On Monday afternoon (10/21/19), the HAB Aquatic Solutions crew began the second of two planned buffered alum applications at Moody Lake. The first half of the alum was applied in October of 2018 ad second half of the application will be completed in October of 2019.
Recall that Moody Lake is a 34-acre lake near Chisago City, MN with an average depth of 14 feet and a maximum depth of 48 feet. Prior to the alum project, the lake was eutrophic, experienced excessive summer algae blooms, and had poor water quality (high phosphorus concentrations, large amounts of algae, low water clarity). Moody was originally listed as impaired for excess nutrients by the Minnesota Pollution Control Agency and had total phosphorus concentrations well above the standard for Minnesota.
A substantial amount of phosphorus had accumulated in the lakebed sediments over the years. The sediments released phosphorus when oxygen levels decrease 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 Moody Lake confirmed that phosphorus was very high in the sediments and available for release into the overlying water column.
The alum application produces a “floc” that settles to the bottom of the lake. The phosphorus in the sediment binds to the floc 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. The goals of the project are to dramatically reduce the internal loading of phosphorus from the sediments, lower the amount of phosphorus in the water column, reduce the amount of algae and improve the recreational opportunities for lake users.
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