APWA Reporter, February 2013
Barbara J. Mahler, PhD, PG, Research Hydrologist, and Peter C.Van Metre, PhD, Research Hydrologist, US Geological Survey Texas Water Science Center, Austin, Texas
Studies by the U.S. Geological Survey (USGS) have demonstrated that coal-tar-based seal coat — a product marketed to protect and beautify the asphalt pavement of driveways and parking lots — contributes polycyclic
aromatic hydrocarbons (PARs) to air, soils, streams and lakes, and homes.
What are seal coat, coal tar, and PAHs?
Pavement sealcoat (also called sealant or driveway sealer) is a black liquid sprayed onto the asphalt pavement of residential driveways, parking lots, and even some playgrounds. Most sealcoat products have a coal-tar-pitch or asphalt (oil) base. SeaI coat used in the central, southern, and eastern U.S. commonly contains coal-tar pitch, and seal coat used in the westem U.S. commonly contains asphalt. Coal-tar pitch, a known human carcinogen, is the residue remaining after distillation of coal tar, a byproduct of the coking of coal. Coal-tar-based seal coat typically is 20 to 35 percent coal-tar pitch and contains from 50,000 to 100,000 milligrams per kilogram (or parts per million) PAHs, about 1,000 times more PARs than in asphalt-based seal coat products. There are hundreds of times more PARs in coal-tar-based seal coat than in tire particles, used motor oil, or other urban sources. Several PARs are toxic, carctnogenic, mutagenic, and / or teratogenic (causing birth defects). At least seven PARs, including benzo[a]pyrene, are probable human carcinogens.
