Application of hydrochemical and isotopic data to determine the origin and circulation conditions of karst groundwater in an alpine and gorge region in the Qinghai–Xizang Plateau: a case study of Genie Mountain

Complex structural systems, lithological differences, and extreme heterogeneity of aquifers combine to create a complex karst aquifer structure in alpine and gorge areas; however, because of the topography, direct investigation of aquifer structure is difficult. In this study, field survey, hydrochemical, and isotopic data are analyzed to reveal the development of karst groundwater and to describe the karst water cycle in Genie Mountain, Qinghai–Xizang Plateau. The results show that groundwater circulation is mainly controlled by active fracture. Atmospheric precipitation and melting ice and snow are the groundwater recharge sources, and recharge areas are mostly located in high mountains above 4500 m a.s.l. The direction of groundwater flow is mostly controlled by the Jinshajiang active Fault, with drainage areas at the intersection of multiple faults. There are two regional karst water runoff conduits. One is along the Edexi-Hongjunshan Fault, where groundwater runs from south to north; the other is along Gangtonglong Fault, where groundwater runs from north to south, and is discharged at Gangtonglong gully. The groundwater cycle can be divided into three levels: epikarst water circulation; mid to deep karst water circulation; and deep geothermal water circulation. The karst springs located in the outlet of the Huolong gully contain markedly higher levels of Na+ and SO42− than other karst springs because of the leaching effect of groundwater on mirabilite. The presence of evaporites in Huolong gully indicates that the plateau planation formed by the structural uplift will change the local climatic conditions, and then affect the groundwater circulation process and the water–rock reaction in the process.


Introduction
Karst aquifers are an important threat to the safety of underground engineering. Globally, a large number of underground projects have experienced water inrush from karst aquifers, resulting in great casualties and property losses (Liu et al. 2019;Lin et al. 2020). There are many reasons for water inrush in karst tunnels, but the most fundamental reasons are: a lack of understanding about the physical characteristics of karst aquifers and the process of groundwater circulation in these systems (Zabidi and De Freitas 2013;Li et al. 2015Li et al. , 2016Fan et al. 2018); and a lack of information about how these characteristics vary at different spatial and temporal scales.
A karst aquifer is the result of karstification, the phenomenon in which carbonate rock dissolves under the action of water, which involves both physical and chemical processes. During karstification, pre-existing discontinuities, fractures, joints, layers or macropores form a hydraulic continuum from the surface to the spring water (Darnault 2008). In general, karst systems exhibit highly variable recharge processes (diffusion and concentration), storage areas (surface karst, aeration zone, and phreatic zone), and flow types (by diffusion and along conduits) (White 2002;Perrin et al. 2003;Ford and Williams 2007). The hydrological systems of karst catchments are highly complex and karst aquifers exhibit strong spatial heterogeneity (Xia 2016). The roles of specific hydrological processes and their effects in different hydrological conditions are often difficult to quantify. The high heterogeneity of karst aquifers also causes challenges for research, because it is not possible to extrapolate from small-scale processes to those that take place at a larger scale.
Owing to their unique hydrogeological characteristics, study of karst aquifers requires special investigation methods (Goldscheider and Drew 2007). Investigation of karst development, such as studies of the characteristics of karst dissolution, measurement of the scale of karst development, and tracer tests, is useful; in addition, hydrochemical and isotopic methods can be applied to accurately identify groundwater circulation. Directly describing the heterogeneity of karst aquifers by means of hydrochemical data is difficult, but environmental isotopes (such as 18 O and 2 H) can provide additional information about specific groundwater flow paths and water transport times of a karst aquifer. This information is essential for describing the different components of an aquifer. Environmental isotopes in water have been studied to provide some insights into the recharge characteristics of karst aquifers under different flow conditions (Aldalla 2009;Yao et al. 2009;Zhang et al. 2015;Bicalho et al. 2019;Rusjan et al. 2019;Tomasz et al. 2019). The mean transit time of karst catchments has also been estimated by isotopic means (Maloszewski et al. 2002;Perrin et al. 2003). Environmental isotopes in water are excellent conservative tracers because they are naturally "injected" throughout the basin by diffusion during rainfall (McGuire and McDonnell 2008;Yin et al. 2011) and do not react chemically in the environment at ambient temperature (Gat 1996). Although there are many uncertainties in isotope geochemistry, these substances are still an important means of improving understanding of karst aquifers.
The study area is Genie Mountain (highest peak 6204 m), which is located in the Hengduan Mountains in the eastern Qinghai-Xizang Plateau. The plateau is the most tectonically active area worldwide. The Jinshajiang Fault Zone and Batang Fault are still active and induce earthquakes (Wu and Cai 1992). Strong tectonic uplift and incision of river valleys have greatly changed the hydrogeological structure, especially of karst aquifers. The karst aquifers developed in the plateau area may be important in the development of high-altitude karst, which is affected by the high altitude above sea level, a cold climate, an extremely complex geological structure, and a series of changes caused by valley incision. The circulation of karst groundwater in the plateau is also affected by the development of karst in the plateau, which intensifies the spatial and temporal heterogeneity of the karst aquifer. Furthermore, the harsh natural environment and geomorphic conditions cause great difficulties for research work. All of these factors are challenges for the accurate characterization of the hydrologic characteristics of a karst aquifer.
In this study, information from field surveys, and hydrochemical, and isotopic data was analyzed to reveal the development of karst groundwater in Genie Mountain. This work was undertaken to describe the karst water cycle process, and to provide reference data for research on karst aquifers with similar characteristics in other regions.

