Study On The Genesis Pattern Of A Typical Karst Geothermal Water System In The Eastern Sichuan Basin And Tunnelling Impacts

Karst geothermal water is one of the important types of geothermal water resources.Compared with other geothermal water types,the characteristics and genesis pattern of karst geothermal water system are significantly affected by aquifer medium features（karst development）.The Tongjing geothermal system of Tongluoshan in eastern Sichuan basin was taken as an example in present research.Intergarated approachs of field investigation,hydrogeochemistry,environmental isotope and numerical simulation were employed to reveal the genesis pattern of karst geothermal water system and its interaction with tunnel construction.This research can provide scientific supports for the safety of traffic engineering construction and the protection of geological environment in karst hydrogeothermal activity area.The main findings of this research are as follows:（1）Tongluoshan anticline is with the typical features of southern karst in China.This region has developed karst at both groundsurface and subsurface.The aquifers of Leikoupo-Jialingjiang Formation and Xujiahe Formation in anticline are laterally divided into three aquifers groups at the core,east and west wings under the control of aquicludes of Jurassic and Feixianguan Formation and the geological structures.Three groundwater flow systems,namely the local,intermediate and regional system,are developed in the region.Groundwaters in Tongjing geothermal area belong to the local and intermediate ground flow system in regional.（2）There are two kinds of groundwaters in Tongjing karst geothermal area,one hot water with temperature above 25℃,the other cold water with temperature below 25℃.These two groundwaters are with significant different characteristics in hydrogeochemistry.The cold groundwaters are fresh with hydrochemical type of HCO₃-Ca type and TDS value below 1 g/L.All cold groundwaters belong to the local groundwater flow system and discharge with the temperature in the range of 15-20℃.The hot groundwaters are much salter with hydrochemcial type of SO₄-Ca and TDS value above 1 g/L.These waters are all from the intermediate groundwater flow system and with the exposure temperature of37-62℃.Strong mixing between cold water and hot water is occurred for geothermal water of the intermediate system in its discharge process due to the karstification development at the shallow depth of aquifers.The mixed cold water ratio can reach to 69%-89%at different depth.（3）The mixing of cold and hot water would potentially destroy the information of thermal groundwater for geothermal system of Tongjing,resulting in underestimating the recharge elevation and reservoir temperature.This underestimation reaches to 78%and40%for recharge elevation and reservoir temperature,respectively,in Tongjing geothermal system.The geothermal system is recharged by precipitation in the exposed karst mountain area at an elevation of 2000-2400 m at the north.Recharged water is heated by the terrestrial heat flow during its deep circulation.The reservoir temperature is estimated ranging from128-146℃.The circulation depth is estimated ranging from 3200-3800 m.Thermal groundwater is controlled by the karst and geological structure of anticline,and flows from the northeast to the southwest.The geothermal water mixes with the local shallow circulating cold water during its discharge.There are two ways of the geothermal water exposure:natural exposure and drilling exposure.（4）Groundwaters in the tunnel zone of Dongquan belongs to the local ground flow system in regional.Dongquan tunnel does not directly cross the intermediate flow system in regional.However,in the discharge process of deep circulation geothermal water in the intermediate flow system,the mixed cold water mainly comes from shallow circulation cold water in local flow system.Tunnel construction will have a strong impact on local water flow system.These effects will change the mixing ratio of hot and cold water of deep circulating geothermal water.Furthermore,it indirectly affects the discharge process of deep circulation geothermal system.The numerical simulation shows that the tunnel construction will cause the groundwater level in the inlet section to decline.It will not change the groundwater level in the middle and outlet sections.After the operation of the tunnel,the groundwater level of inlet section will rise.But the groundwater level will not exceed the bottom elevation of the tunnel.Based on the above understanding,this paper puts forward some suggestions for engineering construction,which can reduce the impact of tunnel engineering on groundwater environment in karst hydrothermal activity area.