Applied multivariate statistical analysis and geochemical modeling for assessing groundwater reactions in the...

Assessing groundwater quality in the semiarid Tucano Central Basin (Brazil) is critical for ensuring sustainable water resource management. The São Sebastião-Marizal Aquifer System mitigates surface water scarcity, yet the processes governing groundwater geochemistry remain underexplored. Therefore, this study integrates hydrogeochemical modeling, mineral stability analysis, and multivariate statistical techniques to characterize water-rock interaction processes and identify the dominant geochemical controls on groundwater composition. The results demonstrate that groundwater is generally undersaturated with respect to primary minerals (e.g., plagioclase, chlorite, calcite, K-feldspar), indicating active dissolution. PHREEQC speciation modeling and mineral stability diagrams indicate that CO₂-rich recharge waters undergo incongruent hydrolysis with aluminosilicates, releasing Na⁺, Ca²⁺, Mg²⁺, K⁺, and HCO₃^-, driving groundwater evolution within the kaolinite stability field. Saturation indices (SI ≈ 0) suggest a progressive approach to thermodynamic equilibrium with secondary mineral phases such as kaolinite, gibbsite, illite, and Ca-montmorillonite. Under supersaturated conditions, Fe²⁺ released by mineral weathering precipitates as hematite and goethite, contributing to lateritic soil development. Principal component and hierarchical cluster analysis revealed two distinct hydrogeochemical trends: in the leaky confined São Sebastião aquifer, cation exchange dominates, leading to Na⁺ enrichment and Ca²⁺Mg²⁺ depletion; in the unconfined Marizal aquifer, reverse ion exchange and weathering of secondary clays result in Ca²⁺ and Mg²⁺ enrichment. Anthropogenic impacts, such as nitrate and sulfate inputs from agriculture, may be present but remain secondary to natural geogenic processes. The combined approach effectively elucidates groundwater evolution pathways, providing essential insights for sustainable groundwater management in the Tucano Central Basin and comparable semiarid regions.