Genesis mechanism of karst water revealed by hydrochemistry and isotopes in Pingliang, Gansu
Karst water is a significant water resource in the Pingliang region. Investigating the formation mechanism of karst water is crucial for the sustainable utilization of karst water resources, the protection of water sources, and the formulation of environmental protection measures in the area. Such studies provide a scientific basis and theoretical support for promoting the sustainable development of regional water resources and maintaining the health of the water environment. The hydrochemical composition of karst water can reveal the processes of recharge, runoff, and discharge, while isotopic tracing methods help delineate the extent and dynamic changes within the karst water system. To uncover the genesis mechanism of karst water in Pingliang City, we conducted a comprehensive analysis of its hydrochemical characteristics and sources using methods such as Piper trilinear diagrams, mathematical modeling, and ion ratio coefficients. The results indicate that the groundwater in the region can be mainly classified into two types: HCO3-Ca and HCO3·SO4-Na, with Ca2+ and Na+ as the dominant cations and ${\rm{HCO}}_3^{-}$ as the primary anion. The pH value ranges from 6.72 to 6.90, exhibiting overall weakly alkaline hydrochemical characteristics. Total Dissolved Solids (TDS) concentrations in the groundwater range from 158.01 mg·L−1 to 1,519.40 mg·L−1. Spearman correlation analysis shows that the hydrochemistry of karst water in the study area is primarily controlled by the dissolution of minerals such as dolomite and evaporites. TDS is highly positively correlated with the contents of Na+, Mg2+, Cl−, ${\rm{SO}}_4^{2-}$ and ${\rm{HCO}}_3^{-}$. Additionally, the content of Mg2+ is significantly positively correlated with the contents of the main anions Cl−, ${\rm{SO}}_4^{2-}$ and ${\rm{HCO}}_3^{-}$ (r > 0.70, p < 0.01), and the content of Na+ also shows a significant positive correlation with the contents of Cl− and ${\rm{SO}}_4^{2-}$ (r > 0.98, p < 0.01). In the analysis of ion proportional coefficients, the hydrochemical characteristics of karst water in Pingliang City are mainly influenced by rock weathering. Na+ not only originates from the dissolution of rock salt, but may also come from the dissolution of sulfate or silicate minerals. Mg2+ is derived from the combined action of calcite and dolomite. Ca2+ is produced not only from the dissolution of dolomite but also from the dissolution of calcite and dolomite. During the rock weathering process, dolomite, calcite, and evaporites dominate the hydrochemical formation of karst water in Pingliang City. The Gibbs diagram combined with the analysis of mineral saturation indices further confirms that the dissolution of evaporites, carbonates, and silicate minerals is the natural source of hydrochemical components in Pingliang City. Ca2+, Mg2+ and ${\rm{HCO}}_3^{-}$ in the karst groundwater of Pingliang City mainly originate from the combined dissolution of calcite and dolomite. Hydrogeological conditions and hydrogen-oxygen isotope analyses show that δD values range from -75.45‰ to -64.80‰, with an average of -69.20‰; while δ18O values range from -11.26‰ to -9.16‰, averaging -10.07‰. These results indicate that all groundwater is recharged by infiltration of atmospheric precipitation.Based on the identification of the gensis model for karst water, it has been determined that there are two genesis modes of karst water in the Pingliang region. Mode 1: Atmospheric precipitation infiltrates through the overlying Quaternary loess layer, continuously dissolving sulfate minerals within the loess, and enters the limestone confined aquifer through karst fissures formed by faults, eventually emerging as springs at the interface between the loess and limestone aquifer. In this mode, the chemical type of karst water is mainly HCO3·SO4-Na. Mode 2: Atmospheric precipitation infiltrates through the overlying Quaternary sandstone layer, continuously dissolving carbonate minerals in the sandstone layer, and enters the limestone confined aquifer through karst fissures formed by faults, eventually emerging as springs at the interface between the sandstone and limestone aquifers. In this mode, the chemical composition of karst water is mainly HCO3-Ca. Although both modes receive atmospheric precipitation recharge from the windward side of the mountain and enter the aquifer through karst fissures in the overlying strata, differences in these strata result in distinct hydrochemical characteristics. This indicates that the mineral dissolution in the overlying strata is the main factor influencing the hydrochemical composition of karst water in Pingliang.
