The Mersey River Basin near Manchester, U.K., is the most plastic polluted watershed in the world, with more than half a million plastic particles per square meter of riverbed. That’s one of the most dramatic findings of the first global map of aquatic plastic pollution, published today in Nature Geoscience. When large storms flood rivers, the plastic collected there washes out to sea. That means rivers are a significant source of plastic polluting the world’s oceans, the study reveals.
To find out how plastic goes from land to sea, researchers counted plastic particles known as microplastics—tiny to microscopic bits of plastic made when sunlight breaks down large pieces of plastic—in the sediment of 10 rivers across 40 sites in the Mersey and Irwell river basins in urban, suburban, and rural northwest England before and after the 2015 Boxing Day Flood—the largest flooding event on record in the region. The flood, which removed all traces of plastic debris at seven of the sites, washed 70% of the plastic—that’s 43 billion particles or about 0.85 metric tons—of plastic out to sea, the scientists found.
When researchers examined the density of plastic pieces in the riverbeds, they found more than a third of the microplastics in the basins, or 17 billion particles, can float in seawater. The researchers estimate this single flooding event contributes 0.5% to the total floating plastic in the world’s oceans. That means the amount of plastic in the world’s oceans is greater than previously imagined, the researchers say. But management strategies like those recently passed in the United States and the United Kingdom that curb use of plastic microbeads—rounded plastic particles found in exfoliating facial washes—could alleviate plastic pollution in rivers, the team says.