Analysis of Sediment Characteristics and Total Phosphorus Models for Shagawa Lake

Shagawa Lake, Minn., received direct discharges of municipal wastewater for nearly 80 years and attained annual average total phosphorus concentrations of 30–60 μg∕L. A tertiary wastewater-treatment facility was constructed in 1973 that removed 80% of the external total phosphorus loading to the lake. The subsequent recovery of the lake was less than expected because of internal loading of phosphorus associated with accumulated lake sediments. Therefore, it is necessary to consider the effects of sediment phosphorus release to predict accurately the dynamics of phosphorus in Shagawa Lake. Measurements of the sediment characteristics have been used to estimate mass transfer coefficients that describe the release of phosphorus from the sediments. Three alternative mass-balance models were developed to determine the significance of the water sediment interactions. Recent field measurements were used to gauge the performance of the models. It was found, even more than 20 years after the reduction of the external loading, that sediment feedback of phosphorus has a significant influence on the dynamics of total phosphorus in the lake. Continued monitoring and data analysis of the sediment are necessary to refine the models and to predict accurately the ultimate fate of the lake.