The mechanisms of contaminants release due to incipient motion at sediment-water interface

Sediments in many rivers and lakes are subjected to resuspension due to a combination of hydrodynamics. However, the roles of contaminant-contained dissolved and particulate sediments during the resuspension release are rarely studied. This study focuses on the release quantity of contaminants in both water phase and solid phase. Conservative tracer （NaC1） and reactive tracer （Phosphorus） were respectively added to cohesive fine-grained sediments and non-cohesive coarse-grained sediments. A range of typical shear stress was conducted to characterize the time-depended release of contaminants in a laboratory flume. When the sediment started to move, the concentration of contaminant in the overlying water increased with the bed shear stress, but the dissolved contaminants responded faster than the particulate ones. The observed contaminant release process can be divided into three main stages： the initial two hours fast mixing： the release contribution of pore water could reach up to 75%; the middle 4-6 h adsorption： the partitioning coefficient of contaminant between water phase and solid phase decreased over the time, and the adsorption of contaminates from resuspended sediment dominated the negative release; the last equilibrium stage： the desorption and adsorption reached equilibrium, and the reactive contaminant made an impact on the water quality in the solid phase. The existing formulas to evaluate the release flux are far from practice meaning as the sediment contaminants undergo a very complex release process.

Sediments in many rivers and lakes are subjected to resuspension due to a combination of hydrodynamics.However,the roles of contaminant-contained dissolved and particulate sediments during the resuspension release are rarely studied.This study focuses on the release quantity of contaminants in both water phase and solid phase.Conservative tracer(NaCl)and reactive tracer(Phosphorus)were respectively added to cohesive fine-grained sediments and non-cohesive coarse-grained sediments.A range of typical shear stress was conducted to characterize the time-depended release of contaminants in a laboratory flume.When the sediment started to move,the concentration of contaminant in the overlying water increased with the bed shear stress,but the dissolved contaminants responded faster than the particulate ones.The observed contaminant release process can be divided into three main stages:the initial two hours fast mixing:the release contribution of pore water could reach up to 75%;the middle 4–6 h adsorption:the partitioning coefficient of contaminant between water phase and solid phase decreased over the time,and the adsorption of contaminates from resuspended sediment dominated the negative release;the last equilibrium stage:the desorption and adsorption reached equilibrium,and the reactive contaminant made an impact on the water quality in the solid phase.The existing formulas to evaluate the release flux are far from practice meaning as the sediment contaminants undergo a very complex release process.