*Article* **Streambank Legacy Sediments in Surface Waters: Phosphorus Sources or Sinks?**

**Shreeram Inamdar 1,\*, Nathan Sienkiewicz 2, Alyssa Lutgen 2, Grant Jiang <sup>2</sup> and Jinjun Kan <sup>3</sup>**


Received: 31 March 2020; Accepted: 9 May 2020; Published: 11 May 2020

**Abstract:** Streambank legacy sediments can contribute substantial amounts of sediments to Mid-Atlantic waterways. However, there is uncertainty about the sediment-bound P inputs and the fate of legacy sediment P in surface waters. We compared legacy sediment P concentrations against other streambank sediments and upland soils and evaluated a variety of P indices to determine if legacy sediments are a source or sink of P to surface waters. Legacy sediments were collected from 15 streambanks in the mid-Atlantic USA. Total P and M3P concentrations and % degree of phosphorus saturation (DPS) values for legacy sediments were lower than those for upland soils. % DPS values for legacy sediments were below the water quality threshold for P leaching. Phosphorus sorption index (PSI) values for legacy sediments indicated a large capacity for P sorption. On the other hand, equilibrium phosphorus concentration (EPC0) for legacy sediments suggested that they could be a source or a sink depending on stream water P concentrations. Anoxic conditions resulted in a greater release of P from legacy sediments compared to oxic conditions. These results suggest that legacy sediment P behavior could be highly variable and watershed models will need to account for this variability to reliably quantify the source-sink behavior of legacy sediments in surface waters.

**Keywords:** legacy sediments; phosphorus; equilibrium phosphorus concentration; sorption; desorption; anoxic; water quality
