Isanti Soil & Water Conservation District

Blue Lake Prepares for an Alum Treatment!

The Isanti SWCD has secured a Clean Water Fund Grant to perform an alum treatment on Blue Lake this year. The treatment would not be possible without the Blue Lake Improvement District, who are providing a necessary 25% grant match and have been working with the Isanti SWCD to make this a reality. The Isanti SWCD has been working to protect the health of Blue Lake by completing conservation projects in the area surrounding the lake. Now, an alum treatment has been identified as the next most logical and cost-effective way to protect Blue Lake.

What is an alum treatment? Alum is short for aluminum sulfate, a common mineral found on earth that has historically been used for drinking water treatment and lake management. It also is sold as a spice in most grocery stores! Alum is a safe and widely studied option for lake treatments to reduce the amount of phosphorus present in the water- a nutrient responsible for algae blooms. Therefore, alum treatments reduce the amount of algae in a lake and improve water clarity. It works by forming a fluffy aluminum hydroxide solid called a floc when applied to water. As the floc settles, it removes phosphorus and particulates (including algae) from the water column. The floc settles on the sediment where it forms a layer that acts as blanket to phosphorus. The phosphorus released from the sediments combines with the alum and is not released into the water.

Why is the alum treatment necessary on Blue Lake? There is an excess of phosphorus in the lake, which fuels algae blooms. A study conducted on Blue Lake in 2019 found that internal loading (the recycling of phosphorus from the lake sediments) significantly contributes to the total phosphorus. Therefore, work must be done to reduce phosphorus from internal loading. The Isanti SWCD has already completed a significant amount of work to reduce phosphorus entering the lake from the upland areas. This has been accomplished through projects such as shoreline restorations, rain gardens, and settling basins. However, the work in upland areas will not be enough to protect Blue Lake. A Blue Lake alum treatment feasibility study conducted in 2020 confirmed an alum treatment could effectively address the excess phosphorus in the lake. The alum treatment will achieve 164% of the phosphorus reduction goal the Isanti SWCD set to keep Blue Lake Healthy!

The alum treatment will take place on Blue Lake in 2022 and 2024. The treatment will have minimal impact to those that use the lake, and is safe for humans, pets, plants, and wildlife. Regular updates are available on as well answers to frequently asked questions.