Palaeo-Recharge Impact on Aquifer Hydrodynamics: Paris Basin Case

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 over the last five million years. For the purpose of this work, a three-dimensional transient modeling of the Paris basin aquifer/aquitard system was developed using the code NEWSAM (Paris School of Mine). The geometry and hydrodynamic parameters of the model originate from a basin model, NEWBAS (Paris School of Mine), built to simulate the geological history of the basin. Geomorphologic evolution is deduced from digital elevation model analysis, which allows estimating of river-valley incision and alpine uptlif. Climate forcing results from palaeoclimate modeling experiments using the LMDz atmospheric general circulation model (Pierre Simon Laplace Institute) with a refined spatial resolution, for the present, the Last Glacial Maximum (21 kyr BP) and the Middle Pliocene (~3 Myr). The water balance is computed by the distributed hydrological model MODSUR (Paris School of Mine) and provides recharge value to aquifer at each time step. We present the simulated evolution of piezometric heads in the system in response to the altered boundary conditions. For the present, the comparison of head patterns between steady state and time dependent simulation shows little differences for aquifer layers close to the surface but suggests a transient state of the current system in the main aquitards of the basin and in the deep aquifers, characterized by abnormally low fluid potentials. The dependence of the boundary-induced transient effects on the hydraulic diffusivity is illustrated by means of a sensitivity study. Time dependant change of recharge induces rapid inversion of leakage orientation through the multilayered aquifers that may induce modification of groundwater quality.