TCE in Mountain View's Air

by Lenny Siegel
January, 2004

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.