PAST-CLIMATIC AND GEOMORPHOLOGIC FORCING INFLUENCE ON PRESENT-DAY HYDRODYNAMICS

Climate and geomorphologic variables are among the main driving force of the recent environmental evolution of sedimentary basin. From an hydrodynamic point of view, those variations find expression by boundary condition changes along time of multi-layers aquifers/aquitards: reduction of the recharge due to climate oscillations (glacial & inter-glacial) with permafrost occurrence, drainage conditions under uplift and river-valley incision processes, fluctuation of the base level due to eustatism. Here we investigate the response of the Paris basin groundwater system to variations in its hydrodynamic boundary conditions induced by past climate and geodynamic changes, we focus on the results obtained for the last cyclic alternation of glacial and interglacial interval while each process is tested independently. To this end, a three-dimensional transient modelling of the Paris basin was developed using the code NEWSAM (Ecole des Mines de Paris, ENSMP). The geometry and the hydrodynamic parameters of the model originate from a basin model, NEWBAS (ENSMP), built to simulate the geological history of the basin. The prescription of spatially and temporally evolving hydrodynamic boundary conditions originates from geomorphologic and climatic scenarios. A sensitivity analysis of each individual mechanism was made over the last 100 ky cycle and a special attention to the permafrost representation. Modification of the recharge is the most likely mechanism, which seems to have an impact on hydraulic head differences, notably the absence of recharge during the permafrost period. Topographic modifications have a low effect on hydraulic head changes and sea level fluctuations have a transient effect localized along the seacoast. Results show a rapid dissipation of perturbation in aquifers while aquitards keep longer memory of their effect, i.e., in the range of several 10 ky depending on the diffusivity of the considered formation. An inter-glacial state over the last climate cycles, lasting less than 10 ky, is also a predominant factor in maintaining a transient regime. Our results show that the Paris basin is definitely not at equilibrium with present-day climate and topographic conditions. Its present transient hydrodynamics is at least a consequence of past glacial and inter-glacial oscillation.