Source-to-sink dynamics of microplastics and heavy metals along a land-use gradient in the tropical river system of India

• Comprehensive assessments integrating microplastics and heavy metals along land use gradient in a tropical river system. • Occurrence of MPs, influenced by land use land cover and hydrological conditions. • Fine-grained sediments played a key role in MP retention across river zones. • PGBs dominated MP morphology, indicating domestic waste as a major source. Microplastics (MPs) pollution in freshwater systems is one of the most pressing environmental concerns worldwide. In this study, the transport dynamics and pathways of MPs in sediment and water samples were investigated from source to sink along the Bharathapuzha River in southern India. Additionally, the concentrations of major and heavy metals (HMs) in water samples were analyzed to understand their spatial relationship with MPs contamination. The results show that MPs are widespread throughout the watershed, with a concentration of 62 ± 31 particles/L in water and 2073 ± 1256 particles/kg in sediments. MPs concentrations in water were primarily influenced by the intensity of human activities and the level of urbanization, while in sediments, both hydrological conditions and anthropogenic inputs played a significant role. A positive correlation between MPs abundance and clay-sized particles across all zones suggests that fine-grained sediments are key to their retention and accumulation. The dominance of pellet/bead morphology (83% in water, 77% in sediment) suggests the influence of household waste inputs from the catchment into the river. Polypropylene, polyethylene, polystyrene, and polyethylene terephthalate were the predominant polymers identified in the river samples. The spatial trend of elemental concentrations shows that the downstream regions, approaching the river’s estuarine zone and characterized by intense human activities, recorded the highest concentrations of Mg, Ca, K, As, and Zn. Notably, Mg and K exceeded the WHO permissible limits in the river system in this zone. Furthermore, no statistically significant correlation was observed between MPs abundance and elemental concentrations. Overall, our findings indicate that human activities and environmental factors influence the distribution of MPs along the river and their subsequent transport to the ocean.