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Karstic aquifers are very sensitive and fragile to environmental changes. With more tunnels excavated in karst areas, groundwater levels are increasingly lowered and resulted in changed natural hydrogeological flow system and groundwater-dependent vegetation, soil and hydrology of surface water systems. Water, especially groundwater, is a limited and key factor and significant driving force for karst ecosystem processes. However, the effects of tunnels excavation on the ecosystems in karst areas remain largely unknown. This study aimed at investigating whether there were variations in soil water contents and shifts of plants water uptake patterns between rainy and dry seasons in a karst trough valley affected by tunnels excavation, and comparing their differences with those in a tunnel-free karst trough valley in southwest China. Monthly soil water contents at two soil layers of the upper 20 cm and lower 20–40 cm were measured, and soil water at two soil layers, subcutaneous water and plant (arbor and shrub) xylem water were sampled in February (mid dry season) and September (late rainy season) 2017, respectively, and the δ2H and δ18O of those waters were analyzed. The IsoSource model based on dual stable isotopes of δ2H and δ18O was used to estimate the contributions of different sources to the plant xylem water. Although the soil water contents at both valleys showed similar vertical and temporal variations resulted primarily from the evaporation and precipitation, the soil water contents of two soil layers in the tunneling affected valley were significantly lower than that of the tunnel-free valley at both seasons. Plants uptake water pattern changed from a dominant soil water source during the rainy season to a dominant subcutaneous water source during the dry season at both valleys. However, plants extracted more water from the subcutaneous zone in the tunneling affected karst trough valley than that of the tunnel-free karst trough valley at both seasons, of which the proportion of plants water uptake from the subcutaneous zone was 33% in rainy season and 76% in dry season, respectively, in the tunneling affected karst trough valley, while the proportion of plants water uptake from the subcutaneous zone was 24% in rainy season and 59% in dry season, respectively, in the tunnel-free karst trough valley. The above results indicated that tunnels excavation decreased the soil water contents and then changed plants water uptake patterns in karst areas.

Soil water plays a crucial role in biogeochemical processes within karst ecosystems. However, geochemical variations of soil waters under different land covers and the related karst critical zone processes are still unclear. In this study, five land covers, including grassland, dry land, shrub land, reforestation land, and bamboo land in the Qingmuguan karst area of Chongqing Municipality, Southwest (SW) China were investigated in order to better understand the spatio-temporal variations of soil water geochemistry and its controlling mechanisms. The hydrochemistry of soil water and stable carbon isotopic compositions of dissolved inorganic carbon (δ13CDIC) in soil water were analyzed by a semi-monthly sampling strategy. The results show that there is remarkable spatio-temporal variation in the hydrochemistry and δ13CDIC of soil waters under different land covers in the studied area. Soil waters collected from shrub, dry, and afforestation lands have higher total dissolved solids (TDS), Ca2+, and HCO3− concentrations and heavier δ13CDIC, which is probably associated with the stronger carbonate dissolution caused by higher soil CO2 and carbonate content in soils under these land covers. However, lower TDS, Ca2+, and HCO3− concentrations as well as δ13CDIC values but higher SO42− concentrations are found in soil waters collected from bamboo land and grassland. The reason is that higher gypsum dissolution or oxidation of sulfide minerals and less soil CO2 input occurs in soils under these two land covers. Under the shrub, dry, and afforestation lands, higher concentrations of Ca2+ and HCO3− in soil waters occur in rainy seasons than in dry seasons, which are probably linked to higher CO2 input due to stronger microbial activities and root respiration in the wet summer seasons. In addition, seasonal variations of NO3− concentrations in soil waters from the dry land are observed, and much higher NO3− concentration occurs in the rainy seasons than that in the dry seasons, which suggest that the agricultural fertilization may lead to high NO3− in soil water. On the vertical soil profile, except for the bamboo land, soil waters under different land covers commonly show an increasing trend of main ion concentrations with the increase of depth. This vertical variation of hydrochemistry and δ13CDIC values in soil waters is primarily controlled by the intensity of carbonate dissolution related to carbonate content in soils and soil CO2 production. The soil waters under different land covers have great variations in δ13CDIC values which ranged from −20.68‰ to −6.90‰. Also, the [HCO3−]/([Ca2+] + [Mg2+]), [NO3−]/[HCO3−], and [SO42−]/([Ca2+] + [Mg2+]) molar ratios in soil waters show a large amplitude of variation. This suggested that carbonic acids could not be a unique dissolving agent and sulfuric/nitric acids may play a role in the weathering of carbonate in the Qingmuguan karst area.

