It is challenging to investigate the transport and dispersion of contaminants in river confluence areas due to the complex flow dynamics. In recent studies on the flow dynamics in river confluence areas, it has been revealed that changes in inflow conditions (discharge ratio, width-depth ratio and concentration difference) can greatly influence pollutant diffusion. In this study, an asymmetric confluence-type river is modeled by a three-dimensional hydrodynamic water quality model, and three hydrodynamic scenarios are numerically simulated. The results show that a higher discharge ratio and width-depth ratio led to an increase in the lateral diffusion area of pollutants, deviation in the trajectory line of the mixing interface towards the opposite bank of the interchange, and an increase in the mixing rate of pollutants. For R = 0.267 and b/h = 3.75, the pollutants at the bottom are completely mixed in the exit end section. However, the difference in the pollutant concentration slightly affects the area, length and shape of the pollutant dispersion zone and notably affects only the concentration in each section.

Asymmetrical river confluence zones play a critical role in water quality protection and remediation. This study develops a three-dimensional numerical model to simulate the hydraulic characteristics and contaminant dispersion processes within river channels. The results indicate that variations in the two geometric factors—the confluence angle and elevation difference—can produce a range of effects. Under the combined influence of these factors, the trajectory line at the pollutant-mixing interface follows a “logarithmic” growth pattern. As indicated by the inhomogeneity index, an increase in the junction angle and elevation difference significantly accelerates the mixing rate of pollutants and enhances dispersion. These insights suggest that, in cases with large confluence angles and significant elevation variations, intense mixing of water flow facilitates the rapid transport and extensive dispersion of pollutants, which may help reduce localized pollution loads. These findings are crucial for developing effective water environment management strategies.