The impact of flood events on the spatio-temporal variability of microplastics in the river sediments of two contrasting streams discharging towards the southern Caspian Sea

Microplastics (MPs), as global emerging pollutants, have received significant attention worldwide due to their wide spread presence in aquatic and terrestrial ecosystems. However, the mechanisms controlling their fate and transport through river networks remains poorly understood. This study investigates the spatio-temporal distribution of MPs in two contrasting rivers (Tajan and Talar) discharging to southern coasts of the Caspian Sea, Iran and identifies pollution sources and their activation. In both rivers, MPs were dominated by black-gray larger-sized (1000-5000 μm) Polystyrene (PS)particles. Spatially, MPs concentrations in both rivers differ from upstream to downstream and showed uneven distribution. The March 2019 flood event affected on the concentration and patterns of MPs in river sediments. The total MPs concentration in both river sediments in all stations significantly decreased from pre to post-flood time (p-value<0.05). It is hypothesized that during the flooding that occurred between spring and summer sampling campaigns, the active surface sediment layer of the streambed is likely to have been mobilized by the increased flow, leading to large scale resuspension of sediments and MPs, releasing MPs into the overlaying water column and consequently, causing a reduction of MPs abundance in streambed sediments. The result indicated that in such stormwater and flood events, both river can act a role of active source for MPs flux for Caspian Sea in downstreams. Our results highlight the importance of different local sources and particle release mechanisms for microplastic transport towards the Caspian Sea, the largest inland lake in the world.

Keywords: Microplastics, River, Sediment, Transport mechanisms, Flooding