Biggest challenge to Willmar Chain of Lakes is the legacy phosphorus hidden under murky waters

May 12—WILLMAR — Water quality challenges for the Willmar Chain of Lakes are many.

Sediment with phosphorus and nitrogen wash into Skataas and Swan Lakes from 11,000 acres of upstream lands, most heavily farmed, and continue on to Willmar and Foot Lakes.

During rain events and snowmelt, pulses of chemicals and nutrients washed from the lawns and streets of much of Willmar pour directly from 27 stormwater inlets into Foot Lake and another 30 inlets in Willmar Lake.

Yet the biggest challenge is hidden under the murky waters of these lakes in the mushy sediment of their bottoms. Phosphorus is distributed in the sediment like sugar dissolved in soda pop.

It's referred to as "legacy" phosphorus because it's been accumulating in the sediment over the course of decades, according to Margaret Johnson, manager of the Kandiyohi County Soil and Water Conservation District. The phosphorus gets stirred up into the water column by carp as they root up the bottom and eat and excrete what they find there.

This phosphorus helps feed the algal blooms that fill the waters and keep swimmers and many others from enjoying the recreational opportunities these lakes would otherwise provide.

What to do about it?

The search for that answer brought Mia Bauer and Katie Kemmitt, environmental scientists with the engineering firm of Stantec, of Plymouth, to the chain of lakes on May 4. Accompanied by Ryan Peterson with the Kandiyohi County SWCD, they used their global positioning system to boat and anchor over predetermined locations in the lakes. At each, they dropped a simple device that punched a tube into the bottom sediment, which they hoisted back up and sealed.

The dozens of sediment-filled tubes they collected have since been shipped to a laboratory at the University of Wisconsin-Stout, where they are being analyzed for their phosphorus content.

Phosphorus tends to settle in the deeper portions of the lake, but it is not necessarily evenly distributed, Katie Kemmitt explained. The sediment-gathering project will determine how much legacy phosphorus fills the sediment and how it is distributed.

Johnson said that data will help inform decisions on how to improve water quality. One possibility being considered is the use of alum, a chemical which binds with the phosphorus and keeps it from feeding algae. The sediment core data would help in effectively targeting the alum to the phosphorus.

Another possibility, but far more challenging to accomplish, would be to conduct a drawdown on the lakes to allow winter kill to knock out the carp and let vegetation re-establish itself and help reset the ecological system overall.

Efforts have already been made to manage the carp population. An upstream control gate was installed a few years ago. In 2018, a commercial fishing crew removed thousands of pounds of carp.

There's a lot to learn yet before any decisions are made, but Johnson said it is known that legacy phosphorus is a very big player in the water quality challenges faced in these waters. Years of water quality testing indicates that 69 percent of the phosphorus in Swan Lake is coming from "internal loading" — introduced from the sediment. In Willmar Lake, 44 percent is from internal loading, she pointed out.

The big picture goal is to improve water quality in the Hawk Creek watershed, Johnson explained. Along with Eagle Lake, the Willmar Chain of Lakes represents the headwaters of the watershed.

She also pointed to the importance of these waters for recreational activities in the area. The lakes are popular fishing destinations in both summer and winter.

Robbins Island Regional Park hosts thousands of visitors and there is no overstating the importance of the waters of Foot and Willmar Lakes to the park. There are 11 to 12 miles of shoreline along Food Lake and the two bays of Willmar Lake within the city of Willmar. That compares to 12 miles of shoreline along Green Lake.