Spatiotemporal patterns, loads, and risks of microplastics in the stormwater system of a subtropical coastal city

Urban stormwater runoff is a critical pathway for microplastics (MPs) to enter marine ecosystems, posing a significant threat. However, systematic studies on spatiotemporal patterns, loads, and risks of MPs in stormwater systems, particularly in subtropical coastal cities, remain limited. This study characterized MPs in Macao's stormwater drains across seasons, using density separation, H2O2 digestion, stereomicroscopy, and μ-FTIR analysis to investigate their abundance and distribution. MPs were ubiquitous across all sample sites, with abundance ranging from 0.86 to 53.03 n/L. Irregular shaped fragments (56.78 ± 0.13.80%) and the color black (38.44 ± 7.47%) were dominant, and the most common polymers were Polyethylene terephthalate (39.55 ± 16.63%), polypropylene (23.05 ± 18.15%), and polyethylene (9.52 ± 8.87%). While abundance of MPs was higher in the wet season, no significant seasonal variation was found. MP characteristics varied significantly with land use. The concentration range of MPs was estimated to be from 2.86 ± 0.38 to 3145.94 ± 766.74 μg/L, with an overall mean of 454.68 ± 572.20 μg/L through the mass concentration models. The daily MP emission load into the receiving water was estimated to range from 8.29 × 103 to 3.09 × 108 particles per day or 2.66 to 10,365.23 g/day. MP risk was high (Polymer Hazard Index/Potential Ecological Risk Index: III/V) due to hazardous polymers, with spatiotemporal variations further observed from fluctuating water flow erosion. Correlation analysis revealed that MP abundance showed a significant negative correlation with dissolved oxygen and a positive correlation with antecedent rainfall. These findings identify stormwater drains as a key MP conduit, influenced by rainfall and land use, calling for targeted management in urban coastal areas.