A group of Minnesota cities is suing several large chemical refiners over the contamination of hundreds of stormwater ponds caused by a black sealant sprayed on parking lots, driveways and playgrounds.

Attorneys for seven cities in the Twin Cities area allege that pavement sealants now banned in Minnesota contain high levels of a carcinogenic chemical that eventually spread into area soils and waters. In separate lawsuits, the cities seek to recover the projected costs associated with cleaning up the stormwater ponds and disposing of the contaminated waste, which could reach into the millions of dollars.

“This is dangerous stuff,” Daniel Shulman of Gray Plant Mooty, an attorney representing the cities, said of the chemical. “The people and the companies that caused this problem should have known that their products were dangerous, and it should be their responsibility to pay for it — not taxpayers.”

Officials in Burnsville, one of the plaintiff cities, estimate that as many as a third of the city’s approximately 270 stormwater retention ponds have high enough levels of the chemical that the contaminated sediment would have to be hauled to landfills rather than sold or disposed of routinely. For larger ponds, the cost of trucking the contaminated sediment could exceed $150,000. Over time, the disposal costs for contaminated dredged waste could reach into the millions of dollars, said Ryan Peterson, Burnsville’s public works director.

“Our costs can go up dramatically,” Peterson said.

Koppers, Inc., a Pittsburgh-based producer of wood treatment chemicals and carbon compounds that is one of the target companies, said the suit is groundless. A spokeswoman responded to questions Wednesday with the written statement, “Koppers does not believe there is merit to these claims and intends to vigorously defend these matters.”

For decades, road-paving companies and some homeowners have used coal tar-based sealants to extend the life of paved surfaces. These sealants are typically sprayed or painted on parking lots, driveways and even playgrounds to protect the surfaces from the elements. However, scientists have found that these sealants contain high concentrations of compounds known as polycyclic aromatic hydrocarbons (PAHs), which harm fish and pose a risk of cancer in humans.

Exposure to PAHs has been linked by health professionals to increased risks of lung, skin, bladder, and respiratory cancers. And early exposure has been linked to developmental delays in children, studies have found.

The toxic chemicals are released into the environment soon after the sealants are applied to pavement, researchers have found. Friction from vehicle tires causes the sealant to wear away into a fine dust, which can be blown by the wind, tracked indoors or washed down storm drains and into streams and waterways.

The U.S. Geological Survey (USGS) has found that coal tar-based sealants account for about half the hazardous PAHs found in 40 lakes sampled across the country.

Banned since 2014

On Monday, seven Minnesota cities — Burnsville, Eden Prairie, Golden Valley, Maple Grove, Minnetonka and White Bear Lake — filed separate but similar lawsuits in U.S. District Court in Minneapolis alleging that the manufacturers of coal tar-based sealant were aware — or should have been aware — of studies showing links between the sealants and a rise in PAH contamination levels in urban lakes and ponds. They are suing Koppers and six other coal-tar refiners and producers.

The Legislature banned the sale and use of coal tar-based sealants in January 2014, but the carcinogenic chemicals continue to be released into the environment as the sealants break up over time, according to the cities’ lawsuits. In some cases, high concentrations of these chemicals have accumulated in stormwater ponds. A report by the Minnesota Pollution Control Agency found that 67 percent of PAHs found in the sediments of 15 metro-area stormwater ponds were from such sealants.

Under state environmental laws, municipalities are required to maintain stormwater ponds, which capture and filter runoff before it reaches streams and lakes — but which periodically fill up with sediment and need to be regularly dredged to operate properly. In the seven-county Twin Cities metropolitan area there are an estimated 15,000 stormwater ponds. Conservative estimates suggest management costs may be measured in the billions of dollars for just these ponds.

The costs of managing these ponds increases if chemicals from the coal tar sealants accumulate in the sediment. In such cases, cities are required to transport the contaminated sediment to area landfills, which can double or triple the cost of disposal, officials said.

The extra disposal costs also threaten surface water quality. In Burnsville, for example, the increased costs of testing and cleaning out stormwater ponds affected by PAH means the city has less money to conduct regular maintenance, Peterson said. As a result, some ponds are shallower, which makes them less effective at preventing runoff pollutants from reaching area lakes, streams and wetlands, he said.

“If we can’t clean them out as quickly, then phosphorus and other contaminants will continue to bypass the ponds,” he warned. “The result will be decreased surface water quality of the lakes and streams that people love.”