Depositional patterns of microplastics in a meandering river: a case study of the Leie River, Belgium

Following the discovery of microplastics (MPs) in river sediments, the number of MP studies on rivers and other freshwater systems has increased rapidly, revealing that MPs are omnipresent in all freshwater environments. The abundance of MPs in freshwater sediments seems to be affected by population density, urban centers, water flow velocity, water catchment size and position and type of sewage and waste management. However, not all of these relations are consistent. For example, while many studies report good correlations between MP source regions (highly populated and industrialized areas) and MP abundance in river sediments, others do not. This is in contrast to the concentrations in the water itself, for which better links with MP source areas were found. What all these studies have in common, is their large-scale approach, in which sediment samples are obtained over distances of tens to thousands of kilometers along the river; and at each site sediments are than retrieved either from the deepest part of the channel or from the river bank (depending on the study). Here, we study MP distribution in a section of the meandering Leie River, in a rural area, a few kilometers upstream of the city of Ghent (Belgium). Multibeam bathymetry and side-scan sonar images allowed selecting three undisturbed across-channel transects where surface sediments were retrieved. Sediment samples were analyzed for MP content, organic-matter content and grain size of the clastic fraction. Overall the MP concentrations are much (up to an order of magnitude) lower in the thalweg compared to samples near the river banks, resulting in an asymmetric distribution at the bend apex, where the thalweg approaches the outer banks. Furthermore, MP concentrations show strong correlations with the organic matter content and grain-size parameters as expected form hydrodynamic sorting. Exceptions to these correlations are the outer bank samples, where MP concentrations are lower than predicted from sedimentological characteristics. We attribute this to the more erosive character of the current in the thalweg near the outer banks, which inhibits MP deposition, but exposes fine-grained and organic-rich flood plain sediments. We highlight that the different hydrodynamic conditions across a river channel greatly influence MP distribution (with an order of magnitude), but in a sedimentologically predictable manner. Care should thus be taken in environmental studies, as local variability in MP concentration across a river bed may be larger then the large-scale variability.