Variation and reason analysis of groundwater hydrochemical characteristics in Beiluhe Basin, Qinghai–Tibet Plateau during a freezing–thawing period

The cryosphere critically impacts the world's climate system and its regional water cycle processes. In China, permafrost occurs in the high-altitude areas of the Qinghai–Tibet Plateau (QTP) where it becomes the headstream of many rivers (Xu et al. 2017). Groundwater reactions to global climate change, which have been sensitive in the QTP, have attracted the attention of scholars in recent years. In the cold regions of the QTP, the groundwater mainly occurs in the active layer, where it is an important contributor to the hydrological circulation in the QTP. The active layer of the QTP is the permafrost topsoil that can generally melt and freeze with seasonal changes and clings to the permafrost. The seasonal freezing–thawing process of the active layer critically influences the surface energy balance, water migration, and groundwater runoff (Anisimov et al. 1997; Hollesen et al. 2011). Zhao et al. (2000), who investigated the active layer in Wudaoliang, found that the freezing–thawing process of the active layers often presents different characteristics during different seasons. They found that the melting of the active layers in the Wudaoliang area begins in late April, achieves the maximum depth in September, and quickly and completely freezes in late October. During the melting and freezing of the active layer, material and energy conversions are quite complex. During the thawing of the active layer, salt migration is consistent with the water. The surface soil water escapes under evaporation, resulting in salt accumulation in the surface soil (Huang et al. 1993; Ni et al. 2015). Salt diffusion is the main process during freezing. Desalination occurs when groundwater freezes, where salt accumulates in unfrozen water, resulting in increased salt in the groundwater near the frozen layer. Salt migrates via the concentration gradient to the unfrozen area that has low salt concentrations (Padilla & Villeneuve 1992; Arenson & Sego 2006; Cui et al. 2019). Freezing of the active layer has an important effect on microorganisms. Changes in the physical properties of the active layer, such as freezing, change the environmental and living conditions of microorganisms, and include changes in the osmotic pressure, temperature, and salt. These changes lead to a decrease in the microbial metabolism and even a hibernation state of the microorganisms (Yang & Jin 2008). The active layer had different functions during different periods; therefore, the constituents of the groundwater also showed different characteristics.

Climate change and human activity have caused changes in ecological phenomena in the QTP in recent years (Fan et al. 2010), including the formation of thermokarst lakes. The formation of thermokarst lakes in the QTP has considerably changed the thermodynamic properties of the surrounding permafrost because of the unique geographical features of the QTP, as compared to high-latitude permafrost. The number of thermokarst lakes on both sides of the Qinghai–Tibet Railway has gradually increased after the completion of the railway, which significantly affects the groundwater around it. For the high temperatures at the bottom of the lake, the thickness of the active layer beneath the lake increases, resulting in the decrease or even disappearance of the permafrost layer. Thermokarst lakes have a close hydraulic relationship with the surrounding groundwater and material and energy exchanges occur frequently (Yang et al. 2013a, 2013b; Gao et al. 2017). Research on the groundwater in the active layer of the QTP must therefore consider the influence of the thermokarst lakes. The close hydraulic connection between thermokarst lake and the groundwater means that stable hydrogen and oxygen isotopes can be used to study conversions between lake water and groundwater.

Even though numerous studies have focused on the water resources in the QTP, the hydrogeochemistry in the QTP is not well understood. The effects of the seasonal freezing and thawing of the active layer on the hydrochemical characteristics of the groundwater are especially poorly understood. This study, therefore, focuses on the variation of the groundwater hydrochemical characteristics during a freezing–thawing period, which is the novelty of this article. In this study, groundwater near thermokarst lakes in the Beiluhe Basin (BLB), which is a typical complete watershed, was considered the research object. The study specifically investigated the hydrochemistry and stable isotope (δ¹⁸O and δD) variations of the groundwater in the BLB during a seasonal freezing–thawing period of the active layer. Groundwater samples were collected in June, July, August, and October during the melting and refreezing processes of the active layer. Hydrogeochemical analyses and mathematical statistics were used to evaluate the variation of the groundwater hydrogeochemical characteristics. The changes in stable hydrogen and oxygen isotopes in the lake water and groundwater revealed the relationship between the lake water and groundwater over different freezing and thawing periods. Using these methods, this study first revealed the changes of groundwater hydrochemical characteristics during a freezing–thawing period in the active layer in a typical watershed, which enriched and improved the relevant research study. Due to the important role groundwater plays in the hydrological circulation system of the QTP, this study will lay a solid foundation for further environmental research in the plateau area and is beneficial to the research of groundwater evolution, utilization, and protection in high-altitude areas.

