According to a new study, microplastics can deposit and remain in riverbeds for up to seven years before washing into the ocean. Researchers originally considered that lightweight microplastics moved quickly through rivers, rarely interacting with riverbed sediments, due to rivers’ near-constant motion.
Hyporheic exchange, a process in which surface water mixes with water in the riverbed, has now been discovered to trap lightweight microplastics that would otherwise float, according to researchers from Northwestern University and the University of Birmingham in England.
The research was published in the journal Science Advances today (Jan. 12). It’s the first study of microplastic accumulation and residence times in freshwater systems, with data collected from sources of plastic contamination across the entire water stream.
The new model focuses on hard-to-measure but abundant microplastics at 100 micrometers and smaller and describes dynamical processes that influence particles, such as hyporheic exchange.
“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.”
These deposited microplastics cause ecological damage, and a 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. This information points us to consider whether we need solutions to remove these plastics to restore freshwater ecosystems.
Aaron Packman
“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 Engineering and director of the Northwestern Center for Water Research. He is also a part of Northwestern’s Institute for Sustainability and Energy’s Program on Plastics, Ecosystems, and Public Health. The study’s first author is Jennifer Drummond, a research fellow at the University of Birmingham and a former Ph.D. student in Packman’s group.
Modeling microplastic movement
Packman, Drummond, and their research teams created a new model to predict how individual particles enter freshwater systems, settle, and then remobilize and redistribute.
The model is the first to incorporate hyporheic exchange activities, which are important in the retention of microplastics in rivers. Despite the fact that the hyporheic exchange process has an impact on how natural organic particles move and flow in freshwater systems, it is rarely thought of as a source of microplastic accumulation.
“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.”
The researchers used global data on urban wastewater discharges and river flow conditions to run the model.
Trapped in headwaters
The researchers discovered that microplastic contamination stays the longest at the source of a river or stream using the new model (known as the “headwaters”). Microplastic particles migrated at a rate of five hours per kilometer in headwaters.
During times of low flow, however, this movement slowed to a crawl, taking up to seven years to move one kilometer. Organisms are more likely to swallow microplastics in the water in these places, thereby compromising ecosystem health.
As microplastics traveled downstream from the headwaters, their residence time decreased. Large creeks had the shortest residency durations.
Packman expects that now that this data is available, researchers will be able to better analyze and understand the long-term effects of microplastic pollution on freshwater systems.
“These deposited microplastics cause ecological damage, and a 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.”
The study, “Microplastic accumulation in riverbed sediment via hyporheic exchange from headwaters to mainstems,” was supported by a Royal Society Newton International Fellowship, Marie Curie Individual Fellowship, the German Research Foundation, the Leverhulme Trust, and the National Science Foundation.
