Colloids moving from the stream into the hyporheic zone may have a negative impact on aquatic ecosystems as they are potential contaminants or carriers of contaminants. Moreover, retained colloids in the hyporheic zone could not only reduce the exchange flux between the stream and streambed but also change the conditions of the bed, affecting the habitats for aquatic organisms. Previous studies focused on the exchange flux across the sediment–water interface, but the colloid transport processes and distribution of retained colloids in the streambed have received little attention. We conducted experiments within a laboratory flume to examine these processes in a streambed driven by bedform‐induced hyporheic flow. Retained colloids measured in the bed at the end of the experiments revealed colloid retention mainly in the shallow layer of hyporheic zone (0–5 cm below the interface). The results demonstrated significant effects of particle trapping and settling on the colloid transport and distribution in the streambed. Retention leads to the formation of a colloid‐filled shallow layer in the bed. Particle paths based on model simulations showed that colloid settling in pore water modifies the direction of colloid transport and allows the colloid particles to move more deeply in the bed.
Contaminants that entered the streambed during previous surface water pollution events can be released to the stream, causing secondary pollution of the stream and impacting its eco‐environmental condition. By means of laboratory experiments and numerical simulations, we investigated density effects on the release of solute from periodic bedforms. The results show that solute release from the upper streambed is driven by bedform‐induced convection, and that density effects generally inhibit the solute release from the lower streambed. Density gradients modify the pore water flow patterns and form circulating flows in the area of lower streambed. The formation of circulating flows is affected by density gradients associated with the solute concentration and horizontal pressure gradients induced by stream slope. The circulating flows near the bottom of the streambed enhance mixing of the hyporheic zone and the ambient flow zone.
Phosphorus is not only an essential nutrient for organisms but also a major contributor to the eutrophication of fluvial ecosystems. Previous studies have focused on the flux of phosphorus at the water‐sediment interface, and very little is known about the transport and distribution of phosphorus in bedforms. In this study, a series of flume experiments and numerical simulations were conducted to investigate phosphorus dynamics in the hyporheic zone. The results show that the phosphorus concentration in the overlying water decreases faster than that of conservative solutes because of sediment adsorption. It is also found that the hyporheic exchange flux of phosphorus is more sensitive to the maximum adsorption capacity than to the equilibrium isotherm distribution coefficient, and the penetration depth is inhibited at higher maximum adsorption capacity. The mass proportion of phosphorus is decreased by 19.5% in the pore water but increased by 95.9% in the sediment with the increase of the maximum adsorption capacity from 0.5 to 1. These findings provide a better understanding of the transport and distribution of phosphorus in the riverbed and the contribution of transient flows such as flood to phosphorus dynamics in the hyporheic zone.
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