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2022 Vol. 41, No. 4

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Influence of initial fractures on the occurrence of karst turbulent flow
JIAO Youjun, HUANG Qibo, YU Qingchun
2022, 41(4): 501-510. doi: 10.11932/karst20220401
Abstract FullText HTML PDF(3962KB) icon1443 icon83
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In karst areas of China, there are many karst caves and underground river conduits in the underground aquifer. The development of large cave and conduit are controlled by many factors and it is not every karst aquifer would develop into conduit system, for example, the southern karst area with ample rainfall has many underground rivers, but in the northern arid and semi-arid karst area, underground rivers are relatively rare. The flow states in conduits may be laminar or turbulent. Turbulent flow is an important condition of forming large-scale caves and conduits. In turbulent state, the water flow begin to have the ability of mechanical transportation, which would carry the solid granule and cause the impact and erosion into surrounding rock. The powerful mechanical erosion of turbulence is very important for karst aquifer to develop into large scale conduits and caves. Furthermore, the dissolution rate of carbonate rock in turbulent flow is at least one order of magnitude faster than laminar flow condition. Therefore, the mechanical erosion and chemical dissolution in turbulent flow make the development of conduit and cave more possible. However, the occurrence of turbulent flow is rigidly affected by the hydraulic condition and the initial medium of the aquifer, such as the initial rock fracture, including the aperture, direction, length and density of initial fractures. So we designed different statistic features of fractures and different hydraulic gradients to study the flow state and dissolution widening rates of fractures by numerical simulation. The cubic law and Lomize equation were used to model the laminar and turbulent flow state in fracture. The Newton-Raphson iteration is high-performance to solve the nonlinear flow equation system of laminar and turbulent flow. Then the dissolution rate equation and the Ca2+ concentration equation system was employed to model the widening of the fractures. The simulation results and discussions were all under given outer environment with PCO2 0.8% and hydraulic gradient 0.02. The modeling results of fracture aperture showed that when the standard deviation of aperture was 0.0005 cm and the mean of aperture is 0.006, there was no turbulence in the modeling period of 5,000 thousand years. But when the standard deviation was increased to 0.001 cm, the turbulent flow emerged on 189 thousand year and the time was greatly shorten. In the 8 modeling aperture statistic situations, as the mean and the standard deviation of aperture increased and the heterogeneity of fractures was more intensive, the turbulent flow began to appear and the time of turbulence became earlier. The existence of primary fractures led to much heterogeneous aquifer and earlier turbulent time. The results of fracture direction modeling scenes showed that when the angle between the direction of primary fractures and the direction of main hydraulic gradient was smaller, the turbulence time would be shorter. If the mean of fracture length is too little, the connectivity of fracture would become poor and the karst dissolution would be heavily restricted. The fracture density, especially the primary fracture density, had much influence on the karst development. Compared with the secondary fractures, if the density of primary fractures was too smaller, the turbulence time would largely increase, and even no turbulence in the whole simulation. We also discussed the influence of hydraulic gradient on the turbulent time. The hydraulic gradient varied from 0.001 to 1 and the mean of aperture was from 0.001 to 0.005. The results showed that for each aperture, it had the corresponding smallest hydraulic gradient to the occurrence of turbulence and the greater gradient the turbulent time would become earlier. Below the smallest hydraulic gradient the turbulence would never occur. If the mean of aperture is less than 0.001cm, no matter how the hydraulic gradient is increased, the flow state in fractures remained laminar and no turbulence occurred, in which karst is nearly not developed in the aquifer. In conclusion, the occurrence time of turbulent flow reflects the possibility of forming large karst conduits and caves in present aquifer. The shorter the time is, the greater the possibility will be.
Genetic mechanism analysis of low Ca/Mg value of acid goaf water in coal mine drainage
SHI Weizhi, ZHAO Chunhong, LIANG Yongping, HAN Zhantao, XIE Hao, TANG Chunlei
2022, 41(4): 511-521. doi: 10.11932/karst2022y10
Abstract FullText HTML PDF(3301KB) icon1660 icon71
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The calculation results of water quality monitoring of the acid mine drainage circulation system of Shandi river in Yangquan City, Shanxi Province for many years show that the Ca/Mg value of the acid goaf water in coal mine drainage is generally low. The average value of 257 groups of acid mine drainage samples was 1.14, the average value of 69 samples of mine drainage in the study area during the same period was 3.73, and the average value of 206 samples of karst groundwater was 5.30, and there is a law that the Ca/Mg value decreased with the increase of acidification degree (increases of SO42− content or decreases of pH value). In response to this problem,combined with the geochemical source conditions of the study area, this paper analyzes the relationship between the saturation index of gypsum, calcite and dolomite minerals and pH value changes in laboratory tests and field monitoring water samples, so as to reveal the genetic mechanism of low Ca/Mg values of acid goaf water in coal mine drainage. The research results show that the scattered magnesite and pyrite in the carbonate interlayers in the Carboniferous-Permian coal measure strata in the study area are the material sources of Ca2+, Mg2+ and SO42− in the goaf water; In the sulfate-dominated acidic solution formed by the oxidative hydrolysis of pyrite (pH value is in the range of 2.0 to 4.5), the saturation index representing the solubility of sulfuric acid to gypsum, calcite and dolomite is in the order of gypsum>calcite>dolomite,at the same time, with the increase of pH value, gypsum will occur in the precipitation reaction, the relative content of Ca2+ would decrease, and the relative content of Mg2+ would increase, which will eventually lead to the decrease of Ca/Mg value in the solution. Its chemical mechanism is, (1) When the pH value is in the range of 2 to 3, the dissolution of gypsum is inhibited by high concentration sulfuric acid, the solubility decreased rapidly with the increase of pH value, gypsum precipitation may occur,which have been confirmed in laboratory tests, and the content of Ca2+ decreased;When the pH value is in the range of 3 to 4.5, the gypsum in the samples is in a reaction equilibrium state with a slight change in the saturation index near zero. A slight increase in pH value will lead to the formation of gypsum precipitation from Ca2+ dissolved from calcite and dolomite. The content of Ca2+ cannot continue to increase, and the relative content of Mg2+ increases. (2) In the pH range of 2 to 4.5, the calcite is affected by the co-ion effect of Ca2+ and the ion-pair balance when the gypsum is saturated, the saturation index of all samples was maintained at about −4.4, irrespective of pH decrease. (3) However, the dissolution of dolomite was not affected by the above inhibition (the solubility of MgSO4 in sulfuric acid solution is greater than that of CaSO4), the Mg2+ and SO42− content of the monitored samples have a significant positive correlation (the linear correlation coefficient reached 0.83). While dolomite forms gypsum precipitation with Ca2+ in the solution, de-dolomitization reaction occured sustainably, and there is dissolution of magnesite, which eventually increased the content of Mg2+ in acid water relative to the content of Ca2+, and the Ca/Mg value decreased. The Ca/Mg value can be used as an indicator of pollution characteristics of acid goaf water in coal mine drainage and applied to environmental impact assessment.
