The Geochemistry Of Carbonate Thermal Waters Hosted By Fault Area Within Southeastern Chongqing,China

The non-renewable energy has facing the resource exhaustion,and the resulting pollution has also caused the serious damage to the environment in the era of economic globalization.In order to practice the concept of“Sustainable Development”and“Harmony of Man with Nature”,the development of thermal waters or other clean-renewable energy is the top priority.Thermal waters are the new-green energy which integrates heat,water and mineral resources.Maybe,The thermal waters will replace fossil fuels as the main energy to mitigate global climate change in the future with the development and utilization of thermal waters worldwide.Because of the carbonate strata have the high porosity and permeability,the carbonate areas are rich in the groundwater.The carbonate thermal waters are also the most important thermal resources except the magmatic activity area.In addition,the mixing of shallow karst water makes the middle-low temperature thermal waters distributed in carbonate area widely.In a number of studies,the medium-low temperature thermal waters have the characteristics of good conditions,multiple use patterns and low technical requirements in the development and utilization.All above,it is an effective way to change the backward economic situation of carbonate areas to exploit the carbonate middle-low temperature thermal waters.Chongqing municipality located in parallel ridge-valley region in the eastern Sichuan and the karst area is half of the total area of Chongqing,nearly.A large area of karst forms a huge underground water storage.Therefore,Chongqing municipality is rich in thermal waters and enjoys a reputation of"the spa capital of the world".The study area located in the southeastern Chongqing municipality called“southeastern Chongqing”,which is a key function zone of ecological and a demonstration zone of poverty alleviation in China.With the implementation of national policies such as"Ecological Civilization and Green Development",the carbonate thermal waters hosted by fault within southeastern Chongqing will probably become the pillar industry to drive the economic development.Up to now,the exploitation and utilization of the thermal waters within southeastern Chongqing are few so that it is very important to carry out scientific research on them.Based on this,combine the field sampling?13 hot springs and 4 thermal wells?,we have tested the main hydrochemical and environmental isotope indexes to studied the geochemical process,water recharge?climate of recharge area and recharge period?,the heat source,the thermal reservoir temperature and the deep circulation characteristics of thermal water within southeastern Chongqing.Through literature review and field investigation,we have found that the thermal reservoir of thermal water within southeastern Chongqing are the Middle and Upper Cambrian and Lower Ordovician strata,which lithology including dolomite and limestone imbedded with evaporite?halite/gypsum/anhydrite?locally.The thermal reservoir cover are composed of the Middle and Upper Ordovician,Middle and Lower Silurian,and Permian and Triassic strata which the lithology including shale,silty shale and siltstone.The Semi-confining layer consists of the shale of Middle Cambrian formation.The temperatures of thermal waters within southeastern Chongqing are 19?54.2?which belong to the low-medium temperature thermal waters.The pH of the thermal waters are 6.8?8,which is weakly alkaline.The thermal waters are divided into two distinct hydrochemical groups:Group A,located to the west of the Yushan fault,are characterized by facies of chloride sodium waters with high TDS;Group B,located to the east of Yushan fault,are dominated by complex geochemical facies with sulfate-bicarbonate-chloride alkaline-earth waters with relatively low TDS.According to the Gibbs figures,the thermal waters in group A were distributed near the evaporation-concentration,and that in group B were distributed near rock weathering.However,the study area is not the arid area,it is speculated that the main ions in the thermal waters within southeastern Chongqing were all derived from the weathering of rock?evaporite?.From PCA and chemical ion proportionality coefficient analysis,the variance contribution rate?42.9%?of Na⁺,K⁺and Cl^–of thermal water in group A was