Hydrochemical and isotopic evidence of groundwater evolution in unconfined aquifer systems of the semi-arid Cariri Valley, Northeast Brazil

Study region The Cariri Valley, located in the Araripe Basin of northeastern Brazil, hosts two stratigraphically distinct unconfined aquifer systems that constitute the primary groundwater source for domestic, agricultural, and industrial water supply in one of the largest semi-arid regions of South America. Understanding the hydrochemical evolution and recharge dynamics of these aquifers is essential for sustainable groundwater management under increasing climatic variability. Study focus This study investigated the hydrochemical and isotopic differentiation between the Missão Velha–Rio da Batateira Aquifer System and the Mauriti Aquifer using groundwater monitoring data collected in 2017 and 2018 within the Brazilian Integrated Groundwater Monitoring Network (RIMAS). Major-ion chemistry, stable isotopes (δ¹⁸O and δ²H), mineralogical characterization by X-ray diffraction, saturation indices calculated with PHREEQC, and hydrochemical indices were integrated to evaluate recharge conditions, water–rock interaction, and groundwater evolution. New hydrological insights for the region Both aquifer systems have a meteoric recharge origin but differ in hydrochemical variability and multivariate composition. The MV–RB showed broader compositional dispersion, whereas Mauriti samples occupied a more restricted domain; however, overlap between systems remained and the small Mauriti sample size limits generalization. Principal component analysis explained 63.34% of the variance in the first two components. PERMANOVA identified a multivariate group effect (pseudo-F = 4.42; p = 0.0071), while a significant dispersion test (p = 0.0376) showed that this result partly reflects the greater heterogeneity of the MV–RB. Silicate weathering and cation exchange are consistent with the mineralogical and ionic evidence, while nitrate and chloride indicate localized surface-derived influence. Stable isotope compositions plot near precipitation-derived meteoric references and support a common recharge source, with the data being consistent with limited pre-infiltration evaporation. The results refine the conceptual model of groundwater evolution in the Cariri Valley while explicitly recognizing analytical-quality and sampling limitations.