A drainage influence scoring framework explains seasonal microplastic patterns in a tropical urban catchment

Urban drainage systems are major pathways for transporting land-based microplastics (MP) into aquatic environments, especially in rapidly urbanizing tropical cities. However, limited attention has been given to how drainage connectivity and proximity to upstream waste distribution influence the seasonal MP flux at the network scale. This study proposes a drainage influence scoring framework that integrates upstream waste proximity, network connectivity, and urban density matrices to explain the MP distribution pattern in Kochi, a tropical coastal city in South India. Water (n = 28 per season) and sediment (n = 10 per season) samples were collected from the city’s drainage channels during the pre- and post-monsoon periods of 2024. Results revealed a significant increase in MP abundance post-monsoon in both water (from 39.65 ± 13.44 to 56.28 ± 15.14 particles/L) and sediment (from 450.22 ± 168.76 to 545.79 ± 220.91 particles/kg dw), as confirmed by the Wilcoxon signed-rank test (α = 0.05). Seasonal change in particle morphology showed an increase in fragments and a relative decline in fibers following the monsoon, indicating enhanced fragmentation and runoff-driven transport. Polymer composition was dominated by PS, PP, and PET, while the presence of styrene–isoprene copolymers suggests contributions from tire wear particles alongside mismanaged plastic waste, textile fibers, and road-derived urban debris. A network-derived influence score, based on upstream waste accumulation along drainage paths, showed a strong correlation with post-monsoon MP levels (ρ = 0.688, p < 0.01). Built-up area and population density also showed significant positive correlations with MP abundance, especially during the post-monsoon period. By integrating spatial predictors with seasonal MP data, this study provides a novel transferable framework for identifying MP hotspots and prioritizing drainage management strategies. The proposed influencing scoring approach enhances existing MP monitoring efforts in developing urban systems lacking a detailed hydrological model. These findings support targeted urban planning strategies, such as culvert redesign, desiltation, and localized waste control, in flood-prone zones of tropical cities.