Spatio-temporal variations in microplastic presence and composition in the sediments of the R. Thames (UK) and its tributaries

Rivers form major conduits for land-derived plastic waste, with the annual emission of plastics to the world’s oceans currently being quantified at as much as 2.7 million metric tons. In particular, riverbed sediments have been found to retain high levels of microplastics (plastics <5 mm in size) and multiple studies reported microplastic concentrations in the sediments to exceed those in the water column by as much as 600,000 times. Emerging evidence suggests that high flows may remobilise some of the sedimentary microplastic pool, resulting in high microplastic loads entering the adjacent seas following flood events. However, the transport mechanisms that microplastics undergo in river settings remain poorly understood.

In this study, we investigated spatio-temporal variations in the microplastic contamination within river bed sediments in the R. Thames (UK) catchment. Sediment samples were taken on a seasonal basis over 3 years (2019-2021) from 12 sampling sites (classified as rural, urban and industrial) located on the main R. Thames and 8 of its tributaries. Microplastics were extracted from sediment using density flotation, then visually counted and investigated using a combination of ATR-FTIR and µATR-FTIR (Attenuated Total Reflectance - Fourier Transform Infrared Spectroscopy). Additional SEM analysis was carried out to describe the morphology (degradational patterns/occurrence of biofouling) of extracted microplastics.

Microplastics were present in most sediment samples (85%), with fragments being the dominant shape (92% across all seasons), followed by beads/pellets (5%) and fibres (3%). Microplastic levels varied on a seasonal basis (0 - 4,200 MP·kg^-1 range), increasing in the summer months and decreasing in the winter by up to 89%. This suggests the occurrence of microplastic accumulation in the summer low-flow conditions followed by subsequent flushing of microplastics as a result of higher winter flows. Inter-site variations in microplastic levels were evident in the summer months, with concentrations increasing in the order of industrial>urban>rural. Microplastics were also more abundant at inner river banks and near point sources (e.g. effluent outlets). In contrast, such inter- and intra-site variations were less clear in the winter. Urban and rural samples were dominated by fragments (96% on average) irrespective of season, with most items made out of polyethylene (PE), but also containing thermoplastic elastomers found in road marking paints (e.g. ethylene vinyl acetate (EVA), polyamide (PA)). Although samples taken from industrial locations were also dominated by fragments (84%), they contained higher numbers of microbeads and industrial pellets (12%) composed of polymers widely used for industrial applications (e.g. poly(methyl methacrylate) (PMMA) used as Perspex glass, poly(diallyl phthalate) used in the processing of thermosetting plastics and resins, or poly(1,4-butylene terephthalate) (PBT) used for insulation purposes in the electrical industry).

Our study reveals a multitude of microplastic sources contributing to the pollution of the R. Thames catchment and confirms the existence of a strong seasonal pattern in microplastic deposition within riverbeds, suggesting a need to account for this process in the global models of microplastic export from land to sea.