Research reveals Rhine transports more litter than expected

New research reveals that the river Rhine transports around 4,000 tonnes of macro litter towards the North Sea each year. That amount significantly exceeds previous estimates. That is the conclusion based on research conducted by researchers from the German universities of Tübingen and Bonn and the Federal Institute of Hydrology. They monitored macro litter in the Rhine River over 16 months, using a litter trap installed in the Rhine River near Cologne.

Macro litter abundance fluctuates significantly and is much higher at times of rising discharge due to flooding or heavy rainfall. “River discharge forecasts could serve as a predictor to guide clean-up efforts”, is the conclusion of the researchers in an article that was published in January in the scientific journal Nature.

All sorts of litter

According to the study, plastics had the highest material share with 69.7%, followed by worked wood (14.9%), glass and ceramics (5.7%), paper and cardboard (2.6%), metal (2.6%), rubber (1.6%), chemicals (1.6%), food waste (0.8%), and cotton and textiles (0.5%). Most of the litter (56%) comes from private consumers. Industry was responsible for 6 % of the macro litter that was found during the study. Industrial waste came mainly from packaging materials, such as small foamed polystyrene fragments.

Citizen science

The researchers collaborated with citizen scientists who volunteered for this project. “Together we collected 20,339 floating macro litter items larger than 1 centimetre that we classified according to international standards. We continuously monitored macro litter using the custom-made litter trap as an interception-based method for 16 months at a fixed location in the Rhine River”, said researcher Leandra Hamann. The stationary litter trap caught floating and submerged macro litter up to 80 cm below the surface and was cleaned every two weeks by volunteers.

Weight measurements

The study determined an average plastic weight per item of 24.3 g, compared to 5.38 g in previous estimates for the Rhine. “This could be due to the measurement of wet weight and the type of scales used for weight measurements with a minimum weight of 1 g, which limited the accuracy of small and lightweight items. This was partly circumvented by measuring several small items of the same category together and then dividing the weight by their number. Yet, with an average weight of 111.6 g per item for all materials, the research results fall within weights reported in beach macro litter monitoring studies where plastic macro litter averaged 15.6 g per item, and other materials ranged from rubber with 76.8 g to wood with 154.7 g”, is stated in the article.

Different research method

The research method in this study differs from research methods in other studies. The long and continuous sampling period of 52 weeks leads to other results. In a previous study researchers based their estimate on only two sampling dates. Another study calculated the average weight based on 508 items and scaled up the total plastic macro litter transport in combination with data from visual observation of only 15 hours.