Decades ago, Thompson Lake was a destination for swimmers, a beach in the developing West St. Paul area.

Today, it's a scenic attraction, a fishing spot and even a place where some harvest snails -- in an urban Dakota County park.

The future of the lake, however, is uncertain.

After decades of serving as a drainage pond as the city evolved around it, Thompson Lake, like many urban lakes and ponds, has a pollution problem.

The sediments that have washed into the lake over the years, settling on the bottom and forming a delta at the lake's northern end, contain a variety of contaminants, including road salt, petroleum products and driveway sealant.

Cleanup, based on a plan that may change as more testing is done, would cost $1.6 million.

"You're seeing long-term impacts of decisions that were made a long time ago about how we deal with storm water," Al Singer, the county's land conservation manager, told Dakota County board members at a meeting last month.

The county, city, a host of other government agencies and even a nearby school are working together to figure out just how harmful the contaminants are to people, to wildlife and to plants.

For Dakota County, the lake's health is essential to plans for improvements to Thompson County Park.

"The quality of the recreation is only as good as the natural resources," said Steve Sullivan, director of the county's parks and open space department.

West St. Paul Mayor John Zanmiller said that he thinks the city has a responsibility to help clean up the lake. "I can't sit by and let that lake continue as it is now," he said.

An older family member recently showed him a photograph of what the lake used to look like.

"To see how it has changed and to tell me about how people swam in the lake, it's definitely a lake that through development we altered," Zanmiller said.

The watershed area -- mostly grassland, marshland and wetland up through 1945 -- has been built up and paved over.

The area that drains into the lake includes Signal Hills Shopping Center, dozens of homes and St. Croix Lutheran School.

The storm water runoff that reaches the lake carries a mix of contaminants that are building up on the bottom and in a sediment delta on the north end, including fertilizer, road salt, yard waste, petroleum products and driveway sealant. Those sealants, particularly those that are coal-tar-based, raise questions.

"One of the things we don't completely understand are what are the human health effects from the contaminants," said David Swenson, water resources director for Dakota County. "This is a problem that all metro lakes that were used for storm water have."

According to the Minnesota Pollution Control Agency, coal-tar based sealants, compared to asphalt-based sealants, contain higher levels of chemicals that "harm fish, and with prolonged exposure, pose a risk of cancer in humans."

The county no longer uses coal-tar sealants on roads, and state agencies are now prohibited from purchasing coal-tar based sealants.

In West St. Paul, Zanmiller said he supports a city ban of coal-tar based driveway sealants, but the issue probably won't be discussed until later this year.

For now, the county and other agencies will keep testing the lake to try to understand what should be done.

St. Croix Lutheran School, which has a campus on the west bank of the lake, is getting in on the effort.

High school students will be gathering samples from the lake and analyzing the results in biology, math and ecology and geology classes.

With more knowledge, Swenson said, the county will look for money to pay for cleanup, possibly grants.

Other funding could also be available if the park is joined up with Kaposia Park and Kaposia Landing to become a regional park. A study of that possibility is planned.

"The important thing is that we are identifying a way to prevent this from becoming more and more of an issue," Commissioner Kathleen Gaylord said. "It's a problem that we're seeing area-wide."

Katie Humphrey • 952-882-9056 Staff writer Nicole Norfleet contributed to this report.