Hydrochemical Characteristics Of The Hot Springs In Western Yunnan And The Factors Affecting The Deposition Of Travertines From The Hot Springs

Deposition of travertine from hot springs is a special natural phenomenon.The hydrochemical characteristics of 14 travertine-depositing hot spring samples and 7 hot spring samples that do not deposit travertine in western Yunnan of China are compared to obtain a better understanding of the hydrochemical factors affecting deposition of travertine from hot springs.It is useful to reveal the migration characteristics of substances in groundwaterThe result shows that all the travertine-depositing hot springs contain relatively high concentrations of CO₂,Ca²⁺and HCO₃^-.Their partial pressures of CO₂?0.03⁰.76 atm?are much higher than that of atmosphere?0.04%atm?and the values of pH range from 5.9 to 7.7.Hydrochemical investigations show that water samples in the travertine-depositing hot springs have relatively high TDS?higher than 590 mg/L?and they are mainly of HCO₃-Ca type.The concentrations of Ca²⁺and HCO₃^-are higher than 80 mg/L and 300 mg/L,respectively,and the values of?Ca²⁺/?HCO₃^-are in the range of 0.1 to 2.The saturation index values of calcite?SIc?are almost positive.By comparison,the concentrations of Ca and HCO₃ in the hot springs that do not deposit travertine are relatively low,the range of value of?Ca²⁺/?HCO₃^-is mostly lower than 1 and the hydrochemical types vary greatly.The Heinitang hot spring is located in the western Yunnan of China and lies in the Tibet-Yunnan geothermal zone and the Tengchong Block.The?²H and?¹⁸O values of the Heinitang hot springs are relatively low and range from-76.5‰to-63.9‰and from-8.9‰to-9.8‰,respectively.The data points fall on and near the GMWL and TMWL.The hot water is of meteoric in origin and obtains heat from heat flow when it undergoes deep circulation of 1495¹682 m.The estimated reservoir temperatures of the Heinitang hot spring range from 84 to 92?.The water samples are of HCO₃-Ca·Na type and the F concentration is relatively high and in the range of 3.8 to 5 mg/L,which are much higher than the World Health Organization?WHO?maximum guideline value?1.5 mg/L?.The F concentration in natural waters depends on such factors as temperature,pH,presence or absence of complexing or precipitating ions and colloids,solubility of fluorine-bearing minerals,anion exchange capacity of aquifer materials?OH for F?,the size and type of geological formations traversed by water,and the amount of time that water is in contact with a particular formation.Minerals which have the greatest effect on the hydrogeochemistry of fluoride are fluorite,apatite,micas,amphiboles,certain clays and villiaumite.Fluorite is the main mineral controlling aqueous fluoride geochemistry in most environments.Field investigation and hydrochemical analyses indicate that the primary factor affecting the deposition of travertine is the concentration of ion pair CaCO₃⁰ in hot springs.The Heinitang hot spring shows a positive correlation between the concentration of CaCO₃⁰?ion pair?and the saturation index with respect to aragonite,calcite and dolomite.They are not linearly related,but are of an exponential relationship.When the aragonite,calcite and dolomite are in the state of dissolved equilibrium,the corresponding concentrations of CaCO₃⁰ are 0.01mmol/L,0.008 mmol/L and 0.012 mmol/L,respectively,indicating that the precipitation of travertine can occur when the concentration of CaCO₃⁰ is higher than0.008 mmol/L.In addition,the concentrations of CaCO₃⁰ in the Heinitang hot springs decrease gradually with the deposition of travertine.The precipitation of travertine cannot occur when the concentration of CaCO₃⁰ decreases to lower than 0.008mmol/L.