Dynamic analysis of hydrochemistry and isotope of the karst spring of Jinlong Cave in the northern section of Taihang Mountains
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摘要: 以太行山北段金龙洞岩溶泉为研究对象,通过数理统计、离子比值及饱和指数等方法,分析泉水水化学和同位素动态特征、水中主要离子来源及演化、水–岩相互作用过程等内容。结果显示:(1)金龙洞泉补给来源为大气降水,泉流量对其响应程度高,泉流量较小时,水中主要离子含量相对高,水化学类型为HCO3 · SO4-Ca·Mg型,泉流量大时,水中离子含量低,水化学类型为HCO3-Ca、HCO3·SO4-Ca型,泉流量增加引起的稀释作用对离子含量影响明显;(2)控制泉水水化学特征的主要因素为溶滤作用和稀释作用,且Ca2+、Mg2+、
${\rm{HCO}}_3^{-}$ 主要来源于碳酸盐岩溶解,${\rm{SO}}_4^{2-}$ 、Na+、K+、Sr主要来源于安山岩中长石、黄铁矿等矿物的风化溶解,${\rm{NO}}_3^{-}$ 则来源于人类活动;(3)降水集中期,泉水主要由灰岩区孔洞裂隙水进行补给,补给路径短,降水较小期,则由海拔相对较高的火山岩分化裂隙水进行补给,径流路径较长;(4)氢氧稳定同位素动态特征表明:水中D、18O含量变化主要受降水和入渗过程中的蒸发及地表水混入影响,且不同季节和时期泉水主要补给来源不同。Abstract:Karst groundwater is an important part of the groundwater system and one of the important resources to ensure human survival and development. The study area is located in the northern part of Taihang Mountains in the west of Baoding City, Hebei Province. The spring water samples were collected monthly in Jinlong Cave, and their dynamic characteristics of hydrochemistry and isotope were analyzed. The study area presents a low-middle mountain landform with erosive structure and warm temperate semi-arid climate. It is a volcano-sedimentary basin with complex geological and structural conditions, which is one of the typical representatives of the thrust nappe structure in the north-central section of Taihang Mountains. The caprock is exposed in a large area. And the strata are well developed with Precambrian, Cambrian, Ordovician, Carboniferous Benxi Formation and Jurassic strata from the bottom to the top. The lithology is mainly composed of dolomite, limestone, shale and andesite. The Fupingian gneiss, in fault contact with the caprock, is exposed around the sedimentary rock mass. The main water-bearing rock formations are medium-thick limestone and dolomite of the middle and upper Ordovician Majiagou formation, Yeli formation and Liangjiashan formation, and limestone of the upper Cambrian Gushan formation. The groundwater type is the fissure water in exposed carbonate karst cave. Through the analysis of spring water samples and spring flow monitoring data, the change of spring flow is basically synchronized with the change of precipitation, indicating that the local precipitation is the main source of supply for the spring. According to the flow monitoring data, the response of spring flow to precipitation lags 14-50 days. The main ion concentration in the spring, the spring flow and precipitation all experience changes in different degrees. Generally speaking, the concentration of each ion in the spring water is relatively low at the period of concentrated precipitation (July and August). However, it is opposite in other periods that the concentration of each ion in water is relatively high with different fluctuation ranges in different periods. In a word, the main controlling factors of the chemical characteristics of spring water are leaching and dilution. In addition, trace elements such as Sr and ${\rm{NO}}_3^{-}$ in spring water also change with the spring flow. Sr is negatively correlated with spring flow, which is mainly controlled by dilution. On the contrary,${\rm{NO}}_3^{-}$ is positively correlated with the spring water volume, indicating that the range of spring water supply changes in different seasons. Based on the dynamic analysis of precipitation isotope, spring water δ18O and δD is positively correlated with the change of spring discharge. The main reason is that the evaporation under cloud is strong in summer, and raindrops experience unbalanced fractionation, which may result in the increase of δ18O value and δD value during the precipitation. Besides, the spring water may also be subject to evaporation in the process of infiltration into underground fissures. According to the analysis of d-excess data, the spring water is mainly supplied by the precipitation of ocean water vapor. The analysis of 34S and 87Sr/86Sr data also proves that the spring