Overview of field monitoring for acid mine water system of the coal mine in Shandi river basin
TANG Chunlei, LIANG Yongping, JIN Hua, ZHAO Chunhong, SHEN Haoyong, WANG Zhiheng, ZHAO Yi, XIE Hao, LIANG CHEN
2022, 41(4): 522-531. doi: 10.11932/karst20220402
Abstract FullText HTML PDF(2382KB) icon1311 icon61
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Acid mine drainage (AMD) is widespread in coal mining areas such as southwest Shandong, Shanxi, Inner Mongolia, Yunnan and Guizhou. The pH value of acidic mine water often ranges between 2 and 5, with high sulfate, Hb, TDS(Total dissolved solids), Fe, Mn and others. The acidic characteristics of acid mine water make Hg, as, CD, Pb, Co, Ni and other trace elements dissolve in the coal seam, and hence accelerate the reaction speed of phenolic organics and increase the toxicological composition. These harmful substances will do severe harm to groundwater, surface water or soil. Shanxi Province, known as the "coal sea", is located in the middle reach of the Yellow River Basin. But the problem of coal resource depletion in Shanxi Province is gradually exposed. In 2010, the mined-out area covered about 20,000 square kilometers, accounting for one eighth of the land area of Shanxi Province. In the coming decades, a large number of acid mine water from closed coal mines in Shanxi Province will overflow and enter the surface water, and the groundwater will become a "permanent pollution source" exerting an extensive and profound impact. With two major water systems—the Yellow River Basin and the Haihe River Basin, Shanxi Province is known as the "water tower of north China Plain". Every year, nearly 5 billion cubic meters of surface water flows overseas, but the polluted surface water and groundwater resources may threaten the ecological environment of other river basins. Significant progress has been made in the study of the mechanism of acid mine water and environmental problems. However, most of the study areas are concentrated in Guizhou and other places in Southwest China, and relatively little research has been conducted in Shanxi, one of the semi-arid and semi humid areas, where the problem of potential acid drainage in coal mines is more serious. In this paper, the Shandi River Basin, a typical abandoned coal mine area in Yangquan City, Shanxi Province, is selected as the study area. Through hydrogeological survey, hydrogeological drilling, hydrogeological profile and other methods, the formation lithology and hydrogeological conditions of the Shandi river basin are described. It is concluded that due to the influence of coal mining, the supply, runoff and discharge conditions of surface water and groundwater in Shandi river basin have changed fundamentally, compared with natural conditions. The recharge mainly migrates vertically through the water-conducting fracture zone generated by destruction. The runoff mainly flows transversely through the tunnel and the water conducting-fracture zone. The drainage takes on two main types, pit drainage and spring overflow. The pH value of acid mine water in mountain basin is 2.47-7.28, averaging 4.17. Ca2+ value is 141.81-525.00 mg·L−1, averaging 408.48 mg·L−1. SO42− value is 1,084.55-13,683.47 mg·L−1, averaging167.82 mg·L−1. The hydro chemical types are Na-SO4, Ca·Mg-SO4, Mg-SO4 and Mg·Ca-SO4. The cations are mainly Ca2+, Mg2+ and Na+, and the anions are mainly SO42−. The acidification reaction rate of FeS2 in closed reduction environment is slow, but fast in open oxidation environment. The pH value of AMD at the main discharge outlet of Shandi river basin is 2.47, and the SO42− value is 3,848.5mg·L−1. The groundwater in this basin is mainly supplied by the infiltration of precipitation, surface water and coal mine drainage in the mined-out area, and finally discharges from Shandi village. Covering an area of 58.4 km2, Shandi river basin is a complete and independent water circulation system where open-pit mining, in-situ mining and closed pit mining are integrated. In this circulation system, water in all kinds converges in the mined-out area, and then discharges. With unique hydrogeological conditions, the basin is a field monitoring and testing site specific for the production, migration and discharge of acid drainage in coal mines. At the same time, it is also a research site for prevention and treatment.
Hydrogeological evolution of typical moniliform karst wetland in the Yunnan-Guizhou Plateau
ZHANG Hua, KANG Xiaoli, PENG Shuhui, HUANG Zhao, GAO Yu, HUANG Qinhui, LIU Shaohua, LIU Tianlun
2022, 41(4): 532-541. doi: 10.11932/karst20220403
Abstract FullText HTML PDF(3572KB) icon1703 icon128
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Geyi town, located in the northeast part of Xuanwei karst fault basin on the Yunnan-Guizhou Plateau, shows its geomorphic type of karst area of hump valley where carbonate strata are widely distributed. Because of the complexity of rock formations and external geological rock stress, various types of morphology are formed by strong dissolution in this area, not only including surface karst landforms such as karst caves, depressions, valleys and sinkholes, but also underground river pipelines and so on. The study area is intermittently uplifted during the neotectonic movement. A short quiet period during the intense uplifting process leads to the development of multi-layer (exactly four layers) horizontal karst caves in this karst area. The study area is distributed in the second layer with a distribution elevation between 2,020-2,050 m. In the typical moniliform wetland of this area, groundwater converges to the syncline structure of basin floor. The nuclear part of the syncline is located in the corrosion trough valley in the depression of the wetland. Eastward and westward groundwater, recharged through joint fissures and runoffs of karst groundwater, discharges to the nuclear part of syncline and converges to the soluble pond or lake wetland like Qiwen lake. Due to the shallowly buried groundwater level, this kind of lake wetland is rich in water resource. As a discharge area as well as a recharge area of the southern underground river system, corrosion trough valley in the depression of the lake wetland maintaines a close hydraulic link between groundwater and surface water in a constant cycle of suppy-discharge-supply of underground water. Under the condition of the same hydrological flow field, keeping the water balance can guarantee the continuous water displacement, and hence maintaining a good water quality in the lake wetland. Through the analysis of hydrological geology, a study on the structure, hydrological geology and karst development characteristics of moniliform lakes of lacustrine type has been conducted. The study shows that the moniliform wetland on the Yunnan-Guizhou Plateau surface experienced three stages. The first stage-the first phase of Nanpan river-is the stage of crustal uplift. In this stage, the sinkhole-karst tube fissure system was developed in terms of dissolution on the basis of structural fissure development. The second stage-the second phase of Nanpan river-is the stage of crust stablity. In this stage, depressions and funnel-cave underground river systems were developed maturely. The third stage is the stage of stone forest in which the new karst uplift led to degradation of the underground river in the layer caused by the capture of a newly developed underground river system; consequently, collapse, siltation and blockage occurred. With the effect of neotectonic movement, the moniliform wetland on the surface was formed under the condition of karst pipeline collapse, silt blockage and uneven karst development in a long time due to crustal uplift and erosion base sapping. In view of the shrinking wetland area at present, protection suggestions are also put forward in this study. These suggestions involve delimiting the water conservation area and the core area of wetland, strengthening wetland protection and its publicity, conducting the study on the evolution of karst wetland to improve the level of wetland scientific research and developing the tourism with characteristics of wetland and "red revolution bases".
