Présentations Colloques

In northern Estonia, groundwater from the Cambrian-Vendian aquifer is the most depleted in 18O in Europe (about - 21‰) and is further characterized by low salinity, high excess air and cold recharge temperatures inferred from noble gases. 14C dating indicates ages ranging from 30 to 19 ky BP. Based on these observations, it was suggested that the Cambrian-Vendian aquifer was recharged by subglacial meltwater beneath the Fennoscandian ice-sheet during the last glaciation.**The Cambrian-Vendian aquifer is a major source of drinking water in Estonia and the sound management of this resource requires more information about its origin and its long-term evolution. For this purpose, a numerical model was built, simulating transient groundwater flow and solute transport. It consists in a 2D cross-sectional model that crosses Estonia along a NW-SE direction. The model was run over the last 25 ky in order to include the Last Glacial Maximum (LGM), during which the entire country was covered by the Fennoscandian ice-sheet. Subglacial recharge is simulated by prescribing hydraulic heads at the land surface that are proportional to the overlying ice-sheet thickness. Calibration of the model is performed using water levels, as well as delta18O and salinity data. **First, the preliminary results of this model show that subglacial recharge is indeed a possible mechanism explaining the presence of fresh water in the deep subsurface even though several thousand years have elapsed since deglaciation. Secondly, inverse modelling was performed using a probabilistic approach to determine the most probable value of some key variables such as the ice-sheet thickness, the initial time of the LGM, the initial delta18O composition of meltwater and the hydraulic diffusivity of the confining layer overlaying the Cambrian-Vendian aquifer. Finally, we discuss whether the present-day hydrodynamics is at equilibrium or still reflects the perturbation of the last glaciation.