Secondary microplastics were prevalent in sediment in a freshwater UK urban river

Plastic pollution has been documented in terrestrial and aquatic environments worldwide, with growing concern for ‘microplastics’ (MPs, <5 mm). Understanding of the sources, fate, and impact of MPs remains limited, particularly in freshwater environments. Furthermore, their small sizes and a lack of standardised methodology hinders monitoring and risk assessment of these emerging contaminants. Here, the distribution of microscopic debris in an urban river close to the marine environment in the West of Scotland was investigated to assess the prevalence of MPs. Bank sediment samples were collected twice from the River Kelvin in Glasgow and were size-fractionated and processed for extraction of MPs by density separation. Light microscopy and scanning electron microscopy with energy dispersive spectroscopy, were employed for characterisation and quantification of microdebris of sizes ranging from 2.8 mm to 0.45 µm. Sample MP spiking and use of procedural blanks allowed the influence of processing on field data quality to be considered. The predominant type of MPs were fibres, comprising >88% of total MP counts, but fibre content in blanks suggested potential contributions from background contamination. Final MP abundances were estimated at 161-432 items per kg dry sediment. In addition, metallic and glass pellets were observed in high abundances in settled material and could be easily misidentified by visual inspection. Thus, compositional analysis is needed to avoid analytical errors from MP misidentification and overestimation.