Hydrochemical Characteristics and Formation Mechanism of Neogene Geothermal Water in the Zhangye–Minle Basin

Abstract

Geothermal resources in arid inland basins are important for clean energy development, yet their circulation and geochemical mechanisms remain insufficiently understood. This study investigates the hydrochemical characteristics and formation mechanisms of geothermal water in the Zhangye–Minle Basin, an arid inland region in northwestern China. A total of nine geothermal water samples were analyzed using major ion chemistry, stable isotopes (δ²H, δ¹⁸O), tritium (³H), and radiocarbon (¹⁴C) to determine recharge sources, flow paths, and geochemical evolution. The waters were predominantly of the Cl–Na and Cl·SO₄–Na types, with total dissolved solids ranging from 3432.00 to 5810.00 mg/L. Isotopic data indicated that recharge originated from atmospheric precipitation and snowmelt in the Qilian Mountains, with recharge altitudes between 2497 and 5799 m. Tritium and ¹⁴C results suggested that most samples were recharged before 1953, with maximum ages exceeding 40,000 years. Gibbs diagrams and ion ratio plots demonstrated that water–rock interaction was the primary geochemical process, while cation exchange was weak. Na⁺ was mainly derived from halite, albite, and mirabilite, while SO₄²⁻ originated largely from gypsum. The calculated reservoir temperatures using cation geothermometers ranged from 57 °C to 148 °C. The deep circulation of geothermal water was closely related to NNW-trending fault zones that facilitated infiltration and heat accumulation. These findings provide new insights into the recharge sources, circulation patterns, and geochemical processes of geothermal systems in fault-controlled basins, offering a scientific basis for their sustainable exploration and development.

1. Introduction

Geothermal energy is a clean, sustainable, and efficient renewable resource that has attracted increasing global attention due to its vast development potential and minimal environmental impact [1,2]. As one of the most promising alternatives to fossil fuels, geothermal energy is expected to contribute significantly to global efforts aimed at achieving carbon neutrality and peak emissions by providing stable and low-carbon baseload energy [3]. Its utilization not only mitigates greenhouse gas emissions but also improves local energy structures and promotes socioeconomic development. Although geothermal energy offers clear environmental and economic benefits, the development of geothermal resources remains uneven across regions. Many promising basins have yet to be thoroughly explored or scientifically evaluated. Geothermal development has achieved notable progress in recent years, particularly in the eastern and southeastern regions, where several demonstration projects and resource assessments have been implemented in China [4]. However, in arid inland regions such as northwest China, geothermal exploration is still limited. The high cost and technical complexity of early-stage prospecting, combined with a lack of detailed geological and hydrogeological data, make it difficult to accurately characterize geothermal systems based solely on surface investigations [5]. As a result, critical questions concerning geothermal water recharge sources, flow pathways, and chemical evolution remain unresolved, which in turn limits the sustainable use of these resources. The Zhangye–Minle Basin, located in the central Hexi Corridor of Gansu Province, is a representative arid inland basin with considerable geothermal potential. Structurally, it is a typical fault–depression composite basin formed during the Meso–Cenozoic, bounded to the south by the Qilian Mountains and divided into a western uplift, central depression, and eastern slope. The basin covers an area of approximately 39,436 km², with crystalline basement depths ranging from 2.5 km in the uplifts to 5.8 km in the depression. Regional heat flow anomalies, warm springs, and elevated geothermal gradients have been identified, indicating significant deep geothermal resources. The local energy structure is still dominated by coal-based resources, and the transition to clean and renewable energy is essential to address environmental challenges and regional energy demands [6]. Although geothermal anomalies have been detected in the basin, the underlying mechanisms governing the formation, circulation, and chemical composition of geothermal water are not well understood. Most existing studies have focused on the geological and tectonic setting or the vulnerability of shallow aquifers, with limited attention paid to the deeper geothermal system and associated hydrogeochemical processes. Recent investigations in comparable basins of northwestern China have demonstrated the utility of integrated hydrochemical and isotopic approaches for identifying recharge sources, residence times, and dominant geochemical processes, but such systematic work is still lacking for the Zhangye–Minle Basin.

In recent years, hydrochemical and isotopic techniques have become essential tools in geothermal studies. These methods are widely used to identify recharge sources, trace groundwater flow paths, estimate residence times, and understand water–rock interaction processes [7]. Major ion chemistry provides valuable insights into the composition and evolution of geothermal fluids, while stable isotopes such as δ¹⁸O and δ²H are useful indicators of recharge origin and altitude. In addition, radioactive isotopes like tritium (³H) and radiocarbon (¹⁴C) are commonly used to assess groundwater age and to distinguish modern recharge from paleo-infiltration events [8,9,10]. A number of case studies have demonstrated the applicability of these methods in complex geothermal environments. For example, Yang et al. analyzed geothermal springs in the Boge Creek area of eastern Tibet and reported significant mixing between rising thermal water and shallow cold groundwater, which altered the isotopic signatures and lowered discharge temperatures [11]. In another study, Yin et al. found that geothermal fluids in the Wuhan area were influenced by ion exchange and the dissolution of silicate and carbonate minerals [12]. Similarly, Duan et al. identified halite and silicate dissolution as dominant geochemical processes in the Lanzhou geothermal system. Their work also revealed that geothermal water was recharged by paleo-infiltration and suggested a hydraulic connection with shallow groundwater through fault structures [13]. Despite the success of these approaches in other regions, the Zhangye–Minle Basin remains understudied in terms of geothermal water chemistry and isotopic composition. Current investigations have primarily focused on structural geology and the thermal field, and there is still a lack of systematic data on the hydrochemical characteristics and evolution mechanisms of geothermal water in the basin [14,15]. Several key scientific issues remain unaddressed. First, the classification of geothermal water types and their spatial variability has not been clearly defined. Second, recharge altitudes, groundwater residence times, and flow depths have not been quantified. Third, the dominant geochemical processes influencing the chemical evolution of geothermal fluids are not well understood. Finally, the role of tectonic features in controlling recharge and circulation pathways requires further investigation.

To address these gaps, the present study conducted a comprehensive assessment of geothermal water in the Zhangye–Minle Basin using integrated hydrochemical and isotopic methods. Data were collected from geothermal wells and shallow groundwater, and a variety of analytical approaches were employed. The study aimed to (i) classify the hydrochemical types of geothermal water and examine their spatial distribution, (ii) estimate reservoir temperatures and circulation depths using geothermometers and temperature gradient data, (iii) identify recharge sources and elevations based on stable and radioactive isotopes, and (iv) evaluate the relative contributions of halite, gypsum, and silicate mineral dissolution, as well as cation exchange, in shaping the geochemical characteristics of geothermal fluids. The results of this study are expected to enhance the scientific understanding of geothermal fluid evolution in arid inland basins. By providing detailed insights into recharge mechanisms, thermal regimes, and geochemical processes, this research supports the rational development and long-term sustainability of geothermal resources in the Zhangye–Minle Basin. Moreover, the findings may serve as a reference for geothermal investigations in similar geological settings across northwestern China and other arid regions.

2. Materials and Methods

2.1. Study Area

The Zhangye–Minle Basin (Figure 1a–c) is located in Zhangye City, Gansu Province, in the central section of the Hexi Corridor, covering an area of approximately 39,436.5 km². Panel (a) shows the regional location. Panel (b) outlines the basin-scale physiography and structural framework that distinguish the western uplift, the central depression, and the eastern slope, with map-scale faults striking NNW to NWW. Panel (c) presents the study-area map with sampling wells and structural domains used throughout this study. The basin lies at the northern foot of the Qilian Mountains and formed as a fault-depression composite basin during the Meso–Cenozoic. Crystalline basement rocks occur at depths of about 2500 m beneath the uplifts and about 5800 m beneath the central depression. These structural and stratigraphic conditions, together with active faults, provide pathways for deep circulation and accumulation of geothermal fluids.