Trend analysis of groundwater quality in major basins of Yunnan Province
GAO Yu, ZHANG Hua, KANG Xiaoli, ZHOU Junrong, WU Hongmei, LIU Haifeng, YE Xian
2022, 41(4): 542-552. doi: 10.11932/karst20220404
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The plateau intermountain basin in Yunnan Province is an important water source gathering area, a population gathering and economic activity center, as well as an environmental hydrogeological unit with index significance of the impact and change of water environment. This article reviews the general monitoring situation of groundwater quality in the main intermountain basins of Yunnan Province. Among nearly 50 basins with an area of more than 100 square kilometers each, 7 have been laid out groundwater monitoring sites, respectively in Kunming, Yuxi, Dali, Chuxiong, Qujing, Jinghong and Kaiyuan. On the basis of dynamic monitoring data for groundwater quality in these basins from 2016 to 2021, the quality of pore water, fissure water and karst water is evaluated. Besides, the groundwater chemical characteristics are analyzed and the change trend of water quality in Yunnan are predicted with data statistics, Piper trilinear diagrams and time series analyses of wavelet neural network. The simulation results of wavelet neural network show that nitrate ion concentration in Hole 152 of Kunming basin increased in a short term, but then slowed down and leveled off. The concentration of manganese ion in Hole 101 of Chuxiong showed an increasing trend, while the concentration of ammonium ion showed a gradual but continual increase in the recent 1 or 2 years after an initial decrease reached the bottom. The high fitting accuracy of wavelet neural network and the average percentage error (less than 10%) demonstrated a satisfactory prediction, which may provide reference for groundwater treatment, protection and prediction, and the building of an early warning platform.It is found that the chemical types of groundwater are complex and diverse, mainly HCO3-Ca·Mg and HCO3-Ca. The content of ammonia nitrogen, manganese, fluoride and nitrate ion exceeds permitted levels, which may mainly indicate the exceedance of an acceptable level of groundwater quality. Over the past six years, the rates of exceeding permitted levels of pore water quality remained stable at about 70%. The components exceeding permitted levels were mainly pH value, total hardness, iodine ion, nitrate, ammonia nitrogen, etc. As for fissure water quality, its rates of exceeding permitted levels were basically unchanged, but the proportion of Class V water was on the rise, mainly in Kunming, Chuxiong and Yuxi. During 2017 and 2019, the rates of exceeding permitted levels were relatively low, but the fissure water quality was deteriorating. The components exceeding permitted levels were mainly manganese ions, pH values, fluorides, etc. The rates of exceeding permitted levels in terms of karst water quality gradually decreased, and in dry season the rate for Class II and Class III water decreased from 23.91% to 15.22%. Components were mainly manganese ions, arsenic ions, ammonia nitrogen, etc. Different indicators show different pollution causes. Pore water may be polluted with infiltration and recharge brought by atmospheric precipitation in pore aquifer. It may also be polluted when laterally recharged by surface water and by agricultural irrigation. Consequently, the deterioration of pore water is closely connected with the discharge of urban domestic sewage, industrial “three wastes” emissions, the use of chemical fertilizer and pesticide, agricultural irrigation and others. Compared with pore water, fissure water was less affected by pollution. Its pollution was mainly caused by the high background values of red beds and metamorphic rocks. Karst water may pollute karst aquifers through low-lying karst fissures, sinkholes and groundwater drainage. According to the statistical results, the water quality in the main basins of Yunnan Plateau shows a stable trend on the whole, which can provide a scientific basis for local economic and social development, territorial space planning, and rational exploitation and effective protection of groundwater resources.
Green energy and its high-efficiency utilization model of surface karst springs in karst plateau mountainous area
ZHOU Cuiqiong, ZHANG Gui, ZHANG Wenjun, GAO Yu, HE Raosheng, WANG Bo, ZHANG Hua, PENG Shuhui
2022, 41(4): 553-561. doi: 10.11932/karst20220405
Abstract FullText HTML PDF(2961KB) icon1230 icon51
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To solve the problem of water shortage in karst plateau mountainous area, this study on the model of development and utilization of surface karst water is conducted. The karst mountainous area in east border of Luxi basin isa typical cluster-peak depression region with an average altitude from 2,100-2,300 m. Depressions and sinkholes are developed there. Outcrops in this area are mailnly composed of limestone and dolomite in sections a, c, d and e of Gejiu formation (T2g), and partially composed of T2g b mudstone, calcareous mudstone, stacked siltstone with marlstone and limestone. The area is generally a monoclinal structure. Detailed hydrogeology research shows that there are 13 exposed epikarst springs with the flow of 0-67.53 L·s−1 due to precipitation. The water leakage in this area is fast, and drought and water shortage are very severe. Thus, surface karst water becomes a very valuable natural resource. This article summarizes a green, high-efficient method of development and utilization of surface karst water as "spring-pool(cellar)-reservoir" optimal management model. In karst concave depression area with siltstone, the grouting with anti-seepage curtain was conducted at the grouting depth of 1.9-16.3 m. An earth dam was built at an anti-seepage foundation and a reservoir with a total storage of 140,000 m3 was constructed to reserve surface water and surface karst water to supply the downstream villages. Below the nozzles of epikarst springs with large flows in Wanbankong depression area, two 1,500-square-meter storage ponds were built to connect the current water supply network for downstream villages. This model of development and utilization of surface karst water is called "spring-pond-cellar" linkage model with an exploitation quantity of around 73,000 m3·a−1. The distance between Aobushan Reservoir and Wanbankong epikarst spring is around 4 km and the reservoir is around 100 m higher than the Wanbankong spring. The flow of the Wanbankong epikarst spring is quite big but very unstable and cannot be reserved. Most of the water in rain season (around 493,700 m3·a−1 accounting for 81.4% of the annual volume) enters to the underground through sinkhole and cannot be utilized. To make the most of reservoir regulation function from surface karst water against rainfall, a pump station and a photovoltaic power plant have been bulit near the Wanbankong epikarst spring. In the rainy season-water stable period with large flows, electric power is generated from the photovoltaic power plant and the pump station will draw surface karst water (around 40,000 m3·a−1) to the upstream Aobushan reservoir. Thus, the reservoir capacity increases from 140,000 m3 to 180,000 m3 under the condition that the reservoir cannot be expanded. In this case, the water supply capacity will increase in dry season. The surface water and underground water are regulated optimally and the total volume of surface spring water and surface water is up to 253,000 m3·a−1. This model may solve the problem of over 13,000 people’s domestic and production water demand, as well as the water demand from seedling over 10,000 Mu in the dry season.
