Sunfish Lake, the body of water that gives the affluent bedroom community its name, has a pollution problem more complicated than most. 

A study has found that the lake’s bottom sediments hold phosphorous pollutants from past uses of the land around the lake, possibly including the four farms that once grew crops and animals there.

Because the primary problem for the lake is this internal source of pollution, improving Sunfish Lake water quality “will need to focus on reducing the release of phosphorous from the sediments,” says the early findings of a report by Barr Engineering of Edina.

Barr’s full report — which will include recommended approaches for tackling the pollution in five north Dakota County lakes — will be presented to the Minnesota Pollution Control Agency this month and released to the public after the agency reviews it.

The city of Sunfish Lake is looking both for direction on how to approach the sediment pollution and for help in finding money to pay for it.

“The citizens want to do something. We just don’t know what is the best thing to do yet, so we are kind of treading water,” said Sunfish Lake Mayor Molly Park.

“Sunfish Lake is an asset in our community,” Park said. “The homes around there are beautiful. It’s our namesake. If it’s degraded, that will affect everyone.”

The lake is ringed by 34 homes on adjoining properties. There is no public access to the water. People must have the permission of a property owner to get onto the lake.

Because of its exclusive nature, property owners have taken care of the lake water — typically by dosing it annually with copper sulfate to kill unwanted green algae.

Addressing the phosphorous held in the lake sediment would also control algae, said Laura Jester, of the Lower Mississippi River Water Management Organization, which is participating in the lake studies.

When high winds mix the lake, phosphorous is churned up from the sediment and promotes the growth of algae, which undermines the water quality, Jester said.

An alum treatment could seal the phosphorous at the bottom of the lake, reducing the growth of algae.

Nontoxic alum forms a fluffy material called floc on contact with water. As it settles through the water to the bottom of the lake, it carries suspended particles with it, increasing the clarity of the lake, according to an explanation published by the Minnehaha Creek Watershed District.

On the bottom of the lake, the floc forms a layer that acts as a phosphorus barrier. This reduces the phosphorus concentrations circulating in the water and limits algae growth. The most common reason alum fails is that it is not distributed in enough quantity. It does not last indefinitely.

It could cost as much as $50,000 to treat the lake with alum, much more than property owners have paid for copper sulfate, Park said.

“We as a city do not have a history of putting money into taking care of the lake,” Park said. But it would be prohibitive for property owners to be paying $50,000 every 10 years to treat the water, she said.

The information about Sunfish Lake has been produced in a study of the water quality of five Dakota County lakes: Sunfish, Pickerel Lake in Lilydale, Rogers Lake, Lake Augusta in Mendota Heights and Thompson Lake in West St. Paul.

The study is funded by the Minnesota Pollution Control Agency with money provided by the Clean Water Land and Legacy Act.

The study identified the source of pollution in all five lakes. Rogers is not polluted and would be in line for a protection plan to keep it clean.

The other lakes, besides Sunfish, have pollutants washed in with water from outside the lake. Sunfish is the only one of the five with “internal loading,” as it’s known.

Sunfish Lake homeowners association president Mike MacDonald said he will wait to see the Barr Engineering report before he comments on what should be done.