The study area belongs to the largest Cenozoic sedimentary basin in the hinterlands of the QTP (Zhou et al. 2007). The phase transition between the different strata and the vertical and horizontal changes between strata are however not evident. All the strata of the quaternary layer are exposed on the surface. The surface rocks have fine particles and a relatively high organic matter content, with a thickness of approximately 0.5–3.5 m. At depths greater than 6 m, the ice content decreases, and a mixed clay layer is exposed. The tertiary lacustrine sedimentary layer underlies the quaternary system. Rock is weathered by strong weathering, and calcareous or argillaceous cements are often seen in the rock layer.

The active layer often undergoes a series of material and energy exchanges during melting and freezing, which affects the hydrochemical composition of groundwater. Water samples were collected in the Beiluhe area in June, July, August, and October to monitor the chemical changes of the groundwater during the freezing–thawing process of the active strata. The sampling dates correspond to the active layer's freezing–thawing stages as follows: the early thawing period (June 2019), the middle thawing period (July 2019), the late thawing period (August 2019), and the freezing period (October 2019).

Groundwater samples were collected from nine boreholes in the research area. These boreholes were divided into three categories based on their geographical locations. ZK01 and ZK02 are located in the southern part of the research area; ZK03, ZK04, and ZK05 are located in the middle part of the research area; and ZK06, ZK07, ZK08, and ZK09 are located in the northern part of the research area. Three water samples were collected from each sampling point. Two 550 mL of water samples were used for general hydrochemical analysis, and one bottle of 50 mL of water sample was used for isotope detection. Further isotopic water samples were collected from eight lakes (Figure 3). The hydrochemical analyses mainly detected the conventional ions in the water and the pH and Total Dissolved Solids (TDS) of the water samples. Isotopic analyses mainly detected the stable isotopes of hydrogen and oxygen in the water samples.

The temperature and pH of the sample were determined directly by a portable water quality analyzer (6250C) at the sites where the samples were collected. The ⁠, ⁠, ⁠, NH₄⁺, and Cl⁻ concentrations of the groundwater samples were determined by conventional titration. The Na⁺ and K⁺ concentrations were determined by flame color reaction and flame atomic absorption spectrometry (TAS990), and the Ca²⁺ and Mg²⁺ concentrations were determined using the Ethylene Diamine Tetraacetic Acid (EDTA) titrimetric method. The traditional drying and weighing approach was used to determine TDS concentrations. The F⁻ concentrations were calculated by the ion-selective electrode method (ICS90A), and NO₂⁻ and NO₃⁻ concentrations were calculated by the ion chromatography (precision ion meter). All the isotopic water samples were analyzed with a Picarro L2140-i isotopic analyzer for δD and δ¹⁸O, whose measurement accuracy values for δD and δ¹⁸O can reach 0.5 and 0.1‰, respectively.

A statistical analysis of the major ions forms the basis of understanding the hydrochemical characteristics of water (Zhou et al. 2016). Table 1 shows the statistical analysis results for the physicochemical parameters of the major ions.

