Pavement sealant a major urban pollutant across America | Local News | Idaho Statesman
A black sealant containing coal-tar turns out to be the largest contributor to the rise of a toxic pollutant in urban lakes and reservoirs across America, according to a U.S. Geological Survey study.
Scientists saw concentrations of polycyclic aromatic hydrocarbons (PAHs) going up rapidly in the 1990s in areas of urban sprawl. PAHs have been known as a probable human carcinogen since the 19th century, when cancer struck chimney sweeps, said Peter Van Metre, a USGS scientist and a principal author of the report. PAHs also are toxic to fish and other aquatic plant and animal life.
The research was based on sampling of sediments from the bottom of 40 lakes and reservoirs in commercial and residential areas in cities and suburbs typical of where most Americans live — not near old industrial sites. Among the cities tested were Anchorage, Alaska; Fort Worth, Texas; Portland; Seattle; Orlando, Fla.; Raleigh, N.C.; Chicago; Newark, N.J.; Detroit; Milwaukee; and Boston. No sites in Idaho were tested.
Cameron Brown, a former seal-coating company owner in Boise, said base sealant products in Idaho are made of asphalt material, not coal-tar, though coal-tar is used in some lines as an additive.
“As a base product, I haven’t seen coal-tar in 15 years,” Brown said. But “even black asphalt that we pave with has some affect on our environment.”
He said if sealant is applied correctly, it holds in place well. But if it’s watered down and misapplied, it breaks up and washes into the environment more quickly, Brown said.
What was striking about the findings was that lakes that had high levels of PAHs also had a large fraction of those PAHs coming from coal-tar-based sealants. The opposite also was true — lakes that had low PAH levels also had a very low fraction of them from the sealant.
The scientists found that coal-tar-based sealants contribute, on average, about half of the PAHs in U.S. urban lakes. Vehicles account for about a quarter.
Coal combustion, the next highest source, is about 20 percent, but varies greatly because of different levels of coal use around the nation.
The study was published in the current issue of the journal Science of The Total Environment.
It’s only been in the past five years that Van Metre and other scientists pinpointed the commonly used coal-tar-based sealant as a major source of PAHs in urban lakes.
Van Metre said that about eight years ago in Austin, Texas, tests of sediment in drainage ditches showed stunningly high levels of PAHs.
“We said, ‘No way — you’ve got a problem with your lab.’ We’d never seen numbers like that,” he said. He and other scientists reported in 2005 that runoff from parking lots with coal-tar-based sealant was a major source of PAHs in the Austin streams.
Austin, Washington, D.C., and several other cities have banned coal-tar-based pavement sealants.
An alternative sealant, an asphalt-emulsion-based one, has PAH levels about 1,000 times lower.
Coal tar is a waste product of the coking of coal during steel making. Coal-tar-based sealant is more commonly used in the Midwest, South and East, and asphalt-based sealant is used more widely on the West Coast.
The Idaho Statesman contributed to this report.
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