Concentrations of polycyclic aromatic hydrocarbons (PAHs) in runoff from pavement with coal-tar-based sealcoat remain elevated for months following sealcoat application, according to a new study by the U.S. Geological Survey.
PAHs are an environmental health concern because they are toxic to fish and other aquatic life. A 2012 human health-risk analysis found that people living near pavement sealed with coal-tar-based products have an elevated risk of cancer.
USGS scientists evaluated concentrations of PAHs and azaarenes (chemicals similar in structure to PAHs but containing a nitrogen atom in the place of a carbon atom) in runoff from test plots sealed with either coal-tar-based or asphalt-based sealcoat starting five hours after sealcoat application and continuing for as long as three months after application. The full report, published in the journal Environmental Pollution, is available online.
Concentrations of PAHs and azaarenes in runoff from the coal-tar-sealcoated pavement were about 20 times higher than in runoff from the asphalt-sealcoated pavement, and about 40 times higher than in runoff from unsealed asphalt. Concentrations and assemblages of PAHs indicated that the asphalt-based sealcoat might have contained a small amount (5-10%) of coal-tar-based sealcoat.
Although the total concentration of PAHs varied relatively little over the three months following application, the concentration of high molecular weight (large) PAHs increased and the concentration of low molecular weight (small) PAHs decreased. The low molecular weight PAHs are acutely toxic to aquatic life, but the high molecular weight PAHs are more likely to cause mutations, birth defects, and cancer. The high molecular weight PAHs in the runoff were mostly in the form of particles.
This study is the first to investigate concentrations of azaarenes associated with sealcoat runoff. Sources of azaarenes include coal-tar and oil-shale processing, wood preserving, and chemical manufacturing. In samples of runoff collected just hours after sealcoat application, concentrations of the azaarene carbazole exceeded those of any other PAH or azaarene measured. Azaarenes have a large range of ecotoxicological effects, including acute toxicity, but have been less well studied than PAHs.
Sealcoat products are widely used in the United States, both commercially and by homeowners. The products are commonly applied to commercial parking lots (including strip malls, schools, churches and shopping centers), residential driveways, apartment complexes and playgrounds.