Lily Lake alum treatment scheduled for May 19

Next week, twenty years of research, planning, community engagement, grant-writing, construction, planting and tending will culminate in a nearly overnight transformation for Lily Lake, a small but storied lake in Stillwater. An alum treatment, scheduled for May 19, will be the final step in a two-decade effort to reduce phosphorus levels in the lake and prevent harmful algae blooms.

“A lot of lake management actually ends up being phosphorus management,” says John Holz, a limnologist with Solitude Lake Management, the contractor conducting the alum treatment. “Lily Lake has been accumulating phosphorus from the surrounding watershed for decades, but the alum will draw phosphorus out of the water column and also seal sediment on the lake bottom so that the nutrient can no longer feed algae.” Local residents can expect to see clearer water in the lake, virtually overnight.

Alum (potassium aluminum sulfate) is a nontoxic compound that you may even have in your kitchen cupboard. Used as an additive in pickling and canning, it helps to keep fruits and vegetables crisp and firm. It is the primary ingredient in Maalox  – say goodbye to heartburn! – and is also used for drinking water clarification in cities like Minneapolis that get their water from a lake or river. “For lake treatment we use the exact same formulation as for drinking water,” explains Holz. “It is completely safe and doesn’t kill anything. In fact, it doesn’t even kill the algae; it merely binds the phosphorus so that it is no longer available to support algal growth.”

Using a custom, specialized barge, equipped with GPS, Solitude Lake Management will deliver approximately 18,000 gallons of alum and 9000 gallons of pH buffer to Lily Lake. The alum will be pumped into the water as a liquid formulation that forms a white, cotton-ball looking substance called flock. The flock then sinks to the bottom of the lake and forms a one-inch thick layer on the bottom. The alum flock attracts phosphorus in the water and has numerous binding sites where the phosphorus can attach. Once it has sunk to the lake bottom, it compresses and creates a seal that prevents phosphorus in the sediment from re-circulating. On average, a typical alum treatment is capable of binding 70-90% of the internal phosphorus load in a lake (the phosphorus that is already in the water).

Above – alum treatment in progress on Moody Lake in Chisago County in 2018.

Over the past twenty years, City of Stillwater, the Middle St. Croix Watershed Management Organization, Friends of Lily Lake, and local government partners have implemented dozens of lake-improvement projects, including stabilizing eroding gullies, redesigning the city-owned boat launch with porous pavers and native plants, installing raingardens in surrounding neighborhoods, and retrofitting parking lots and stormwater ponds in commercial areas. Community volunteers have also rallied to adopt storm drains and care for neighborhood raingardens.

“So many little projects over the years have all added up to a big change for Lily Lake,” says Jay Michels, an erosion and sediment control specialist with EOR, Inc., who has worked on several Lily Lake protection projects. “My kids learned to swim in Lily Lake and I actually won a race, swimming across the lake years ago,” he says. “After so much work to restore water quality, this alum treatment is the frosting on the cake.”

Most recently, EOR, Inc. designed and installed an infiltration basin at the south end of Lily Lake Park that collects and treats stormwater from the surrounding area. This “mega raingarden” reduces phosphorus flowing into Lily Lake by 32 pounds per year and was the final project needed to achieve a 145lb phosphorus reduction goal for external loading (phosphorus coming from watershed runoff) that was set in 2006. Both the infiltration basin and the alum treatment are funded, in part, through a $513,500 Clean Water Fund grant from the Minnesota Board of Water and Soil Resources. Additional funding and staff support come from the City of Stillwater and the Middle St. Croix Watershed Management Organization.

The new infiltration basin at Lily Lake Park is essentially a giant raingarden that captures and filters stormwater runoff from the surrounding area.

Contractors will be setting up equipment at the Lily Lake boat launch on Wednesday of next week and will be conducting the alum treatment on Thursday and Friday, May 19-20. The park and the lake will remain open during this time.

Want to help restore Lily Lake? Volunteers are needed to plant the new infiltration basin at Lily Lake Park on Wednesday, June 1, 10am-2pm. RSVP to Brett at