Spatial distribution of microplastics in floodplain sediments of the river Loire (France)

, Croiset Camille, Dhivert Elie, Phuong Ngoc-Nam, Grosbois Cécile, Zalouk-Vergnoux Aurore, Baltzer Agnès, Gasperi Johnny.

Sediments from rivers, lakes, and other continental hydrosystems constitute important sinks for microplastics but remain a poorly studied compartment (Dris et al., 2018). Microplastic spatial distribution depends both on their intrinsic properties, and also on hydro-sedimentary processes. However, the understanding of MP depositional mechanisms within sediments remains fragmentary. In the vast majority of studies, these sediments are sampled without questioning hydro-sedimentary processes which may control the abundance of MP at different spatial scales (Dekiff et al., 2014; Mani et al., 2018). In this context, 16 surface sediment samples were collected in a floodplain of the river Loire (over an 8 km² area). These samples correspond to different depositional environments with various connectivity levels and sediment compositions. After treatment of samples (organic matter digestion and densimetric separations), the quantities, types and sizes of microplastics in each sample were analysed by µFTIR (Fourier Transform Infrared Spectroscopy coupled to microscopy). In parallel, grain size distributions and total organic carbon contents were also analysed. The connectivity rate to the river flow was determined by combining sample altitudes and water level variations over the last decade. Results show MP levels ranging between 1,110 and 19,342 items/kg dw (dry weight). A substantial spatial variability is illustrated over the floodplain. Analyzes are ongoing to explain this variability regarding : (i) the connectivity rate linked with the local topography and hydrological conditons, (ii) sediment composition as a proxy of hydro-sedimentary processes, (iii) the geomorphological functionning of sampled environments, and potentially (iv) surface land cover to determine whether vegetation has a significative impact on MP abundance in these flooplain depositional environments.

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