The region is characterized by a continental arid climate, with an average annual precipitation of less than 150 mm and a potential evaporation exceeding 2000 mm. Precipitation is mainly concentrated from June to September, and the climate is typified by low humidity, high evaporation, and large temperature fluctuations between day and night. These climatic conditions contribute to limited natural groundwater recharge and strong spatial heterogeneity in groundwater distribution. Hydrogeological, the basin contains multilayered aquifer systems composed of loose clastic sediments, including gravel, sand, silt, and mudstone. Shallow unconfined aquifers are mainly distributed in the piedmont alluvial–proluvial fans, with groundwater depths ranging from 2 to 20 m. Confined and semi-confined aquifers occur in the basin center and low-lying areas, where groundwater exists under artesian conditions. Recharge is primarily derived from atmospheric precipitation, snowmelt from the Qilian Mountains, and lateral inflow along fault zones. Groundwater flows generally from the mountainous recharge areas in the south and west toward the central depression and eastern slope zones.

2.2. Data Source and Methods

Sampling was carried out between 2023 and 2024, during which nine geothermal water samples were collected following the completion of well productivity testing (Figure 1c). For hydrochemical analysis, 4000 mL of geothermal water was collected in natural-colored polyethylene containers, fully filled underwater to avoid gas exchange, then sealed and transported to the Lanzhou Mineral Resources Testing Center, Ministry of Natural Resources (Gansu Provincial Central Laboratory). The measured parameters included pH, total dissolved solids (TDS), total hardness, Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻, SO₄²⁻, and NO₃⁻. For isotope analysis, 500 mL of water was sampled in colorless polyethylene bottles for δ¹⁸O, δ²H, and ³H testing, while ¹⁴C samples were collected on-site using the BaCO₃ precipitation method and stored in 750 mL polyethylene bottles. All isotope samples were analyzed at the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences. In addition to geothermal water, 15 reference water samples were also collected, including 3 river samples (B), 2 spring samples (C), 6 groundwater samples (D), and 4 confined water samples (E). Hydrochemical composition was analyzed using an inductively coupled plasma optical emission spectrometer (iCAP 6300, Thermo Fisher Scientific, Waltham, MA, USA). Stable isotopes δ¹⁸O and δ²H were measured with an isotope ratio mass spectrometer (L2130-i, Picarro Inc., Santa Clara, CA, USA), with analytical precisions of ±0.1‰ and ±1.0‰, respectively. Tritium (³H) and radiocarbon (¹⁴C) were analyzed using an ultra-low background liquid scintillation spectrometer (Quantulus 1220, PerkinElmer, Waltham, MA, USA), with precisions of ±0.1 TU and ±0.3 pMC. To interpret the hydrogeochemical data, Piper, Durov, and bivariate ion concentration plots were employed to identify water types and ion sources [16,17]. Gibbs diagrams were used to assess the dominant hydrogeochemical processes [18]. Stable isotopes (δ¹⁸O and δ²H) were used to determine recharge sources and elevations, while tritium and ¹⁴C provided age estimates of geothermal water [19].

Integration of geological context, well characteristics, and aqueous geochemistry underpins the analysis. Wells are situated within the basin’s structural domains (western uplift and central depression) on the basis of reported locations, temperature–depth behavior, and reservoir attribution, which provide the structural constraint for interpretation. Cation-geothermometric outputs (Na–K, Na–K–Ca, K–Mg) are evaluated under uniform criteria: estimates yielding non-physical Na–K–Ca temperatures are excluded from quantitative use; agreement between Na–K and K–Mg defines a quantitative temperature bracket, whereas divergence is treated as qualitative evidence of thermal maturity without assigning a numerical reservoir temperature. Positions on Gibbs diagrams furnish basin-scale constraints that distinguish evaporative concentration from rock-dominated solute acquisition and relate the distributions of TDS and major ions to the structural and stratigraphic architecture.

Cation geothermometers (Na–K, Na–K–Ca, K–Mg) were applied following the formulations cited in this study. To ensure interpretability within the scope of these formulations, results were classified as follows: (i) non-physical Na–K–Ca outputs (e.g., negative or unrealistically high temperatures) were excluded from quantitative use; (ii) concordant Na–K and K–Mg estimates were taken as quantitative brackets, and their spread reported; (iii) divergent Na–K vs. K–Mg results were treated qualitatively as maturity indicators rather than assigned numerical reservoir temperatures; and (iv) all quantitative values were cross-checked against measured wellhead temperatures to avoid over-interpretation when large mismatches occurred.

2.3. Geological and Well Context

The Zhangye–Minle Basin is a faulted Cenozoic depression where piedmont alluvial fans along the Qilian Mountains grade basinward into Neogene sand–silt successions with local evaporitic interbeds. NNW trending faults provide fracture corridors that focus recharge, deep circulation, and upward discharge. Within this framework, meteoric waters infiltrate at the mountain front, reside at kilometer depth in Neogene clastics, and ascend through permeable fault damage zones toward discharge areas. For each sampled well we report the two verifiable metadata that constrain the thermal regime, namely wellhead temperature and total depth, compiled from field notes and public reports. Detailed completion information, including screened or open hole intervals, is generally not available in the public domain. To ensure robust interpretation without relying on undocumented details, we frame mineral sources at the basin-scale facies and relate the observed chloride and sulfate behavior to Neogene stratigraphy and evaporitic layers (

Table 1. Wellhead temperature and total depth of sampled wells.

Monitoring Point	Well Depth (m)	T (°C)
A1	2269.18	77
A2	1567	55
A3	2350.76	61
A4	2357.2	57
A5	1701.75	47
A6	2053.08	46
A7	1500.59	45
A8	2174	78
A9	2601.22	56

).

3. Results and Discussion

3.1. Hydrochemical Characteristics of Geothermal Water

3.1.1. Characteristics of Main Chemical Components

The statistical characteristics of the major chemical components in nine geothermal water samples collected from the Zhangye–Minle Basin during the period from 2023 to 2024 are presented in

Table 2. Characteristics of water chemical component contents in monitoring wells.

Project	Average	Maximum	Minimum	Standard Deviation	Variance	Coefficient of Variation
	Chemical Composition (mg/L)
Na+	1341.78	1655.00	1017.00	218.65	47,807.51	0.16
K+	26.38	61.70	8.00	16.82	283.07	0.64
Ca2+	178.76	409.00	56.20	116.05	13,466.64	0.65
Mg2+	67.12	123.00	35.60	30.14	908.50	0.45
HCO3−	386.97	773.10	133.00	237.95	56,620.27	0.61
Cl−	1658.22	2276.00	1285.00	377.51	142,516.84	0.23
SO4−2	977.69	1442.00	701.60	276.38	76,388.29	0.28
PH	7.60	8.24	7.10	0.31	0.10	0.04
TDS	4667.33	5810.00	3432.00	899.65	809,367.56	0.19

. The geothermal water was characterized as weakly alkaline, with pH values ranging between 7.10 and 8.24. A low coefficient of variation (0.04) indicated minimal spatial variability in pH levels. The concentrations of total dissolved solids (TDS) ranged from 3432.00 to 5810.00 mg/L, and a moderate coefficient of variation (0.19) suggested relatively consistent mineralization across the sampled locations. Among the major cations, Na⁺ concentrations ranged from 1017.00 to 1655.00 mg/L, with a standard deviation of 218.65 mg/L and a low coefficient of variation (0.16), indicating a relatively uniform spatial distribution. In contrast, Ca²⁺ and K⁺ exhibited higher coefficients of variation at 0.65 and 0.64, respectively, reflecting localized differences in mineral dissolution and water–rock interactions. Mg²⁺ concentrations were relatively low, ranging from 35.60 to 123.00 mg/L, with moderate variability (CV = 0.45). For the anions, Cl⁻ and SO₄²⁻ showed the highest concentrations, ranging from 1285.00 to 2276.00 mg/L and 704.01 to 1442.00 mg/L, respectively. Although Cl⁻ exhibited the highest absolute concentration, its coefficient of variation remained moderate (0.23), while SO₄²⁻ displayed slightly greater variability (0.28), likely influenced by complex lithological or tectonic factors. HCO₃⁻ concentrations ranged from 133.00 to 773.10 mg/L and demonstrated higher variability (CV = 0.61), suggesting notable differences in carbonate equilibrium conditions across the basin. NO₃⁻ was not included among the listed parameters, and no signs of nitrate contamination were observed.

