The effects of stream water velocity, streambed celerity, and particle properties on microplastic deposition in streams

Microplastic (MP) is ubiquitously found in aquatic environments and poses a risk to organisms and potentially also to human health. While MP in oceans was studied extensively in recent years there is still a lack of knowledge especially on how MP is being transported and deposited in streams. We conducted flume experiments to study how plastic properties and bedform movement influence the deposition dynamics of MP. We used a recirculating stainless-steel laboratory flume (650 cm x 20 cm), that was packed with homogeneous sand (D50=0.65 mm). We compared the deposition dynamics of MP under stationary bed and under fast moving bedforms. We used aged MP fibers made from PET, PP, PA at various lengths (25, 100, 200 μm). The deposition rates were quantified by adding MP into the water and tracking their concentration in the stream water over time. In addition, streambed samples were taken to quantify and analyze the locations and concentrations of MPs within the sediment. It was found that the physical properties of MP including size, density, and type had a relatively minor influence on their deposition rate because the mechanism was dominated by the movement of the bedform and not due to their transport within the porous medium. Only a slight difference in deposition rate was observed for the different types of MP. The MP particles that we used are too large to be efficiently transported into the porous media and the resulting patterns were deposition close to the water-sediment interface in stationary bed, and below the moving fraction of the bed for moving bedforms. These experimental results represent unique observations of the transport mechanisms of MP in streams with moving bedforms. They are important for the understanding of the transfer of MP from its source toward the oceans, for understanding the life cycle of plastics in the environment, to develop sampling strategies in streams, and to find long-term solutions for reducing their concentrations and the associated risks.