Floods can flush out large quantities of microplastic pollution from riverbeds, reports a study published online this week in Nature Geoscience.

Microplastics are plastic particles that are less than 5 mm in length. Some are deliberately manufactured for the cosmetics industry, while others form when larger plastics break apart. Microplastics contaminate the world’s oceans, threatening both the environment and ecosystem. Although approximately 90% of microplastic contamination in the oceans is thought to originate from land, not much is known about the role rivers play in this process.

Rachel Hurley and colleagues examined the microplastics in river sediments from 40 sites across northwest England. They found microplastics in all riverbeds, even in the smallest rural streams, and identified various urban pollution hotspots with concentrations of up to 517,000 plastic particles per square metre. After severe flooding in the winter of 2015/16, the authors sampled the same locations again. They found that the microplastic pollution had decreased at 70% of the sites. The flooding transported away around 70% of the total microplastic load from the riverbeds - amounting to about 0.85 tonnes of plastic - and flushed all microbead pollution from seven of the sites.