Geographic setting and climate setting
Genie Mountain is located in the western part of Sichuan Province, China, and on the eastern bank of the Jinshajiang River (30°10′-30°50′N, 98°30′-99°25′E) (Fig. 1). The highest peak in the region is Mount Dangjiezhenla (6204 m a. s. l.), and the lowest point is the Jinshajiang River (2240 m a. s. l.). The height difference between the highest and lowest points is 3820 m. The gradient is generally ~ 45°, and up to 60° in some areas. The valley, which is a typical V-shaped mountain canyon landform, generally does not exceed 1000 m in width.
The climate of the study area is influenced by altitude, the presence of mountains, and atmospheric circulation. As a result of these factors, and the region experiences an alpine plateau climate with an annual mean temperature of 8.0-12.3 °C. The mean temperature in January is − 1.6 °C (mean monthly temperature of long-term from 1953 to 2019). The temperature rises rapidly in spring, and sunshine is abundant in summer, with a maximum temperature of > 35 °C. In winter, the lowest air temperature is below − 10 °C. The annual maximum rainfall is 829 mm (1998), the minimum rainfall is 292 mm (1994), and the mean rainfall is 504 mm. The rainy season is May to September; approximately 90% of total rainfall occurs during that interval. The average annual evaporation is 1711 mm.
According to the karst classification of the Chinese Academy of Sciences, the karst in Genie Mountain is a temperate arid climate-type denudation karst of the Qinghai-Xizang Plateau .
The main river in the study area is the north to southflowing Jinshajiang River, which is the terminal discharge point of groundwater from Genie Mountain. The mean flow in the rainy season is about 2344m 3 /s (July, 1972(July, ~ 2017, and the mean flow in the dry season is 294 m 3 /s (January,

Geology
The study area is part of the Songpan-Ganzi block, and is located to the east of Himalayan east structural junction and to the west of Ganzi-Litang deep fault. The main fault structure in the area is the Jinshajiang Fault Zone.
The Jinshajiang Fault Zone is approximately 700 km long and approximately 80 km wide. The structure of the zone is complex: it is composed of six to seven main faults, including the Jinshajiang East Boundary Fault, the Gangtonglong Fault, and the Edexi-Hongjunshan Fault. The maximum fault displacement is 25 km. The fault zone generally trends north-south and dips at a high angle. The zone contains many branch faults and traction structures.
Among them, the Edexi-Hongjunshan Fault, the Gangtonglong Fault and the Jinshajiang East Boundary Fault are active faults. These faults were most recently active in the Pleistocene. Chalo-Songduo Fault is a derivative fault of the Jinshajiang East Boundary Fault. Its activity is unknown. Some studies believe that its activity time should not be earlier than the Jinshajiang East Boundary Fault (Cao 2020). The Edexi-Hongjunshan Fault and the Gangtonglong Fault pass through the Genie Mountain and control the development direction of Huolong Gully and Gangtonglong Gully.
Cambrian-to Quaternary-aged sedimentary rocks occur in the study area, but Jurassic and Cretaceous rocks are absent. The main karst strata are Devonian-to lower Permian-aged limestones, dolomites, and a small amount of marble. The total thickness of the karst-bearing strata is more than 5000 m. The strata mostly crop out in faultcontrolled, north-south-trending blocks. The karst-bearing strata mainly occur between the Edexi-Hongjunshan Fault and the Jinshajiang East Boundary Fault. West of the Edexi-Hongjunshan Fault and east of the Jinshajiang East Boundary Fault, there are Devonian and Carboniferous sandstones, schists, and phyllites. Pure carbonate rocks are rare and of limited spatial extent.
Under the influence of the structural uplifting and glacier plucking, a planation surface has formed above 4600 m a.s.l. The surface extends NNW, with its northern end at the outlet of the Huolong gully and the southern end located near the east-west surface watershed. The total length of the planation surface is approximately 21 km. The surface is U-shaped overall, 200-400 m wide, and contains many glacial lakes in a relatively flat terrain. The thickness of the moraine is ≥ 1.5 m.

