2009 CPEO Military List Archive

From: Lenny Siegel <lsiegel@cpeo.org>
Date: Fri, 20 Mar 2009 13:49:26 -0700 (PDT)
Reply: cpeo-military
Subject: [CPEO-MEF] EPA Interim Guidance on TCE and Vapor Intrusion
EPA's January 2009 Memo on TCE and Vapor Intrusion
Summarized by Lenny Siegel
[Please excuse duplicate postings.]

On January 15, 2009 outgoing EPA Assistant Administrator for Solid Waste and Emergency Response Susan Parker Bodine signed a significant memorandum, "Interim Recommended Trichloroethylene (TCE) Toxicity Values to Assess Human Health Risk and Recommendations for the Vapor Intrusion (VI) Pathway Analysis." This document provides what many of us have been asking for, for years: A national interim standard for TCE in indoor air.

Unlike other 11th hour Bush environmental actions, this one was not rushed through to undermine the ability of the incoming administration to take action. This had been in the works for many months. It reportedly had buy-in from other federal agencies, including the Defense Department. But it was held up, for no clear reason, in the White House.

Indoor Air Action Level

They key element of the memo is the establishment of an interim action level of For TCE in indoor residential air: 1.2 micrograms per cubic meter (ug/m3), based upon California EPA's inhalation unit risk value. More precisely, this is the "point of departure for determining preliminary remediation goals," associated with a 10^-6 (one in a million) excess lifetime cancer risk.

While officially the memo serves as guidance for EPA alone, I anticipate that other federal agencies, including those responsible for cleaning toxic sites, and state regulators will follow EPA's advice.

The memo states, "This guidance ... does not represent a new, independent review of TCE toxicity, which EPA has currently underway as part of the IRIS program." Bodine was referring to the review that the National Research Council discussed in July 2006. The NRC found, "enough information exists for the U.S. Environmental Protection Agency to complete a credible human health risk assessment now."


The new standard is slightly higher than the 1.0 ug/m3 that EPA Region 9 has been using since 2005, also based on California's studies. That level has proven generally protective, though I believe that science-based studies will eventually drive the standard lower, at or below the 0.2 ug/m3 concentration typically found in outdoor air throughout the United States. The Obama EPA must move quickly to complete a human health risk assessment for TCE. It is long overdue.

Because one cannot mitigate indoor air to levels below those in outdoor air, a standard below background levels has no practical meaning. I believe communities will accept 1.2 ug/m3 as an interim standard, as long as no one who has benefited from risk management decisions based upon 1.0 .8, or .4 ug/m3 is forced to accept a weaker standard.

In the long run, I still argue that homes and other sensitive uses should be mitigated wherever indoor contamination levels caused by vapor intrusion exceeds toxic concentrations in outdoor air, if mitigation is practical. The goal should be to eliminate the exposure, not to find a numerical level that scientists have calculated to be within risk ranges set by policy-makers.

Furthermore, homes, businesses, and schoolrooms that test below the 1.2 ug/m3 threshold, may still require mitigation, for either of two reasons: 1) Soil gas levels beneath the structures may be high enough to cause vapor intrusion should a crack or hole open up in the slab, floor, or basement wall. Or 2) The building or room may be located among other structures that test high enough to require mitigation. Measurements vary over time, and occupants and property-owners should not be told they do not qualify for a depressurization system when their neighbors on two sides do.

Multiple Lines of Evidence

The Bodine memo also endorses the Multiple Lines of Evidence approach to investigating vapor intrusion. EPA staff has been advocating this strategy for some time. Demonstrating that vapor intrusion is causing unacceptable exposures is a complex task. I've called it "rocket science," because it is difficult for most members of impacted communities to understand. The solution, however, is not to simplify the investigation, but instead to devote more effort to explaining it. Cooperation of community members is essential, because sampling usually depends upon their assenting to intrusive activity on their property.

The memo does a good job of explaining Multiple Lines of Evidence:

"Our experience with vapor intrusion investigations indicates that no single media data set, whether it be ground water, soil gas, sub-slab gas or indoor air, can be used reliably to fully evaluate the potential for risks from VI above health risk-based levels due to the large number of variables affecting the transport of vapors from the subsurface to indoor air and the confounding influence of indoor sources of common subsurface contaminants. Our investigations have found that spatial and temporal impacts on volatile organic chemical (VOC) concentrations are highly variable. Some of this variability is due to vertical and horizontal differences in subsurface conditions and the differences in structural conditions, such as foundation cracks, and ventilation rates from one building to another. Variation in weather conditions, such as rainfall and barometric pressure, can also have a significant impact. All these factors strongly suggest that multiples lines of evidence are important to evaluate VI as an exposure pathway of concern at sites where hazardous VOCs have been released to the subsurface.

"Lines of evidence to evaluate the VI pathway may include: site history and geology, groundwater data, soil gas data, sub-slab soil gas data, crawlspace sample data, chemical ratio data, modeling results, building/home surveys, chemical use inventory, and other support information, as appropriate...."


Lenny Siegel
Executive Director, Center for Public Environmental Oversight
a project of the Pacific Studies Center
278-A Hope St., Mountain View, CA 94041
Voice: 650/961-8918 or 650/969-1545
Fax: 650/961-8918

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