中国岩溶

Experiment of monitoring multi-layer groundwater at a karst hill slope and its scientific significance

GUO Fang, JIANG Guanghui, LIU Fan, LI Zhijie

2024, 43(3): 491-499. doi: 10.11932/karst20240301

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The heterogeneity of karst aquifers and the complexity of karst hydrogeological conditions pose significant challenges to groundwater observation compared to the groundwater in pore and fissure aquifers. In the areas of springs or underground rivers in which lack natural groundwater outcrops, borehole drilling is crucial for us to reveal aquifer structures and monitor groundwater. Clustered single-hole drilling inadequacies are pronounced in karst regions, because we need numerous times of single-hole drillings to comprehensively reveal different aquifer conditions, which may in turn result in land occupation and high costs. Besides, the difficulty in accurately locating karst conduits or fractures, due to complex hydrogeological conditions, may contribute to potential incompleteness of single-hole representativeness. Finally, drilling for groundwater monitoring may traverse multiple geological layers, causing interconnection of groundwater, thereby making it impossible for us to distinguish water quality conditions in different layers or channels during groundwater quality monitoring.The drilling of one shaft with multi-layer borehole refers to the technique that can achieve segmentation of multiple layers within a single borehole drilling operation. This technique, also known as Continuous Multichannel Tubing (CMT), is an internationally recognized method for layered drilling monitoring, is widely utilized in North America but seldom used in China. Because the technique of one shaft with multi-layer monitoring demonstrates its high ability in differentiating water head or solute distribution, it has been extensively applied in monitoring in pore and fissure aquifers, but seldom in karst media because of its complex structure. Therefore, there are no reported cases of application of this technique in karst regions, and its feasibility needs to be confirmed.The Yaji karst hydrogeological experimental site (hereinafter referred to the Yaji experimental site) represents a typical bared karst aquifer system in Southern China. The aquifer comprises three parts: the upper vadose zone, lower vadose zone, and saturated zone, playing the role of rainfall storage and regulation. Eighteen clustered-distributed boreholes established at the experimental site can reflect the heterogeneity of karst development or water level changes on a planar scale, yet they can hardly reveal the vertical groundwater movement. It is difficult for us to use open boreholes in defining how the differences in karst development vertically affect hydrodynamics. Therefore, this study selected one borehole (ZK6) from the runoff area on the western slope of the Yaji experimental site to implement the one shaft with multi-layer drilling experiment, aiming to explore the application of layered drilling monitoring technique in karst aquifers. The study result can provide conditions for understanding groundwater stratification, vertical distribution of water heads, and layered pumping, and more importantly, provide experience in applying borehole layering techniques and schemes in highly heterogeneous karst water-bearing media.Firstly, based on the data of borehole core, pumping tests, and results from hydrochemical and temperature logging, ZK6 was divided into four layers, with karst development characteristics as the main basis for division. Secondly, hydrological and hydrochemical characteristics of the four layers were compared and analyzed. Significant differences were found in the water level dynamics of the four layers in response to rainfall. The first layer predominantly alternates between being dry and being filled with water throughout a hydrological year, occupying 79% and 21% of the time respectively, which indicates alternating dry and wet conditions in the karst media, with a tendency towards dryness. The second layer exhibits significant lags in water level response. Differences in water heads between the third and fourth layers reflect the vertical distribution characteristics of groundwater discharge zones. Differences in hydrochemical characteristics among the four layers suggest that discrepancies in karst media structures affect both hydrological processes and hydrochemical characteristics. The electrical conductivity in the fourth layer remains relatively stable at 450 μS·cm⁻¹, while in the second layer, due to surface element accumulation and slow water renewal rates, the electrical conductivity is twice that of the fourth layer. The drastic changes in electrical conductivity in the first layer reflect the strong influence of rainfall recharge in the surface karst zone. Finally, based on the hydrological and hydrochemical characteristics of borehole layers, it is concluded that the first to fourth layers respectively represent the epikarst zone, fissure-matrix zone, upper saturated zone and lower saturated zone. Although the technique of one shaft with multi-layer poses operating challenges in karst mountain slopes, once achieved, it will not only facilitate our understanding of the vertical hydrodynamic mechanisms of karst stratification, but will also achieve possible breakthroughs in technology for layering development and utilization of groundwater in heterogeneous karst aquifers. The use of borehole layering monitoring technique enables the measurement of vertical distribution of water heads within boreholes, thereby providing the potential for utilizing layering isolation techniques to facilitate runoff utilization from epikarst zones and vadose zones.

Spectral characteristics and indication of dissolved organic matter in the karst water system of Jinan

LIU Dan, TIAN Chanjuan, CHEN Xuequn, XU Dandan, GUAN Qinghua, XIN Guangming

2024, 43(3): 500-512. doi: 10.11932/karst2024y022

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Dissolved organic matter (DOM) plays an important role in the biochemical cycle of karst water systems. In this paper, the karst water system of Jinan spring, a typical karst area in North China as well as an important source of drinking water in Jinan, was selected as the research area. In the southern part of the area, the metamorphic rocks of the Taishan Group of Archean Eonothem are exposed. The carbonate rock strata of the Cambrian and Ordovician, which incline toward the north with monoclinic structures and are hidden under the Quaternary in the north part, are located above the metamorphic rocks. The regional groundwater flows from south to north under the control of topography. The main aquifers in the sampling area of this study are the middle Cambrian Zhangxia Formation, the upper Cambrian Fengshan Formation and the Ordovician aquifers, which contain carbonate fissure water with lithology mainly composed of limestone, dolomite limestone and argillaceous limestone. According to the hydrogeological conditions in the source area and the formation process of the source, the study area was successively divided into the indirect recharge area (IRA), direct recharge area (DRA) and discharge area (DA) from south to north. The direct recharge area can directly be recharged by surface water leakage. In the indirect recharge area, recharged by atmospheric precipitation, surface water and groundwater flows into the direct recharge area in the form of surface and subsurface runoffs. Groundwater was blocked by magmatic rocks in the north, and fissures were developed in the contact zone between aquifers and the magmatic rocks, where karst groundwater rose and was exposed, and springs came into being.This study aims to investigate the spectral characteristics, spatial distribution and indicator of DOM in karst groundwater in recharge, runoff and discharge areas at a spring-area scale. The composition, source and spatial distribution in the karst water system of Jinan spring were analyzed by absorption spectrum, fluorescence spectrum and PARAFAC, combined with water chemical index and correlation analysis. In this study, three main fluorescence components have been identified in the Jinan spring area, namely, C1, C2 and C3, which are humus, protein-like tryptophan and protein-like tyrosine, respectively. Groundwater in the indirect recharge area is dominated by humic substances in low molecular weight, while groundwater in the direct recharge area and the confluence and discharge area are dominated by protein-like substances. In these two areas, the change in protein-like substances caused by microbial activities is the main driving force for the change in geochemical characteristics of DOM. The order of endogenous contributions is DA>DRA>IRA, which is consistent with the order of water circulation quantity in the karst water system. In this karst water system, aromatic substances are mainly present in hydrophobic components. The humus component C1 is an exogenous input, and is significantly positively correlated with the protein-like tryptophan component C2. These two components from different sources are subject to the same factors. The protein-like tyrosine component C3 has no obvious correlation with all indices. The humus component C1 and the total fluorescence intensity can be used as natural indicators to trace the karst water cycle. Besides, the protein-like tryptophan C2 and the protein-like tyrosine C3 may provide important biogeochemical information to evaluate the vulnerability of karst aquifers.

Evaluation of groundwater pollution risk based on the optimized DRASTIC model: A case study of the areas along the route of South-to-North Water Diversion Project in Shandong Province

LIU Hong, ZHANG Jun, YU Sang, ZHANG Li, CHEN Hao, LU Xiaowei, QIAO Yue

2024, 43(3): 513-526. doi: 10.11932/karst20240302

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Based on the requirements of guaranteeing water quality and sustainably developing ecology proposed by the South-to-North Water Diversion Project, the prevention and treatment of groundwater pollution has become a research focus. In this study, the prevention of shallow groundwater pollution along the route of South-to-North Water Diversion Project in Shandong Province was taken as an example, and the system of evaluating shallow groundwater pollution risk in the study area was constructed from the aspects of groundwater pollution source load, groundwater ecological vulnerability and groundwater functional value. The pollution load was quantitatively and qualitatively analyzed in terms of toxicity, emission and emission possibility of pollutants. Based on land use data and soil oxygen contents, an optimized DRASTOL model was constructed after the optimization of a traditional one. Habitat quality data and night light coefficients simulated by the InVEST model, and the sensitive F⁻ and SO$_4^{2-}$ concentration distributions in groundwater in the study area were used as factors to evaluate the groundwater functional value. The results show low values in both shallow groundwater pollution loads and groundwater functions, and show medium values of groundwater vulnerability. Combined with groundwater pollution load, groundwater vulnerability and groundwater functional value, the distribution of groundwater pollution risk was obtained. The results show that the groundwater pollution risk in the study area is generally at a low or a medium level. The area with high pollution risk totals 7,444.88 km², accounting for 20.17% of the whole study area, which is mainly distributed in central Heze City, the center and southwest of Jining City, the northwest Zaozhuang City, the center and east of Tai'an City, and the south of Gangcheng City. Reasons for groundwater pollution in these areas mentioned above are as follows. Because groundwater levels in these areas are relatively shallow, when much precipitation occurs in the natural environment, surface pollutants easily infiltrate into the ground, which can pose a threat to the groundwater quality. Besides, industrial and mining land is densely distributed in these areas whose strata are rich in water; consequently, a large number of strong toxic pollutants are likely to be emitted. At the same time, pollutants are also possibly generated by social and economic activities with high intensity. The combined influence of the above factors leads to high risk of groundwater pollution in the study area. The results obtained in this study provide a reference for the zoning of groundwater pollution risk in the areas along the route of South-to-North Water Diversion Project in Shandong Province.

