10 rivers on Earth may be responsible for around 90% of oceanic plastic pollution in the World.
Plastic waste has become a major environmental concern because of the long-term persistence in oceans around the globe, and the adverse consequences to marine life and human health.
Almost four million tonnes of plastic are dumped into the seas each year.
But establishing the source of the pollution remains difficult. As well as calculating with precision the amount of plastic waste in our oceans.
Estimates tend to cover wide ranges, but previous research found that about 1/5th of total ocean plastic trash is tossed from fishing boats, ships, drilling platforms…
The remaining 4/5ths are carried from land to the sea by some of the World’s main waterways.
The Joys of Plastic
Since the 20th Century, plastic use has increasingly been used in wide-ranging applications, with a global production exceeding 300 million tonnes per year since 2014.
Unfortunately, due to the durability of the product, low-recycling rates, poor waste management and its use in the maritime industry, a massive portion of plastics produced around the World, enters and persists in marine ecosystems.
Release of plastics into the sea occurs through a variety of pathways, including river and atmospheric transport, beach littering and via aquaculture, shipping and fishing activities.
Those polluted ecosystems include shoreline, seabed, water column and sea surface environments.
From Rivers to the Seas
A paper published in the journal Environmental Science and Technology by scientists from the Helmholtz Centre for Environmental Research and the Weihenstephan-Triesdorf University of Applied Science, both in Germany, calculates that rivers contribute between 410,000 and four million tonnes a year to oceanic plastic debris, with 88 to 95% coming from only 10.
Plastic debris loads, both microplastic (particles <5 mm) and macroplastic (particles >5 mm) are correlated to the mismanaged plastic waste (MMPW) generated in river catchments, where large rivers with population-rich catchments delivering a disproportionately higher fraction of MMPW into the sea.
The river catchments most at fault are the
- Yangtze, Xi and Huanpu (China)
- Ganges (India)
- Cross (Cameroon and Nigeria)
- Brantas and Solo (Indonesia)
- Amazon (Brazil)
- Pasig (Philippines)
- Irrawaddy (Myanmar).
Mismanaged Plastic Waste
Using MMPW as a predictor, the authors calculated that the global plastic debris dumped from rivers into the World’s oceans average between 0.41 and 4 × 106 tons per year.
The researchers note that making more precise estimates of how much plastic pollution is ending up in our oceans is bedevilled by unknowns, including the “missing plastic” problem.
Plastic concentrations in different parts of the ocean can differ significantly.
The “Missing Plastic” Problem is the obvious mismatch between the large estimated plastic inputs into the sea – based on what is known about plastics produced, used, recycled and dumped in landfill – and the amount actually observed in the water.
The researchers used a similar methodology employed in an earlier study, published in Nature Communications, based on work by researchers from the Netherlands-based Ocean Cleanup project.
This study calculated 1.15 to 2.41 million tonnes of plastic waste is now entering the ocean every year from rivers; 67% of the global total, they suggested, came from the top 20 polluting rivers.
The German team, led by Christian Schmidt of the Helmholtz Centre, compiled a larger data set – analysing plastic concentrations in 240 individual samples from 79 sites covering 57 rivers.
They classified plastic particles into those smaller or larger than five millimetres in size, and treated each cohort separately to make their calculations.
The scientists concluded that only a few river catchments contribute to the vast majority of the total plastics load.
Therefore, targeted mitigation measures would be highly efficient at tackling the problem.
“Reducing plastic loads by 50% in the 10 top-ranked rivers would reduce the total river-based load to the sea by 45%,” they stated.