water of Jinlong Cave is supplied by regional atmospheric precipitation, and the${\rm{SO}}_4^{2-}$ in water is not from gypsum dissolution, while the spring water is mixed with the groundwater from the silicate area and the groundwater from the carbonate area. Furthermore, according to the analysis of ion ratio and mineral saturation data of spring water,${\rm{HCO}}_3^{-}$ , Ca2+and Mg2+ in spring water mainly come from the dissolution of carbonate rock dominated by sulfuric acid;${\rm{SO}}_4^{2-}$ mainly comes from the oxidation dissolution of sulfur-containing minerals such as pyrite in Jurassic andesite. The PHREEQCI software was used to calculate the saturation indexes of calcite, dolomite, gypsum, aragonite and quartz in the groundwater of Jinlong Cave. Results show that the saturation indexes of gypsum and quartz do not change significantly with the change of precipitation and spring water, but gypsum is in an unsaturated state and quartz in a slightly saturated state. Calcite and dolomite are in unsaturated states when the spring flow is large, while they are in saturated states at other times. Aragonite is always in an unsaturated state, but when the spring flow is large, the degree of unsaturation increases. These phenomena prove that dilution is one of the main factors controlling the chemical dynamics of spring water. Therefore, it is considered that the groundwater of Jinlong Cave can be divided based on two periods: during the period of intense precipitation, the groundwater of Jinlong Cave is mainly recharged by the precipitation infiltration in the carbonate rock area where karst fissures such as karst caves are very developed and can quickly convert precipitation into groundwater; during the period of small precipitation, the groundwater is mainly recharged by the groundwater with relatively slow flow rate in the volcanic sedimentary rock area dominated by weathered fissures at high altitude. -
表 1 金龙洞泉流量及水化学组分
Table 1. Spring flow and hydrochemical parameters in Jinlong Cave
样品
编号取样
日期水温/
℃当月降
水量/mm流量/
m3·h−1pH Ec/
μs·cm-1TDS Na+ K+ Ca2+ Mg2+ ${\rm{HCO}}_3^{-}$ Cl− ${\rm{SO}}_4^{2-}$ ${\rm{NO}}_3^{-}$ Sr H2SiO3 mg·L−1 JLDQ01 2019.9.23 13.8 38.6 973.9 7.76 490.0 279.92 9.50 1.75 66.72 13.98 152.34 5.42 69.79 24.60 0.63 11.00 JLDQ02 2019.11.21 12.6 6.2 521.3 7.83 525.0 285.75 13.98 1.77 64.65 14.90 170.62 5.55 79.40 20.20 0.65 13.74 JLDQ03 2019.12.12 13.1 1.6 693.7 7.70 450.0 300.33 14.12 1.73 65.49 14.52 207.18 7.24 74.72 18.92 0.58 11.98 JLDQ04 2020.1.14 13.3 2.4 568.8 7.85 388.0 273.76 16.46 1.71 63.74 14.95 194.99 5.24 70.41 16.11 0.77 13.74 JLDQ05 2020.4.23 13.4 0.4 398.9 7.32 506.0 290.83 15.20 1.88 57.80 17.94 205.93 5.59 75.69 13.77 1.25 12.89 JLDQ06 2020.5.18 14.0 61.6 655.2 7.61 513.0 297.16 14.49 1.83 65.57 16.57 175.65 7.02 77.31 14.63 1.20 12.89 JLDQ07 2020.6.16 13.7 46.0 224.3 7.80 517.0 295.08 14.30 1.85 65.62 16.65 151.42 7.60 80.46 15.02 1.28 13.45 JLDQ08 2020.7.16 14.7 137.8 1061.1 7.46 513.3 311.07 14.59 2.16 68.39 16.26 205.93 7.40 79.81 19.49 0.91 12.55 JLDQ09 2020.8.22 14.7 203.5 3330.0 7.29 437.7 264.89 5.27 1.59 63.02 12.25 159.16 5.60 75.39 22.19 0.62 9.83 JLDQ10 2020.9.28 14.2 25.3 1524.6 7.58 447.0 279.20 5.20 1.55 63.63 12.85 198.95 5.60 70.74 20.17 0.65 11.19 JLDQ11 2020.10.18 13.5 0.0 1160.2 8.00 400.8 264.50 5.67 1.46 62.73 12.89 189.45 5.38 70.12 18.28 0.65 10.85 JLDQ12 2020.11.20 13.4 28.7 663.1 7.90 499.0 287.04 14.22 1.68 65.56 14.14 185.69 5.99 79.00 13.61 0.79 10.34 JLDQ13 2020.12.14 13.4 0.4 501.6 7.91 474.0 282.30 13.77 1.68 63.36 14.36 179.05 4.86 77.71 17.03 0.51 11.01 表 2 金龙洞泉同位素测试数据
Table 2. Isotopic data of the spring in Jinlong Cave
样品编号 取样时间 δ18OVSMOW/‰ δDVSMOW/‰ d-excess δ34S /‰ 87Sr/86Sr JLDQ01 2019.10.14 −9.0 −62.4 9.6 −0.5 0.710197 JLDQ07 2020.6.16 −10.2 −69.5 12.1 JLDQ08 2020.7.16 −9.9 −66.8 12.4 JLDQ10 2020.9.28 −9.6 −68.0 8.8 JLDQ11 2020.10.18 −9.1 −66.8 6.0 JLDQ12 2020.11.20 −10.1 −70.6 10.2 JLDQ13 2020.12.14 −9.5 −64.7 11.3 -
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