Flow pattern and residence time of groundwater within the south-eastern Taoudeni sedimentary basin (Burkina Faso, Mali)

Abstract

Summary The knowledge about groundwater flow conditions within the Southeastern Taoudeni Basin Aquifer shared by Burkina Faso and Mali is relatively limited with very little information on potentiometric heads, recharge processes, residence time and water quality. A better evaluation of groundwater resources in this area is a strategic point for water resources management in the entire Soudano-Sahelian region which endures since the beginning of the twentieth century a continuous decrease in precipitation amount. This paper provides a transboundary synthesis using water (18O, 2H and 3H) and carbon isotopes (13C and 14C) in conjunction with hydrogeological and hydrochemical data. The objectives are to improve the conceptual model of groundwater recharge and flow within this sandstone reservoir, and to assess the changes in the aquifer due to water abstraction and recent climate changes including an insight into Sahelian aquifers palaeorecharge processes. The local meteoric water line for the Bobo-Dioulasso station is proposed: δ2H = 8.0 (±0.5)δ18O + 10.2 (±2.1). Two main tendencies can be derived from groundwater chemistry. First, a slight evolution from the Ca–Mg–HCO3 type towards a Na–K–HCO3 type that indicates developed interactions between groundwater and clay minerals related to the residence time of groundwater. A second tendency towards Cl–NO3–SO4–HCO3 water types indicates the anthropogenic influence on groundwater related to the poor sanitary conditions observed around wells. The carbon-14 activity measured on the TDIC varies between 0.3 and 122 pmC, so our record contains samples covering a wide period from Actual to Pleistocene suggesting a continuous recharge of the system through time even if the Sahel region has endured many different climate phases which have influenced the infiltration and recharge processes. All groundwater samples have stable isotope compositions in the range of the present day regional and global meteoric water line which suggests that the sampled groundwater was not significantly affected by evaporation during recharge. Evolved waters are depleted relative to unevolved samples by 1.5–2‰ in δ18O and 10–15‰ in δ2H. The whole dataset support the hypothesis of a largely unified homogeneous aquifer system with a multilayered structure but it also points out the very low renewability of the resource and a strong anthropogenic contamination of the shallowest horizons.