Spring Lake is a 600-acre lake located in central Minnesota near the town of Prior Lake. Spring Lake is a highly valued recreational resource that supports swimming, boating and fishing activities. However, the high nutrients have led to excessive amounts of algae that interfere with the use of the lake. In 2011 the Prior Lake-Spring Lake Watershed District completed a Total Maximum Daily Load (TMDL) study and determined Spring Lake was impaired/threatened by an excess of phosphorus and recommended that the phosphorus concentration should be reduced by about 85%.
It is the concentration of the nutrient phosphorus which controls the level of algae growth. The objective of most successful lake management projects is to lower the lake water column phosphorus concentrations, which in turn lowers the amount of algae. Comprehensive studies documented that a substantial amount of phosphorus has accumulated in the lakebed sediments over the years. The sediments release the phosphorus when conditions are right at the lake bottom. This leaching of phosphorus from the lakebed is called internal loading and ultimately increases the amount of phosphorus available for algal uptake and growth. Samples from the bottom of the lake confirmed that phosphorus was very high in the sediments and available to be released into the overlying water column. The studies also determined that approximately half of the phosphorus in Spring Lake comes from the surrounding land and streams that drain to the lake, and the other half is released internally from the bottom sediments of Spring Lake.
In order to address the internal phosphorus inputs, the PLSLWD contracted with HAB Aquatic Solutions to conduct an alum application over an eleven-day period in October 2013. Aluminum Sulfate, or alum, is a nontoxic material that is commonly used in water treatment plants to clarify drinking water. In lakes, alum is used to lower the concentration of phosphorus in the water. The application produces a “floc” that settles to the bottom of the lake. The floc has sites where phosphorus in the sediments become chemically bound as it leaches from the bottom. The floc effectively intercepts and binds the phosphorus, which makes it unavailable for the algae to use for growth.
In 2013, HAB applied half of the total amount of alum needed to reach the internal loading reduction goal. The application greatly improved the lake’s water quality by reducing phosphorus, reducing algae and increasing water clarity. HAB will be back at Spring Lake in May 2018 to apply the next 25% of the total required alum amount to increase the longevity of the water quality improvements. The goals of the project are to significantly reduce the internal loading of phosphorus from the sediments, lower the amount of phosphorus in the water column and reduce the amount of algae and improve the recreational opportunities for lake users.
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