Microplastics can be deposited and remain in river beds for up to seven years before reaching the ocean, a new study reveals.

Because rivers are in almost constant motion, researchers previously assumed that lightweight microplastics flowed rapidly through rivers and rarely interacted with riverbed sediments.

Now, researchers led by Northwestern University and the University of Birmingham have discovered that hyporheic exchange, a process in which surface water mixes with water in the riverbed, can trap lightweight microplastics that would otherwise be expected to they will float

The study was published in the journal Science Advances. It marks the first assessment of microplastic accumulation and residence times within freshwater systems, from sources of plastic pollution throughout the watercourse. The new model describes dynamic processes that influence particles, including hyporheic exchange, and focuses on hard-to-measure but abundant microplastics 100 micrometers in size and smaller.

"Most of what we know about plastic pollution comes from the oceans because it’s so visible there."Northwestern’s Aaron Packman, one of the study’s lead authors, said in a statement. "Now, we know that tiny plastic particles, fragments, and fibers can be found almost everywhere. However, we still don’t know what happens to the particles discharged from cities and sewage. Most of the work so far has involved documenting where the plastics are found. particles can be found and how much is reaching the ocean.

"Our work shows that a large number of microplastics from urban wastewater end up depositing near the river source and take a long time to be transported downstream to the oceans.".

To conduct the study, the researchers developed a new model to simulate how individual particles enter freshwater systems, settle, and then remobilize and redistribute.

The model is the first to include hyporheic exchange processes, which play an important role in the retention of microplastics in rivers. Although it is well known that the hyporheic exchange process affects the way natural organic particles move and flow through freshwater systems, the process is rarely considered microplastic accumulation.

"The retention of microplastics that we observed was not a surprise because we already knew that this happens with natural organic particles."Packman said. "The difference is that natural particles biodegrade, while many plastics simply accumulate. Because plastics do not degrade, they remain in the freshwater environment for a long time, until they are carried away by the flow of the river.".

To run the model, the researchers used global data on urban wastewater discharges and river flow conditions.

Using the new model, the researchers found that microplastic pollution resides longest at the source of a river or stream (known as the "headers"). At the headwaters, microplastic particles moved at an average speed of five hours per kilometer. But during low-flow conditions, this movement slowed down, taking up to seven years to move just one kilometer. In these areas, organisms are more likely to ingest microplastics in the water, which could degrade the health of the ecosystem.

Residence time decreased as the microplastics moved away from the headwaters, downstream. And the residence times were shorter in the larger streams.

Now that this information is available, Packman hopes researchers can better assess and understand the long-term impacts of microplastic pollution in freshwater systems.

"These deposited microplastics cause ecological damage, and the sheer number of deposited particles means it will take a long time to remove them from our freshwater ecosystems.", He said. "This information tells us that we need to consider whether we need solutions to remove these plastics to restore freshwater ecosystems.".

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