Evaluation of optimal conditions for managed aquifer recharge in the west African basement area.

Most African populations depend on groundwater in rural areas for their drinking water. Indeed, in the face of climate change and strong demographic growth, groundwater is increasingly in demand. The sustainability of water resources in this type of environment is becoming a major challenge. Managed Aquifer Recharge (MAR), which consists of inducing water infiltration through appropriate developments to replenish an aquifer's water stock, is therefore one of the measures that can be implemented to secure water supplies, combat the effects of climate change and, more generally, contribute to improving groundwater availability. However, the issue of the effectiveness of MAR systems, depending on the type of environment, still remains. The aim of this research is to determine the influence of aquifer and infiltration basin properties on artificial recharge, with a view to identifying the optimum conditions for setting up such a system.

Using synthetic modeling, we designed a representative domain of basement aquifers incorporating an infiltration basin. The hydrogeological properties of the different layers of the defined alteration profile were then determined, including the boundary conditions of the domain. The study involved varying the various physical characteristics of each layer, such as hydraulic conductivity (homogeneous and inhomogeneous), vadose zone thickness, storage, water table thickness and hydraulic gradient, and the characteristics of the infiltration basin, such as effective infiltration, recharge time, geometry and loading conditions (constant hydraulic head, variable hydraulic head). These simulations were carried out under the FEFLOW numerical model in both saturated and unsaturated zones, in order to test different solutions. The results of the simulations were then compared with those obtained using the Hantush analytical solution. These comparisons not only validated the model results, but also enabled us to carry out a sensitivity analysis of the validity range of the analytical solution by reproducing the different scenarios. This study presents an original approach both in terms of its methodology (analytical model, numerical model, application model) and its implementation in a basement zone and West African context.

The results show that simulations in saturated and unsaturated conditions are virtually identical. Most scenarios show a strong relaxation (1 to 2 days) after the injection time. The main parameters influencing recharge and relaxation are hydraulic conductivity, storage, unsaturated zone thickness and effective infiltration in the infiltration basin, while the hydraulic gradient has no significant influence. In addition, infiltration basins with variable hydraulic head (flow injection) performed better in terms of recharge (13 m difference) than basins with constant hydraulic heading. Finally, these results have enabled us to establish different hydraulic head curves as a function of different aquifer and seepage basin parameters, enabling effective inter-comparison.

Keywords: Basement area, FEFLOW, Infiltration basin, MAR, Modelling.