Aquifers overridden by ice sheets and groundwater recharged into permeable beds from the melting ice base are important components of the hydrological cycle in regions affected by glaciations. Research on subglacial groundwater is very scarce and often disputable, but the available data allow the following generalizations:

1 Glaciers pressurize groundwater and impose regional head gradients driving groundwater toward the ice margin where it is released into subaerial drainage systems. If the glacier margin rests on permafrost, the zone of pressurized ground-water may extend for tens of kilometres into the ice forefield.

2 Groundwater systems are significantly if not entirely reorganized during glaciations as compared with the non-glacial conditions. Local topographic catchments are replaced by large, ice-shape controlled regional catchments that extend from glacier termini to the ice divides or to cold-based inner parts of ice sheets. Groundwater flow velocities increase several times, and flow depth may reach several kilometres flushing deep aquifers and aquitards. Non-glacial meteoric groundwater may be replaced completely by glacially fed groundwater.

3 Drainage capacity of the bed influences the ice movement mechanism by controlling the sediment strength and the extent of basal coupling. A substratum of low hydraulic conductivity facilitates fast ice flow by bed deformation and/or basal sliding on a thin water sheet. Indirectly, the drainage capacity of the bed influences glacier transport processes ranging between the subglacial transport in the deforming bed and englacial transport when the deforming bed is absent.

4 Large Pleistocene ice sheets overrode beds whose transmis-sivity was typically insufficient to evacuate all the subglacial meltwater as groundwater flow. The system responded by formation of channels, the remnants of which, in the form of tunnel valleys (some over 500 m deep and hundreds of kilometres long) and eskers, occur abundantly across Europe and North America.

5 Pressurized groundwater may trigger profound disturbances in sediments and rocks such as hydrofracturing and glacio-tectonic thrusting and folding, and it may create specific landscapes of soft-sediment extrusion.

6 Glacial groundwater is characterized by a distinct chemical composition, notably low 18O and 2H content and high content of dissolved oxygen, and rocks affected by glacial meltwater may show specific mineralogical and chemical signatures. Old glacial groundwater trapped in aquitards has been documented from several bedrock and sediment areas in Europe and North America, down to depths of several hundreds of metres and more.

7 The predicted reorganization of groundwater flow in areas affected by future glaciations, especially its increased flow dynamics, deep penetration and highly oxidizing chemistry, is of major concern for planning repositories of radioactive waste because many radionuclides have half-lives well outside the next glacial cycle.

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