Study maps export of microplastics by rivers

A research team led by scientists from Peking University has developed the world's first global daily dataset of riverine microplastic concentrations and exports, revealing that rivers deliver far more microplastics to the ocean than most previous estimates suggested.

The study, published in the journal Science on Sept 11, highlighted the urgency of curbing plastic production at the source and improving waste management, particularly as climate-driven extreme rainfall is expected to intensify the transport of microplastics into the ocean.

Rivers are key conduits in the global microplastic cycle, carrying plastic particles through waterways and into marine environments, where they can move through food webs and pose risks to ecosystems and human health. However, despite the importance of understanding riverine microplastic exports and the factors that drive them for pollution control and climate adaptation, their global scale has remained uncertain.

To address this knowledge gap, the research team compiled a global dataset by standardizing diverse observations of riverine microplastics. The researchers harmonized different sampling methods, mesh sizes and reporting units, then developed a global model incorporating daily data on runoff, rainfall, land use, wastewater treatment coverage and plastic waste generation.

The results showed that global rivers transported about 263,000 metric tons of microplastics to the oceans in 2022. The exports were highly concentrated, with just 20 rivers accounting for about 50 percent of the total. The Yangtze River was the single largest contributor.

The study also found that rivers in developing countries accounted for 96 percent of global riverine microplastic exports. Southeast and East Asia, which together cover only 14 percent of the world's land area, contributed more than half of the total.

"The results have greatly narrowed the uncertainty of previous estimates about global total river exports. By introducing more robust observational constraints and finer modeling resolution, our study has also reduced the uncertainty of microplastic transport to the ocean in key basins," Liu Maodian, a co-corresponding author of the study and a researcher at Peking University's College of Urban and Environmental Sciences, told Guangming Daily.

Liu added that although the Yangtze remains the largest single contributor, the model has revised downward earlier estimates of its microplastic exports.

The study further revealed that riverine microplastic exports vary considerably over time. In the rivers with the highest exports globally, about 70 percent of annual exports occur during the rainy season.

Heavy rainfall and resulting high flows can sharply increase microplastic concentrations and transport in rivers by washing accumulated plastic from urban surfaces, farmland and riverbanks into waterways. They can also re-suspend particles that have settled on riverbeds, carrying them downstream toward the ocean.

The researchers noted that previous models based on annual or monthly average hydrological conditions may underestimate the contribution of short-lived extreme events to global riverine microplastic exports.

These intense pulses of transport are particularly pronounced where heavy rainfall coincides with high plastic consumption, inadequate waste management and efficient runoff generation, the study found.

The findings suggest that global riverine microplastic pollution is driven not by any single factor but by the combined effects of human activities and hydroclimatic processes. They also help explain why developing countries account for the overwhelming majority of riverine microplastic exports.

Liu said future pollution control efforts should prioritize high-emission river basins and major rivers in countries of the Global South. Measures should focus on intercepting and managing plastic waste before and after the rainy season and extreme rainfall events, rather than allocating resources solely according to annual average emissions.

Wang Xuejun, another co-corresponding author of the study, told Guangming Daily that as climate change continues to alter global patterns of extreme rainfall, floods and monsoon activity, the effects of short-term hydrological events on the movement of land-based plastics into rivers and oceans may also change.

Understanding these transport mechanisms is essential to improving predictions of global marine plastic pollution and providing a scientific basis for more targeted and effective pollution control strategies, Wang said.