Long dry periods may trigger microplastic formation in seasonal rivers

2024年07月31日13:48 来源：cen

Scientists are only now starting to understand whether plastics degrade more when riverbeds run dry

Credit: Shutterstock

Scientists remain unsure about how droughts in rivers affect plastic pollution. A new study suggests that the drying up of riverbeds could increase plastic weathering caused by the sun’s ultraviolet radiation, resulting in increased microplastic formation (Environ. Sci. Technol. 2024, DOI: 10.1021/acs.est.4c00528).

Intermittent rivers and ephemeral streams (IRES)—waterways that dry out periodically across seasons or years—exist in every continent and climate type across the world. In the past 5 decades, many perennial rivers and streams have become intermittent because of climate change, changes in land use patterns, and natural causes. Yet researchers have not studied the intermittent ecosystems to the same extent as their perennial counterparts.

During dry events, the water column no longer shields the plastic waste in IRES from direct sunlight, so the materials become weakened, says Nans Barthélémy, a PhD student at the French National Center for Scientific Research and the paper’s lead author. When the water flow returns after a dry period, a process known as rewetting, fluid pulses carrying sediments and organic matter exert mechanical forces on the already-embrittled plastics, causing them to fragment into microplastics. Therefore, the researchers say, IRES that remain dry for long periods could be hot spots of plastic fragmentation.

In laboratory experiments, the researchers exposed polyvinyl chloride (PVC) disks to varying dry durations over 6 days—from 0 (no dry days) to 6 (all dry). The PVC was exposed to UV radiation simultaneously. After the simulated drying event, the plastic was combined with water and sediments, and the combination was rigorously mixed to re-create the harsh rewetting events in IRES. The researchers then measured the mass loss in the PVC disks as well as the size and abundance of microplastics.

The plastic exposed to up to 4 days of dry conditions showed no differences in fragmentation after the simulated rewetting process. But when the dry duration exceeded the 4-day threshold, fragmentation increased.

To have a point of comparison, the researchers also performed the experiment in a river catchment—an area that collects rainwater that drains into a lake, river, or reservoir—during a dry period. “Every week, I took some subsamples and put them through the fragmentation process,” Barthélémy says. The researchers found that 12 weeks of direct sunlight exposure produced similar abundances and sizes of microplastic as obtained after 6 full days of laboratory experiments using a UVC lamp.

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