This report investigates the sensitivity of simulated permafrost thickness and dynamics to avariety of climatic, geological and hydrogeological conditions for two geological environments,basement under sedimentary cover and a low permeability succession of Mesozoic shales andsiltstones (Case 1 and Case 2 respectively). A combination of one dimensional heat conductionmodelling, including the effects of freeze-thaw, and two dimensional heat conduction-advectionmodelling, including freeze thaw, has been undertaken to simulate permafrost development inthese two contrasting geological environments. This enables an assessment of the sensitivities toa range of possible geological parameters, advective heat flow, and the effect of glaciation withand without the influence of glacial loading.In this report, permafrost is defined as the sub-surface in which ice is present even in very smallamounts, i.e. ice content is greater than 0%, and in the model, this is at the zero degree isotherm.The maximum permafrost thickness is strongly dependent on the mean annual surfacetemperature, the presence of ice that will insulate the system and the duration of the cold phase.By scaling the minimum temperature of 57 Pliocene-Pleistocene globally distributed benthic未18O records to temperatures of 14掳C, 18掳C and 25 掳C below the present day mean annualtemperature, the maximum permafrost thickness for Case 1 is simulated to reach 171 m, 248 m,and 475 m, and for Case 2 80 m, 138 m, and 238 m respectively. The difference in permafrostthickness between the two Cases is attributed to the variation in subsurface rock properties.Deeper permafrost depths than for Case 1 and 2 can be expected where the thermal conductivityis higher than for Case 1 and 2.A sensitivity study of the geological parameters has shown that there is a strong, non-linear,relationship between thermal conductivity, latent heat and geothermal heat flow for a series oftemperatures representative of the glacial cycles of the past one million years. This is in contrastto a steady state temperature profile, where permafrost thickness relates linearly to thermalconductivity, heat flow and ground surface temperature. Thickest permafrost under unchangedclimatic conditions is to be expected where there is a low heat flow, a high thermal conductivityand a low porosity, such as for example in the north of Scotland.The results of the modelling show that when the temperature regime is dominated by heatconduction, such as for the low permeability Case 2, a heat conduction only model is sufficientto estimate the thickness and distribution of permafrost. However, when heat advection is likelyto be important, such as in Case 1, the coupling of permafrost and groundwater flow is necessaryto simulate the permafrost distribution during freeze and thaw, or during shallow permafrostevents. This particularly holds true when permafrost is modelled to be relatively permeable,where modelling suggests that heat advection of cold water at recharge points (interfluves)results in cooling and thicker permafrost compared to discharge points where discharge ofwarmer water results in thinner permafrost. However, these variabilities in local permafrostthickness are of minor importance for the question of freezing of the repository. However, whenassessing the broader influences of permafrost on a geological environment, local variations inpermafrost extent of thickness can have consequences on the biosphere.Glaciation influences the thermal regime of the ground surface. If the glacier bed is undergoingpressure melting, as found in the ablation zone, a reduction in permafrost depth can be expected.If the glacier bed is cold based, as often found in the accumulation zone or at ice divides wherestrong vertical advection of cold ice has a cooling effect, then the maximum permafrost thicknesscan be expected to be similar to the scenario without glaciation. It may even increase if thetemperatures at the glacier bed are colder than the ground surface temperatures, which may occur.when the temperature in the area where the ice is forming is colder than that prevailingdownstream.Recharge and discharge decrease considerably during periods when permafrost is present. In thecase of a model with an open model boundary to one side, representing the coast for example,and a high topographic gradient (Case 1), a large drop in hydraulic heads is observed beneath thepermafrost. This results in lower groundwater flows at depth compared to unfrozen conditions.Where a modelled area is closed on all sides (Case 2), a decrease in flow at depth is alsoobserved, however the hydraulic heads do not decrease to the same extent as the hydraulicgradient is less than for Case 1. During permafrost thaw, hydraulic heads rise, resulting in anuptake of groundwater into elastic storage from recharge over the top boundary of the modeldomain.When taliks underneath surface water bodies develop, the groundwater flow system remainsmore active than during continuous permafrost. Recharge and discharge are focused on the lakesand a regional groundwater flow system connecting the lakes can develop. Heat advectionremains more important during thick permafrost when through taliks remain open.In the model, during periods of glaciation, hydraulic heads increase by ~1500 m at depth forCase 1 and Case 2 when ice loading is applied. When ice-sheet loading is not accounted for, thehydraulic head signal in low permeability layers is dampened. During glacial advance,groundwater recharge increases by up to two orders of magnitude, and during glacial retreatdischarge increases. During ice advance, groundwater flow is in a downward direction but duringice retreat it is in an upward direction. Depending on the flow direction of the glacier,groundwater flow directions can be reversed during a glaciation. Modelling the AnglianGlaciation (middle Pleistocene glaciation, equivalent to the Elsterian or Mindel glaciation inEurope and the Alps, most extensive glaciation in the British Isles, MIS 12), the hydraulic headand groundwater flow magnitude are affected by the glaciation for tens of thousands of years,whereas after the Devensian glaciation (late Pleistocene glaciation, equivalent to theWeichselian/Vistulian or W眉rm glaciation in Europe and the Alps, MIS 5d to 2), the signalremains for thousands of years.High hydraulic heads that may be present during glaciation are likely to modify the groundwaterflow around a GDF. The modelling presented here based on two settings and typical thermal andhydraulic properties for the rocks present, demonstrates that the depth of permafrost couldextend up to a depth of 300m below the surface and, depending on specific characteristics (largethermal conductivity and low porosity) and an exceptionally long cold period, could extend togreater depths. Permafrost to these depths may affect the engineering properties of some rocktypes and could lead to the development of new fracture pathways in more brittle formations.Permafrost could also affect some of the engineered components of a GDF in similar ways, suchas the properties of clay materials.
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