By Morgan Linn | February 8, 2017
Coal tar sealants are commonly applied to asphalt driveways and parking lots. Image: MPCA Photos, Flickr
By Morgan Linn
Milwaukee banned coal-tar sealants Tuesday after a study blamed them for contaminating streams.
The Milwaukee Public Works Committee recommended a city ordinance to the general council that would ban the use of coal-tar sealants. The council approved the ban unanimously.
The ban was proposed in the wake of a recent study that found that as many as 78 percent of Milwaukee streams have toxic levels of polycyclic aromatic hydrocarbons, or PAHs.
PAHs are organic contaminants that in high amounts can harm aquatic animals, the study says. They can cause cancer, genetic mutations and reproductive issues.
There are more than 200 different PAHs, and they come from a wide variety of sources, said Austin Baldwin, a research hydrologist with the U.S. Geological Survey and co-author of the study.
“They’re naturally occurring in things like oil and tar,” he said. “They’re also produced anytime a carbon based material is burned.”
This includes the burning of coal, gasoline and wood, he said. Even grilling or cooking meat produces PAHs.
To track down the main source of the contaminants, researchers used a variety of forensic tests and source identification methods.
One test used ratios to compare PAHs in stream samples to the PAHs in potential sources.
Stream samples have a specific ratio of different PAHs, said Steve Corsi, a research hydrologist with the U.S. Geological Survey and co-author of the study.
“Some sources might have high concentrations of compound A, but low concentrations of B and C, for example,” Baldwin said. “You can look at the ratios of the different PAH compounds to get an idea of the source.”
Researchers also looked at how land was being used around the streams to find a correlation between types of land use and PAH levels.
“We looked at different characteristics of the watersheds that drain to the areas where we sampled,” Corsi said. They looked at residential areas, roads, commercial areas, industrial areas and parking lots.
“All the methods that we used pointed to the same thing, and that was coal-tar pavement sealant,” Baldwin said.
Coal-tar sealant contributes up to 77 percent of the PAHs found in streambeds, the study says. It’s commonly used on asphalt parking lots in the Great Lakes region.
“Pavement sealers are spread on driveways, parking lots, and over the years they just wear off eventually from abrasion,” Corsi said. “They lose their adhesion to the pavement and end up flaking off.”
Then the wind, water or tires carry the chipped sealant elsewhere, which is how the PAHs end up in nearby streams, he said.
The results of the study led Alderman Jim Bohl to sponsor the city-wide ban on coal-tar sealants. Bohl is a city council member on the Public Works Committee and a commissioner for the Milwaukee Metropolitan Sewerage District, which worked with the U.S. Geological Survey to support the study.
“It’s time for momentum to carry here in Wisconsin as it has in a number of other communities,” Bohl said to the council just before the ban passed.
There is the potential to take the ban further so that it applies to Milwaukee County, not just the city, he said. The city of Milwaukee constitutes around 60 percent of the county’s population.
The county government is weighing the idea of creating a ban, which may be proposed in March, he said. A county ban means multiple layers of government would be able to ensure enforcement.
A county ban on top of a city ban may seem duplicative, but it’s not a problem, Bohl said.
“I sort of see it as you lying down on the bed and pulling a blanket over a sheet,” he said. “If we have a sheet that covers you, and they want to pull a blanket over you, so be it, it’ll keep you warm.
Bohl was warned by city council members in other areas that there would be backlash against a city ban, but there has not been any so far.
“[The industry] has made some efforts in other communities to try to fight any bans or restrictions on coal-tar,” he said. “Various councils have sometimes backtracked or not enacted bans because of the legislative pushback.”
There are alternative products that can be used, Bohl said.
“A number of communities don’t use coal-tar sealants and they still have options for sealing their driveways,” Corsi said. “The one that’s most common is an asphalt-based sealant.”
PAH amounts are 1,000 times lower in asphalt-based products than in coal-tar based products, Bohl said.
Researchers hope to continue looking at the issue of coal-tar sealants and their connection to PAH runoff, Corsi said.
“PAH contamination in the Great Lakes region is an important issue to consider,” he said. “We’re definitely thinking about future studies within the Great Lakes region to better understand how widespread this problem is.”