Bass and Pomerleau Lakes Alum Treatment

Bass and Pomerleau Lakes in Plymouth couldn't be more different. Bass is a large, heavily used shallow-deep recreational lake surrounded by homes with an active lake association. Pomerleau Lake is a quiet, deep lake mostly surrounded by wetlands and used by only a few knowledgeable anglers. But both lakes are Impaired Waters for excess nutrient concentrations, and both share the same source: phosphorus that is released from the sediments when water on the bottom of the lake becomes low in oxygen. The most common method of reducing this "internal sediment release" is to apply aluminum sulfate - alum - to the lake. The liquid alum slowly filters down to the lake bottom. As it falls, it binds with phosphorus in the water column to form a floc. That floc settles on the bottom and forms a protective barrier between the sediments and the lake, inhibiting the further release of phosphorus. Each of these lakes have received two alum treatments. The first was applied in May 2019 and the second in fall of 2020. Monitoring shows an immediate improvement in water clarity, and a significant reduction in phosphorus release from the sediments. The project also includes treatment of invasive curly-leaf pondweed.

This project was funded by a $267,040 grant from the Clean Water Land and Legacy Amendment administered by the Minnesota Board of Water and Soil Resources, and by the Shingle Creek Watershed Commission. Special thanks to the Bass Lake Improvement Association for volunteer assistance.

Liquid alum is dispensed from tanks on a barge that slowly travel a programmed path across the lake. The alum very slowly sinks to the bottom of the lake, and as it falls it binds phosphorus in the water column. When on the lakebed it acts as a barrier to further release of alum from the sediments.

This graph shows the concentration of total phosphorus (TP) at the lake bottom. Prior to the first alum treatment in 2019, by mid-summer, the TP release from the lake sediments became very high, fueling algae blooms and reducing water clarity. In 2019 and 2020, the concentration at the lake bottom was a fraction of what it had been in previous years.