80% of Plastic in the Ocean Comes From 1,656 Rivers

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Plastic pollution is a severe threat to the world’s waterways and oceans. Most of the plastic ever produced in the world has never been recycled and is discarded either into landfills or makes its way into the natural environment.

Most of the plastic that enters the ocean is not from fishing materials but comes from land-based sources. A team of researchers with the Ocean Cleanup Project and academic research institutions analyzed where geographically rivers are the biggest sources of plastic pollution entering the ocean.

While previous studies have attempted to analyze plastic emissions from the world’s river, this study attempted to refine estimates of which rivers are the biggest sources of those emission through a combination of field observations and a distributed probabilistic model.

Various types and sizes of plastics collected from the Kinnickinnic River, Milwaukee, WI. Photo: S. Mason, State University of New York at Fredonia. Public domain. Source: USGS.
Various types and sizes of plastics collected from the Kinnickinnic River, Milwaukee, WI. Photo: S. Mason, State University of New York at Fredonia. Public domain. Source: USGS.

To develop the model, researchers looked at drivers of plastic transport such as wind, runoff, and river discharge and then factored in other variables such as land use, terrain, and proximity of settlements along rivers close to ocean access. The model was then calibrated against field observations (observers stand near rivers and counting the number and types of macroplastics floating by). The study did not calculate for the presence of microplastics.

The model analyzed 100,887 outlets of rivers and streams (only rivers with a discharge above 0.1 m3/s  were considered) and found that, of those,  31,904 emitted plastic pollution into the ocean of at least  0.1 metric tons per year.

Geography of Plastic River Pollution

Geography played a significant role in determining which rivers had a higher plastic pollution emission. The probability of a river emitting plastic pollution ranged from “0% for land-locked regions and up to 80% for coastal urban centers located near a river.”

Researchers found that the presence of high precipitation rates, a higher proportion of paved surfaces, and proximity of urban settlements to a river was a higher predictor of plastic pollution emission than the size of the river basin.

Map showing the geospatial distribution of plastic entering the ocean through rivers. The 1656 rivers accountable for 80% of the total influx are presented. The gray shading indicates the probability for plastic entering the ocean [P(E)] on a 10 × 10–km resolution. Map: Meijer et al., 2021.
Map showing the geospatial distribution of plastic entering the ocean through rivers. The 1656 rivers accountable for 80% of the total influx are presented. The gray shading indicates the probability for plastic entering the ocean [P(E)] on a 10 × 10–km resolution. Map: Meijer et al., 2021.

Researchers point out this example comparing the Ciliwung River in Indonesia with the Rhine River in Western Europe shows how these factors result in a higher rate of plastic emission. Even though the Ciliwung River basin (591 km2) is much smaller than the Rhine River basin (163,000 km2) and generates less plastic waste (19,590 metric tons/year  versus 34,440 metric tons/year), the amount of plastic being emitted from the Ciliwung River was substantially higher both the observed (308 MT year) and modeled (205 MT year) than the Rhine River (3 MT year observed and 5.4 MT year modeled).

The researchers point to a close proximity of waste generation to the river, closer proximity of the river to the ocean, and a higher annual precipitation in Indonesia as factors for why plastic emissions are much higher for the Ciliwung River.

In total, researchers found that 1,656 of the world’s rivers are responsible for 80% of plastic emissions into the ocean. These rivers represent a variety of discharge rates.

The study found the countries with a small surface area compared to the length of their coastline coupled with high precipitation rates were more likely to have a higher emission rate of plastic pollution. Archipelago countries like the Philippines and Indonesia as well as areas in the Caribbean were such countries with higher plastic emission rates.

Visit: Interactive map of the world’s highest plastic polluting rivers.

Map of total emitted plastic into the ocean ME (MT year−1) per country. Map: Meijer et al., 2021.
Map of total emitted plastic into the ocean ME (MT year−1) per country. Map: Meijer et al., 2021.

The Study:

Meijer, L. J. J., van Emmerik, T., van der Ent, R., Schmidt, C., & Lebreton, L. (2021). More than 1000 rivers account for 80% of global riverine plastic emissions into the ocean. Science Advances7(18), eaaz5803. https://doi.org/10.1126/sciadv.aaz5803

Watch the Video on Mapping Plastic Polluting Rivers

The Ocean Cleanup project has an informative video that discusses the research that led to the non-profit understanding which rivers are emitting the most plastic waste.

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