Simulating groundwater flow and contaminant migration in heterogeneous alluvial sediments

Simulating groundwater flow and contaminant migration in heterogeneous alluvial sediments

Supervisors Jared West, Ian Burke and James Graham (NNL)

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.

Image: Concrete cap over shaft containing radioactive wastes at one of the UK nuclear legacy sites.  The cap was installed to allow cement grout injection via boreholes into the underlying sandstone rock (seen in foreground) to reduce the migration of radionuclides from the shaft into the sea (photo: Ritches Ltd).  This is an example of the remedial work that can be required to prevent subsurface migration of contaminants from nuclear legacy 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.

Objectives

You will aim to address three main questions:

  1. What is the best approach for predicting contaminant transport in heterogeneous glacio-fluvial sediments?
  2. How can we reconstruct of the spatial distribution of permeability and sorption properties based on the available core sample data?
  3. By applying the above in combination with future climate, what is the envelope of possible contaminant impacts from the site arising from subsurface migration?

Background to the UK situation and Sellafield

The UK Nuclear Decommissioning Authority is mandated to oversee the removal of buildings and radioactive wastes present at ~20 UK nuclear sites. Its purpose is to deliver the decommissioning and clean-up of the UK’s civil nuclear legacy in a safe and cost-effective manner, and where possible to accelerate programmes of work that reduce hazard. The NDA does not directly manage the UK’s nuclear sites. It oversees the work through contracts with specially designed companies known as Site Licence Companies.

The NDA determines the overall strategy and priorities for managing decommissioning. Its annual budget is £3.2 billion. The vast majority of the NDA budget is spent through contracts with Site Licence Companies, who also sub contract to other companies which provide special services.

The Sellafield site is a large nuclear fuel reprocessing, nuclear waste storage, nuclear decommissioning and former nuclear power generating site on the coast of Cumbria, England. The site covers an area of two square miles and comprises more than 200 nuclear facilities and more than 1,000 buildings. It is Europe’s largest nuclear site and has the most diverse range of nuclear facilities in the world situated on a single site. It is due to be fully decommissioned by 2120 at a cost of £121bn.

Many of the UK nuclear sites, including Sellafield, are located on coastal plains and are underlain by thick fluvial-glacial alluvium deposits. These sediments are highly heterogeneous in nature and consist of layers ranging from clay size right up to cobble sized materials. There is also often considerable lateral variability in sediment texture reflecting changing depositional environments present during their formation. Groundwater flow through the coastal plain including the sediments and the underlying Triassic Sherwood Sandstone is driven by recharge from streams flowing off the lake district massif to the east (Medici et al., 2016; 2018).

Historically there has been several events where unauthorised releases of radionuclide and/ or hydrocarbon containing fluids have occurred. These have contained radionuclides such as Tc-99 and H-3 that are highly mobile in groundwater, others such as Sr-90, C-137 and U which display sorption controlled transport. Other chemical contaminants, such as hydrocarbons as Non-Aqueous Phase Liquids and their resulting dissolution products, have also been accidentally released into groundwater. At Sellafield, over 200 groundwater monitoring boreholes exist, where aquifer materials and groundwater data has been recorded. This has provided evidence for the existence of extensive radioactive and chemical contamination plumes in a very complex unconsolidated aquifer structure relating to different releases from specific waste types and processes over time.

Understanding groundwater flow patterns and reactive transport of contaminants represents a considerable risk to closure plans for nuclear sites. Regulatory planning commonly assumes little or no sorption, leading to unrealistically high estimates for the rates of plume migration (such that ‘safe to walk away’ end state conditions cannot be guaranteed). On the other hand, sorption data from experiments using only <2mm sediments may lead to overly optimistic estimates of plume retardation, especially if groundwater flow is mostly occurring through courser sediments with >2mm grainsize. Interaction of plumes with saline influenced groundwater at site boundaries also adds additional complexity to understanding how long term migration of contaminants will occur.

This project will thus aim to develop modelling approaches which considers both the 3 dimensional complexity of unconsolidated fluvial aquifers and the variable groundwater compositions present.

Methodology and Approach

A study will be undertaken to establish the influence of sediment heterogeneity on the migration of radionuclides, chemical contaminants in glacio-fluvial sediments.  This will include a review of the previous work on modelling solute migration in heterogeneous porous media, in order to identify the fundamental science that can be applied to the specific problems at this and similar sites.  In the first phase of the work, fundamental approaches to modelling solute transport such as particle tracing, advection-dispersion, and Continuous Time Random Walk (CTRW) will be assessed for their suitability for tackling these problems (Bottrell et al., 2006; Berkowitz et al., 2006; Medici et al., 2019).  Once the most appropriate modelling approach has been identified, it will be necessary to review the available codes for implementation.