Hydrochemical characteristics and genetic analysis of the Xianju basin in southern Zhejiang Province

ZHOU Shiyang, DONG Haogang, LI Lixiang, YUAN Dongfang, LU Li, YAO Feiyan, XIANG Fan, CHEN Lin, WANG Zhenwei, WU Xin

2024, 43(3): 527-537. doi: 10.11932/karst20240303

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The Xianju basin is a typical hydrogeological unit and the main basin in the watershed of southern Zhejiang in which socio-economic activities are concentrated. Therefore, studying the hydrochemical characteristics of this basin is important for the development and utilization of groundwater resources and for the water environmental protection in southern Zhejiang. To reveal the hydrochemical characteristics of surface water and groundwater in the basin, we comprehensively analyzed the hydrochemical characteristics and origins of surface water and shallow groundwater in the Xianju basin with mathematical methods, Piper trilinear diagrams, principal component analysis, and ion ratios. The results show that HCO₃-Ca·Na dominates the hydrochemical type of the Xianju basin, with Ca²⁺ and Na⁺ as the dominant cations, and HCO$_3^{-}$ as the dominant anion. The pH values range from 6.48 to 7.81, indicating an overall hydrochemical characteristic of being slightly alkaline to neutral. The values of Total Dissolved Solids (TDS) of groundwater range from 45.5 mg·L⁻¹ to 288 mg·L⁻¹, while TDS values of surface water range from 20.1 mg·L⁻¹ to 95.8 mg·L⁻¹. The variation coefficients of major ions in groundwater are generally higher than those in surface water, with K⁺, Na⁺, Cl⁻, and NO$_3^{-}$ showing larger variations, especially NO$_3^{-}$ with a maximum coefficient of 36.80 mg·L⁻¹ and a minimum of 0.13 mg·L⁻¹. TDS values range from 45.50 mg·L⁻¹ to 288.00 mg·L⁻¹ in groundwater, and from 20.10 mg·L⁻¹ to 95.80 mg·L⁻¹ water.Principal component analysis indicates that the hydrochemical characteristics in the Xianju basin are mainly influenced by rock weathering and human activities. Rock weathering accounts for 67.89% of the influence, while human activities contribute 12.74%. Silicate weathering plays a predominant role in rock weathering processes, followed by carbonate weathering, with negligible influence from evaporite dissolution. In terms of correlation analysis of hydrochemical parameters, strong correlations are observed among values of TDS, K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO$_4^{2-} $, and HCO$_3^{-}$ in surface water, while NO$_3^{-}$ values show weaker correlations with those of other hydrochemical components. In groundwater, values of TDS correlate significantly with those of Na⁺, Ca²⁺, Mg²⁺, Cl⁻, and HCO$_3^{-}$, whereas values of HCO$_3^{-}$ show strong correlations with those of TDS, Na⁺ , Ca²⁺, and Mg²⁺. Values of NO$_3^{-}$ show weak correlation with those of other chemical components. Ca²⁺, Mg²⁺, and HCO$_3^{-}$ primarily originated from silicate rock dissolution, and secondarily from calcite dissolution in carbonate rocks. K⁺, Na⁺, Cl⁻ mainly originated from silicate rock dissolution, and partially from anthropogenic sources.The reverse exchange of Na⁺ with Ca²⁺ or Mg²⁺ contributes to an increase of Na⁺ contents in hydrochemical compositions, and groundwater exhibits stronger ion exchange capacity compared to surface water. NO$_3^{-}$ primarily originated from anthropogenic activities. Surface water quality is mainly influenced by industrial and mining activities, possibly related to fluorite mining in Jiaoshan township and central Bulu township in the upstream basin. Groundwater, on the other hand, is affected by a complex array of factors including industrial and mining activities, agriculture, and domestic sewage. Spatially, there is a distinct zonation of major ion contents in groundwater and surface water in the basin. From the upper reaches to the lower reaches of Yong'an Creek, the transition occurs from the phenomenon that major ion contents are influenced by natural dissolution to phenomenon that the contents are jointly influenced by natural dissolution and human activities. From the edge to the center of basin, natural dissolution processes in groundwater gradually intensify. The areas with a dense population and with intensive agricultural activities are more susceptible to human activity.In conclusion, the findings provide a scientific basis for the rational exploitation and protection of regional water resources. Understanding the hydrochemical characteristics and spatial distribution of water bodies in the Xianju basin is crucial for the sustainable utilization of water resources and environmental conservation in southern Zhejiang.

Geochemical characteristics of spring water in Baishuitai of Yunnan and their indicative significance on climatic environment

ZHAO Xi, XU Hongyang, GAN Hailing, YE Aihua, ZHOU Limin, ZHENG Xiangmin

2024, 43(3): 538-551. doi: 10.11932/karst20240304

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Understanding the geochemical characteristics and dynamic changes of groundwater (e.g., springs, etc.) is an important scientific reference for accurately interpreting the paleoclimatic and environmental information on karst deposits (e.g., stalagmites, travertine, etc.). In this study, we continuously collected local rainwater samples and S3 spring water in Baishuitai area of Yunnan Province on a monthly frequency for nearly 5 years (January 2018 to October 2022). We analyzed the temporal variations of stable isotopes of hydrogen and oxygen (δD and δ¹⁸O) compositions and trace element contents in the two types of water bodies to identify the recharge source and hydrogeochemical evolution process of Baishuitai spring water, and to reveal the source differences of trace elements in spring water and their indicative significance on climate and environment. The results showed as follows. (1) Values of stable isotope composition of rainwater in Baishuitai region of Yunnan had obvious seasonal variation characteristics—high in the dry season and low in the rainy season. This result was mainly affected by water vapor source and evaporation conditions. During the dry season, water vapor mainly came from the stable continental air mass, and the evaporation effect was strong, which resulted in the enrichment of D and ¹⁸O in the remaining water vapor mass. During the rainy season, multiple fractionation and condensation processes occurred when water vapor migrated from the sea to the land, due to the influence of the southwest monsoon and the southeast monsoon. D and ¹⁸O were severely scoured, thus making δD and δ¹⁸O light in rain during the precipitation process. (2) The stable isotopic compositions of hydrogen and oxygen in Baishuitai spring water were on or near the atmospheric precipitation line, indicating that the spring water was mainly supplied by atmospheric precipitation. During the infiltration of atmospheric precipitation into the zone of shallow circulation runoff, new and old water from different channels and fissure networks was continuously mixed to regulate and store hydrogen and oxygen isotopes in groundwater, resulting a much smaller variation amplitude of δD and δ¹⁸O in spring water than in rainfall. During the rainy season in 2018, 2019 and 2022, δD and δ¹⁸O in spring water witnessed a similar trend to δD and δ¹⁸O in rainwater with a lag of about one month, indicating that δD and δ¹⁸O in water evolved to some extent in underground runoff, so that δD and δ¹⁸O in spring water correctly inherited some of the characteristics of δD and δ¹⁸O in rainwater. The δD-δ¹⁸O scatter points of spring water in Baishuitai region all shifted to the area near the left side of the atmospheric precipitation line. This means that δ¹⁸O in spring water became lighter under the exchange of lighter oxygen isotopes with CO₂ gas from deep sources, while δD basically did not change, indicating that atmospheric precipitation was fully mixed with deep source CO₂ during the infiltration process. The smoothing effect of isotope regulation and storage generated by the mixing of old water, as well as the exchange of oxygen isotope with CO₂ from deep sources, indicated that the Baishuitai region contains a wide spring area, numerous underground passageways and fissures, and intricate karst features. (3) The trace elements in Baishuitai spring water show two types of dynamic change characteristics. Elements such as Ca, Mg, Sr, Ba, and Si constitute one category, mainly originating from the dissolution of surrounding rocks under karst water erosion. After entering the rainy season, the water–rock interaction was weakened by the dilution of rainfall. Concentrations of these elements decreased to varying degrees in the early rainy season, and increased in the late rainy season to the transition from the rainy season to the rainy and dry season, while these concentrations stayed stable or slightly fluctuated in the dry season. The decrease in precipitation promoted the precipitation of calcium carbonate caused by degassing, which increased the values of Mg/Ca, Sr/Ca and Ba/Ca. At the same time, water within the fractures between soil and bedrock was less disturbed by fresh water inputs. This reduction in the rate of water migration enhanced the interaction between water and rock, resulting in a preferential leaching of elements such as Sr and Ba. The consequent differential changes in the concentrations of these elements suggest that variations in their levels could be indicative of shifts in precipitation patterns. Elements like Fe, Al, and Mn constitute another category, primarily originating from the leaching effects of atmospheric precipitation on the overlying soil. The occurrence of peaks in these elements during the rainy season corresponds to periods of heavy annual precipitation, while peaks observed in the dry season may be associated with the pulse-like action triggered by a single instance of heavy precipitation.

