What is an Alum Application?
Aluminum Sulfate, commonly referred to as Alum, is a material similar to an antacid that permanently binds to phosphorus, a nutrient that commonly drives algae growth. The goal of any application of alum to a lake, including the proposed application to Spring Lake, is to reduce the amount of phosphorus available to algae and reduce the likelihood of algae blooms.
How does Alum work?
Spring Lake, like many relatively deep lakes in Minnesota, stratifies in the winter and summer. During these times, the bottom of the lake and the surface of the lake have different temperatures, and the bottom loses most of its oxygen (becomes anoxic). Without oxygen around, phosphorus in lake bottom sediments (approximately the first 6 to 10 centimeters) dissolves out of the particles it was attached to and into the water, increasing the concentration of phosphorus in the bottom of the lake. When the lake mixes in the spring and the fall, the high-phosphorus bottom water mixes with the whole lake, and becomes available for algae to use near the surface.
When alum is added, a relatively thin layer of fluffy white material called flocculant or floc is created on the bottom of the lake, and partially mixes with the sediments. This floc binds with phosphorus in a way that does not allow the phosphorus to dissolve into the water when the oxygen is removed. That breaks the cycle, so that when the spring and fall mixing occurs, the bottom water has much less phosphorus in it to drive algae growth.
What will Alum do?
Spring Lake has very high phosphorus concentrations. The state goal for lakes like Spring is concentrations no greater than 40 micrograms of phosphorus per liter of water (40 ug/L); Spring frequently exceeds 140 ug/L during the summer. These high phosphorus concentrations allow very high algae concentrations, which cause bad smells, can create fish kills, and in the later summer/early fall of 2012 resulted in large deposits of rotting plant material in the Spring Lake Outlet Channel containing high levels of microcystin, a potentially toxic substance secreted by certain kinds of algae.
The Total Maximum Daily Load (TMDL) study completed in 2011 determined that approximately half of the phosphorus in the lake comes from the land and streams that drain to the lake, and the other half is released from the bottom of the lake. The alum application will address the phosphorus from the bottom of the lake, reducing both the average amount of algae in the summer and the likelihood of bad smells, fish kills, and potentially toxic materials.
You may have noticed that the Spring Lake phosphorus concentration is often over 140 ug/L, and the alum application is intended to address half of that. Since half of 140 is 70, and the goal is to keep concentrations of phosphorus below 40 ug/L, we cannot meet the goals for Spring Lake just by applying alum. It will take a combination of land, stream, and in-lake practices to meet the goal. In the meantime, the alum application should reduce, but not eliminate, the issues that arise from high algae concentrations.
During the summer of 2012, the Prior Lake – Spring Lake Watershed District contracted Barr Engineering to develop a study examining a potential application of alum to Spring Lake. The study is located here:
That study was completed and presented to the Board October of 2012; the recommendation was to move forward with an alum application in 2013 or 2014.
Other watershed districts in the Twin Cities Metro area have considered or applied alum as well, such as the Minnehaha Creek Watershed District.