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.
In this study, both laboratory experiments and numerical simulations were conducted to investigate the effect of density-driven flow on the transport of high-concentration pollutants in the hyporheic zone. The results show that the density gradient can change the flow of pore water and the strong density-driven flow can lead to an unstable flow, which increases the effect of preferential flow and thus causes the appearance of solute fingers in the hyporheic zone. Notably, these solute fingers become more obvious with the increase of depth. The appearance of solute fingers depends on the relative strength of the pumping exchange and density gradient, which are represented by the dimensionless number M* and N* respectively. Finger flows appear near the interface when M* is less than 0.5 N*. This study may contribute to better understanding the transport and destination of solutes and thus may provide some insights into the assessment on pollution incidents.
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.