Overall, the spatial variability of the geothermal water chemistry was classified as weak to moderate. Na⁺ and pH levels were relatively stable across the study area, reflecting the influence of the basin’s regional hydrogeological background, whereas Ca²⁺, K⁺, and HCO₃⁻ exhibited more pronounced heterogeneity, suggesting the impact of localized geochemical reactions. Ion ratio relationships, such as Na⁺ versus Cl⁻ and SO₄²⁻ versus Ca²⁺, indicate that halite and gypsum dissolution are important mineral sources, consistent with the Neogene evaporitic layers described in Section 2.3. These geochemical patterns support the interpretation that geothermal water composition was jointly controlled by large-scale hydrogeological conditions and site-specific processes including mineral dissolution and cation exchange.

Figure 2 illustrates the heatmap distribution of seven major chemical components (Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻, and SO₄²⁻) across nine geothermal water sampling points within the Zhangye–Minle Basin. The sampling sites were geographically dispersed, with A1, A2, and A3 located in Minle County; A5, A6, and A7 in Linze County; and A8 and A9 in Zhangye City. This spatial layout provides a representative overview of the hydrochemical characteristics across different subregions of the basin. Overall, Cl⁻ concentrations were the most prominent among all ions, especially at site A5 in Linze County, which exhibited the highest intensity (over 2200 mg/L), followed by moderate levels in A3, A4, A8, and A9. This suggests a possible influence of strong evaporite dissolution or anthropogenic input in the central and northern parts of the study area. SO₄²⁻ also showed elevated concentrations at A1, A5, and A9, indicating either sulfur-bearing mineral dissolution or deep geothermal water mixing at these points. Na⁺ levels were consistently high across all sites, particularly at A5, A8, and A9, reflecting the overall dominance of sodium-type groundwater in the basin. In contrast, the concentrations of K⁺ and Mg²⁺ were uniformly low and showed minimal spatial variation, indicating their limited role in controlling water chemistry. Ca²⁺ exhibited moderate variability, with slightly higher levels at A3 and A5, which may relate to localized carbonate dissolution. HCO₃⁻ concentrations were highest at A6 and A7, suggesting greater influence from carbonate equilibrium or shallow recharge processes in the southern part of Linze County. The observed spatial patterns suggest that geothermal water chemistry in the Zhangye–Minle Basin is not only shaped by regional geological settings but also influenced by site-specific geochemical processes, including mineral dissolution, mixing with deep fluids, and anthropogenic factors.

3.1.2. Hydrochemical Types of Geothermal Water

The geothermal water in the Zhangye–Minle Basin was mainly classified into two hydrochemical facies: Cl·SO₄–Na and Cl–Na. Although the concentrations of major ions such as Na⁺, Ca²⁺, Cl⁻, and SO₄²⁻ vary among different sampling points, the overall hydrochemical composition remains relatively stable, reflecting the dominant influence of evaporite dissolution and water–rock interactions in the region. Based on the Piper diagram results in Figure 3, the spatial distribution of the hydrochemical types reveals clear regional differentiation. In Minle County, samples A1, A2, and A3 were identified as Cl·SO₄–Na type, suggesting the involvement of both chloride and sulfate sources, which may be attributed to the dissolution of mixed evaporites such as halite and gypsum. Sample A4, also from Minle, exhibited a Cl–Na water type, indicating a relatively stronger contribution from halite dissolution and a lower input from sulfate-bearing minerals. In Linze County, sample A5 was categorized as Cl–Na type, reflecting dominant halite dissolution with limited sulfate influence, while A7 exhibited a Cl·SO₄–Na type, indicating co-dissolution of halite and gypsum. In Zhangye City, samples A6, A8, and A9 were all determined to be Cl·SO₄–Na type. The co-occurrence of high Na⁺, Cl⁻, and SO₄²⁻ concentrations in these samples indicates the significant role of both halite and gypsum dissolution, as well as possible long-term water–rock interactions within deep aquifers. Overall, the basin is dominated by Cl·SO₄–Na hydrochemical facies, with Cl–Na types occurring locally due to relatively simpler solute sources.

Statistically, seven out of nine samples were classified as Cl·SO₄–Na type, accounting for 77.8% of all geothermal water samples. The remaining two samples, A4 and A5, were identified as Cl–Na types, representing 22.2%. The predominance of the Cl·SO₄–Na facies suggests that the geochemical evolution of geothermal water in this basin is mainly influenced by the dissolution of both halite and sulfate-bearing evaporites such as gypsum, along with prolonged water–rock interactions. In contrast, the presence of the Cl–Na facies in certain locations indicates a relatively simpler solute source that is dominated by halite dissolution. These findings reflect the spatial heterogeneity of geothermal water types across Minle, Linze, and Zhangye, and also provide important insights into the geological and hydrological processes that control groundwater chemistry in arid inland basins.

The Durov diagram presented in Figure 4 provides a comprehensive visualization of the hydrochemical characteristics of geothermal water in the Zhangye–Minle Basin. The diagram integrates multiple hydrochemical parameters including cations, anions, pH, and total dissolved solids (TDS), allowing for a detailed assessment of water types and their geochemical processes. The majority of geothermal water samples are clustered in the lower left quadrant of the diagram, indicating that Na⁺+K⁺ and Cl⁻ are the dominant ions. This distribution pattern is consistent with the results of the Piper diagram and confirms that the geothermal water in the study area is predominantly of the Cl–Na and Cl·SO₄–Na types. In terms of cation composition, most samples plot in the Na⁺+K⁺-dominant region, reflecting extensive ion exchange or prolonged water–rock interaction. For anions, a high proportion of samples are located in the Cl⁻-dominated area, particularly samples A1, A5, A6, A7, A8, and A9, suggesting contributions from halite dissolution or deep circulation processes. The TDS values of the geothermal water samples range from approximately 3600 to 5800 mg/L. Samples A5 and A7 from Linze County, as well as A6, A8, and A9 from Zhangye City, exhibit relatively high TDS, indicating stronger mineralization, likely due to deeper circulation or longer residence time. In contrast, samples A1, A2, A3, and A4 from Minle County show moderate TDS levels, suggesting shorter flow paths or weaker geochemical evolution. The pH values range from 7.1 to 8.5, generally reflecting weakly alkaline water. Samples A2 and A9 display relatively higher pH, possibly due to degassing of CO₂ or dissolution of carbonate minerals, while A1, A4, and A7 exhibit slightly lower pH values, suggesting variations in redox conditions or microbial activity. Overall, the Durov diagram confirms the dominance of Na⁺ and Cl⁻ in geothermal water chemistry across the region, while also highlighting spatial differences in TDS and pH that reflect variations in geothermal circulation depth, rock–water interaction, and aquifer conditions.

