Characteristics And Genesis Of The Thermal Groundwater Of Basin-anticline Outcropping Type Near The Main Urban Area Of Chongqing In Eastern Sichuan Basin

Thermal groundwater of a basin-anticline outcropping type occurs near the main urban area of Chongqing in eastern Sichuan Basin.This paper mainly discusses the unique occurrence and emergence characteristics of the thermal groundwater in the study area,analyses the hydrogeochemical characteristics and evolution of the hot water,reveals the genesis of thermal groundwater,and clarifies the characteristics of groundwater flow system under the influence of the anticlines,which is helpful to enrich the understanding of the occurrence and circulation of the thermal groundwater,and provides an important scientific evidence for the development and utilization of the thermal groundwater in Chongqing.The thermal groundwater in the carbonates of the Lower Triassic Jialingjiang Group and Middle Triassic Leikoupo Group near the main urban area of Chongqing has a unique occurrence that is called the“basin-anticline outcropping”type.Its occurrence and emergence are strongly controlled by the nearly north-south trending anticlines.The thermal groundwater of the basin-anticline outcropping type is similar to that of the basin type but also is characterized by the outcropping type because of the anticlines.The hot springs in the study area exist mainly in the outcropping areas of the carbonates,in the middle and the plunging ends of the anticlines where the topography was cut by rivers.They can also rise through the overlying sandstones and form up-flow springs in the plunging ends of the anticlines.Geothermal wells tapping the carbonates geothermal reservoirs on the flanks of the anticlines also produce thermal groundwater?Drilling hot springs?.The thermal groundwater of low-to-medium temperature?32.9-57??near the main urban area of Chongqing is weak acid to weak alkaline?pH,6.2-8.46?with SO₄-Ca type and TDS of1,620-2,929 mg/L.Hierarchical cluster analysis?HCA?divides the water samples in the study area into three categories,namely:high TDS-high Ca and SO₄ water;medium TDS-high Na and Cl water;low TDS-high HCO3 water,respectively.Factor analysis?FA?show that the dissolution of gypsum and anhydrite in the carbonates geothermal reservoir is the main hydrogeochemical processes,which restrains the dissolution of carbonate rocks.The reverse simulation of the hydrochemical paths quantitatively confirms the above water-rock reactions.Combined with the hydrochemical characteristics of the hot water in the same geothermal reservoir in the adjacent area?Cl-Na type?,it is found that the hot water in the study area is gradually desalinating,but has not yet reached the fresh water of HCO₃-Ca type.The range of reservoir temperature obtained by silica geothermal geothermometers is 56-98.7??average 75.8??,from which the circulation depth of the thermal groundwater in the study area is estimated to be 820-2419 m.The multi-mineral saturation indices modeling indicates that the temperature is larger,and the results obtained by the Fixed-Al method are basically consistent with those calculated by the geothermometers.Silica-enthalpy mixing equation and silica-enthalpy model indicate that the mixing proportion of the cold water in the thermal groundwater may be larger with the average of 0.79.The thermal groundwater receives recharge from precipitation in the outcropping areas of the carbonates in the cores of the nearly north-south trending anticlines,and undergoes the subsurface circulation along the eastern and western flanks of the anticlines to varying depths.After obtaining heat from the heat flow from below,the groundwater converges to the low terrain cut by rivers and rises to the surface in the form of hot springs.Along the anticlinal structures,the groundwater flow system can be divided into three levels:???shallow circulation system with groundwater of HCO₃-Ca type and low TDS discharging through normal temperature springs,???middle circulation system with groundwater of SO4-Ca type and TDS of 2-3 g/L discharging through hot springs and???deep circulation system with groundwater of Cl-Na type and high TDS discharging through hot springs or wells.