Hydrogeochemical and isotopic characterizations of an aquifer in the semi-arid region of the Mexican Highlands

This work addresses hydrogeochemical processes in shallow aquifers, represented by the Quaternary alluvial deposits, from a part of the semi-arid Mexican Highlands through the evaluations of physicochemical parameters as well as the δ¹⁸O and δ²H compositions of groundwater. Mifflin diagram separates the Na-bicarbonate, sulphate and mixed groundwater into one group showing interactions with volcanic lithology in the recharge zones and another group interacting with the evaporite rich sedimentary formation. In the Gibbs diagram, the samples with Cl^?/(Cl^? + HCO₃^?) < 0.4 showed higher influence of ion exchange and the samples with Cl^?/(Cl^? + HCO₃^?) > 0.4 showed higher effects of evaporation. All of them were oversaturated with carbonate minerals (i.e., calcite and dolomite) and unsaturated with evaporites (i.e., gypsum and halite). Evaporation (earlier stage) occurred before the water-rock interactions (later stage). Evolution occurs through three different routes such as up to 7.5‰ enrichment in δ¹⁸O caused by the infiltration with a certain delay and subsequently, the water-rock interaction became dominant. Ternary mixing models revealed dominant influence of local recharge (C₁) on the water system, contributing 70.4%. Water-rock interaction (C₃) with 18% and evaporation (C₂) with 11.6% had comparatively less influences. The presence of nitrate (2–60 mg/L) in the groundwater indicated variable degrees of anthropogenic pollution.