Hydrogeology
The fracture system is the main control on water flow in the karst system in the study area. The spatial development of karst strata is primarily controlled by the Jinshajiang Fault zone. Devonian, Carboniferous, and Permian carbonate rocks crop out in nearly north-south-directed blocks. The fault is bounded by Lower and Middle Triassic-aged sandstones and slates to the east, and by lower and middle Cambrian-aged schists, phyllites, and volcanic rocks to the west, forming a boundary for karst water. The second factor that affects the distribution of karst features is the influence of the fault zone, especially the development of structural fissures within the zone which provide space for groundwater. After a long period of water-rock interaction, dissolved gaps and pores develop gradually in carbonate rocks that form underground conduits for groundwater movement. Therefore, karst development in the fault-affected zone is much more rapid than that in unaffected areas. From the regional geological data, nearly 70% of karst is developed in the zone of structural influence. Furthermore, the distribution of karst water follows the Jinshajiang Fault. Faults to the east of the surface watershed are very well developed, whereas faults are relatively sparse to the west of the surface watershed. The main groundwater discharge area is also in the eastern part of Genie Mountain. Consequently, karst water is abundant in the eastern part of Genie Mountain, and less abundant in the western area.
Karst water in Genie Mountain is mainly supplied by atmospheric precipitation and melted ice and snow. This water infiltrates and runs along the karst conduits formed along the structures. The altitudes of the Jiangqu and Xiqu rivers control the discharge height level of karst water, and are also mainly recharged by groundwater. Karst water forms a concentrated drainage zone at the intersection of multiple faults. The locations of karst springs are mainly concentrated in the mouth of Huolong gully and its northern part, and Gangtonglong Gully. The main discharge points can be divided into the northern, Huolong, eastern, and Gangtonglong groups on the basis of their location. Thermal springs SM04 and Q75 are exposed on the eastern side of the Jiangqu River.

Flow measurement and sampling
A total of 42 groups of samples were collected: 16 from karst springs, 2 from thermal springs, 23 from river water, and 1 snow sample from the top of the mountain (Fig. 1). To avoid the impact of heavy rainfall events on rivers and groundwater, all sample collections and flow measurements were completed in 3 days (between July 31 and August 2, 2019), and there was no precipitation in the study area from July 28th to August 2nd. The current measurement data are used as the same period data for analysis.
Due to accessibility issues in the study area, the hydrometeorological monitoring data in the study area could not be obtained. As for comparison, flow measurements of Q309 were also carried out on May 31 (the end of the dry season), July 10, 2020 (the middle of the rainy season), and March 31, 2021 (the middle of the dry season).
The flow rate of the main karst springs in the study area was measured using the section-velocity method. The equipment used was a FP111 portable flow meter (Global Water, US). The flow meter range is 0.1-6.1 m/s, and the test accuracy is 0.03 m/s. Sampling bottles were rinsed with sample water at least three times before collection. The samples were filtered through 0.45-μm membrane filters and poured into 1.5-L and 250-mL high-density polyethylene bottles for analyses of major and trace elements. The 250-mL samples were acidified by adding double-distilled nitric acid until the pH was below 2; the 1.5-L samples were untreated. Samples were stored at 4 °C until analysis. Samples for stable isotope analysis (δ 18 O and δD) were collected in 50-mL glass bottles, which were sealed with airtight caps.

Analytical methods
At the time of sample collection, the water temperature, pH, and electrical conductivity (EC) were measured using a HANNA HI 991,301 pH/EC/Temperature multi-parameter instrument (Hanna Instruments ® , Woonsocket, RI, USA). Major anions, except HCO 3 − , were analyzed with a Thermo Scientific Dionex ICS-4000 (precision ± 1%). HCO 3 − levels Stable isotope ratios are reported in parts per thousand (‰) using the conventional δ notation: δ sample (‰) = [(R sample − R standard )/R standard ] × 1000, where R represents the 2 H/ 1 H or 18 O/ 16 O ratio of the sample and the standard. The δ 18 O H2O and δD values for water samples were measured using a Picarro L2130-i Analyzer at the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences. The results are reported in ‰ relative to Standard Mean Ocean Water, with a precision of ± 0.1‰ for δ 18 O H2O and ± 1‰ for δD.
Tritium samples analyzed by an ultra-low background liquid scintillation spectrometer (1220 Quantulus) after lowtemperature electrolysis enrichment. The detection limit and test precision are 1 TU and ± 0.5 TU, respectively.

Age of groundwater
Tritium was used to determine the age of groundwater. According to the classification method of International Atomic Energy Agency, when the tritium content is less than 3 TU, it means that the groundwater may have been derived from rainfall recharge before 1954; when the tritium content is 3-20TU, it means that the groundwater is likely to have received rainfall recharge during the period 1954 to 1961. Shigehiko Kimura further divides it and considers that the tritium content is n × 10 0 TU (n is a natural number, the same below in this paper), indicating that it is mixed water (mixing stagnant water and n × 10 0 ~ n × 10 2 TU water), and n × 10 1 TU indicating that it is a mixture of recent precipitation or stagnant water and n × 10 2 TU water (Wang, 2009). In this paper, the division methods mentioned above were used to evaluate the age of groundwater.

Spring flow
The flow measurement results show that the spring flow can be divided into three distinct categories. The spring flow rate of the Level 1 category is greater than 600L/s, which includes sampling sites Q309 (627 L/s on August, 2019) and G06 (640 L/s). the spring flow rates of the Level 2 category are about n × 10 1 L/s, which include sampling sitesG11 (21.6 L/s) and G09 (9.79 L/s). The other springs are in the Level 3 category, and have flow rates of ≤ 3.0 L/s (Table 1).
Springs numbered Q309 and G06 of Level 1 are located near the exits of Huolong Gully and Gangtonglong Gully, respectively. The flow accounts for 99.64% and 99.60% of the total flow of the two springs groups respectively. The flow of Q309 is not very sensitive to atmospheric precipitation fluctuations. The spring numbered G11 has the largest flow in the Northern group, which accounts for 78.69% of the group. Except for spring A10, most of the groundwater in Eastern group is discharged in linear seepage faces, and the flow rate is too small to be measured.

