U.S. Department of the Interior
U.S. Geological Survey Fact Sheet 2011–3010 February 2011
Coal-Tar-Based Pavement Seal Coat, Polycyclic AromaticHydrocarbons (PAHs), and Environmental Health
• Dust from pavement with coal-tar-based seal coat has greatly elevated PAH concentrations compared to dust from unsealed pavement.• Coal-tar-based seal coat is the largest source of PAH contamination to 40 urban lakes studied, accounting for one-half of all PAH inputs.• Coal-tar-based seal coat use is the primary cause of upward trends in PAHs, since the 1960s, in urban lake sediment.• Residences adjacent to parking lots with coal-tar-based seal coat have PAH concentrations in house dust that are 25 times higher than those in house dust in residences adjacent to parking lots without coal-tar-based seal coat.• PAHs move from a seal coated surface into our environment by many mechanisms: storm runoff, adhesion to tires, wind, foot traffic, and volatilization. Studies by the U.S. Geological Survey (USGS) have identified coal-tar-based seal coat—the black, viscous liquid sprayed or painted on asphalt pavement such as parking lots—as a major source of polycyclic aromatichydrocarbon (PAH) contamination in urban areas for large parts of the Nation. Several PAHs are suspected human carcinogens and are toxic to aquatic life. Seal coat is the black, viscous liquid sprayed or painted on the asphalt pavement of many parking lots, driveways, and playgrounds.
How does Seal coat get from Driveways and Parking Lots into Streams and Lakes, Homes, and the Air?
Friction from vehicle tires abrades pavement seal coat into small particles. These particles are washed off pavement by rain and carried down storm drains and into streams. Other seal coat particles adhere to vehicle tires and are transported to other surfaces, blown offsite by wind, or tracked indoors on the soles of shoes. Some of the PAHs in seal coat volatilize (evaporate), which is why sealed parking lots and driveways frequently give off a “mothball”smell. Seal coat wear is visible in high traffic areas within a few months after application, and seal coat manufacturers recommend reapplication every 2 to 4 years. Gray asphalt pavement shows through where seal coat has worn off the driveway of an apartment complex.
What are Sealcoat, PAHs, and CoalTar?
Pavement seal coat (also called sealant) is a black liquid that is sprayed or painted on some asphalt pavement. It is marketed as protecting and beautifying the underlying pavement, and is used commercially and by homeowners across the Nation. It is applied to parking lots associated with commercial businesses, apartment and condominium complexes, churches, schools, and business parks, to residential driveways, and even to some playgrounds. Most seal coat products have a coal-tar-pitch or asphalt (oil) base. Coal-tar-based seal coat is commonly used in the central, southern,and eastern United States, and asphalt-based seal coat is commonly used in the western United States.
PAHs are a group of chemical compounds that form whenever anything with a carbon base is burned, from wood and gasoline to cigarettes and meat. PAHs also are in objects and materials, such as automobile tires and coal tar, the production of which involves the heating of carbon-based materials. PAHs are of environmental concern because several are toxic, carcinogenic, mutagenic,and/or teratogenic (causing birth defects) to aquatic life, and seven are probable human carcinogens(U.S. Environmental Protection Agency, 2009).
Coal tar is a byproduct of the coking of coal for the steel industry and coal-tar pitch is the residue remaining after the distillation of coal tar. Coal-tar pitch is 50 percent or more PAHs by weight and is known to cause cancer in humans (International Agency for Research on Cancer, 1980). Coal-tar-based seal coat products typically are 20 to 35 percent coal-tar pitch. Product analyses indicate that coal-tar-based seal coat products contain about 1,000 times more PAHs than seal coat products with an asphalt base (City of Austin, 2005).
Runoff from seal coated pavement (black surface) enters storm drains that lead to local streams. Drain grate (inset) is marked“DUMP NO WASTE” and “DRAINS TO WATERWAYS.”
Does product type really matter?
PAH concentrations in the coal-tar-based seal coat product are about 1,000 times higher than in the asphalt-based product(more than 50,000 milligrams per kilogram [mg/kg]in coal-tar-based products and 50 mg/kg in asphalt-based products [City of Austin, 2005]). Anecdotal reports, such as Web sites, blogs, and comments by industry representatives, indicate that the coal-tar-based product is used predominantly east of the Continental Divide and the asphalt-based product is used predominantly west of the Continental Divide.During 2007 – 08, the USGS swept dust from seal-coated and unseal coated parking lots in nine cities across the United States and analyzed the dust for PAHs. For six cities in the central and eastern United States, the median PAH concentration in dust from seal coated parking lots was 2,200 mg/kg, about 1,000 times higher than in dust from seal coated parking lots in the western United States, where the median concentration was 2.1 mg/kg. Although both product types are available nationally, these results confirm the regional difference in use patterns (Van Metreandothers, 2009).
PAHs are increasing in urban lakes across the United States.
To better understand why this might be happening, USGS scientists collected sediment cores from 40 lakes in cities from Anchorage,Alaska, to Orlando, Florida, analyzed the cores for PAHs, and determined the contribution of PAHs from many different sources by using a chemical mass-balance model. The model is based on differences in the chemical “fingerprint” of PAHs from each source.Coal-tar-based seal coat accounted for one-half of all PAHs in the lakes, on average, while vehicle-related sources accounted for about one-fourth. Lakes with a large contribution of PAHs from seal coat tended to have high PAH concentrations; in many cases, at levels that can be harmful to aquatic life. Analysis of historical trends in PAH sources to 8 of the 40 lakes indicates that seal coat use is the primary cause of increases in PAH concentrations since the 1960s. Identifying where PAHs are coming from is essential for developing environmental management strategies(Van Metre and Mahler, 2010). Concentrations of PAHs in dust swept from sealed parking lots in central and eastern U.S. cities, where coal-tar-based-seal coat use dominates, were about 1,000 times higher than in western U.S. cities, where asphalt-based-seal coat use dominates.Concentrations shown on the map are the sum of 12 PAHs, in milligrams per kilogram (VanMetre and others, 2009).