Distribution and influencing factors of karst underground rivers in the Pearl River Basin
YANG Yang, ZHAO Liangjie, XIA Riyuan, WANG Ying
2022, 41(4): 562-576. doi: 10.11932/karst20220515
Abstract FullText HTML PDF(8281KB) icon1595 icon109
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The Pearl River Basin is the area of first-class water resources at the southernmost end of China. Its geographical location is 102°14′-115°57′ E and 21°35′-26°50′N. The main stream of the Pearl River, with a total length of 2,214 km, flows through Yunnan, Guizhou, Guangxi and Guangdong. The total area of the Pearl River Basin is about 438,100 km2, including Nanbeipan river, Hongliu river, Yujiang river, Xijiang river, Beijiang river, the Pearl River Delta and the dongjiang river basin. The water-bearing rocks in the Pearl River Basin are divided into karst water of carbonate rock, fissure water of clastic rock, pore water of loose rock and fissure water of magmatic rock. The distribution area of exposed carbonate rocks is about 149,500 km2, and that of buried carbonate rocks is about 39,600 km2, accounting for 43.16% of the total area. Because there are 1,036 karst underground rivers, and the discharge of karst underground rivers of the Pearl River Basin in the dry season is about 47.39 million m3·d−1. the basin is rich in groundwater resources. Therefore, it is of great significance to explore the distribution and karstification characteristics of underground rivers for the exploitation and utilization of karst water resources in southern China. Based on a large number of field investigations in the southwest karst area, 200,000 hydrogeological survey reports and the 1∶250,000 and 1∶50,000 karst hydrogeological surveys conducted from 2003 to 2016 in the Pearl River Basin, we selected 348 groups of rock samples and 1,036 karst underground rivers as study objects. Then, taking Dalongtan and Sanqiutian underground rivers in Yunnan Province, Sixiaojing, Tianshengqiao and Huachu underground rivers in Guizhou Province, and Disu, Zhaidi and Dizhou underground rivers in Guangxi as examples, we analyzed and summarized the development law, distribution characteristics and influencing factors of underground rivers in the Pearl River Basin from the perspectives of lithology, landform, structure, hydrodynamic conditions and neotectonic movement. According to the analysis, the outlets of 310 underground river are at an elevation between 200-400 m, accounting for the largest proportion. Among them, more than 30 underground rivers, mainly located in the river basins of Hongliu river, Yuhe river, South Panjiang river and north Panjiang river, respectively cover a catchment area greater than 200 km2. Their discharge in the dry season is greater than 1,000 L·s−1 and the length of main pipeline is greater than 10 km. The study result shows that the underground river is most developed in fine-grained oolitic bioclastic pure limestone with a specific solubility of 0.84-1.2, moderately developed in dolomite with a specific solubility of 0.62-0.83, and weakly developed in argillaceous limestone with a specific solubility of 0.43-0.61. Landform and surface river network determine the trend and direction of karst underground river movement. The structure controls the spatial pattern of underground river development. The development of underground river is particularly obvious in the reverse composite part of the structure, the fracture zone on both sides of the compressional torsional fault, the contact zone with the non-soluble rock, the part with the largest fold bending, and the fracture zone in the anticline axis and the syncline axis. Hydrodynamic characteristics affect the development scale and depth of underground rivers. The neotectonic movement facilitates the constant changes of cycle and alternation conditions of groundwater, as well as the deep, inherited and new development of underground rivers. Finally, according to the morphology of underground river and the evolution conditions of water circulation, the underground river can be divided into four types: single conduit type at the initial stage of development, multi-stage feather type, neotectonic control network type and mature dendritic type. All in all, this study is expected to provide data support for the exploitation and utilization of karst underground rivers, monitoring and evaluation of water resources, and the selection of backup water sources to meet urban emergency in the Pearl River Basin.
Water flow characteristics of Maocun underground river basin based on environmental isotopes
GUO Yongli, WU Peiyan, HUANG Fen, SUN Ping’an, MIAO Ying, LIU Shaohua
2022, 41(4): 577-587. doi: 10.11932/karst20220406
Abstract FullText HTML PDF(1504KB) icon1283 icon66
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Underground river basins in peak-cluster depressions are main water supply sources of villages in Guangxi. But there exist problems of groundwater exploitation, groundwater pollution and natural disasters in these areas due to a special geological structure. Besides, the karst hydrological process is critical driving force for the sustainable development of ecological environment in karst areas; therefore, understanding mechanism of hydrological process is the key to the problem of ecological environment. Hydrochemical characteristics are closely related to hydrodynamics in a karst groundwater system, and hydrochemical tracers have been successfully used to indicate recharging sources, flow paths and water flow velocities.Taking Maocun underground river basin in peak-cluster depression in Guilin, Guangxi as an example, we interpret water flow characteristics in the study area based on the hydrodynamic processes indicated by hydrochemical environmental tracers. Temporal and spatial variations of isotopes and their influence factors, especially those of stable environmental isotopes as natural tracers of water cycle, are suitable for indicating water flow characteristics and hydrodynamic processes in karst areas. Variation ranges of δD and δ18O of karst water samples fall in the ranges of local precipitation, indicating that precipitation is the main recharge source in the study area. Dissolved inorganic carbon (DIC) in karst water is approximately equal to ${\rm{HCO}}_3^{-}$, and the concentration of ${\rm{HCO}}_3^{-}$ is mainly affected by dissolution of carbonates and soil CO2. Based on isotopes of δ13CDIC and the linear mass conservation, the average value of DIC from carbonates dissolution is 52.13‰, which can be used to interpret the water-carbonate interaction.In southwestern karst areas, karst is well developed with the characteristics of special dual structure. Surface water and shallow groundwater is well connected, the retention time of shallow groundwater is short, and hydrological processes are sensitive to precipitation or artificial influences. 222Rn with the half-time of 3.82 d is the decay product of radioactive uranium and radium, which can be used to determine the characteristics of karst hydrological processes on a short-time scale. Different types of water bodies are of significant differences in the values of 222Rn, which can indicate the seasonal variation characteristics of groundwater levels, fractured karst networks, karst water cycles and water flow patterns.Electrical conductivity (EC) in the karst groundwater system is determined by interaction time of water and carbonate, which could be used to interpret ratios of recharing sources, groundwater flow patterns and structure characteristics of aquifer system. The complex spatial structures of karst aquifers interpreted by the responding characteristics of 222Rn and EC values to the precipitation show that karst water is recharged by different sources and water flow paths under different meteorological and hydrological conditions.Precipitation is driving force of water flow in karst areas. Accumulated precipitation can drive fractured water of deep karst flow to the downstream with strong dilution. The larger the amount of precipitation is, the faster the velocity of karst water flow is. The dispersed fissure flow is the main pattern in the karst aquifer system under the situation of less precipitation in August. Because the flow time in the conduit is longer than the half-time of 222Rn (3.82 d), decay characteristics of 222Rn (3.82 d) cannot be used to calculate the flow velocity of karst water. Accumulated precipitation could drive fractured water of the deep karst and conduit water to flow to the downstream under the situation of heavy precipitation in June. The flow time of conduit water from Dayanqian to outlet is 0.82 d (<3.82 d). The influence of water-carbonate interaction on 222Rn can be ignored, and the decay characteristics of 222Rn can be used to interpret flow patterns of conduit water in the rainy season. Based on the decay model of 222Rn in June, the effective water flow velocity in the underground conduit is 2,427.49 m·d−1, which is in the same order of magnitude as the results calculated by artificial tracers. Therefore, the decay characteristics of 222Rn can effectively reflect water flow patterns of conduits in the rainy season.Water flow patterns in the underground river of southwest China are similar to those of surface water, and the underground river responds sensitively to precipitation. Due to the limitations of spatial characteristics indicated by artificial tracers, they cannot be used to interpret spatial structure characteristics of a karst aquifer system. However, hydraulic connection of the karst aquifer system can be interpreted by the interrelationships among SI, 222Rn, δ18O and δ13CDIC of water samples located in the transition zone from non-karst areas to karst areas. Karst water in Xiaolongbei, Laolongshui, Bianyan and Shanwan are well connected hydraulicly. The good linear relationships among Beidiping-Shegengyan-Outlet and Laolongshui-Dayanqian-Outlet by environmental tracers indicate that there are complex water flow paths recharging the outlet in the underground river basin. Environmental isotopes can divide karst water samples into several groups, and better interpret water flow paths and spatial structure characteristics of karst aquifer system. Therefore, hydrochemical environmental tracers can provide important information of a karst aquifer system, interpret multiple flow characteristics and compensate for drawbacks of hydrodynamic methods.
