The effects of stream bed motion and fiber length on microplastic deposition

Abstract

The key processes controlling the transport and deposition of microplastics (MP) in streams and rivers remain poorly understood, particularly under conditions with moving sediments. Experiments were conducted in a 6 m-long and 0.2 m-wide stainless-steel recirculating flume filled with 25 cm of sand (median grain size of 0.6 mm) and water. We conducted two main experiments at stream water velocities of 13 and 53 cm/s, which resulted in stationary and moving bedforms, respectively. MP particles were added to the stream water, and the rates of MP deposition were determined by monitoring the decrease in their concentration over time. The tested MPs included polypropylene (PP) fibers with a diameter of 25 μm and lengths of 25, 100, 200, and 2000 μm. We also used carboxylated polystyrene (PS) microspheres (1 μm) that served as a reference particle. 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 the fibers, the microspheres had approximately the same amount of deposition under stationary and moving bedforms at the end of the experiment. 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 benthic organisms.