Long-term changes in sediment phosphorus below a rural effluent discharge

Abstract. Effluent discharge often increases the amount of phosphorus (P) in the water column and bed material of receiving water bodies. The goal of this study was to evaluate changes in sediment-P interactions in an effluent-driven stream over a 4-year period where hydrology and watershed P management changed dramatically. Specifically, this study evaluated (i) the equilibrium between benthic sediments and stream water dissolved P; and (ii) the amounts of select P fractions in the bed material within the fluvial channel. Sediment and water samples were collected at Columbia Hollow in northwest Arkansas from October 2003 through September 2007, and the sampling site was approximately 3 km downstream from the Decatur wastewater treatment plant (WWTP). Monthly average effluent total P (TP) concentrations were highly variable (0.30–4.80 mg L⁻¹) from October 2003 until December 2005; however, the Decatur WWTP implemented new P management strategies in 2006 that reduced the variability in effluent TP (0.28–0.95 mg L⁻¹). Soluble reactive P (SRP) concentrations at Columbia Hollow 3 km downstream from the effluent discharge followed the same pattern; these concentrations were positively correlated to the effluent TP (r=0.73; p<0.001). Sediment equilibrium concentrations (EPC₀) were significantly less (ln transformed data, p<0.001) after the WWTP effluent reduced TP concentrations, and sediment EPC₀ suggested that the stream bed material acted as a P source to the overlying water at Columbia Hollow. The effects of this effluent discharge and the WWTP management changes on sediment P dynamics were profound. Prior to implementation of WWTP P management, the effluent TP concentrations were the driving factor related to SRP concentrations in the water column and sediment EPC₀. Conversely, after the P management changes the benthic sediments became the important factor likely regulating dissolved P concentrations in the stream water.