Evidence that the concentration of an important class of toxic chemicals, polycyclic aromatic hydrocarbons (PAH), has been increasing in urban lakes and streams has been growing for four decades, but the sources haven't been pinned down. PAHs are probable human carcinogens, and are toxic to fish and other aquatic life. A study scheduled to be published shortly in the journal Science of the Total Environment has found that coal tar-based sealcoats used on pavement are the primary source.
- "Contribution of PAHs from Coal-Tar Pavement Sealcoat and Other Sources to 40 U.S. Lakes Evaluated Using Mass-Balance Receptor Modeling,"  by Peter C. Van Metre and Barbara J. Mahler (abstract from Oct. 31, 2010, presentation at Geological Society of America meeting in Denver).
- Science of the Total Environment. 
Some highlights of the study:
- Advanced technology was used to pinpoint coal tar-based sealcoats as the dominant culprit, providing information that hadn't been available.
- The findings are based on 40 lakes in diverse urban settings around the country (specifically identified in the Supplement to the study), suggesting the problem occurs throughout the country wherever coal tar-based sealcoating dominates the market (primarily the central and eastern US).
- Asphalt-based sealcoats, which are used in some areas (primarily the western US), leach far less PAH contamination (though it's unknown if other contaminants from such products may be a significant problem).
Less-toxic alternative sealcoat products are not yet widely available, says lead author and USGS research hydrologist Peter Van Metre,  512-927-3506. The option of coating coal tar-based sealcoat with asphalt-based sealcoat isn't viable, since PAHs still leach through, he adds. Much remains unknown about asphalt-based sealcoat. Some cities have become concerned about the building evidence against these products, and are taking various actions, he says.
The late SEJ founding board member Kevin Carmody was one of the first to explore the sealcoat issue in a series of stories about urban water pollution  in the Austin American-Statesman in 2003-4.