Records of glacier activities by the alpine travertine landform in the Yulongxi area along Gongga mountain

CHAI Peiran, ZHAO Xueqin, WANG Fudong, JIANG Yaoxi, ZHANG Qiang, ZHU Heyan

2024, 43(3): 552-562, 584. doi: 10.11932/karst2024y027

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As a carrier of geological historical records and climatic information, travertine has played a crucial role in the study of global climate change, and scholars have made significant progress in research on travertine over the past few decades. Travertine deposits are widely distributed on the Qinghai-Tibet Plateau, often referred to as the "Third Pole in the World". They are especially distributed in high mountains along the eastern edge of the plateau, forming a belt of travertine from northeast to southwest. These travertine formations in the high-altitude regions are predominantly found above the Quaternary ancient glaciers, closely associated with the development and distribution of these glaciers. Hence, the travertine geomorphology in these high-altitude areas may serve as evidence of the Quaternary glacial activities. Therefore, aiming to provide evidence for the Quaternary glacier activities, this study selects surface-exposed travertine in the Yulongxi area on the western side of Gongga mountain as the research subject to analyze records of glacial activities in high-altitude travertine.The specific research methods employed in this study are as follows: (1) conducting field geological work to survey glacial landforms surrounding travertine, such as glacial striations, sheep back stones, etc., and collecting samples of moraines on and near the old travertine bodies; (2) utilizing GF-05 satellite hyperspectral remote sensing data to interpret the lithology of travertine developed in the fault zone of the Yulongxi area, thereby obtaining the distribution characteristics of travertine in the study area; (3) employing drones to perform low-altitude photogrammetric surveys on several typical travertine bodies in the study area, constructing refined DEMs and 3D models to analyze the distribution patterns and geomorphological characteristics of travertine, and combining terrain features to analyze the formation of sheep back stones and whalebacks; (4) using XRD testing technology to analyze the compositions of moraines and comparing the similarities and differences between the compositions of moraines deposited at different locations on the old travertine bodies; (5) integrating previous chronological data on travertine belts developed in the north-south direction on the eastern edge of the Qinghai-Tibet Plateau, and comprehensively discussing the potential records of glacial activities in travertine in the study area during the last Glacial Maximum.The research findings are summarized as follows. (1) Travertine in the study area is mainly distributed on both sides of the fault zone within the altitude range of 4,000 m to 4,500 m. Large travertine formations are more concentrated within a 200-meter elevation difference. Some residual travertine bodies are characterized morphologically by elongated shapes, surface striations, etc., consistent with the appearance of whalebacks and sheep back stone. (2) Based on XRD test results and terrain features, it is inferred that the deposits covering the travertine bodies are likely the products from glacial activities. During glacial periods, material weathered and eroded from the upper part of the Zagunao Formation was transported onto the travertine bodies through glacial action. Simultaneously, with the Quaternary glacier activities, early travertine formations underwent intense glacial erosion, forming glacial erosional landforms.Combining previous research on travertine and moraines in the Gongga mountain area, this study draws the following conclusions regarding the potential records of glacial activities during the last Glacial Maximum in the study area. During interstadial periods of the last Glacial Maximum, warming climate led to glacier melting and retreat. The melting of ice and snow, coupled with rainfall, provided favorable material conditions for the formation of travertine deposits in the study area. Meltwater and rainfall infiltrated down to lower elevations along synclines of the Zagunao Formation, upwelled along the fault in the Yulongxi area, overflowed near the fault and downstream valleys, and finally formed extensive travertine deposits. During glacial periods, the entire region was covered by glaciers. Under the pressure of the upper glaciers, bedrock debris entrained at the glacier base eroded and abraded the bedrock (travertine). Early travertine formations were eroded, forming glacial erosional landforms, with travertine fragments and bedrock debris covering the travertine bodies in some areas. With global warming and rising temperature, travertine growth ceased in most areas, due to the rapid retreat of glaciers and the reduction of precipitation and meltwater. Early glacially eroded travertine landforms were subjected to erosion and weathering by water flow, and some moraines remained on the top of old travertine bodies.

Research progress of nitrate tracing in karst groundwater based on CiteSpace

YANG Pingheng, HUA Maosong, LUO Weiqun, GUO Wenjing

2024, 43(3): 563-574. doi: 10.11932/karst2024y005

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Nitrate (${\rm{NO}}_3^{-}$) is one of the main pollutants in groundwater; therefore, once ${\rm{NO}}_3^{-}$ concentrations exceed an acceptable level, it will have adverse effects on water ecology and human health. Identifying the sources and biogeochemical transformation process of nitrate in groundwater is of important theoretical and practical significance for pollution prevention and control. The karst groundwater system is characterized by high vulnerability. At present, studies on nitrate in karst groundwater based on nitrogen and oxygen isotope technology have made great progress but most of them were presented by case study, lacking a systematic summary of research progress. CiteSpace, the software of knowledge network analysis, uses bibliometrics and visualization methods, co-citation network, association rules, cluster analysis to analyze the research hotspots in a field in Chinese and foreign literature. CiteSpace enables readers to better understand the knowledge framework, research topics and future hot topics, and plays a very good role in promoting the analysis of research progress in different professional fields.To investigate the research progress of isotope tracing of nitrate sources in karst groundwater in recent years (from 2003 to 2022), and comprehensively analyze the research hotspots of nitrate in karst groundwater, "karst groundwater", "karst aquifer", "nitrate", "nitrate-nitrogen", "nitrogen isotope", "oxygen isotope", "MixSIAR", "nitrate sources", "nitrogen transformations", "nitrate pollution", and "dual nitrate isotopes" were selected as subject terms in CNKI, and 1,191 papers were retrieved after the unrelated subject terms had been excluded. Meanwhile, subject terms of "karst groundwater", "karst aquifer", "nitrate", "NOx-N", "nitrogen isotope", "oxygen isotope" "MixSIAR", "nitrate source", "nitrogen transformation", "nitrate contamination", and "dual nitrate isotopes" were searched in Web of Science (WOS) core collection database, and a total of 2,232 papers were retrieved. CiteSpace was employed to analyze publishing institutions, authors, keywords and research hotspots, and co-citations of both the retrieved Chinese literature and English literature.The results show that the Institute of Karst Geology of the Chinese Academy of Geological Sciences had a great advantage in studies of nitrate in karst water system, because this institute has published the largest number of academic papers written in Chinese. The keyword clustering of "#1 karst groundwater", "#2 karst water", "#5 karst water resources", "#6 karst hydrological model" and "#11 karst aquifer" can be summarized as a research hotspot of karst hydrology and water resources. The keyword clustering of "#7 hydrogeochemistry", "#8 hydrochemistry" and "#10 hydrochemical characteristics" can be summarized as a research hotspot of karst hydrochemistry and water environment. The keyword clustering of "#0 nitrate pollution", "#3 karst", "#4 groundwater", "#9 stable isotope", "#12 nitrogen and oxygen isotope" and "#13 source analysis" can be summarized as a research hotspot of nitrate pollution tracing in karst groundwater. In terms of English literature, the United States, China and Germany had the largest number of publications, and there was a relatively close cooperative relationship between them. Academic papers published by the Chinese Academy of Sciences ranked top among the researching institutes. The keyword clustering of "#0 karst", "#5 macrophytes", "#6 ecohydrology", "#11 karst hydrology" and "#3 groundwater recharge" can be summarized as a research hotspot of karst ecology and hydrology research. The keyword clustering of "#10 groundwater", "#2 nitrogen cycle", "#9 nitrogen" and "#8 karst water" can be summarized as a research hotspot of nitrate transformation in karst groundwater (nitrogen cycle). The keyword clustering of "#1 source appointment", "#12 nitrate pollution", "#4 delta n15", "#7 stable isotope" and "#13 nitrogen isotope" can be summarized as a research hotspot of nitrate pollution tracing in karst groundwater. The technology of dual nitrate isotopes provides a strong technical support for the research of nitrate tracing and biogeochemistry in karst groundwater. Isotopic fractionation effect should be considered to improve the accuracy of nitrate pollution tracing results. This paper aims to provide reference for the source tracing and effective prevention of nitrate pollution in karst groundwater in the future.

Characteristics of travertine profiles and their environmental indications in Munigou, Sichuan, China

HE Jingwen, DAI Qunwei, XU Ronglin, CUI Jie, AN Dejun, DU Weiqi, ZHANG Ting, JIANG Ruiyang, RAN Yue

