Coal-Tar-Based Sealants on Pavements are Primary Source of PAHs in Great Lakes Tributaries

According to a recent study published in Environmental Toxicology and Chemistry, runoff from pavement with coal-tar-based sealant is the most likely primary source of polycyclic aromatic hydrocarbons, or PAHs, found in the majority of streambed sediments of Great Lakes tributaries. PAHs are a group of chemicals found in crude oil and coal and occur as a byproduct of burning and can have harmful effects to organisms in the environment under certain conditions. So, it is important to understand their sources, distribution, and magnitude in the Great Lakes Basin.

Scientists with the US Geological Survey, the US Environmental Protection Agency, and the University of Helsinki collected sediment samples from 71 streambed sites throughout the US portion of the Great Lakes Basin. The sites were selected to represent watersheds with a range of land uses, from 0.7 to 100% urban. The authors employed multiple lines of evidence to identify the most likely source of PAHs to sediment. They found that, based upon relative concentrations of the particular PAHs found in the sediments, dust from coal-tar-sealant was most likely the dominant PAH source for 57 of the sites, with the remainder of PAHs coming from sources such as vehicle emissions.

Pavement sealant is a black, shiny liquid sprayed or painted on asphalt parking lots, driveways, and playgrounds to improve the appearance and protect the underlying asphalt. Coal-tar sealants have significantly higher levels of PAHs and related compounds compared with alternatives such as asphalt-based sealants and contribute more PAHs to the watershed than other urban sources, including vehicle emissions, used motor oil, and particles abraded from tires of moving vehicles. PAHs from coal-tar sealants are transported to streams through stormwater runoff.

PAH exposure has been linked to an increased risk of lung, skin, bladder, and respiratory cancers, especially when the exposure occurs in early childhood. Exposure to runoff from pavement with coal-tar-based sealant (even 3 months after application) is acutely toxic to flathead minnows and water fleas, both of which are commonly used to assess toxicity to aquatic life more generally. Even highly diluted runoff can cause DNA damage and impair DNA repair. 

Concentrations of PAHs in 62% of the samples exceeded screening criteria for aquatic life. The research indicates the need for further analysis to determine the potential effects of PAHs on aquatic life in that area in order to improve habitat for aquatic organisms and underscores the need to understand the source of the PAHs.

An earlier study, published in 2016 in the same publication, found coal-tar-based sealant was the major source of PAHs in Milwaukee streambed sediments. US Geological Survey Hydrologists Austin Baldwin and Steven Corsi were co-authors on both the 2016 study and the study published today, with others co-authoring each study.