Modeling the Fate of Sediments and Attached PAHs in a Base-Flow-Dominated River

Polycyclic aromatic hydrocarbons (PAHs) are a group of persistent organic pollutants that ubiquitously occur in rivers and, due to their strong hydrophobicity, are mainly sorbed to sediments. Thus, transport of sediments with the water flow, e.g. as suspended particles, facilitates PAH transport and needs to be considered to explain the pollutant fate along the river. In this study, we therefore present an integrated model that describes sediment transport and particle-facilitated pollutant transport to investigate the fate of sediments and attached PAHs. The model considers surface runoff generated particles and attached PAHs. We assume one active bed sediment layer where PAHs are in local equilibrium. We apply this model to a base-flow-dominated river, the Ammer River located in southwestern Germany. The models are calibrated to stream flows, suspended-sediment concentrations, and PAH concentrations. Results show that urban particles dominate the total suspended sediment load and the contribution of rural sediments is very small due to the weak rural surface runoff. Net sediment trapping was found in very mild river reaches during the simulation period. Sediment-bound PAHs constitute approximately 75% of the total annual PAH load, which confirms the sediment-facilitated fate of PAHs. Sediment turnover determines the turnover of attached PAHs in steep reaches whereas the diffusion of PAHs from sediments to the water phase is relevant in very mild river segments. Sediment trapping pools have PAH legacy potentials, which may remain 10-20 years after their environmental regulation in the 1970s since these river reaches contain old and highly PAH-polluted particles, which may act as a secondary PAH source. This study is useful to develop appropriate river management strategies and provides insights to evaluate the environmental exposure of persistent organic pollutants to river systems.