Microplastics storage at the sediment-water interface in a gravel-bed river: Importance of local hydro-sedimentary conditions in downwelling, upwelling, and sedimentation zones.

Rivers are often seen as vectors for microplastic transport from land to the ocean; however, the sediment-water interface of rivers also provides opportunity for the temporary storage and retention of plastic debris. Hydrological, sedimentary, physical, chemical and biological factors influence microplastic fate and transport, yet the influence of the interactions between these different drivers and mechanisms on microplastic transport at the sediment-water interface remains poorly understood. This study investigates how hydrological exchanges impact microplastics distribution in streambed sediments by comparing microplastic concentrations in zones with surface-groundwater exchange fluxes (upwelling or downwelling) and zones with sediment accumulation along gravel bars situated in the lower catchment of the Ain River (France). Sediment samples were collected from 15 locations (five per hydro-sedimentary condition) at two different depths (i.e., sediment surface and -20 cm), and microplastics between 20 and 500 µm in size were analyzed. Our results indicate that hydrological flow conditions significantly control microplastic concentrations in streambed sediments. In the top sediment layer, microplastic concentrations in sediment size fraction smaller than 2 mm were similar across hydrological zones. However, upwelling zones showed significantly higher concentrations at 20 cm depth in comparison to downwelling zones, likely due to an abundance of smaller microplastics in upwelling zones (mean particle size of microplastics = 251 ± 154 µm, median particle size= 179 µm). This suggests that water exchanges influence particle retention in upwelling zones. Additionally, sedimentation zones exhibited much higher microplastic concentrations than upwelling and downwelling zones, possibly due to accumulation with fine sediments (D = 221 µm). These insights emphasize the importance of sedimentological and hydrological conditions in shaping microplastic distribution and sampling. It also highlights that sediment load of gravel bed rivers can serve as potential reservoirs for sub-pore size microplastics which pose risks to interstitial fauna, groundwater quality, and aquatic food webs.