The Possible Source of Abnormally High Sodium Content in Low-Salinity Geothermal Water

The dissolution of sodium-containing minerals in high-temperature geothermal systems can cause Na⁺ to exceed 400 mg/L with high salinity. But the Na⁺ of low-salinity geothermal water is mostly less than 100 mg/L in medium-low temperature geothermal systems. However, geothermal water with Na⁺ up to 325.4 mg/L and total dissolved solids less than 650 mg/L was found in the Huangshadong geothermal field, which is a typical medium-low temperature hydrothermal system in South China. The water chemistry results indicate that thermal groundwater is uniformly HCO₃ -Na type with high sodium content (average 240.06 mg/L). All the thermal groundwater and shallow groundwater have the same meteoric origin based on δD and δ¹⁸ O. According to water chemical geothermometers and multicomponent mineral equilibrium (MME) method, the reservoir temperature is estimated to be 100 to 130 °C at a maximum depth of 2.43 km. The estimation of the Cl^- mixed indicator suggests that geothermal water has mixed with 51% to 72% of shallow groundwater, resulting in the reduction of Na⁺ content in real geothermal water (Na⁺ up to 685.2 mg/L). The simulated results of water-rock interactions indicate that mineral dissolution and ion exchange have minor contributions to Na⁺ enrichment in geothermal water. Hydrochemical simulations and Gibbs diagrams suggest an additional source of high sodium: granite fluid inclusions are fractured into geothermal water at high temperatures. Granite fluid inclusions may only account for 3% to 5% of geothermal water, but they provide the main source of Na⁺ in geothermal water.