October 2006

ARTICLES  
     

  Bad News for TCE Fans
  Keith Tolson

      

It seems like every year a new study is released that reverses a previous health risk/benefit report. Such is the nature of applying scientific methods to evaluate these questions. As new research is conducted, we expand our knowledge base and refine our understanding of complex problems. Ultimately, we hope to arrive at the correct conclusion.

A similar scientific re-examination is currently underway with the pervasive compound trichloroethylene (TCE). On 27 July 2006, the National Research Council (NRC), an arm of the National Academy of Sciences (NAS), issued a report recommending that EPA press forward with a reassessment of the cancer potency of TCE. It is widely anticipated that this will result in substantially lower acceptable cleanup criteria. Such a change will drive up remediation costs and force reopening closed sites to address residual contamination.

TCE is a chlorinated solvent that was used extensively by industry in degreasing applications. It is relatively resistant to degradation and readily migrates through soil to contaminate groundwater. TCE contamination is found at more than half of all EPA Superfund sites and many state program sites around the country.

The question is - how toxic TCE is to humans? Specifically, how much TCE constitutes an unacceptable risk for developing cancer. Animal studies show that exposure to TCE is associated with an increased incidence of cancer. Supporting studies in workers exposed to TCE show a relationship between exposure and cancer. In 1985, EPA used the studies available at that time to develop a cancer potency estimate. In the face of new studies suggesting TCE posed a greater threat, EPA withdrew the cancer estimate and set out to complete a reassessment. EPA completed the reassessment in 2001 and concluded that TCE was approximately 30 times more potent than previously thought.

Considerable debate surrounded the release of this reassessment with advocates from the Department of Defense and the Department of Energy (agencies with a large numbers of TCE contaminated sites) heavily criticizing the scientific basis for the reassessment. EPA sent the matter to the NAS for it to resolve the issue of whether the science was acceptable.

The NRC panel concluded that the scientific studies examined by EPA are scientifically sound and suitable for use. They recommended further study to determine the mechanism by which TCE causes cancer and to identify populations most sensitive to these effects. Perhaps most importantly, they urged EPA to complete the potency reassessment using "currently available data." The implication here is that the data presently available to EPA, and previously interpreted by EPA, will lead to a similar reassessment conclusion. TCE cleanup criteria will go down.

The key numerical criteria that is likely to change as a result of this assessment is the federal drinking water standard or Maximum Contaminant Level (MCL). The time and cost required to complete TCE remediation will increase dramatically if a lower MCL is promulgated. Current remediation techniques employed to address TCE contaminated groundwater often become less effective at lower levels of contamination. More innovative approaches using active bioremediation or nano-scale zero valent iron may be required to achieve site closure. These techniques actively break down TCE in the subsurface and are capable of dealing with low level TCE contamination over a wide area.

EPA currently projects to issue a draft TCE potency estimate for public comment in December 2008 with a finalized value in August 2009, a measured pace reminiscent of the arsenic reassessment of several years ago. However, several regulatory jurisdictions currently use or endorse the higher withdrawn EPA cancer potency estimate. Florida has elected to use the lower (less stringent) original cancer potency estimate and has adopted a wait-and-see approach. While we are waiting for EPA to make its final decision on the potency estimate for TCE, strategic planning should begin now. These plans should consider the implications that the new TCE cleanup criteria will have on remedial decisions, property acquisition, and an assessment of legal liability associated with TCE contaminated property.

Dr. Keith Tolson is a toxicologist working for GeoSyntec Consultants in Tampa Florida. His practice focuses on human health risk assessment, applied toxicology and regulatory affairs. He can be reached at (813) 558-0990 or by email at ktolson@geosyntec.com.

 

 

 

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