Effect of water–sediment regulation of the Xiaolangdi Reservoir on the concentrations, bioavailability, and fluxes of PAHs in the middle and lower reaches of the Yellow River

[Display omitted] •Water–sediment regulation is beneficial to sediment/PAH ejection from the reservoir.•Total dissolved PAHs in river water increased during water–sediment regulation.•Bioavailability of PAHs during water–sediment regulation increased 2–11 times.•Environmental risk of PAHs downstream of the reservoir increased during regulation. The water–sediment regulation of the Xiaolangdi Reservoir is conducted to control the relationship between riverine runoff and sediment transport of the Yellow River; however, there is no research about the effect of water–sediment regulation on the bioavailability and fluxes of hydrophobic organic compounds (HOCs). In this study, water and suspended sediment (SPS) samples were collected downstream of the Xiaolangdi Reservoir before, during, and after the water–sediment regulation in 2013. The 16 priority polycyclic aromatic hydrocarbon (PAH) concentrations of freely dissolved, total dissolved, and SPS-associated were determined. During water regulation, water discharged from the reservoir at a high flow rate led to the resuspension of downstream sediment. During sediment regulation, the sediment ejected from the reservoir resulted in higher SPS concentrations than that during water regulation. Both the freely and total dissolved PAH concentrations in river water during sediment regulation were the highest, followed by the concentrations during water regulation and before regulation. The freely dissolved PAH concentrations in river water during the water–sediment regulation were 2–11 times higher than those before water–sediment regulation. This was due to the fact that the resuspended sediment during water–sediment regulation could release PAHs into water phase, and more contaminants were released from the SPS during sediment regulation than during water regulation. The fluxes of sediment and the 16 priority PAHs (Σ16PAHs) during water–sediment regulation contributed to 32.4% and 35.7% of their annual fluxes, respectively, which were higher than the contribution (22.6%) of water discharge. This study suggested that the water–sediment regulation might reduce the long-term retention of sediment and PAHs in the reservoir. However, the environmental risk of PAHs as well as other contaminants downstream of the reservoir and in the estuary might increase during that period. Therefore, the effect of water–sediment regulation on the bioavailability and environmental risk of HOCs should be considered in the operation and ma