Blinded by Science

Industry members say an examination of supporting data in the USGS 2005 follow-up report reveals a somewhat different picture than what its authors claim. In the report one table (Table S2) contains data for yield of PAH's in micrograms per square meter for each of the pavement surfaces and events sampled. While average yields per unit area of PAH collected in runoff from test plots sealed with coal tar sealer were somewhat higher than the other test plots sampled, comparing this data with the land use data in another table (Table S3), would project PAH levels in Austin's environment due to runoff from coal tar sealed pavements to be very small.

For example, PAH's contributed by vehicle exhaust, a well-studied urban PAH source, can be derived from published EPA emissions factors for PAH compounds and Texas Transportation Institute data on vehicle miles traveled in the Austin metropolitan area. A comparison of these figures would indicate PAH's contributed by vehicle exhaust as 34 times the amount projected by runoff from sealed pavements. If the potential contribution of coal tar sealed pavements is only 3% that of vehicle exhaust, how can pavement sealer be a "major" PAH source to Austin's waterways? The impact of PAH contribution by pavement sealer would be even further dwarfed if other known major PAH sources are included in the analysis.

But even more importantly, the major contribution of vehicle traffic to PAH contamination in Austin's waterways was recognized in an earlier study in 2000 by USGS researchers, who noted the effect increased traffic had on PAH levels in the area: "The large increases in traffic offer an explanation for why PAH's have more than doubled in Town Lake from 1975 to 1990 while percent of urban land use only increased by 5%. This suggests that urban sprawl in outlying areas may affect traffic patterns and water quality in the inner city." Accordingly, this particular study attributes recent PAH increases in Austin's waterways to burgeoning traffic, not parking lots.

So while the coal tar issue percolates, contractors need to realize the issue is far from settled. Coal tar sealer is not banned anywhere in the country except for Austin, TX; the studies that resulted in the ban are being challenged; and a third study, which relies not only on scientific principles but also input from a variety of organizations, is underway.