Distribution, sources, and transport of microplastics in a karst river of Southwest China

Microplastics (MPs) have emerged as a significant pollutant in riverine ecosystems, yet their transport and distribution in karst rivers remain largely unexplored. This study examines MPs (20–500 μm) in surface water and sediments of the Wuma River, a karst river in Southwest China. Key findings indicate that abundance of MPs ranged from 4 to 122 n/L in surface water and 1,702.25 to 16,594.73 n/kg in sediments, with fragments (~90%) dominating. More than 85% of MPs fall within the 20–100 μm size range, with polyurethane (PU) and polyethylene terephthalate (PET) as the most prevalent polymers. MPs mainly originate from residential activities, agriculture, riverbank tourism, and tire and road abrasion. Karst river channel characteristics play a significant role in MPs retention. Overflow dams increase retention of smaller-sized MPs (20–50 μm), while underground river networks, large boulders, and riparian vegetation influence downstream movement and redistribution. Additionally, river dredging effectively reduces MPs accumulation in sediments. The results advances understanding of MPs behavior in karst fluvial systems, emphasizing the necessity for targeted mitigation strategies to protect these ecologically fragile environments. • Microplastics (20 -100 μm) dominate sediments and surface waters in Wuma River. • Polyurethane (PU) and polyethylene terephthalate (PET) was the most common plastic polymer in Wuma River. • Urban discharge, agriculture, riverbank tourism, and road wear are key MPs sources. • Dams, underground rivers, large boulders, and vegetation enhance MPs retention, reducing downstream transport.