Table 1

Statistical analysis results for major ion concentrations (mg/L) in groundwater

.	Parameter .	pH .	TDS .	Na+ .	Ca2+ .	Mg2+ .	.	Cl− .	.
June	Mean	7.77	715.33	130.08	87.74	45.98	451.30	193.23	42.41
	Max	7.94	1080.00	188.00	132.00	72.90	555.00	464.00	96.10
	Min	7.66	520.00	75.50	64.10	24.30	329.00	67.40	24.00
	SD	0.09	182.36	34.99	22.29	19.00	79.71	127.04	24.42
	CV	1.15	25.49	26.90	25.48	41.34	17.66	65.74	57.57
July	Mean	7.60	1017.00	161.38	119.04	73.01	549.75	317.88	67.83
	Max	7.78	1760.00	275.00	200.00	194.00	744.00	995.00	125.00
	Min	7.36	644.00	126.00	68.10	30.40	427.00	68.00	33.60
	SD	0.15	377.63	46.70	37.17	50.96	92.36	282.75	34.10
	CV	2.01	37.13	28.93	31.22	69.80	16.80	88.95	50.28
August	Mean	7.78	598.57	147.27	98.48	65.62	546.20	214.40	80.25
	Max	7.95	1432.00	232.00	170.00	113.00	830.00	595.00	173.00
	Min	7.58	380.00	79.70	40.10	17.00	299.00	47.00	19.20
	SD	0.13	290.05	40.98	35.70	26.86	129.84	155.85	43.61
	CV	1.64	48.54	27.83	36.25	40.93	23.77	72.69	54.34
October	Mean	7.52	1435.50	273.75	142.18	64.24	426.65	576.13	81.59
	Max	7.83	4100.00	787.00	317.00	221.00	738.00	2225.00	149.00
	Min	7.22	576.00	108.00	48.10	26.70	195.00	93.00	9.61
	SD	0.21	1097.84	225.96	81.24	60.71	182.51	659.31	49.31
	CV	2.86	76.48	82.54	57.14	94.51	42.81	114.44	60.44

.	Parameter .	pH .	TDS .	Na+ .	Ca2+ .	Mg2+ .	.	Cl− .	.
June	Mean	7.77	715.33	130.08	87.74	45.98	451.30	193.23	42.41
	Max	7.94	1080.00	188.00	132.00	72.90	555.00	464.00	96.10
	Min	7.66	520.00	75.50	64.10	24.30	329.00	67.40	24.00
	SD	0.09	182.36	34.99	22.29	19.00	79.71	127.04	24.42
	CV	1.15	25.49	26.90	25.48	41.34	17.66	65.74	57.57
July	Mean	7.60	1017.00	161.38	119.04	73.01	549.75	317.88	67.83
	Max	7.78	1760.00	275.00	200.00	194.00	744.00	995.00	125.00
	Min	7.36	644.00	126.00	68.10	30.40	427.00	68.00	33.60
	SD	0.15	377.63	46.70	37.17	50.96	92.36	282.75	34.10
	CV	2.01	37.13	28.93	31.22	69.80	16.80	88.95	50.28
August	Mean	7.78	598.57	147.27	98.48	65.62	546.20	214.40	80.25
	Max	7.95	1432.00	232.00	170.00	113.00	830.00	595.00	173.00
	Min	7.58	380.00	79.70	40.10	17.00	299.00	47.00	19.20
	SD	0.13	290.05	40.98	35.70	26.86	129.84	155.85	43.61
	CV	1.64	48.54	27.83	36.25	40.93	23.77	72.69	54.34
October	Mean	7.52	1435.50	273.75	142.18	64.24	426.65	576.13	81.59
	Max	7.83	4100.00	787.00	317.00	221.00	738.00	2225.00	149.00
	Min	7.22	576.00	108.00	48.10	26.70	195.00	93.00	9.61
	SD	0.21	1097.84	225.96	81.24	60.71	182.51	659.31	49.31
	CV	2.86	76.48	82.54	57.14	94.51	42.81	114.44	60.44

CV, coefficient of variability; SD, standard deviation.

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The concentration of increased gradually from 42.41 mg/L in the early melting period of the active layer to 81.59 mg/L during the freezing and thawing of the active layer. The concentrations of the and the Mg²⁺ increased from 549.75 and 73.01 mg/L to 451.3 and 45.98 mg/L, respectively, during the early thawing period of the active layer. Thereafter, it decreased gradually from the late thawing period to the freezing period of the active layer, reaching concentrations of 426.65 and 64.24 mg/L, respectively. The concentrations of the Cl⁻, Na⁺, and Ca²⁺ fluctuated and reached their minimum values during the early thawing period of the active layer, reaching 193.23, 130.08, and 87.47 mg/L, respectively. In the middle of the thawing period, the ion concentrations rose to 317.88, 161.38, and 119.04 mg/L, respectively. The concentrations of the Cl⁻, Na⁺, and Ca² decreased during the late thawing period of the active layer and finally increased to 576.13, 273.75, and 142.18 mg/L in the freezing period, respectively.

As shown in Figure 6, cations were mainly distributed in the B region of the Piper during the freezing–thawing process of the active layer, indicating that none of the cations in the groundwater was predominant. Anions, with and Cl⁻ being dominant, were mainly distributed in regions E and G of the Piper. Most of the water samples were distributed in zones 1 and 4 of the Piper, with the hydrochemical type being mainly HCO₃·• Cl–Na •·Ca •·Mg during the thawing period which changed to Cl·• HCO₃–Na •·Ca •·Mg type water during the freezing period.

