Microplastics ‘can linger in rivers for years’, study finds
Tiny fragments of plastic pollution known as microplastics could linger in riverbeds for years, scientists have warned.
Researchers had previously assumed that the tiny fragments flowed rapidly through rivers to the sea. They said that the lingering plastic pollution could damage life in rivers.
But a new study, conducted at Northwestern University and the University of Birmingham, has found that plastic fragments can linger in riverbeds for up to seven years.
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Lead author Aaron Packman. of Northwestern University, said: "Most of what we know about plastics pollution is from the oceans because it's very visible there.
"Now, we know that small plastic particles, fragments and fibres can be found nearly everywhere. However, we still don't know what happens to the particles discharged from cities and wastewater.
“Most of the work thus far has been to document where plastic particles can be found and how much is reaching the ocean.
"Our work shows that a lot of microplastics from urban wastewater end up depositing near the river's source and take a long time to be transported downstream to oceans."
The researchers found that hyporheic exchange - a process in which surface water mixes with water in the riverbed - can trap lightweight microplastics that otherwise might be expected to float.
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The researchers developed a new model to simulate how individual particles enter freshwater systems, settle and then later remobilise and redistribute.
"The retention of microplastics we observed wasn't a surprise because we already understood this happens with natural organic particles," Packman said.
"The difference is that natural particles biodegrade, whereas a lot of plastics just accumulate. Because plastics don't degrade, they stay in the freshwater environment for a long time - until they are washed out by river flow."
To run the model, the researchers used global data on urban wastewater discharges and river flow conditions.
Using the new model, the researchers found microplastic pollution resides the longest at the source of a river or stream (known as the "headwaters").
During low-flow conditions, this movement slowed to a crawl - taking up to seven years to move just one kilometre.
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In these areas, organisms are more likely to ingest microplastics in the water, potentially degrading ecosystem health.
The residence time decreased as microplastics moved away from the headwaters, farther downstream.
Packman hopes researchers can better assess and understand the long-term impacts of microplastic pollution on freshwater systems.
"These deposited microplastics cause ecological damage, and the large amount of deposited particles means that it will take a very long time for all of them to be washed out of our freshwater ecosystems," he said.
"This information points us to consider whether we need solutions to remove these plastics to restore freshwater ecosystems."
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