Hydrogeochemical characteristics of geothermal water in Hunan Province

Hunan Province hosts abundant geothermal water resources，while little research of hydrochemistry on them has been made. research is not thorough. The purpose of this work is to analyze the occurrence of geothermal water in Hunan Province，estimate the temperature of the heat storage in the study area，the mixing ratio of cold and hot water，and the depth of hot water circulation. Using the ion ratio method，phreeqc calculation of mineral saturation and the silicon enthalpy model， we analyze hydro-geochemestry of geothermal water at 69 sites in the study area. The results indicate that the main hydrochemical types of geothermal water are HCO3-Ca and HCO3-Ca.Mg， followed by SO4.HCO3-Ca.Mg and SO4-Ca； The ratio of calcium to magnesium ions in 77% of all geothermal water in the study area is greater than 3，reflecting the long storage time of geothermal water in Hunan Province. Using phreeqc to calculate the multi-mineral saturation index finds that the silica mineral is closest to the saturated state. Using the silicon-enthal pymixing model to estimate the heat storage temperature and the mixing ratio of cold water in the study area，the results show that the thermal storage temperature ranges 32 ℃~226 ℃，140 ℃ on average，and the average cold water mixing ratio is 85%，which is relatively large. The geothermal gradient is used to calculate the geothermal water circulation depth， yielding of 5-6 km， 5.34 km on average. In general，the geothermal water in Hunan Province has a long evolution time，a relatively long runoff time，and a large regional circulation depth. After the hydrothermal fluid is warmed by the thermal storage a long period of runoff and water-rock interaction，the geothermal water is driven by high pressure and thermodynamics. The geothermal water circulates to the surface of the ground，where it is mixed with cold water near the surface to form "immature" medium-low temperature hot water， dominated by low temperature.