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Migration of microplastics in hyporheic zone sediments: Beiluo River, China
Summary
A study of China's Beiluo River found microplastics at all sediment depths within the hyporheic zone, with abundance declining with depth and small fibrous particles primarily redistributed via hyporheic exchange between surface water and subsurface sediments. Hyporheic zones act as ecological transition spaces critical for aquatic biodiversity and water purification, and understanding how microplastics migrate into these zones reveals a previously underappreciated route of deep sediment contamination.
Microplastics (MPs) are plastic particles smaller than 5 mm that settle from surface waters into sediments. They can then be transferred to deep sediments via hyporheic exchange. Most studies on MPs in hyporheic zone sediments have been Limited to surface sediments. This study investigated the pollution, sources, and migration modes of MPs in sediments at various depths within the hyporheic zone of the Beiluo River. MPs were present in the surface water and sediments at different depths in the hyporheic zone. The average abundances of MPs in surface water and surface, middle, and deep sediments were 12 items·L−1, 313 items·kg−1, 206 items·kg−1, and 166 items·kg−1, respectively. The abundance of MPs in the sediments decreased with increasing depth. The physical characteristics of MPs were mainly fibrous, a small size (0.05–0.50 mm), and a blue color. The main polymer types of MPs were PE, PP, and PET. The main sources of MP pollution in the Beiluo River Basin were washing wastewater and agricultural mulch. Small MPs primarily migrated via hyporheic exchange. This process strongly influenced their vertical spatial distribution. The results of this study provide a basis for further research on the effects of hyporheic exchange on the vertical distribution of MPs in sediments.