As shown in Figure 6, the Na⁺/(Na⁺ + Ca²⁺) of the groundwater was between 0.4 and 0.7, and the Cl⁻/(Cl⁻ + ⁠) was between 0.1 and 0.9. The groundwater in the study area is controlled by rock weathering and evaporative crystallization. The relationship between TDS and Cl⁻/(Cl⁻ + ⁠) indicates that some groundwater sample points are controlled by rock weathering, whereas some groundwater sample points are controlled by evaporation and concentration.

The ratios of major ions in groundwater at different stages can be obtained by calculating the equivalent concentrations of each of the major ions in groundwater. The changes in regional groundwater quality over time could be investigated by analyzing the hydrochemical characteristics of the groundwater formation.

The Na/Cl ratio measures the extent of salt leaching and accumulation as well as the degree of Na enrichment in groundwater.

Figure 8 shows the changes in the Na/Cl ratios of the groundwater in each borehole during the freezing–thawing process of the active layer. Here, the changes of Na/Cl ratio in the groundwater were not evident because the molarity of Na⁺ and Cl⁻ changed in similar ratios from the melting to the freezing of the active layer. The fluctuation of the Na/Cl ratio around 1 indicates that the Na⁺ and Cl⁻ in the groundwater may be directly related to the dissolution of halite. The Na/Cl ratio at borehole ZK03 was the lowest, which is consistent with the conclusion that Na/Cl ratio is less than 1 in high-TDS groundwater. The high groundwater salinity in borehole ZK03 is due to the strong evaporation at this site.

The high consumption of during the freezing phase of the active layer resulted in a considerable increase in the (Ca + Mg)/HCO₃ ratio.

The HCO₃/Ca ratios of groundwater in the study area were all less than 4 (Figure 10), indicating that part of Ca²⁺ and in the groundwater may be directly related to the dissolution of calcite. Microbial activity in the active layer increased from the middle thawing period to the late thawing period. The observed increases in the concentration may be related to the respiration of microorganisms. The CO₂ produced by respiration increases the solubility of dolomite in groundwater. The decrease in HCO₃/Ca ratio that was observed from the late thawing period to the freezing period of the active layer may be related to the groundwater reactions of Formulae (1) or (2), which was more intense in the groundwater of ZK08.

As shown in Figure 11, the Ca/Cl ratio of ZK03 was the lowest, indicating that compared with other areas, the groundwater flow in this area was blocked and subjected to strong evaporation. The Ca/Cl ratio rose slightly from the middle thawing period to the late thawing period of the active layer, probably because the average temperature was the highest during this stage. The melting depth of the active layer achieved its maximum during this stage, which meant that part of the CO₂ could be released from the melting process of the active layer with the CO₂ produced by respiration. This led to an increase in the CO₂ content in the groundwater. The ability of groundwater to dissolve carbonates consequently increased. The Ca/Cl ratio decreased considerably during the freezing period of the active layer because the groundwater temperature decreased significantly and the groundwater flows almost stagnated. The reaction of Formula (1) to the groundwater results in a decrease in the Ca/Cl ratio.

During the melting of the active layer to its refreezing, the SI of anhydrite in the different boreholes (Figure 12(a)) was greater than 0 and increased slowly over time, indicating that the concentrations of Ca²⁺ and gradually increased during melting and refreezing. This increase in Ca²⁺ concentration may be due to the production of CO₂ by the respiration of microorganisms in the soil and the CO₂ released after the melting of the active layer. The elevated CO₂ increases the ability of groundwater to dissolve calcite. The increase in concentration may be related to the evaporation or desalination of the active layer during the freezing period.

The SIs of calcite (Figure 12(b)) and dolomite (Figure 12(c)) were greater than 0 and reached their maximum in the late thawing period of the active layer. These results indicate that dolomite and calcite control the hydrochemical composition of groundwater in the study area. Over time, both dolomite and calcite increased gradually from the early to the late thawing period and decreased rapidly during the freezing of the active layer. The SI changes in the calcite and dolomite may be due to the following reasons: High temperatures from the early thawing period to the late thawing period of the active layer melted the active layer and released CO₂. At the same time, respiration increased due to an increase in soil microbial activity, which released CO₂ into the groundwater, increasing the ability of groundwater to dissolve carbonate during runoff. During the freezing of the active layer, the temperature decreased, slowed or even stopped microbial respiration, and the groundwater mainly reacted with Equations (1) and (2), decreasing the SIs of the dolomite and calcite.