2024, 43(3): 575-584. doi: 10.11932/karst2024y021

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The travertine profiles are located in the Zhaga waterfall in Munigou, Sichuan Province, where the highest altitude ranges from 4,070 m to 2,800 m, and the average annual temperature is 7℃. The study area is situated in the Minjiang fracture zone, with Paleozoic carbonate construction on the east side of the fracture, and shallow metamorphic rocks and slates of the Middle and Upper Triassic Xikang Group on the west side. Travertine is deposited by the stream fed by karst springs, and the mouth of the spring is located on the left bank of the hook upstream of Zhaga waterfall. The travertine deposits are distributed from the mouth of the spring down to the mouth of the gully. Their depositional patterns fall into striated travertine, clastic travertine, etc., and the type of depositional phase is a dam. The overall travertine profile is about 30-meter thick with horizontal sedimentary construction. In this study, the travertine profiles were investigated in terms of mineralogy and isotope geochemistry. UV spectrophotometry and X-ray fluorescence spectroscopy were used to determine the organic matter, total nitrogen, total phosphorus and chemical elements in travertine profile sediments so as to comprehensively analyze the characteristics of the travertine profiles and their significance of environmental indications. The results show that the travertine profiles can be divided into four stages based on sedimentary features such as sediment color, texture and inclusions. Stage 1: 1,080–1130 cm, overall dominated by a black muddy travertine layer interspersed with yellow travertine, grey travertine, and white travertine, containing twigs, leaves, and roots; Stage 2: 1,130–1,230 cm, overall dominated by a grey travertine layer interspersed with yellow travertine and black muddy travertine layers containing tree roots; Stage 3: 1,230–1,353 cm, overall dominated by yellow travertine layers interspersed with black muddy travertine, white travertine and grey travertine; Stage 4: 1,353–1,480 cm, overall dominated by a white travertine layer interspersed with yellow travertine and black muddy travertine layers containing roots and leaves.The testing and analysis of the sediments from the travertine profile in Munigou revealed that the overall organic matter content of travertine sediments ranged from 294.66 g·kg⁻¹ to 2,366.52 g·kg⁻¹ with a mean value of 1,031.93 g·kg⁻¹; the TN content ranged from 0 g·kg⁻¹ to 0.89 g·kg⁻¹ with a mean value of 0.25 g·kg⁻¹; the TP content ranged from 0.03 g·kg⁻¹ to 0.25 g·kg⁻¹ with a mean value of 0.12 g·kg⁻¹. In order to fully understand the relationship between organic matter, TN and TP, and to better reveal the environmental characteristics when the travertine was deposited, we carried out Pearson correlation analyses of organic matter, TN and TP, which showed that the organic matter and TN content exhibited a significant correlation (r=0.822, n=48, P<0.01); the organic matter and TP content were positively correlated (r=0.574, n=48, P<0.01); there was a good correlation between TN and TP contents (r=0.644, n=48, P<0.01). Travertine sediments with four different colors at different depths were selected for elemental analysis, and the results showed that the main chemical compositions of the travertine profile were CaO and SiO₂, followed by Al₂O₃, Fe₂O₃, MgO, Na₂O, and TiO₂. The CaO content ranged from 97.79% to 27.51%, with an average value of 70.25%. The SiO₂ content ranged from 50.10% to 0.76%, with an average value of 19.52%. CaO is chemically active and easy to migrate. The high CaO activity indicates that it is easily lost under warm and humid conditions, making its content relatively low, and under dry and cold conditions, the weak surface chemical action enriches CaO, making its content high. SiO₂ is chemically stable and difficult to migrate; therefore, its high content can indicate much precipitation, warm and wet climate, and its relatively high enrichment, and vice versa.The travertine profiles at 1,080–1,130 cm, 1,250–1,280 cm and 1,460–1,480 cm were dominated by black muddy travertine, with high content of organic matter, TN and TP, and their chemical elements were dominated by SiO₂. The environment at the time of travertine deposition is presumed to be of wet climate during the rainy season with abundant precipitation. The travertine profiles at 1,130–1,250 cm and 1,280–1,460 cm were dominated by grey travertine, white travertine and yellow travertine, with low levels of organic matter, TN, TP, and their chemical elements were dominated by CaO. The environment at the time of deposition is presumed to be of a dry season with little precipitation and dry and cold climate. The above conclusions are intended to reveal the environmental conditions of Munigou at the time when travertine was deposited and to lay the foundation for an in-depth study of the ancient environment of Munigou.

Influence of different data types and dimension reduction on the recognition accuracy of travertine hyperspectral images

XU Menghui, WANG Weihong, TIAN Shuojuan, ZI Yingkun, WU Zhouhang, WANG Xiaomeng, XIANG Hongyao, FAN Jing

2024, 43(3): 585-594. doi: 10.11932/karst20240305

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Travertine is a kind of travertine carbonate precipitate that is generated when huge quantities of carbon dioxide are released from the surface of the earth. The formation of a large-scale landscape from this type of precipitate often takes a considerable length of time. Therefore, the travertine landscape may be used as a significant carrier for the study of crustal movement, paleoclimate, and other geological settings. Furthermore, the large-scale travertine landscape, which is considered as a natural heritage, is significant for conservation with a high tourist value. This study focuses on the Huanglong Scenic Area in China, which is recognized as a global natural heritage site by the United Nations Educational, Scientific, and Cultural Organization (UNESCO). This area is renowned for its expansive surface travertine landscapes that include a wide variety of distinctive formations and vibrant colors. The travertine in Huanglong, on the other hand, has been experiencing major deterioration in recent years, such as blackening and algal erosion. Therefore, the recognition and monitoring of travertine is urgent. This study proposes a method of recognizing travertine based on hyperspectral reflectance data in order to facilitate the protection and restoration of travertine resources. This method can be used to effectively tackle the problems brought about by traditional field surveys that are time-consuming, labor-intensive and likely destructive to travertine landscapes. This study was conducted in the following procedure. Four types of data were taken as classification objects, that is, original data and other three types of data that were converted respectively by multiple scattering, first-order derivative and second-order inverse for the original data. Then, these four types of data were respectively reduced to their corresponding dimensions by Principal Component Analysis (PCA) and Linear Discriminant Analysis (LDA), according to the magnitude of the cumulative variance of the data. Finally, for the classification of these four types of data after dimension reduction, they were respectively put into four kinds of classifiers, namely, Support Vector Machines (SVM), Random Forests (RF), BP Neural Networks, and Convolutional Neural Networks (CNN). Overall Classification Accuracy (OA) was used as an evaluation index. In addition, Particle Swarm Algorithm (PSO) was used to optimize the penalty coefficient C and the Gammer parameter values of SVM. Afterward, the optimized SVM was applied to develop a recognition model of classification. Moreover, three indicators, namely, F1-Score, Kappa coefficient, and OA were utilized to assess the performance of SVM recognition model. In terms of the data type and the method of dimension reduction, the classification results of the recognition model established in this study were studied. In the aspect of the method of data dimension reduction, it was discovered that dimension reduction of the original data by PCA was superior to that by LDA. Furthermore, the classification model of the original data by PCA dimension reduction was generally more accurate than that by LDA. With regard to the type of data, the mean value of the model accuracy with MSC data as input was 88%, which was the second largest among the four types of data, only 0.1% lower than the first one. However, its variance and standard deviation were 0.043 and 0.042, respectively, much smaller than those of the models with the other three types of data, which indicated that the recognition model with MSC data was much more stable. Finally, the SVM classification model that was optimized by PSO demonstrated its outstanding performance when evaluated from the three performance indexes: F1-score, kappa coefficient, and OA. In general, this performance is superior to that of the unoptimized SVM recognition model, with the SD-PCA-PSO-SVM model the best performance among the three. Values of F1-Score, Kappa and OA of the classification results by optimized SVM were 0.93, 0.92, and 0.98, respectively. In conclusion, it is easier for the unoptimized classifier to acquire a high-precision recognition model, if the MSC data or the original data processed by PCA dimension reduction were selected as the input in the recognition of travertine. Additionally, selecting an appropriate theory to optimize the model can also improve the recognition performance of the model.

Geomorphic information extraction and morphological characteristics analysis of karst peak-cluster depressions based on DEM

HE Jiyong, TIAN Yichao, ZHANG Qiang, WANG Donghua, ZHANG Yali, ZHOU Huijuan

2024, 43(3): 595-605. doi: 10.11932/karst20240306

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The largest area of karst in the world is distributed in China. In the karst area, the peak-cluster depression is regarded as one of the typical landforms. The positive and negative landforms in peak-cluster depressions control the spatial distribution pattern of soil and water resources, and significantly affect regional landforms, soil erosion and quality of regional ecological environment.In the southwest of Guangxi Zhuang Autonomous Region (hereinafter referred to as southwest Guangxi), the distribution of karst landforms is relatively concentrated, and their unique and steep terrain greatly restricts the development of regional economy. Consequently, this remote and border region became a poverty-stricken area in which are located old revolutionary bases and is inhabited by minority nationalities. The landform of karst peak-cluster depression together with urban architecture formed a unique mosaic landscape in southwest Guangxi, which is vulnerable in ecological environment. Although the traditional method of artificial vectorization can accurately obtain the positive and negative topographic features of peak-cluster depressions, this method has its disadvantages such as high working intensity, low working efficiency and long working period, which may pose challenges to the information extraction of peak-cluster depressions on a large regional scale, and which may also limit the detailed study on the ecosystem of peak-cluster depression basins. Therefore, it is of great significance for us to explore the formation mechanism of peak-cluster depressions and the evolution of regional geographic environment so that we can provide a scientific basis for the protection and sustainable development of regional ecological environment.In this study, with the use of software such as ArcGIS and Google Earth, information of peak-cluster depressions was extracted based on Digital Elevation Model (DEM) and satellite remote sensing images by two methods: hydrology method and saddle method. By means of spatial analysis and fractal theory, the structural morphological characteristics and spatial distribution of peak-cluster depressions were quantitatively analyzed. The results show as follows. (1) In terms of extraction methods, the saddle method can extract the depression boundary based on the features of the saddle, while the hydrology method can effectively identify the depression by simulating the convergence process of water flow. Compared with the hydrologic method, the saddle method can improve the accuracy of extracting saddle points by 29.80%. (2) In the morphological analysis of peak-cluster depressions, 94% of the depressions in the study area are basin-shaped, and the rest are deep cone-shaped and dish-shaped. The main morphology of peak-cluster depressions in the study area is basin-shaped with a shallow depth and a small area. In addition, the analysis of the relationship between the circumference and the area of the depression patch finds that there is a high correlation between the two, and its fractal dimension is 1.15. The mosaic structure of the depression patch is relatively stable. (3) In terms of spatial distribution, 80% of the peak-cluster depressions in the study area are concentrated in the interbedding areas of limestone and dolomite and in the contiguous areas of limestone in the south and north of the study area, while the development of peak-cluster depressions in the central clastic rock strata is not obvious.

