cal-EPA) released its Draft Supplemental Vapor Intrusion Guidance (“Draft Guidance”)  – designed to promote consistency in the investigation of vapor intrusion among different state agencies responsible for overseeing the remediation of contaminated properties. It has been widely criticized by stakeholders, however, who claim it would require substantial investigation and impose significant costs at sites that pose little or no risk. Although the Draft Guidance has not yet been finalized, some regulatory agencies have already adopted key features of the Draft Guidance – and, as predicted, costs and liabilities are already increasing for parties responsible for cleanups.
The most controversial feature of the Draft Guidance is a proposed attenuation factor of 0.03 for soil and soil gas. The attenuation factor is applied to concentrations of contaminants in the soil and soil gas to extrapolate the concentrations of those contaminants that – based on assumptions factored into the attenuation factor – could be found inside nearby buildings. The 0.03 attenuation factor is already in use by the U.S. EPA but is significantly more conservative than the attenuation factor that has been used by California agencies.
Some agencies are already requiring use of the 0.03 attenuation factor – either as a screening tool to determine when indoor air sampling will be required and/or to identify potential risk for purposes of risk-based cleanups. That is, the attenuation factor as applied to soil and soil gas can trigger a vapor intrusion investigation, including indoor air sampling. Even if no vapor intrusion is detected inside a building, however – and thus no actual risk – the 0.03 attenuation factor is also in some cases being required for use to calculate current and/or future risks in a risk assessment (e.g., an agency may rely on the Office of Environmental Health Hazard Assessment ("OEHHA") to review and comment on risk assessments, and OEHHA may recommend use of the 0.03 attenuation factor). Thus, the attenuation factor also affects cleanup levels for risk-based cleanups.
Where a potential risk exists, some agencies are beginning to require remediation of the source area – consistent with the Draft Guidance – rather than relying solely on mitigation of vapor intrusion. That is, rather than simply installing a vapor intrusion mitigation system in a building, responsible parties are being required to remove contaminants from the affected media. Moreover, where a potential risk is associated with a neighboring building, agencies may require responsible parties to collect indoor air samples from the neighboring property – an exercise that often can be fraught with difficulties.
In conclusion, even sites that may have been under agency oversight for many years may suddenly be identified as presenting a potential health risk to occupants of on-site buildings or of neighboring properties. Responsible parties will need to evaluate specific agency policies applicable to their site – and, particularly where conservative attenuation factors apply, make strategic decisions about how to mitigate risks as well as minimize associated potential liabilities.
 The following agencies have contributed to the Draft Guidance: the Department of Toxic Substances Control (“DTSC”), the State Water Resources Control Board (“SWRCB”), the Bay Area Regional Water Quality Control Board (“Bay Area RWQCB”).  Some of the commenters who submitted public comments on the Draft Guidance complained the attenuation factor was developed using data collected from sites outside California and thus is not representative of conditions in California.