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Microplastic particles can be deposited in river floodplains and penetrate into deeper areas of the soil. The number of particles identified depends in particular on surface plant cover, the frequency of flooding, and soil properties. Researchers from the Universities of Bayreuth and Cologne determined this during investigations in the Rhine floodplain of Langel-Merkenich north of Cologne. The study, published in the journal "Science of the Total Environment", is the result of interdisciplinary cooperation in the DFG Collaborative Research Centre 1357 "Microplastics" at the University of Bayreuth.
When microplastics are transported via rivers towards the sea, the particles can accumulate not only in river sediment but also on-shore. The research team from Bayreuth and Cologne investigated the regularly flooded Rhine floodplain of Langel-Merkenich to determine whether significant numbers of particles remain in the flooded areas in the event of more frequent flooding. The scientists were particularly interested in how microplastics are distributed in the flooded soils and whether they reach deeper areas of the soil. To this end, with increasing distance from the river, they collected soil samples at two different depths: at a depth of up to five centimetres from the surface of the soil, and at a depth of between five and 20 centimetres. After thorough rinsing, the abundance of microplastic particles and their size were then measured in the laboratories of the University of Bayreuth using micro-Fourier transform infrared spectroscopy (microFTIR spectroscopy).
"MicroFTIR spectroscopy is a technically sophisticated technique that can characterise the chemical composition of any microplastic particle larger than ten micrometres contained in a sample. Even a particle a tenth of the diameter of a human hair can still be clearly identified as to which type of plastic it belongs to. The methodology we have developed for soil samples also enables the analysis of quite large and thus representative sample quantities,” says Dr Martin Löder, Head of Research at the University of Bayreuth. In the end, the measurements within the scope of the study showed that the number of microplastic particles in the soil samples varied considerably. At a depth of up to five centimetres, between 25,502 and 51,119 particles per kilogram of dry soil could be detected; at a depth of between five and 20 centimetres, there were between 25,616 and 84,824 particles. About 75 per cent of the particles were smaller than 150 micrometres.
The researchers also found out why the microplastic particles are very unevenly distributed in the soils examined in the Rhine floodplains. It is especially in the depressions that microplastic particles accumulate in the course of flooding. In places that were protected from erosion by grass cover and had comparatively high earthworm activity, particularly large numbers of particles had moved into deeper layers of the soil. "The mechanisms that determine the transport of microplastics between different environmental compartments are incredibly complex. With the method developed in Bayreuth, we were able to detect even the smallest microplastic particles in floodplain soil and show what factors play a role in the deposition of microplastic particles," says Bayreuth doctoral student and co-author Julia Möller M.Sc., who specialises in researching microplastic particles in soils.
Professor Dr Christina Bogner coordinated the research at the Institute of Geography at the University of Cologne and emphasises the pilot character of the study: "Our interdisciplinary approach stands to be transferable to other floodplains to allow relevant processes to be elucidated. Information from such studies is essential both for the localisation of potential microplastic sinks for sampling plans, and for the identification of areas with increased bioavailability of microplastics for appropriate ecological risk assessment."
Dr. Martin Löder
Animal Ecology I
Head of Plastics Group
University of Bayreuth
Phone: +49 (0)921 55-2209
E-mail: martin.loeder@uni-bayreuth.de
Markus Rolf, Hannes Laermanns, Lukas Kienzler, Christian Pohl, Julia N. Möller, Christian Laforsch, Martin G. J. Löder, Christina Bogner: Flooding frequency and floodplain topography determine abundance of microplastics in an alluvial Rhine soil. Science of the Total Environment (2022), DOI: https://doi.org/10.1016/j.scitotenv.2022.155141
Sampling in the Rhine floodplain of Langel-Merkenich
Photo: Lukas Kienzler.
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Sampling in the Rhine floodplain of Langel-Merkenich
Photo: Lukas Kienzler.
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