Water composition
The pH values of the analyzed samples were between 6.74 and 8.79, which is neutral or weakly alkaline overall, and the range of TDS values was large, between 114.7 and 1331 mg/L ( Table 2). The mean TDS value of the Huolong group was 589.60 mg/L, higher than those of the northern (236.38 mg/L), Gangtonglong (132.14 mg/L), and eastern groups (330.34 mg/L), but markedly lower than that of thermal springs (1008.86 mg/L). The Na + (128.85 mg/L) and 2− (300.29 mg/L) concentrations in springs of the Huolong group were higher than those of other groups, whereas the Ca 2+ , Mg 2+ , and HCO 3 − levels of the Huolong group were similar to those of the other groups. There are seven springs in the Huolong group, of which Q201, Q309, and Q312 are of the Na·Ca-SO 4 , Na-SO 4 , and Na-SO 4 ·HCO 3 hydrochemical types (Fig. 2), and the others are mainly Ca·Mg-HCO 3 type. The springs of the northern, eastern, and Gangtonglong groups are Ca·Mg-HCO 3 type. This difference may indicate greater water-rock reactions in the Huolong group.
The pH values of river water were between 7.01 and 8.33, and the TDS values ranged from 101.8 to 1234 mg/L, mean 336 mg/L, which is similar to the pH values of groundwater. The river water samples from sites H202, H205, H206, and H207 near the karst springs in the Huolong group exhibited Na-SO 4 or Na-SO 4 ·HCO 3 water chemistry. The karst springs numbered Q201, Q309, and Q312 and the four surface water points numbered H202, H205, H206, and H207 plot in the same area (Fig. 2), showing that the river water and groundwater in the Huolong group are strongly correlated. Other points plot relatively close together. The rest of the river water samples was Ca·Mg-HCO 3 type, with TDS values of about 101.8-239.97 mg/L.
The TDS values of thermal springs exceeded 1000 mg/L. However, thermal springs Q75 and SM04 possessed Na-HCO 3 and Ca-Na-HCO 3 water chemistry, respectively. Except for thermal spring SM04 (45 °C) and Q75 (64 °C), the temperatures of springs are 5.8-13.2 °C (the atmospheric temperature during the measurement was 14-23 °C). The differences in water type and temperature indicate that, on the one hand, the circulation depth of thermal spring water is large and, on the other hand, the circulation processes of the two thermal springs are different.   The δD and δ 18 O characteristics of water points in the study area were compared with those of the global meteoric water line, the Mount Gongga (approximately 500 km east of the work area) water line (GGMWL), and the eastern Qinghai-Xizang Plateau meteoric water line (EQXMWL) (Fig. 3). However, Mount Gongga is part of the eastern Qinghai-Xizang Plateau, and there is a large difference in the isotopic characteristics of precipitation between the two meteoric lines, which arises from the different origins and proportions of water and the precipitation caused by the local water cycle in high mountain and valley areas. Additionally, most of the samples fall between the EQXMWL and the GGMWL, indicating that there is a close relationship between groundwater and precipitation. The slope of the linear relationship between δD and δ 18 O of all karst springs is 4.37, lower than that of the GGMWL (9.40) and EQXMWL (6.79), which results from the relatively arid climate and high evaporation in the study area. The annual rainfall of the eastern Qinghai-Xizang Plateau is 892 mm (Li et al. 2018) and that of Gongga Mountain is 1938 mm (Song et al. 2015), both larger than the value of 504 mm in the study area.

Isotopic composition of waters
The water samples can be divided into three groups ( Fig. 3; Table 3). Group A contains thermal springs, and groups B and C include both karst springs and river water. Obviously, the circulation depth of samples in group A was greater than that in groups B and C. Group B can be divided  into two parts: the lower left part includes karst springs with large flows, such as Q309, G06 and G11, and the circulation depth is slightly larger than the water sample in the upper right part. Most of the surface water samples are located in the upper right part of the isotopic plot, indicating that they are closely connected to karst water samples in the upper right part of the plot. Group C includes three spring water samples from the Eastern group and Northern group of samples. The circulation depth is the shallowest among these three groups.
Most of the tritium values of surface water and groundwater were between 3 and 10 TU (Fig. 4), indicating that the groundwater is mainly derived from the mixing of pre-bomb water and modern precipitation. The river water is mainly supplied by groundwater discharge. The tritium values of thermal springs were all approximately 1 TU, suggesting that the water supply is mainly older than 1954, and the water experiences a a long residence time in the aquifer.
The tritium values of samples G09 in the northern group and A10 in the eastern Group were greater than 10 TU, indicating that the groundwater is mainly supplied by modern precipitation and that there is a short groundwater residence time in the aquifer.

