Microplastic deposition controlled by fluvial sedimentary facies in an urban river

Samuel Roudbar , Daniel Le Heron, Michael Wagreich, Ronald Poeppl

Microplastic particles (MP) are characterised by their irregular shapes, lower density relative to natural grains, often failing at subscribing to sedimentological transport laws under controlled experimental conditions. Mismanagement of plastic waste, including associated environmental and health concerns, underpins the importance of systematic field-based behavioural observations on their riverine transport. Sediments from a highly anthropogenic river were sampled (N = 25) from 13 diverse sedimentary facies in the suburbs of Vienna to assess depositional patterns in terms of microplastic abundance and composition, sediment grain size distribution, total organic matter content (TOC), and seasonality. An alternative sedimentology-based grain shape classification, together with a new Depositional Capacity Index (DCI) were applied to determine preferential depositional patterns of MP. Microplastic abundance in river sediments reached 4,720 ± 4,644.5 MP kg⁻¹ (mean ± SD, n +17) in spring and decreased significantly after the wet season to 1,606 ± 622.6 MP kg⁻¹ (n = 8) in autumn. Deposition varies strongly across sedimentary facies and correlates with the newly DCI: throughflow-exposed riverbed with low DCI contained as little as 314 MP kg⁻¹, whereas vegetation-trapped backwater deposits with high DCI reached up to 15,544 MP kg⁻¹. Microplastics exhibit hydraulic size compensation relative to natural grains during transport, resulting in a systematic relationship between microplastic and sediment grain size. Their abundance is primarily controlled by sedimentary facies that favour the retention of organic matter, suggesting co-accumulation with organic-rich fine sediments, whereas microplastic composition (shape and density) shows no clear environmental control. These findings enhance our understanding of the environmental fate of microplastic particles as sediment analogues and help identify contamination hotspots relevant for targeted mitigation efforts.