Genesis of Geothermal Water in Southwestern China: Insights from Hydrochemistry and Stable Isotopes

The research objective is to constrain the genesis and evolution of medium–low temperature geothermal waters hosted in carbonate rocks within complex fault-controlled settings, which represent cost-effective geothermal resources yet remain poorly understood in terms of their hydrochemical behavior and recharge mechanisms. The methodology involves a comprehensive multi-isotopic and hydrogeochemical investigation of 17 geothermal water samples (13 hot springs and 4 wells) collected from southeastern Chongqing, southwestern China. Analytical techniques include ion chemistry, stable water isotopes (δDH2O, δ18OH2O), sulfate isotopes (δ34SSO4, δ18OSO4), and 87Sr/86Sr ratios, complemented by chemical geothermometry and principal component analysis. Field investigations and laboratory measurements were systematically conducted to ensure data robustness. The results, along with their implications, reveal several key trends. Isotopic altitude effects indicate that recharge occurs in mid-altitude mountains (elevations of 767–1267 m). Water–rock interactions are predominantly governed by halite and gypsum dissolution, with sulfate isotopes and 87Sr/86Sr ratios (ranging from 0.7090 to 0.7098) consistently confirming that Sr2+ and sulfate are largely derived from evaporite dissolution. Geothermometer calculations yield average reservoir temperatures of approximately 79.3 °C across the study area. Notably, during upwelling, mixing with shallow karst water results in an average cold-water fraction of 35% (up to 95% in the most diluted sample), substantially lowering its surface temperature. Based on these trends, a conceptual model is proposed wherein meteoric water infiltrates, is heated at depth, rises along fault systems, and undergoes continuous cold-water mixing during ascent. The significance of these findings lies in providing a refined understanding of geothermal water evolution in fold-thrust belts, thereby supporting the sustainable exploration and utilization of low-enthalpy geothermal resources in carbonate aquifers.