Just around the river bend: the fate of plastic bottle pollution in the Thames Estuary

Previously considered to be the main pathway of plastic pollution into the marine environment, large river systems are increasingly thought to act as major traps for plastic debris. The River Thames is polluted by plastic debris, including plastic drink bottles, but the proportion of items entering the North Sea is unclear. This study investigates the movement of plastic drink bottles in the Thames estuary using a combination of numerical modelling and in-situ sampling data. The model explores different factors to explain the pathways of the plastic bottles, including the bottle fullness (empty, half-full and full), the wind, and the river discharge (high/low). The model results are validated by in-situ bottle collection data from citizen science cleanups along the banks of the river. Empty bottles are found to be more affected by the wind than the river discharge and more likely to strand along the Thames estuary shoreline. Conversely, full and half-full bottles are more likely to be carried downstream by the residual currents and leave the estuary into the North Sea, particularly during periods of high river discharge. The results suggests that 53 % of full bottles and 41 % of half-full bottles entering the Thames estuary leave the estuary over a 150-days period, contributing to plastic pollution in the marine environment. Understanding the transport and accumulation patterns of debris in rivers, as well as gaining insight into the factors influencing their movement, helps identify optimal clean-up locations to stem the flow of plastic waste entering the marine environment. • We used models and in-situ data to study the movement of plastic bottles in the Thames. • Empty bottles were more affected by the wind, and full bottles by the river flow. • Empty bottles were more likely to get stranded along the river shore. • Over half of the modelled full bottles left the estuary into the North Sea.