The downstream evolution of macro and meso plastic (from Rhine to Waal) : RWS information - Results of the Kor-net fishing in 2023

Since 1950, the demand for plastic has increased dramatically. However, poor plastic management has resulted in significant plastic pollution across various environments. The aquatic environment is particularly affected, with approximately 9 to 23 million metric tons of plastic ending up in water bodies annually worldwide. Rivers play a crucial role as transport routes, carrying plastic waste to the oceans, where most accumulation occurs. Despite this, there is limited research on how plastic evolves as it moves along rivers. To develop effective mitigation strategies and improve modelling approaches, more knowledge is needed about the plastic transport within rivers. To address this knowledge gap, samples were taken in the water column and along the shores of both the rivers Rhine and the Waal.

This study investigates changes in plastic concentration, size, and composition between the two locations and identifies the hydrological factors driving these differences. Additionally, this study examines the accumulation of plastics on riverbanks. It also evaluates whether bridge counting, a simpler and more cost-effective method, can be used for plastic monitoring. The analysis reveals that plastic concentrations do not differ significantly between the two locations, but the size of plastic debris does, with notable differences in diameter and perimeter in the Waal. Distribution patterns show higher plastic concentrations in the inner bends of the river, as well as at the bottom and surface layers. Smaller particles, such as foils, are more prevalent at the surface and middle column, while larger particles tend to sink. In terms of composition, the most common OSPAR categories found include small and large plastic foils, small unidentifiable rigid plastics, ropes with a diameter less than 1 cm, sanitary wipes, and other textiles. Textile and sanitary wipes were the only categories that differed between the two rivers in the top 5 most common OSPAR categories.

This study also finds a positive correlation between discharge rates and plastic concentrations during peak discharge periods, indicating that rising water levels can remobilize stranded plastics. This is supported by the increased number of OSPAR categories found during higher discharge rates. The plastic composition on riverbanks showed greater diversity and more recreational waste compared to the water column. The bridge counting method was found to underestimate the amount of plastic in the river, making it less reliable than trawl nets. While this research offers new insights into how plastic evolves as it moves downstream in the Rhine, further studies should increase the number of samples and ensure they are taken at comparable hydrological conditions.