Hydrochemical characteristics and water quality evaluation of karst groundwater in typical industrial cities
LU Li, CHEN Yudao, ZOU Shengzhang, FAN Lianjie, LIN Yongsheng, WANG Zhe
2022, 41(4): 588-598. doi: 10.11932/karst20220407
Abstract FullText HTML PDF(1326KB) icon1453 icon65
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As economic pillars of the karst area in south China, industrial cities located in this area play an important role in the promotion of national strategy and economic development. Karst groundwater, the main water source of industrial cities in the karst area, is vital for urban development. Therefore, studies on hydrochemical characteristics of karst groundwater and solutions to water pollution problems have always been working focuses of local governments, but little attention to the chemical problems of karst groundwater in industrial cities has been paid in previous studies. This study takes karst groundwater in Liuzhou City, Guangxi as the research object. Based on karst hydrogeological survey, a total of 119 groups of karst underground water samples were collected, including 31 groups of urban underground water samples and 88 groups of non-urban underground water samples. Using mathematical statistics, hydrochemistry method (Piper diagram, Gibbs diagram, ion ratio coefficient, mineral saturation index calculation), factor analysis and fuzzy comprehensive evaluation method, we analyzed the hydrochemical characteristics and formation mechanism of karst groundwater in industrial cities, and carried out the quality evaluation of karst groundwater. Results show that karst groundwater in the study area is generally medium-weak alkaline water with a small amount of acidic water. The main cations are Ca2+ and Mg2+, and the main anions are HCO3- and SO42-. In terms of concentration and variation degree, karst groundwater in urban areas is larger than that in non-urban areas. In terms of water chemistry types, there are also obvious differences between the two kinds of areas, that is, water chemistry types in urban areas are mainly HCO3-Ca+Mg type, while those in non-urban areas are mainly HCO3-Ca type. The proportion of SO42- type water in urban areas is much higher than that in non-urban areas, reflecting that urban areas suffer serious pollution from more pollution sources than non-urban areas. The chemical composition of groundwater in the study area is mainly controlled by rock dissolution, industrial pollution, urban pollution and agricultural activities, with contribution rates of 31.52%, 25.15%, 18.12% and 10.74%, respectively. Dissolution factors of rock are mainly carbonate minerals such as calcite and dolomite, primarily from dolomite dissolution in urban areas and calcite dissolution in non-urban areas. The dissolution factors are mainly distributed in Liujiang district and northwestern Luzhai county of Liuzhou City. Industrial pollution factors are mainly distributed in Liunan district, Liubei district and other areas of Liuzhou City. The industrial pollution in these areas is related to the discharge of wastewater, waste gas and waste residue by a number of heavy industry enterprises, leading to the excessive concentration of heavy metals. Urban living pollution factors are mainly distributed in Chengzhong district, Yufeng district, Luzhai county, Liucheng county and other areas, and are related to domestic sewage discharge and domestic garbage leaching. The factors of agricultural activities are mainly distributed in Liujiang river, Luoqing river, Longjiang river and other river valleys, and are related to the use of chemical fertilizers and pesticides in agricultural activities and the direct discharge of animal feces from breeding. Hydrochemical sensitivity indexes of different functional areas are diverse, among which the industrial area is dominated by heavy metals, agricultural area by trinitrogen, and living area by K+, Na+, Cl- and SO42-. Mineral saturation indexes show that calcite and most dolomite are saturated, while gypsum and salt rocks are dissolved. The karst groundwater quality is good as a whole, mainly I-III water, accounting for 86.29%, but the water quality varies greatly in different areas. The water quality in the urban area is poor, with the proportion of IV-V water as high as 35.48%. The places with water exceeding permitted levels are mainly distributed in Liunan district and Liubei district of Liuzhou City, and factors exceeding levels are Al, Mn, Pb, Fe and Hg. Reasons for exceeding levels are related to the discharge of "three types of waste" and geochemical background of heavy industry enterprises. The water quality in non-urban areas is better, and the proportion of IV-V water is only 6.45%. The places with water quality exceeding permitted are only distributed in Liujiang river valley and Luoqing river valley, and the factor exceeding levels is trinitrogen, which is caused by the excessive use of chemical fertilizers and pesticides.
Analysis of groundwater runoff patterns in Zhangguan-Paihuadong karst water system in the south of the Mingyue gorge anticline
YU Siyao, QIN Zixuan, YANG Yanna, MAO Weina, HAO Chao, XU Mo, LIU Yang
2022, 41(4): 599-609. doi: 10.11932/karst20220408
Abstract FullText HTML PDF(2293KB) icon1397 icon86
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The long-term interaction between groundwater and karst development in eastern Sichuan has created a unique groundwater runoff pattern and caused a complex hydrogeological problem of karst engineering. Hence, finding out the groundwater runoff pattern in the southern section of Mingyue gorge anticline is of great significance for guiding the site selection and construction of tunnel engineering in this area.Based on previous studies, hydrogeological conditions of the study area are systematically sorted out. Mingyue gorge anticline is a typical closed one in eastern Sichuan, and there is no fault in the area. Atmospheric rainfall is the main recharge source in the water system. Controlled by the boundary of the non-soluble rock formation on the two sides of anticline and topographic relief, groundwater converges and is recharged through the karst structures such as depressions and sinkholes, flowing toward the river runoff at the lowest discharge datum along the tectonic line within the region. The outlet of S05 (Paihuadong), exposed on the bank slope of Yulin river, is a main discharge point of groundwater in the system. The discharge is about 365 L·s-1 in the dry season, accounting for 95.94% of the total discharge of all natural groundwater outcrops in the study area. Main aquifers in the area are composed of carbonate rocks of Jialingjiang formation and Leikoupo formation. The lithology of T1j1 and T1j3 is mainly limestone, while the lithology of T1j2 , T1j4 and T2l is mainly dolomite. The difference of rock solubility results in different karst development in each aquifer. From the perspective of surface karst phenomenon, the area of depression and the number of falling holes developed in T1j1 and T1j3 strata respectively account for 67.38% and 69.84% of the total amount of the system. From the perspective of underground karst phenomenon, 121 karst caves have been revealed by 22 boreholes in the study area. The number of karst caves distributed in T1j1 and T1j3 strata accounts for 75.8% of the total. Statistical results of surface and underground karst phenomenon reflect that the degrees of karst in T1j1 and T1j3 strata is significantly higher than those in T1j2 , T1j4 and T2l strata. Because aquifers with different degrees of karstic distribution are intersectional in the plane, the heterogeneity of water-bearing media leads to relatively weak hydraulic connection in adjacent strata, and the groundwater level revealed by boreholes and spring points is obviously different in different layers.Results of hydrochemical analysis of 18 groups of water samples in the study area further illustrate the discontinuity of groundwater in each aquifer. HCO3-Ca type water mainly exists in T1j1 and T1j3 strata. Their conventional hydrochemical and isotope characteristics indicate that the water-rock interaction time is relatively short. Hydrochemical types of T1j2, T1j4and T2l strata are mainly HCO3·SO4-Ca and HCO3·SO4-Ca·Mg. Their conventional hydrochemical and isotope characteristics indicate that the water-rock interaction time is relatively long, and the hydrochemical characteristics of groundwater outcrop in strong and weak karstified strata are obviously different. From the hydrogeological profile of S01 (Zhangjiadong) to S05 (Bahuadong), the hydrochemical types of water samples S01 and ZK07 in the recharge and runoff area of the system are HCO3-Ca type, and the hydrochemical characteristics of the two are very similar. When sampling sites are located within a drainage area of the system, the hydrochemical type of groundwater near sampling sites ZK10 and S05 is changed to HCO3·SO4-Ca type, and corresponding TDS values are significantly higher than those of recharge and runoff area, indicating that groundwater in different aquifers is mixed in the system discharge area.Based on the analysis of karst hydrogeological conditions and hydrogeochemical characteristics in the study area, a runoff law of groundwater in Zhangguan-Paihuadong karst water system is summarized (groundwater in the study area generally shows a broom-like runoff pattern). In system recharge and runoff areas, a hydraulic connection between aquifers is not strong. Multiple groundwater flow systems have formed with the stratum as a unit, and groundwater carries out relatively independent bedding runoff in each layer. In the drainage area of the system, the mutual capture of each water flow system is increasingly strong. Hence, these water flow systems gradually merge into one, discharging to Yulin river through the drainage hole. In addition, according to previous research, groundwater inside the system vertically presents a multi-order nested flow system. Controlled by the discharge datum of different cutting depths, this system is successively developed into local flow system, intermediate flow system and regional flow system from the shallow part to the deep.
