Hydrochemical characteristics and genesis analysis of hot springs in the Changtan area, Chongqing
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摘要: 渝东北地区高隆起背斜两翼发育的地下热水系统具有独特的水化学特征与成因机制。为揭示其水化学特征及其与典型地下热水的成因差异,选取方斗山背斜西北翼长滩地区天然温泉及钻孔温泉为研究对象,通过水文地球化学测试、水化学图解、地热温标和地下热水循环深度计算,结合区域水文地质条件探讨其形成机理。结果表明:(1)研究区温泉水化学类型为Cl-Na型,主要离子含量呈现出Na+(40.27 g·L−1)与Cl−(60.72 g·L−1)显著富集特征;(2)基于SiO2地热温标计算显示热储温度为54~78 ℃,结合地温梯度推算地下水循环深度达
2000 m;(3)地下水系统接受方斗山背斜核部灰岩出露区大气降水入渗补给,热源机制以围岩传导热为主。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. -
表 1 研究区地下水类型及其赋存环境一览表
Table 1. Summary of groundwater types and their host environments
地下水类型 地下水亚类 赋存地层 赋存环境 碳酸盐岩溶水 碳酸盐岩类溶洞裂隙水 T2j3 赋存于方斗山背斜核部及北西翼近核部,表层岩溶作用导致
地形上形成溶蚀沟谷、溶洞、垄脊状沟谷等地貌类型碳酸盐岩夹碎屑岩溶洞裂隙水 T2j4 碳酸盐岩夹碎屑岩裂隙-孔隙水 T2b1、T2b3 基岩裂隙水 碎屑岩层间裂隙水 T3xj 赋存状况受制于砂岩含水层厚度、裂隙发育程度及地貌条件 红层承压水 红层承压水 J1z、J1zl、J2x 赋存于泥岩之间的砂岩含水层中,独立的补径排系统 网状裂隙水 基岩风化带网状裂隙水 J2s 赋存于侏罗系红层浅部风化带网状裂隙中,其富水性极弱 表 2 水化学综合指标数据表
Table 2. Comprehensive index data of water chemistry
水样采集点 pH 标准差 TDS/mg∙L−1 标准差 总硬度/mg∙L−1 标准差 Eh/mV 标准差 钻孔温泉 7.26 0.30 112.50×103 6197 617.1×10 16.92 −214 22.79 天然温泉 7.87 0.32 879.62×102 4591 264.8×10 19.38 −117 24.83 磨刀溪地表水 6.82 0.38 763.20 123.31 386.60 21.47 369 36.50 表 3 钻孔温泉物质含量表
Table 3. Ion contents of artificially drilled hot springs
物质类别 物质种类 SY01 SY02 SY03 SY04 SY05 SY06 均值 主要阴阳离子 Na+/ g∙L−1 38.01 43.04 38.02 36.60 47.20 38.75 40.27 Ca2+/ g∙L−1 1.69 1.67 1.73 1.72 1.69 1.67 1.69 Cl−/ g∙L−1 56.83 67.48 56.32 56.32 66.13 61.24 60.72 ${\rm{SO}}_4^{2-}$/ g∙L−1 5.44 4.73 5.46 5.91 5.62 5.49 5.44 K+/ mg∙L−1 109.86 1119.73 919.95 932.03 1058.00 1015.26 1015.26 Mg2+/ mg∙L−1 476.61 451.34 439.57 435.29 549.12 501.27 475.53 ${\rm{HCO}}_3^{-}$/mg∙L−1 286.91 377.51 285.93 237.00 326.15 280.43 298.99 微量离子 F−/ mg∙L−1 3.44 3.48 3.80 3.92 3.60 3.50 3.62 ${\rm{NO}}_3^{-}$/ mg∙L−1 2.82 1.19 0.12 0.14 0.21 0.15 0.77 Li+/ mg∙L−1 0.46 0.41 0.54 0.51 0.49 0.55 0.49 Sr2+/ mg∙L−1 41.69 22.60 42.25 43.67 41.52 30.65 37.06 Zn2+/ mg∙L−1 0.15 0.15 0.36 0.19 0.22 0.11 0.20 Cu2+/ mg∙L−1 0.23 0.28 0.26 0.21 0.16 Cr3+/ mg∙L−1 0.59 0.04 0.39 0.14 0.02 0.20 Mn2+/ mg∙L−1 0.34 0.32 0.31 0.43 0.28 0.01 0.28 硫化物 H2S/ mg∙L−1 153.86 117.86 133.47 121.64 140.26 123.15 131.71 表 4 钻孔温泉热储温度计算表
Table 4. Calculation of thermal storage temperature for artifically drilled hot springs
取样批次 SiO2浓度/ g∙L−1 T1/℃ T2/℃ T3/℃ Ta/℃ SY01 31.48 50.31 81.62 51.85 61.26 SY02 34.81 54.80 85.26 55.81 65.29 SY03 40.80 62.14 91.17 61.82 71.71 SY04 32.55 51.79 82.82 53.18 62.60 SY05 46.63 68.57 96.30 66.69 77.19 SY06 26.63 43.12 75.71 44.93 54.59 均值 35.48 55.12 85.48 55.71 65.44 表 5 钻孔温泉地下热水循环深度计算表
Table 5. Calculation of depth of underground hot water circulation in artificially drilled hot springs
取样批次 g/m·℃−1 T/℃ t0/℃ h0/m h/m SY01 40 61.26 17 25 1795.39 SY02 40 65.29 17 25 1956.60 SY03 40 71.71 17 25 2213.43 SY04 40 62.60 17 25 1848.89 SY05 40 77.19 17 25 2432.45 SY06 40 54.59 17 25 1528.41 -
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