Numerical simulation of groundwater in karst tunnel areas based on MODFLOW-CFPv2
Southwest China is a typical karst development area, and the groundwater system is extremely complex, with a high degree of heterogeneity and complexity. The karst geological features in this area usually show a fissure-pore dual-media structure, and this geohydrological feature leads to the obvious nonlinear behavior of groundwater flow patterns. These special geological conditions not only pose significant challenges to the development and utilization of groundwater resources, but also complicate the accurate prediction of groundwater dynamics. In particular, when the tunnel construction is conducted in karst areas, it is easy to affect the groundwater flow field and produce water inrush disasters, which will have a far-reaching impact on the local groundwater resources, ecological environment, and even social and economic activities. Therefore, it is of great practical significance to study and understand the law of groundwater flow in karst areas and to accurately predict the amount of groundwater inflow and flow modes during tunnel construction, so as to ensure the safety of engineering construction, the protection of water resources, and the prevention of groundwater pollution. In this study, the MODFLOW-CFPv2 dual-media coupling model has been adopted. This model integrates two mechanisms-pipeline flow and matrix flow-enabling effective simulation of the complex water flow interaction between the fractured pipeline and the surrounding matrix in karst systems. The introduction of this model enables researchers to more accurately describe the laminar and turbulent flow characteristics in karst groundwater systems, and provides a powerful tool for in-depth understanding and simulation of the dynamics of karst groundwater. By integrating Gempy 3D geological modeling technology, this study further improves the fitting ability of the model to the complex geological structures of karst areas, thereby providing a more realistic depiction of groundwater flow characteristics within the tunnel area. During the simulation, key parameters, including the hydraulic conduction coefficient, recharge coefficient, pipeline size, and friction coefficient, have been calibrated through actual geological and hydrological data to ensure the accuracy and reliability of the model results. The main objective of this study is to quantitatively evaluate the water inflow and its influence on the groundwater flow field during tunnel construction in karst areas through numerical simulation, so as to provide theoretical support for the hydrological management of tunnel construction. In order to ensure the reliability of the simulation results, a large number of field monitoring data were used for model calibration. Specifically, the long-term monitoring data of borehole water level and spring flow were used as calibration objective functions to reflect the hydrological trend before and after tunnel construction. After calibration, the results showed that the Pearson correlation coefficient between the simulated water level and the measured water level is more than 0.98. This indicates that the model can accurately reflect the actual groundwater dynamics and provides a reliable basis for subsequent predictions. Simulation results revealed a significant variation in water inflow during tunnel construction. At the beginning of tunnel excavation, the predicted peak water inflow exceeded 20,000 m3·d-1 due to the drastic changes in the groundwater flow field caused by excavation, indicating a substantially increased risk of water inrush disasters. The simulation also showed that the drainage channel formed after the tunnel excavation caused a rapid drop in groundwater levels, far exceeding natural fluctuations. Even after the completion of the construction, groundwater levels had not returned to pre-excavation levels and remained approximately two meters lower, indicating a lasting impact of tunnel construction on the hydrological system. These findings provide important evidence for the hydrological management during tunnel construction, highlighting the need to consider the long-term effects of such projects on groundwater flow and water levels, especially in karst areas.Furthermore, this study revealed the profound impact of tunnel construction on groundwater flow and spring flow variations. As the tunnel excavation progresses, groundwater is rapidly discharged through karst pipelines, leading to significant changes in regional groundwater flow and spring discharge. The research findings indicate that during tunnel construction, groundwater flow is influenced not only by fissure flow but also interacted with matrix flow, further complicating the groundwater flow patterns. By analyzing the model results, researchers can more accurately identify the extent of the impact of tunnel construction on the groundwater flow field and propose corresponding hydrological management measures.
