Modelling subglacial hydrogeology in the European lowland: Where did all this water go?

Pleistocene ice sheets have profoundly affected groundwater flow in Europe and North America. Local sub-aerial catchment areas were replaced by orders-of-magnitude larger catchments controlled by ice sheet topography, and pressurized groundwater flowed much faster and deeper than during interglacial conditions. Glacially-fed groundwater flushed aquifers and penetrated deep aquitards leaving a characteristic isotopic and chemical signature. Subglacial groundwater flow, by evacuating meltwater from the ice-bed interface may have strongly influenced ice sheet stability.
A series of numerical experiments was conducted to learn more about possible subglacial groundwater drainage conditions under the Elsterian ice sheet at the southern fringe of the Scandinavian Ice Sheet (SIS). The modelling was 3D, steady-state simulation performed for different palaeo-glaciological scenarios with a range of realistic ice thicknesses over an area of about 300.000 km2 between the present Baltic Sea in the north and the Karpathian/Sudetian Mountains in the south. Hydrogeology of the modelled area was generalised into 6 major units down to the impermeable substratum (Permian salt or bedrock) at a maximum depth of up to ca. 4.6 km.
The most plausible simulation scenario shows a total re-organization of the groundwater flow field under the ice sheet in relation to the modern (interglacial) situation. The groundwater flowed from the Baltic Sea towards the mountains in the south (i.e. in the opposite direction than today) and discharged at the ice sheet margin. Due to the generally low hydraulic conductivity of the substratum, only a small fraction of the basal meltwater likely drained as groundwater flow whereas the remaining amount of the water must have been evacuated to the ice margin by some other mechanisms remaining elusive at present. In extensive areas along the southern and western fringe of the Scandinavian Ice Sheet with similar palaeo-hydrogeological conditions tunnel valleys occur, which drained the surplus meltwater from the ice-bed interface. However, in the study area only very few, ill-developed, tunnel valleys of Elsterian age occur suggesting widespread subglacial meltwater storage and, therefore, instabilities in ice sheet behaviour leading to surges and ice streaming.