Plastic in rivers: new method provides better estimates of the problem
A new paper warns that plastic pollution in rivers may be an even more serious problem than previously thought. The authors of the paper, including IHE Delft Senior Lecturer Daniel Valero Huerta and IHE Delft alumni Biruk S. Belay, reviewed conventional assumptions for the transport of plastic in rivers and found that the actual amount of plastic waste in rivers could be up to 90% greater than previously assumed. The new findings should help improve monitoring and remove plastic from water bodies.
Rivers play a key role in the transport of plastic in the environment. "As soon as plastic enters a river, it is transported rapidly and can spread throughout the environment," said Valero, who in addition to being Senior Lecturer in Hydraulic Engineering at IHE Delft also is a Research Associate at Karlsruhe Institute of Technology’s Institute of Water and River Basin Management.
"But, depending on its size and material, plastic can behave very differently in the process. It can sink, be suspended in the water, remain afloat or be stopped by obstacles," said Valero, lead author of the paper published in Water Research.
Because current methods for estimating plastic pollution in rivers are mainly based on surface observations, this means that what happens under the water surface is missed.
Plastic particles are transported very differently
Belay, who graduated from IHE Delft with an MSc in Water Supply Engineering in 2021 and now is pursuing a PhD in Hydraulic Engineering at the Helmut-Schmidt University in Germany, meticulously prepared 3,000 pieces of plastic and conducted turbulent transport experiments as part of his MSc thesis research.
Valero, Belay and other partners from Deltares and UNSW Canberra, then investigated the behaviour of plastic particles in river-like flows. In laboratory models, each individual particle was tracked in 3D with millimetre precision using a multi-camera system, that recorded the entire water column - from the water surface to the bottom. With this experiment, the researchers were able to statistically prove that plastic particles behave very differently depending on where they are located in a river. Plastic that is transported below the water surface behaves as predicted by common models for turbulent flows. "The particles are dispersed like dust in the wind," Valero said.
As soon as plastic emerges on the surface, however, the situation changes: "The particles are caught by the surface tension like flies in a spider's web. Then they cannot escape easily," Valero said.
If you can estimate where the most plastic is, then you also know where a clean-up is most effective
Better models for visual monitoring
On the one hand, the results of the experiment show that it is not enough to consider only floating plastic on the surface to estimate the amount of plastic in rivers.
"The bias is significant. If the turbulent character of the transport of plastic particles under the water surface is not considered, then the amount of plastic waste in rivers can be underestimated by up to 90%," Valero said.
On the other hand, the results confirm that existing knowledge about the behaviour of particles in turbulent flows applies to the transport of plastic in rivers, meaning that it can help to estimate the total amount more realistically.
The researchers quantified the ratio between concentrations of plastic particles at the water surface and at greater depths with different transport conditions. Using this basis, visual observation of the water surface can be used to calculate the total quantity of transported plastic.
In addition, the results can support the development of new approaches for plastic removal: "If you can estimate where the most plastic is, then you also know where a clean-up is most effective," Valero said.