“For the ocean science community, it’s important to understand how plastic debris spreads in the oceans,” Schmidt told Seeker. “[Until now] it has been sufficient to know that 80 percent of ocean plastic is generated from land, but current research is more directed to the sources of the problem.”

The problem of our plastic-filled oceans has been studied for years, but until now there has been little information on how the plastic gets there. Christian Schmidt, a hydrogeologist at the center who led the study, believes that to get plastic out of the ocean, we must first figure out how it is getting in.

Researchers from the Helmholtz-Center for Environmental Research in Germany, think they’ve nailed down a crucial detail about plastic pollution: how it gets into the ocean. And, they found, just 10 rivers account for 90 percent of the plastics flowing into the seas. Their study is published in the journal Environmental Science & Technology.

Over the last six decades, according to one estimate, 9.1 billion tons of plastic has been produced worldwide and 7 billion tons of it has ended up as waste. Much of that waste has infiltrated our oceans. And, according to another estimate, more than 8 million tons of plastic is dumped into the sea each year.

There are extremely small plastic particles found in virtually every sea and river in the world. Plastic particles have even been found in the tap water of more than a dozen countries, including the US. Because plastic essentially never breaks down completely in the ocean, it is extremely harmful to marine ecosystems. It often kills fish, birds, and marine mammals when they mistake it for food.

It would be impossible to clean all the plastic already present in the ocean, Schmidt noted, so the only solution is to quickly reduce the input in the hope of preventing further harm.

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Schmidt and his colleagues analyzed 73 previous studies on aquatic systems around the world. Each study included information on the quantity of plastic that had been improperly disposed of in various catchment areas — the location where rainfall flows into a river. The analysis included 240 individual water samples from 79 sites, covering 57 rivers.

Microplastics, particles less than five millimeters in length, were detected in 98.5 percent of catchment area samples and large plastic particles were found in 55 percent. The amount of waste that ended up in the ocean, the researchers found, corresponded to the amount of plastic found in a catchment area. And larger rivers made a much more significant contribution than smaller ones because the quantity of plastic per cubic meter of water was much higher in large ones.

Just 10 river systems are responsible for 90 percent of the plastic flowing into the oceans. Eight are in Asia and two are in Africa. Ranked from the highest amount of plastic waste to the lowest, they are: the Yangtze River, Indus River, Yellow River, Hai He River, Nile River, Ganges River, Pearl River, Amur River, Niger River, and the Mekong River.

The 10 river systems have several common characteristics. They run adjacent to large concentrations of people, where public awareness about proper waste disposal and recycling is lacking, as is sufficient waste infrastructure.

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According to Schmidt, even halving the amount of plastic waste in rivers would be a major achievement.

“Implementing better waste management is of paramount importance, as well as reducing plastic inputs into the environment, including rivers,” Schmidt said. “But improving waste infrastructure should go hand in hand with actions to make people aware that whatever is dumped into rivers will not disappear. It will potentially end up in the oceans,” he said.

Schmidt pointed to one effort in India that has banned the use of disposable plastic in the capital of Delhi as a good example of how to reduce plastic pollution. But, he cautioned, more developed countries certainly aren’t innocent of polluting with plastic. “In industrial countries, it is assumed that 2 percent of the waste is littered, despite a well-developed waste infrastructure.”

Schmidt and his colleagues plan to find out how long plastic debris takes to reach the sea once it’s in our rivers, information that is crucial for understanding how to stop it.

“One of our key questions is, ‘How long does plastic need to travel from the source to the oceans?’” he said. “This is important because, and I’m optimistic here, we will see actions to reduce inputs into our river systems soon, but it’s unclear how long it will take until we see an effect at the river mouths. It could be months, years or even decades.”