New findings on the monitoring of plastic waste in rivers
A research team from the Karlsruhe Institute for Technology (KIT) has, in a study carried out with partners from the Netherlands and Australia, examined conventional assumptions as to what quantity of plastic waste is present in rivers, and how this plastic litter is further transported. According to their results, the quantity of plastic litter actually present in rivers could be up to 90 percent larger than previously assumed. The new findings from the study should help to improve the monitoring of plastic litter in rivers and to remove it from the waterways.
According to the current status of research, rivers play a major role in the discharge of improperly disposed-of plastics into the environment. "As soon as plastic gets into a river, it is transported further at high speed and spread around in the environment," explained Dr. Daniel Valero from the Institute of Water and River Basin Management at the KIT, who is the chief author of the study. Depending on the size and the nature of the objects or pieces, plastic can behave in very different ways. To estimate the contamination of rivers, use has until now been made primarily of data relating to the surface of water. This means, says Valero, that it has been possible to effectively monitor large rivers with, for example, cameras from bridges.
In the study published at Science Direct in the journal "Water Research", Valero examined with his research team in river models in the hydraulic engineering laboratory of the Technical University of Delft the behaviour of a total of 3,000 plastic parts that are frequently found in river environments. The objects examined in the study ranged from 30 mm pieces of plastic or plastic film to complete objects such as plastic beakers or face masks. The test specimens were made of polypropylene, as with the beakers, and high-density polyethylene, as with the plastic film. Several cameras recorded how each of the plastic specimens behaved in water under different flow conditions, whereby, according to the study, the entire water column from the water surface down to the lower layers was documented. It was shown statistically through extrapolation that conventional monitoring practices that concentrate only on the water surface and the areas close to the surface could underestimate the total quantity of plastics by as much as 90 percent. Pieces of plastic behave very differently depending on where exactly they are situated in a river. Plastic objects below the surface of the water are, according to the results, transported in the way predicted in normal models. "The particles are spread about like dust in the wind. Only few pieces find their way back to the water surface again," explains Valero. As soon as the plastics do come up to the surface, the situation changes: "On contact with the water surface, the pieces of plastic are caught by the surface tension like flies in a spider’s web. They then drift on together." With this mode of transport, surface tension thus plays a major role.
The results of the study show, according to Valero and his team, that, to estimate the quantity of plastic litter in rivers, the transport of pieces of plastic below the water surface must also be taken into account, and that the quantity of plastic actually transported can be calculated relatively accurately. Over and above this, the results could, according to Valero and his colleagues, also help in a very practical matter with the targeted cleaning of waterways, because it is now possible to estimate where most of the plastic is situated.
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