July 9, 2005
Parking Lots Paving the Way for Pollution
Widely used sealants add carcinogens to local streams, study finds
America's love affair with the road may be polluting its fields and streams: Researchers say runoff from sealant-covered parking lots and driveways can flood nearby water systems with known carcinogens.
Now, a joint study by the U.S. Geological Survey and the city of Austin, Texas, has found that soil tests near parking lots covered with coal-tar emulsion sealcoat contain PAH levels that are 65 times higher than soil near unsealed asphalt or cement lots.
"What was so striking was the difference in the concentrations of the PAH levels in the particles of unsealed parking lots compared to the particles washing off the asphalt-coated and coal-tar based lots," said study author Barbara J. Mahler, a research hydrologist with the U.S. Geological Survey in Austin, Texas.
Her team's tests also showed that PAH levels in runoff from lots sealed with an asphalt-based emulsion -- the other sealant on the market -- was still ten times higher that levels near unsealed asphalt and cement parking lots.
"We are talking here about water quality in urban streams," Mahler pointed out. "This is not isolated to industrial areas, but everywhere we drive -- where we shop, work, go to school and live -- wherever there are parking lots," she added.
Mahler said there's no hard evidence yet on how many of the nation's thousands of parking lots are protected by sealants, which are applied to extend the life of the lot and give it that shiny look. But the Texas researcher said that, anecdotally, asphalt-based sealants seem to be more prevalent in the western states, while coal-based sealants are used more in Midwestern and eastern states.
The study appears in the current online edition of Environmental Science and Technology.
Calls to Peter Sebaaly, director of the industry- and University of Nevada-funded Pavement Coating Technology Center (PCTC) were not returned. But in a letter to the researchers last winter, Kevin Hardin, chairman of PCTC's executive committee, said he disputed the findings, writing that "coal-tar sealers are being inappropriately singled out as the main source of PAHs, which is contrary to the scientific consensus that the activities of urbanization (vehicle exhaust, rubber tires, asphalt roads, etc.) are the primary source of PAHs, not any single material."
In their study, researchers from Austin's Watershed Protection and Development Review Department and the U.S. Geological Survey gathered run-off samples near 13 urban parking lots that were sealed with either coal-tar or asphalt-based sealants. Also included in the study were four test plots at a local airport that has been closed since 1999. Three of these lots were sealed just prior to testing, and the fourth was left unsealed.
Distilled water was sprinkled on the lots to simulate a light rain, and testings were done of the subsequent runoff.
"This is a solid scientific study, very thorough, and the public is well-served that the U.S. Geological Survey is taking it upon itself to bring attention to this issue," said Morton Lippmann, a professor at New York University School of Medicine's department of environmental medicine.
Lippmann noted that coal tar is a known carcinogenic in humans, and that workers spraying coal tar onto hot pavement in the Texas heat could be exposing themselves to inhalation exposure, as well.
"Roofers don't use coal tar because there's a lot of inhalation exposure," he pointed out.
Nancy McClintock, assistant director of the Watershed Protection and Development Review Department for the City of Austin, who worked with the U.S. Geological Survey on the study, said the findings prompted the city to ask local hardware stores to voluntarily cut back on selling coal tar products.
"We're starting to see a change in what they're selling," she said.
The findings could help people decide to take control of the issue of PAH contamination themselves, Mahler hopes.
"Most of the other sources of PAH in urban environments are diffuse and hard to control, like car exhaust," Mahler said, "but the use of sealants is voluntary."
Head to the The City of Austin's ( www.ci.austin.tx.us ) Web site for a full report on the PAH study findings.