Leveraging Sedimentary Process Insights to Enhance Understanding of Microplastic Deposition in Rivers

River networks are the major pathways for microplastic (MP) transport from terrestrial environments to oceans. However, the ability to quantify the water–sediment exchange of MPs, locations of deposition, and the time scales over which burial occurs is limited and thus often our estimation of where MP deposit is biased. To fill this gap, previous work on processes that control MP deposition will be briefly reviewed, with the aim of enhancing our understanding of the dynamic interplay between flow, sediment transport, and MP deposition. Detailed studies on MP deposition onto surficial sediment show that MP transport can be explained by the shear stress theory, hyporheic exchange, and bioturbation. Nevertheless, these processes cannot fully explain the observed distribution of MPs in deeper river sediments. It is proposed that bedform movement, channel reworking, bar formation, and aggradation/degradation at the river network scale should be included when estimating MP deposition. Results from flume experiments and a numerical model will be shown to explain potential processes that can lead to the burial of MP beneath the moving streambeds, which provides a mechanism for long-term accumulation and may explain resuspension events characterized by high MP loads during floods. It is argued that incorporating data on MP distribution in riverbeds with fluvial geomorphological and particle transport models will improve the current evaluation of MP transport in river networks.