Dissolution characteristics of gypsum salt in the red beds of central Yunnan Province and their effects on groundwater quality: A case study of Xiejiahe valley in Chuxiong City
HUANG Shengdong, ZHAO Long, HUANG Guiren, CHEN Jipu, WANG Yu
2022, 41(4): 610-622. doi: 10.11932/karst20220409
Abstract FullText HTML PDF(5306KB) icon1410 icon59
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Red beds in central Yunnan Province are mainly deposited of continental red clastic rocks. Soluble salts in the red beds are enriched in some sections, generating undrinkable salt water or brackish water. Besides, during water resources exploitation, the improper design and construction of water supply drilling will often lead to serial pollution of groundwater or the formation of inferior wells, affecting the project benefit and safety of water supply.To explore dissolution characteristics of gypsum salt in the red beds of central Yunnan and their effect on groundwater quality, a representative area of Xiejiahe Valley in Lucheng Town of Chuxiong City was selected in this study. Comprehensive survey methods such as geological survey, mapping and drilling were used to analyze lithology, geological structures, aquifers, groundwater types, classification and distribution and dissolution characteristics of gypsum salt in the study area. Based on indicators of sulfur isotope (δ34S), ${\rm{SO}}_4^{2 - }$ concentration and TDS of groundwater, the groundwater quality was studied and the effect of gypsum salt dissolution on groundwater was analyzed.The Quaternary loose sedimentary layer (Qpal), early Cretaceous Puchanghe formation (K1p) mudstone intercalated with sandstone, and Gaofengsi formation (K1g) sandstone intercalated with mudstone compose the main strata in the study area. Groundwater media types include pore water of loose layer, bedrock fissure water and karst water. Bedrock fissure water is composed of weathering fissure water, interlayer fissure water and structural fissure water. Pore water of loose layer and weathered fissure water constitutes a shallow circulation of groundwater with fast circulation speed. The deep circulation system is mainly composed of interlayer fissure water with pressure and slow circulation speed. The Yi-gan fault (F8), mainly containing vein fissure water, runs through the whole study area. Associated structures developed near monitoring wells lead to water diversion in this section of the fault. There are two types of gypsum salt in red beds of the study area: scattered gypsum salt and fissure gypsum. K5, K7 and K9 boreholes reveal the pseudocrystal of gypsum salt in Puchanghe formation. K1, K2, K6, K8 and K19 boreholes reveal fissure gypsum in Gaofensi formation. Sulfur isotope (δ34S) is positively correlated with ${\rm{SO}}_4^{2 - }$ concentration of borehole water samples, indicating that the high concentration of ${\rm{SO}}_4^{2 - }$ in groundwater is derived from dissolution of gypsum filling in the surrounding rock of aquifer. The plane distribution of fissure gypsum shows that gypsum mainly occurs in the Yi-gan fault and its associated tectonic fractures. Dissolution forms of gypsum and salt are mainly dissolution pores and cracks. Dissolution pores are mainly formed by dissolution of scattered gypsum salt, and dissolution cracks are mainly developed above the underground section of 100m depth. The dissolution intensity of gypsum salt is mainly controlled by the development degree of pores in water-bearing media and the groundwater circulation rate of groundwater. The effect of gypsum salt dissolution on groundwater quality in the study area is significant. A fast speed of shallow fissure groundwater circulation results in basic dissolution of gypsum salt, and soluble salt minerals are strongly dissolved, hence weakly influencing groundwater quality, and contributing to a generally high quality of groundwater. Due to the slow circulation of deep confined groundwater, sulphate is enriched in confined aquifer after gypsum salt dissolution. The concentration of ${\rm{SO}}_4^{2 - }$ is high and most of groundwater exceeds permitted levels. Deep confined water in the fault and in the water diversion section of the influence belt of fault is transported to shallow groundwater, which results in the pollution of shallow groundwater and poor water quality in the area.Dissolution forms of gypsum salt mainly include dissolution holes of scattered gypsumsalt and dissolution cracks of fissured gypsum, and the dissolution intensity is mainly controlled by development degrees of pores in water-bearing media and groundwater circulation rates of groundwater. Because the speed of shallow fissure groundwater circulation is fast, although soluble salt minerals are strongly dissolved, the influence on groundwater quality is still weak and the area of groundwater exceeding permitted levels is small. Conversely, if the circulation speed of deep fissure groundwater is slow, and soluble salt is enriched in deep confined aquifers after dissolution, resulting in low groundwater quality. Water diversion in the fault and in its influence belt causes deep confined water to flow over to shallow groundwater, which leads to the deterioration of shallow groundwater quality.
