Lightweight microplastics can be trapped in freshwater sediment and stay longest at “headwaters” or sources of rivers and streams, a new study has found.
It can then take up to seven years for such pollutants to travel just a kilometre further towards the sea or ocean, a study by researchers at the University of Birmingham, Northwestern University and Loyola University Chicago in North America calculates.
The research published in the journal Science Advances follows a similar study released last autumn by the University of Leicester.
As reported by Afloat, the University of Leicester research found microplastics may travel at less than 0.01km per hour.
The new research published this week notes that swirling river waters can trap lightweight microplastics that otherwise might be expected to float – depositing them in riverbeds.
“As rivers are in near-constant motion, researchers had previously assumed that lightweight microplastics were swept rather swiftly towards the ocean and rarely interacted with riverbed sediments,” the authors state.
They set out a new model describing processes that influence particles, including hyporheic exchange, involving widely abundant microplastics which are 100 micrometres in size and smaller.
The scientists used global data on urban wastewater discharges and river flow condition to discover that microplastic pollution resides the longest at the source of a river or stream - known as the ‘headwaters’ that are furthest away from the ocean.
In headwaters, microplastic particles move at an average rate of five hours per kilometre, but can then take up to seven years to move one kilometre under low-flow conditions.
Stefan Krause, professor of ecohydrology and biogeochemistry at the University of Birmingham, noted that the slow movement of microplastics downstream” makes it more likely that aquatic species ingest microplastics and propagate them through the food-web.
This can "potentially cause harm for environmental and public health”, Krause said.
“Our findings highlight that we need to develop strategies to reduce future microplastic inputs into rivers, and find effective solutions to remove the existing legacy of plastics from our rivers in order to restore freshwater ecosystems,” he said.
The study was led by Dr Jennifer Drummond at the University of Birmingham, and supported by a Royal Society Newton International Fellowship, Marie Curie Individual Fellowship, the German Research Foundation, the Leverhulme Trust and the National Science Foundation.
The paper ‘Microplastic accumulation in riverbed sediment via hyporheic exchange from headwaters to mainstems’ by Jennifer D. Drummond, Uwe Schneidewind, Angang Li, Timothy J. Hoellein, Stefan Krause and Aaron I Packman is published in Science Advances.