Spatial and temporal variations of vegetation carbon sinks in southwest karst and non-karst regions and climate impact factors
Terrestrial ecosystems, as a core component of the global carbon cycle, play a crucial role in carbon source/sink dynamics, which are significant for mitigating climate change and formulating strategies on sustainable development. Karst ecosystems, characterized by unique geological features such as carbonate rock dissolution, shallow soil, and high ecological fragility, exhibit distinct carbon sequestration mechanisms compared to non-karst regions. The southwest region of China boasts the world’s largest contiguous karst area. Through rock weathering and vegetation restoration, the regional carbon sink capacity has been significantly enhanced. However, its carbon cycle is vulnerable to climate fluctuations and human activities. Although existing studies have focused on the carbon sink effect of karst ecosystems, the long-term spatiotemporal comparative analysis of carbon sources/sinks in karst and non-karst regions is still insufficient, and the influence mechanism of climate factors on carbon fluxes has not yet been fully clarified. In addition, the climate conditions associated with cloudy and rainy weather restrict the acquisition of traditional remote sensing data, thereby impeding the development of large-scale dynamic monitoring. Addressing these gaps is essential for optimizing ecological restoration strategies and advancing global carbon cycle models. Net Ecosystem Productivity (NEP), defined as the difference between Net Primary Productivity (NPP) and Heterotrophic respiration (Rh), serves as a key indicator for quantifying carbon sink/source dynamics. This study utilized the Google Earth Engine (GEE) platform and integrates MODIS MOD17A3HGF net primary productivity data, with a spatial resolution of 500 meters, spanning from 2003 to 2019, along with the ERA5-Land climate dataset, which included temperature and precipitation variables. The spatiotemporal evolution characteristics of vegetation carbon sources/sinks and their climatic driving factors in the karst and non-karst regions of Southwest China were systematically analyzed. All datasets were reprojected to the WGS_1984_UTM_Zone48N coordinate system and resampled to 500-meter resolution with the use of nearest-neighbor interpolation. The research methods include: (1) quantifying the carbon source/sink intensity through the NEP model, where soil Heterotrophic respiration (Rh) was estimated through an empirical formula incorporating temperature (T) and precipitation (R); (2) identifying the long-term trend of NEP by using Theil-Sen median slope estimation method and the Mann-Kendall significance test, and categorizing the results into nine significance levels; (3) analyzing the spatial heterogeneity of temperature and precipitation effects on NEP based on partial correlation analysis. Regions with p-values less than 0.05 were mapped to visualize spatial heterogeneity. The research findings show that: (1) From 2003 to 2019, the average annual NEP in the southwest region was 718.18 gC·m−2, showing a significant overall upward trend at a growth rate of 3.11 gC·(m−2·a)−1. Notably, the growth rate in the karst area was 4.27 gC·(m2·a)−1, significantly higher than the 2.76 gC·(m2·a)−1 observed in non-karst regions. (2) Carbon sink areas accounted for 99.994% of the total study area, while the carbon source areas (0.006%) were sparsely distributed, primarily in urbanized or high-altitude non-karst regions. (3) Spatially, NEP displayed a latitudinal gradient, decreasing from the tropical southern regions (e.g., Yunnan, Guangxi) to temperate northern zones. High NEP values (1,152.5 gC·m−2 to 1,920.84 gC·m−2) were correlated with dense forests and favorable hydrothermal conditions, whereas low values (<384.17 gC·m−2) were found in the alpine regions of Sichuan. Over 73% of the study area showed increasing NEP trends, concentrated in the Sichuan Basin and karst-dominated provinces such as Guizhou and Hunan. Declines (23.5%) were prominent in Xishuangbanna (Yunnan), where elevated temperatures intensified soil respiration. (4) Positive correlations between NEP and temperature dominated 86.89% of the regions, particularly in central karst zones where moderate warming enhanced photosynthesis. Negative correlations (13.11%) were localized in southern Yunnan, where excessive heat impaired enzymatic activity. Negative correlations prevailed in monsoon-dominated regions (e.g., Guangdong, 51.7%), where heavy rainfall exacerbated soil erosion and anthropogenic disturbances. Conversely, positive correlations (48.3%) occurred in arid karst hills (e.g., Guangxi), where precipitation alleviated water stress. This study combines high-resolution remote sensing data with climate models to reveal the spatial differentiation of carbon sink functions and their climate-driven mechanisms in the karst regions of Southwest China. It provides a scientific basis for regional ecological restoration policies, such as the management of rocky desertification. The integration of GEE and MODIS data enabled high-resolution, long-term monitoring of carbon fluxes in ecologically fragile karst landscapes, overcoming traditional limitations posed by cloud-contaminated imagery. Further research indicates that moderate warming can enhance the productivity of karst vegetation, whereas extreme high temperatures or heavy precipitation may suppress the carbon sink capacity. Future studies should incorporate factors such as land-use change and human interference to comprehensively assess carbon management strategies for fragile ecosystems and provide references for carbon cycle research in comparable karst landform regions worldwide.
