A NEW VIEWPOINT CONCERNING GENESIS OF KARST THERMAL GROUNDWATER UNDER COLD-HOT WATER MIXING

Karst geothermal system is one of the most important geothermal types. The particularity of karst media makes geothermal groundwater easier to be influenced by the shallow circulating cold groundwater during its formation, as a result, differs the genetic mechanism of karst geothermal system from others. In present research, the karst geothermal system of Tongluoshan anticline at the margin of Sichuan Basin is taken as an example to reveal the potential misunderstanding of geothermal groundwater genesis caused by the mixing of cold and hot water. A comprehensive approach using of hydrogeochemistry and isotope coupled with field survey is performed to get insight into the mixing process of cold and hot water at the discharge zone of geothermal system. The effects of the mixing on the understanding of geothermal system are deeply discussed. A conceptual model of karst geothermal groundwater system with cold-hot water mixing was established. The results show that the mixing effect of cold and hot water is extremely strong at the discharge zone of geothermal system. The mixing ratio of cold water can be up to 73% ~89% in Tongluoshan geothermal system. The mixing of cold and hot water would potentially destroy the information of thermal groundwater for geothermal system, resulting in underestimating the recharge elevation and reservoir temperature. This underestimation reaches to 78% and 45% for recharge elevation and reservoir temperature, respectively, in Tongluoshan geothermal system. The true model of Tongluoshan karst geothermal groundwater system is recognized by eliminating the effects of cold and hot water mixing. The geothermal system is recharged by precipitation in the exposed karst mountain area at an elevation of 2100~2400 m at the north. Recharged water is heated by the terrestrial heat flow during its deep circulation. The reservoir temperature of carbonate aquifers of lower Triassic Jialingjiang formation(T₁j) and middle Triassic Leikoupo formation(T₂l) is estimated ranging from 128~172 ℃. Thermal groundwater is controlled by the karst and geological structure of anticline, and flows from the northeast to the southwest. Thermal water mixes with the local shallow circulating cold water during its discharge. As a result, the temperature of exposed thermal water declines to 38~62 ℃.