Bedload transport contributions to the microplastic load of the River Waal, Netherlands

Microplastics in rivers and streambed sediments represent a significant risk to ecosystem health and functioning locally, but also contribute to the total load of plastic waste transported towards the oceans. Despite increasing recognition of the importance of rivers as major conduits that are connecting terrestrial sources of mismanaged plastic waste across river basins to the oceans and that can also form important long-term sinks, actual estimates of riverine plastic waste contributions to the oceans have focused exclusively on floating, and sometimes suspended, plastic debris. However, the role of plastic waste, and in particular of microplastic particles when transported as streambed sediments, and their contribution to the total riverine plastic particle load has not yet been established. Indeed, the mechanisms of microplastic deposition and accumulation, and the resulting spatial patterns within riverine sediments, remain poorly understood.Here, we present a first attempt of combining observations of microplastic concentrations in riverbed sediments with geophysically aided quantifications of bedload sediment transport to estimate contributions of bedload-transported microplastics to the total plastic waste load of a major European river, the River Waal, a Dutch tributary of the River Rhine. We therefore analysed microplastic concentrations in the top 20 cm of streambed sediments at 18 locations across a sequence of transects covering characteristic dune bedforms in the River Waal to establish characteristic ranges and spatial distributions of microplastic concentrations at the riverbed. Analysis of a 420cm deep sediment core was used to quantify the vertical microplastic distribution in the active zone and beyond. Following organic matter digestion and density separation, identified microplastics were counted and characterised for their particle size and shape using fluorescence microscopy aided by Nile Red staining. Additionally, polymer identification was performed on identified microplastic particles using micro-Raman spectroscopy.Time series of multibeam (MBES) bathymetric information, together with sub-bottom profiler data (parametric echo sounder, PES) of subsurface sedimentary structures, were used to characterize the transport dynamics of recent alluvial dune sediments of the active river channel, where migrating dunes represent the main bedform that control microplastic transport and burial. This information of alluvial dune movement was used to derive bedload transport budgets for the River Waal. When combining these sediment transport estimates with the ranges of microplastic concentrations observed at the surface of streambed sediments, our analysis yields first insights into the potential ranges of microplastics transported as bedload, revealing that these can be substantial and represent an under-recognized fraction of the total plastic waste load transported towards the oceans. While creating budgets at these scales remains highly uncertain given that sample locations and times are restricted by the challenges inherent to microplastic analyses in natural media, our results highlight the need for further exploring the mechanistic drivers of riverine microplastic transport and highlight the importance of streambed sediments as long-term storage zones and legacy pollutants of microplastics that can have profound impact on downstream ecosystem health and functioning.