TCE in Mountain View's Air

[Lenny Siegel <lsiegel@cpeo.org>]

[Please excuse the cross-posting. The Mountain View sites include both
federal and private contaminated properties. - LS]


The investigation of trichloroethylene (TCE) in the air in Mountain
View, California - my community - has come a long way, but there are
still more questions than answers. At two community meetings this month,
representatives and contractors for U.S. EPA, the Navy, NASA, and the
Bay Area Air Quality Management District reported on the results of
recent studies. Over the next year or two, I anticipate that we will
have a much better picture of local exposures to volatile organic
compounds, and a better model for where they are coming from. Hopefully,
remedial activities will be adjusted to reduce those exposures. In
addition, I expect that the lessons from Mountain View will prove
valuable at other locations where vapor intrusion is likely to occur.

Though the Mountain View air investigations began as a search for indoor
air pollution, the recent presentations focused on outdoor air
contamination - largely at the request of community members of the
Moffett Field Restoration Advisory Board and the umbrella Northeast
Mountain View Advisory Council. The outdoor air issue originally emerged
when the Navy suggested that TCE readings inside unoccupied units of the
Moffett Field housing area were the result of ambient TCE concentrations
throughout the region. Some of us in the community questioned the Navy's
preliminary conclusions, but more important, we found the prospect of
constant regional exposure to TCE, at levels exceeding provisional EPA
screening levels, to be of great concern. As I have written before, I
believe the investigations of vapor intrusion should include all
sources, all receptors, and all pathways in the area.

Here is my selective summary of key results, as discussed at the January
meetings of the two advisory groups:

* Detections of TCE in outdoor air are low, apparently posing no
unacceptable acute or short-term health risk, but high enough to cause
concern about the possible health effects of long-term exposure.

* TCE contamination is consistently found, in the ambient air in areas
above known shallow groundwater plumes, at levels exceeding U.S. EPA's
stringent provisional screening levels for TCE: .017 micrograms per
cubic meter for residential or unrestricted use areas and .048
micrograms per cubic meter for occupational exposures.

* TCE contamination above the screening levels is sometimes found in
outdoor air at nearby reference locations, including a local elementary school.

* Sometimes indoor air concentrations of TCE are greater than those
immediately outside, but not always.

* Some of the parties have shown that it is possible to bring the
detection limit down below the provisional EPA screening level, but the
Air District reports that its labs - which generate the regional
background figures of "non-detect" - cannot measure TCE in air with such
precision without a major investment in new equipment.

* By analyzing TCE and a number of other compounds, NASA contractors
have demonstrated that more frequent sampling - at some locations three
eight-hour samples a day several days a week - can help resolve
anomalous results.

* The Air District has thus far been unable to find a source that
explains unusually high measurements of TCE at NASA's background
reporting station in late September, 2003. The District looked for
current users of TCE, calculated the impact of Mountain View's former
landfill, and considered the contribution of the one remaining scrubbed
air stripper, part of the treatment system for the "regional plume" from
the former operations of local electronics companies and the Navy.

And here are my conclusions for now, essentially hypotheses that I
believe should influence future investigations:

Since TCE in air dissipates due to degradation (a reported half-life of
four days), advection (wind), and diffusion, there must be persistent
sources in the northeast Mountain View area. While we may still yet find
current sources, it is more likely that the sources are the historical
contamination of groundwater and soil. There is a large volume of TCE
underground in Mountain View, and it "wants" to come to the surface. The
local indoor air investigations suggest that preferential pathways such
as cracks and utility lines predominate over homogeneous vertical
migration, but I believe such pathways exist outdoors as well as indoors.

I appreciate the extent to which the responsible parties, property
owners, and regulatory agencies are studying this problem. Some of the
work - in lowering detection limits and conducting repeated sampling -
is cutting edge. But I've made one more request: All of the parties
should create a combined data base, and U.S. EPA should orchestrate
additional sampling, with the purpose of mapping the concentration and
extent of the outdoor TCE-in-air plume in Mountain View. This may be
more difficult than mapping a groundwater plume, because the temporal
variations are much more significant. On the other hand, while local
groundwater studies have necessarily been three-dimensional, at this
point there seems no reason to map the vertical migration of TCE in air.

Air contamination above shallow groundwater plumes seems high enough and
consistent enough to merit an additional response. If continuing studies
bear this out, more cleanup - not just mitigation techniques such as
venting or land use controls - may be necessary. The existing remedies
are conventional - slurry walls and pump-and-treat. To protect the
public, the regulators and responsible parties should consider newer
cleanup technologies that are designed to treat or remove contamination
near the surface.

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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