Verena Levy Sturm

Microplastics (MPs) are widespread in aquatic environments, yet the factors controlling their deposition and burial in dynamic riverbeds remain poorly understood. This study investigates how streambed motion and fiber size affect MP deposition in sandy streambeds. Experiments were conducted in a stainless-steel flume (650 × 20 cm) with a 25 cm silica sand bed (D50 = 0.6 mm) and 12 cm water depth. One experiment involved moving bedforms (water velocity = 0.53 m/s, bed celerity = 4 m/hr), while the other used stationary bedforms (0.15 m/s). Polypropylene (PP) fibers (25–2000 μm) and carboxylated polystyrene (PS) microspheres (1 μm) were introduced into the flow. MP concentrations were monitored over three days, and deposition was inferred from their decline in the water. Control experiments without sediment were also conducted. Results show different deposition patterns between fibers and spheres. MP fibers deposited faster and with less resuspension under stationary bedforms as compared to moving bedforms. Furthermore, increasing the lengths of MP fibers resulted in more deposition under moving bedforms but did not have a significant effect under stationary bedforms. In contrast to fibers, microspheres could escape resuspension due to the mobility in porous media and their amount of deposition wasn’t influenced by moving bedforms. Understanding and identifying key processes that control MP deposition in streams with moving bedforms will enhance our ability to predict their transport mechanisms, providing also crucial insights for evaluating risks to streambed organisms. Due to climate change a scarcity of fresh water is predicted, which increases the importance of protecting sweetwater streams from any pollution, including MP.