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Coal-tar-based pavement sealant is the largest source of toxic polycyclic aromatic hydrocarbons (PAHs) found in 40 urban lakes recently studied by the U.S. Geological Survey.
PAHs are an environmental health concern because several are probable human carcinogens, they are toxic to fish and other aquatic life, and their concentrations have been increasing in urban lakes in recent decades, USGS reports.
Study: From AK to FL
Coal-tar-based pavement sealant is the black, shiny substance sprayed or painted on many parking lots, driveways and playgrounds. USGS scientists evaluated the concentration of PAHs from many different sources to lakes in cities from Anchorage, AK, to Orlando, FL. The full report can be found in the journal Science of the Total Environment.
USGS scientists collected sediment cores from 40 lakes, analyzed the cores for PAHs, and determined the contribution of PAHs from many different sources using a chemical mass-balance model.
On average, coal-tar-based sealcoat accounted for one-half of all PAHs in the lakes, while vehicle-related sources accounted for about one-quarter. Lakes with a large contribution of PAHs from sealcoat tended to have high PAH concentrations—many at levels that can harm aquatic life.
Analysis of historical trends in PAH sources to a subset of the lakes indicates that sealcoat use since the 1960s is the primary cause of increases in PAH concentrations.
Runoff and Dust Transmission
“These findings represent a significant advance in our understanding of the sources of these contaminants in streams and lakes,” said USGS scientist Peter Van Metre. “Identifying where contaminants are coming from is the first step in designing effective management strategies.”
Coal tar is made up of at least 50% PAHs. Pavement sealants that contain coal tar, therefore, have extremely high levels of PAHs compared to other PAH sources such as vehicle emissions, used motor oil, and tire particles, according to USGS.
Small particles of sealcoat wear off the surface relatively rapidly, especially in high traffic, and are transported from parking lots and driveways to streams and lakes by storm runoff. Manufacturers recommend resealing surfaces every three to five years.
Runoff isn’t the only path by which PAHs are leaving parking lots. A recent USGS study found that use of coal-tar-based sealcoat on parking lots was associated with elevated concentrations of PAHs in house dust.
2 Formulations
Sealcoat products are widely used in the U.S., both commercially and by homeowners. The products are commonly applied to commercial parking lots (including strip malls, schools, churches and shopping centers), residential driveways, apartment complexes and playgrounds.
The City of Austin, TX, banned coal-tar-based sealcoat in 2006; until then, about 600,000 gallons of sealcoat were applied every year in the city.
Two kinds of sealcoat products are widely used: coal-tar-emulsion based and asphalt-emulsion based. End users can determine whether a product contains coal tar by reading the label. The coal-tar products have PAH levels about 1,000 times higher than the asphalt products.
Usage Patterns
National use numbers are not available; however, previous research suggests that asphalt-based sealcoat is more commonly used on the West Coast and coal-tar based sealcoat is more commonly used in the Midwest, the South, and the East.
The results of the USGS lake study reflect this East-West difference. For example, sealcoat contributes more than 80% of PAHs in Lake Anne, VA, where PAH concentrations are about 20 times higher than in Decker Lake, UT, even though the areas have similar population density and level of urban development.
Furthermore, PAH levels in pavement dust from sealcoated parking lots in Virginia are about 1,000 times higher than those from sealed parking lots in Utah, USGS reports.
To learn more, visit the USGS National Water Quality Assessment Program website on PAHs and sealcoat.
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