Hydrogeochemical characterization and quantification of the groundwater mixture in a hydrogeological basin of the middle zone of San Luis Potosí

The hydrogeochemical characterization and water-rock interaction processes in the subbasins of Santa Catarina and Ocampo-Paraíso in the Zona Media of San Luis Potosí have allowed identifying the types of groundwater. The results of the chemical analysis and the regional geological study show the possible rock-water interaction mechanisms. These results indicate that the composition of the groundwater in each of these basins reflects the dissolution of the carbonate and magnesium minerals of the carbonate rocks of Cretaceous age (El Abra Formation), by rainwater, and of the groundwater through a granular medium with a strong influence of the material of volcanic origin. The Ca-HCO₃, Ca-Mg-HCO₃, water type are modified during the groundwater flow by incorporation of Na⁺ and SO₄^2- ions. The San Nicolás (subbasin III) has covered by fluvial deposits in a climatic conditions of semiarid regions. The hydrogeochemical analysis of major ions, lithium and bromine allowed identifying that the extracted water from the region is a product of mixing process, involving three endmembers (ternary mixture). The first, Santa Catarina (C₁), has the lowest concentrations of bromine and lithium and its recharge occur in the highest altitude land in the region, towards the Sierra de Álvarez. The second end member (C₂), displays high values of bromine, low in lithium and originates towards the Ocampo Paraíso Valley. The third end member (C₃), has low bromine concentrations, high in lithium, and comes from San Nicolás Tolentino area. The mixing percentages indicate that C₁ contributes 50%, C₂ gives 31% and C₃ adds 19% water to the system.