Role of bank materials as potential source and carrier of phosphorus

Abstract

Lake Pepin is a natural impoundment on the Upper Mississippi River, 80 km south of the Twin Cities, Minnesota. Two major water quality concerns for this lake are the higher rates of sedimentation and elevated eutrophic nutrient conditions. Although the majority of sediments in the Minnesota River and then Lake Pepin are coming from river banks, there is a perception that a significant amount of particulate phosphorus is coming from agricultural lands. The goal of this research was to assess the role of river bank materials as a source and carriers of phosphorus to Lake Pepin. In this study, we characterized the river bank materials in Blue Earth County for equilibrium phosphorus concentration (EPC0), total phosphorus (TP) content, particulate bound P fractions, and the potential of bank materials to adsorb and desorb soluble P. Results showed that bank materials are inherently high in TP content (>400 mg/kg), have low EPC0 (<0.1 mg/L) values, strong P binding ability, and high P adsorption and low P desorption potentials. Since the dissolved P concentrations in river waters contributing to Lake Pepin are >0.1 mg P/L part of the year, low EPC0 values suggest continued P adsorption by bank materials from river waters even under current conditions. Particle enrichment of bank materials during transport explained TP concentrations in Lake Pepin sediments prior to 1850. After 1850, we outlined scenarios using particle enrichment and historical river pollution as potential reasons for higher TP concentrations in Lake Pepin sediments. We conclude that river bank materials are a significant source of TP but not that of dissolved P to either the river system or to Lake Pepin. These materials do act as scavengers of dissolved P from river waters and carry it to downstream locations. The dissolved P in the river could be from the treatment plants, some point sources or agricultural lands. This adsorbed P will be stored in locations where sediments settle and will be a continuous source of dissolved P in the future. One way to prevent sediment from semi-sequestering dissolved P from river waters is to make sure dissolved P concentrations in river water are less than EPC0 values of bank materials.