[Please excuse the duplicate posting. - LS]
In my December 30, 2004 Military Environmental Forum article about TCE
in Moffett Field's Orion Park Housing area, I wrote:
"The fact that other units above tainted groundwater didn't show vapor
intrusion may be because they are effectively sealed off or vented, or
because there are preferential pathways leading the vapors in another direction."
It occurred to me today (December 31) that the last clause may not be
clear. It's actually based upon a conceptual site model that is emerging
from investigations at the GTE site, a nearby civilian residential
development which also has attached units.
When EPA and GTE began their vapor intrusion investigations at the site,
they found high levels of indoor TCE in one housing unit in a building
that contains five or six attached townhouses. The readings exceeded the
modeled levels, even at a location with some of the highest
concentrations of TCE in groundwater on the property. So they installed
a venting system and undertook a much more comprehensive investigation
of vapor intrusion throughout the subdivision.
That investigation showed that other units within the same building had
levels below those predicted by the model. That suggests that TCE soil
gas collected under the slab for the entire building, and that most of
it migrated through a preferential pathway into the single unit, leaving
the others relatively clean, despite their proximity.
Most of the diagrams that are presented to the public at potential vapor
intrusion sites show a single home sitting above contamination. I'd like
to see more diagrams with attached units, as well as procedures for
sampling such buildings. Sampling a single unit in such a building, as
the Navy did at Orion Park, is akin to "Russian Roulette." All the risk
may be concentrated in a fraction of the units. One doesn't know, at the
single targeted unit, whether it's a hit or a miss.
Lenny
--
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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