Water origin
The precipitation in the study area comes from the Indian Ocean, Pacific Ocean, and Central Asia (Song et al. 2015). Excess deuterium in Genie Mountain spring water was basically consistent with global values (Table 4), but lower than the values in the Western Pacific, Central Asia, and Gongga Mountain, and slightly higher than that in the Indian Ocean, indicating that the water vapor source of local internal circulation is important. The short cycle leads to relatively insignificant fractionation effects, and the D values are low. The deuterium surplus of the Huolong group is the largest,  and there is little difference among the other groups, which indicates that the karst water of the Huolong group has experienced marked evaporation.
In the process of water vapor transport, isotope fractionation occurs, resulting in the depletion of δD and δ 18 O in the study area. As a result of the high mountain and valley topography, the study area is mainly dominated by the local water cycle. The stable isotopic signatures of all karst springs fall between the EQXMWL and the GGMWL (Fig. 3), providing evidence of a meteoric origin for these waters.
The isotopic composition of groundwater in mountainous areas is often affected by altitude effects, which depend on the altitude of the groundwater recharge area. According to Li et al. (2019), the oxygen isotopic composition of atmospheric precipitation in the eastern part of the Qinghai-Xizang Plateau exhibits the following relationship: where h is the altitude (m). According to formula (1), the recharge area elevations of the northern, Huolong, and Gangtonglong groups are not much different (Fig. 5). The average values are 4679, 4751, and 4727 m, respectively, but the average difference in elevation between the recharge and discharge areas for the Gangtonglong group is 1924 m, more than for the Huolong group (1533 m) and the northern group (1568 m). The eastern group karst water has the shortest vertical circulation distance: the elevation of the recharge area is only 4446 m, and the vertical difference is 961 m. Hence, the replenishment elevation and vertical circulation distance of groundwater sample A10 are shorter than for other samples. The elevation of approximately 4446 m does not reach the planation surface or the watershed of Genie Mountain, so the recharge area can only be located on the hillside. A10 is supplied by modern water (Fig. 5). Thus, A10 is recharged from the hillside on the east side of Genie Mountain, from shallowly circulating groundwater that runs through the shallow karst system.

Groundwater flow process and water-rock reactions
Generally speaking, under certain conditions, such as stratum lithology and hydraulic gradient, the greater the circulation depth of karst groundwater, the greater the extent of water-rock interactions, and the higher the TDS value. However, there is no obvious correspondence between the karst water circulation depth of Genie Mountain and the TDS value of karst springs (Fig. 6). With the exception of sample site Q309, the TDS values of other karst springs with a flow rate greater than 9L/s are not greater than 400 mg/L. The water type (SO 4 -Na) of water sample Q309 is also significantly different from other karst springs (mostly HCO 3 -Ca). It shows that the groundwater flow process of Huolong Group is different from other groups.

Characteristics of groundwater flow
Karst in Genie Mountain has mainly developed along structural features in carbonate rocks. Based on the topography, Page 11 of 18 291 geological structure and the position of springs, the groundwater of the Northern group mainly flows along the fault on the north-eastern side of the Edexi-Hongjunshan fault and is discharged near the Jinshajiang East Boundary Fault. The exposure of Eastern Group's karst spring is affected by the Jinshajiang East boundary fault, but there is no obvious fault exposure between the recharge area and the discharge point. From the analysis of geological conditions in the area, it is concluded that spring A10 may only groundwater flow from an epikarst feature.
Both in Gangtonglong group and Huolong group exposed karst springs have flow rates of more than 500L/s, indicating that a concentrated regional groundwater takes place is a significant karst conduit. These two passages are most likely to extend along the Gangtonglong fault and the Edexi-Hongjunshan fault, respectively. The vertical flow distance of the karst spring numbered G06 is greater than 2000 m, and the horizontal flow distance is about 10-20 km; the vertical flow distance of the karst area numbered Q309 is about 1800 m, and the maximum horizontal flow distance can reach 30 km. However, the water chemistry type of Q309 is significantly different from other karst springs, indicating that there are differences in the water-rock interactions that occur during groundwater flow.

