Prior Lake Spring Lake Watershed District
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Alum Treatments

Barge applying alum on Spring Lake 2018Alum treatments have been identified as a way to address high levels of internal phosphorus stored in the lake bottom sediment on Spring and Upper Prior Lakes. Both lakes are listed by the State of Minnesota as impaired lakes due to the high levels of phosphorus.

While phosphorus is an essential nutrient for plants, too much phosphorus can lead to excessive algae growth in lakes. Excessive algae is not only unsightly and undesirable for recreation, it can also reduce lake oxygen levels leading to fish kills and kill beneficial aquatic plants by blocking sunlight. An alum treatment reduces internal phosphorus by applying aluminum sulfate (alum) to the lake; the alum binds to the phosphorus making it unavailable for algae growth.

Alum treatments are only one of many ways the Prior Lake-Spring Lake Watershed District is working to improve and protect water quality in your local lakes and wetlands. Reducing the incoming sources of phosphorus and nutrients to the lakes is also important.

District projects including the ferric chloride facility, 12/17 wetland restoration, the Raymond Park shoreline & habitat restoration project and the Spring Lake parcel shoreline restoration project all work to reduce phosphorus entering the lake and improve habitat. The District also works with farmers to use best management practices to reduce runoff, including cover crops, contour farming, reduced/no-till tillage, nutrient management planning and grassed waterways.

Landowners, including lakeshore owners, are an important part of the solution to cleaner lakes. What you do in your yard is reflected in the health of the lake.

Help improve the water quality of your lake and keep it healthy by: planting a buffer along your shoreline; minimizing fertilizer and pesticide use in your yard; picking up promptly after pets; installing a rain garden or area of native plants; adding a rain barrel; and letting aquatic plants grow off your shoreline. More information and ideas for your home & yard, shoreline & more on our website.

Spring Lake Treatment

The third and final dose of the alum treatment on Spring Lake began May 4 and was completed May 14. The treatment took 11 days to complete and experienced some slight delays due to windy conditions as the contractor cannot apply alum if it is too windy. The project was staged at the Spring Lake boat launch and the launch remained open for public use during treatment.

For project updates visit the alum treatment website.

The first two doses of alum were applied on Spring Lake in 2013 and 2018. The decision to treat Spring Lake with alum was made following a recommendation from a 2012 District study done by Barr Engineering which identified internal loading of phosphorus as a major contributor to poor water quality in the lake. Phosphorus in Spring Lake has decreased by 48% since the first treatment in 2013.

Upper Prior Lake Treatment

The Upper Prior Lake alum treatment is largely funded by grant from the Minnesota Board of Water & Soil Resources (BWSR). Treatment on Upper Prior is currently scheduled to began May 26 and was completed June 3 ahead of schedule thanks to favorable weather conditions. The treatment time depends on weather conditions as the contractor cannot apply the alum if it is too windy. This is the first time Upper Prior Lake has been treated with alum. The project is staged out of the lot just north of Charlie’s.

For project updates visit the alum treatment website.

The 2020 alum treatment on Upper Prior is the first of two doses planned for the lake. The application of the second dose will depend on lake conditions and the response to the first treatment; the earliest second dose would be applied is 2022 – two years after the first dose.

Frequently Asked Questions:

For answers to these and other FAQs, please visit the alum treatment website.

  • What is alum and how does it work?
  • Where does all this phosphorus come from?
  • What are causes and symptoms of poor water quality?

Projects and Programs

  • Rules and Permitting
  • Projects
    • CR 12/17 Wetland Restoration
    • Spring Lake Shoreline Restoration
    • Ferric Chloride Treatment Facility
    • Lower Prior Lake Protection Projects
    • Carp Management
    • Raymond Park Restoration Project
    • Sutton Lake Stormwater Storage Project
    • Fish Lake Shoreline Enhancement & Prairie Restoration Project
    • Alum Treatments
  • Monitoring
  • Municipal Separate Storm Sewer System (MS4)

Prior Lake – Spring Lake Watershed District

The District covers about 42 square miles in Scott County, MN. Water in the PLSLWD flows mainly from the southwest to the northeast through Spring, Upper Prior and Lower Prior Lakes, and then north through the Prior Lake Outlet Channel to the Minnesota River near Valley Fair amusement park.
Clean Water Land & Legacy Amendment

Prior Lake - Spring Lake Watershed District
4646 Dakota Street SE
Prior Lake, MN 55372

Phone: (952) 447-4166
Email: info@plslwd.org

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