The fate of microplastics in Rhine floodplains: process and patterns of deposition, distribution and vertical migration in the soil

Rivers are known to transport microplastics and other contaminants and deposit them partly in their riverbed sediments. With increasing water levels, rivers can remobilize deposited microplastics and flood their banks and adjacent floodplains. Therefore, floodplains are exposed to microplastic deposition during flooding. The resulting microplastic abundance in floodplain soils and the lateral and vertical distribution of microplastics are not yet fully understood. Therefore, this thesis analyzed three Rhine floodplains in Cologne (Germany) to answer three main research questions. Namely, (I) how do microplastics distribute laterally in the Rhine floodplains? (II) Does meadow vegetation filter out microplastics during flooding? (III) How are microplastics distributed in floodplain soils, and can microplastics be transported vertically? To understand the lateral distribution pattern of microplastics, we analyzed floodplain topographies and the resulting flood frequencies to relate them to the microplastic numbers found in the floodplain soil. We analyzed the microplastics with fourier-transform-infrared (FTIR) spectroscopy and pyrolysis gas chromatography/mass spectroscopy (pyrolysis GC/MS), to obtain the microplastic numbers and their mass concentrations. For the evaluation of microplastic retention by floodplain vegetation, we sampled vegetation and sediments deposited on the vegetation after one flood event. We determined the above-ground biomass of vegetation and analyzed the found microplastics with FTIR spectroscopy. Finally, to study the vertical distribution and transport of microplastics, we sampled an 1.1 m deep soil profile. We applied opitically stimulated luminescence (OSL) dating to estimate the sediment deposition ages of coarse quartz. To obtain information on vertical transport, the microplastics found were analyzed with FTIR spectroscopy and the resulting vertical microplastic distribution was related to the deposition ages in the soil profile. The results obtained show that the lateral distribution of microplastics is influenced by local topography, flood frequencies, and vegetation cover. This led to lower microplastic numbers and mass concentrations on the river banks (3,300–20,876 microplastics kg−1 and 9.7–110.9 mg kg−1) and accumulations in the floodplains depressions (8,516–70,124 microplastics kg−1 and 46.2–141.6 mg kg−1). Furthermore, our analysis indicates that the increase in above-ground vegetation is capable of filtering microplastics from floodwater. In particular, atmospheric and fluvial depositions can be distinguished in microplastic numbers, shape, and size. This microplastic input from fluvial depositions (1,220–44,054 microplastics m−2) can potentially infiltrate the floodplain soil and contributes partly to the microplastic budget. We found microplastics throughout the entire 1.1 m deep soil profile and an accumulation of 798,141 microplastics kg−1 in a depth of 38– 45 cm. Furthermore, sediment deposition ages reached the 1950s in the first 20 cm, thus suggesting vertical transport of microplastics by preferential flowpaths and bioturbation by earthworms. This research contributed to the understanding of the lateral and vertical distribution of microplastics and their transport in floodplains and floodplain soils, which are at the interface between terrestrial and aquatic ecosystems. These floodplains are subject to microplastic accumulations and provide a basis for future ecological risk assessments.