Large-scale spatial reconstitution of groundwater fluxes in a karst aquifer on the basis of hydrodynamic and hydrodispersive measurements

This article presents a multiphysics-based approach for a large-scale spatial reconstitution of head and tracer tests responses measured in a regional scale karst aquifer in Southern France. The main dataset consists in hydraulic heads measured in 11 wells during 6 weeks and 4 tracer tests. A pump station withdraws groundwater from this aquifer at varying flow rates during days and nights to supply water to Montpellier agglomeration, generating a periodic signal which propagates in the whole aquifer. Some of the measurements show a periodic response, some others not. As a first step, this specificity is exploited to localize spatially the preferential flow paths network with a structural deterministic inversion approach. Then, inversions of the weekly averaged head variations are performed on the first and the last weeks of the dataset (showing different drawdown trends) to assess the hydraulic diffusivity field. Finally, the tracer restitutions at the spring are inverted in order to optimize the conduits diameters associated to the fastest flow paths. The proposed approach permits a satisfying reproduction of the hydraulic heads and tracer measurements. Simulations with the optimized model on a week of the dataset not used in the inversion allows to validate the inversion result. The inverted model suggests that daily periodic responses represent efficient constraints for the localization of the preferential flows, while the integration of weekly head variations provides a characterization of the hydraulic properties between each well and the closest preferential flow paths, and tracer restitutions permit to characterize the velocities in the preferential paths.