Geochemical and Isotopic Marks for Tracing Groundwater Salinization: Santiago Island, Republic of Cape Verde, Case Study

In Santiago Island, the overexploitation of coastal aquifers and pollution are among the main problems related to groundwater resources assessment and management. The national water authorities are dealing with brackish groundwater for agriculture and human supply in numerous parts of the Island, as it is the only type of water available. Solutes and environmental isotope obtained in different aquifer systems were used in the identification/sources of groundwater resources degradation. Chemical and isotope analyses were carried out in groundwater samples in order to understand the role of anthropogenic activities and the main origin of salts in the groundwater units. The results obtained indicate water-rock interaction mechanisms as the major process responsible for the groundwater quality (mainly Ca–HCO₃-type), reflecting the lithological composition of the Island’s geological formations. The geochemistry, together with the isotopic data, gives essential information on groundwater recharge, as well as on the identification of salinization mechanisms (e.g., seawater intrusion, salt dissolution, and marine aerosols). The isotopic pattern presented by part of the groundwater samples can be related to direct infiltration of irrigation waters undergoing significant evaporation, during the dry periods. Carbon-14 determinations (six boreholes) indicate an apparent groundwater ages between 1.3 and 3.1 ka BP.