Microplastics can be deposited in river floodplains and transported to deeper soil horizons. Local topography, flood frequency, and soil characteristics are responsible for the amount of plastic particles deposited and their possible transport into deeper soil. That is the central finding of a study by researchers at the Universities of Cologne and Bayreuth, led by Professor Dr. Christina Bogner at the University of Cologne’s Institute of Geography and Dr. Martin Löder at the University of Bayreuth. The research team investigated the Rhine floodplain Langel-Merkenich north of Cologne for microplastics contamination and sampled soil in three transects at increasing distances from the river, each in two soil depths. The research team is part of the German Research Foundation’s Collaborative Research Centre 1357 Microplastics. The article detailing the research, ‘Flooding frequency and floodplain topography determine abundance of microplastics in an alluvial Rhine soil’, has been published in the journal Science of the Total Environment.
It is well-known that microplastics are transported to the oceans via rivers. Yet, it is unclear whether all particles ultimately end up there. On their way to the sea, microplastics interact with river sediments, and can also be deposited in the shore areas. The research team investigated whether some microplastics remain in the floodplain of the Rhine after major floods. The scientists were particularly interested in how the microplastics are distributed in the flooded soils and whether they reach deeper soil horizons.
They collected soil samples in two different depths (0-5 cm and 5-20 cm) along three sampling transects at an increasing distances from the river, and determined the abundance of microplastics using micro-Fourier transform infrared spectroscopy (microFTIR spectroscopy). This method allows to unambiguously determine the plastic type of each examined particle down to a minimum particle size of 10 µm by measuring its chemical fingerprint. The amount of microplastics per kilogramme of dry soil varied between 25,502 and 51,119 particles in the top 5 cm and between 25,616 and 84,824 particles in the deeper soil (5-20 cm). About 75 per cent of the particles were smaller than 150 µm.
The researchers found that the distribution of microplastics depended on two main factors: on the one hand, on the topography and vegetation of the soil surface, and on the other hand, on how frequently the site under investigation is flooded. Thus, during flooding, microplastics can accumulate in the floodplains of the Rhine, especially in the depressions, and are also transported into deeper layers at sites which are protected from erosion by grass and where earthworm activity has been detected.
‘The smaller the microplastics, the more likely they are taken up by soil organisms, and can potentially harm them. In our Collaborative Research Center, we are looking into issues of formation and transport behaviour of microplastics in the environment. Additionally, we are analysing how precisely and in what quantities microplastics could be harmful to soil organisms,’ said doctoral researcher Markus Rolf. Professor Bogner added: ‘Our interdisciplinary approach can also be applied to other floodplains to elucidate similar processes. This kind of information is crucial both for locating potential microplastics sinks for sampling protocols and for identifying areas of increased bioavailability of microplastics for ecological risk assessment.’