Technology of classifying lithology of typical karst areas based on machine learning: Taking the Pingguo area, Guangxi as an example

DU Wei, MENG Xiaoqian, TU Jienan, LIU Song, HU Wei, ZHANG Yiming, DAI Yuanyuan, WU Yang

2024, 43(3): 606-616. doi: 10.11932/karst2024y025

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Karst is strongly developed in Southwest China, and the geological hazards specific to karst areas may cause serious damage to the local construction facilities; therefore, the rapid identification of carbonate rocks is of great significance for the planning of infrastructure construction such as electric power, transportation, etc. At present, the classification of lithology through remote sensing is still one of the most efficient ways. In this study, on the basis of visible to short-wave infrared multispectral information, texture information such as the second order moments of entropy and angular, and topographic features such as curvature and slope, we proposed a method to classify carbonate rocks, clasolite rocks, Quaternary sediments and water bodies by remote sensing in the Pingguo area to realize automatic extraction of carbonate rocks. This method presents its advantages of obtaining data sources with high accuracy in simple realization path in terms of lithology classification of typical karst areas, and this study can provide a new idea for rapid lithology classification in karst areas. Located in the southwest of Guangxi with subtropical monsoon climate, the Pingguo area is one of the most developed karst areas in China, in which mainly develops limestone, dolomite and their interbedded and interlayered layers of the Triassic Beisi Group and Luolou Group. Karst landforms are distributed in the north, central and southwest of the Pingguo area, and are dominated by the karst peaks and depressions, significantly different from the morphological characteristics of clasolite rocks in the same area.The process of lithology classification and carbonate rock identification in this study mainly included the following steps: data source selection and processing, texture feature extraction, terrain feature extraction, sample sketching and training, and final mapping. The data was mainly from Landsat 8 OLI and AW3D 30 DSM available for the public. The 8-channel data was obtained, based on the synthesis of five multispectral bands including BLUE, GREEN, RED, NIR, SWIR bands, texture features of second-order moments of entropy and angular with Principal Component Analysis, and topographic features such as curvature and slope. Then the maximum likelihood classification was used to carry out the lithology classification of the study area, and the distribution of carbonate rocks was finally obtained.The results show that the overall accuracy of the method in the study area is over 90%, and the kappa coefficient is more than 0.85, which proves that the classification model is effective and practical for the extraction of carbonate rocks in the study area. In addition, results of accuracy evaluation show that the accuracy is high for classification of carbonate rocks, with an accuracy more than 94%, but the accuracy of classifying water bodies and Quaternary is relatively poor. The main reason for this difference is that the Quaternary sediments are scattered in the carbonate and clasolite areas, and part of Quaternary sediments may exist in the area covered with vegetation which is easy to be judged as the area of carbonate or clasolite rocks. The main reason for the slightly lower accuracy in classification of water bodies is that some water bodies and the areas shaded by carbonate rocks are easily confused, and there are also cases where carbonate rocks are misclassified as water bodies.Based on multi-source remote sensing data, we conducted the maximum likelihood estimation with the consideration of spectral, topographic, textural and other multi-factors in order to develop a fast and accurate automatic extraction method for carbonate rocks in typical karst areas. In addition, we used this method to automatically classify carbonate rocks, clasolite rocks, Quaternary sediments and water bodies in the Pingguo area and evaluated the classification accuracy. We drew the conclusions as follows. (1) Based on data of Landsat and AW3D 30 DSM, we adopted multi-factor synergistic analysis of spectral information, textural features and topographic features of typical carbonate areas, and carried out automatic extraction of carbonate rocks, clasolite rocks, etc. in the Pingguo area, a typical karst area. The overall classification accuracy was greater than 93%. (2) There was a great difference in the surface morphology of carbonate rocks and clasolite rocks in the study area, which is one of the important factors for distinguishing these two types of rocks. Meanwhile, the slope has a significant effect on the accuracy of carbonate rock extraction, so it is suggested that a slope should be used as one of the factors in the classification in the process of comprehensive extraction of carbonate rocks. (3) Lithology is difficult to be automatically extracted with multi-spectral remote sensing information, due to the complex multi-solution of extraction and the spatial heterogeneity of lithology. Most of the current studies differentiated lithology through the inversion of mineral compositions from the perspective of mineralization and alteration. With the gradual development and improvement of machine learning and deep learning, the comprehensive extraction method integrated with spectral bands, texture and morphology will provide a new idea for the significant improvement of the accuracy of lithology extraction, which may be one of the important directions for future research on lithology extraction.

Study on land use change of Dajiu lake and its impact on ecological environment in the last 60 years

MENG Xiaojun, YUAN Jingjing, SUN Xinyue, HUANG Wei, AI Dong, SHEN Lulu, MENG Yuling

2024, 43(3): 617-626, 649. doi: 10.11932/karst20240307

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Dajiu lake of Shennongjia is a typical, unique and rare subalpine peat-swamp wetland in China. From 1985 to 2002, due to the building of farms, the area of Dajiu lake had been transformed from wetlands to farmlands, for example, transforming natural river channels, straightening curved river sections, building north-south main canals, draining water in wetlands, cultivating croplands in a large amount to expand the farmland area, and growing high-altitude vegetables. With China increasingly giving priority to ecological conservation, the government of Shennongjia Forestry District established the Nature Reserve of Dajiu Lake Wetland at a district level in December 2003, initiating the protection of ecological environment in Dajiu lake. During this period, the ecological environment in the area of Dajiu lake, especially the center of this area, underwent a dramatic transition from extensive development to large-scale conservation. In order to fully understand the change in the ecological environment of Dajiu lake before and after large-scale development and conservation, the center of Dajiu Lake National Wetland Park was selected as the study area. Keyhole satellite data, Landsat7 ETM data, and unmanned aerial vehicle images from 1963, 2002, and 2021, respectively representing the undeveloped period, development period, and protection period of Dajiu lake, were compared to analyze changes in land use and in ecological environment quality within the study area during the three time periods. Research results show that during the primitive agricultural period in the area of Dajiu lake, wetlands and grasslands covered the largest area of total land use, with less land used for construction, resulting in a relatively better ecological environment. However, during the period of large-scale agricultural development, a significant number of grasslands and wetlands were converted into arable lands. With a sharp increase of arable lands and human activities, the ecological environment suffered considerable damage. After implementing large-scale ecological environment protection in the area of Dajiu lake, initiatives such as returning cropland back to grasslands and wetlands contributed to a significant recovery of grasslands and wetlands, a notable increase in water body area, and a considerable improvement of ecological environment. The index of regional ecological environment quality was determined based on the land use transfer matrix data about the area of Dajiu lake from 1963 to 2002. The results indicate that the index of regional ecological environment quality in the study area decreased from 0.601 in 1963 to 0.473 in 2002, but rose to 0.651 in 2021. This shows that over the past 60 years, the overall ecological environment quality in the study area has exhibited a pattern of an increase after an initial decrease, with a sustained improvement in the overall ecological environment after 2002. The quality of ecological environment during the protection period surpassed that of the large-scale development period and was better than that in the undeveloped period, which demonstrated significant achievements in the 18-year protection of ecological environment in the area of Dajiu lake. This study analyzed in detail the driving forces behind the changes in the ecological environment quality of the study area, explored the specific reasons for and predicted the trends of changes in land use and ecological environment quality. Remote sensing interpretation of land use changes in the area of Dajiu lake was generally based on remote sensing data after 2000. In the remote sensing interpretation of this study, the time of historical data was advanced to 1963, which extended the time span of land use changes in Dajiu lake to over 60 years from more than 20 years, longer than that in previous studies.

Hydrological characteristics of dripping water in karst caves under different land covers

FAN Hongdai, HU Yundi, ZHOU Zhongfa, HU Deyong, ZHANG Yuchao, HUANG Zhengzhou, WANG Dengchao, SHANG Yun, LI Hang

2024, 43(3): 627-639. doi: 10.11932/karst2024y018

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In recent years, the deterioration of global climate has caused a series of natural environment problems, which urgently requires researchers to forecast the future climate and put forward effective management measures. Previous studies have shown that the study and reconstruction of the earth paleoclimate can help people understand the patterns of past climate change and its dominant mechanism. Therefore, as to effectively predict the future climate change and the way the contemporary ecosystem responds to the future climate change, the restoration and reconstruction of ancient climate and environment have gradually become the focus of today's research on earth sciences. At present, an increasing number of researchers have reconstructed paleoclimate and environment based on various geochemical indexes of stalagmites and secondary carbonate sediments in karst caves. Cave dripping water is the material basis for the formation of stalagmites, and it is also the main source of trace elements in stalagmites. The high frequency signal caused by the change of external climate environment will affect the ancient climate information recorded by stalagmites. Therefore, it is of great importance to establish a long-term monitoring system based on cave dripping water and to analyze the relationship between the hydrologic characteristics of cave dripping water and the changes of external environment during the study of stalagmite indication. Cave dripping water is an important part in the water cycles of karst areas, and its hydrological characteristics are closely related to the geochemical isotope indexes which carry climate information. The land cover type is an important factor affecting the regional water cycle processes. However, most of the current studies discussing the impact of land cover on water cycle processes are mainly concentrated on the watershed scale in non-karst areas, and very few studies focus on the impact of land cover on the hydrological characteristics of cave dripping water in karst areas. In order to explore the influence of different land covers on hydrological characteristics of cave dripping water in karst areas, this study selected three karst caves with different types of topsoil and vegetation in Guizhou Province, Southwest of China. These three caves are Qingjiang cave with thick soil layer and moderate vegetation cover), Guangshun cave with thin soil layer and dense vegetation cover, and Shijiangjun cave with thin soil layer, sparse vegetation cover and thick carbonate roof. This study was conducted based on drip discharge rate, soil water content and meteorological data during one hydrological year. Study results show: (1) The drip discharge rates of the three caves display obvious seasonal variations that are synchronized with the input of precipitation, i.e. high in the rainy season, low in the dry season, because they were influenced by meteorological conditions, e.g. precipitation, temperature and humidity. The dripping water flow is vulnerable to dry up in the dry season with less precipitation input. (2) Soil has a strong retention and storage effect on precipitation infiltration. Compared with Guangshun cave and Shijiangjun cave, Qingjiang cave with thick soil layer on the roof shows a unique low flow value during the rainy season from August to October, due to high external temperature and long exposure time after rain, which leads to strong evaporation effect. Besides, the lowest annual soil water content also occurs during this period. (3) The abundance of drip discharge rates of the three caves can be ranked as follows: Shijiangjun cave (1.185 mL·min⁻¹)>Qingjiang cave (0.600 mL·min⁻¹)>Guangshun cave (0.236 mL·min⁻¹), according to their annual mean values. The annual stability of their drip discharge can be obtained by their Cv values, indicating that the drip discharge is the most stable in Shijiangjun cave (0.072), followed by the discharge in Guangshun cave (0.481) and Qingjiang cave (1.264). (4) The influence of vegetation on hydrological characteristics of dripping water is weaker than that of soil, and the dripping water flow in caves with better soil cover development are more likely to dry up throughout the year. For example, the dripping water flow in Qingjiang cave with thick soil cover is more likely to dry up than in Guangshun cave with thin soil layer but with dense vegetation cover. In addition, caves with thick carbonate roof would have a better regulation effect on dripping water flow, e.g. Shijiangjun cave where its dripping water flow is the most stable and abundant throughout the year. In summary, the discharge rate of dripping water is affected not only by meteorological conditions such as precipitation, temperature and humidity, but also by land cover and thickness of aquifer roof above a cave. Therefore, the results of this study could provide crucial scientific guidance for the selection of dripping water points with relatively abundant and stable flow in karst caves, and would be of great significance for the use of cave dripping water in climate research.

