Hydro-Environmental Controls on Microplastic in Coastal Deltas

Abstract

Microplastics (<5 mm synthetic polymer particles) are pervasive global pollutants, yet their dynamics within river floodplains, a key but underexplored depositional environment, remain unquantified. This thesis provides the first systematic investigation into the role of flooding as the primary driver of the transport, weathering, retention, and chemical fate of microplastics in deltaic floodplains. The study integrates a systematic literature review, geospatial flood analysis, and empirical fieldwork coupled with extensive laboratory analysis. A systematic literature review of seventy-six studies revealed consistent contamination across river deltas, yet few have addressed the role of flooding in microplastic accumulation. To address this gap, a new conceptual model was developed and tested through soil sampling across fifteen sites representing five zones of flood exposure. Surface and subsurface soils were analysed for polymer composition, degradation features, and co-occurring elemental concentrations using Raman spectroscopy, scanning electron microscopy, and inductively coupled plasma techniques. The results revealed widespread microplastic contamination across the study area, with concentrations ranging from 72 to 4,032 MPs/kg and a mean concentration of 911 MPs/kg. The concentrations increased with greater flood duration and frequency, with the exception of the lowest flood exposure zone, which exhibited the highest concentrations due to artificial elevation and dense human settlement. The dominant polymer types identified were polypropylene (51 %), polyethylene (20%), polyvinyl chloride (19%) and polyethylene terephthalate (6%). More advanced surface degradation was observed on microplastic particles in less inundated soils, likely because they remain longer under dry and aerated conditions, allowing greater physical and chemical weathering. Elemental analysis using energy-dispersive X-ray spectroscopy revealed the presence of iron, aluminium, titanium, arsenic, and antimony on microplastic surfaces. Although several elements, including aluminium, iron, arsenic, and antimony, showed statistically significant correlations with microplastic abundance, no consistent co-occurrence was observed between microplastic and metal concentrations in soil or water samples. This research demonstrates that flooding plays a central role in the spatial retention and surface alteration of microplastics in deltaic landscapes. By combining geospatial flood data with particle-scale chemical analysis, the thesis provides a mechanistic understanding of contaminant dynamics in flood-prone landscapes. The results contribute new knowledge to the study of terrestrial microplastic pollution and offer insights applicable to environmental monitoring and land-use management across low-lying coastal deltas worldwide.

Date of Award	16 Mar 2026
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Mo Hoque (Supervisor) & Mike Fowler (Supervisor)