An integrated dual-model framework for microplastic flux estimation and source apportionment in tropical island river systems.

Microplastics (MPs) have emerged as a major global environmental concern, with rivers serving as a key pathway transporting land-based MPs to the ocean. However, river-to-ocean MP fluxes and their sources in tropical island river systems remain poorly understood due to limited hydrological observations and heterogeneous human activities. To address this gap, this study applied two complementary approaches for flux estimation, including a runoff-based model and a socioeconomic-driven model, and constructed a composite-feature source library integrating multiple MP characteristics from representative land-based sources, with the results verified using an optimized Positive Matrix Factorization model. A total of 144 sampling sites were established, including 39 riverine sites along 10 coastal rivers for flux assessment and 104 land-based sites for source library construction representing domestic, industrial, and agricultural sources. Riverine MP abundances ranged from 0.27 to 6.00 n/L, with rayon, polypropylene, and polyethylene terephthalate dominating and occurring mainly as fibers and fragments. Estimated annual MP fluxes ranged from 6.30 × 10¹¹ to 1.61 × 10¹³ n/a using the runoff-based model and 1.31 × 10¹⁰ to 1.32 × 10¹¹ n/a using the socioeconomic-driven model, with a significant positive correlation between the two model estimates. Both source apportionment methods consistently indicated that domestic (e.g., sewage discharge) and industrial sources (e.g., industrial discharge outlets) are the primary contributors to MP inputs, while agricultural contributions are negligible. Overall, this study offers new insights into MP transport and sources in tropical island river systems and provides a practical framework for riverine MP flux estimation and source apportionment.