Driveway sealants are turning streams toxic
Coal-tar sealants MILWAUKEE applied to blacktop parking lots and driveways are the primary source of toxic chemicals found in the muck at the bottom of area waterways, according to a study from the U.S. Geological Survey and the Milwaukee Metropolitan Sewerage District.
Tests of muck samples collected at 40 locations along 19 creeks and rivers in the metropolitan area and dust from six parking lots found that coal-tar sealants contributed up to 94% of all polycyclic aromatic hydrocarbons, or PAHs, in streambed sediment, according to the study, published Thursday in the journal Environmental Toxicology and Chemistry.
Fully 78% of the samples contained enough PAHs to be considered toxic and capable of causing adverse effects in aquatic animals, said Austin Baldwin, a USGS scientist and lead author of the study.
Rain and melting snow rinse PAHs and other contaminants off the pavement and into stormwater storage basins or directly into storm sewers that carry the load to waterways.
Even before the study was published, early circulation of its findings boosted support for local restrictions or bans on the use of coal-tar sealants and a switch to sealants containing asphalt emulsions, said Chris Magruder, a retired Milwaukee sewer district scientist who is science advisory committee coordinator for the Southeastern Wisconsin Watersheds Trust.
Coal tar, a byproduct of converting coal to coke, is a solidcarbon fuel and carbon source for the steelmaking industry, and it’s a known human carcinogen.
As coal is heated to produce coke, coal tar vapors are released.
Pavement sealants made with coal tar contain much higher concentrations of PAHs — up to 1,000 times more — than available substitute products made with asphalt emulsions, according to other studies.
Asphalt sealant products are known as seal coats.
They are used to improve the appearance and maintain the surface of parking lots and driveways.
As of this month, Milwaukee’s sewer district no longer allows its contractors to use coal-tar sealants for sewer district projects.