Water-rock reactions
In karst areas, a series of water-rock reactions occur between groundwater and the rock matrix in aquifers. The molar concentration relationship of Ca 2+ /Na + -HCO 3− /Na + and Ca 2+ /Na + -Mg 2+ /Na + can be applied to qualitatively distinguish the influence of rock weathering and dissolution on groundwater in the study area (Zheng et al. 2020). Most of the samples fall within the dissolution range of carbonate rocks, whereas samples Q201, Q309, and Q312 of the Huolong group fall between the salt rock evaporation zone and the silicate rock zone (Fig. 7). Thus, these three springs experience different water-rock interactions from other karst springs.
A Gibbs diagram and the quantitative relationships of index ions in water were applied to analyze the process of groundwater circulation. Most of the karst springs are dominated by rock weathering (Fig. 8a and b).
The milliequivalent concentration relationship between (Na + − Cl − ) and (Ca 2+ + Mg 2+ )-(HCO 3 − + SO 4 2− ) can indicate the occurrence of cation exchange between groundwater and carbonate rocks. When the slope is near − 1, there is obvious cation exchange, and the degree of cation exchange can be determined from the absolute value of the difference between the slope of the fitting curve and − 1. From fitting the data with the least-square method, we conclude that obvious cation exchange takes place in karst springs, except for the eastern group. From the slope of the linear fitting results, cation exchange is greatest in the Huolong group, lower in the northern group, still lower in the Gangtonglong group, and lowest in the eastern group (Fig. 8c, the x-axis is in log-scale).
The three karst springs Q201, Q312, and Q309, which are located at the outlet of the Huolong gully, exhibit different hydrochemical characteristics from other karst springs, indicating that they have experienced different groundwater flow conditions and water-rock interactions. The three karst springs of the Huolong gully and their downstream surface water points fall in the area of evaporation-crystallization dominance. The springs at the outlet of the Huolong gully mainly experienced evaporation or Plots of the relationship of HCO 3 − /Na + versus Ca 2+ /Na + and Mg 2+ /Na + versus Ca 2+ /Na + in groundwater, the circles and ellipses in the picture are schematic representations of typical water-rock interactions evaporite dissolution (Fig. 8d). Springs Q201, Q309, and Q312 possess Na-Ca-SO 4 -HCO 3 water, Na-SO 4 water, and Na-SO 4 -HCO 3 water, respectively (Table 2), and have markedly higher Na + and SO 4 2− concentrations than other karst springs.
In general, the source of SO 4 2− in the groundwater of carbonate rock areas is likely to be from the dissolution of gypsum that is present in some of the carbonate rocks. The milliequivalent ratio of (Ca 2+ + Mg 2+ ) − (HCO 3 − ) and (SO 4 2− ) − (Na + − Cl − ) can be used to determine whether the SO 4 2− ions come from gypsum dissolution. (Ca 2+ + Mg 2+ ) − (HCO 3 − ) represents the concentration of Ca 2+ derived from gypsum dissolution, and (SO 4 2− ) − (Na + − Cl − ) represents the concentration of SO 4 2− derived from gypsum dissolution (Fig. 9). The karst springs at the outlet of the Huolong gully plot far from the gypsum dissolution line, indicating that the SO 4 2− in karst water is not mainly derived from gypsum dissolution.
The ratio of (Ca 2+ + Mg 2+ )/(HCO 3 − ) versus (SO 4 2− )/ (HCO 3 − ) can be used to determine the contribution of carbonate rock and gypsum dissolution to groundwater components. In this diagram, the Huolong group is different from other karst water groups (Fig. 10), which mainly reflects dissolution of carbonate rocks and dissolution of gypsum. The Huolong group is located in the area above the 1:1 line in Fig. 11a, indicating that it is not Ca 2+ or Mg 2+ but other cations are the main ions to balance the solution. After adding Na + , all the points fall near the 1:1  (Fig. 11b), meaning that (Ca 2+ + Mg 2+ + Na + ) is the main cation. The source of SO 4 2− in groundwater is possibly from the dissolution of sodium-containing minerals.
The Na + in the Huolong group groundwater has several possible sources: atmospheric precipitation, dissolution of sodium-containing minerals in rocks or sediments along Huolong gully, such as halite, mirabilite and albite. The concentrations of Na + , SO 4 2− and Cl − in snow sample were 0.21 mg/L, 0.63 mg/L and 0.25 mg/L, respectively, which were all lower than 320.2 mg/L, 695.9 mg/L and 1.75 mg/L in Q309. The multiples of are 1525 times, 1105 times and 7 times respectively. The ion balance cannot be achieved by the concentration of atmospheric precipitation without the dissolution of other minerals.
The dissolution of albite follows the following process (Zheng et al. 2020): However, in the diagram of the milligram equivalent concentration relationship of Na + and HCO 3 − (Fig. 12a), all points fall far from the dissolution line of albite, so the dissolution of albite contributes little to Na + in groundwater. From Fig. 12b and the multiple difference between the Na + and Cl − concentrations of snow and Q309, halite dissolution is not the main source of Na + .
From the diagrams in Figs. 11 and 12, the source of Na + in the karst springs at the outlet of the Huolong gully is sodium sulfate minerals, which may be mirabilite (Na 2 SO 4 ·10H 2 O) or glauberite (Na 2 SO 4 CaSO 4 ). Ca 2+ is derived from the dissolution of carbonate minerals (Fig. 11); therefore, the source of Na + in karst water at the outlet of the Huolong gully is the dissolution of mirabilite.
Software PHREEQC was used to analyze the water-rock interaction process from snow to Q309. The results showed that mirabilite dissolved 6.858 × 10 -3 mol and albite dissolved 1.684 × 10 -5 mol per solution unit, and the concentration of the solution caused by water evaporation was 3.91 times.
Overall, there are two main chemical sources of karst water in Genie Mountain. Most of the karst water is derived from filtration through carbonate karst, but the Huolong gully karst springs are also influenced by mirabilite dissolution.
The study area is in an alpine and gorge region. The height difference between the groundwater recharge area and the discharge area is generally more than 1500 m, and it is not easy to generate a slow-flow, stagnant water environment. Formation of mirabilite requires an arid environment with relatively stagnant water; therefore, the groundwater recharge area of the Huolong group is likely to be located on the planation surface of Genie Mountain above 4600 m a.s.l. The surface extends in NNW direction, the terrain is relatively flat, there are some moderately thick moraines, and the area includes some lakes. The lakes are mainly replenished by atmospheric precipitation and seasonal melting of ice and snow. In the arid environment, the lake water and pore water in moraines continuously undergoes evaporation and filtration, which increases the ion concentration in groundwater. Lake water, pore water, and melt water from snow and ice continuously seep into the karst groundwater circulation system.

