Trusler, Megan M.
(2025)
Investigating Plastic Degradation and Spatio-Temporal Microplastic Accumulation in Sediments of the Thames Estuary (UK), its Tributaries, and its Salt Marshes.
PhD thesis, University of Nottingham.
Abstract
Context
Plastics are diverse, widespread materials used across a range of sectors from packaging to construction. However, their durability allows them to be preserved in the environment and slowly weather into increasingly small particles. Microplastics are any plastic particle less than 5 mm in size, and they have been found to accumulate in environments across the globe. River networks are known to be plastic transport corridors, and coastal estuaries in particular are thought to contain high a high abundance of microplastics, although understanding of microplastic behaviour and accumulation trends remains limited.
This thesis presents the first record of microplastic sediment storage in the Thames estuary (UK), its tributaries and salt marshes, with the aim of identifying spatio-temporal trends in microplastic storage behaviour within transitional urban-industrial estuarine environments. Additionally, degradation pathways of polystyrene plastics exposed to different environmental conditions will be explored with the aim of understanding the mechanisms and timescales behind the aging of plastic waste and their breakdown in the environment. Finally, the utility of computed tomography (CT) in the study of microplastics in sediments will be explored as a novel non-destructive screening approach.
Methods
Firstly, the chemical degradation of polystyrene was tracked monthly using Fourier-transform infrared (FTIR) spectroscopy through attenuated total reflectance (ATR) over a period of 24 months under a combination of treatments that compared the effect of solar irradiation and soil interaction on the degradation rate. Microplastic sediment accumulation was then quantified across the Thames estuary, three of its tributaries (Barking Creek, Bow Creek, and the River Medway) and two of its salt marshes (Swanscombe and Rainham salt marshes) through the examination of surface sediment and sediment core transects via density separation methods. Finally, a suite of experimental cores were scanned using CT scanners. These were artificially layered and spiked sediment cores with different types and sizes of microplastics, plus two environmental samples.
Results
FTIR spectrum absorbance at native peaks and analysis of the carbonyl index indicated that after 24 months of exposure, polystyrene cup lids exposed to natural ultraviolet (UV) sunlight were more degraded than samples that were buried and not exposed to UV. Variations in the degradation rate across the year were also identified, suggesting that it was closely related to seasonal cycles in temperate regions. A multiple regression found that soil temperature (discarding measurements of site UVB and rainfall rate) was the best model for predicting degradation rate (via the carbonyl index) of the polystyrene treatment surface without shade (P = 0.002). In the next chapters, work to investigate microplastic abundance in the Thames estuary found microplastics in all samples, with a dominance of low-density plastic types. However, the abundance of microplastics varied both between sites and with depth, indicating spatio-temporal heterogeneity across the estuary. Microplastic abundance in the estuary was found to be controlled by an urbanisation-distance gradient (P = 0.03, linear regression), specific site hydrodynamics, and human activity. For example, combined sewer overflows (CSOs) were characterised as significant sources of microplastic fibres and therefore hotspots of accumulation. A correlation between microplastic abundance and total organic carbon (TOC %) in the estuary surface sediment was also established (P = 0.004, linear regression).
A linear regression comparing the abundance of microplastics in tributary transects of Barking and Bow Creeks was not statistically significant and had opposite trends, suggesting that more complex factors were affecting abundance and creating dynamic sediment systems in these environments (P <0.001 Barking Creek, P = 0.056 Bow Creek, linear regressions). Analysis of sediment cores at Swanscombe and Rainham salt marshes suggested that microplastic accumulation was influenced by elevation-distance and vegetation zone, with the high marsh zone containing a higher abundance of microplastics compared to the intertidal zone (P = 0.002 Swanscombe salt marsh when excluding sample TS2, P = 0.026 Rainham salt marsh, both linear regressions). High marsh zones were also found to act as temporal indicators of microplastic accumulation, showing increases in microplastic abundance within each sediment slice since the 1960s for all four radiometrically dated cores, mirroring increasing historical plastics production trends. A statistically significant relationship was identified between microplastic abundance and sediment depth on both salt marshes (P < 0.001 for 3 cores, and for TS4 P = 0.026, all linear regression). Finally, the suite of CT scans revealed that the technique presents a promising opportunity to investigate sediment microplastics with wide scope for further research. Large microplastic particles could be digitally isolated in randomly spiked artificial cores, and environmental cores were able to highlight the contextual data lost using conventional microplastic isolation techniques.
Main Conclusions
Ultimately, this investigation makes significant progress into understanding microplastic abundance within the Thames estuary and their spatio-temporal accumulation trends in tidal urban-industrial estuaries. Wider discussion indicated further study should expand upon this knowledge base to fully elucidate microplastic behaviour in estuaries, using interdisciplinary projects to contextualise a wider pollutant base and inform effective future microplastic management strategies.
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