DTSC Issues Final Guidance for Vapor Intrusion Evaluation and Mitigation

November 11, 2011
The California Department of Toxic Substances Control (“DTSC”) recently published its final Vapor Intrusion Guidance (“VI Guidance”)1 for stakeholders to use in evaluating whether vapor intrusion is occurring at properties contaminated with volatile organic compounds (“VOCs”). Vapor intrusion is:

often the most significant exposure pathway for VOCs in the subsurface. . . . [W]ith the exception of household use of contaminated groundwater, vapor intrusion is the exposure pathway that usually poses the greatest risk from long-term (chronic) exposure for this class of chemicals.” [VI Guidance, at p. C-1 (emphasis added)]

Although the DTSC notes that “[u]se of the Guidance is optional,” the VI Guidance provides a new level of scrutiny that will be applied in a number of ways to sites with VOC contamination. The DTSC “anticipates that this Guidance will be used by regulators, responsible parties, environmental consultants, community groups and property developers.” [VI Guidance, at p. iii]

1. A Step-Wise Vapor Intrusion Evaluation Process

The VI Guidance establishes a step-wise evaluation procedure to be used in conjunction with other DTSC guidance. The process begins with Step 1: “Identify all spills and releases.” The intermediate steps include: (i) identifying imminent hazards and, if such hazards exist, proceeding immediately to Step 11a; (ii) performing a screening evaluation using default vapor attenuation factors to determine if a potential risk exists; and (iii) if a potential risk exists, (x) collecting additional site data, such as from the subsurface, below the slab, or within the crawl space, but not indoor air data, or (y) collecting indoor air samples. The process ends with Step 11, which addresses:(a) if there is an existing building, mitigating indoor air exposure, implementing engineering controls and remediating contamination, as appropriate; (b) if no building exists, remediating contamination or instituting institutional controls to reduce or prevent potential exposures; and (c) in both cases, instituting long-term monitoring.

2. Significant Changes in the 2011 VI Guidance from the 2004 Interim VI Guidance

Probably the most significant change from the 2004 Interim VI Guidance is that default attenuation factors have been revised to be more conservative. Thus, the 2011 default attenuation factor for an existing commercial building for a subslab sample is 0.05, whereas the 2004 default attenuation factor was 0.01. This means that the estimated indoor air concentration of a VOC based on a subslab sample using the 2011 default attenuation factor will be five times greater than the concentration would have been using the 2004 default attenuation factor.

Other significant changes from the DTSC’s 2004 interim final guidance are the following:

  • Multiple lines of evidence should be used to produce technically sound risk-based vapor intrusion decisions. The Guidance lists 13 different lines of evidence as some of lines of evidence that can be used. Examples of the lines of evidence are open-field soil gas data, soil matrix data, spatial and temporal variability of data, indoor and outdoor air data and radon data.
  • Installation of permanent soil gas monitoring wells should be installed rather than relying on soil gas grab samples in certain situations.
  • Utility corridor assessment should be included in the evaluation.
  • Confirmation sampling should be conducted to confirm that the cleanup actions reduced subsurface VOC concentrations to levels that are protective of human health.

In summary, the VI Guidance will likely result in more frequent implementation of engineering and/or institutional controls for existing buildings.

If you have any questions about the VI Guidance or addressing VOCs at contaminated sites in California, please contact a member of Bingham’s Environmental and Natural Resources Group.

For more information, please contact:

Edward Strohbehn Jr., Partner, 415.393.2059

Rick Rothman, Partner, 213.680.6590

1 “Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (Vapor Intrusion Guidance)” (Final) (October 2011).

This article was originally published by Bingham McCutchen LLP.