Microplastic deposition and transport in a highly-modified stream: insights from geomorphic depocenters and sediment analogues

Continuous human intervention in the environment has profoundly reshaped natural landforms, often disrupting their environmental functions to serve anthropogenic needs. The Krotenbach, located just south of Vienna (Austria), is a 14.2 km long, highly modified stream engineered to efficiently transport its water discharge. Further downstream from its production zone in the Wienerwald, the river partly flows through and beneath an urban–industrial area before continuing through agricultural land. These two distinctive sections are separated by one of the main highways in Austria, with the sampling area located downstream of this boundary. At the upstream limit of the study section, a wastewater treatment plant continuously contributes approximately 78% of the total river discharge, creating a persistently altered flow regime. This strongly anthropogenic system offers an ideal flume-like field laboratory to study the transport and accumulation of microplastics in riverbed sediments.

Despite its constricted planar morphology, low slope gradient, and limited sediment supply, the most significant depocenters were sampled for microplastic concentrations (>63 µm), sediment grain-size analysis, and total organic carbon content. In addition, all large wood elements and anthropogenic obstacles with sediment retention potential were mapped and quantified using field-based geomorphological indices of sediment connectivity, defined here as the efficiency of sediment transport from point A to point B within a defined system. A sediment connectivity model was therefore developed based on cumulative drainage area, channel slope, and retention indices for large wood and anthropogenic obstacles, to help predict and explain MP deposition patterns.

The primary objective of this research is to evaluate which fluvio-geomorphological sinks have the highest capacity to store microplastic particles (MPs) and to assess the extent to which their preferential occurrence can be explained by particle morphometrics, density, and their response to the relative degree of sub-watershed connectivity. Alongside sedimentary facies characterization, MP datasets were evaluated using a newly developed semi-quantitative sediment retention index to predict the sedimentological component of their riverine transport behaviour. Results show that the index developed for this study satisfactorily captures the physical principles likely governing microplastic transport. Comparisons between observed concentrations and model predictions highlight the sedimentary behaviour of microplastics deposited in riverbed sediments. Both MPs and mineral sediments appear to respond, through their respective fragmentation capacities, to hydraulic sorting under the same energy gradient.

Samples collected six months later, in October 2025, show a net decrease of over 50% in MP concentrations, particularly in sinks characterized by high bed shear stress. This second sampling campaign highlights the short residence time of MPs and the overall high connectivity of such a man-made river system. The role of small urban catchments in plastic pollution dynamics should therefore be prioritized in plastic pollution mitigation and remediation strategies.

All MP concentrations were corrected for contamination using six laboratory blanks and two field blanks. Recovery tests conducted with high-density 125 µm polyethylene yielded a recovery rate of 87.6%. Microplastics were identified through manual mapping using micro-FTIR analysis, with an analytical variability of 2.22% (standard deviation) based on 25% sub-sampling.