Short-term Changes in Sediment and Water Chemistry After a Partial Aluminum Sulfate Treatment to Long Lake, Wisconsin

Abstract

Long Lake, a shallow lake (5.2 m) located in Polk County, Wisconsin historically exhibited poor water quality with seasonal cyanobacterial blooms. Persistent blooms have been linked to high concentrations of phosphorus (P) in the sediment and water column. For Long Lake, internal loading from sediment was the primary contributor of P. A 105 g/m2 dose of aluminum sulfate (alum) was split into lower dosages to treat internal loading over the next two to three years. The first dose of 60 g/m2 was applied on June 11, 2018. Limnological monitoring and sediment chemistry were conducted prior to the treatment and continued post treatment. The first alum treatment was successful in reducing P from Long Lake. There was an 83% decrease in mobile P from the top 0.5 cm sediment layer from pre- and post-alum treatment (0.612 mg/g pre-treatment versus 0.102 mg/g post-treatment), and a 338% increase in aluminum bound P (0.123 mg/g pre-treatment versus 0.539 mg/g post-treatment). Compared to the previous year, there was a 76% decrease in mean seasonal bottom total P (0.315 mg/L in 2017 versus 0.075 mg/L in 2018), and a 99% decrease in mean seasonal bottom soluble reactive P (0.223 mg/L in 2017 versus 0.002 mg/L in 2018). Additionally, chlorophyll concentrations (37.03 μg/L in 2017 versus 16.31 μg/L in 2018) decreased which aided a 0.52 m (1.33m in 2017 vs 1.85 m in 2018) increase in Secchi transparency. The first alum treatment to Long Lake has, thus far, been successful in reducing P and improving water quality.