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Water quality treatment set for Bass, Pomerleau lakes

Post Date:05/13/2019

The City of Plymouth has partnered with the Shingle Creek Watershed Management Commission to apply an alum treatment to two lakes in Plymouth this week to improve water quality.

Treatment of Pomerleau Lake is set for Monday, May 13. Treatment of Bass Lake is set for Wednesday, May 15.

The project will improve water quality by reducing concentrations of nutrients, particularly phosphorus. It will reduce the amount of algae, increase water clarity and improve fishing and recreational opportunities for lake users.

Residents Invited to Demonstration May 15
Residents who live near the lakes are invited to attend a demonstration of the alum application project 8 a.m. Wednesday, May 15, at the Bass Lake boat launch, 11560 54th Ave. N. in Plymouth.

Impaired Waters Listing
The Minnesota Pollution Control Agency listed Schmidt, Bass, and Pomerleau lakes as impaired waters for excessive nutrients in 2002. Schmidt Lake was delisted in 2016, thanks to the combined efforts of the city, lakeshore property owners and the Shingle Creek Watershed Management Commission to reduce the levels of pollutants entering the lake. However, Bass and Pomerleau lakes remain impaired.

According to the PCA, about 40% of Minnesota’s lakes and streams are impaired (do not meet water quality standards). Lakes and streams may be impaired due to high levels of phosphorus, bacteria, chlorides (salt), fertilizers and sediments, as well as leaves and grass, which add excess nutrients.

In the case of Pomerleau and Bass lakes, a substantial amount of phosphorus has accumulated in the lakebed sediments through the years. The sediments release the phosphorus when oxygen levels decrease at the lake bottom – which ultimately increases the amount of phosphorus available and may lead to excessive algae growth.

During the treatment process, alum (aluminum sulphate) added to the lakes binds to the phosphorus and settles on the bottom of the lake, preventing the release of additional phosphorus from the sediment in the lakebed.

Alum is a common food additive shown to be safe for humans, according to the Food and Drug Administration. Aluminum, a main ingredient of alum, is the third most abundant element in the Earth’s crust and naturally occurs in lake sediments.

Alum application on Pomerleau and Bass lakes includes a half dose – the second half will be applied in 2020.

For more information about the project, visit or email the Plymouth Public Works Department at

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