More than a decade of research has revealed that coal tar in paving sealants may be negatively impacting human health and the environment. But now, environmental engineer Charles Werth is directing a study to determine if the risks to aquatic organisms actually diminish over time.
“The answer will help officials know whether active cleanup will be needed or whether lakes can in fact ‘repair’ themselves if coal tar sealant use were discontinued,” he said.
Americans apply roughly 99 million gallons of coal tar sealcoat to parking lots, driveways and playgrounds each year to maintain a clean look and protect the asphalt or concrete underneath from water and ice.
“These sealcoats are worn off by driving, sun and freezing temperatures,” said Werth, who relocated from Illinois to the University of Texas at Austin during the study. “And the abraded particles are washed into nearby waterways when it rains.”
Coal tar sealcoat particles are rich in polycyclic aromatic hydrocarbons (PAHs)—a large suite of compounds created when wood, gasoline and other carbon-based materials are burned. Scientists have linked these chemicals to mutations, birth defects, cancer and death in animals. Several PAHs are also considered probable human carcinogens by the U.S. Environmental Protection Agency and the International Agency for Research on Cancer.
Their threat to aquatic wildlife, though, may alter after PAHs enter the water. Depending on the chemical makeup of the sediment, Werth said, the compounds may break away from the coal tar particles and form stronger bonds with other materials.
“When this happens, the organisms living in the sediment are directly exposed to lower concentrations of PAHs because the chemicals aren’t being released,” he added.
Werth, University of Illinois’ Michael Plewa and graduate student Victoria Boyd have seen signs of this chemical redistribution in Whitnall Park Pond in Wisconsin. But it will be at least another year before they can quantify the impact of that shift.
The project began in 2011 with a grant to the Illinois Water Resources Center from the U.S. Geological Survey and the National Institutes for Water Resources. It was one of only five selected that year nationwide.
But Werth’s work on PAHs from coal tar sealcoat started more than a decade ago with a study that traced the sources of compounds in Texas’ Lake Como and the stream that feeds into it.
That study—published in the journal Environmental Science & Technology—was among the first to show that coal tar sealcoat can be the primary source of PAHs in urban lakes. Nearly 84 percent of the PAHs found at the bottom of Lake Como came from coal tar sealcoat.
To track these compounds back to their sources, researchers used an approach often referred to as PAH fingerprinting. The exact makeup and molecular weight of PAHs in a product are unique to that material—car tires, for example, have a different arrangement of compounds than motor oil or coal tar sealcoat.
“You can take advantage of that individual signature to determine which materials are most likely to produce the mix of PAHs you see in the sediment,” said Werth.
The fingerprinting results were confirmed with a less common and more time-intensive detection method. Instead of working backwards from the PAHs, the researchers suspended sediment samples in resin before examining them under a microscope.
“It isn’t easy, but if you are very careful and methodical, you can actually count the different types of source particles,” Werth added.
Illinois Water 2016
Photo: Victoria Boyd