Hydrochemical Characteristics And Hydrogeochemical Simulation Of Hot Springs Along The Bijiang River In Yunnan

The emergence of geothermal waters along the Bijiang river transmit from depths and closely impact by the central axis fault of the Lanping basin.The hot springs emerge in the region with characteristics of well-mineralized and semi-drought red layers where is famous for this special landform. Mixing with shallow meteoric water in the flow system is a dominant hydrogeological process leading to the thermal water temperature decreasing and hydrochemical characteristics different ranging from HCO3-Ca?Mg water type to Cl-Na water type with a wide range of total dissolved solid(TDS, 0.644-8.17 g/L). According to the Silicon-enthalpy graphic method, the geothermal reservoir temperatures are estimated between 118℃ and 215℃, and mixing ratios between non-thermal and thermal groundwaters are 0.76 on the average. The high concentrations of Na and Cl indicate there are some well mineralized cold water in the flow system to circulate to the thermal water due to intense water-rock reactions in depth. High concentrations of Ca2+ and SO42- reflect progressively longer flow path in circulation to the surface with the well solubility of gypsum and the poor solubility of Ca-Montmorillonite. Hierarchical clustering and multivariate statistical methods recognized three hot springs groups and four parameters groups which can reflect the similar hot spring discharge zone.Based on the geochemical evolution and reaction simulation, a kind of travertine deposition model was established. The Lianchangping hot spring water was investigated in detail in order to examine travertine deposition and establish the downstream variations in the measured parameters. The dissolution and precipitation of minerals can dominate the concentrations of Ca2+, Mg2+and HCO3- to some extent. The Lianchangping hot springs are of SO4?HCO3-Ca?Na type with total dissolved solids(TDS) ranging from 3560 to 4240 mg/L. Hot water samples collected along travertine slope show that PCO2, TDS and contents of HCO3 and Ca decrease downstream along the travertine slope duo to travertine deposition. The initial milliequivalent ratio of γCa2+/γHCO3- is greater than 1.5 before travertine precipitation and there is an increasing trend in the ratio downstream along the travertine slope. Hydrochemical evolution along the slope is calculated with the Phreeqc. Sets of minerals calcite, dolomite, chalcedony, halite, gypsum, kaolinite, Ca-montmorillonite and CO2 gases reacted in appropriate amounts are found through reaction simulation, quantitatively accounting for the variations in chemical compositions, especially depositing minerals along the flow path on the travertine slope. The influence of the deep tectonic and hydraulic discontinuities connection over the circulation evolution of the subsurface waters is shown by the different hydrochemical compositions and the minerals possibly participated in the water-rock reactions. Minerals of kaolinite, Ca-montmorillonite, calcite, dolomite, chalcedony, plagioclase and CO2(g) play an important part in the spring thermal water flow path. The establishment of appropriate geochemical modeling can help restoring and exploring the deep circulation pattern and providing quantitative analysis of travertine deposition which is significant for enrichment of the theory of hydrochemical circulation.