Decoding microplastic risks through land use: A multi-index and spatial analysis in a tropical watershed

Microplastic (MPs) pollution has become a critical environmental concern in freshwater ecosystems due to its persistence, ubiquity, and ecological toxicity. This study investigates the abundance, characteristics, and ecological risks of MPs in surface water and sediments of the Gembong River, a small tropical watershed in East Java, Indonesia, characterized by a distinct land-use gradient from upstream agricultural to downstream urban areas. A total of 26 sites were sampled during the dry season to minimize hydrological variation. MPs were extracted using density separation and Wet Peroxide Oxidation (WPO), visually identified under a stereomicroscope, and characterized using Fourier-transform infrared spectroscopy (FTIR). The results revealed that MPs concentrations were higher in sediments (mean: 406 particles/100 g) than in surface water (mean: 141 particles/L), with increasing abundance from upstream to downstream. Fibers dominated surface water (40 %), while fragments were prevalent in sediments (36 %). Transparent MPs were most common in both matrices, and particles sized 100–500 µm were the most frequently detected. Polyethylene (PE) and polyvinyl chloride (PVC) were the most abundant polymers. Redundancy analysis (RDA) showed strong associations between residential land use and MPs traits such as fragment shape, transparent color, and intermediate particle size. To quantify ecological risk, three indices—Polymer Hazard Index (PHI), Pollution Load Index (PLI), and Potential Ecological Risk Index (PERI)—were applied. Sediment samples exhibited higher PHI and PERI scores than surface water, with downstream sites reaching “considerable to very high risk” categories. These findings underscore the role of land use in shaping MPs distribution and highlight the urgent need for integrated risk management strategies in small-scale riverine systems.