<p>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.</p><p>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 &#181;-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.</p>
<p>Although the occurrence of microplastic particles (MPs) and their impact on different environments have become a widely recognized research topics, their transport mechanisms in terrestrial environments are still understudied. While first research in this field have focused on the abundance of MPs in soils and its vertical distribution, only little is known about the mechanisms of MP transport on sediment and soils surfaces. This might be explained by the challenges of detecting MPs in terrestrial settings.</p><p>Therefore, we investigate the surface transport mechanisms and patterns by using fluorescent MP particles that can be tracked by an advanced complementary metal&#8211;oxide&#8211;semiconductor (CMOS) high-resolution camera. Within this study we used an experimental set-up including a flume box with surfaces of different roughness and several rates of surface discharge. We traced the pathways of environmentally pristine and biofouled fluorescent amorphously shaped Polystyrene (PS) and Polymethyl methacrylate (PMMA) to analyze how polymer type, biofilm, surface roughness and film thickness influence their transport. Subsequently, time series analysis of the images were performed and evaluated using R software. This included the calculation of particle size, estimation of pathways and path lengths. First results suggest a large influence of the water film thickness of the runoff and the surface roughness.&#160;</p>
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