Modelling permafrost thickness in Great Britain over glacial cycles.

Like other countries, the UK has opted for deep geological disposal for the long-term, safe management of higher-activity radioactive waste. However, a site and a geological environment have yet to be identified to host a geological disposal facility. In considering its long-term safety functionality, it is necessary to consider natural processes, such as permafrost development, that have the potential to alter the geological environment over the time-scale of glacial-interglacial cycles. We applied a numerical model to simulate the impact of long-term climatic variability on groundwater flow and permafrost dynamics in two contrasting geological settings in Great Britain: (i) higher strength rocks (HSR) overlain by higher permeability sandstones with a high topographic gradient (GS1); (ii) a mixed sedimentary sequence of high and low permeability rocks resting on igneous HSR with a very low topographic gradient (GS2). We evaluated the sensitivity of simulated permafrost thickness to a variety of climatic and subsurface conditions. Uncertainty in the scaling of the surface temperature time-series, 10-25 °C below present day temperature, has the largest impact on maximum permafrost thickness, PF, compared to other variables. However, considering plausible parameter ranges for UK settings, PF is up to twice as sensitive to changes in thermal conductivity and geothermal heat flux than to changes in porosity. Heat advection only affects modelled PF for high hydraulic conductivity rocks and if permafrost is considered to be relatively permeable. Whilst local differences in permafrost thickness of tens of meters, caused by variations in heat advection, are of minor importance over glacial-interglacial cycles, heat advection can be important in the development of taliks and the maintenance of a more active groundwater flow system. We conclude that it is likely to be important to simulate the effect of heat advection on coupled permafrost and groundwater flow systems in settings containing higher permeability lithological sequences.