A Study Of The Hydrochemical Characteristics And Geneses Of Geothermal Water In The Western Edge Of The Sichuan Basin In China

A study of the hydrochemical characteristics of groundwater and the formation of geothermal water in the western Sichuan Basin is of great significance for enriching the understanding of groundwater circulation and water-rock interactions,and promoting the development and utilization of geothermal resources.This article takes 18 springs(cold springs,hot springs and geothermal wells)in the western Sichuan Basin as the research object.By understanding the natural geographic characteristics,stratigraphic characteristics,structural geology,geothermal geology,and hydrogeological background of the springs in the study area,the hydrogeochemical and isotopic characteristics of the springs are analyzed and summarized,and the formations of representative hot springs are explored.The study area starts from Guangyuan city in the north,Ya'an city in the west,and Yibin city in the south.Except for Yufuguodu hot spring(SC5),which is located in the basin,the other springs are on the western edge of the basin.The geothermal reservoirs of the Zhiyuanguan hot spring(SC1),Huashuiwan hot spring(SC4),Yuantou hot spring(SC14),Shunan hot spring(SC14),and Xunsi hot spring(SC18)are composed of Permian and Triassic carbonate rocks.The geothermal reservoir of the Lutang hot spring(SC7)consists of Jurassic rock.The temperature of the geothermal water in the study area ranges from 40.6 to 78?,showing geothermal water of low temperature.The pH of the geothermal water ranges from 5.7 to 7.8 with an average value of 6.99,indicating weakly acidic to weakly alkaline.The pH of the cold spring is 6.9-8.3,showing medium to weakly alkaline.Cluster analysis and factor analysis show that the springs in the study area falls into three categories:Type ? water with high TDS and high Cl water,which is mainly affected by the dissolution of halite and strontium-rich aragonite.Type ? water is medium TDS and high SO₄water,which is mainly affected by the dissolution of gypsum and anhydrite.Type ? water is low TDS and high HCO₃ water,which is mainly affected by the dissolution of calcite and potassium feldspar.Reverse simulation of the hydrochemical path shows that the dissolved amount of halite is 1705 mmol/L,and 167.5 mmol/L precipitation of albite occurs in the path SC13?SC12.The distribution pattern of rare earth elements of the hot springs in the study area shows a fluctuating increase.The total rare earth element content varies from 0.02?g/L(SC6)to 78.53?g/L(SC12),and the content is affected by pH.The isotope characteristics of the water samples in the area show that most of the springs are of meteoric origin.The deuterium excess parameter(d)of the springs in the study area varies between d=10‰and d=10‰.Based on the?²H and?¹⁸O values,the average elevation of the springs'replenishment area(the mountainous areas in the western edge of the basin)is estimated to be 810-2640 m.The temperature of the geothermal reservoirs of the hot springs estimated by the SiO₂ geothermal temperature scale,cation temperature scale,and multi-mineral balance diagram method ranges from 51 to 108?.By using the silicon-enthalpy mixing equation method,the mixing ratio of the cold water in the thermal groundwater is calculated as 43%-95%.According to the reservoir temperature,the circulation depth of the thermal groundwater is estimated to be1478-3792 m.The ²²⁶Ra-²²²Rn method is used to estimate the groundwater age in the study area,and 26-628 a is obtained.This article summarizes the formation of the cold springs,natural hot springs,and geothermal wells in the study area.The formation of the cold springs is relatively simple with shallow circulation of groundwater,with strong water alternation,and incongruent dissolution of salt-bearing rocks hardly occurs.For the natural hot springs,the Xunsi hot spring(SC18)is taken as an example.The formation is roughly summarized as follows:groundwater receives recharge from infiltration of precipitation in the nearby mountain area and undergoes deep circulation with strong water alternation,and incongruent dissolution of salt-bearing rocks occurs,and it is heated by the geothermal heat and rises to the surface and emerges as a hot spring in the river valley.For the geothermal well,the Zhiyuanguan hot spring(SC1)is taken as an example.The formation is summarized as follows:precipitation in the remote mountainous area infiltrates and replenishes the groundwater.It undergoes deep circulation and flows towards the basin.The subsurface flow and the water alternation are slow.The incongruent dissolution of salt-bearing rocks occurs.The groundwater is heated by the geothermal heat and rises to the land surface when is trapped by the geothermal wells.