Peking University, September 24, 2026: Rivers around the world carried about 260,000 metric tons of microplastics into the ocean in 2022, according to a new study led by researchers at Peking University (PKU). Published in Science, the study finds that this pollution is not released at a steady rate throughout the year. Instead, heavy rainfall and floods can trigger short, concentrated surges of microplastic transport.
Led by Professor Wang Xuejun and researcher Liu Maodian of PKU's College of Urban and Environmental Sciences, the team used river microplastic observations to model global patterns and daily changes in microplastic transport from rivers to the ocean.
Why it matters Microplastics are now widespread across the world's oceans and may affect marine ecosystems and human health through food webs. Rivers are an important pathway for land-based microplastics entering the ocean, yet reliable global estimates of how much they transport—and when this pollution reaches the sea—have remained limited.
The new study shows that both geography and timing matter: just 20 rivers accounted for about half of the global riverine microplastic export to the ocean.
Key findings Across most river basins worldwide, higher rainfall and runoff were associated with higher microplastic concentrations. Rather than simply diluting pollution, intense rainfall can wash plastic particles accumulated on urban surfaces, in soils, and along riverbanks into waterways, creating short periods of concentrated transport, or "pulses."
The effect is especially pronounced during rainy seasons. About 70 percent of annual transport occurred during the rainy season on average. The researchers suggest that models relying on annual or monthly average hydrological conditions may therefore underestimate the contribution of short-term extreme events.
The impact of rainfall also differs across regions. In areas where plastic use is growing rapidly and waste and wastewater treatment infrastructure remains relatively limited, rainfall may be more likely to raise river microplastic concentrations. In some regions with more developed infrastructure, rainfall may instead mainly have a dilution effect.
The study also found that greater vegetation cover was generally associated with lower river microplastic concentrations, while urbanized surfaces may facilitate the movement of plastic into rivers through surface runoff.
Future implications The findings suggest that future plastic pollution control should focus more on high-emission river basins in the Global South, while strengthening pollution interception and management around rainy seasons and extreme rainfall events.
As climate change may further alter patterns of extreme rainfall, flooding, and monsoons, the influence of short-term hydrological events on plastic transport from land to rivers and oceans may also change. Understanding these pollution pulses could improve predictions of global marine plastic pollution and provide a stronger scientific basis for more targeted interventions.
*This article is featured in PKU News "Why It Matters" series. More from this series. Read more: https://www.science.org/doi/10.1126/science.aeb4487
Written by: Ashley Leung Edited by: Chen Shizhuo Source: PKU News (Chinese)
