Hydrogeochemical Characterization of Thermal Waters from the Guaraní Aquifer in Uruguay and Their Potential Use in Balneology

Thermal groundwater resources constitute valuable health-oriented georesources, particularly when integrated into regional strategies for wellness, balneotherapy, and therapeutic tourism. This study presents the first comprehensive and integrated hydrochemical, geospatial, and balneological characterization of thermal groundwater systems in Uruguay, enabling their classification from a medical hydrology perspective and supporting the assessment of their potential use in balneotherapy. Seven thermal groundwater sources located in northwestern Uruguay were investigated, mainly associated with the Guaraní Aquifer System (GAS), together with the singular Almirón spring, which represents a distinct hydrogeological setting. Field measurements and laboratory analyses were conducted to determine physicochemical parameters, major ions, and gases. Hydrogeochemical facies were identified using Piper and Gibbs diagrams, while multivariate statistical techniques, including Principal Component Analysis (PCA) and hierarchical clustering, were applied to discriminate water types and support their balneological classification. The results indicate that most thermal waters associated with the GAS are characterized by sodium–bicarbonate facies, weak to medium mineralization. Dry residue to 180 °C, (311–734 mg/L), and mesothermal to hyperthermal temperatures (36.3–44.5 °C), reflecting deep confined circulation and prolonged water–rock interaction. By comparison, the Almirón spring exhibits a chloride–sodium facies with strong mineralization. Dry residue to 180 °C, (6590 mg/L) and hypothermal (32 °C), consistent with a distinct hydrogeological origin involving crystalline basement and Devonian sedimentary units and reflecting more evolved geochemical conditions. Based on the obtained results, and by analogy with comparable international hydrothermal profiles, the main balneological indications of these waters include musculoskeletal and rheumatic disorders, dermatological disorders, and other emerging indications such as stress, sleep disorders, obesity, and Long COVID. In conclusion, this study reveals the hydrochemical diversity of Uruguay’s thermal groundwater and its possible use in balneology. Future research should focus on controlled clinical and balneological studies to validate specific therapeutic effects.