The SI of halite (Figure 12(d)) was less than 0 during all the stages of the freezing–thawing process of the active layer. This indicates that the halite in the soil strata in the study area has been completely dissolved. The SI of halite increased gradually over time but increased significantly from the late thawing period to the freezing period of the active layer. Evaporation and precipitation mainly influenced the changes in the SI of halite. The evaporation of groundwater was supplemented to an extent during the early to the late thawing period of the active layer, which slightly changed the SI due to large precipitation. During the freezing of the active layer, salt accumulated in the unfrozen water, leading to a significant increase in the Na⁺ and Cl⁻ concentrations of the groundwater.

Both groundwater indices in the borehole in the northern part of the study area were greater than 0 (Figure 13). Alternating cation adsorption may therefore occur in these areas. Most of the boreholes in the middle and southern parts of the study area did not have cationic alternating adsorption. The molarity of the Na⁺ in the northern borehole was greater than that of the Ca²⁺; therefore, Na⁺ may replace the Ca²⁺ that is adsorbed on the surface of the solid particles in the groundwater during groundwater flow.

These results show that Ca²⁺ may have replaced Na⁺ adsorbed on the surface of solid particles in the groundwater of the northern borehole in June, July, and August, and that Na⁺ probably replaced Ca²⁺ in the groundwater in October.

Modeled simulation method is widely used in the hydrology research, including water quantity (Burgan & Aksoy 2022), water quality (Rehana & Dhanya 2018) and water energy (Kitessa et al. 2021). Reverse hydrogeochemical simulation uses the material balance model to determine the amount of intermineral precipitation or dissolution at two different points on the groundwater flow process (Lv et al. 2014), that is, water and rock composition identification and quantitative hydrogeochemical reactions are applied to explain the formation and evolution of hydrochemical composition (Soumya et al. 2011). In this study, a simulation path was selected, and Phreeqc was used to conduct reverse hydrogeochemical simulation to represent water–rock interaction quantitatively and verify the hydrogeochemical process from the recharge area to the discharge area on the basis of the hydrogeological data of the study area and the existing hydrochemical analysis.

ZK06 was selected for the reverse hydrogeochemical simulation, and a simulated path of recharge–runoff–discharge was determined: June → July → August → October.

The selection of mineral phases may be a key step for the success of reverse hydrogeochemical simulation, which is mainly based on hydrochemical analysis, rock and mineral identification, and characteristics of water-bearing medium (Yang et al. 2018). According to the above hydrochemical analysis, the main mineral phases in the study area are calcite, dolomite, halite, and anhydrite. Aragonite and anhydrite are dismissed for their minimal content. In the process of hydrochemical evolution, cationic alternating adsorption is essential and should be regarded as a mineral phase. To simulate evaporation (or dilution), the phase of the H₂O(g) component must be included. In consideration of the constant dissolution of CO₂(g) into groundwater, CO₂(g) was regarded as a possible mineral phase.

The simulation results are shown in Table 2.

The simulation results were analyzed as follows:

(1) June–July

Along the seepage path, the main water–rock action that causes the change of groundwater chemical composition is the dissolution of dolomite, calcite, anhydrite, and halite. The dissolution rates are 0.501, 0.354, 0.128, and 0.095 mmol/L. The cationic alternating adsorption of Ca–Na results in 0.571 mmol/L Na⁺ entering the groundwater and 0.286 mmol/L Ca²⁺ leaving the groundwater. The dissolution amount of CO₂ is 1.517 mmol/L, and 4.234 mol/L of water is evaporated. The water type is HCO₃ Cl-⁻Na Ca.

(2) July–August

Along this period, the following water–rock interactions occur mainly between these two points: The precipitation of dolomite, calcite and anhydrite. The precipitation rates are 0.221, 1.436, and 0.065 mmol/L. The negative value of halite is due to the large amount of precipitation. The cationic alternating adsorption of Ca–Na results in 0.288 mmol/L Ca²⁺ entering the groundwater and 0.575 mmol/L Na⁺ leaving the groundwater. The escape amount of CO₂ is 2.049 mmol/L, and 12.84 mol/L of water is evaporated. The water type is HCO₃ Cl–Na Ca.

