Effects of alum treatment on water quality and sediment in the Minneapolis Chain of Lakes, Minnesota, USA

The effects of aluminum sulfate (alum) treatment on water quality in 4 lakes of theMinneapolis Chain of Lakes (MN, USA) were examined. Lakes Harriet and Calhoun (treated in 2001) and Cedar Lake and Lake of the Isles (treated in 1996) all showed initial water quality improvement based on surface water total phosphorus (TP), chlorophyll a (Chl-A), and Secchi disk depth. Three lakes (Harriet, Calhoun, and Cedar) were at or below historical estimates of growing season average TP after treatment and showed continued improvement in surface water quality through 2005. Lake of the Isles, which received the lowest alum dose, returned to pretreatment conditions after 6 years. Estimates of sediment phosphorus (P) release rates, however, indicated that alum treatment still limited internal P release in all 4 lakes. Although the alum application to Lake Harriet was a littoral-only treatment, water quality improved in this lake as well. The aluminum hydroxide floc drifted to the deeper part of the lake, reducing internal P release from deeper sediments by 85% in the 2 years following treatment, leading to unexpected improvements in surface water Chl-A and TP concentrations.