Sediment-Water Interfaces as Traps and Sources of Microplastic Fragments and Microfibers─Insights from Stream Flume Experiments

Microplastic pollution has been found to negatively impact water quality and ecosystem health in numerous riverine environments at different spatial and temporal scales. However, many of the underlying principles controlling microplastic transport and retention mechanisms are still poorly understood. Here, we study the deposition behavior of nylon fibers and fragments (small and large) in flow-controlled stream flume experiments with gravel or mixed sediment. We use a stochastic modeling approach and Latin hypercube sampling to optimize the parameters describing microplastic deposition and resuspension and relate deposition rates to settling rates calculated using Stoke's law. Our experiments show that lower streamflow velocity leads to faster microplastic deposition, an effect that is shape-dependent and more pronounced for fibers. In experiments with similar flow velocity, large fragments were more quickly deposited in flumes containing gravel compared to mixed sediment. Stoke's settling rates and model-based deposition rates can differ by several orders of magnitude, especially for fibers. For our flume experiments, these differences are attributed to transitional and turbulent flow near the streambed. Results emphasize that microplastic net deposition and near-bed transport cannot be well described by Stoke's law. Results will further our understanding of microplastic fate and transport in riverine environments.