Microplastic pollution in salt marsh and urban tributary sediment cores of the River Thames estuary, UK: Spatial and temporal accumulation trends

Microplastics in sediment cores from urban tidal tributaries, Barking and Bow Creek-London and salt marshes Swanscombe, Kent, and Rainham, Essex, Thames estuary (UK), were quantified by density separation and ATR-FTIR spectroscopy. All eight tributary cores were dominated by low-density microplastics, polypropylene, polyethylene, and polystyrene with the greatest abundance (mean 360.0 ± 12.0 particles 100 g^-1 dwt (0-10 cm depth) observed furthest from the confluence with the Thames due to storm tank combined-sewer-overflow input. Salt marsh core microplastics were highest at Swanscombe (mean 267.1 ± 10.2 particles 100 g^-1 dwt at 0-10 cm depth) in the high-marsh vegetation zone. Marsh sediment radionuclide dating (Pb²¹⁰, Cs¹³⁷) suggested a presence of microplastics in the sediment since at least the late 1950s, with increasing abundance towards surface sediments. Tidal tributaries and salt marshes of the Thames act as natural filters, with salt marshes accumulating microplastics over time and tributaries acting as both stores and sources depending on individual site conditions and hydrodynamic variability.