Chemical detectives have tracked down the main source of a troublesome family of organic pollutants. Their "fingerprinting" of polycyclic aromatic hydrocarbons (PAHs) shows that road- and sidewalk-surface sealers based on coal tar are responsible for most of the noxious substances by far.
PAHs have accumulated rapidly over the past 40 years in many US stream and lake beds, and many are toxic or carcinogenic. Suspected culprits had included used motor oil and the burning of wood, coal and oil.
Current US guidelines consider certain PAHs dangerous to organisms that live at the bottom of streams and lakes if there is more than 22.8 milligrams of the chemical per kilogram of sediment. Many urban and suburban lakes have higher levels; worse still, other kinds of PAH are air pollutants.
Yet controlling that pollution has been difficult. "There are so many small sources that people had thrown up their hands," says Peter Van Metre of the US Geological Survey (USGS) Texas Water Quality Center in Austin. Then Austin water-quality officials suggested that coal-tar roadway sealer might have caused local PAH problems.
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