Reducing Dissolved Phosphorus in Stream Water May Not Influence Estimation of Sediment Equilibrium Phosphorus Concentrations

Core Ideas The potential for stream sediments to adsorb or release P depends on the sediment equilibrium phosphorus concentration (EPC). Accurate estimations of sediment EPC are important for watershed modeling and total maximum daily load development. Existing methods for estimating sediment EPC often require extrapolating past a known dataset. This study used alum to promote desorption from sediments during EPC estimations. Results suggested current methods of EPC estimation are acceptable. The potential for stream sediments to adsorb or release phosphorus (P) depends on the equilibrium P concentration (EPC), which is the water column P concentration where P is neither (net) adsorbed nor released from the sediments. Current methods of measuring EPC in streams include adding known P concentrations to stream water, mixing with fresh sediments, and allowing the mixture to reach equilibrium through P sorption. Sediment EPC is usually calculated as the x ‐intercept of the linear relationship between initial P concentrations and P adsorbed by the sediments. However, the x ‐intercept is often an extrapolation past the known data set (i.e., no desorption of P from the sediments is simulated). The purpose of this study was to use aluminum sulfate (alum) to decrease ambient P concentrations and encourage P desorption from the sediments and to compare EPC estimations with and without the use of alum treatments. The results from this study found optimal alum dosages to be between 5 and 25 mg‐Al 2 (SO 4 ) 3 L −1 to provide a range of P removal for the EPC experiments. Above 25 mg‐Al 2 (SO 4 ) 3 L −1 , stream water pH began to decrease. The other notable changes to water chemistry after alum treatments were increases in sulfate and residual concentrations of aluminum. However, EPC estimations were not significantly different between traditional calculation methods and the use of P desorption data points. These results suggest current methods for EPC estimations are acceptable, and spending time and money to promote desorption of P from the sediments is not necessary to estimate EPC.