Assessment of deep aquifer complexity by long period numerical path lines
Résumé
In the large sedimentary basins such as the Aquitain Basin (Southwest of France), the hydrogeological behaviour of the multi-layered aquifer system is complex. The hydrodynamic and isotopic chemistry coupled data can bring some answers on the main flow and the recharge of the whole system. The confrontation of hydrodynamic parameters with isotopic geochemical data was carried out on the Infra-Molassic Sands (IMS) groundwater of the Aquitain Basin where the apparent water age, for most of the samples, ranges from -15 000 to -35 000 years BP. Moreover, this main upper aquifer is located in a regional multi-layered aquifer system. The solution adopted to analyse the behaviour of this system consists in heuristic numerical modelling of this aquifer in order to carry out "numerical" path lines over several thousands years. The variability of the depth of the aquifer of the IMS (from + 300 down to - 2000 meters) needs to take into account the thermal parameters. The model so generated covers a surface of approximately 32 000 km². In a first step, we investigated a model with a recharge only on the outcrops areas and a flow model by "piston" effect in a mono-layer system. The calculated water's age distribution obtained by this model of horizontal flow does not agree with the observed age data. In a second step, the aquifer of IMS was integrated into a multi-layered system. The results obtained highlighted the principle of a recharge of the groundwater of the IMS by vertical leakage on long transient regimes through the overlying aquitard formation. These models allowed to show the respective importance of vertical and horizontal flows on a regional scale, and thus to better understand the behavior of this system.
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