Typology of hard rock ground waters within the Lower Sassandra, a main catchment in humid tropical West Africa
Résumé
Randomness of fracture networks still makes channelized flow a challenge to track in hard-rock aquifers. While not underestimating geological and hydrological criteria that are also handled here through mapping exercises, this study raises an issue of water quality encountered in lifelong boreholes. Chemical classification checked against a recent conceptual model of bedrock aquifers gives birth to a new typology of groundwater in a complex granitic aquifer system located in the SW of Ivory Coast (West Africa). Major ion chemistry, borehole completion data, digital elevation model and satellite images are used to interpret the geochemical water facies as an expression of connexions between the saprolite and the saprock, or transient insulation. From major ions ratios, cumulate mineralization, carbonate equilibrium, stable isotopes, the maturation of ground waters and mixing between bedrock layers are described at seasonal and local scales. The results highlight some vertical feeding of the water table into the main saprock aquifer owing to shortcuts through the saprolite, along with the existence of dead-ends in the hydraulically active fracture network. Also, some influence of fault zones, either drain or barrier, is confirmed on the (Ca, Mg) bicarbonate water facies within the saprock.