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California’s
February 2020 public draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion is
a step forward. It took more than two years to get it to the point of public
review, for no clear reason, but it’s a relief to see it. It summarizes the contents: “This Supplemental Guidance incorporates
information from recent technical and regulatory publications that have
highlighted the variable nature of vapor behavior and lessons learned in the
assessment of VI. “This Supplemental Guidance provides
information and recommendations on the following topics: “*
Using United States Environmental Protection Agency (USEPA) 2015 attenuation
factors *
Establishing a four-step evaluation process to assess VI *
Considering sewers as a potential VI migration route and pathway of exposure * Building a California-specific VI
database” After hearing rumors that consultants for
responsible parties and developers had convinced California’s agencies to adopt
less protective default attenuation factors (the ratio of indoor air
concentrations of vapor-forming chemicals to subsurface concentrations), I was
pleased to see that the draft Guidance
sticks with EPA’s default factors.
Medium
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Attenuation Factor
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Crawl Space Gas
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1
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Subslab Soil Gas
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0.03
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Soil Gas
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0.03
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Groundwater
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0.001
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I support use of those attenuation
factors, particularly at development sites where there is no indoor air to
measure. The uncertainty due to the spatial and temporal variability of vapor
intrusion supports the imperative of a protective approach. It also warns against using models, such
as the Johnson-Ettinger model, to develop site-specific attenuation factors for
initial site screening. However, it states, “alternative approaches may be used
if supported by adequate technical and site information.” The Guidance points out the weaknesses of
EPA’s database used to create the default attenuation factors, and it establishes
a system for collecting California data to eventually develop California
attenuation factors, using the State Water Board’s Geotracker data base,
currently used to automatically share site documents with the public. My biggest concern, one I have been
expressing for 17 years, is the preference for using soil gas sampling –
particularly exterior soil gas sampling – to determine whether to sample indoor
air. Not only is exterior soil gas sampling a notoriously inaccurate predictor
of indoor air contamination, compared to subslab soil gas, but there is a long
history of investigators failing to fully delineate both soil gas and
groundwater plumes. Perhaps more important, stakeholders – the
people who live, work, recreate, study, and pray at potential vapor intrusion
sites – have long told me that they are uncomfortable with “all-clear” findings
based on calculations instead of indoor air sampling. It’s true
that background sources – both indoor and outdoor – can create false positives
for vapor intrusion, but there are now many accepted methods for distinguishing
subsurface sources from other sources of vapor-forming chemicals. I am also concerned about the
indeterminate risk management approach for risk levels between an excess
lifetime cancer risk of one in ten thousand (10^-4) and one in a million
(10^-6). Decision-makers are offered a menu of risk-management approaches,
including institutional controls, additional investigation/sampling,
monitoring, refining risk assessment, mitigation, remediation, and “none.” I
believe that there should be a preference for action. Mitigation
makes sense because the near certainty of protection against unacceptable
exposure costs very little under most circumstances. Remediation is
usually required because California has non-degradation laws for groundwater,
which is usually the source of subsurface vapors. The Guidance also contains what appears to
be new, valuable material on investigating large buildings, planning for future
buildings, parking garage sampling, and sewer lines as a pathway - all worth
reading if those apply at your site. Lenny Siegel
Begin forwarded message:
Subject: CalEPA Draft Supplemental Vapor Intrusion Guidance
Date: February 14, 2020 at 9:30:17 AM PST
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Dear Stakeholder:
Department of Toxic Substances Control (DTSC), State Water Board, and San Francisco Bay Regional Water Quality Control Board have developed supplemental vapor intrusion guidance for conducting vapor intrusion evaluations in California. The draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion (Draft Guidance) is available for review and public comment from February 14 to April 30, 2020.
Availability of Draft Guidance
Electronic Copies
Hard Copies Hard copies of the Draft Guidance are available for review at various locations. Please see the Public Notice (English version; Spanish version) for details. Public Meetings & Webinars
DTSC and State Water Board will host a series of informational public meetings and webinars state-wide to answer questions and solicit public comments/feedback in April 2020. See the Public Notice (English version; Spanish version) for details regarding the public meetings and webinars.
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Lenny Siegel Executive Director Center for Public Environmental Oversight A project of the Pacific Studies Center P.O. Box 998, Mountain View, CA 94042 Voice/Fax: 650-961-8918
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Lenny Siegel Executive Director Center for Public Environmental Oversight A project of the Pacific Studies Center P.O. Box 998, Mountain View, CA 94042 Voice/Fax: 650-961-8918
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