|From:||Lenny Siegel <firstname.lastname@example.org>|
|Date:||3 Aug 2006 17:22:48 -0000|
|Subject:||[CPEO-BIF] The Implications of the NRC Report on TCE|
[Please excuse the multiple postings. - LS]|
THE IMPLICATIONS OF THE NRC REPORT ON TCE Lenny Siegel August 3, 2006
Last week (on July 27, 2006) the National Research Council (NRC), an arm of the National Academies of Sciences, issued its report, "Assessing the Human Health Risks of Trichloroethylene: Key Scientific Issues." The report is likely to have enormous implications for federal, state, and private cleanup programs, because TCE is found in groundwater at thousands of contamination sites throughout the United States.
The NRC concluded that the evidence that TCE causes cancer is becoming stronger. Furthermore, it recommended that EPA extrapolate the cancer risk using a linear model, which among other things means there is no concentration so low that it doesn't pose a cancer risk - the risk just goes down in proportion to the concentration.
The NRC Committee recommended "that federal agencies finalize their risk assessment with currently available data so that risk management decisions can be made expeditiously." Significantly, this seems to suggest that EPA update and complete its 2001 Human Health Risk Assessment, rather than start anew. I assume that this lead recommendation was made to "federal agencies" because the study was sponsored by the Defense Department, Energy Department, and NASA, as well as EPA. However, of those agencies, only EPA has the statutory authority to issue this type of risk assessment.
The Committee made this recommendation even though it documented scientific uncertainty in a number of key areas. It defined several important areas for future research, as well as methodological suggestions. In particular, it laid out guidelines for a new meta-analysis, a study that combines data from multiple studies.
Most observers seem to think that the completed risk assessment will lead to a new federal drinking water standard - a maximum contaminant level, or MCL. Some suggest that the MCL will be lowered from its current 5 parts per billion (ppb) to 1 ppb. In 2003, the Air Force evaluated the impact of such a change on groundwater remediation alone. It estimated that the Defense Department's cost for addressing TCE would double from $5 billion to $10 billion, primarily because of the ongoing costs of operation and maintenance.
I believe those numbers are exaggerated. There is substantial evidence that conventional remediation - "pump and treat" - declines in effectiveness over time. Systems are likely to be turned off when the concentration vs. time graph reaches an asymptote, and in a majority of cases this will be above 1 ppb or even 5 ppb - of TCE in groundwater.
Also, in 2003, when it appeared that EPA was going to promulgate more protective TCE standards, EPA toxicologists calculated a TCE indoor air screening level of under .02 micrograms per cubic meter to correspond to one excess cancer for every million people exposed to this concentration over thirty years. This number was included in EPA's 2002 draft Vapor Intrusion Guidance, and or a short time, it served as a default screening level at many sites where vapor intrusion investigations were underway.
However, since background - ambient air in many metropolitan areas - is at least an order of magnitude above the .02 level, outdoor readings were the de facto screening levels, since there is no practical way to keep indoor air cleaner than the outdoor air that inevitably drifts in.
When EPA management decided not to continuing using the 2001 Draft Risk Assessment, EPA field personnel shifted to indoor air screening levels averaging about 1 microgram per cubic meter.
Vapor intrusion projects are more sensitive to health standards than drinking water response. It seems that a large number of people are exposed at levels near the screening levels, so any change in the screening levels affects how many homes or other structures are tested as well as how many structures are mitigated. To responsible parties, the cost differential can indeed be enormous.
But the area where more stringent TCE standards are likely to have the greatest impact is the intersection between air exposure and groundwater contamination. It is not addressed often, because air and water are dealt with by different regulatory organizations under different statutes. As I wrote above, TCE is persistent in outdoor air in many metropolitan areas, despite its half life of three to eight days. As I documented in my recent report on TCE in the Los Angeles area, (see http://www.cpeo.org/pubs/TCE-LA.doc) monitoring stations in Azusa, Burbank, and Los Angeles most recently detected an average concentration of .3 micrograms of TCE in each cubic meter of air. This is startling because manufacturers in the region report almost no current use of TCE, and because consumer products contain relatively small quantities of the compound. (In other states, such as Kansas and Georgia, TCE is still being used in significant quantities.)
Since TCE does not occur in nature, it has to be coming from somewhere. The best explanation, at least in California, is that is it making its way to the surface from landfills and groundwater plumes. The pathways include vapor intrusion, outdoor vapor migration, and releases from treatment systems.
The key point is not that one can't clean indoor air because outdoor air is contaminated, but that millions of people are breathing unsafe levels of TCE around the clock. The levels are low, but using a linear cancer model, the likely health impacts add up.
To minimize releases into the air, therefore, there is a need to accelerate the cleanup of subsurface TCE. Conventional responses may be adequate to limit the spread of groundwater contamination, but they are painfully slow and in a majority of cases may never even meet the current MCL.
Fortunately, new technologies - active bioremediation, advanced oxidation, permeable iron barriers, etc. - in many scenarios not only destroy the contamination, but they reduce concentrations much more rapidly. It might seem unrealistic to get responsible parties to switch over to alternative technologies, but it turns out that in many cases the new, more protective technologies are less expensive, because the costs - such as electricity - don't keep rising and don't continue indefinitely. Many polluters, including the Air Force and Navy, now routinely evaluate and sometimes replace old pump-and-treat systems with innovative technologies. This could be done on a much larger scale.
It is likely that the findings of the NRC committee will lead EPA to calculate a more protective action level of TCE in indoor air than the current average 1 microgram per cubic meter level. If so, that level should be applied to outdoor air that is at or above that level. I don't expect Clean Air Act regulators to take the lead on this issue, because 1) they're more concerned about diesel exhaust and 2) they don't regulate the subsurface sources. So it's up to the those responsible for investigating and remediating subsurface TCE pollution to systematically review the treatment systems in place and find ways to use newer technologies to accelerate the reduction of risk.
Furthermore, there needs to be a comprehensive approach. Government agencies should compile a complete list of TCE sites throughout the country, including what is known about continuing releases, even at low concentrations. Right now that information is fragmented among agencies, and few agencies know to what degree the sites they regulate are emitting TCE vapors.
TCE doesn't have to remain a permanent blight on public health - if we admit the risk and decide to eliminate it cheaper, faster, and better.
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