Multi-year cleanup of West St. Paul lake now complete


A forebay was created to collect contaminated sediment before it goes into Thompson Lake. The forebay was part of a roughly $2.6 million restoration project aiming to improve water quality at the lake in Thompson County Park. (courtesy of Lower Mississippi River Watershed Management Organization)

Work included installing large underground chambers, which remove sediment, debris and trash from stormwater before it reaches the lake. The chambers will periodically be cleaned out.

There was so much sediment deposited in the Thompson Lake that there are a noticeable differences between aerial photos from 1997 and 2017. (photos courtesy of Lower Mississippi River Watershed Management Organization )

A June 9 celebration marks the completion of a multi-year project to clean up Thompson Lake in West St. Paul. 

The $2.6 million worth of work done at Thompson County Park aimed to improve the water quality of the lake and surrounding wetlands after decades of sediment buildup.

Joe Barten, a resource conservationist with the Dakota County Soil and Water Conservation District, and also an administrator with the Lower Mississippi River Watershed Management Organization, said the lake had been on the watershed’s radar for over a decade because of its numerous issues.

 

The problems

Barten said when looking at the lake from a watershed basis, the land around it was its main problem.

“When it rains, [storm water is] coming off this whole landscape and essentially being washed into Thompson Lake,” he said. 

The lake is in a suburban watershed, between St. Croix Lutheran Academy and Highway 52, with lots of hard surfaces surrounding it that don’t soak in storm water.

Pollutants from such runoff, such as polycyclic aromatic hydrocarbons, which come from coal-tar based sealants that are now banned, Barten said, were building up in the lake sediment.

“All these driveways and sidewalks for the last 50 years, they were using something called coal-tar driveway sealant ... That eventually degrades and wears off, and it washes into Thompson Lake,” Barten said. 

The lake was also experiencing high levels of phosphorus, also due to runoff.

“There’s just too much phosphorus going into the lake and just causing the lake to be green, full of algae throughout the summer,” Barten said, adding the lake was neither a good habitat nor good for recreation.

More recently, chloride — which is found in road salt — has also been a problem in the lake. Barten said the Thompson Lake project won’t necessarily fix that, because it’s more dependent on controlling the amount of salt placed on the roads.

In the fall of 2018, work began on the restoration and cleanup of the lake, which involved the removal of thousands of cubic yards of contaiminated sediment that contained polycyclic aromatic hydrocarbons, a potential carcinogen. 

 

Improving water quality

Barten said it took 10 years for the watershed, city and county to come together — for “all the stars to align” — to make the project possible. Another important piece was a $576,000 Clean Water Fund grant the watershed district received to carry out the work.

A large portion of the grant went toward sidement removal. Other restoration work included creation of a new forebay, a smaller pool of water in front of the main basin, wetland construction, a new trail and construction of underground chambers, which are designed to capture pollutants from stormwater before they hit the lake.

“Basically, it stops the pollutants and sand and grit and dirt from getting in [the forebay],” Barten said, pointing out it’s cheaper to remove sediment from the chambers than excavating it from the forebay or lake.  

The work’s goal, Barten said, is to catch as much sediment as possible in the underground chambers, while whatever isn’t caught there will be caught in the forebay. Whatever makes it past the forebay will go into the new wetland, which is also meant to slow down water as it heads into the lake. 

“The more we can catch, the more we’re going to improve the water quality of the lake and the clarity of the lake, and the recreation capacity of the lake,” Barten said. 

He noted the lack of phosphorus and other excess nutrients will be noticable to the general public as clearer water, and that in the future the lake will heal, offering better fishing and more biodiversity.

 

Not just the water

Besides water quality improvement, work was also done on the surrounding habitat and recreation areas. 

Barten said a goal was to alter the park as little as possible, though the horseshoe pit was lost. A boardwalk was installed as part of trail improvements, he said, and some aesthetic updates were made in the park, with a focus on using native plants.

“We want this to be as natural as possible, long term,” he said.

 

–Hannah Burlingame can be reached at 651-748-7824 or hburlingame@lillienews.com.

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