Structural characteristics of Triassic carbonate geothermal reservoir and genesis of thermal water in the Tongluo mountain anticline of Guang’an City, China
YUAN Jianfei, LIU Huizhong, DENG Guoshi, LI Minghui
2022, 41(4): 623-635. doi: 10.11932/karst20220410
Abstract FullText HTML PDF(3206KB) icon1288 icon47
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There are abundant geothermal resources at low-medium temperature in the fold mountain areas of eastern Sichuan, and the main geothermal reservoirs are Leikoupo and Jialingjiang formations. Generally, karst geothermal reservoirs have the advantages of large water volume, easy reinjection of tail water after exploitation, and less damage to geothermal reservoirs. Recently, a great breakthrough in geothermal exploration of the Tongluo mountain anticline in fold mountain areas of eastern Sichuan has been achieved. Two deep geothermal wells with a depth of more than 1,700 m have been consecutively drilled to obtain karst geothermal resources with high development and utilization value. However, although good results have been obtained in terms of geothermal exploration, there are relatively few studies on the characteristics of geothermal reservoirs and the mechanism of geothermal water circulation. In the study on structure characteristics of Triassic karst geothermal reservoir, geothermal water chemistry and stable isotope composition, geothermal reservoir temperature, and mechanism of geothermal water circulation, methods including geothermal drilling, hydro-geochemistry, diagram analysis of thermal water components, and geo-thermometer assessment are used to analyze the occurrence condition of karst geothermal water and genetic model in this study area. Results show that the geothermal reservoir structure in the study area is relatively intact, and the cap layer of thermal reservoir, the thermal reservoir, and the lower water barrier layer are formed as independent geothermal hydrogeological units. Karst geothermal water is mainly SO4-Ca·Mg and SO4-Ca types and rich in contents of F, Sr, Li, B, and SiO2. The oxygen and hydrogen isotope analysis indicates that karst geothermal water is of meteoric origin, and the recharge area is located in the Daba mountainous area to the north of Tongluo mountain. The elevation of deep geothermal water recharge is more than 1,100 m; the average annual temperature of the recharge area is 9℃; the horizontal distance reaches 220 km. According to the SiO2 geo-thermometer, the karst geothermal reservoir temperature is 56-76℃; the circulation depth of geothermal water is 2,013-3,030 m, and the karst geothermal water is immature. Moreover, the dissolution of carbonate and evaporite and the mixing of cold groundwater are major hydro-geochemical processes in the geothermal water circulation path, and the mixing ratio of cold groundwater is higher than 0.8. Based on the results of geothermal geology, geothermal drilling, hydro-geochemistry, and environment isotopes, the genesis mechanism of geothermal water in this area is summarized as follows, groundwater receives water recharge from rainwater through karst depressions, sink, and pit of the Leikoupo and Jialingjiang formations in the anticline core region, heated by geothermal gradient and hot rock during the deep circulation of the anticline wing. During water circulation, major hydro-geochemical processes are water-rock interaction and thermal-cold water mixing. After then, karst geothermal water occurs along the anticline axis and wing region in forms of hot springs and geothermal wells. This study not only enriches the theory of the karst geothermal reservoirs system in the fold mountainous areas of eastern Sichuan but also provides a scientific basis for the exploration and exploitation of deep karst geothermal resources in this area.
Imprints of millennial-scale events during the MIS3 revealed by stalagmite δ13C records in China
LUO Xuelin, ZHOU Ying, FU Tianxiang, NIE Minmin, LIANG Yijia, DONG Jinguo
2022, 41(4): 636-647. doi: 10.11932/karst20220411
Abstract FullText HTML PDF(3298KB) icon962 icon35
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During the last glacial period, a series of millennial-scale abrupt climatic events, including Heinrich events and Dansgaard-Oeschger events, exerted influential and profound impacts on the global climate systems. Due to advantages of high resolution, multiple proxies and 230Th dating methods, Chinese stalagmite δ18O records reveal distinct teleconnections between the climates in the northern high latitudes and the Asian monsoon domain. Generally, during cold Stadials in the northern high latitudes, Asian summer monsoon was weak and stalagmite δ18O values shifted positively, and during warm Interstadials, Asian summer monsoon was strong and stalagmite δ18O values shifted negatively. However, accompanied with the wide application comes a hot debate on the interpretation of stalagmite δ18O. It is suggested that Chinese stalagmite δ18O could possibly reflect Asian summer monsoon which is related with the average monsoonal intensity or the overall moisture transport to China, but could not merely represent local precipitation changes. For instance, during Stadials, under the influence of weak Asian summer monsoon, precipitation in southern China might increase, indicating inconsistent changes of "rainfall" and "wind". Climates in the monsoon marginal regions, namely northern China, are found in consistent behavior in terms of "rainfall" and “wind” changes. Besides, calcite δ13C is also potential for the reconstruction of paleoclimatology and paleo-environment, thus, to some extent, could compensate the shortage of calcite δ18O which lacks changing signals of local environment.Yangquan City in Shanxi Province is located at the Loess Plateau and the northern edge of the Asian monsoon. Multi-proxy records induced from stalagmites in this region can provide us a better understanding of the "wind" and "rainfall" aspects of the monsoonal climate. At an elevation of 1,200 m above sea level, Lianhua Cave (38°10′N, 113°43′E) is developed in the Ordovician limestone, with a narrow entrance and passages. Relative humidity in the inner cave reaches 98%-100%, and the temperature in July reaches 11°C, close to the mean annual ground temperature. Average annual precipitation is 515 mm (AD 1970-AD 2000; recorded in a meteorological station of Yangquan, 20 km from the cave). Sample LH36 is candle-shaped, 206 mm in length and 80–110 mm in diameter. After halved and polished, we find clear growth layers in the sample and it is composed of milky-white and yellowish calcite. Alternating changes of the petrography and brown weathered layers are observed at the depth of 33-35 mm and 145-150 mm, indicating two growth hiatuses. Considering the hiatuses, we use the depth section of 37-145 mm for this study, which grew in the (Marine Isotope Stage) MIS3. On the polished profile, we drill 109 samples with a 0.3 mm-diameter carbide dental bur at 1 mm intervals, 50 μg each, for stable isotope analyses. Measurements are carried out by the usage of a Finnigan-MAT 253 mass spectrometer equipped with an automated Kiel Carbonate Device at College of Geography Science, Nanjing Normal University. The analytical errors are better than ± 0.06‰ for δ18O and ± 0.05‰ for δ13C. Eight powder samples for 230Th dating are drilled with 0.9 mm-diameter carbide dental bur, 70-130 mg each. Chemical procedures and U-Th isotopic measurement are performed on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Thermo Finnigan NEPTUNE, at the High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, and at the Isotope Laboratory, College of Geography Science, Nanjing Normal University. Uncertainties in isotopic data and dates are relative to AD 1950, and are given at the 2σ level.Based on 8 230Th dates and 109 sets of stable isotope data of LH36, we obtain a paleoclimate record with an average resolution of 120 years from 54.5 to 41.1 ka BP during the MIS3. Both Hendy test and Replication test indicate an equilibrium fractionation of isotopes during the stalagmite deposition. Comparison with other four independently-dated, high-resolution stalagmite δ13C records between 29°N and 41°N in the Asian monsoon region shows that the stalagmite δ13C records from different caves have good reproducibility during the overlapped growth period. We suggest that speleothem δ13C effectively indicates soil CO2 production in the overlying area of the cave, reflecting changes in the cave’s external environment and Asian summer monsoon. Five millennial-scale Asian summer monsoon intensification events correspond to the Dansgaard-Oeschger (DO) 10-14 cycles recorded in the Greenland ice core within dating errors, and the two weak monsoon processes are closely related to the Heinrich Event 5 and Heinrich Event 5a in the North Atlantic. The spatial consistency of stalagmite δ13C records in China suggests that the Asian summer monsoon and the related regional ecological environment fluctuations sensitively respond to climate changes at northern high latitudes through sea-air coupling on the millennial timescale.
