Microplastics in Rhine Floodplains: How Flooding and Landforms Influence Soil Contamination(Rolf et al., 2022)

By Sophia Schmitz, Sean Steinhauer, Markus Wirtz, Juliane Weiling

Microplastics in Floodplain Soils

Markus Rolf and his interdisciplinary research team from the Institute of Geography at the University of Cologne investigated in 2022 how local topography and flooding frequency influence the accumulation of microplastics in floodplain soils. Their study, titled “Flooding frequency and floodplain topography determine abundance of microplastics in an alluvial Rhine soil”, provides new insights into the causes and mechanisms of microplastic distribution in floodplains – using the floodplain area of Langel-Merkenich, north of Cologne, as a case example.

Dig In and Measure

To explore this, the team collected soil samples along three transects – measurement lines across the landscape (Fig. 1) – at two different depths (0–5 cm and 5–20 cm) in order to analyze the vertical distribution of microplastics. In the lab, microplastics were filtered out of the samples and identified using infrared spectroscopy (FTIR spectroscopy). In addition, flooding frequency at each sampling site was estimated through hydrodynamic modeling using MIKE 21 software (DHI, Hørsholm, Denmark), combined with the analysis of long-term water level data.

Figure 1: A and B: Study area. C: Land cover map showing the three transects and sampling points. D: Digital elevation model of the area (ROLF et al. 2022).

Where the Microplastics Gather

The results are clear: The amount of microplastics found in the soil is strongly influenced by local terrain and the resulting flooding patterns. In depressions and low-lying areas, where water – and the particles it carries – tend to collect and settle, significantly higher concentrations of microplastics were found.

Interestingly, the concentration of microplastics was mostly higher in the deeper soil layers (5–20 cm) than at the surface. The researchers suggest that this could be due not only to water movement, but also to soil-specific characteristics – such as texture, vegetation cover, and even the activity of earthworms – which may influence how particles move and migrate downward.

Stopover or Final Sink? - Tiny Particles, Big Questions

The study also highlights a key point: Floodplains are not just temporary “storage zones” for microplastics – they can become long-term sinks where the particles accumulate in deeper layers of the soil. This insight is crucial for environmental assessments of river landscapes, and for developing future ecological risk strategies. If microplastics become permanently embedded in certain areas, the likelihood increases that they may enter the food chain – with consequences that are still largely unknown.

Figure 2: Results for the three transects across the two investigated soil depths (ROLF et al. 2022).

Reference

Rolf, M., Laermanns, H., Kienzler, L., Pohl, C., Möller, J.N., Laforsch, C., Löder, M.G.J., Bogner, C., 2022.Flooding frequency and floodplain topography determine abundance of microplastics in an alluvial Rhine soil. Science of the total environment. DOI: 10.1016/j.scitotenv.2022.155141