(3) August–October

Along the seepage path, the main water–rock action that causes the change of groundwater chemical composition is the dissolution of dolomite, calcite, anhydrite, and halite. The dissolution rates are 0.617, 0.593, 0.063, and 0.951 mmol/L. The cationic alternating adsorption of Ca–Na results in 0.233 mmol/L Ca²⁺ entering the groundwater and 0.466 mmol/L Na⁺ leaving the groundwater. The dissolution amount of CO₂ is 2.432 mmol/L, and 7.757 mol/L of water is evaporated. The water type changes from HCO₃ Cl⁻Na Ca to HCO₃ Cl–Ca Na.

Using reverse hydrogeochemical simulation, the main water–rock action of groundwater that can be obtained at different times is the dissolution or precipitation of calcite, dolomite, anhydrite, and halite. Cation exchange, the escape or dissolution of carbon dioxide and water evaporation and recharge, and the final simulation results confirm the previous qualitative analysis.

Stable isotopes have been widely used to explore the sources of various water types and the relationship between groundwater and surface water (Horita & David 1994; Qian et al. 2007; Lin et al. 2016; Wang et al. 2022). The statistical analysis results for isotopic content detection are shown in Table 3.

The hydrogen and oxygen isotopes in atmospheric precipitation tend to change synchronously, and δD and δ¹⁸O may present different values under the influence of atmospheric precipitation, topographic factors, monsoon, and other factors. Therefore, the relationship between δD and δ¹⁸O often varies in different regions of meteoric precipitation.

As shown in Table 3, at the early thawing period (June), δD (44.83‰) in the lake is greater than the average δD (60.64‰) of groundwater, and the average of δ¹⁸O in the lakes (5.83‰) is greater than the average δ¹⁸O (7.80‰) in the groundwater. The concentration of isotope in lake water is greater than groundwater, the reasons for which are probably as follows: since the average temperature of the active layer is relatively low in the early melting stage, most of the active layer is frozen, and most of the groundwater is stored below the active layer, with less contact with the atmosphere and less evaporation of groundwater. During this period, the lake is mainly replenished by atmospheric precipitation and permafrost meltwater, and evaporation is the main drainage of water resources in the regional water cycle. With continuous evaporation, δD and δ¹⁸O in the lake gradually increase and becomes higher than in the groundwater.

The average value of δD in the lake (−33.02‰) is higher than the average value of δD in the groundwater (−57.37‰), and the average value of δ¹⁸O in the lake (−4.42‰) is higher than the average value of δ¹⁸O in the groundwater (−8.21‰) during the period of active layer melting. Compared with what occurred during the early thawing period of the active layer, δD and δ¹⁸O in the lake showed a certain increase probably because in the middle thawing period of the active layer, the average temperature and precipitation reach the maximum, and the water circulation in the region reaches the maximum. Although the melting speed of the active layer reached its maximum in this period, it did not completely melt through the active layer. The runoff effect of groundwater was considerably enhanced compared with the previous period, but it did not reach its maximum, and the flow of groundwater was still hindered to some extent. However, in the middle thawing period of the active layer, lake surface evaporation played a dominant role. In this process, although a large amount of rainwater with low isotopic content is replenished into the lake, it is still unable to balance the water consumption caused by evaporation during this period.

The change of δD and delta δ¹⁸O in groundwater is not evident compared with the previous period, in which δ¹⁸O shows a slight decrease, whereas δD shows a slight increase. Although δD and δ¹⁸O in the groundwater increase due to the increase in temperature and evaporation, the lower δD and δ¹⁸O rain enter the groundwater, and isotopic mixing occurs, making the changes of δD and δ¹⁸O in the groundwater irrelevant.

The average value of δD in the lake (−46.46‰) is higher than the average value of δD in the groundwater (63.68‰) in the late thawing period of the active layer, and the average value of δ¹⁸O in the lake (−5.93‰) is higher than the average value of δ¹⁸O in the groundwater (−8.83‰). Compared with the middle thawing period of the active layer, δD and δ¹⁸O in the lake are reduced possibly because in the late thawing period, the thickness of the unfrozen soil layer below the surface reaches its maximum; this period is often believed to be the period when groundwater runoff has the strongest effect and the greatest impact on the thermokarst lake. Large quantities of groundwater and atmospheric precipitation with low δD and δ¹⁸O enter the lake, and isotopic mixing with the lake balances the evaporation process, further reducing the concentration of δD and δ¹⁸O in the lake. The decrease in δD and δ¹⁸O in groundwater may be related to the replenishment of rainwater and the enhancement of groundwater mobility.

