Microplastics in freshwater environments: Influence of topography and water velocity on their distribution in river systems

This study examined the distribution and interaction of microplastics (MPs) between water and sediment matrices across lake and river systems in Krishnagiri District, Tamil Nadu, India, emphasizing the influence of hydrodynamic conditions on their transport and deposition. A total of 3,050 MP particles were detected, with water samples contributing 47.47% (1,185 particles from 21,115 L) and sediments 52.53% (1,865 particles/kg). Fibers dominated water samples (92.4%), whereas sediments exhibited greater MP diversity, including fragments, fiber bundles, beads, films, and foams (26.5%), indicating enhanced accumulation and heterogeneity due to long-term deposition. A strong negative correlation was found between water velocity and MP concentration in water (ρ = –0.82, p = 0.001), suggesting that higher velocities reduce MP retention. No significant correlation was observed between water velocity and sediment MPs (ρ = 0.18, p = 0.5), implying that topography and depositional conditions exert stronger control on MP accumulation. Spatial analysis of water-to-sediment MP concentration ratios revealed that eight of eleven river sites exhibited higher MP loads in sediments, particularly in wider and low-velocity zones, where reduced turbulence promotes MP settling. Confluence points showed elevated MP concentrations in water due to enhanced hydrodynamic mixing, sediment disruption, and resuspension of buried particles. Lakes also exhibited higher MP concentrations per litre than rivers, reflecting their role as long-term sinks. Overall, results demonstrate that hydrodynamics, geomorphology critically govern MP transport, retention, and distribution within freshwater ecosystems, providing insight into their environmental fate and informing mitigation strategies for aquatic plastic pollution.