New studies defend use of refined tar-base sealers

PCTC went to the “man who wrote the book” on PAH forensics, Paul Boehm, and asked him to apply forensic methods to the question of whether RTS are a significant source of PAHs in sediments. 

In the September 2010 issue of Stormwater magazine, Boehm and his team published their first evaluation. In Figure 2 of their article (Boehm and others, 2010), refined tar-based sealant clearly appears to have a distinct PAH fingerprint that is not uniquely reflected in PAH analysis of a variety of environmental samples with the possible exception of sediments in the immediate vicinity of sealcoated lots. The article concludes:

“PAHs are ubiquitous and have multiple sources in urban environments. As a result, PAHs are common constituents in stormwater and in urban sediments. While the atmospheric deposition of combustion products is typically the most significant pathway that introduces PAHs to urban sediments, a number of other point and nonpoint sources also contribute… While pavement seal-coating can introduce PAHs into runoff and have measurable localized contributions, the available literature suggests that other sources, especially combustion sources, represent the vast majority of PAH inputs to urban sediments.”

Boehm and his team are conducting a more in-depth forensic analysis of PAHs in sediments, which will be one of the topics at the free seminar C-21, “Sealcoating: Regulatory Challenges and Industry Initiatives” at National Pavement Expo February 4 in Nashville.  Preliminary results were presented at the annual meeting of the Society for Environmental Toxicology and Chemistry in November 2010, with the conclusion that PAHs in sediment samples collected throughout the country were more similar to PAHs typically found in atmospheric particles than in PAHs in samples associated with refined tar-based sealant.  They concluded that sealants are not a primary source of PAHs in urban sediments.

Given the results of studies sponsored by PCTC, it’s reasonable to ask how it is that some scientists continue to claim that PAHs in sediments throughout the United States have their origin in application of refined tar-based sealant to parking lots and driveways.  PCTC believes that by basing all later calculations on the flawed conclusions first presented in the incomplete 2005 study in Austin, those scientists have constructed a theory with the shakiest of foundation.

 In the most recent publications, those scientists attribute volumes of PAHs to refined tar-based sealant that are supported by no facts or figures. Those imaginary volumes are then fed into a mass balance model (previously only used to model atmospheric effects) to come up with equally imaginary “percentages” of refined tar-based sealant -source PAHs in sediments.  There’s an old saying about mathematical models:  “Garbage in – garbage out.”

Anne LeHuray is executive director of Pavement Coatings Technology Council, 2308 Mount Vernon Avenue, Suite 134 Alexandria, Virginia 22301. She can be reached at www.pavementcouncil.org.