P Speciation in Sediments of the Minnesota River Basin.

Sediments are a dominant P reservoir in aquatic ecosystems and play a critical role in governing dissolved P concentrations. This study assessed the likely sources of change in sediment P concentrations as well as sections of streams that are contributing to this increase in the Greater Blue Earth River Basin to the Minnesota/Mississippi confluence. The procedures involved analyzing the bed load and suspended sediments for TP concentrations. Bed load sediments were used to analyze for legacy effects whereas the suspended sediments were used to assess changes in a storm event. Results showed that bed load sediments sometimes had a higher total P concentration downstream than the upstream of the sewage treatment plant. In some locations these differences were due to particle size differences. In other cases, these differences were due to P adsorption from effluent discharged from the sewage treatment plant. To further confirm the P adsorption from a point source, the clay fraction of the bed load sediments were separated and then analyzed for various P fractions using the sequential extraction procedures.  The clay particles showed an increase (25%) in loosely bound P, thus confirming a recent P adsorption from the point source. In general, the suspended sediments showed nearly uniform particulate P concentrations consistent with measured TP concentration in Lake Pepin sediments. Two stretches of rivers showing higher TP concentrations in suspended sediments were likely a result of dissolved P losses from hog manure applied agricultural fields surrounding those stretches. These dissolved P losses possibly occurred through surface inlets either in the field or in adjacent road ditches. Overall, these results confirm our earlier findings that most TP in Lake Pepin sediments is likely from bank materials in the basin and higher concentrations over time were the result of particle enrichment in combination with P adsorption from point sources along the way.