Parking-lot sealcoat is the black liquid you see sprayed or painted on many parking lots, driveways, and playgrounds. Intended as a barrier to protect paved surfaces from the elements, which can cause cracking during freezing weather, they contain extremely elevated levels of polycyclic aromatic hydrocarbons (PAHs).

A recent joint study shows they can affect the quality of downstream water resources.

Abrasion by tires causes the coating to break into small particles that wash into urban streams in rain and runoff.

A recent joint study in Texas by the USGS National Water-Quality Assessment Program and the City of Austin found 65 times higher PAH concentrations in runoff from coal-tar based sealcoat treated parking lots compared to non-coated parking lots.

Asphalt-based sealcoat treated parking lots contribute 10 times the PAH laden particles compared to non-coated parking lots.

The large differences observed suggest that abraded sealcoat is a major and heretofore unrecognized source of PAHs in urban and suburban water bodies, many of which have been showing increasing PAH concentrations over the past 30-35 years.

PAHs are an environmental concern because they are toxic to aquatic life and several are suspected human carcinogens. Aquatic life is affected in several ways through exposure of PAHs bound to sediments.

The USGS study did not evaluate human health impacts but believes the risk to human health from PAH in drinking water is small because of their tendency to attach to sediment rather than dissolve in water. Nor do they tend to bioaccumulate in fish consumed by humans.

Besides water impacts, several studies are showing PAHs have an effect on air quality and human health. Homes using the sealant were found to be 14 times more likely to have carcinogenic doses of PAH in housedust.

For further details, visit:

-- USGS website, Parking Lot Sealcoat: A Major Source of Polycyclic Aromatic Hydrocarbons (PAHs) in Urban and Suburban Environments;

-- Coal-Tar-Based Pavement Sealcoat, Polycyclic Aromatic Hydrocarbons (PAHs), and Environmental Health; and

-- Effects of PAHs on Human Health.

These findings and other scientific studies are available at the USGS National Water Quality Assessment webpage.

(Written By: Diane Oleson, Natural Resources Educator, Renewable Natural Resources Team, Penn State Extension, York County, and reprinted from Penn State Extension Watershed Winds newsletter.)

7/29/2013