Evaluating the Possible Role of Phosphorus Release from Sediments on Stream Restoration

Elevated phosphorus concentrations can lead to algal blooms which impair waters for consumption, recreation, industry and agricultural uses. Along with sources of phosphorus such as runoff from agriculture and the effluent from wastewater treatment plants, there are reservoirs of phosphorus stored in stream sediments. As phosphorus inputs from agriculture and industry are decreased, there is the potential for these phosphorus reserves in the sediment to be released and prolong the time for restoration. Mill Creek in Central Wisconsin has water phosphorus concentrations ranging from 0.1 mg/L to 0.5 mg/L. These high phosphorus concentrations are the result of both wastewater discharges and agriculture runoff. Often a model used to develop a Total Mass Daily Load (TMDL) does not include a sediment reserve as a source of phosphorus. This study evaluates two methods of estimating sediment phosphorus reserves and incorporates them within a model for the phosphorus concentration in a stream. Two methods for estimating phosphorus equilibration with the stream were examined: sorption isotherms with equilibrium phosphorus concentration estimation; and pore-water profiling with equilibrators. This study compares these two approaches along a phosphorus concentration gradient in the stream and examines the sensitivity of stream phosphorus concentration reductions to sediment phosphorus reserves.