Interplay of topography, flood frequency and soil properties determine the distribution of microplastics in a Rhine floodplain

Rivers are important transport pathways of microplastics from terrestrial to marine environments. They also interact with terrestrial ecosystems, in particular during flood events, when microplastics can be deposited in or eroded from floodplains. The spatial distribution of these riverine microplastics in alluvial floodplains remains widely unclear. However, the knowledge on their abundance and distribution in floodplain soils is essentially important for ecological risk assessment.

We analysed the distribution of microplastics in three transects of a floodplain soil in a nature reserve in Cologne (Germany). We took soil samples in two different depths (0-5 cm and 5-20 cm), described the soil profiles and plant cover and determined the soil texture. Additionally, we used a hydrodynamic model (MIKE21 software by DHI) and time series of Rhine's water level to analyse the frequency of past flood events from 1950 to 2020. We analysed concentrations of microplastics via ATR-FTIR and µ-FPA-FTIR spectroscopy after density separation and enzymatic-oxidative purification of soil samples. We found elevated microplastic concentrations per kg of dry soil with increasing distance to the river ranging from 25,616 particles/kg to 84,824 particles/kg. Combining the analysis of flood events, the digital terrain model and quantification of microplastics, we show how the local topography (e.g., depressions), flood frequency and soil properties (e.g., grain size) interact and affect the spatial and vertical distribution of microplastics.