Comparative analysis of carbon sink effects in karst water systems in three-dimensional climate zones: Taking the Lijiang Basin-the Jinshajiang karst area as an example
To actively address global climate change and achieve the strategic goals of carbon peak and carbon neutrality, Chinese geologists have recently confirmed from multiple perspectives that the potential of karst carbon sinks is substantial. Studying the factors influencing karst processes is fundamental to accurately quantifying the intensity of karst carbon sinks. This study took multiple karst springs within the same aquifer, characterized by similar land use but different elevations in the Lijiang Basin–the Jinshajiang karst area as the research objects. Based on hydrological and hydrochemical data, the mass concentration of CO2 consumed by carbonate rock weathering and the carbon sink flux at each level of the groundwater system were calculated by the hydrochemical-runoff method. The results show that climatic conditions are the main factors controlling karst carbon sink effects. The mass concentration of CO2 consumed by carbonate rock weathering is mainly influenced by temperature and is negatively correlated with altitude. In other words, the lower the altitude, the higher the mass concentration of CO2 consumed by carbonate rock weathering. Additionally, the rate of increase in the mass concentration of CO2 consumed by carbonate rock weathering tends to rise as altitude decreases. The carbon sink flux is mainly controlled by temperature and precipitation. In high-altitude mountainous regions where carbonate rocks are mainly affected by freeze-thaw weathering, the carbon sink flux is mainly controlled by precipitation and shows a positive correlation with altitude; that is, higher altitude receive greater precipitation, resulting in an increased carbon sink flux. Conversely, when carbonate rocks are primarily affected by chemical weathering, the carbon sink flux is mainly controlled by temperature and affected by precipitation, exhibiting a negative correlation with altitude. In this case, lower altitudes correspond to a greater carbon sink flux from carbonate rock weathering, with an increasing trend in magnitude. However, this change is less pronounced than the variation in the mass concentration of CO2 consumed by carbonate rock weathering, which decreases due to the reduction of precipitation. This indicates that the variations in altitude in a three-dimensional climate zone give rise to different climate types,such as precipitation form and amount, temperature, air pressure, evaporation, sunshine, etc.,thereby controlling the distribution of vegetation types and organisms, the intensity of weathering, and indirectly controlling the development and formation processes of karst and distribution of soil. Therefore, differences in altitude alter nearly all external environmental factors that control and affect karst development except for the hydraulic gradient. This results in the formation of diverse karst morphologies and distinct karst water system characteristics, which further amplify variations in karst features and carbon sink intensity. This is the main reason for the differences in karst and carbon sink intensity observed within the same area. The hydrological characteristics of karst water systems, along with vegetation and soil cover conditions, are important factors influencing the mass concentration of CO2 consumed by carbonate rock weathering and the carbon sink flux. These factors can even have a greater impact than climatic conditions. Therefore, human modifications of the external environment can change the intensity of karst carbon sinks, supporting strategic goals related to carbon peak and carbon neutrality. This study provides a geological basis for accurately assessing the intensity of karst carbon sinks, which is of great significance for understanding the global carbon cycle and advancing efforts toward carbon peak and carbon neutrality.
Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
Founded in 1982 bimonthly
ISSN: 1001-4810
CN 45-1157/P
Editor-in-chief: Jiang Zhongcheng
Sponsor: Chinese Academy of Geological Sciences
Sponsored by: Institute of Karst Geology, Chinese Academy of Geological Sciences
Cosponsor: International Research Center on Karst under the Auspices of UNESCOKarst Geology Committee of Geological Society of ChinaSpeleology Committee of Geological Society of China
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