3.2. Characteristics of Ground Temperature Variations and Circulation Depth

3.2.1. Temperature Estimation Method

Geothermometers can be used to estimate the temperature of geothermal reservoir fields. For medium to low-temperature geothermal systems, cation geothermometers are commonly applied. These geothermometers are based on empirical functional relationships between the ratios of cation concentrations such as potassium, calcium, sodium, and magnesium in water and the corresponding formation temperatures. The concentrations of these cations vary with temperature, as shown in Equations (1)–(3). Typical cation geothermometers include the Na–K, K–Mg, Na–K–Ca, and Na–Li types [20,21,22]. By comparing the measured temperature with the calculated temperature using geothermometers, one can determine which estimation better reflects the actual subsurface conditions. This preliminary estimation approach is useful for evaluating geothermal potential before conducting detailed site investigations.

$$T_{Na-K-Ca}={\displaystyle \frac{1647}{\lg (\frac{C_{Na}}{C_K})+\beta (\lg (\frac{\sqrt{C_{Ca}}}{C_{Na}})+2.06)+2.47}}-273$$

(1)

$$T_{Na-K}={\displaystyle \frac{1217}{\lg (\frac{C_{Na}}{C_K})+1.483}}-273$$

(2)

$$T_{K-Mg}={\displaystyle \frac{4410}{14-\lg (\frac{\sqrt{C_K}}{C_{Mg}})}}-273$$

(3)

3.2.2. Geothermal Gradient

Geothermal gradients were influenced by multiple geological and hydrogeological factors, including tectonic morphology, basement undulation, magmatic activity, lithology, caprock thickness, folding, faulting, and the movement of deep groundwater [23]. Even when the lithology of the thermal reservoir remained relatively uniform, these factors contributed to pronounced vertical variations in geothermal well temperature profiles. According to Figure 5, the average geothermal gradient in the western uplift zone of the Zhangye Basin, where wells A1 and A3 were located in Minle County, ranged from 2.04 to 2.58 °C per 100 m. In comparison, the central depression zone, represented by A4 in Minle County and A5 in Linze County, exhibited slightly higher gradients of 2.32 to 2.67 °C per 100 m. For instance, the overall gradient of well A1 was 2.32 °C per 100 m; however, within the depth interval of 1200–2300 m, the temperature rose more slowly. In this section, the temperature increased from 42.9 °C to 55.3 °C, corresponding to a reduced gradient of approximately 1.35 °C per 100 m. This pattern was likely driven by convective heat transfer associated with deep circulating groundwater superimposed on a conductive heat regime, and coincided with the main thermal reservoir depth (1225–2290 m) inferred from borehole geophysical data. Additionally, A7, located in Linze County, exhibited a relatively high temperature gradient throughout the profile, suggesting efficient heat transfer or weaker caprock confinement. A6, A8, and A9, all situated in Zhangye City, presented the steepest temperature curves, with bottom-hole temperatures approaching 70 °C at depths beyond 2300 m. These conditions might have resulted from localized tectono-thermal anomalies or vertical upwelling of deep thermal fluids. The spatial variability observed among the wells indicated that geothermal gradients across the basin were significantly controlled by geological structure and subsurface hydrothermal activity.

3.2.3. Temperature Estimation

Table 3. Shows the calculated reservoir temperatures using three different cation geothermometer methods.

Sample Site	T(Na + K + Ca)	T(Na + K)	T(K + Mg)	Tm	(TK-Mg − Tm)/Tm
A1	66.07	150.29	79.54	77.00	3.29
A2	70.66	90.41	60.20	55.00	9.46
A3	−38.54	101.38	59.32	61.00	−2.76
A4	−40.29	100.53	59.30	57.00	4.03
A5	329.14	112.10	72.87	47.00	55.03
A6	−2.49	84.00	55.96	46.00	21.65
A7	82.18	65.19	46.69	45.00	3.75
A8	622.72	129.91	80.19	78.00	2.81
A9	696.03	75.19	59.13	56.00	5.59

presents the estimated reservoir temperatures of nine geothermal wells using three cation geothermometers: Na + K + Ca, Na + K, and K + Mg. These geothermometers are based on empirical relationships between ion concentrations in geothermal water and subsurface temperature, providing important insights into the geothermal characteristics of the Zhangye Basin [24]. The Na + K + Ca geothermometer produced the most inconsistent and extreme values. For instance, sample points A3 and A4, both located in Minle County, yielded negative temperature estimates (−38.54 °C and −40.29 °C), while A8 and A9, situated in Zhangye City, produced abnormally high values exceeding 600 °C. These results suggest that the Na + K + Ca method is likely affected by non-equilibrium conditions or unsuitable water chemistry, making it unreliable in this region. In comparison, the Na + K method produced more moderate and plausible temperature estimates. Most values fall within a reasonable geothermal range, such as A2 (90.41 °C) in Minle County and A5 (112.10 °C) in Linze County. However, some results still appear elevated, such as A1 with 150.29 °C, which exceeds the typical outlet temperatures observed in local wells. Despite this, the method better approximated actual temperatures in several samples compared to the Na + K + Ca method. Among the three, the K + Mg geothermometer yielded the most stable and realistic results. Its predictions closely matched the measured wellhead temperatures (Tm), especially in samples from Minle (A1), Zhangye City (A6, A8), and Linze County (A5, A7). For example, A5, A6, and A7 produced predicted temperatures of 72.87 °C, 55.96 °C, and 46.69 °C, closely aligning with their respective measured values of 47.00 °C, 46.00 °C, and 45.00 °C. The deviation index was relatively low across most sites, with A1 and A8 showing values of 3.29 and 2.81, respectively. Although A3 showed a negative deviation, this may result from localized hydrological or thermal anomalies disrupting ion equilibrium. In summary, while the Na + K + Ca and Na + K methods provided mixed results, the K + Mg geothermometer demonstrated better reliability and consistency with field data, indicating its suitability for estimating geothermal reservoir temperatures in the Zhangye Basin.

3.2.4. Reservoir Temperature and Circulation Depth

Based on temperature measurement data from geothermal wells in the area, the geothermal anomaly zone in the Zhangye Basin exhibits a geothermal gradient of 2.04 to 2.67 °C per 100 m. The depth of the isothermal zone is approximately 30 m, with an isothermal temperature of 7.6 °C. The circulation depth of the underground geothermal water can be determined using Equation (4) [25]:

$$H={\displaystyle \frac{T_1-T_2}{G}}+h$$

(4)

In Equation (4):

H—circulation depth of geothermal water (m);

T₁—reservoir temperature (°C);

T₂—temperature of the isothermal zone (°C);

G—geothermal gradient (°C/m);

h—average depth of the isothermal zone (m).

The calculated circulation depth of geothermal water in the Zhangye Basin geothermal field ranges from 1588.91 to 2813.27 m, which is approximately 500 to 800 m deeper than the bottom boundary of the reservoir identified in existing exploration wells. This indicates that geothermal water circulation still occurs within fracture zones at certain depths below the reservoir. This conclusion is basically consistent with the findings of Yin et al. [26].

3.3. Isotopic Caracteristics

3.3.1. Hydrogen and Oxygen Isotopes

Stable isotopes (¹⁸O and ²H) provide important information about the sources and processes of groundwater [27]. The results of stable isotope analyses are listed in

Table 4. Contents of stable hydrogen and oxygen isotopes.

River Water Samples (Labeled as B)	Average	Min	Max
δ18O (‰)	−8.17	−9.7	−6.9
δ2H (‰)	−56.33	−62	−46
Spring water samples (labeled as C)	average	min	max
δ18O (‰)	−6.70	−7.4	−6
δ2H (‰)	−50.00	−53	−47
Groundwater samples (labeled as D)	average	min	max
δ18O (‰)	−7.37	−8.2	−6.5
δ2H (‰)	−49.17	−52	−44
Confined water samples (labeled as E)	average	min	max
δ18O (‰)	−9.03	−9.9	−7.8
δ2H (‰)	−59.50	−63	−56

and Figure 6. The δ¹⁸O and δ²H values of the collected water samples range from −6.00‰ to −10.50‰ and −44.00‰ to −78.00‰, respectively. Specifically, river water samples (B1–B3) range from −6.90‰ to −9.70‰ for δ¹⁸O and −46.00‰ to −62.00‰ for δ²H; spring water samples (C1–C2) range from −6.00‰ to −7.40‰ for δ¹⁸O and −47.00‰ to −53.00‰ for δ²H; groundwater samples (D1–D6) range from −6.50‰ to −8.20‰ for δ¹⁸O and −44.00‰ to −52.00‰ for δ²H; and confined water samples (E1–E4) range from −7.80‰ to −9.90‰ for δ¹⁸O and −56.00‰ to −63.00‰ for δ²H. Therefore, spring water samples have the highest δ¹⁸O and δ²H isotopic values. The δ¹⁸O and δ²H values of geothermal water samples are significantly lower than those of spring and river water samples, suggesting that the geothermal water is derived from recharge occurring at higher elevations where temperatures are lower.