Thermal springs
The study area is located at the western edge of the Litang-Batang geothermal zone in the eastern part of the Qinghai-Xizang Plateau, and experiences relatively intense geothermal activity. Two thermal spring samples were collected, in which the main types of groundwater are Na-HCO 3 and Ca-Na-HCO 3 and the main source of water is atmospheric precipitation. These two thermal springs have experienced a long thermal cycle at depth (3300-3700 m) (Zhao et al. 2019). Groundwater flows through the deep karst or fissure conduits controlled by the Chalo-Songduo Fault (Cao 2020), and is heated by deep magma heat sources . Driven by deep thermal power to circulate to the surface, more intense ion exchange occurs with the surrounding rock. The water from depth is mixed with cold water in the shallow region, with a mixing ratio with cold water of 64-68% (Zhao et al. 2019).

Circulation rate
According to the tritium content and hydrochemical characteristics, Genie Mountain karst water cycle can be divided into four categories (Fig. 13).
Catgeory A: The tritium content is greater than 10TU, and the TDS value is less than 400 mg/L. Water is supplied from modern atmospheric precipitation or snow/ice melting on the hillside or near the top of Genie Mountain. The water enters the epikarst system, the direction of runoff is mainly affected by topography, and the water is discharged near the excretion base level, with short runoff distance, shallow circulation depth, and a rapid circulation rate. Typical examples of these karst springs are A20 in the eastern group and G09 in the northern group.
Category B: The tritium content is 3-9 TU, concentration of SO 4 2− is less than 100 mg/L and the TDS value is less than 400 mg/L. The circulation depth is great, and the circulation rate is fast. The groundwater mainly dissolves carbonate rocks, and is affected little by evaporation. The groundwater flow and discharge of this type of groundwater are obviously controlled by structural features. The Gangtonglong group, Northern group and part of Huolong group of springs belong to this type.
Category C: The tritium content is 3-10 TU. The water is SO 4 -Na or SO 4 ·HCO 3 -Na type, and concentration of SO 4 2− is larger than 500 mg/L and the TDS value is larger than 1000 mg/L. In the process of groundwater flow, the water also mixes with groundwater that is over 60 years old. This water is discharged at the junction of the fractures near the mouth of the Huolong gully. The horizontal groundwater flow distance is 15-25 km, the circulation depth is large, and the circulation rate is fast. This water type is strongly influenced by leaching and evaporation processes.
Category D: The tritium content is about 1 TU. The water is of HCO 3 -Na type, and the TDS value is larger than 1000 mg/L. The groundwater flow depth is large and the circulation rate is slow.

Groundwater circulation
Genie Mountain is located in the karst area of the gorge in the eastern plateau of the Qinghai-Xizang Plateau. The existence of the Jinshajiang River structural belt has exacerbated the complexity of the groundwater flow system in the area.
The karst water in the Genie Mountain area is mainly supplied by atmospheric precipitation and melted ice and snow. The groundwater flow and drainage of karst water are mainly controlled by active geological structures (Fig. 14). Most of the karst phenomena develop along structural features in bedrock, forming multiple karst water circulation conduits distributed along the faults. Among them, there are two  Fig. 13 Plot of the relationship of TDS and SO 4 2− versus tritium in groundwater. A groundwater recharged by modern precipitation, with rapid circulation; B groundwater recharged by a mix of modern and more than 60-year-old precipitation, with long circulation; C ground-water recharged by a mix of modern and more than 60-year-old precipitation, with evaporation and filtration and long circulation; D groundwater recharged by precipitation and melt water, influenced by deep thermal geological processes regional karst conduits. One is along the Edexi-Hongjunshan Fault, where groundwater runs from south to north, and is mainly discharged at the outlet of Huolong gully; the other is along Gangtonglong Fault, where groundwater runs from north to south, and is discharged at Gangtonglong gully. The karst spring flow of Huolong gully (Q309) and Gangtonglong gully (G06) accounted for 47.8% and 48.8% of Genie Mountain's total karst water excretion, respectively.
In addition to the two regional karst water flow conduits, there are also karst water flow conduits distributed along other faults (mostly distributed in the Northern group and Huolong group), and epikarst water (such as the karst springs numbered A10 and G09).
According to the process and rate, karst water cycle in Genie Mountain can be divided into three levels and four types. Type I (the first level) is epikarst, the recharge altitude is relatively low (4400 ~ 4600 m), and generally cannot reach above the surface watershed; the cycle distance is short, the altitude difference between the recharge area and the drainage area is less than 1500 m, and the horizontal flow distance is ≤ 5 km; cycle speed fast (the age of groundwater is generally n × 10 0 ~ n × 10 1 yr), the water chemistry type is HCO 3 -Ca, and the TDS value is ≤ 350 mg/L. Karst springs in type I are distributed in the north and north-eastern part of Genie Mountain. Some karst springs cannot be measured due to the sporadic water flows and because the flow rate of individual springs is lower than the detection limit of the flow measuring equipment. Type II (the second level): Tectonic karst groundwater cycle, representing the main cycle type of karst water in Genie Mountain. The recharge area is 4600-4900 m a. s. l, the altitude difference between the replenishment area and the drainage area is 1200-1900 m. The horizontal flow distance is from 5 km to more than 20 km (Gangtonglong flow conduits); the circulation speed is relatively fast, and the groundwater age is n × 10 1 ~ n × 10 2 yr. Water-rock interaction is mainly the dissolution of carbonate rock. The chemical type of groundwater is HCO 3 -Ca-Mg, and TDS Value < 400 mg/L. The karst water of Gangtonglong group, northern group and part of Huolong group (karst springs numbered Q310 and SM02) belong to Type II. The total flow of these karst springs accounts for about 51% of Genie Mountain's karst water. Type III (the second level): Cycle of tectonic karst groundwater affected by the glacial landform. Mainly refers to the karst water that supplies the Huolong group of springs. The area of Huolong gully above 4600 m above sea level is flat, with a certain thickness of moraine. There are also a large number of glacial lakes, and groundwater flowrates are relatively slow. Atmospheric precipitation and seasonal melting of ice and snow replenish glacial lakes. In arid environments, the lake water and pore water in the moraine are continuously evaporated and filtered, increasing the ion concentration in groundwater. Lake water, pore water, and melting water from snow and ice constantly seep into karst water. Affected by this, the TDS value of Type III (≥ 1000 mg/L) is greater than that of Type II. Influenced by mirabilite dissolution, the type of groundwater is SO 4 -Na.
Type IV (the third level): Geothermal water cycle. Atmospheric precipitation and melted ice and snow enter the Chalo-Songduo Fault belt through karst conduits or fissures at various levels, and are heated by deep geothermal sources at a depth range of approximately 3300-3700 m. Under the action of deep groundwater flow dynamics, the water flows along the branch faults and interacts with other waters during the ascent to discharge areas. Mixing with shallow cold water takes place. The depth of groundwater flow is large, the groundwater age is more than 60 yr, and the cycle rate is slow.

