How The Ocean Cleanup Mapped the World's Rivers | Research | The Ocean Cleanup

• 1000 rivers account for nearly 80% of global riverine plastic emissions into the ocean, according to our study, published in  Science Advances .
• Our model suggests that instead of a handful of large continental rivers contributing the most emissions, a high number of small and medium-sized rivers play a significant role in the influx of plastic from rivers to the ocean.
• These 1000 rivers can present very different characteristics, including river width, flow dynamics, marine traffic, and urbanization.
• A wide range of mitigation measures must be applied to these rivers across the globe to substantially decrease the amount of waste entering our oceans from rivers.
• Our study results are accessible in this interactive map, where you can find and help to address your nearest polluting river.

To substantially decrease riverine plastic emissions into the ocean, in how many rivers do we need to implement mitigation measures, where are these rivers located, and what are the characteristics of these rivers? For the last three years, researchers at The Ocean Cleanup have been working on answering these questions to effectively reduce oceanic plastic pollution. Our study – published today in  Science Advances  – estimates that 1000 rivers account for nearly 80% of global riverine plastic emissions into the ocean, made up of a high number of small and medium-sized rivers. While this number is much higher than previous estimations (100 times), it is only 1% of rivers worldwide, which means solving the problem is feasible. By collectively taking a global approach with various technologies to target these most polluting rivers, we can drastically reduce the influx of plastic into the ocean.

These high-resolution data allow for The Ocean Cleanup and other organizations worldwide to develop focused mitigation strategies and technologies to reduce riverine plastic emissions. In 2019, The Ocean Cleanup launched our river cleanup technology, the Interceptor, and have been using this data as a compass to find suitable locations to deploy Interceptor solutions. With this study now accepted, The Ocean Cleanup can use its findings to further support and accelerate global plastic extraction initiatives, policy, and consumer changes and awareness.

MODELING GLOBAL RIVERINE PLASTIC EMISSIONS

The Ocean Cleanup’s mission is to rid the world’s oceans of plastic. Decreasing the inflow of plastics into the ocean is one part of our solution to solve the problem of plastic accumulating in our oceans. But how are plastic emissions distributed over the 100,000 rivers, creeks, and canals that exist worldwide? Two previous studies (2017) gathered data on mismanaged plastic waste generated within a river basin and applied a correction factor to estimate the river’s plastic emission. These studies predicted that as little as 5 or 47 rivers account for 80% of riverine plastic emissions.

Since 2017 more data on macroplastics in rivers became available from several research groups, including The Ocean Cleanup. These data, and new insights, suggested that global riverine plastic emissions could be more widespread than previously thought. Therefore, to effectively impact pollution in rivers and oceans, we needed to update our global river plastic emissions model by including more details on plastic transport dynamics and data at a higher spatial resolution.

In collaboration with researchers from various institutions and organizations, we created a new model framework and included data from 136 field measurements, representing 67 rivers in 14 countries of 3 different continents, collected between 2017 and 2020 to calibrate and validate our model. We used analogous research and insights from field campaigns to conceptualize plastic transport and worked with 60 earth science and plastic experts to parametrize our probabilistic model. Our updated model now suggests that  1000 rivers account for nearly 80% of global annual emissions, ranging between 0.8 million and 2.7 million metric tons per year , with small urban rivers among the most polluting.

To build a model that simulates global riverine plastic transport, we needed global data sets. By utilizing data on terrain slope, elevation (DEM), flow direction (HydroSHEDS), runoff data (GRUN), land use data (GLC2000), precipitation and wind data (WorldClim2), we calculated a global river network, river classes, terrain characteristics, the distance from every location on Earth towards the nearest river and ocean, and the corresponding mobilization and transport probabilities. We then figured the probability for mismanaged plastic waste to reach the ocean for each grid cell. This plastic emission probability is then multiplied with the corresponding mismanaged plastic waste and accumulation within a river basin, resulting in the annual plastic emissions per river mouth.

Abstract

Plastic waste increasingly accumulates in the marine environment, but data on the distribution and quantification of riverine sources required for development of effective mitigation are limited. Our model approach includes geographically distributed data on plastic waste, land use, wind, precipitation, and rivers and calculates the probability for plastic waste to reach a river and subsequently the ocean. This probabilistic approach highlights regions that are likely to emit plastic into the ocean. We calibrated our model using recent field observations and show that emissions are distributed over more rivers than previously thought by up to two orders of magnitude. We estimate that more than 1000 rivers account for 80% of global annual emissions, which range between 0.8 million and 2.7 million metric tons per year, with small urban rivers among the most polluting. These high-resolution data allow for the focused development of mitigation strategies and technologies to reduce riverine plastic emissions.

INTRODUCTION

Plastic pollution in oceans and rivers is an emerging environmental hazard ( 1 ), and accumulation on riverbanks, deltas, coastlines ( 2 ), and the ocean surface ( 3 ) is rapidly increasing. Of all the plastics ever made to date, it was estimated that 60% has been discarded in landfills or in the natural environment ( 4 ). Plastic pollution poses threats on aquatic life, ecosystems, and human health ( 5 ,  6 ). Plastic litter also causes severe economic losses through damage to vessels and fishing gear, negative effects on the tourism industry, and increased shoreline cleaning efforts, adding up to US$1.26 billion per year for the Asian-Pacific Rim alone ( 7 ). Work on the origin and fate of plastic pollution in aquatic environments suggests that land-based plastics are one of the main sources of marine plastic pollution ( 8 ), either by direct emission from coastal zones ( 9 ) or by transport through rivers ( 10 ,  11 ). Riverine plastic transport remains understudied, especially in areas that are expected to contribute most to global plastic emissions into the ocean ( 12 ). A better understanding of pathways and transport mechanisms of plastic waste to and within rivers and the global distribution of riverine plastic emissions into the ocean is a prerequisite to developing effective prevention and collection strategies.

Previous attempts to estimate the distribution of global riverine emissions of plastic into the ocean ( 10 ,  11 ) relied on empirical indicators representative of waste generation inside a river basin. These assessments demonstrated a significant correlation between (micro)plastic concentration data collected by surface trawls in rivers, national statistics on mismanaged plastic waste (MPW) generation, and population density. For both studies, an empirical formulation was presented on the basis of this correlation, which was extrapolated to other rivers where data were not available. This resulted in predicted plastic (micro- and macroplastics combined) emissions of 1.15 million to 2.41 million metric tons (MT) per year ( 10 ) and 0.41 million to 4 million MT year–1 ( 11 ). These studies did not account for spatial distribution of plastic waste generation or climatological or geographical differences within river basins. According to these studies, the 10 largest emitting rivers contribute 50 to 61% and 88 to 94% to the total river emissions. Both models agreed on a disproportional contribution of Asian rivers to global plastic emissions. While these modeling efforts have provided a first approximation of the magnitude and spatial distribution of global riverine plastic emissions, they emphasized the scarcity of data on macroplastic contamination in freshwater ecosystems. Available measurements used for calibration of emission predictions were not always collected directly at the river mouths, and studies reported data on plastic contamination using varying units and methods, including surface trawling from boats or bridges ( 13 – 15 ).

Sampling methods using surface net trawls for freshwater plastic contamination may be well suited for monitoring microplastic concentrations (size,