3.3.2. Carbon and Tritium Isotopes

The results of geothermal water age dating in the Zhangye–Minle Basin are summarized in

Table 5. ¹⁴C and ³H contents of geothermal water samples from different wells.

Sample	A1	A2	A3	A4	A5	A6	A7	A8	A9
14C (ka)	29.31 ± 2.72	13.29 ± 0.97	25.15 ± 3.02	40.16 ± 3.74	27.85 ± 5.30		40.08 ± 4.67	29.31 ± 2.72	13.29 ± 0.97
3H (TU)	<0.5	<1	<1	<1	<1	<0.5	1.5 ± 0.7	1.3 ± 0.5	<1

. Except for samples A8 from Zhangye City and A7 from Linze County, which indicate a mixture of submodern and recently recharged water, most geothermal fluids in the basin were identified as old water recharged prior to 1953. During groundwater circulation, hydrogeochemical processes such as mineral dissolution, precipitation, and ion exchange occur between water and aquifer materials. The geothermal water in this region formed under relatively closed and deep geological conditions, and radiocarbon dating (¹⁴C) provides a key foundation for tracing the thermal water evolution. Among the samples, A4 from Minle County and A8 from Zhangye City exhibit the oldest ¹⁴C ages of 40,160 and 40,080 years, respectively. Although A8 and A7 show relatively old apparent ages, their elevated ³H contents confirm the involvement of recent recharge, which results in younger ¹⁴C values than would be expected for unmixed deep water.

To further investigate the formation history of geothermal water, both tritium (³H) and radiocarbon (¹⁴C) isotopes were analyzed. Tritium has a half-life of 12.26 years, and its concentration is expressed in Tritium Units (TU). A ³H value below 0.8 TU indicates submodern water. Values between 0.8 and 4.0 TU correspond to mixtures of submodern and recently recharged water, while values from 5.0 to 15.0 TU reflect modern recharge that occurred within the past 5 to 10 years [28]. The half-life of ¹⁴C is approximately 5760 years, enabling age estimates of up to 40,000 to 50,000 years. Since nuclear weapons testing began in 1953, large quantities of ³H have entered the atmosphere and subsequently infiltrated groundwater systems through precipitation. As a result, groundwater with very low ³H concentrations (e.g., <2 TU) is identified as “old water” formed before 1953, while samples with high ³H content necessarily contain a component of “new water” recharged after that date [29]. In this study, samples A1, A2, A3, and A4 from Minle County displayed low ³H levels, confirming their origin as submodern groundwater. Conversely, A7 from Linze County and A8 from Zhangye City exhibited higher ³H contents, indicating the influence of modern recharge. These spatial differences reflect varying recharge histories and highlight the combined utility of ³H and ¹⁴C in tracing groundwater age.

3.3.3. Recharge Sources of Geothermal Water

Craig [30] studied the compositions of δ²H and δ¹⁸O in atmospheric precipitation and surface water worldwide, revealing a linear relationship between δ²H and δ¹⁸O, based on which the Global Meteoric Water Line (GMWL) was established as δ²H = 8δ¹⁸O + 10. Huang et al. [31] proposed the atmospheric precipitation line for the Zhangye region as δ²H = 6.76δ¹⁸O − 4.50 (R² = 0.94), with a slope and intercept lower than those of the China Meteoric Water Line (CMWL) δ²H = 7.9δ¹⁸O + 8.2 proposed by Wang et al. [32]. The slope of the meteoric water line reflects the relative fractionation rates of δ¹⁸O and δ²H. The local atmospheric precipitation line slope of 6.76 deviates from the GMWL, indicating secondary evaporation occurred during precipitation processes. The main reason is that Zhangye is far from the ocean and has an extremely arid climate, with a considerable amount of water vapor originating from local evaporation. In arid regions, δ¹⁸O and δ²H values in surface waters are elevated, thus evaporated vapor also shows high δ¹⁸O and δ²H. Additionally, under arid climatic conditions, isotope enrichment occurs during raindrop evaporation, contributing to elevated δ¹⁸O values in Zhangye’s spring and summer precipitation.

Isotopic fractionation causes different isotopic relationships in various water bodies. Differences in stable isotopes (¹⁸O and ²H) can indicate the recharge sources and recharge elevation of water. Estimating groundwater recharge elevation based on the altitude effect of δ²H and δ¹⁸O in precipitation is an effective approach. Currently, ¹⁸O is primarily used for calculating groundwater recharge elevation [33]. The formula for estimating recharge elevation using ¹⁸O is as follows:

$$H={\displaystyle \frac{\delta G-\delta P}{K+h}}$$

(5)

In Equation (5):

H is the isotopic recharge elevation (m);

h is the elevation of the sampling point (well or spring) (m);

δG is the δ¹⁸O value of the groundwater (spring water), in ‰;

δP is the δ¹⁸O value of atmospheric precipitation near the sampling point, in ‰;

K is the altitude gradient of δ¹⁸O in atmospheric precipitation (‰/100 m), representing the change in δ¹⁸O value per 100 m elevation increase.

The recharge elevation of geothermal water was estimated using the oxygen isotope altitude gradient method, where h represents the ground elevation at the sampling point. In this study, δP is taken as the δ¹⁸O value of local atmospheric precipitation in Zhangye (−6.3‰), while δG denotes the measured δ¹⁸O value of geothermal water. The average isotopic lapse rate K for δ¹⁸O in atmospheric precipitation typically ranges from −0.12‰ to −0.24‰ per 100 m. Based on these parameters, the calculated recharge elevations of geothermal wells span from 2497.09 m to 5798.96 m. These results are presented in

Table 6. Summary of recharge elevations of geothermal wells.

Sample Site	18O(%)	δP(%)	K1 (%/100 m)	K2 (%/100 m)	Wellhead Elevation (m)	H1 (m)	H2 (m)
A1	−10.3	−6.3	−0.12	−0.24	1780	5113.33	3446.67
A2	−10.4	−6.3	−0.12	−0.24	1598	5014.67	3306.33
A3	−10.1	−6.3	−0.12	−0.24	1777.82	4944.49	3361.15
A4	−10.4	−6.3	−0.12	−0.24	1762.29	5178.96	3470.62
A5	−8.6	−6.3	−0.12	−0.24	1538.76	3455.43	2497.09
A6	−9.5	−6.3	−0.12	−0.24	1644	4310.67	2977.33
A7	−10.5	−6.3	−0.12	−0.24	1463	4963.00	3213.00
A8	−10	−6.3	−0.12	−0.24	1536	4619.33	3077.67
A9	−10.4	−6.3	−0.12	−0.24	1487	4903.67	3195.33

. The estimated elevation range aligns closely with the altitude of the Qilian Mountains (approximately 3000–5000 m), where snow and glacial ice persist above 4500 m. This correspondence further supports the conclusion that geothermal water in the Zhangye–Minle Basin is primarily recharged by atmospheric precipitation and partial snowmelt originating from high-elevation zones in the Qilian Mountains. These findings are consistent with previous research on regional geothermal recharge mechanisms [34].

3.3.4. Conceptual Circulation of Geothermal Water

Existing exploration data confirm that the stratigraphy of the Zhangye Basin, from top to bottom, consists of Quaternary, Neogene, Cretaceous, and Paleozoic formations. Among these, the Neogene is characterized by red clastic rocks typical of an inland basin, mainly comprising mudstone, sandy mudstone, sandstone conglomerate, and conglomerate. The Neogene is divided into the Upper Suolehe Formation (N₂s) and the Middle Baiyanghe Formation (N₁b). The sedimentation is continuous, thinning gradually from the basin center toward the edges, with a thickness of 800–1000 m in the center, reducing to 500–600 m near the basin margins. The main geothermal reservoir in the Zhangye–Minle Basin is the clastic porous reservoir of the Middle Baiyanghe Formation of the Neogene. The lower Jianquanzi section of the Baiyanghe Formation mainly consists of poorly cemented gravelly sandstone and conglomerate, which are relatively stable in distribution and have a thickness ranging from 100 to 500 m. This section serves as the core reservoir of the basin, overlain by the Suolehe Formation of the Quaternary to Neogene, which acts as an effective cap rock.

