Release of Sediment Phosphorus in Mill Creek of Portage and Wood Counties, Wisconsin

Abstract

This research characterized the sorption of P by sediment in Mill Creek, of Portage and Wood counties, Wisconsin. Mill Creek is an impaired stream where the sediment has been exposed to high P concentrations for at least 10 years. P sorption is important because the release of P from stream sediments could prolong efforts to reduce P concentrations in streams. This research can be used to estimate how long sediment P release would affect stream P concentrations after a reduction in P inputs. Laboratory sorption and desorption experiments evaluated the buffering of P with sediment. Mill Creek was sampled at five locations. Each location had a different dissolved P concentration. The results showed a decrease in EPC (equilibrium P concentration) with a decrease in stream P concentration. The sorption/desorption measurements were combined and fit to a non-linear Freundlich isotherm model. The KEPC (change in sorption at EPC) had an inverse relationship with EPC. While there were differences in the EPC and KEPC values between locations, the results showed that the sediment along Mill Creek had a relatively similar buffering capacity when equilibrated with similar stream P concentrations. The different EPC and KEPC values between locations could be due to the different ranges of stream P concentrations the sediments have been exposed to. A simulation model was developed to predict how the sediment P released over time could influence stream P concentrations. The characteristics of the sediment were based on a linear fit to the Freundlich curve at the concentration of interest. Freundlich curves were based on both the P desorption and sorption data. The sediment layer used in the simulation was representative of the oxic sediment layer as determined with pore-water equilibrators. The simulation suggests sediments could have an effect on stream P concentrations after a reduction in external loading but the duration may only be a few years. That time could be increased with greater stream P reductions, slower desorption rates or if there are large areas of transient flow.