Conclusion
Genie Mountain is located in the Hengduan Mountains region of China, a tectonically active area. The marked tectonic activity and complex tectonic system have shaped the complex karst aquifer structure in this area. As a result of the high mountain and valley topography, with a difference in altitude between the mountain peak and the valley in the study area of more than 2000 m, transportation and research is extremely difficult. In the present study, hydrochemical and isotopic methods were applied to understand the groundwater recharge sources, the dynamics of groundwater flow, and the hydrochemical evolution of groundwater in the Genie Mountain area. In total, 18 groundwater samples, 23 surface water samples, and 1 snow sample were collected in July and August 2019.
Genie Mountain's karst groundwater is mainly derived from atmospheric precipitation and melted ice and snow. The groundwater is mainly recharged by rainfall from local water circulation and is supplemented by water vapor from the Middle East, the Indian Ocean, and the Western Pacific. Marked isotopic depletion occurs. Groundwater recharge areas are mostly located in high mountains above 4500 m, and the direction of groundwater flow is mostly controlled by the Jinshajiang Fault, with drainage areas at the intersection of multiple faults.
On the basis of the hydrochemistry, isotopes, and the location of groundwater discharge zones, karst water is divided into five groups: the Northern, Huolong, Eastern, and Gangtonglong groups and thermal springs. Groundwater mainly dissolves carbonate rocks during flow within carbonate aquifers. The levels of Na + and SO 4 2− in karst water of the Huolong group are markedly higher than in other groups. Through the analysis of water chemical indicators, it could be concluded that the source of Na + and SO 4 2− is the dissolution of mirabilite. We infer from this result that the karst water replenishment area of the Huolong group is an arid evaporative environment, and the groundwater experiences slow flow in Huolong gully.
The karst groundwater cycle in Genie Mountain is mainly controlled by active faults and can be divided into three levels and four types. The first level is the circulation of epikarst groundwater, in which circulation is rapid, water-rock interactions are limited, and the groundwater is of Ca-HCO 3 type. Level two is the structure-controlled deep karst groundwater cycle, which can be divided into two types. The first type is Huolong group karst groundwater, for which the recharge area is located on the planation surface of Genie Mountain. Groundwater interacts with evaporites and is mostly Na-SO 4 ·HCO 3 type with TDS value ≥ 1000 mg/L. Modern precipitation and older groundwater become mixed in circulation. The other type is karst water flowing in conduits along structural features, which is Ca·Mg-HCO 3 type. The circulation distance is generally more than 10 km, and this will also mix with older groundwater. The TDS value is generally ≤ 400 mg/L. The third level is thermal springs heated by a deep heat source, for which the circulation rate is slow.
In areas where it is difficult to conduct direct investigations, such as alpine and gorge areas, analysis of complex groundwater cycle processes and the combined use of water chemistry and isotopes can effectively extract key information from water samples and provide explanations about the evolution of hydrochemistry. In future research, additional new isotopic methods can be included to further improve the accuracy of circulation data.