Cave CO2 is an important part of the carbon cycle in a karst system. From 2008 to 2009 the partial pressure of CO2 (PCO2) of the cave air and cave water (cave stream and dripwater) in Xueyu Cave was studied. The PCO2 for cave air and cave water over two years showed that CO2 concentrations vary seasonally in all field sites, with higher CO2 concentrations in the wet season and lower concentrations in the dry season. These seasonal CO2 variations of cave air and cave water are attributed to seasonally variable soil CO2 production that is controlled by outside temperature fluctuations. Cave ventilation controls synchronous sharp variations of PCO2 from wet season to dry season or dry season to wet season in cave water and cave air. In the wet season, there were no obvious spatial differences of mean PCO2 in cave stream and cave air from upstream to downstream cave. Conversely, there were significant spatial differences of mean PCO2 in cave stream and cave air from upstream to downstream cave in the dry season because of cave ventilation. Mean dripwater PCO2 in D1 site where near cave entrance is much higher than that of the D2 and D3 sites where located in inner cave because D1 has a shorter flowpath from cave ceiling to soda-straw tip and lesser CO2 degassing along the flowpath. Further work in Xueyu Cave needs to clarify the quantitative relationship of CO2 cycle between cave air CO2 and cave water and focus on the influence of CO2 cycle on speleothem form.

A prerequisite for using cave speleothems to reconstruct palaeoenvironmental conditions is an accurate understanding of specific factors controlling calcite growth, in particular the isotopic partitioning of oxygen (δ 18 O) and carbon (δ 13 C) which are the most commonly used proxies. An in situ monitoring study from April 2008 to September 2009 at Xueyu Cave, Chongqing, SW China, provides insight into the controls on calcite growth rates, drip water composition, cave air parameters and δ 18 O and δ 13 C isotopic values of modern calcite precipitation. Both cave air P CO 2 and drip water hydrochemical characteristics show obvious seasonality driven by seasonal changes in the external environment. Calcite growth rates also display clear intra-annual variation, with the lowest values occurring during wet season and peak values during the dry season. Seasonal variations of calcite growth rate are primarily controlled by variations of cave air P CO 2 and drip water rate. Seasonal δ 18 O -VPDB and δ 13 C -VPDB in modern calcite precipitates vary, with more negative values in the wet season than in the dry season. Strong positive correlation of δ 18 O -VPDB vs. δ 13 C -VPDB is due to simultaneous enrichment of both isotopes in the calcite. This correlation indicates that kinetic fractionation occurs between parent drip water and depositing calcite, likely caused by the variations of cave air P CO 2 and drip rate influenced by seasonal cave ventilation. Kinetic fractionation amplifies the equilibrium fractionation value of calcite δ 18 O (by ∼1.5‰) and δ 13 C (by ∼1.7‰), which quantitatively reflects surface conditions during the cave ventilation season. These results indicate that the cave monitoring of growth rate and δ 18 O and δ 13 C of modern calcite precipitation are necessary in order to use a speleothem to reconstruct palaeoenvironment.

Cave air CO2 is a vital part of the cave environment. Most studies about cave air CO2 variations are performed in caves with no streams; there are few studies to date regarding the relationship of cave air CO2 variations and drip water hydrochemistry in underground stream–developed caves. To study the relationship of underground stream, drip water, and cave air CO2, monthly and daily monitoring of air CO2 and of underground stream and drip water was performed in Xueyu Cave from 2012 to 2013. The results revealed that there was marked seasonal variation of air CO2 and stream hydrochemistry in the cave. Daily variations of cave air CO2, and of stream and drip water hydrochemistry, were notable during continuous monitoring. A dilution effect was observed by analyzing hydrochemical variations in underground stream and drip water after rainfall. High cave air CO2 along with low pH and low δ13CDIC in stream and drip water indicated that air CO2 was one of the dominant factors controlling stream and drip water hydrochemistry on a daily scale. On a seasonal scale, stream flows may promote increased cave air CO2 in summer; in turn, the higher cave air CO2 could inhibit degassing of drip water and make calcite δ13C more negative. Variation of calcite δ13C (precipitated from drip water) was in reverse of monthly temperature, soil CO2, and cave air CO2. Therefore, calcite δ13C in Xueyu Cave could be used to determine monthly changes outside the cave. However, considering the different precipitation rate of sediment in different seasons, it was difficult to use stalagmites to reconstruct environmental change on a seasonal scale.

