Spatial analysis of riverine microplastic in a Rhine floodplain soil in Germany

The Rhine River flows through six European countries and is in exchange with diverse land use forms and human activities that potentially release microplastics (MPs). The Rhine interacts permanently with its surrounding banks and floodplains by changing water-levels. Several studies have documented the presence of MPs in the Rhine along its course as well as in its tributaries. However, the spatial distribution of MPs due to certain flood events in alluvial floodplains remains widely unclear. The knowledge about the amount and distribution of MPs and on their potential entry pathways into Rhine floodplains is essentially important for an ecological risk assessment. In this study, we analysed the amount and distribution of MPs in a floodplain soil in the nature reserve Merkenich-Langel, in the northern periphery of Cologne (Germany). We hypothesize that MPs are transported by the Rhine and are deposited at the site during flood events. For spatial analysis we used the MIKE software (DHI A/S, Hørsholm Denmark) merged with a digital terrain model of the study site to analyse past flood events and their potential deposition of MP. We chose three sampling transects located within the past flooded area each with three sampling spots with increasing distance and elevation to the river. Samples were taken from two different soil depths (0–5 cm and 5–20 cm) and the samples of the three sampling spots and same depth were combined to one mixed soil sample per transect. MP concentrations were analysed via ATR-FTIR and µ-FPA-FTIR spectroscopy after density separation and enzymatic-oxidative purification. We found an increase of MP concentration per kg of dry soil in the depth 5–20 cm with increasing distance to the river ranging from 25.612 particles/kg to 85.076 particles/kg. The results of MP concentration in 0–5 cm topsoil layer will be compared to the concentration in the soil depth of 5–20 cm. We correlate these results to the frequency of flood events.