Hydrogeological Characterisation of a Sedimentary Aquifer and Springs Near the City of Bobo-Dioulasso, Burkina Faso: Isotopic Approaches

The aim of this study was to gain a better understanding of the functioning of the aquifer system and springs in the Pala basin through hydrochemical and isotopic analyses. Thirteen samples were analyzed, including two rainwater samples, three spring samples and eight borehole samples. The stable isotopic composition (<sup>2</sup>H and <sup>18</sup>O) of rainwater shows that it is virtually unaffected by evaporation as it falls. Groundwater d-excess values are high, with an average of 11.28 ‰, and all samples have values higher than the local rainfall average of 7.99 ‰. This result suggests direct infiltration of precipitation of oceanic and slightly continental origin, through a favourable geological context. The isotopic study shows that there are two different recharge periods: a cold, wet period with a depletion in <sup>18</sup>O and the current period. Around borehole E2, the oldest with a tritium content of 1.8 TU, recharge took place during this cold period. The other waters are the result of mixing with waters from the current period. The springs observed in the study area come from Sotouba sandstone aquifers after erosion exposed the piezometric water level in the Kou basin, hence their origin. Most of the boreholes tap the aquifers of both geological formations, which makes it difficult to distinguish the water from these two aquifers by analysis.