Monsoon-driven spatio-temporal dynamics of microplastics in Brahmaputra riverbank sediments: Quantification using spectroscopic, pollution index and multivariate analysis.

Microplastics (MPs) are plastics that are smaller than 5 mm in size, have become a major issue among scientists and regulatory bodies due to their pervasive presence in aquatic environments, sediments, and the atmosphere. Despite a recent surge of MP studies that are more prevalent in freshwater ecosystems, riverine ecosystems, particularly monsoon-influenced regions, remain underexplored. This study investigates MP accumulation in the Brahmaputra River bank sediments near Guwahati, India, based on three years of field data collected between 2021 and 2023. Sediment samples were gathered from 21 locations (S1-S21) during the pre-flood (March) and post-flood (November) seasons each year to assess spatial patterns across zones classified as urban (<10 km from city center), semi-urban (10-20 km), and rural/remote (>20 km). A second dataset, focused on temporal trends, was built from monthly sampling (excluding peak flood months) at seven representative sites (T1-T7). Across all sites, MP concentrations varied from 45 to 437 items/kg, Different forms of MPs including fibers, fragments/beads, foams, and films, sizes ranging from 100 μm to 1 mm, and polymers such as polyethylene, polypropylene, polyester, and polyamide were observed with fibrous forms being the most prevalent. Polymer analysis indicated a dominance of polyester, polyethylene, polyamide and polypropylene, reflecting inputs from textiles, domestic waste, and packaging materials. Sites closer to urban centres exhibited consistently higher concentrations. Seasonal shifts were pronounced, with post-monsoon samples showing marked declines in MP abundance (p < 0.01), likely due to redistribution driven by hydrological redistribution. For both seasonal and temporal studies, the onset of monsoon caused MPs spikes, and with receding flood water, the quantity plummets. Assessment of ecological and contamination risk, using Pollution Load Index (PLI), Polymer Risk Index (PRI), and Potential Ecological Risk Index (PERI) revealed moderate-to-high contamination at urban sites, with synthetic fibers contributing significantly to hazard profiles. Multivariate analysis, Principal Component Analysis and Hierarchical Cluster Analysis revealed distinct spatio-temporal clusters. These findings provide a critical baseline for managing plastic pollution in dynamic river systems.