Origin and recharge estimation of groundwater using chemistry and environmental isotopes in the Mahafaly sedimentary aquifer, district of Betioky Southwestern Madagascar

Hydrochemical and environmental isotopes studies are used to determine the groundwater origin and recharge estimation in the sedimentary aquifer of Mahafaly, southwest of Madagascar. The groundwater chemical type in the study zone varies according to the geological formation and the altitude of the sampling site. Groundwater at high altitude of the Isalo formation represents freshwater with Na-Ca-HCO₃-Cl type and it changes to more saline downwards while following the direction along the flow paths, where in the border of the Onilahy River the water type becomes Na-Ca-Mg-SO₄-Cl-HCO₃. Highly mineralized deep groundwater mixed with local rainwater occurs in the Sakamena basin except for the groundwater samples near the Onilahy River, which represents a mixing with the river. Groundwater isotopic composition results in the study area show that the aquifer is partially recharged by local rainfall during the summer season. Mixing and evaporation are the processes dominant which control the groundwater mechanism recharge. Besides, the tritium results have a mean value of around 0.83TU, which confirms that the majority of the samples, around 60%, are produced by mixing of recent and old groundwater from different areas. Hot springs have a tritium value less than 0.2TU indicating a paleo-recharge from a distant recharge zone. We propose a mean groundwater residence time in the study zone of around 75 years, which corresponds to a mean recharge rate of 60mm/year giving a total recharge of 2.7*108 m³/year over the total study area.