Formation Mechanism Of Thermal Groundwater And Characteristics Of Geothermal Field In Gaoligong Mountain, Dali-Ruili Railway

Newly-built Dali-Ruili railway will get through Gaoligong Mountain areas in the form of tunnel, and may experience high-temperature heat and water hazards because hydrothermal activity is strong in tunnel zone. Study on thermal activity and geothermal field characteristic in tunnel zone is of great significance, for it can not only solve basic geothermal geological problems but predict the heat-harm problems of the linear project. Field investigation on Gaoligong Mountain is carried out during June ~ July 2010, and simple analysis, race elements and isotopes results of 84 groups hot spring water, 135 groups cold spring water, 9 groups gully water, 7 groups reservoir water, 3 groups of gas composition test results, 50 groups borehole temperature data, 96 rock thermal conductivity test results, were taken as the basic data to study formation mechanism of thermal groundwater and spatial distribution of the geothermal field. Conclusions are as follows:(1) Exposed features and chemistry characteristics of hot water. The study area was divided into five hydrothermal activity according to hot springs explosion:ⅠNu River SN-trending tectonic hydrothermal activity zone,ⅡHuangcaoba hydrothermal activity zone,ⅢZhaoyang - Pingda hydrothermal activity zone,ⅣLuxi Basin hydrothermal activity zone andⅤLongchuan River hydrothermal activity zone. The order of water temperature isⅡ>Ⅲ>Ⅴ>Ⅰ>Ⅳ, and salinity increases with temperature. Hydrochemical types of thermal groundwater inⅠandⅣzone are mainly HCO3-Ca·Mg, and thermal groundwater inⅡ,ⅢandⅤare mainly HCO3-Na.(2) Supply, runoff and discharge of thermal groundwater. The supply of thermal groundwater mainly is precipitation. Groundwater runoff and drainage are controlled by geological structures. Hot water flows along north-south fractured zone, and discharges at junctions of faults or the contact surface between complex and surrounding rock. Huangcaoba fault behaves as acquiclude, and shallow cool groundwater and deep-runoff geothermal water are independent. It is calculated out based on the altitude effectiveness ofδ18O andδD, that the recharge height of hot water inⅠ~Ⅴareas are 750~1400m, 1500~2300m, 1400~2700m, 1050~1350m, 1500~2200m respectively. Deep-runoff geothermal water inⅡandⅢis recharged mainly by ancient meteoric water at the age of about 50 years, hot water ofⅠ,Ⅳ,Ⅴis mainly from modern water at the age of <5 to 10 years.(3) Hydrothermal genesis. Groundwater inⅡis heated in the deep circulation along fault. Hot water inⅢandⅤbelongs to magmatic heating mode. Some springs inⅡandⅢbelong to mixed model of deep cycle-heating and magmatic-heating. Hot water inⅠandⅣis the mix of deep-cycle heating and shallow meteoric water mixing.(4) Geothermal system type. Study area belongs to medium ~ low temperature convective system. Silicon - enthalpy mixing model indicated that part of hot water exposed in Q strata of Nu River valley and Luxi Basin is mixed by shallow cool water with the ratio of 60%~78%. Calculated temperature of hot water before mixture is higher than the value obtained by geothermal thermometric scale of SiO2 significantly.(5) Geothermal field characteristics: There are two block sections of high ground temperature(> 50℃) in Gaoligong Mountain zone. The former is centered by hot springs in Banglazhang and Wuhe on right bank of Longchuan River, the latter is Chaoyang-Pingda complex body area. Taking into account the role of heat conduction and convection, two zones of high temperature gradient greater than 6℃/ 100m exist in north and south of Huangcaoba fault. Geothermal gradient along the Nu River, Luxi basin and Huangcaoba fault zone is relatively low. Heat flow according to heat conduction in Gaoligong Mountain zone is average 39.24mW/m2, in the range of normal to slightly lower. Two anomalies of high earth temperature locate in the intensive north-south Lushui-Ruili fault zone and north of Huangcaoba fault zone.(6) Prediction on heat-harzerd of proposed tunnel. The proposed tunnel is an optimum arrangement from the view of thermal point. The tunnel will get through zones of relatively low geothermal gradient and avoid zones of high earth temperature and high heat flow. Segment of rock temperature on tunnel bottom <28℃accounts for 25% of the total length, 28 ~ 37℃is for 68%, and 37 ~ 50℃is for 7%. Earth temperature in tunnel is mainly of low and normal temperature.