Origin and mineralization processes of groundwater in metamorphic hardrock aquifers in West Africa: Insights from stable isotopes (δ2H and δ18O) and major ions

The groundwater contained in the basement formations of sub-Saharan countries is the main source of drinking water for the population in rural and semi-rural areas. Captured by boreholes and wells, it is used for domestic purposes, irrigation, and other socio-economic activities. Given the increased pressure on the resource due to population growth and urbanization, assessing its origin, availability, and quality for sustainable management is imperative. This study was carried out in the Kara River Watershed (KRW) in northern Togo. The study aimed at determining the recharge and mineralization processes of the groundwater in the watershed using conventional graphs, multivariate statistical analyses, and geochemical modelling. Physico-chemical (pH, temperature, and TDS), chemical (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^-, SO₄^2-, Cl^-, NO₃^- and SiO₂), and isotopic (δ²H and δ¹⁸O) analyses were carried out on 149 groundwater samples (boreholes and wells). The results showed that the isotopic composition of the groundwater suggests recharge of meteoric origin, often influenced by secondary evaporation and important mixing processes. From a hydrochemical point of view, the groundwater is generally low mineralized with TDS ˂ 1000 mg/L. These waters' main mineralization processes are water/rock interactions and human activities influence. The heterogeneity of the geological formations is responsible for the spatial variability of the water chemistry, with CaMg-HCO₃^-, intermediate, and Na-HCO₃^- water types predominating. Hydrolysis of silicate minerals, ion exchanges, and dissolution of carbonate minerals (calcite and dolomite) are responsible for water mineralization. Nitrates of human and animal origin often strongly degrade water quality. The results of this study will enable decision-makers to implement a relevant strategy for the sustainable management of groundwater resources, particularly in the city of Kara, where human activities massively impact groundwater quality.

Keywords: Recharge processes, hydrochemistry, water/rock interactions, hard-rock aquifer, human activities, Kara River, Togo.