During the freezing period of the active layer, the average value of δD in the lake (−49.00‰) is higher than that in the groundwater (−64.88‰), and the average value of δ¹⁸O in the lake (−6.22‰) is higher than the δ¹⁸O in the groundwater (−8.79‰). Compared with what occurs during the late thawing period of the active layer, δD and δ¹⁸O in the lake and groundwater continue to decrease possibly because during the freezing period of the active layer, the average temperature and precipitation are remarkably reduced, the freezing of the active layer is almost complete, the groundwater flow is considerably slow, and the amount of runoff supply to the lake is small. On the one hand, evaporation allows δD and δ¹⁸O to enter the air. However, heavy isotopes in groundwater and lake water may result in isotopic fractionation; moreover, a part of them transfers to the ice, resulting in a decrease in the content of δD and δ¹⁸O.

This study systematically analyzed the variation of hydrochemical characteristics of groundwater and the isotope compositions (δ18O and δD) of thermokarst lake water and groundwater during the freezing–thawing process of the permafrost in the QTP. The following conclusions can be drawn:

• From the early thawing period to the late thawing period of the active layer, the groundwater is mainly a mixed water sample of HCO₃ • Cl–Na·• Ca·• Mg, and the groundwater gradually transforms into Cl·• HCO₃–Na·• Ca·• Mg type water during the freezing of the active layer. Based on the Gibbs diagram, the hydrochemical components of groundwater in the study area are influenced by simultaneous evaporation, concentration, and rock weathering.

• The main ion proportionality coefficients of the groundwater show that the Na+ and the Cl– may derive from the dissolution of halite. Ca²⁺ and Mg²⁺ may derive from the dissolution of dolomite, and the HCO₃/Ca ratio shows that groundwater may be directly related to the dissolution of calcite. The increase in the HCO₃– concentration may be related to the respiration of microorganisms. The CO₂ produced by respiration increases the solubility of groundwater to dolomite. The Ca/Cl ratio rose slightly from the middle thawing period to the late thawing period of the active layer because the average temperature is at its highest during this stage. During the freezing period of the active layer, the Ca/Cl ratio however decreased considerably because the temperature decreased significantly while the groundwater flow almost stagnated.

• The SIs of related minerals in the groundwater indicates that dolomite and calcite control the relevant hydrochemical groundwater components in the study area during the different stages of thawing to freezing. Halite has, however, not reached saturation, indicating that the halite has been completely dissolved in the soil strata of the study area.

• Using reverse hydrogeochemical simulation, the reaction amount of minerals can be obtained. The main water–rock action of groundwater at different times is the dissolution or precipitation of calcite, dolomite, anhydrite, and halite, the cation exchange, the escape or dissolution of carbon dioxide and water evaporation and recharge. The simulation results quantitatively verify the main water–rock action obtained by qualitative analysis.

• The δD and δ¹⁸O isotopes in June are higher than in August, probably because of continuous precipitation, which led to the dilution of δD and δ¹⁸O in the atmospheric precipitation in August. The δD and δ¹⁸O of the groundwater and lake water fall between the GMWL and the PTPL, and the precipitation contributes to the formation of thermokarst lakes in the study area. The δD and δ¹⁸O fluctuations in the lake water and the groundwater are mainly affected by the runoff, temperature, and evaporation of groundwater.

This study revealed the changes of groundwater hydrochemical characteristics during a freezing–thawing period in the active layer in a typical watershed. Due to the severe climate and working conditions in these high-altitude areas, water sample collection and analysis for this paper were only carried out over 1 year. In the future, the sampling time nodes should be increased to analyze the inter-annual variation of the hydrochemical characteristics.

This research was sponsored by the National Natural Science Foundation of China, Grant No. 41730640—Environmental and Hydrological Effects of the Thermokarst Lakes in the Permafrost Region of the QTP and the Fundamental Research Funds for the Central Universities, CHD, Grant No. 300102290401.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.