Colloids are often in forms of inorganic and organic particles whose particle sizes (dp) are ranging from 1.0 nm to 10.0 μm. On the one hand, their transport processes in the hyporheic zone such as deposition, resuspension, clogging and release are substantially affected by hyporheic exchange. On the other hand, the existence of colloids can modify the hydraulic structure of the hyporheic zone due to clogging. Despite the general acknowledgement that particle size plays an important role in contaminant adsorption and clogging, it remains unclear how the particle size of colloids changes during their transport processes in the hyporheic zone. This study aims to investigate the variation of the particle size of colloids in the overlying water and the effects of settlement and convection-diffusion via laboratory experiment and numerical simulation. The results show that both settlement and convection-diffusion play roles in the exchange of colloids between the stream and the streambed. More specifically, settlement is the dominant factor affecting the exchange process of large-sized particles (dp > 3.06 μm) in the hyporheic zone as the high settling velocity dominates the outflux of colloids in the overlying water; the exchange process of small-sized particles (dp < 1.10 μm) is more affected by convection-diffusion and some of them can be released from the streambed to the overlying water; while the exchange process of middle-sized particles (1.10 μm < dp < 3.06 μm) is affected by both convection-diffusion and settlement. Thus, this study may provide important insights into the variation of the particle size of colloids in the overlying water and the effects of settlement and convection-diffusion.

Hyporheic exchange plays a critical role in driving the transport of colloids in the hyporheic zone, but it remains unexplored how the particle size distribution of colloids varies during their transport process. This study aims to investigate the particle size variations and effects of factors on different sized colloids via laboratory experiment and simulation. The results show that both settlement and convection-diffusion play a role in the exchange of colloids between the stream and the streambed. Settlement is the dominant factor determining the exchange of large-sized particles due to their high settling velocity; the transport of small-sized particles is affected more by convection and diffusion, and part of them can be released from the streambed to the stream; while the exchange of middle-sized particles is affected by both convection-diffusion and settlement. As the colloids can affect the environment and eco-system of hyporheic zone in both positive and negative ways, the knowledge of how size variations affect the transport patterns of colloids is significant to future studies.

Colloids exist widely in rivers which can act as contaminants or carries of contaminants. Hyporheic exchange drives colloids to transport into the hyporheic zone. However, the variation of the particle size of colloids has seldom been considered in previous transport theories of colloids. This study aims to investigate the variation of the particle size of colloids and functions for different sized particles via laboratory experiments and simulations. The results show that the settlement and convection-diffusion of colloids play a dominant role in the exchange of colloidal particles between the stream and the streambed. Large particles can settle into the streambed more rapidly as settlement domains the process, which however can hardly be detected in the overlying water during the later period of the experiment; the exchange process of small-sized particles is affected more by convection and diffusion, and a retarded trace release can be monitored from the streambed to the stream; while for middle-sized particles, mass transfer coefficient and settling velocity affect exchange process together. Because the variation of particle size (affecting specific surface area, surface electrochemical characteristics) can have a substantial effect on the contaminants carriers (colloids) in the hyporheic zone, the knowledge of it should be taken into account in future studies.