Cave dripwater hydrochemistry responds to environmental changes, both within and outside of the cave, and thereby conveys this information to any stalagmite fed by the drips. As stalagmites are important archives of climate proxy information, understanding how dripwater hydrochemistry responds to environmental forcing is critical. However, despite the large number of speleothems in SW China, the response of dripwater to regional climate variability is not yet adequately understood. A 3-year study of three drip sites in Xueyu Cave, Chongqing Municipality, SW China, revealed the most important mechanisms controlling dripwater chemical variability. The principal chemical indices (pH, specific conductivity, Ca2+, Mg2+, Sr2+ and ) in collected dripwaters and the local climate data were analysed in this study. The principal controls on the hydrochemistry were found to be the external climate and its changes, groundwater residence time, cave ventilation and prior calcite precipitation (PCP) processes. Dripwater hydrochemistry showed strongly coherent seasonal patterns despite the fact that all sites are Ca–HCO3 type waters and supersaturated with calcite. Seasonal changes in dripwater hydrochemistry were influenced by the soil and vadose zone CO2 content as well as groundwater residence time in the upper karst zone. Cave-air CO2 seasonal variations were consistent with changes in dripwater PCO2 and cave ventilation. Trace element ratios (Mg/Ca and Sr/Ca) of dripwater were controlled by PCP processes. Seasonal variations in dripwater Mg/Ca and Sr/Ca ratios in Xueyu Cave showed inverse changes with the Asia Monsoon Index during the monitoring period, reflecting the seasonal climate changes that may have been recorded in the speleothems. Based on a linear regression of PCO2 and the Ca2+ data in the cold–dry winter season, a 130-ppm shift in cave-air PCO2 results in a 1-ppm shift in dripwater Ca2+ concentration in Xueyu Cave. This study illustrates the importance of understanding factors controlling the changes in the composition of dripwater before it reaches the speleothem.

A systemic study was conducted to understand the links between the host rock, water, and speleothems of Xueyu Cave in Southwestern China. An outcrop section of the host rock of Xueyu Cave was surveyed, the stratum that the host rock belongs to was subdivided into 11 lithofacies, and the Xueyu Cave is developed in the grainstone facies segment. Lithological characteristics and chemical components of the host rock indicate that its predominant mineral component is low magnesium calcite (LMC). To understand the hydrochemical characteristics of percolating water in Xueyu Cave, three drip water monitoring sites were taken to collect water samples for a whole monsoon year. The temperature (T), pH, electronic conductivity (EC), drip rate, Ca2+, Mg2+, Na+, K+, Ba2+, Sr2+, HCO3 −, SO4 2−, NO3 −, Cl−, partial pressure of CO2 (pCO2), saturation indexes, and precipitation rate of carbonate minerals were investigated monthly. The data reveals that the pH, EC, drip rate, Ca2+, Mg2+, HCO3 −, pCO2, saturation indexes, and precipitation rate of carbonate minerals exhibit seasonal variation; these are due to the seasonal change of precipitation and soil CO2 concentration that are driven by monsoon climate. The petrological and geochemical characteristics of speleothems were also investigated, and results indicate that the main chemical and mineral components of speleothems are quite similar to the host rock; this is because the main material source of drip water and speleothems are governed by the host rock. This study reveals the main mineralogical and geochemical characteristics of host rock and speleothems, further discussing the seasonal variation of hydrochemical indexes of drip water. All these works will be helpful to more thoroughly understand the operation regularity of karst dynamic system (KDS) of the Xueyu Cave.

