Because rivers are in near-constant motion, researchers previously assumed lightweight microplastics quickly flowed through rivers, rarely interacting with riverbed sediments. With new simulations, researchers now have discovered 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.

Because rivers are in near-constant motion, researchers previously assumed lightweight microplastics quickly flowed through rivers, rarely interacting with riverbed sediments.

Now, researchers led by Northwestern University and the University of Birmingham in England, have found 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.

The study was published today (Jan. 12) 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 entire water stream. The new model describes dynamical processes that influence particles, including hyporheic exchange, and focuses on hard-to-measure but abundant microplastics at 100 micrometers in size and smaller.

«Most of what we know about plastics pollution is from the oceans because it’s very visible there,» said Northwestern’s Aaron Packman, one of the study’s senior authors. «Now, we know that small plastic particles, fragments and fibers 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.»

Packman is a professor of civil and environmental engineering at Northwestern’s McCormick School of Engineeringand director of the Northwestern Center for Water Research. He also is a member of the Program on Plastics, Ecosystems and Public Healthat the Institute for Sustainability and Energy at Northwestern. Jennifer Drummond, a research fellow at the University of Birmingham and former Ph.D. student in Packman’s laboratory, is the study’s first author.

Story Source:
Materials provided by Northwestern University. Original written by Amanda Morris. Note: Content may be edited for style and length.