Nutrient and metal cycling in the Severn Estuary

Abstract

Estuaries are the continuum between terrestrial freshwater and marine environment and play a critical role in modulating the dissolved and particulate meterials transported from land to ocean. Therefore, it is important to gain a better understanding of biogeochemical cycling in the estuarine systems. The Severn Estuary is Europe’s largest hypertidal estuary, situated in the southwestern region of the UK. Its highly dynamic nature presents a unique opportunity to gain deeper insights into biogeochemical processes in an estuarine environment characterized by high turbidity, strong tidal forces, and complex interactions between physical, chemical, and biological components.

One key process known to be active in tropical estuaries is reverse weathering, or precipitation of authigenic siliactes. Here, a sediment incubation experiment, combined with the geochemical and electroscopic analyses, has demonstrated that reverse weathering can occur in a turbulant temperate estuary like the Severn Estuary. In addtion, the results revealed that reverse weathering could occur without the presence of microbes, and this process could fractionate the silicon (Si) isotopic compositions in the porewater from +2.20‰ to +4.61‰.

The groundwater discharge (GD) has a great potential in supplying a wide range of nutrients and metals into the Severn Estuary. However, little is known about the processes that control the chemical composition of GD, or the flux of groundwaters into the Estuary. This thesis presents the first-time analyses of the unconventional stable isotopic compositions of the groundwater samples in the UK to address these gaps. The geochemical dynamics in the groundwater within a Quaternary sand and gravel aquifer from Cardiff is influenced by a range of factors, including redox conditions, formation of secondary minerals, biological utilization and humen activities. The construction of the Cardiff Bay Barrage has altered the hydrodynamics of the surrounding groundwater system by raising the water table and enhancing the hydraulic gradient, which has restricted seawater intrusion into the aquifer while increasing the GD flux into the Severn Estuary. The GD flux into the Severn Estuary was estimated for the first time using an inverse model, which is 3.08 ± 0.34 km³ yr⁻¹. This substantial flux, coupled with the enriched solute concentrations observed in groundwater samples from Cardiff, suggests that GD can deliver nutrient and metal loads to the Severn Estuary that are comparable to, or even exceed those from riverine inputs.

By investigating the sources and sinks of nutrients and metals in the Severn Estuary, this research provides valuable insights into its biogeochemical cycles and highlights the complex interactions that regulate the transport, transformation, and retention of these solutes. These findings contribute to a better understanding of estuarine dynamics and their broader implications for coastal and marine ecosystems.

Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Sponsors	China Scholarship Council
Supervisor	Katharine Hendry (Supervisor), Christopher D Coath (Supervisor), James M Byrne (Supervisor) & Jamie Lewis (Supervisor)