The Effects of Large Roughness Elements on the In-stream Transport and Retention of Polystyrene Microplastics

Abstract The dominant mechanisms controlling the concentration and transport of microplastics (MPs) in riverine systems are not fully understood yet. Polystyrene (PS) is a highly abundant MP in streams and rivers. This study investigated the impact of large roughness elements (LREs) on in-stream transport and retention patterns of polystyrene-microplastics (PS-MPs). Experiments were conducted with and without LREs under a range of shear Reynolds numbers (𝑅𝑒∗) in an eco-hydraulics flume. We found a clear dependence of MPs’ velocity on 𝑅𝑒∗ in LREs-dominated channel. The results also revealed that the LREs-generated turbulence kinetic energy (TKE) can be a good predictor of PS-MPs transport and retention rate. This indicates the effectiveness of TKE in retaining PS-MPs as they travel through streams and rivers. The presence of LREs increases the PS-MPs capture and decreases their velocity of transport. This suggests that PS-MPs retention can be increased by increasing the LRE density in reverine systems.