Explaining the groundwater salinity of hard-rock aquifers in semi-arid hinterlands using a multidisciplinary approach

Shallow crystalline groundwater in the semi-arid hinterland of Ceará is brackish or saline with mixed chloride or sodium chloride facies. Very few hydrochemical data are available for the area and the drivers behind this salinity are not clearly identified. In this study, extensive field data collection was performed to provide new information about the hydrogeological functioning and the salinization processes, through the implementation of piezometric, hydrogeochemical, isotopic (18O, 2H) and multitracer dating (14C, 3H, CFC, SF6) monitoring. Piezometric and isotopic data evidence fast flow circulation processes and a high contribution of evaporated surface water to aquifer recharge. Multitracer dating shows the groundwater is essentially composed of seasonal vertical infiltration flows that mix with older waters stored in the aquifer. Chemical analyses suggest the groundwater, originally low mineralized, has become progressively saltier due to leaching of salts that were evapoconcentrated in either surface waters or the unsaturated zone during drier periods.