These driveway sealants polluted Minnesota ponds. Who should pay?

For years, Minnesotans sealed their driveways and parking lots with products made from refined coal tar to keep them looking shiny, black and free of cracks.

Those products contained harmful chemicals that ended up in stormwater ponds, where they have become a costly problem. Now some metro-area cities want to hold the companies that make those products accountable for the cleanup costs.

Seven cities filed separate lawsuits last month in U.S. District Court against a group of seven companies that refine coal-tar sealants. They're asking for help to pay for cleaning up and removing contaminated material from their stormwater ponds.

"They have this problem, it's not going away, and it's going to cost money," said Robin Greenwald, lead attorney at the New York-based law firm Weitz & Luxenberg, which represents the seven cities. "They either have to have the taxpayers burdened with that cost, or the polluter."

Minnesota banned sealants made from refined coal tar in 2014, amid growing concern over the presence of chemicals known as polycyclic aromatic hydrocarbons, or PAHs, in the environment.

PAHs form when organic materials, such as wood and coal, combust. They even form when you're cooking meat on a grill, said Raymond Hozalski, an environmental engineering professor at the University of Minnesota.

"When it starts to turn black like on the edge of the chicken skin, you're actually making so-called PAHs," Hozalski said. "Of course, it makes the food taste good."

Some PAHs are known carcinogens, substances that can cause cancer in humans. While that charring on grilled chicken probably doesn't have enough of the chemicals to give someone cancer, coal tar sealants contain significantly higher levels of PAHs, and could pose a health risk to humans exposed to them.

Over time, coal tar sealants break down and turn into fine particles that wash away in wind or rain, often ending up in stormwater retention ponds.

Minnesota has roughly 30,000 such ponds designed catch the water that runs off roads, roofs and lawns after it rains.

Stormwater carries pollutants — sediment, phosphorus and nitrogen from leaves and lawn fertilizers, and PAHs — into the retention ponds, which give the pollutants a chance to be used up by plants or settle to the bottom.

The major purpose of the ponds is to stop phosphorus from moving downstream, said Ryan Peterson, public works director in Burnsville, one of the cities that filed suit.

"That's what really wrecks the water quality and makes algae in lakes and streams," Peterson said.

To keep the ponds working effectively, cities dredge their stormwater ponds every 20 years or so and remove all that sediment. They usually take the extra dirt and use it as fill, or spread it on a field, where the nutrients can be beneficial.

But that's not possible if it's loaded with PAHs, Hozalski said. Under state regulations, cities must test their stormwater ponds. Sediment with high PAH levels is considered hazardous waste and must be taken to a landfill, he said.

"Now, all of a sudden, your costs go up by roughly a factor of two to three," Hozalski said.

For a city like Burnsville with hundreds of stormwater ponds, over time that can add up — to millions of extra dollars. The cities are seeking to recover those additional costs of monitoring, testing and disposing of PAH-contaminated sediment.

"We're not trying to say that the ponds filling in with sediment are the fault of the coal tar providers," Peterson said. "It's the fact that we have to manage in a different way because of it."

Along with Burnsville, the cities of Bloomington, Golden Valley, Maple Grove, Eden Prairie, White Bear Lake and Minnetonka also filed lawsuits.

One of the companies named in the lawsuits, Pittsburgh-based Koppers, responded with a statement saying it doesn't believe the cities' claims have merit, and it intends to fight them.

Figuring out just who should be held responsible — and for how much of the contamination — could be a difficult task, since sealants aren't the only source of PAHs in stormwater ponds. They are present throughout the environment and come from different sources, including vehicle exhaust, wood-burning stoves and coal gasification plants.

A 2010 report by the Minnesota Pollution Control Agency found about two-thirds of PAHs in sediment in stormwater ponds studied in the metro area came from coal tar sealants.

Hozalski's recent study sampled sediment from 60 stormwater ponds in the Twin Cities, St. Cloud, Duluth and Rochester and used forensic techniques to track the source of the PAHs, like using fingerprints to track a criminal.

"We found that, actually, within some ponds we had very little, if any, of what we think are signatures of these coal tar sealants," Hozalski said. "But in some, it was as high as 90 percent. So it really can be quite variable from pond to pond and even within a pond."

The lawsuits filed by the Minnesota cities are believed to be the first in the country against coal tar sealant refiners. They could have national implications since the products are still legal to use in most states.

Ken Ashfeld, public works director for the city of Maple Grove, said he hopes it raises awareness of the expensive problem coal tar sealants leave behind.

"Part of this effort is a bit of public awareness, as well as to get other states to ban this material for their own good," he said.

Ashfeld and other city officials caution that PAHs in stormwater ponds don't pose a human health risk. They tend to stick to the sediment at the bottom of a pond, so it's unlikely that they're escaping into the groundwater or rivers and lakes.

But there is a risk to the environment. Because of the higher disposal cost, some cities aren't cleaning out contaminated ponds as often as they were before the PAH regulations.

"If a pond isn't functioning as intended, then some of those things are just going to pass through and get into the environment," Ashfeld said.

That's why the cities are asking for financial help. They say if the ponds aren't maintained regularly and fill up with sediment, they won't work as effectively to capture pollutants before the water goes downstream.