3.4. Hydrogeochemical Formation Mechanism

3.4.1. Gibbs Diagram

Figure 7 illustrates the Gibbs diagrams constructed using two indices: (K⁺ + Na⁺)/[(K⁺ + Na⁺) + Ca²⁺] versus TDS (a), and Cl⁻/(Cl⁻ + HCO₃⁻) versus TDS (b), to explore the hydrochemical genesis of geothermal water in the Zhangye–Minle Basin [35]. As shown in Figure 7a, all nine geothermal samples have (K⁺ + Na⁺)/[(K⁺ + Na⁺) + Ca²⁺] ratios greater than 0.5, with the majority ranging from 0.77 to 0.96. The corresponding TDS values are concentrated between 1000 mg/L and 9000 mg/L. These data points are mainly distributed in the upper-middle part of the diagram, within the evaporation crystallization zone, suggesting that evaporation plays a dominant role in regulating the chemical evolution of the geothermal water. In particular, samples A5 and A7 from Linze County, and A6, A8, and A9 from Zhangye City, exhibit higher TDS values (>3000 mg/L) and elevated (K⁺ + Na⁺)/[(K⁺ + Na⁺) + Ca²⁺] ratios (>0.8), indicating intensive evaporative concentration in these areas. Samples A1–A3 from Minle County and A4 from the central basin show slightly lower TDS values (mostly 600–1500 mg/L) while still maintaining high (Na⁺ + K⁺) proportions, placing them near the boundary between rock dominance and evaporation dominance fields. This reflects a coupled process where water–rock interaction contributes to the release of Na⁺ and K⁺, and subsequent evaporation further concentrates solutes.

In Figure 7b, Cl⁻/(Cl⁻ + HCO₃⁻) ratios fall between 0.64 and 0.92, with most samples above 0.7. This distribution again places nearly all samples within the evaporation dominance region. Samples A8 and A9 from Zhangye City are particularly notable, with Cl⁻/(Cl⁻ + HCO₃⁻) ratios exceeding 0.9 and TDS above 4000 mg/L, consistent with advanced evaporative concentration. Similarly, samples A5 and A7 from Linze County and A4 from Minle County exhibit elevated TDS (>2500 mg/L) and Cl⁻ enrichment, reinforcing the influence of evaporative crystallization. In contrast, samples A2 and A3 from Minle County show relatively lower TDS (below 1500 mg/L) and Cl⁻/(Cl⁻ + HCO₃⁻) ratios around 0.6–0.7, indicating that water–rock interaction remains a significant contributor in these areas, with less pronounced evaporation effects. The strong correlation between increasing TDS and both ion ratios across the dataset confirms that evaporation crystallization is the dominant mechanism for solute accumulation, while ion exchange and mineral dissolution also play secondary roles in shaping groundwater chemistry.

To better understand the hydrogeochemical processes influencing geothermal water composition, a correlation analysis was conducted among major ions and physicochemical parameters. The results highlight distinct interrelationships that reflect underlying geochemical mechanisms, as visualized in the ion correlation heatmap (Figure 8). A notably strong positive correlation is observed between Na⁺ and TDS (r = 0.89), indicating that sodium is the dominant contributor to the total dissolved solids in the geothermal system. Similar strong correlations are found between TDS and K⁺ (r = 0.81), Mg²⁺ (r = 0.82), Cl⁻ (r = 0.71), and SO₄²⁻ (r = 0.68), underscoring their roles as primary dissolved constituents derived from mineral dissolution and evaporative concentration. The correlation between Mg²⁺ and Ca²⁺ (r = 0.79) suggests a common origin, likely from the dissolution of carbonate or silicate minerals. Mg²⁺ also correlates strongly with SO₄²⁻ (r = 0.66) and moderately with Na⁺ (r = 0.52), pointing to concurrent contributions from gypsum dissolution and cation exchange. The positive relationship between Na⁺ and HCO₃⁻ (r = 0.68) implies that sodium enrichment is accompanied by bicarbonate release, possibly through silicate weathering and exchange reactions involving Ca²⁺. In contrast, the negative correlation between Ca²⁺ and HCO₃⁻ (r = –0.34) supports this mechanism, as Ca²⁺ is likely displaced from exchange sites while HCO₃⁻ accumulates in the aqueous phase. Moderate correlations between Cl⁻ and Na⁺ (r = 0.58), as well as with Ca²⁺ (r = 0.69), suggest conservative transport behavior and potential contributions from halite dissolution. The slightly negative association between Cl⁻ and HCO₃⁻ (r = –0.13) further indicates differing hydrochemical origins for these anions. pH exhibits a consistent negative correlation with most major ions, particularly Na⁺ (r = –0.53) and TDS (r = –0.55), implying that increased ionic strength may be associated with lower pH values due to acid-base buffering or intensified water–rock interactions. Overall, the correlation structure reveals that the chemical composition of geothermal water is predominantly governed by processes such as mineral dissolution, cation exchange, evaporation, and silicate weathering, providing a robust basis for interpreting geothermal hydrochemistry in the study area.

3.4.2. Chemical Formation Mechanism

The origins of major ions in geothermal water were further investigated by examining the proportional relationships between representative cations and anions (Figure 9), combined with mineral dissolution reactions (Equations (6) and (7)) [36]. As shown in Figure 9a, the majority of the geothermal water samples plot near or slightly above the 1:1 fitting line of γNa⁺/γCl⁻, indicating that Na⁺ and Cl⁻ were primarily derived from the dissolution of halite (NaCl) in rock salt. A4 from Minle County and A5 from Linze County display the highest concentrations of Na⁺ and Cl⁻, suggesting strong halite dissolution, while A3 and A1 from Minle County and A9 from Zhangye City exhibit relatively lower values, reflecting weaker input from halite. However, the γ(Na⁺ + K⁺)/γCl⁻ ratios of most samples exceed 1.0, implying that silicate weathering may also contribute to Na⁺ enrichment. As shown in Figure 9b, the γNa⁺/γSO₄²⁻ ratios of most samples approximate the 2:1 fitting line, which is consistent with the stoichiometry of mirabilite (Na₂SO₄) dissolution. A5 (Linze County) and A8 (Zhangye City) fall above this line, suggesting additional Na⁺ contributions from albite weathering (Equation (6)). This is further supported by the presence of granite intrusions in the basin, providing a potential source of sodium-bearing silicate minerals.

$$\begin{array}{c} \mathrm{NaCl}(Rock salt)⟷{\mathrm{Na}}^{+}+{\mathrm{Cl}}^{-}\\ 2{\mathrm{NaAlSi}}_3{\mathrm{O}}_8(Albite)+2{\mathrm{CO}}_2+11{\mathrm{H}}_2\mathrm{O}⟷2{\mathrm{Na}}^{+}+{\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4(Kaolinite)\\ {+4\mathrm{H}}_4{\mathrm{SiO}}_4+2{{\mathrm{HCO}}_3}^{-}\\ { \mathrm{Na}}_2{\mathrm{SO}}_4(Mirabilite)⟷2{\mathrm{Na}}^{+}+{{\mathrm{SO}}_4}^{2-}\end{array}$$

(6)

$$\begin{array}{c}{\mathrm{CaSO}}_{4}2{\mathrm{H}}_2\mathrm{O}(Gypsum)⟷{\mathrm{Ca}}^{2+}+{\mathrm{SO}}_4{}^{2+}+2{\mathrm{H}}_2\mathrm{O}\\ {\mathrm{CaCO}}_3(Calcite)+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}⟷{\mathrm{Ca}}^{2+}+2{{\mathrm{HCO}}_3}^{-}\\ \mathrm{CaMg}{\left({\mathrm{CO}}_3\right)}_2(Dolomite)+2{\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}⟷{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}+4{{\mathrm{HCO}}_3}^{-}\end{array}$$

