Solid waste management practices in riparian communities are a key driver of microplastic concentrations in the Mekong River, Cambodia

Rivers are major conduits of plastic pollution, however the role of community level waste collection on riverine microplastic concentrations remains poorly understood. We investigated seasonal patterns and drivers of microplastic pollution along the Lower Mekong River, Cambodia, with a focus on riparian waste collection services. Surface water samples were collected during dry and wet seasons using a pump and filter method (100 µm minimum aperture), and more than 15,000 particles were chemically identified using Fourier Transform Infrared spectroscopy (FTIR). Statistical models evaluated the relationships between microplastic concentrations and upstream waste collection availability, population, household income, and season. Microplastic concentrations varied across sites and showed significant seasonal differences in particle size, morphology, and polymer composition. Dry season samples were dominated by small (<100 μm), diverse polymers, whereas wet season samples contained larger (<300 μm) particles associated with single-use plastics, consistent with monsoon-driven mobilization of household waste. Across seasons, microplastic concentrations showed a strong negative correlation with upstream waste collection availability. Model results indicate that modest increases in waste collection access in low-service riparian communities could reduce downstream water surface microplastic concentrations by approximately 50%. These findings identify waste collection infrastructure as a critical, actionable control for reducing riverine microplastic pollution.