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.

But researchers from the University of Birmingham in England and Northwestern University and Loyola University Chicago, in the United States, have discovered that hyporheic exchange — a process in which surface water mixes with water in the riverbed — can trap lightweight microplastics in sediment.

Publishing their findings in Science Advances, the experts set out a new model describing processes that influence particles, including hyporheic exchange, and focuses on hard-to-measure but widely abundant microplastics at 100 micrometres in size and smaller.

The research marks the first assessment of microplastic accumulation and residence times within freshwater systems, from sources of plastic pollution throughout the entire stream, from the headwaters of a river to its confluence to the sea.

Stefan Krause, Professor of Ecohydrology and Biogeochemistry at the University of Birmingham, commented: "We've learned that rivers can store microplastics for a long time as they wash downstream to the ocean - up to seven years to travel just one kilometre.

"Lightweight microplastics accumulate substantially in riverbed sediments, remaining trapped for up to many years. Their slow movement downstream makes it more likely that aquatic species ingest microplastics and propagate them through the food-web, potentially causing harm for environmental and public health.

"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."

Researchers developed a new model to simulate how individual particles enter freshwater systems, settle and later remobilise and redistribute. The model is the first to include hyporheic exchange processes, which play a significant role in retaining microplastics within rivers.

Although it is well-known that the hyporheic exchange process affects how natural organic particles move and flow through freshwater systems, the process has rarely been considered in the context of microplastic accumulation.

The scientists used global data on urban wastewater discharges and river flow conditions, discovering 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 it can take them up to seven years to move one kilometre under low-flow conditions. The residence time decreased as microplastics moved away from the headwaters.

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.