Hydrochemical characteristics and genesis analysis of hot springs in the Changtan area,Chongqing

LIU Cao; XU Gaohai; LI Delong; LUO Fuheng; XU Haifeng; CHEN Cheng; ZOU Jianshu

doi:10.11932/karst20250602

Abstract：This study takes the natural and artificially drilled hot springs as the research objects located in the northwest wing of the Fangdoushan anticline in Changtan Town,Wanzhou District,Chongqing City.It systematically examines their hydrochemical characteristics,heat storage conditions,and genetic mechanisms,revealing the unique formation process of the low-temperature Cl-Na type hot springs in the high-uplift anticline region.The hot springs in the study area are located within the carbonate rock layers of the Lower Triassic Jialingjiang Formation(T1j),and are controlled by the combined effect of the Fangdoushan anticline structure and local hydrogeological conditions.Through hydrogeochemical analysis,geothermal temperature scale calculations,and hydrogeological conceptual model analysis,the following key conclusions are drawn: 1.Chemical characteristics and genesis of water (1)The chemical composition of the hot spring water is classified as Cl-Na type(according to the Piper three-line graph).The pH is weakly alkaline(7.26 to 7.87).The mineralization degree is significantly higher than the regional background level,TDS up to 11.25 g·L-1.The proportions of Cl-and Na+are 36.72％and 55.37％,respectively.(2)The high mineralization degree stems from the concentrated halogen dissolution and filtration in the tidal flat-lagoon facies strata of the Triassic Jialingjiang Formation(T1j),as well as the water-rock interactions during the deep circulation processes(such as dolomite dissolution and sodium growth petrification).Additionaly,sulfate reduction reactions involving microorganisms(e.g.,H2S generation)and long-range migration(＞10 km)further enhance ion enrichment.(3)The content of Sr2+among the trace elements is notably high(37.06 mg·L-1),and its source is closely related to the water-rock interactions involving carbonate rocks and gelatinate layers in the Triassic Jialingjiang Formation(T1j). 2.Heat storage conditions and circulation mechanism (1)The calculation of geothermal temperature scales(quartz,chalcedony,and quartzite)shows that the heat storage temperature ranges from 54 ℃ to 78 ℃,with an average of 65.4 ℃,classifying it as a low-temperature geothermal system.Based on the ground temperature gradient,the circulation depth is estimated to be approximately 2,000 meters.(2)The heat storage layer is composed of interbedded limestone and dolomite from the Triassic Jialingjiang Formation(T1j),which overlies the mudstone of the second section of the Badong Formation(T2b2),the clastic rocks of the Triassic Xujiahe Formation(T3xj),and the sand-mudstone of the Jurassic system.Together,these formations create a water-proof and heat-insulating cover layer,thereby forming a closed heat storage structure. 3.Groundwater recharge sources and heat sources (1)Recharge sources:Atmospheric precipitation in the exposed area of the Triassic Jialingjiang Formation(T1j)at the core of the Fangdoushan anticline infiltrates in through karst fissures and sinkholes,generating deep runoff.(2)Runoff path:Groundwater migrates deeper along the fault-fracture system at the wing of the anticline,absorbs heat from the surrounding rock,and undergoes dissolution and filtration,with a retention time of several hundred years.(3)Heat source:There is no additional heat source,such as magmatic activity.The heat originates from the normal geothermal gradient(2.5 to 3.0 ℃·100 m-1),and the formation of the heat reservoir is driven by the warming of the stratum. 4.Three-dimensional genesis conceptual model (1)After the infiltrating as atmospheric precipitation,the water migrates deeper along the fracture network at the core of the anticline.It undergoes long-path circulation(approximately 2,000 meters)and slow warming,accompanied by intense water-rock interaction with salt-bearing carbonate rocks.(2)Deep faults,such as the outer dam reverse fault,act as secondary hydrothermal channels that locally supplement solutes and heat;however,the main heat source remains the natural warming of the crust.(3)Hydrogeological sections reveal that the combination of an aquifer and impermeable layer under the control of anticline structures,is the key to the occurrence and migration of thermal fluids.The mudstone of the Triassic Badong Formation(T2b)effectively blocks vertical heat loss and maintains the stability of thermal reservoirs.