Density effects on solute release from the streambed

Contaminants can be released to the stream from the bed by previous pollution events. The releasing contaminants may cause secondary pollution for a river, which impacts the river’s 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 the solute released from the upper streambed is driven by convection induced by the bedforms, and that density effects generally inhibit the solute release from the lower streambed region. Density gradients modify the pore water flow patterns and form circulating flows in the deep streambed area. The formation of circulating flows is affected by the density gradients induced by solute concentration and horizontal pressure gradients due to the the slope of the river. The circulating flows enhance the mixing near the bottom of the streambed and allow exchange between the hyporheic zone and ambient flow zone.