closed to that of SO₄^2–,Ca²⁺and Sr²⁺?40.2%?,both are very high.Moreover,the thermal waters in group A were distributed in the lines of Na⁺:Cl^–=1:1 and Ca²⁺:SO₄^2–=1:1,which suggesting the main ions of thermal waters were affected by the halite dissolving,at the same time evaporite?gypsum/anhydrite?weathering was enhanced because of the dissolution of halite.For the group B,the variance contribution rate of Ca²⁺,Mg²⁺and SO₄^2–in the thermal water was 45.6%,and that of Na⁺,K⁺and Cl^–was 33.8%.All the waters were distributed near the line of Ca²⁺:SO₄^2–=1:1,indicating that the dissolution of evaporite?gypsum/anhydrite?was the main factor in the thermal water of group B.Also there was a little halite dissolution.According to?³⁴S_SO4 in thermal waters?25.3‰–37.4‰?and?¹⁸O_SO4?9.17‰–16.2‰?,the SO₄^2–in thermal waters were derived from the dissolution of evaporite?gypsum/anhydrite?formed during the Cambrian and Ordovician periods.Another,the⁸⁷Sr/⁸⁶Sr ratios of thermal waters were 0.7090–0.7098,averaging at 0.7093.And the concentration of Sr²⁺was positively correlated with the concentrations of Ca²⁺and SO₄^2–,while that was negatively correlated to the concentration of HCO₃^–,indicating that the dissolution of evaporite?gypsum/anhydrite?played a major role in thermal water,but the dissolution of carbonate was inhibited from dissolution of evaporite?gypsum/anhydrite?.The?D_H2O?–62.1‰––41.8‰?and?¹⁸O_H2O?–9.2‰––7.0‰?of thermal water are distributed near the precipitation line of Chongqing and the world which indicating the thermal water came from the recharge of atmospheric precipitation.The recharge elevations of thermal waters were concentrated in 767–1267 m,with an average of 1094m by the formula of recharge elevation except No.16.It is speculated that the recharge area was the middle mountainous area in the study area where elevations were 1000–1500 m.The calibration ages of thermal waters were 9–21 ka BP except for No.16,averaging at 14.8 ka BP,that is,the thermal water recharge was the late Pleistocene meteoric precipitation with a relatively humid and cold climate from the A¹⁴C_DIC which calibrated by ALK model.The average thermal reservoir temperature of thermal water in Group A was 69?,and that in Group B was 81?from geothermometers of quartz-correlation and theoretical chemical.According to the geothermal gradient and circulation depth formulas,the average geothermal gradient in the study area was 21?/km.The circulating depthes of thermal waters in Group A were 2.1–3.5 km,with an average of2.5 km,that were 2.5–4.2 km for Group B,with an average of 3.1km.From the Na-K-Mg Giggenbach plot and the values of sulfur isotopes,both No.16 thermal water in group A and thermal waters of group B were evenly distributed in the vicinity of Mg²⁺in the Na-K-Mg Giggenbach plot.The?¹⁸O_SO4 of thermal waters?Nos.2,3,7,9 and 10?weres lower than that of the value from the dissolution of evaporite?gypsum/anhydrite?,but gradually closed to?¹⁸O_SO4 of river water in the study area?5.78–12.89,mean 8.43,unpublished?,indicating the mixing phenomenon between the thermal water and cold water.Based on the mixing model of ⁸⁷Sr/⁸⁶Sr ratio and the concentration of Sr²⁺,the average dilution of thermal water in the study area was calculated to be 65%.Based on the above research,the paper put forward a conceptual model of the carbonate thermal waters hosted by the fault within southeastern Chongqing:The meteoric precipitation penetrated continuously to received the heat conduction in the deep of the earth when it had reached the carbonate rock which exposed in the recharge area.Then the meteoric precipitation flowed from the recharge area to the discharge area through the karst subsurface morphology due to the influence of gravity?topography?with the water-rock interaction between the surrounding rock and thermal waters.The water-rock interaction has changed the hydrochemical characteristics of thermal water.The thermal waters in group A are characterized by facies of chloride sodium waters with higher TDS;the thermal waters in group B are dominated by complex geochemical facies with sulfate-bicarbonate-chloride alkaline-earth waters with relatively lower TDS.The mix between thermal waters and shallow karst waters has occurred as the thermal waters rose.Finally,the thermal waters exposed in the deep fault and cross-cutting of rivers,forming the thermal water resources.