The Formation Mechanism And Hydrogeological Parameters Of Karst Thermal Water

Nanyang thermal spring is a unique type of groundwater in the Xiangxi River Karst Basin of San Xia area.Due to its special geographical location,its formation mechanism and the sustainable exploitation of thermal groundwater resources have been paid much attention.Hydrogeological investigation in karst area can be carried out to find out the spatial structure of water-bearing system in karst area,Hydrogeological investigation in karst area can be carried out to find out the spatial structure of aquifer system in karst area,identify the genetic mechanism of karst underground hot water,and determine the hydrogeological parameters of geothermal water in karst area,which is of great theoretical and practical significance for the sustainable development and utilization of Nanyang thermal spring.Based on the project of China Geological Survey and taking Nanyang thermal spring in Xiangxi River basin as an example,this paper studies the hydrodynamic characteristics of karst groundwater from the qualitative and quantitative aspects based on hydrogeology,hydrogeochemistry and groundwater hydraulics.The qualitative research aims to analyze the chemical characteristics of groundwater in karst area by means of hydrogeological exploration,hydrochemistry and environmental isotopes,and to identify the movement of geothermal water in San Xia karst area,so as to develop the conceptual model of the formation mechanism of karst geothermal water.Combining with the conceptual model of geothermal water in karst area and the pumping test in the field,the quantitative research aims to develop the analytical model and hydrothermal numerical model of groundwater in karst area,to explore the inversion method of hydrogeological parameters of karst aquifers and determine the parameters of thermal water aquifers,which can provide theoretical basis and technical guidance for the sustainable development and utilization of Nanyang thermal spring in Xiangxi River Basis.The main results are as follows.Precipitation around the mountains of Shennongjia Group at the northwest end of the Jiuchong Fault is thought to be the main recharge source for the thermal water.From the hydrochemical characteristics,the thermal groundwater is determined to be generally Cl-Na type.The dissolution of minerals such as rock salts and dolomite results in high concentrations of Na⁺?590.0 mg/l??Cl^-?780.5mg/l?,TDS?1806.7 mg/l?and minor elements?Sr,F?.The thermal water flows through a longer subsurface path,and the water-rock interaction is more adequate,which belong to the regional groundwater circulation.The shallow cold groundwater are characterized by HCO₃-Ca or HCO₃-Ca·Mg type,which belong to the local flow system defined by shorter slow paths and low water-rock interaction.Taking into account the results of different chemical geothermometers,fluid-mineral equilibrium method and silica-enthalpy mixing model,the actual estimated mean reservoir temperature of the Nangyang thermal spring is136.4?145.0°C?average 141.8°C?.The circulation depth of the thermal spring was measured at about 3,623 m.Furthermore,the silica-enthalpy mixing model and graphic method show that Nanyang thermal spring is a mixture of approximately 84.9%shallow cold groundwater and 15.1%deep thermal groundwater within this geothermal area.Based on the above analysis,the formation of Nanyang thermal spring is summarized as follows:the thermal spring is recharged by atmospheric rainfall in the middle and upper reaches of the Jiu Chong River in Shennongjia Mountain area,infiltrating into the fissure aquifer system of metamorphic rocks in Shennongjia Group,and flows into the Jiuchong fault.The groundwater is transported and heated along the path,and flow southward to the intersection of Xinhua fault in the north of Nanyang town,then the thermal groundwater flow upwards to the surface after mixing with the carbonate cold shallow groundwater of the local water system as it encounters the impermeable shale of Cambrian Shipai Formation and Nantong Formation.The carbonate aquifer in the study area presents dual media characteristics.The caves developed in dissolved pores,medium-wide fissure and fracture zone are the channels for the rapid movement of groundwater,the fissure network composed of micro-fissures and the pore media are the main part of aquifer,which plays a role of water storage during the movement of groundwater.Based on this,the dual-porosity flow concept model and analytical model of partially penetrating well in karst aquifer have been proposed,and the hydraulic parameters of carbonate aquifer are evaluated by field pumping test data.The results show that the hydraulic parameters of Cambrian Shilongdong formation:the radial hydraulic conductivity of fissure??is 0.41 m/d,the the vertical hydraulic conductivity of fissure??is 0.41 m/d,the specific storage coefficient of the matrix??is 0.1 m^-1,and the exchange rate?u?is 0.8.The parameters of Cambrian Qinjiamiao formation:the radial hydraulic conductivity of fissure??rangs from 0.31 to 0.82 m/d,the vertical hydraulic conductivity of fissure??is 0.05m/d,the value of the specific storage coefficient of the matrix??rangs from 4×10^-3 to8×10^-3 m^-1,and the exchange rate?u?rangs from 0.005 m^-2 to 0.02 m^-2.The error ranges are between 0.31 and 1.304,and the fitting results meet the accuracy requirements.It provides a novel and practical approach to estimate the aquifer parameters in fracture-karst aquifers with a partially penetrating well.Based on the formation model of Nanyang thermal spring,a hydrogeological conceptual model was established.Combing with the results of the shallow carbonate aquifer parameters,an analytical solution model and a three-dimensional numerical model of geothermal water have been developed.The hydrogeological parameters of geothermal water are identified and verified by using the experimental data of two pumping test from 5 August 2016 to 12 August 2016.The results show that the hydraulic conductivity of Ordovician Dengying formation rangs are 532.32?781.21m²/d,and the pressure conductivity coefficient value rangs from 3.825×106 m²/d to5.475×106 m²/d.The hydraulic conductivity values of deep thermal groundwater in Nanyang thermal spring are from 95.06 m²/d to 204.50 m²/d.The relative error range between the simulated and measured values is 1.7%-15%,and the results are reliable.The research results of this paper deepen the understanding of the hydrodynamic characteristics of Nanyang thermal spring,which not only provide theoretical basis and technical guidance for the sustainable development and utilization of Nanyang thermal spring,but also provide new technical methods for the field investigation,hydrogeological survey tests and the determination of hydrogeological parameters of such geothermal water resources in karst areas.