This question used to be a classic on beach holidays: “Mom, Dad, how does the salt get into the sea?” Today, parents probably hear much more often: “Where does all that plastic come from?” The countless disposable cups and plastic lids, straws and flip-flops still on the most remote beaches, soft drink bottles even in Antarctica and plastic parts in deep-sea trenches. Not to forget the free-floating ghost nets in which birds, fish and dolphins get caught, as well as the many transparent bags that turtles fatally mistake for jellyfish and eat.
Now as then, the answer to the children’s question is almost identical. The salt is released from rocks by weathering processes and transported to the sea via streams and rivers, where, depending on the extent of evaporation, it accumulates to a greater or lesser extent or is deposited in sediments on the ground. Similarly, with plastic, while the remains of fishing nets come from boats and deep-sea fishing vessels, most other plastic ends up in the oceans from land. Sometimes waves take plastic from the shore, but rivers carry far more into the sea. There, the plastic floats with the currents around the world, is thrown onto distant beaches, crumbles into microplastics or finally sinks to the seabed.
According to calculations by researchers at the Norwegian environmental agency GRID-Arendal, which works closely with the United Nations Environment Program and whose data is quoted in the so-called plastic atlas of the Heinrich Böll Foundation and the German Federal Environment and Nature Conservation Agency, around 86 million tons of plastic waste have been discarded so far into the seas: 29 million of them lie on coasts or on the seabed, 23 million drift in coastal waters, 34 million in the open ocean and up to 440,000 tons near the sea surface – including in five large eddies in the Pacific, Atlantic and Indian Oceans .
A truckload per minute
The most recent and extensive global study focusing on the role of rivers in waste transport was published in the journal Science Advances in 2021 by a team of Dutch and German experts. According to this, rivers transport between 0.8 and 2.7 million tons of plastic waste into the oceans every year. To put it bluntly, it’s like a medium-sized truck tipping its load into the world’s oceans every minute.
“However, we only examined plastic parts that are larger than five millimeters,” reports Lourens Meijer, who led the study for the organization The Ocean Cleanup in Rotterdam and now works as director for sustainability at the electronics company Philips: “Microplastics under five millimeters was not in focus.” The reason is simple: Another study had shown that the input of microplastics through rivers, at almost 50,000 tons per year, is significantly lower than that of macroplastics. “Many parts are only ground into microplastics in the ocean,” explains the water management expert.
The effort involved in Meijer’s study was enormous: measurements were taken on 67 rivers on three continents over a period of several years. Some observers counted the plastic parts of bridges, some they caught the garbage with nets and noted their size and weight. These on-site investigations served to calibrate and optimize a highly complex computer model that includes all of the approximately 100,000 rivers that flow into the sea anywhere in the world. What’s more, the model covers the entire globe with an extremely fine grid, the elements of which each cover less than 100 by 100 meters of the earth’s surface.
For each of these billions of grid areas, a computer calculates the probability that plastic waste will end up in the next river and finally in the sea. For this, not only local data on population density, land use, the amount of plastic waste and the more or less successful waste management must be included. Equally important are the wind conditions and precipitation, the degree of inclination of the areas and the currents in the rivers and their distance from the ocean. Heavy monsoon rains and floods, for example, can wash a lot of garbage into the sea at certain times of the year.
Small rivers are the culprits
Around the world, around 70 million tons of plastic end up in open, unsupervised landfills or immediately as waste in the environment, but only a few percent of it drifts into the open sea. A previous 2017 study found that there are just 10 rivers in the world that dump most of the plastic waste into the oceans – including China’s Yangtze, Pakistan’s Indus, and Africa’s Nile and Niger. But the more recent study from 2021 comes to an amazingly different conclusion: “The real culprits are relatively small rivers that flow through highly polluted coastal cities,” reports Lourens Meijer.