Assessment of geochemical processes in the shared groundwater resources of the Taoudeni aquifer system (Sahel region, Africa).

The Taoudeni basin occupies a large portion of the West African Craton in Mauritania, Mali, SW Burkina Faso, and Algeria. The multi-layer Taoudeni aquifer system (TAS) occurs in geological formations extending in age from the Infra-Cambrian to the Quaternary. These groundwater resources support local livelihoods, maintain vital ecosystems, and strongly affect terrestrial water and energy budgets in the region. Hydrogeochemical and stable (δ2H, δ13C, δ18O) and radioactive (3H, 14C) isotope analyses were carried out with the aim of identifying the major geochemical processes controlling groundwater quality and the recharge mechanisms. The analyzed groundwater samples showed a wide range of chemical compositions based on major ions, with limited variability, regardless of the aquifer lithology or stratigraphy. The predominant water type, Mg–Ca–HCO3, was found in most geological and hydrogeological settings. A principal component analysis of the hydrochemical data revealed the two major processes governing the geochemistry of groundwaters in the TAS. These are water–rock interaction (leaching, weathering, ion exchange) and anthropogenic contamination. Environmental isotopes revealed the presence of groundwaters with heterogeneous signatures, reflecting different recharge processes and varying groundwater ages. A widespread present-day recharge by direct infiltration of rainwater and/or by surface water was highlighted by significant tritium levels in most wells and high 14C activities occurring mainly in shallow aquifers. However, isotopically depleted δ18O contents with low 14C activities were found in the confined part of the Continental Intercalaire aquifer, particularly in the Algerian part, indicating the presence of fossil groundwater, recharged under colder past climatic conditions. [ABSTRACT FROM AUTHOR]