Journal Article

Assessing the Behavior of Microplastics in Fluvial Systems: Infiltration and Retention Dynamics in Streambed Sediments

Fleckenstein, Jan H., Gilfedder, Benjamin Silas, Boos, Jan‐Pascal, Frei, Sven, Dichgans, Franz

2024

Overview

Microplastics (MPs) have been detected ubiquitously in fluvial systems and advective transfer has been proposed as a potential mechanism for the transport of (sub‐) pore‐scale MPs from surface waters into streambed sediments. However, the influence of particle and sediment properties, as well as the hydrodynamic flow regime, on the infiltration behavior and mobility of MPs in streambed sediments remains unclear. In this study, we conducted a series of flume experiments to investigate the effect of particle size (1–10 μm), sediment type (fine and coarse sand), and flow regime (high/low flow) on particle infiltration dynamics in a rippled streambed. Quantification of particles in the flume compartments (surface flow, streambed interface, and in the streambed) was achieved using continuous fluorescence techniques. Results indicated that the maximum infiltration depth into the streambed decreased with increasing particle size (11, 10, and 7 cm for 1, 3, and 10 μm). The highest particle retardation was observed in the fine sediment experiment, where 22% of the particles were still in the streambed at the end of the experiment. Particle residence times were shortest under high flow conditions, suggesting that periods of increased discharge can effectively flush MPs from streambed sediments. This study provides novel insights into the complex dynamics of MP infiltration and retention in streambed sediments and contributes to a better understanding of MPs fate in fluvial ecosystems. Quantitative data from this study can improve existing modeling frameworks for MPs transport and assist in assessing the exposure risk of MPs ingestion by benthic organisms. Plain Language Summary Microplastics (MPs) (small plastic particles) are present in river systems worldwide. The processes that lead to their transport and retention in rivers are not fully understood. Scientists have proposed that the infiltration of surface water into the streambed can carry MPs with it. In this study, we conducted experiments in a controlled environment that resembles a stream and its streambed. We investigated how different sizes of plastic particles (1, 3, and 10 μm), the types of sediment (fine and coarse sand), and water flow rates (low and high) affect how far particles travel in a streambed. We found that the size of MPs played a significant role in their depth of infiltration. Larger particles did not infiltrate as deeply as smaller particles, and were also retained in the streambed. Fine sand trapped particles for a longer time than coarse sand, and 22% of the particles remained in the streambed until the end of the experiment. Faster flowing water quickly removed MPs from the streambed. Our research helps understand how MPs spread in river systems and how long they remain in the streambed. The data can be used to improve transport models and assess the risk MPs pose to aquatic organisms. Key Points (Sub‐) Pore‐scale microplastics were advectively transferred from the surface water into the streambed sediments in flume experiments Infiltration patterns depend on microplastic size, streambed sediment type and surface flow velocities Microplastic retention was observed for 10 μm beads, 1 μm beads were considerably retarded in fine sediments

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: advective transfer / Aquatic organisms / Benthos / Flow rates / flow velocities / Flumes / Fluorescence / fluvial systems / High flow / hydraulic flumes / hydrodynamics / Infiltration / Ingestion / Low flow / Microplastics / overland flow / Particle size / Plastic pollution / Retention / risk / Risk assessment / River systems / Rivers / Sand / Sediment / Sediments / Soil properties / stream channels / streambed sediments / Streambeds / Surface flow / Surface water / Trapped particles / water / Water flow