Hydrogeochemical evolution of a semiarid endorheic basin, with intense agricultural and livestock activities

• Four groundwater flow systems identified: local, local-sulfated, intermediate and regional. • Gypsum and plagioclase dissolution drive groundwater chemistry; halite plays a role too. • Surface water irrigation in agriculture alters groundwater chemistry in deep wells. • Ion exchange impacts groundwater evolution linked to alluvial and lake sediments. • Nitrate addition and reduction in irrigation areas affect the groundwater in deep wells. Water resources in semi-arid endorheic basins are increasingly at risk due to human activities that significantly alter natural systems. The Comarca Lagunera Region (CLR) in northern Mexico exemplifies this issue, where extensive groundwater extraction for agriculture and rapid urbanization have led to a continuous decline in the water table and deteriorating water quality. Recent studies emphasize the urgent need to address groundwater pollution in the region. This study aims to characterize the physical and chemical properties of groundwater and assess the factors and processes influencing groundwater chemistry. Using a hydrogeochemical framework, mixing rates were quantified through endmember mixing analysis, and inverse hydrogeochemical models were developed to explain groundwater evolution. The results likely suggest four primary groundwater flow systems − local, local-sulfated, intermediate, and regional − with a predominant Na-SO 4 type, followed by Ca-HCO 3 -SO 4 type. Five endmembers were identified, including contributions from the Nazas River, which supplied 7–44 % of pumped groundwater in wells within irrigation district DR017, correlating with elevated NO 3 − concentrations. Inverse hydrogeochemical modeling revealed distinct flow paths and quantified key geochemical processes. Carbonate, gypsum, pyrite and albite dissolution/precipitation played a crucial role in the local and intermediate flows, while halite and gypsum dissolution predominated in regional flow. Ion exchange reactions influenced hydrochemical changes across all flow compartments. Regional mass transfer analysis indicated that the principal ions were SO 4 2− (8.0–17.5 mmol/l), Na + (3.2 to 50.0 mmol/l), Ca 2+ and Mg 2+ (4.2 – 8.1 mmol/l), and NO 3 − (0.0 – 3.9 mmol/l). The findings have significant implications for groundwater management in the CLR and similar semi-arid endorheic basins. By understanding the complex interactions between natural processes and anthropogenic influences, this research provides critical insights into groundwater evolution. The results can be used to propose strategies to mitigate groundwater pollution, enhance water resource management, and promote sustainable agricultural practices in groundwater-dependent regions.