A semi-analytical approach to characterize the effects of unsteadiness and dune migration on microplastics fate

Streams and rivers, together with their bed (i.e. benthic) and subsurface (i.e. hyporheic zone) environments represents the natural connectors between terrestrial and aquatic environments through which transport and transformation processes control the fate of a multitude of elements, including nutrients and contaminants, commonly produced by human activities. In recent years, particular attention was given to a new class of pollutants known as Contaminants of Emerging Concern (CECs) that include, among others, microplastics. Microplastics enter the streams and rivers after escaping degradation from wastewater treatment plants (WWTPs) and during the time they spend within the riverine environments they may have potential adverse effects on human and freshwater ecosystems. Among the different processes that affect their fate, burial within the streambed sediments is still the subject of current research; especially considering that under some hydrodynamics conditions small scale bedforms (i.e. dunes and ripples) can migrate causing hyporheic exchange to depend on both bedform migration, called turnover, and near-bed pressure gradients, called pumping. Here, we analyze the effects of turnover and pumping in sand-bedded streams with mobile dunes by proposing an analytical solution of the process. The proposed analytical solution allows us to determine: i) the pressure head distribution, ii) the velocity field and iii) the residence time distribution within a homogeneous stream bed sediment under transient conditions of the stream flows. The solution allows to characterize and quantify the penetration depth and the release (i.e. the resuspension process) of microplastics due to the trapping-releases successions as hyporheic pathways (connecting downwelling and upwelling zones) change in time.