In the second phase of the work the permeability and sorption properties of the glacio-fluvial sediments at the Sellafield site will be estimated empirically from the available data (including from a currently-running PhD project on sediment characterisation). The spatial properties at the site scale will be determined (horizontal and vertical co-variance, sedimentary domains etc, Merritt and Auton, 2000; McMillan et al., 2000).  Available approaches for the generation of permeability/sorption property fields will be reviewed, and the selected approaches will be applied (these may include approaches that use a priori geological architectures e.g. see Columbera et al., 2018).

In the third phase of the work contaminant transport modelling will be applied to constrain the envelope of possible contaminant impacts from the site.  Site groundwater and stream monitoring data will be used to constrain modelled hydraulic gradients and boundary conditions. The influence of heterogeneous sediment permeability and sorption properties on three specific solute transport scenarios relevant to contaminant impacts may be investigated i) migration of radionuclide plumes off-site (non-sorbing contaminants such as U and H-3; sorbing contaminants such as Sr-90, Tc-99 and Cr-137)  ii) transport, distribution and degradation of dissolved phase hydrocarbon contaminants including density effects (relevant to the longevity of such contaminants on site in terms of the 100 year period to closure) and iii) behaviour of the saline-freshwater interface at this coastal site. Finally, the impacts of possible changes to the hydrological driver (ie rainfall) over the decadal timescale to site closure will be modelled, based on the downscaled IPCC climate change scenario predictions for the region.

Impact

Project outcomes will represent an advance our knowledge and understanding of the physicochemical processes occurring in alluvial sediments that will assist in designing closure plans for nuclear legacy sites. This project will to produce highly innovative research in reactive transport in heterogeneous porous media which may be more widely applicable. This knowledge is urgently needed to increase our ability to assess the impacts from nuclear legacy sites and from contaminated industrial sites more generally.

Student profile

The project would suit a numerate student with a background in earth or environmental sciences, geology, or geophysics. You should have a strong interest in assessing environmental impacts, and a strong background in a quantitative science. Willingness and excitement for taking up the challenge to work at the boundary of geochemistry and hydrogeology is a prerequisite.

You will undertake training in state-of-the-art numerical simulation approaches to prediction of fluid flow and solute transport, and analysis of geostatistical properties of sediments and sedimentary architectures, as well as undertaking a placement at the partner organisation – National Nuclear Laboratories.

References

Berkowitz B.,  Cortis A., Dentz M.,  Scher, H. 2006. Modeling non‐Fickian transport in geological formations as a continuous time random walk. Reviews of Geophysics. 44, 2. https://doi.org/10.1029/2005RG000178

Bottrell SH, West LJ, Yoshida K. 2006. Combined isotopic and modelling approach to determine the source of saline groundwaters in the Selby Triassic sandstone aquifer, UK. , pp. 325-338

Columbera et al, 2018 Colombera L, Mountney NP, Medici G, West LJ. 2019. The geometry of fluvial channel bodies: Empirical characterization and implications for object-based models of the subsurface. AAPG Bulletin. 103(4), pp. 905-929

Fuller A. J., Shaw S., Peacock C. L., Trivedi D., Small J. S., Abrahamsen L. G. and Burke I. T. 2014. Ionic strength and pH dependent multi-site sorption of Cs onto a micaceous aquifer sediment. Applied Geochemistry, 40, 32-42.

McMillan, A.A., Heathcote, J.A., Klinck, B.A., Shepley, M.G., Jackson, C.P. and Degnan, P.J., 2000. Hydrogeological characterization of the onshore Quaternary sediments at Sellafield using the concept of domains. Quarterly Journal of Engineering Geology and Hydrogeology, 33(4), pp.301-323.

Medici G, West LJ, Mountney NP. 2016. Characterizing flow pathways in a sandstone aquifer: Tectonic vs sedimentary heterogeneities. Journal of Contaminant Hydrology. 194, pp. 36-58

Medici G, West LJ, Mountney NP. 2018. Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK. Hydrogeology Journal. 26

Medici G, West LJ, Chapman PJ, Banwart SA. 2019. Prediction of contaminant transport in fractured carbonate aquifer-types; case study of the Permian Magnesian Limestone Group (NE England, UK). Environmental Science and Pollution Research. 26(24), pp. 24863-24884

Merritt, J.W. and Auton, C.A., 2000. An outline of the lithostratigraphy and depositional history of Quaternary deposits in the Sellafield district, west Cumbria. Proceedings of the Yorkshire Geological Society, 53(2), pp.129-154.

Wallace S.H., Shaw S., Morris K., Small J. S., Fuller A. J. and Burke I.T. 2012. Effect of groundwater pH and ionic strength on strontium sorption in aquifer sediments: Implications for 90Sr mobility at contaminated nuclear sites, Applied Geochemistry, 27 (8), 1482-1491.