Simulating groundwater flow and contaminant migration in heterogeneous alluvial sediments

In this project, you will use groundwater level monitoring and chemistry data combined with the results of detailed analysis of sediments to predict the long-term impacts of radionuclides at one of the UK’s most important nuclear legacy sites. You will undertake numerical modelling to constrain the impact of sediment heterogeneity on the rate of movement of key radionuclides including uranium and tritium; Sr-90, Tc-99 and Cr-137. Results from your research will have direct implications for assessing the envelope of possible impacts from this and similar sites.

At a time when many nuclear legacy sites around the world are being assessed to allow their safe closure, assessment of the potential of the subsurface for migration of contaminants is critical. This project aims to utilize pre-existing extensive datasets on groundwater physics and chemistry, and detailed sediment characterisation to develop numerical models in order to learn how best to assess the impacts of subsurface contaminant migration. The specific site investigated is underlain by highly permeable fluvio-glacial sediments. The project results will be widely applicable to similar systems in the UK and across the world.

Our ability to safely close nuclear legacy sites in order to avoid burdening future generations with their ongoing management is a difficult societal problem both nationally and internationally. Both radioisotopes and other chemical contaminants have leaked or been released in the subsurface at many such sites including the Sellafield site in the UK, Fukishima in Japan and Chernobyl in the Ukraine. The long-term impacts of such contaminants are highly dependent on the status and evolution of fluid flow through the underlying sediments through time and space, and the interactions between these contaminants and the sediment particles. Fluid flow is controlled by the permeability of the sediment, whereas the contaminant interactions, such as sorption of contaminants to sediment particles, are highly dependent on sediment and groundwater chemistry (Wallace et al., 2012; Fuller et al., 2015). Both the permeability and the extent of chemical interactions are strongly dependent on the grain size of the sediment. While the behaviour of contaminants in a uniform grain-size sediment can be predicted, heterogeneous sediments such as fluvio-glacial and other alluvial deposits typically consist of both coarse-grained elements, which are highly permeable and permit rapid fluid flow, and fine grained clay-rich units which show slower flow and a higher sorption potential for contaminants. Contaminant transport in such heterogeneous grain-size sediments is a complex problem dependent on the spatial arrangement of these units, which is amendable to study using numerical simulations.

To address the question of the potential impacts of a plume of contaminants in heterogeneous sediments, the project will study the examples of the plumes of radionuclide and other contaminants released at the Sellafield nuclear legacy site, which have the potential to migrate off-site towards the Irish Sea. The work will help evaluate and predict the envelope of possible migration rates of various contaminants, and thus whether and when remedial action will need to be taken within the multi-decadal timespan of planned site closure. You will initially construct a conceptual hydrogeological model of the study area, by identifying the most appropriate methods for reconstructing spatial distribution of coarse versus fine grained sediment units based on the available sediment-core data. Then you will move on to investigate plume migration rate, initially by looking at the historical chemical monitoring data where this is available, then by selection of appropriate simulation software and the development of scenario models based on different possible sedimentary architectures that are compatible with the observed data. You will then run numerical simulations of subsurface reactive transport based on these scenario models coupled with predictions of future hydrological forcing derived from the IPCC climate change predictions for future rainfall trends in the study area.