The role of CO2 in karst has been of interest for decades, and emphasized by IGCP 379, International Geoscience Programme, UNESCO started in 1995. There are still open questions about the dynamics of carbon in karst systems, particularly the flux of carbon between the surface and subsurface and between different components in the karst subsurface. This research report focuses on the variations of hydrochemistry and PCO2 (partial pressures of carbon dioxide) in subtropical karst groundwater, using high-resolution auto-monitoring hydrochemical data (15-min intervals). The aim of this study was to understand how hydrochemistry and PCO2 in karst systems respond to recharge over different time scales and what the controlling factors are. An auto-monitoring hydrochemistry station was installed about 300 m upstream from the exit in the active stream channel of Xueyu Cave, a typical subtropical karst cave. Four years of high-resolution continuous pH, specific conductivity (Spc), temperature and water-level data were collected. A thermodynamic model was used to link the continuous data to monthly water quality data, allowing the calculation of CO2 partial pressures and calcite saturation (SIc) levels on a continuous basis. Seasonal, diurnal and storm-scale variations were captured in the hydrochemistry and calculated PCO2 records, indicating that the cave stream is a dynamic and variable system. Seasonal features (higher specific conductivity and lower pH in summer; lower specific conductivity and higher pH in winter) tend to covary with temperature which influences the production of CO2 in soils, thus being the driving force for the variations (the soil CO2 effect). Due to the buffer effect of a thick vadose zone and large void cave space, diurnal variations are not obvious compared with epikarst springs in SW China. Storm-scale fluctuations due to storm events occur during the summer rainy season. Piston flow effects, dilution and soil CO2 effects determine the variations in different storm events. At the beginning of the rains, the piston effect drives the variations, characterized by increase in Spc, SIc and pH in the cave stream and decrease in PCO2. With heavy rainfall, decrease in Spc shows control by the dilution effect, while decrease in SIc and pH and increase in PCO2 indicates the greater influence of soil CO2. These results imply that the soil and cave voids are important factors influencing the hydrochemical evolution of karst groundwater. Future works need to use such high-resolution technology widely for tracing the PCO2 and hydrochemical variations in different karst aquifers.

Concentration of dissolved nitrate (NO3 −) and stable nitrogen isotopic composition of nitrate in water samples from 19 epikarst springs in the Jinfo Mountain area were analysed in order to understand the temporal–spatial distribution of nitrate in karst spring waters and trace nitrogen sources for the springs that have suffered from nitrate contamination. The results show that hydrochemistry of epikarst spring water is Ca·Mg–HCO3 facies, reflecting the control of water–carbonate rock interaction. NO3 − concentrations in water of springs within the national nature reserve (NNR) in the Jinfo Mountain area range from 0 to 8.16 mg/L, suggesting that these springs within the NNR have good water quality. However, most springs outside the NNR have high NO3 concentrations (c: 10.5–46.18 mg/L) that have exceeded the value of Chinese national drinking water quality standard (NO3–N ≤10 mg/L), revealing that these springs have suffered from nitrate contamination to a significant degree. δ 15N–NO3 values of these contaminated springs outside the NNR range from −1.2 ‰ to 11.27 ‰, higher than those of unpolluted springs within NNR (−5.2 ‰ –8.6 ‰). Two main sources of nitrate in these contaminated springs have been identified, including animal manure/sewage and chemical nitrogen fertilisers that are from anthropogenic activities. Our study also shows that the nitrate fate and origin are associated with the land use in the recharge catchment of individual epikarst springs, and the establishment of national nature reserve can protect the water quality of epikarst spring.

Wentang Valley Anticline is selected as study area and thermal water resources in this area will be discussed in this paper. Because of the same geological backgrounds, the hydrogeochemical features of the thermal water is similar in this study area, and its hydrochemical type is SO 4 -Ca. In addition, the thermal water contains multiplicate and sanative microelements and gases, such as Sr, F, Li, B, and Rn, so it is superior iatrical thermal water. The thermal water source is atmosphere precipitation. And its aquifer may be the stratum of Jialingjiang formation in Lower Trias (T 1 j). Na-K-Mg diagram shows that five water samples fall in the region of immature water. So it can be said that it has low tempertures in the fluid-rock processes and the thermal water maybe mixed. Mineral equilibrium diagram (log(Q/K)-T diagram) indicates that the log(Q/K)-T curve of quartz is closest to saturation line, and the equilibrious temperature is 65∼77°C. This temperature is within the reservoir temperature (65∼82°C) calculated by quartz geothermometer.

Over geological time scales, Earth degassing has a significant impact on atmospheric carbon dioxide (CO2) concentrations, which are an important component of global carbon cycle models. In Tibet, structural conditions and associated widespread geothermal systems lead to carbon dioxide degassing during geothermal water migration. We characterized the hydrochemical conditions of two geothermal fields on the Tibetan Plateau. The chemical composition of geothermal waters was controlled by K-feldspar and albite. Geothermal waters in the Langjiu geothermal field were sodium chloride type and those of the Dagejia geothermal field were sodium bicarbonate type. Simulations of CO2 partial pressure within the two hydrogeothermal systems showed that CO2 degassing occurs during hot water migration from the aquifer to the surface. Carbon dioxide degassing flux from the Langjiu geothermal field was estimated to be ∼3.6×106 kg km−2 a−1, and that from Dagejia was ∼3.3×106 kg km−2 a−1.