Assessment of Microplastic Susceptibility Zones in the Gadilam River Basin, Tamil Nadu, India, Using a GIS-Based Analytic Hierarchy Process

Abstract

Microplastic occurrence in river basins is heterogeneous; however, South Asia basin-scale assessments have relied on point-based sampling, limiting coverage. This study delineates microplastic susceptibility zones across the Gadilam River Basin (1,396.76 km²) in Tamil Nadu by integrating twelve source, pathway, and terrain parameters such as runoff, land use/land cover (LULC), population density, the normalized difference built-up index (NDBI), slope, drainage density, flow accumulation, and distances to dump sites, industries, markets, waterbodies and hospitals within a GIS-based AHP. The Pairwise comparison assigned the largest weights to runoff (17.61%), distance to dump sites (14.29%), LULC (14.02%) and population density (14.01%). Weighted linear combination produced a susceptibility index ranging from 176.47 to 482.17 were classified into five zones: Very Low (19.64%), Low (35.60%), Moderate (30.39%), High (8.84%) and Very High (5.54% of basin area). Among the 342 revenue villages within the basin, 27 villages, accounting for 7.89%, were classified as High, while 20 villages, representing 5.85%, were categorized as Very High. These villages are predominantly located in the central and lower regions of the basin, where waste sources, developed land, and monsoon-driven runoff converge over areas of low-gradient terrain.  Kruskal–Wallis tests with false-discovery-rate correction showed that eleven of the twelve factors differed significantly among classes, and epsilon-squared effect sizes ranked distance to dump sites (ε² = 0.39), population density (0.36), runoff (0.26) and LULC (0.26) as the strongest discriminators, converging independently with the AHP weighting. The 47 High and Very High villages constitute a bounded priority set for field-based microplastic monitoring, dump-site remediation and urban runoff management.