[CPEO-BIF] Region 9's TCE air standard

[Lenny Siegel <lsiegel@cpeo.org>]

U.S. EPA's Region 9 officials have recently clarified the health
standard they are using for TCE in indoor air.

They describe the EPA Region's Provisional Risk Range, for TCE in
residential air, as from .017 micrograms per cubic meter (which
represents a 10^-6 additional lifetime cancer risk) to 1.7 micrograms
per cubic meter (corresponding to a 10^-4 additional lifetime cancer risk).

They also describe Ca/EPA's Risk Range, which is enforceable, as from
.96 micrograms per cubic meter (which represents a 10^-6 additional
lifetime cancer risk) to 96 micrograms per cubic meter (corresponding to
a 10^-4 additional lifetime cancer risk).

EPA has chosen an "Interim Action Level" of 1 microgram per cubic meter,
which is within the overlap of the two Risk Ranges. That is, it meets
the requirements of EPA's more protective range while remaining
enforceable because it is within California's risk range.



I view this as backsliding from two years ago, when the Region announced
the Provisional Preliminary Remediation Goal of .017. It did so on the
assumption that EPA's national Health Risk Assessment for TCE would move
forward, but it didn't,  so the .017 remains unpromulgated and
uneforceable. EPA project managers may still use the number to help
guide vapor intrusion investigations, but they cannot force responsible
parties to reduce air concentrations to that level.

In practice, the difference between the two numbers may not be as great
as it seems.

FOR MITIGATION: I don't know of any location that requires ventilation
as a mitigation for concentrations near .017, because in such situations
the ambient air is usually above that level. That is, ventilation
wouldn't work because similarly contaminated air would move in from
outside. Still, there are numerous situations in which I would support
mitigation at .2 micrograms per cubic meter (TCE in indoor residential air).

FOR REMEDIATION: The action level of 1.0 may be used to argue for
improved or accelerated source remediation, if that level is found in
any home impacted by the contamination. In fact, participants in our
community advisory group in Mountain View are beginning to argue that
additional cleanup is necessary to address the POTENTIAL for vapor
intrusion. That is, there is evidence that passive mitigation -
primarily impermeable liners on concrete slabs - is usually protective
in the short run, but residents are concerned that earthquakes, age, or
even human activity may open up preferential pathways in the future. At
a minimum, monitoring is necessary, but it would make more sense to
eliminate the high levels of TCE in groundwater or soil (verified by
high soil gas measurements) and thus reduce the need for long-term stewardship.

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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