Exchange of pollutants between rivers and the surrounding environment: Physical processes, modelling approaches and experimental methods

The fate of solute and pollutants is controlled by a broad number of different transport and storage mechanisms, ranging from simple processes (i.e. molecular diffusion, advection etc.) to more complex phenomena (i.e. evapotranspiration, groundwater flows, etc.). Different mathematical models, accounting for different exchange processes, have been developed and applied to specific experimental studies to assess transport and storage parameters. Experimental research focused on transport and retention processes induced by the transient storage in the dead zones, by the river bed topography and vegetation, by evapotranspiration. The analysis of these physical processes is generally conducted observing the behavior of solutes in field environments or in scaled laboratory models, using artificial or environmental tracers to track the fate of transported substances and assess transport and retention parameters. To improve the knowledge of pollutant exchange mechanism between a river and the surrounding environment, new experimental techniques focusing on long timescale retention and investigating the link between river biology and hydrodynamics are required. The development of new protocols for tracer tests design and the use of new specific tracers will open future research perspectives.