Characteristics of soil carbon, nitrogen and their components under different vegetation types in the karst reservoir area: A case study of Huaxi reservoir in Guiyang

ZHOU Wei, JIANG Xia, YAO Shiju, ZHANG Yan

2024, 43(3): 640-649. doi: 10.11932/karst2024y030

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The study area is located in the water conservation area of Huaxi reservoir in Guiyang City, Guizhou Province. Soil in this area is developed from limestone with the soil thickness varying greatly from 0 cm to 103.5 cm. Rocks are exposed in some parts of the area. Because the study area has been closed off to prevent deterioration, the vegetation is well protected. The main vegetation types are grassland, shrubs and bamboo forest. In order to understand the relationship between soil nitrogen and phosphorus contents and their components in the karst reservoir area, we analyzed the decomposition characteristics of soil organic matter in the karst area. The result can provide theoretical support for ecological restoration and conservation of water supply in the karst area.In Huaxi reservoir, we chose three typical vegetation types—grassland, shrubbery and bamboo forest—as the research objects. We selected 3–5 duplicates from every different vegetation plot, and studied the soil carbon and nitrogen contents and active components of these three vegetation types in two layers (Layer A and Layer B). The results showed that there were significant effects for different vegetation on carbon and nitrogen and their components (except for AN). With the change of vegetation types (grassland→shrubbery→bamboo forest), the content of organic carbon and its active components gradually increased (Layer A>Layer B). TOC contents in bamboo forest (36.16 g·kg⁻¹ and 29.68 g·kg⁻¹) were higher than those in grassland (20.48 g·kg⁻¹ and 18.24 g·kg⁻¹). MBC contents in grassland was 54 percent and 49 percent of that in bamboo forest. The proportion of WSOC in total organic carbon was the smallest (0.04%–0.11%), among which the content of WSOC in bamboo forest accounted for a relatively high proportion (0.11%). The proportion of microbial biomass nitrogen was the second, which accounted for a relatively higher proportion in shrubbery (0.99% in layer A and 1.42% in layer B), but the lowest in bamboo forest (0.88% in Layer A and 0.89% in Layer B). The proportion of microbial biomass nitrogen was the least, accounting for only 0.02%–0.20% of total nitrogen. With the restoration (grassland→shrubbery→bamboo forest), MBN contents increased gradually, and the content of ${\rm{NO}}_3^{-}$−N in Layer A gradually reduced. The contents of microbial biomass nitrogen in bamboo forest (3.48 mg·kg⁻¹ and 3.12 mg· kg⁻¹) were significantly higher than those in shrubbery and grassland. The content of ${\rm{NO}}_3^{-}$−N in grassland was higher than that in bamboo forest (2.24 mg·kg⁻¹). The content of NH$_4^{+}$−N was much higher than that of ${\rm{NO}}_3^{-}$−N. The content of nitrate nitrogen was more than 10 times that of NH$_4^{+}$−N. With the layer deepening, the contents of MBN and AN reduced, but the TN content increased. Among the ratios of different nitrogen components to total nitrogen, the ratio of AN to TN was the highest (3.38%–6.93%). The ratio of NH$_4^{+}$−N to TN was higher than that of ${\rm{NO}}_3^{-}$−N to TN, and the ratio of microbial nitrogen was the lowest, accounting for only 0.03% in shrubbery and grassland. In Layer A, the ratio of C to N was from 8.49 to 21.52, showing as grassland<shrubbery<bamboo forest, and layer A>layer B. In layer B, the ratio was between 8.49 to 11.06, showing as shrubbery<grassland<bamboo forest.The results indicate that the vegetation types can affect the contents of carbon and nitrogen and their components. The vegetation of bamboo forest and shrubbery is conducive to the decomposition and absorption of soil carbon and nitrogen and can reduce the loss of soil and water. Therefore, the vegetation of bamboo forest and shrubbery can maintain water quality in the reservoir. Grassland is not suitable for ecological restoration in the karst area. However, this study only analyzes carbon and nitrogen and their components in different vegetation types, but does not explore changes of soil phosphorus content, microorganism content and plant root exudate, which needs further research.

Heavy metal pollution of soil and its ecological risk evaluation in the phosphate mining area in central Guizhou

JIANG Feng, LI Qiang, GAO Feng, JIQIN kebuzi, LIU Hanwu, WANG Ruofan

2024, 43(3): 650-660. doi: 10.11932/karst2024y029

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To investigate the current status of heavy metal pollution of soil in the phosphate mining area in central Guizhou and to comprehensively evaluate the potential pollution risks of eight heavy metal elements, a systematic sampling approach was employed in a plum blossom pattern. A total of 24 topsoil samples were collected from the surrounding areas of the phosphate mining area and from different strata in the non-mining area. The concentrations of Cu, Pb, Zn, As, Hg, Se, Cd, and Cr were determined. Based on the test results, comprehensive evaluations of the degree of heavy metal pollution and potential ecological risks in the topsoil of the mining area were conducted in terms of the Nemerow Pollution Index (NPI), Geo-accumulation Index (I_geo), and Hankanson Potential Ecological Risk Index (RI).The results indicate that the average concentrations of Hg, Se, Cd, As, Pb, Zn, and Cr in the topsoil of the phosphate mining area in central Guizhou were 3.83 mg·kg⁻¹, 8.42 mg·kg⁻¹, 2.29 mg·kg⁻¹, 240.6 mg·kg⁻¹, 65.83 mg·kg⁻¹, 141.4 mg·kg⁻¹, and 145.8 mg·kg⁻¹, respectively. These values were 37.58, 24.40, 19.26, 18.09, 2.67, 2.17, and 1.78 times higher than the corresponding background values of soil in Guizhou, indicating significant enrichment of these heavy metals in the study area. In contrast, the average concentration of Cu did not exceed the background value of the surrounding soil. The evaluation of heavy metal pollution in the topsoil showed that NPI values for Hg, Se, Cd, As, Pb, Zn, and Cr ranged from 4.59 to 69.32, indicating a general level of heavy pollution. Specifically, NPI values for Hg, Se, As, Cd, and Pb were 75.73, 55.21, 35.36, 19.75, and 10.32, respectively, all demonstrating heavy pollution. NPI values of Cr and Zn were 2.17 and 2.11, respectively, suggesting moderate pollution. The lowest NPI values were observed in Cr and Cu, with averages of 1.89 and 1.21, respectively, indicating light pollution. The I_geo analysis revealed that the average I_geo value for Hg was 4.45, showing severe to extreme pollution. The average I_geo values for As, Se, and Cd ranged from 3.35 to 3.65, indicating heavy pollution. I_geo values of Pb, Zn, and Cr averaged between 0.21 and 0.44, suggesting a range from unpolluted to moderate levels of pollution. The average I_geo value for Cu was -0.83, indicating an unpolluted level. The evalution of potential ecological risk showed that the single-element RI for Hg in the topsoil was 1,533.33, indicating an extremely high ecological risk. The single-element RI value for Cd was 298.91, indicating a high ecological risk, while the value of As is 180.92, indicating a very high ecological risk. The values for Cu, Pb, Zn, and Cr were all below 10, suggesting minor risk levels. Overall, the average RI value for the topsoil in the phosphate mining area was 2,285.50, indicating an extremely high potential ecological risk. Hg was identified as the primary ecological risk element, followed by Cd and As, warranting significant attention and management measures.

Zoning of ecological treatment in typical karst areas based on ecosystem vulnerability assessment

