River bank materials as a source and as carriers of phosphorus to lake Pepin

Lake Pepin, a natural impoundment on the Upper Mississippi River, has water quality problems of high sedimentation rates and elevated phosphorus (P) levels. The majority of sediments in Lake Pepin come from river banks consisting of fine tills in the Minnesota River Basin. Since 1850, inorganic P concentrations in lake sediments have continuously increased. This study explored whether the increase in inorganic P concentrations can be explained through selective transport of fine particles combined with in-stream P adsorption. The measurements included total P (TP) content, P adsorption/desorption isotherms, and the solution equilibrium P concentration at zero adsorption (EPC₀) for various bank materials. Results showed that till bank materials are inherently high in TP (> 400 mg kg^-1), have strong P binding ability, and have low EPC₀ (< 0.1 mg L^-1). Selective transport of fine particles from bank materials explained the inorganic P concentrations of Lake Pepin sediments before 1850. After 1850, P adsorption from the water column simulating historic river pollution and additional selective transport of fine particles further explained the increasing inorganic P concentrations in lake sediments. We conclude that the increasing P concentrations in Lake Pepin sediments are largely the sewage and industrial waste P that was picked up by fine particles of bank materials from river waters. Because a substantial reduction in sediment load from bank materials may be difficult to achieve, we suggest directing efforts toward insuring that P additions to rivers upstream of Lake Pepin do not result in P concentration greater than the EPC₀ value of bank materials.