Rivers may be carrying more microplastics into the world's oceans than previously thought - Earth - Earth Sciences - Environment

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Microplastics are a modern environmental menace and a potential threat to human health. These tiny pieces of plastic, measuring from 1 to 5,000 micrometers, come mainly from larger plastics as well as sources such as paints and cosmetics. They are found in almost every corner of the planet, and a new study published in Science suggests rivers may be carrying far more of them into the ocean than previously thought.

Many microplastics make their way from land into rivers and ultimately, the oceans. Once there, they can disrupt coastal ecosystems and food webs and be ingested by marine animals.

Estimates of the amount of microplastics flowing from rivers to oceans have varied enormously over the years, ranging from 6,100 to 2.31 million metric tons annually. Reasons for the wide gap include a lack of field data, differences in sampling methods and computer models that failed to capture sudden changes caused by rainfall.

"Rivers are key conduits in the global microplastic cycle, but how much they export to the ocean and what controls this export remain unresolved," the study authors wrote in their paper.

Building a better model

Led by Hehao Qin of Peking University, the team adopted a more comprehensive approach that tracks river conditions daily. The researchers developed a computer model that estimates how much microplastic flows from these waterways to oceans.

This new model addresses some of the core problems that plagued earlier estimates. It uses a mathematical adjustment to standardize measurements across different net sizes and sampling methods. It also tracks daily rainfall and river flow, allowing it to capture sudden changes that earlier models could miss.

It was trained on 1,386 real-world microplastic measurements collected from rivers worldwide. To make predictions, it combines this information with daily weather patterns, land-use data and river flow to calculate particle counts for rivers around the world. It can then convert these totals into metric tons.

A much bigger problem

Using their new model, the research team calculated that rivers dumped roughly 263,000 metric tons of microplastics into the world's oceans in 2022. Around 96% of that plastic originated from countries in the Global South, with Southeast and East Asia accounting for more than half. And the river which empties the most microplastic waste into the sea? The Yangtze River.

The team also uncovered something that past global models struggled to capture: the impact of rainfall. In many watersheds, heavy downpours can cause sudden spikes in microplastic levels, triggering what the scientists call pulse enrichment. Intense storms can flush microplastics from land into rivers and stir up plastic already trapped in soils and riverbeds.

Based on their findings, the team offers a stark warning: "Without stronger upstream controls on plastic production and waste management, climate-driven increases in extreme rainfall may intensify these pulses, worsening marine microplastic pollution." 

by Paul Arnold, Phys.org

edited by Lisa Lock, reviewed by Robert Egan 

Source: Rivers may be carrying more microplastics into the world's oceans than previously thought