HU Zhaoxin, LUO Weiqun, JIANG Zhongcheng, WU Zeyan, TANG Qingjia

2024, 43(3): 661-671, 703. doi: 10.11932/karst2024y026

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Guohua Demonstration Zone for Ecological Restoration of Desertification (hereinafter referred to as Guohua Demonstration Zone) is located in Guohua town, Pingguo City, Guangxi, with a total area of 30 km². It is a typical area of karst peak-cluster depression. Before the treatment of rocky desertification, Guohua Demonstration Zone suffered from severe rocky desertification and soil erosion. Few crop varieties grew there with low land productivity. This area was covered with less than 10% of vegetation and less than 1% of forest. Consequently, it was extremely fragile in ecological environment. After more than 20 years of systematic governance, the vegetation coverage in the demonstration area has increased to 85%, and the proportion of rocky desertification area has decreased from the original 76% to 12%. The ecological environment has been greatly improved.This study aims to further provide a scientific basis for the ecological protection and restoration in the demonstration area, and to provide reference for the development of ecological governance strategies in karst areas based on the results of ecosystem vulnerability evaluation at a micro scale. With these purposes, this study has established an index system for ecosystem vulnerability evaluation. Based on the "cause-effect" model, this study selected seven indicators including rock type, slope, comprehensive soil nutrient level, land use status, vegetation coverage, soil erosion intensity, and degree of rocky desertification. In a 10 m×10 m grid taken as a unit, GIS technology and Analytic Hierarchy Process (AHP) were used to conduct a high-resolution ecosystem vulnerability evaluation in Guohua Demonstration Zone. By single parameter analysis, the contribution rate of each evaluation index to ecosystem vulnerability was calculated to analyze the main influencing factors causing ecosystem vulnerability in the study area.The results showed that the proportions of areas with micro, mild, moderate, severe, and extremely fragile ecosystem in the study area were 36.43%, 33.90%, 18.91%, 9.34%, and 1.43%, respectively. The main factor causing micro vulnerability of ecosystem in the study area is natural background, and ecological treatment in micro-vulnerable area should mainly focus on the maintaining of current situation, especially the protection of existing forest and grass vegetation. The main factors causing mild and moderate vulnerability of the ecosystem in the study area are natural background and human activities. Ecological treatment in mild and moderate vulnerable areas should focus on the protection of existing forest and grass vegetation and the limiting of human activities. The main factor causing severe and extremely vulnerability of the ecosystem in the study area is degradation of ecological environment. Ecological treatment in severe and extremely vulnerable areas should focus on the strict restriction of human activities and the restoration of forest and grass vegetation. According to statistical analysis of the overall vulnerability of ecosystem in different land uses in the study area, the main directions in ecological treatment for different land uses were proposed. Paddy fields should be mainly developed and utilized. Dry land, grassland and gardens should be developed and utilized under the premise of protection and restoration. The existing vegetation in forest land should be protected to achieve a positive vegetation succession, with appropriate development and utilization. Mining land should be restored ecologically. Other land types should be maintained in their current status.Based on the slope and land area, the study area was divided into nine zones: the mountainous zone closed off for afforestation, the artificial afforestation zone, the under-forest planting zone, the zone for grassland improvement, the zone for garden transformation, the zone for special economic plants, the zone for returning cropland back to forests, mining restoration zone, and non-treatment zone. Specific measures for treatment were proposed for each zone. To protect existing vegetation and promote natural positive succession of vegetation, human activities such as land reclamation, grazing, and firewood cutting should be prohibited in the mountainous zone closed off for afforestation. In the artificial afforestation zone, heliophilous species such as Dalbergia odorifera T. Chen., Delavaya toxocarpa Franch. and Leucocephala (Lam.) de Wit, which need light in plots with low vegetation coverage can be planted. In plots with high vegetation height and moderate coverage, neutral and sciophilous species such as Cyclobalanopsis Oerst. and Caesalpinia sappan L., which need some shade can be planted. Vine shrubs such as Euonymus fortunei (Turcz.) Hand.-Mazz. and Ficus pumila L. can be planted on side slopes and steep slopes to form plant hedges. In the under-forest planting zone, characteristic economic plants suitable for karst areas, such as Erythropalum scandens Bl. and Lonicera japonica Thunb can be introduced. In the zone for grassland improvement, high-quality perennial grass with strong resistance such as Guimu No.1, Cichorium intybus L. and Pennisetum purpureum, can be cultivated to develop livestock husbandry. In the zone for garden transformation, low-stem economic crops such as Erythropalum scandens Bl. and Lonicera japonica Thunb. can be intercropped under the forest. In the zone for special economic plants, evergreen wild vegetables suitable for growing in karst areas, such as Erythropalum scandens Bl. and Sauropus androgynus (L.) Merr. can be introduced to replace traditional crops such as corn. In the zone for returning farmland to forest, afforestation and grass planting should be carried out to promote the restoration for forest and grass vegetation according to local conditions. Ecological slope protection, site leveling, greening, and backfilling with netted soil can be implemented for reclamation and greening in the mining restoration zone. In non-treatment zone, the current situation can be maintained.

Extraction of land use information in karst areas based on Sentinel-2 images

LONG Ziwei, WANG Hong, JIA Yu, WU Yongjun, PENG Junjie

2024, 43(3): 672-683. doi: 10.11932/karst20240308

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Accurate land use information is the foundation of land management. Remote sensing data, characterized by its ease of acquisition, low cost, and high efficiency, has been widely used by scholars at home and abroad in the research of land use classification in combination with machine learning algorithms. Karst landforms are widely distributed in Southwest China. This region is fragile in ecological environment, due to its rugged terrain, large surface undulations, fragmented distribution of land plots. In addition, because of the long-term influence of its topography, the level of land use in the region is relatively low, and its economic development remains sluggish. Although extraction of accurate land use information is crucial for land resource management and planning in karst areas, the complex terrain and fragmented distribution of land plots in karst areas pose challenges for the extraction. Therefore, building on previous research, this study selected the southwestern part of Guiyang City, Guizhou Province—an area with karst landform characterized by complex terrain distribution and fragmented land plots—as a study area. With the use of Sentinel-2 satellite imagery as the basic data, the optimal object-oriented segmentation scale was calculated. The ReliefF algorithm was utilized to select features to input into the random forest algorithm, and land covers obtained from remote sensing images in different years were classified based on stratified classification.This study proposed a method that determined the optimal segmentation scale through joint evaluation, selected features from the prior feature dataset by random forest algorithm, and carried out classification with the use of ReliefF algorithm and a stratified masking strategy. Firstly, the optimal segmentation scale was determined as 80 by a joint evaluation with the combination of homogeneity and Moran's I. Subsequently, the ReliefF algorithm was employed to rank the importance of the initial features, with the top 15 significant features being selected. On this basis, the superiority of the random forest algorithm was verified by comparing multiple machine learning algorithms. Then, taking the Sentinel-2 images in 2022 as experimental data, this study designed three schemes for object-oriented classification to validate the superiority of the method with the combination of optimal segmentation scale, feature selection, and stratified masking in land use information extraction in karst areas. With the same samples, Model A completed the experiment by random forest algorithm without feature selection, and selected all 25 user-defined features, spectral features, shape features, and texture features. Model B selected the top 15 features after feature selection by ReliefF algorithm, and completed the random forest classification. Model C selected the top 15 features after ReliefF algorithm optimization and employed stratified masking under the random forest algorithm. In the principle of starting from the easiest area, non-vegetation areas were classified first. After the already classified areas had been ruled out by masking, the vegetation areas were classified, and finally, the classification results are merged. This method was then applied to the images in 2023 to verify its applicability in the extraction of land use information in karst areas. Through experimental research, the following conclusions can be drawn. Using the ReliefF algorithm to optimize the 25 classification features of remote sensing images can effectively improve classification accuracy and efficiency when training/validation samples are the same. In this study, the overall accuracy of Model B after feature selection was improved by 6.2%, compared to that of Model A with the original feature dataset, and Kappa was improved by 0.081. Multi-scale segmentation is the foundation of object-oriented classification and can avoid the "salt and pepper phenomenon". The evaluation of segmentation quality with the use of homogeneity and heterogeneity indices indicate that the optimal segmentation scale is 80. This approach can minimize the subjectivity of manual visual inspection. The random forest algorithm is superior in extracting land use information on different types of regions. Combining this algorithm with stratified masking can further reduce interference from already classified features to unclassified ones. The study demonstrates that the stratified masking method can achieve 88.6% of accuracy, the highest.

Differences of carbon and oxygen isotopes in coarse-grained calcite from the Permian Maokou Formation in different regions of Sichuan Basin and their significance in paleokarst environment

GAO Zhaolong, DAN Yong, ZHANG Xihua, CAO Hua, DENG Min, CHEN Cong, HU Luojia

2024, 43(3): 684-693. doi: 10.11932/karst20240309

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Oil and gas fields in karst carbonate reservoirs are widely distributed in China. A number of karst fracture-vuggy reservoirs have been found in the Tarim Basin, the Ordos Basin and the Sichuan Basin. Therefore, research of reservoirs in karst fractured caves has become one of the focuses in recent years, which indicates a promising future and huge potential for exploring karst carbonate reservoirs in China. Reservoirs in fractured caves of the Maokou Formation have always been one of the key areas for oil and gas exploration in the Sichuan Basin. Since the last century when reservoirs in fractured caves in the southern Sichuan region were discovered, breakthroughs have been made in this field in the Yuanba area of northern Sichuan, Yunjin area of southern Sichuan, and the Penglaiba area of central Sichuan, demonstrating good exploration prospects. However, there has always been controversy over the origin of fractured caves. One view suggests that the exposure of the Maokou Formation caused by the Dongwu Movement and the occurrence of freshwater karst are the main reasons for the development of fractured caves in the Maokou Formation, while another view suggests that fractured caves in the Maokou Formation are related to the tectonic fluid activity during the burial period. these two views are mostly based on research in the southern Sichuan region, and further consideration of the entire Sichuan Basin is needed to search for relevant evidence from geological phenomena to geochemistry.To understand the causes and differences of these fractured caves, we systematically sampled calcite for data of carbon and oxygen isotopes from outcrop profiles or from drilling fractured caves in different areas of the Sichuan Basin, in order to determine the paleokarst environment in which fractured caves were developed. Study results show that fractured caves of the Maokou Formation in this basin are mainly composed of small-and-medium-sized dissolution pores, fractures, and dissolution fractures. The carbon and oxygen isotopes of calcite in the Maokou Formation can be divided into four types, indicating four types of paleokarst environment with different fluid properties. Type I is the quasi syngenetic karst environment in which the oxygen isotope values of calcite in fractured caves are basically consistent with the values of limestone or of Middle Permian seawater in the Maokou Formation, with δ¹³C ranging from 1.80‰ to 3.83‰ and δ¹⁸O ranging from −8.95‰ to −4.11‰. Type II is the freshwater karst environment in which high negative δ¹³C and δ¹⁸O values are present in calcite of fractured caves, with δ¹³C<0‰ and δ¹⁸O ranging from −12.00‰ to −9.00‰. Type III is the buried karst environment in which lower δ¹⁸O values are significantly negative compared to the values of bedrock. However, δ¹³C values are basically consistent with those of bedrock, with δ¹³C ranging from 0‰ to 3.83‰ and δ¹⁸O ranging from −12.00‰ to −8.50‰. Type IV is the high-temperature hydrothermal environment in which δ¹⁸O values are significantly negative (<−12.00‰).Regional comparison shows that the oxygen isotopes in central and southern Sichuan are more negative than those in northern Sichuan, while the carbon isotopes in northern and central Sichuan are more negative than those in southern Sichuan. Research suggests that buried karst is the main type in southern Sichuan, followed by freshwater karst and quasi syngenetic karst, all together forming reservoirs dominated by fractures. In addition to quasi syngenetic karst and freshwater karst, high-temperature hydrothermal karst is also developed in central Sichuan. The high-temperature hydrothermal karst has formed dissolution pore reservoirs related to dolomite. The northern Sichuan is mainly characterized by quasi syngenetic karst and freshwater karst, forming reservoirs mainly composed of dissolution pores.

