Re: [CPEO-BIF] My preliminary comments on California VI guidance

[Lenny Siegel <lsiegel@cpeo.org>]

Alpha Oversimplified

In my earlier comments on California's Vapor Intrusion guidance, I
discussed "attenuation factors." See comments 7 and 10. For those who
are unfamiliar with that term, here's an explanation:

In the language of the Johnson-Ettinger model and other models for
projecting indoor air contamination from subsurface conditions, alpha
(usually represented by the Greek letter) is the attenuation factor. It
represents the ratio of the indoor air concentration of a vapor
contaminant to the concentration of the same vapor in shallow soil gas,
near but not at the surface. Thus, if the concentration of TCE in indoor
air is 1 microgram per cubic meter and the concentration of TCE vapor in
the soil at 5 feet below ground surface is 100 micrograms per cubic
meter, the measured alpha is .01.

The various vapor intrusion guidances offer "default" alphas, ratios
based on site conditions (such as whether the structure is has a slab,
crawlspace, or basement) to predict indoor air concentrations based upon
soil gas measurement. If the soil gas measurement multiplied by alpha is
sufficiently below the applicable health standard for the contaminant,
then agencies will generally not conduct or order indoor air sampling.
Thus, the lower the default alpha used at a site, the less likely it is
that indoor air sampling will take place.

LS

-- 


Lenny Siegel
Director, Center for Public Environmental Oversight
c/o PSC, 278-A Hope St., Mountain View, CA 94041
Voice: 650/961-8918 or 650/969-1545
Fax: 650/961-8918
<lsiegel@cpeo.org>
http://www.cpeo.org
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