Low abundance but high microplastic toxicity and potential ecological risks in a drinking-water source

Microplastics (MPs) in drinking-water sources pose potential health risks, yet their ecological threats under low-intensity anthropogenic activities remain poorly understood. To address this, this study focused on the Miyun Reservoir (Beijing's primary drinking-water source), and systematically collected surface water and sediment samples from the reservoir, its upstream protection zones, and downstream reaches. MP abundance and characteristics were determined using stereomicroscopy and micro-Fourier transform infrared spectroscopy. Potential sources and fragmentation characteristics were identified by comprehensively applying characteristic-based source identification, diversity index, and a conditional fragmentation model. Ecological risks were evaluated using pollution load index (PLI), polymer hazard index (PHI), and potential ecological risk index (PERI). Results showed low MP abundance (water: 0.56 ± 0.36 items/L; sediment: 74.48 ± 54.91 items/kg), dominated by fibers and films (<1 mm). Major polymers included rayon, polyethylene, polypropylene, and polyethylene terephthalate. Domestic wastewater and agriculture were considered as the primary potential contributors. PLI indicated Level I (lowest risk) at all sites, whereas PHI and PERI revealed high risks at multiple sites, and correlated positively with agricultural and built-up land area, underscoring land-use control over toxicity. Within the reservoir, all sites except R2 had PHI/PERI values at Levels IV-V, exceeding upstream river sites located at the inflow, thus displaying a pattern of low abundance but high toxicity and potential ecological risks. Effective management in drinking-water source areas should therefore prioritize reducing high-hazard inputs over merely lowing MP abundance. Notably, omitting rayon from current hazard frameworks may underestimate actual ecological risks.