Hydrogeochemical Characteristics and Evolutionary Mechanisms of the Nanping Geothermal Field, Southeastern Hainan Island, China

The southeastern Hainan Island boasts abundant hydrothermal resources, most of which are exposed as thermal springs. Analyzing the hydrochemical characteristics, hydrochemical evolutionary mechanisms, and material transition of these resources is significant for their exploitation and utilization. This study investigated the Nanping geothermal field in southeastern Hainan Island, using five groups of geothermal water samples collected in 2022, as well as seven groups of geothermal water samples, one group of shallow groundwater samples, and one group of surface water samples taken in 2023. Specifically, this study examined water–rock interactions in the geothermal field using the Gibbs model, ion ratios, chloro-alkaline indices (CAIs), and the sodium adsorption ratio (SAR). Moreover, the mineral transfer process in groundwater was analyzed using inverse hydrogeochemical simulation. The results indicate that in the study area the geothermal water temperatures range from 64 °C to 80 °C, pH values from 8.32 to 8.64, and TDS concentrations from 431 mg/L to 623 mg/L. The primary hydrochemical types of geothermal water in the study area include Cl-Na and Cl·HCO₃-Na, suggesting low-temperature, slightly alkaline geothermal water. The hydrochemical components of geothermal water in the study area are primarily affected by water–rock interactions. Besides the dissolution of silicate minerals and halite, cation exchange reactions contribute greatly to the formation of Na⁺ and K⁺ in geothermal water. Geothermal water receives recharge from the atmospheric precipitation of the Diaoluo Shan area in the northwest of the study area, with the recharge elevation ranging from 967 to 1115 m. The inverse hydrogeochemical simulation results reveal that during the water–rock interactions, silicate minerals, clay minerals, gypsum, and halite dissolve, while quartz and carbonate minerals precipitate. Additionally, these processes are accompanied by cation exchange reactions dominated by the replacement of Na⁺ in surrounding rocks by Ca²⁺ in geothermal water. This study can provide a geological basis for the exploitation, utilization, and management of the Nanping geothermal field.