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 HCO -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 δ 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.