(7)

In contrast, the relationship between Ca²⁺ and SO₄²⁻ (Figure 9c) reveals that most samples lie below the 1:1 line, suggesting that SO₄²⁻ predominantly originates from gypsum (CaSO₄·2H₂O) dissolution, while Ca²⁺ is partially removed through secondary processes such as carbonate precipitation. A4 (Minle) is an exception, closely aligned with the 1:1 line, indicating a stronger contribution from gypsum. A7 (Linze) and A9 (Zhangye) show distinctly low Ca²⁺ concentrations relative to SO₄²⁻, consistent with ion removal or alternative SO₄²⁻ sources such as mirabilite. The relationships involving bicarbonate and divalent cations further clarify the sources of Ca²⁺ and Mg²⁺. As shown in Figure 9d, most samples plot above the 1:1 and below the 1:2 line of γCa²⁺/γHCO₃⁻, indicating that calcite dissolution is not the sole contributor to Ca²⁺. This is corroborated by A3 and A4 (Minle), which are located closest to the 1:2 line, indicating weak calcite dissolution, while A8 and A9 (Zhangye) fall below it, implying limited carbonate contribution.

In Figure 9e, the γ(Ca²⁺ + Mg²⁺)/γHCO₃⁻ ratios for most samples range between the 1:1 and 2:1 lines, indicating that dolomite and calcite are not the dominant sources of Ca²⁺ and Mg²⁺. Notably, A1 and A3 (Minle) display relatively higher values, suggesting greater influence from dolomite weathering. Meanwhile, A7 (Linze) and A9 (Zhangye) show lower γ(Ca²⁺ + Mg²⁺)/γHCO₃⁻ ratios, possibly due to Mg²⁺ retention in mineral matrices. Figure 9f integrates the contributions of SO₄²⁻ and HCO₃⁻ to estimate the net influence of dolomite and sulfate mineral dissolution. The samples generally cluster near the 1:1 line, with A4 and A3 (Minle) closest, indicating that the combined weathering of dolomite and gypsum governs the concentrations of Ca²⁺ and Mg²⁺. A6 (Zhangye) and A2 (Minle) slightly deviate from the line, reflecting minor influence from silicate weathering. Collectively, the analysis confirms that Na⁺ in the Zhangye–Minle Basin geothermal water primarily derives from halite dissolution, with supplementary input from albite and mirabilite. SO₄²⁻ mainly originates from gypsum and mirabilite, while Ca²⁺ and Mg²⁺ are predominantly controlled by the dissolution of carbonates and sulfates rather than cation exchange. This is supported by the absence of reverse cationic trends in Figure 9e,f. The distinct distribution patterns across counties (A1–A4 in Minle, A5 and A7 in Linze, and A6, A8–A9 in Zhangye) highlight the spatial variability of hydrogeochemical processes influenced by lithology, structural controls, and recharge pathways.

To further evaluate the presence and extent of cation exchange processes in the geothermal system, two ionic ratio diagrams were constructed as shown in Figure 10. These diagrams are interpreted in conjunction with Equation (8), which describes the mechanism of ion exchange between Na⁺ and alkaline earth metals during groundwater–rock interaction [37]. Figure 10a displays the relationship between γ(Na⁺ − Cl⁻) and γ[(Ca²⁺ + Mg²⁺) − (HCO₃⁻ + SO₄²⁻)], where γ(Na⁺ − Cl⁻) quantifies the surplus of Na⁺ beyond halite dissolution, and γ[(Ca²⁺ + Mg²⁺) − (HCO₃⁻ + SO₄²⁻)] indicates the potential contribution of Ca²⁺ and Mg²⁺ exceeding their associated anions. A strong negative correlation is observed (R² = 0.96), and the majority of samples are distributed near or above the cation exchange line, suggesting that Na⁺ enrichment and Ca²⁺ and Mg²⁺ depletion occurred as a result of cation exchange. The spatial distribution of the samples further supports this interpretation. Samples A1, A2, A3, and A4 located in Minle County, as well as A5 and A7 from Linze County, exhibit moderate deviations that are consistent with partial cation exchange processes. In contrast, samples A6, A8, and A9 from Zhangye City lie in the upper-right quadrant of the plot and are characterized by significant Na⁺ enrichment and strong depletion of Ca²⁺ and Mg²⁺, indicating a more intense exchange reaction possibly associated with longer water–rock interaction pathways and deeper circulation. Sample A4 shows a distinct pattern, falling below the fitting line with negative values on both axes, suggesting that ion exchange is not the dominant process in this case and that the ionic composition may be controlled more by carbonate dissolution.

Further validation is provided by Figure 10b, which displays γ[(Ca²⁺ + Mg²⁺) − (Na⁺ + K⁺)] versus γ[HCO₃⁻ − (SO₄²⁻ + Cl⁻)]. This diagram helps to differentiate between ion exchange and mineral dissolution as the controlling processes. Most samples plot in the lower-left quadrant, supporting the conclusion that Na⁺ accumulation is not accompanied by significant contributions from Ca²⁺ and Mg²⁺, which is consistent with the exchange of these divalent cations for Na⁺. This is particularly evident in samples A6, A8, and A9, which again show the strongest signals of ion exchange. The results from both subfigures, in combination with Equation (8), confirm that cation exchange is an important geochemical mechanism in the Zhangye–Minle geothermal system, especially in the central and northern portions of the basin where deeper circulation and prolonged water–rock interactions enhance this process.

$${\mathrm{Ca}}^{2+}or{ \mathrm{Mg}}^{2+}+{\mathrm{Na}}_2\mathrm{X}(Medium)⟷2{\mathrm{Na}}^{+}+\mathrm{CaX}or \mathrm{MgX}$$

(8)

To further evaluate the role of cation exchange in the hydrochemical evolution of geothermal water in the Zhangye and Minle Basin, the Chloro-Alkaline Indices CAI-I and CAI-II were calculated [38,39,40], as presented in Figure 11. These indices are useful indicators for identifying the occurrence and direction of cation exchange reactions. When both CAI-I and CAI-II values are negative, the results suggest a normal exchange process in which Ca²⁺ and Mg²⁺ in groundwater are exchanged with Na⁺ and K⁺ adsorbed onto mineral surfaces. Conversely, positive values for both indices suggest a reverse exchange, where Na⁺ and K⁺ in groundwater are replaced by Ca²⁺ and Mg²⁺ released from the aquifer matrix.

As illustrated in Figure 11a, seven of the nine groundwater samples fall within the lower-left quadrant, where both CAI-I and CAI-II are negative. These include A1, A2, A3, A4 from Minle County, A5 from Linze County, and A6, A8, A9 from Zhangye City. This pattern indicates that a normal cation exchange process is prevalent throughout most parts of the study area. Sample A3, located in Minle County, is close to the vertical axis but still exhibits a negative CAI-II value, reflecting a weaker but consistent exchange effect. In contrast, sample A7 from Linze County is the only one showing positive values for both indices, suggesting that reverse cation exchange may be occurring locally. In this case, Na⁺ and K⁺ in groundwater are being replaced by Ca²⁺ and Mg²⁺ derived from the surrounding rock matrix. This interpretation is supported by the observed ionic concentrations at A7, where Ca²⁺ is relatively high and Na⁺ is comparatively low.