Relationship between sedimentation and red karst landform during the Ordovician in Hunan, Chongqing and Guizhou

CHEN Youzhi, JIANG Fuwei, CHEN Ying, YANG Guilai, YU Ning, SU Xiaoliang

2024, 43(3): 694-703. doi: 10.11932/karst20240310

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The Ordovician red karst is a newly discovered geological landscape in Hunan, Chongqing and Guizhou. Its appearances, distributions and rhythms are distinctive. With the development of red stone forest parks in the study area, red karst landforms have attracted great attention in recent years. Researchers have analyzed red karst landforms from their appearance characteristics, geomorphological types, surface corrosion, geological structures, climatic and neo-tectonic movement. However, there is still lack of research on the relationship between formation of red karst landforms and sedimentation. Integrating the field geological survey, observation of thin sections, odontolite analysis and X-ray diffraction analysis, this study tried to discuss the inner connection between weathering and red karst landforms in the view of sedimentation.During the syndiagenetic stage, the increase of oxygen content in the atmosphere caused the oxidation at the bottom of shallow continental shelf. During this process, Fe₂O₃ that was formed by the oxidation of iron ions in terrigenous debris entered sedimentary strata, which contributed to red-colored landform. Controlled by the distribution of lithofacies in the early-to-mid Ordovician, karst landforms were only developed in the carbonate arc blet of the shallow continental shelf along the area of Wuhan–Songzi–Songtao–Huangping. Pore sizes and structures of carbonate rocks affected by sedimentary microfacies would influence weathering, and hence the differential weathering caused concave and convex terrains. The Milankovitch cycle resulted in the overlapping of argillaceous limestone and bioclastic micritic limestone in the Guniutan Formation, and the meter-scale cycle of circum-tidal flat caused the rhythms of karst landforms. Types and contents of clay minerals within the limestone indicate the differences in sediment sources and burial histories and also affects hydration expansion and weathering resistance of the formation. In summary, the sedimentation process that controls the rock formation and weathering is one controlling factor to the formation and distribution of red karst landforms.

Influence of multiple karst caves at the pile end on the bearing capacity of pile foundations based on ABAQUS

GUO Bin, YIN Ou, CHEN Xian, LIU Qinghua

2024, 43(3): 704-716. doi: 10.11932/karst2024y024

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As an important load-bearing structure of buildings, the stability of pile foundations will directly impact building safety. With the expansion of engineering construction, foundations are frequently laid in an encounter with multiple karst caves. However, research on the influence of complex geological conditions with multiple karst caves at the pile end on the ultimate bearing capacity of pile foundations lags behind engineering practice. Therefore, studying this impact is crucial for engineering practice. In this study, a model of pile end containing multiple karst caves was established by the large finite element analysis software ABAQUS, with reference to a specific karst pile foundation project in the Hongshan district, Wuhan City, Hubei Province. This study aimed to explore the influence of cave shape, geometric characteristics, and spacing between karst caves on the ultimate bearing capacity of pile foundations under complex geological conditions with multiple karst caves at the pile end. This study involved the following steps. 1. Geological models of piles and karst caves were established. This involved firstly determining the premise assumptions for finite element analysis, selecting appropriate constitutive models, setting relevant parameters of pile foundations and engineering geological conditions based on a specific karst pile foundation project; secondly, setting constraints and boundary conditions, performing grid division, and determining calculation analysis; finally, establishing the model of comparative working condition groups according to the research purpose. 2. The finite element analysis was conducted with ABAQUS to simulate the displacement process of pile foundations under various karst geological conditions subjected to external forces. 3. Data compilation and analysis were completed. Load-displacement curves of pile foundations under different karst geological conditions were compiled. Based on Technical Code for Testing of Building Foundation Piles and the failure mode of pile foundations in simulation, the standard for selecting the ultimate bearing capacity of pile foundations was determined. Ultimate bearing capacities of pile foundations under different working conditions were extracted, organized, and graphed to analyze the influence of different cave parameters on the ultimate bearing capacity of pile foundations.The results indicate that under the same conditions of a single karst cave, square caves exert a greater adverse impact on the ultimate bearing capacity of pile foundations than circular ones. However, when the cave cap thickness reaches 5 times the pile diameter, the impact of cave shape on the pile foundation becomes negligible. As the single-direction size of the karst caves increases, their adverse effect on the ultimate bearing capacity of the pile foundation gradually intensifies. If two karst caves are present at the pile end with centroids at the same elevation, the appearance of a side cave will significantly reduce the ultimate bearing capacity of the pile foundation. The ultimate bearing capacity of the pile foundation when both caves are elliptical is greater than when they are rectangular. If a constant change of single-directional size for the side cave maintains, when the thickness of the cave cap H is ≤3d, the change in cave height will exert a much greater impact on the ultimate bearing capacity of the pile foundation than the change in span; when H is equal to 5d, the influence of the change in the span of side cave on the ultimate bearing capacity of the pile foundation will exceed that of change in the cave height. As the cave spacing increases, the influence of side caves on ultimate bearing capacity gradually decreases; when the spacing reaches five times the pile diameter, this effect can be ignored. This study employs the numerical simulation method with ABAQUS to analyze the ultimate bearing capacity of pile foundations with multiple karst caves, delving deeper into the influence of geometric characteristics of side caves and cave spacing on the ultimate bearing capacity of pile foundations. It provides theoretical support and technical guidance for practical engineering. Additionally, it offers valuable references and insights for the design and construction of pile foundations under similar complex geological conditions.

Stability analysis of karst tunnel surrounding rock considering spatiotemporal effects

WANG Kaixiang, WANG Xiaotao, GONG Liang, HUANG Dan

2024, 43(3): 717-726. doi: 10.11932/karst20240311

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During tunnel construction, various geological problems may be encountered, and karst development can cause great harm to the tunnel. Because the lithology of the rock mass in the karst area is poor and its structure is relatively broken, it is likely to occur the deformation of surrounding rock and even the instability and damage for the whole tunnel during the tunnel excavation. Due to the limitation of exploration technology and exploration cycle, as well as the irregularity of karst development itself, it is difficult to determine the karst morphology and properties. Therefore, it is very important for us to accurately detect the geometric characteristics of karst so as to propose a reasonable scheme of karst disposal and tunnel excavation, which can ensure the stability and construction safety of the surrounding rock in karst tunnel excavation. The deformation of the surrounding rock of tunnel follows the evolution law of "accelerated deformation–rapid deformation–slow deformation–stable deformation". The longitudinal deformation curve of the convergence–constraint method reveals the spatial effect of deformation of surrounding rock during tunnel excavation, and provides a theoretical basis for us to determine the best time for tunnel excavation support by analyzing the interaction between the surrounding rock and the supporting structure. In Maojiapo tunnel, a single-line single-hole railway tunnel from Dangyang to Yuan'an, a large karst cave is situated at the position of the bottom plate on the right side of the tunnel face. To ensure a clear understanding of the location and size of the cave, and the directions of cave branches, a holographic 3D laser scanner was used to obtain the 3D point cloud data on accurate survey of the karst cave, such as the basic data of geometric characteristics and spatial distribution of the karst cave. In this large and dry karst cave, there are long extended pipelines impacting little on the overall stability of the tunnel. In addition, the karst hall is located under the tunnel. Based on the distribution characteristics of the karst cave and the relative position of the tunnel, this study adopted different reinforcement schemes for the karst hall, and proposed four tunnel karst treatment measures, namely, bridge span, backfill with C20 concrete, backfill with C20 rubble concrete, and ballast backfill. After the four schemes were compared in terms of construction technology and economic benefits, the scheme of backfill with C20 concrete was adopted for karst cave treatment. MIDAS/GTS-NX was used to numerically simulate and analyze the excavation and support of the karst section of Maojiapo tunnel after the karst cave treatment had been completed. An empty-cave model was established based on shape, location and size of the karst cave detected by high-precision 3D laser scanning.The longitudinal deformation law of the surrounding rock during the karst tunnel excavation was obtained by the convergence–constraint method, and the stress and failure characteristics of surrounding rock before and after the tunnel excavation of the karst section were studied. The supporting stability of surrounding rock in the karst section and the non-karst section was analyzed. The results show that compared with the vault subsidence and peripheral convergence of the non-karst section and the karst section of the tunnel, the occurrences of vault subsidence and horizontal peripheral convergence of the karst section reduced after treatment, which indicated the treatment measures of the karst section were reasonable and feasible. Compared with the longitudinal deformation of surrounding rock at different footage of karst tunnel excavation, the footage is negatively correlated with the force of supporting structure. Choosing optimal excavation footage can not only ensure that the deformation of surrounding rock meets the requirements of the specification, but can also contribute to the reasonable use of supporting. This study makes a more complete evaluation of the treatment effect of karst caves and the basis of karst tunnel excavation.

2024 Vol. 43, No. 3

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