Riverine plastic pollution

Unsplash / Naja Bertolt Jensen

Key messages:

  • Despite ongoing research, there is great uncertainty about the amount of plastic in the environment
  • Plastics accumulate in terrestrial and aquatic environments, making them a long-term source to freshwater and the oceans even if the mismanagement of waste is stopped.
  • Small urban rivers can contribute substantially to plastic export to the oceans
  • Local actions to reduce inputs to rivers in urban coastal areas can effectively reduce plastic export to the oceans.
  • Monitoring plastics in rivers, even by simple means such as counting floating objects, helps to shed further light on plastic transport in rivers and to confirm, for example, the success of measures to reduce plastic pollution in river


Plastics everywhere

Plastics as a product are a success story. Production of plastics has grown faster than GDP (Geyer et al. 2017). The properties of plastic that make it so successful, its durability, light weight and low cost production, are also the cause of its mismanagement and leakage. For example, about 40% of its production are single use items.

Since the presence of plastics in the oceans was first identified in the 1970s (Carpenter et al. 1972), plastics in the environment are now considered a global environmental problem. A growing number of studies have shown that plastics are found almost everywhere (Morales-Casalles et al. 2021). Their widespread occurrence and the fact that pollution of soils, lakes, rivers, and oceans is irreversible make plastic pollution a global environmental threat (MacLeod et al. 2021). Consequently, plastic production, waste generation and its fate in the environment are addressed by the Sustainable Development Goals (SDGs) established by the United Nations dealing with Sustainable Cities and Communities (SDG 11), Responsible Consumption and Production (SDG 12) and Life Below Water (SDG 14).

Furthermore, SDG 6 (Clean Water and Sanitation) has an indirect relation to plastics, as plastic garbage may block waterways and cause hygienic problems and plastics-associated chemicals can enter drinking water resources.

How much plastic is there

Plastic production has increased from 2 million tons in 1950 (Geyer et al. 2017) to 431 million tons in 2019 (OECD 2022). In 2019, global plastic waste generation was 353 million tons (OECD Global Plastics Outlook Database, https://doi.org/10.1787/c0821f81-en). While plastic production and waste generation figures can be estimated relatively reliably using manufacturer data and national statistics, the fate of plastics in the environment depends on more assumptions, which increases the uncertainty of estimates.

It is estimated that 82 million tons/year of plastic waste is mismanaged, of which 13 million tons/year end up in the terrestrial environment and 5.8 million tons/year end up in rivers (OECD Global Plastics Outlook Database, https://doi.org/10.1787/c0821f81-en). Another study suggests that there is no difference between terrestrial and aquatic pollution, as potentially terrestrial plastic can be mobilised by runoff and wind, while mismanaged plastics far from rivers are less likely to enter waterways. The amount of plastic entering the aquatic environment annually ranges from 19 to 23 million tons (Borelle et al. 2021). Although the range of numbers is quite high, several million tons of plastic are likely to end up in rivers around the world every year.

Export from rivers to oceans

Rivers are inherently a natural pathway, carrying nutrients and sediments from the land to the ocean. In this light, it is likely that rivers are also a major transport pathway for plastics from land. However, increased observational evidence suggests that most plastics may accumulate in and around rivers rather than exported to sea.

Several studies have estimated the amount of plastic - either microplastics or macroplastics or both - in rivers worldwide and have come to very different conclusions. (Figure 1). The midpoint estimates for microplastics range from 31 kilo tons to 2.31 million tons per year. The range for macroplastics is 0.15 tons to 1 million tons per year. And the midpoint estimates for total exported plastics range from 0.13 to 2.46 million tons.

All studies related observed plastic concentrations in rivers to river catchment characteristics such as plastic waste generation, population, or settlement distribution. However, different observational data and the inclusion of different factors lead to different results, although all models adequately reproduced their underlying observational data (Roebroek et al. 2022).

All estimates of riverine export are substantially lower than estimates of plastics entering the aquatic environment. This suggests that plastics accumulate on land and in rivers (van Emmerik et al. 2022), forming a secondary, potentially long-term source of plastics even after the primary leakage of mismanaged plastic waste has ended.

Fig 1
Figure 1.Global export of plastic (Micro, Macroplastic and Total plastics) into the oceans based on different studies. 