Figure 11b further illustrates the distribution of CAI values among the samples. Most points cluster near the zero line, indicating that the overall intensity of cation exchange in the basin is limited. Although both normal and reverse exchange processes are evident in different locations, their magnitude is not sufficient to dominate the chemical composition of geothermal water. Therefore, cation exchange acts as a secondary geochemical process in this region and is likely superimposed upon other mechanisms such as mineral dissolution and water–rock interaction.

$$\begin{array}{c}CAI-I={\displaystyle \frac{{\mathrm{Cl}}^{-}-({\mathrm{Na}}^{+}+K^{+})}{{\mathrm{Cl}}^{-}}}\\ CAI-II={\displaystyle \frac{{\mathrm{Cl}}^{-}-({\mathrm{Na}}^{+}+K^{+})}{{{\mathrm{SO}}_4}^{2-}+{{\mathrm{HCO}}_3}^{-}+{{\mathrm{CO}}_3}^{2-}+{{\mathrm{NO}}_3}^{-}}}\end{array}$$

(9)

To further investigate the hydrogeochemical evolution of geothermal water in the Zhangye–Minle Basin, the ionic ratios γMg²⁺/γCa²⁺ and γMg²⁺/γNa⁺ were analyzed, as illustrated in Figure 12. These ratios reflect the dominant geochemical processes such as the dissolution of sodium and calcium salts, the intensity of water–rock interaction, and possible evaporative effects. The spatial distribution and chemical signatures of the nine sampling sites demonstrate distinct hydrogeochemical pathways corresponding to their geographic locations.

Samples A1, A2, A3, and A4 are located in Minle County. Sample A1 shows a relatively low γMg²⁺/γNa⁺ ratio and a moderate γMg²⁺/γCa²⁺ ratio, indicating that its water chemistry is influenced by the combined dissolution of albite and gypsum. A2 and A3 display similar but slightly lower γMg²⁺/γNa⁺ ratios, suggesting a more pronounced contribution from sodium salt dissolution. In particular, A3 shows a lower γMg²⁺/γCa²⁺ ratio, implying a stronger influence from the dissolution of calcium-bearing minerals such as calcite or dolomite. A4 lies in the lower-right quadrant of the diagram and shows the lowest values for both γMg²⁺/γCa²⁺ and γMg²⁺/γNa⁺, reflecting a stronger influence from calcium salt dissolution, likely involving gypsum or calcite, combined with relatively elevated Na⁺ input. Its geochemical characteristics indicate that evaporative concentration and the dissolution of highly soluble salts may be more influential than prolonged water–rock interactions. Samples A5 and A7 from Linze County and A6 from Zhangye City exhibit higher γMg²⁺/γCa²⁺ ratios and relatively low γMg²⁺/γNa⁺ values. This distribution suggests that the groundwater in these areas is primarily controlled by the weathering of magnesium-rich carbonates, while the influence from sodium-bearing minerals is comparatively limited. Among them, A6 has the lowest γMg²⁺/γNa⁺ value, indicating a relatively greater input from sodium sources than the others in the same region. Samples A8 and A9, also located in Zhangye City, are positioned in the upper-left area of the diagram. They exhibit the highest γMg²⁺/γCa²⁺ values and moderate γMg²⁺/γNa⁺ ratios. These characteristics suggest a dominant influence of dolomite weathering and a limited contribution from sodium salts. The elevated Mg²⁺ concentrations, coupled with comparatively lower Na⁺ values, indicate that water–rock interactions involving magnesium-bearing carbonates play a central role in this area.

In summary, the ion ratio patterns presented in Figure 12 reveal considerable spatial variability in the controlling geochemical processes across the Zhangye–Minle Basin. Groundwater in Zhangye City (A6, A8, and A9) is primarily influenced by dolomite dissolution, while in Linze County (A5 and A7), magnesium-carbonate weathering is the dominant process. In contrast, samples from Minle County (A1–A4) reflect a more complex interplay of halite, gypsum, and carbonate mineral dissolution, indicating that multiple geochemical processes jointly shape the composition of geothermal water in this region.

3.5. Discussion

The spatial co-variation in temperature and salinity in the Zhangye–Minle Basin is most consistently explained by circulation guided by north-northwest trending fault zones within a two-domain structural framework that distinguishes the western uplift from the central depression. Wells in the depression exhibit steeper temperature gradients with depth and higher maximum well temperatures than wells on the uplift. A basinward increase in total dissolved solids is observed, and the samples plot predominantly within evaporation-dominant fields on Gibbs diagrams. These relationships indicate longer residence times and deeper flow paths in the depocenter, where high-permeability fault damage zones induce upward flow. Interaction with Neogene successions that include evaporitic interbeds accounts for conservative chloride enrichment and sustained sulfate loads along the inferred paths.

Thermometric bounds are constrained by internal consistency within the cation-geothermometry framework adopted in this study. Concordant Na–K and K–Mg estimates are retained as quantitative temperature brackets. Divergent Na–K and K–Mg pairs are interpreted qualitatively as indicators of thermal maturity, and Na–K–Ca outliers are excluded from quantitative use. Under these criteria, K–Mg predictions agree closely with measured temperatures at representative sites in Linze and Zhangye City, exemplified by wells A5, A6, and A7. The agreement supports the bracketed reservoir temperatures and is compatible with circulation focused by fault-controlled pathways.

Independent depth estimates obtained from the governing equations indicate that flow extends several hundred meters beneath the mapped reservoir base. This depth extent is consistent with fracture-controlled conduits and explains the steepening of temperature-depth curves toward the basin center. The conjunction of higher temperatures, elevated salinity, and evaporation-biased chemistry therefore reflects fault-controlled channelization of upward flow together with residence-time control on water–rock interactions.

These findings have practical implications for appraisal and development. Production targeting should prioritize corridors proximal to mapped north-northwest faults along the flanks of the central depression where higher temperatures and transmissive horizons are expected. Well spacing that accounts for structural anisotropy can reduce thermal interference, and reinjection positioned down-gradient within the depression can maintain pressure support with limited risk of premature cooling. The evaporation-biased chemistry, together with chloride and sulfate enrichment, warrants materials selection and operational control aimed at scaling and corrosion management. Concordant geothermometers constrain the reservoir temperature to a finite range; for engineering design we use the lower bound of this range as a conservative design temperature for direct use systems. Continued surveillance of temperature and major-ion chemistry at key wells will refine operational strategies within the methodological scope established in this study.

4. Conclusions

This study integrated hydrochemical analysis, isotopic tracing, and ion-ratio diagnostics to resolve the composition, origin, and formation mechanisms of geothermal water in the Zhangye–Minle Basin.

(1) Geothermal waters exhibit elevated Na⁺, Cl⁻, and SO₄²⁻. The γ(Na⁺ + K⁺)/γCl⁻ ratios mostly exceed 1, and Cl⁻/(Cl⁻ + HCO₃⁻) values are generally above 0.7. Together with the Piper classification, with seven of nine samples as Cl·SO₄₋Na and two as Cl-Na, these signatures indicate mixed evaporite dissolution (halite, gypsum, mirabilite) accompanied by prolonged water–rock interaction during deep circulation.

(2) Stable isotopes trace recharge to high-elevation precipitation and snowmelt in the Qilian Mountains, with recharge elevations from 2497.09 m to 5798.96 m. Tritium indicates that most waters were recharged before 1953, and the oldest radiocarbon age reaches 40,160 years, evidencing deep and long-residence circulation. Shallow unconfined aquifers fringe the alluvial and proluvial fans, whereas confined artesian aquifers occupy the basin center.

(3) Ion sources are consistent with mineral control: Na⁺ mainly from albite, mirabilite, and halite; SO₄²⁻ from gypsum and mirabilite; Cl⁻ from halite; and Ca²⁺ and Mg²⁺ from calcite and dolomite. Ion-ratio diagnostics, together with the Gibbs fields and cation-exchange indices, show mineral dissolution as the primary control and cation exchange as a limited secondary process. The composite ratios γ(Na⁺ − Cl⁻)/γ[(Ca²⁺ + Mg²⁺) − (HCO₃⁻ + SO₄²⁻)] and γ[(Ca²⁺ + Mg²⁺) − (Na⁺ + K⁺)]/γ[HCO₃⁻ − (SO₄²⁻ + Cl⁻)] reinforce this interpretation.

Collectively, these results clarify the genesis, recharge pathways, and circulation architecture of geothermal water in an arid inland basin and provide a robust basis for geothermal resource assessment and sustainable development in northwestern China.