The river Rhine is estimated to carry between 3,000 and 4,700 tonnes of macrolitter — pieces of litter larger than 25 millimetres in size — towards the North Sea every year, according to research published in Communications Sustainability. The upper estimate, extrapolated from the results of 12 months of continuous monitoring in collaboration with citizen scientists in Cologne, is more than 250 times higher than some previous estimates, and suggests that long-term physical litter collection is a crucial monitoring method for estimating how much litter rivers transport.
Manmade litter negatively affects the environment, human health, and crucial infrastructure such as drainage systems. Rivers play a key role in transporting litter into other aquatic and marine environments, but there has so far been little long-term observation of the quantity of litter transported by rivers.
Leandra Hamann and colleagues used a floating litter trap anchored in Cologne to monitor the litter carried by the Rhine between 19 November 2022 and 18 November 2023. The trap captured litter floating on the surface and submerged up to 80 centimetres below it, and each day filtered approximately 0.08% of the mean daily river flow. Any captured litter larger than 1 centimetre in size was collected, weighed, and categorised.
Over the year, the trap captured 17,523 pieces of litter with an estimated total mass of around 1,955 kg (excluding water weight). Around 70% of the individual pieces were made of plastic, but these pieces accounted for only around 15% of the overall mass of litter. The authors identified 56% of the items captured as originating from private consumers, with around 28% of items being food or drink related. Other major sources of litter included fireworks (10.7% of all items) and cigarette-related waste (6.5%).
The authors then extrapolated their results to estimate that annually, the Rhine carries between 3,010 and 4,707 tonnes of litter to the North Sea each year, of which between 446 and 697 tonnes is plastic. These estimates for plastic are between 22 and 286 times more than previous estimates based on single-day litter observations. The authors say that their results show that realistic estimates of river litter are best made with long-term continuous monitoring methods which physically collect the litter.
In a separate Comment piece, Dilek Fraisl and colleagues argue that citizen science projects worldwide could help gather large quantities of the data used by the UN to monitor the Sustainable Development Goals (SDGs). The termination of the Demographic and Health Surveys formerly backed by the United States Agency for International Development has left a major shortfall in this data, but the authors argue that as many as 60% of the SDG indicators reliant on household survey data could be supported by citizen science. They say that, "especially in turbulent times", official statistics should rely on citizen science as well as government-run surveys and monitoring, and that as such we need to invest more in citizen science.