Small rivers turning big

In addition to the total amount of plastics entering the oceans via rivers, information on which rivers carry the largest share of the total plastic load is necessary for planning interventions. There are 520 major river basins that drain into the world's oceans, with the Amazon being the largest at 6 million km² (GRDC, 2020). The total number of rivers and streams draining to the oceans is much higher, about 100,000 when smaller streams are considered (Meijer et al. 2021).

Similar to the total amount of plastics transported by rivers, the distribution of plastic loads in rivers is still uncertain and is the subject of ongoing research. The first studies on global plastic export by rivers by Lebreton et al. (2017) and Schmidt et al. (2017) found that large rivers in particular transport large amounts of plastics. Lebreton found that the top twenty polluting rivers account for about 67% of the global total. The figures found by Schmidt et al. suggest an even more extreme focus, with the top ten rivers accounting for 90% of the total riverine plastic load.

The observed amounts of plastic in rivers can be related to the amount of mismanaged plastic waste that occurs upstream of the observation points. Mismanaged plastic waste (Jambeck et al. 2015) was estimated globally by combining data on solid waste, population density, and economic status. Global solid waste data is provided in Hoornweg and Bhada-Tata (2012).

The relationship between observed plastic load and mismanaged plastic waste is used to estimate plastic loads for rivers without observational data, which are the majority of global rivers.

In a study by Mai et al. (2019), observed plastic loads were related to the human development index (HDI) rather than to mismanaged plastic waste. Again, the top 10, mostly large rivers, contribute a significant 41% of the total riverine plastic load. Regardless of the underlying explanatory data, whether it is mismanaged, managed plastic or HDI, these studies assumed that plastic available for transport reaches the oceans regardless of the size of the river basin or the distance of the source location to the coast.

The Meijer et al. 2021 study provided new insight into which rivers transport most plastic; they analyzed a new dataset focused on macroplastics. In their model to extrapolate to rivers without observational data, they considered various river basin characteristics that affect the connectivity of the mismanaged plastic waste to the river network, such as land use and precipitation. They also found that retention within the river systems affects plastic transport into the oceans. Therefore, sources of plastics that are closer to the river network and sources that are close to the river basin outlet have a higher likelihood of reaching the oceans. This gives a very different picture of which rivers contribute the most to plastic in the ocean. Meijer et al. 2021 estimated that more than 1000 rivers are responsible for 80% of global annual riverine plastic emissions, with small rivers flowing through large coastal cities ranking in the top ten most polluting rivers. So it is likely that not a few large rivers, but many small, urban rivers cause most of the river-transported plastic in the oceans.

This suggests that policies and actions that focus on urban coastal areas reducing plastic waste through improved litter collection, street litter removal and prevention, and river cleanup may effectively reduce ocean pollution.

The need for more and consistent data

One thing runs through all the figures on plastic pollution of rivers: Uncertainty. Plastic pollution is complex and depends on many factors, making it difficult to measure and model. Any model requires input data, and more and better observational data will allow us to build better models. Compared to other pollutants, data on plastics in rivers are still sparse. So there is a need for more data involving more rivers, but also for regular monitoring to detect changes in plastic transport over time and to monitor the success of cleanups and efforts to reduce waste. In addition, the use of different techniques for sampling and analysis makes it difficult to compare different data sets, thus harmonising monitoring techniques is pivotal. While increased efforts in collecting and harmonising data are cost-intense, respective monitoring programmes will help design strategies and measures to effectively address the plastic pollution problem for sustainable development in its ecological, social, but also economic dimension.

However, results from workshops and surveys conducted by WWQAs Plastics Workstream indicate that barriers to plastics monitoring are generally quite high. Factors preventing the implementation of monitoring programmes are, in fact, widespread and can include, besides financial issues, also a lack of technical and human resources just as much as specific knowledge gaps for data collection and laboratory analysis. It is therefore recommended that monitoring programmes for plastics in rivers also use rather simple methods such as counting macroplastics visually from bridges. This also opens the options for participatory monitoring approaches such as citizen science to help address the data issue. Furthermore, to enable comparable monitoring data on a global scale, consistent guidelines are necessary. The UNEP (2020) report Monitoring Plastics in Rivers and Lakes: Guidelines for the Harmonization of Methodologies" provides guidance on the timely development and implementation of relevant monitoring programmes for plastics in freshwaters tailored to the different baseline conditions in different countries.

Christian Schmidt, Tim van Emmerik, Sabrina Kirschke and Katrin Wendt-Potthoff

F urther reading

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