Factors influencing the recrystallization of aragonite stalagmites and their implications for paleoclimate
LI Jiayan, TIAN Yiping, GUANG Kaiyue, ZHU Shanying, LI Yunxia, GAO Yongli, RAO Zhiguo
2022, 41(4): 648-659. doi: 10.11932/karst20220412
Abstract FullText HTML PDF(1709KB) icon900 icon62
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The karst geomorphology is widely distributed in central south and southwestern China, especially in western Hunan Province, where speleothems (especially stalagmites) have become one of the most important archives for high-resolution paleoclimatic studies. Stalagmites are mainly composed of calcite and aragonite, and aragonite stalagmites can provide precise chronology with high uranium content. However, aragonite is easily transformed to calcite if continuously infiltrating and leaching by dripping water in wet cave environment. Some stalagmites in western Hunan of China initially deposited in aragonite minerals, which are prone to recrystallization (especially transforming into calcite), and could change the relevant chemical element signals, limiting the application of some aragonite stalagmite proxy indicators in paleoclimate research. Here, we summarize and sort out the influencing factors of the recrystallization of aragonite stalagmites and their influence on the stalagmites paleoclimate from published literature to define the applications of aragonite stalagmites in paleoclimate research.The determination of mineral phases and recrystallization of aragonite stalagmites is the first prerequisite for stalagmites paleoclimatic research. The stalagmite profile characteristics, XRD results, microscope observations and geochemical element characteristics could be used as the basis for discriminating the recrystallization of stalagmites.The influencing factors of the recrystallization of aragonite stalagmites include, (1) The saturation of cave dripping water and pore water in stalagmite. When the water is in the state of unsaturation for aragonite and infiltrates into the porous aragonite stalagmites, it will dissolve aragonite and reprecipitate to calcite. (2) Aragonite crystal defects and the existence of calcite cement between crystals can facilitate aragonite transform to calcite. (3) The recrystallization of aragonite stalagmite normally occurs in lower concentration of Mg2+ in karst water. (4) Other factors, such as organic matter and α-recoil, will also affect the mineral transformation of aragonite stalagmites.Besides, the recrystallization of aragonite stalagmites can modify some geochemical signals initially preserved in aragonite. (1) Due to the difference of crystal fabrics of calcite and aragonite, the uranium element will be lost when aragonite is transformed to calcite, and the losing will cause abnormal or reverse chronology. (2) The recrystallization of aragonite stalagmites can result in depleted or abnormal δ18O signals, and δ13C values of recrystallized calcite present more complex characteristics, which will be depleted or similar to primary aragonite. (3) Compared with recrystallized calcite, the primary aragonite is enriched in Sr but depleted in Mg in some caves. However, trace element concentration of recrystallized calcite in other caves does not differ greatly with that of primary aragonite. In brief, the changes of these proxies before and after recrystallization may vary between caves. Consequently, the process of aragonite to calcite transformation will weaken the accuracy of dating and the reliability of these proxies as environmental indicators. (4) Due to the different precipitation conditions of aragonite and calcite, the variation of stalagmite mineral phase may indicate climate and environmental evolution, but more studies are needed to confirm for recrystallization stalagmites.Finally, we present results of mineral analysis and 230Th dating of RM2 stalagmite with 200 cm length from Remi cave, Longshan, Hunan Province. RM2 stalagmite has undergone inhomogeneity recrystallization process. Meanwhile, we find that recrystallization does have a certain influence on the 230Th dating. However, the mechanism of recrystallization and the effect of stalagmite recrystallization on other proxies needs further studies.
Progress in reconstruction of karst rocky desertification by stalagmite δ13C
XU Yuzhen, LI Tingyong, LI Junyun, YANG Yan
2022, 41(4): 660-674. doi: 10.11932/karst2022y11
Abstract FullText HTML PDF(1731KB) icon1302 icon54
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The formation and evolution mechanism of karst rocky desertification is a scientific problem that has been paid increasing attention, and the ecological environment in karst areas is not only fragile, but also unstable. Therefore, the study of karst rocky desertification is of practical significance for the restoration of ecological environment in karst areas. Cave stalagmite δ13C is affected by many factors and can respond sensitively to the changes of surface ecological environment and karst hydrological conditions. Hence, the use of stalagmites δ13C on the study of evolution history of ecological environment in karst areas has become an important direction. In this paper, we discuss the main factors affecting stalagmite δ13C from two aspects, i.e., the overlying surface environment and cave deposition. Combining the results of modern cave monitoring with model simulation, we analyze the main factors and mechanisms affecting δ13C in cave drop water and sediment. Under the influence of multiple factors, the environmental significance of stalagmites δ13C has multiple implications. According to the published research results, in this paper, we summarize the indicative significance of stalagmite δ13C from three aspects, different time scales, different regional distribution, and different cave sedimentary environments. In order to accurately interpret the environmental significance of stalagmite δ13C, we put forward the solutions-comprehensive analysis, modern monitoring and model simulation. In this paper, we discuss the concept, genesis, development process and environmental effects of karst rocky desertification, and analyze the close relationship between surface rocky desertification and stalagmite δ13C records in caves. We also summarize published research results about the application of stalagmites δ13C to the reconstruction of regional rocky desertification. Meanwhile, we discuss main problems faced in the current research, (1) How to interpret the indicative meaning of stalagmite δ13C correctly? This problem is the premise that stalagmite δ13C can be used to reconstruct the history of regional rocky desertification. (2) Spatially, the area of the overlying surface reflected by stalagmites δ13C records is limited, so it is necessary for us to carefully consider whether the selected stalagmite region reflects the same spatial distribution as that of the study area, and whether it represents the environmental changes of the target study area, when using stalagmite δ13C to reconstruct the evolution history of rocky desertification in a certain region. (3) Karst rocky desertification can develop rapidly on the decadal time scale, while a certain age error may occur in stalagmite dating. Can stalagmite δ13C record sensitively record the changes of surface environment in such a short period and reconstruct the process of regional rocky desertification? In order to accurately reconstruct the regional karst environment and the evolution history of rocky desertification, we put forward the following suggestions, (1) In order to avoid the uncertainty of stalagmite δ13C in paleo-environment reconstruction, the comparative analysis of stalagmite δ13C and δ18O, trace elements and mineral structure can be integrated with modern monitoring and model simulation to correctly interpret the indicative significance of stalagmite δ13C. On this basis, the karst hydrological process can be reconstructed more accurately and the evolution history of rocky desertification can be determined. (2) By comparing the evolution process of rocky desertification recorded by stalagmites from multiple caves in the study area and multiple stalagmites from the same cave, the regional representativeness of single stalagmite record can be reduced. (3) Multi-index stalagmite records with high resolution and high precision chronological control can accurately record the changes of the land surface environment during the rapid occurrence and development of the rocky desertification process.

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