In river floodplains, microplastic particles can be deposited and also penetrate into deeper areas of the soil. The number of particles detected depends in particular on the vegetation on the surface of the soil, the frequency of flooding and the composition of the soil. Researchers from the Universities of Bayreuth and Cologne found this out during investigations in the Rhine floodplain Langel-Merkenich north of Cologne. The study, published in the journal Science of the Total Environment, resulted from interdisciplinary collaboration in the DFG Collaborative Research Center 1357 “Microplastics” at the University of Bayreuth.
When microplastics are transported by rivers toward the sea, the particles can be deposited not only in river sediments but also in riparian areas. The research team from Bayreuth and Cologne investigated in the regularly flooded Rhine floodplain Langel-Merkenich whether a larger number of microplastics also remain in areas that are flooded more frequently. The scientists were particularly interested in how microplastics are distributed in the flooded soils and whether they reach deeper areas of the soil. Therefore, with increasing distance from the river, they collected soil samples at two different depth ranges: from the soil surface to a depth of five centimeters and at a depth between five and 20 centimeters. The abundance of microplastic particles and their size were measured using micro-Fourier transform infrared spectroscopy (microFTIR spectroscopy) after a complex purification process in the laboratories of the University of Bayreuth.
“MicroFTIR spectroscopy is a technically sophisticated technique that can be used to characterize the chemical composition of each microplastic particle larger than ten micrometers in a sample. Even if a particle is as tiny as one-tenth the diameter of a human hair, we can still clearly determine 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. Overall, the measurements in the study showed that the number of microplastic particles in the soil samples varied significantly: At a depth of up to five centimeters, between 25,502 and 51,119 particles could be detected per kilogram of dry soil; at a depth of between five and 20 centimeters, there were between 25,616 and 84,824 particles. About 75 percent of the particles were smaller than 150 micrometers.
The researchers also found out why the microplastic particles are very unevenly distributed in the soils of the Rhine floodplains that were studied: Especially in the depressions, microplastic particles can accumulate in the course of flooding. At sites protected from erosion by grass cover and showing comparatively high earthworm activity, particularly large numbers of particles had moved into deeper layers of the soil. “The mechanisms that condition the transport of microplastics between different environmental compartments are incredibly complex. With our method developed in Bayreuth, we were able to detect even the smallest microplastic particles in the floodplain soil and show which factors play a role in their relocation to deeper soil layers,” says Bayreuth doctoral student and co-author Julia Möller M.Sc., who specializes in research on microplastic particles in soils
“Our interdisciplinary approach can also be applied to other floodplains to elucidate the relevant processes. Information from such studies is essential both for locating potential microplastic sinks for sampling plans and for identifying areas of increased microplastic bioavailability for appropriate ecological risk assessment.”
- Professor Dr. Christina Bogner
