Hydrochemical Processes, Mineral Scaling and Water Quality of Geothermal Waters in Sichuan Basin, Southwestern China

Abstract

Geothermal resources are significant natural resources for achieving carbon neutrality. In this study, we collected eight groups of geothermal water samples from a Sichuan sedimentary basin. Major and trace elements were measured for hydrochemical analysis. SO₄²⁻ and Ca²⁺ are the major anion and cation, respectively, in geothermal waters with the hydrochemical type Ca–SO₄. The dissolution of calcite and gypsum, silicate weathering and positive cation exchange were responsible for hydrochemical processes. Saturation indices showed the unsaturated affinity of geothermal waters. Carbonate scaling would be the main problem during geothermal exploitation. The water quality index indicated that most of the geothermal water samples, except G3 and G8, were suitable for drinking purposes. The poor water quality of the G3 and G8 samples was attributed to elevated Na⁺ and K⁺ concentrations. The weights of affecting factors followed the order of NH₄⁺ (3.803) > Cl⁻ (2.823) > Na⁺ (2.677) > pH (2.224) > Ca²⁺ (1.506) > SO₄²⁻ (1.169) > F⁻ (1.127) > Mg²⁺ (0.850) > TDS (0.808). The results of this study provide an important insight for geothermal exploitation in sedimentary basins worldwide.

1. Introduction

Geothermal resources, as a sustainable and renewable energy source, play a pivotal role in the global transition from fossil fuels, offering significant potential for mitigating climate change and enhancing energy security [1,2,3,4,5,6]. Research in this domain spans diverse areas, including geothermal reservoir exploration and characterization [7,8,9], as well as the assessment of environmental impacts to optimize resource utilization and address management challenges [10,11]. Advancing geothermal energy research is essential to unlocking its full potential, thereby contributing to a more resilient and sustainable energy future.

Hydrochemical analysis of geothermal resources is essential for elucidating the geochemical processes governing the formation, composition, and behavior of geothermal fluids [7,12,13]. This approach provides critical insights into reservoir characteristics, such as temperature, pressure, and mineral content, which are fundamental for optimizing resource management and utilization [14,15]. Through the analysis of geothermal water chemistry, researchers can trace heat and solute sources, evaluate fluid–rock interactions, and assess risks of scaling or corrosion in production systems [9,16,17]. Furthermore, hydrochemical analysis is indispensable for environmental impact assessments, ensuring geothermal development adheres to ecological and regulatory standards [18,19]. As a foundational tool, it advances the sustainable and efficient exploitation of geothermal resources, supporting energy security and climate change mitigation efforts.

Sedimentary basins, characterized by stratified sedimentary rock layers, often harbor substantial geothermal resources due to their favorable thermal and fluid dynamic conditions [20,21]. A comprehensive understanding of these properties is critical for assessing geothermal energy potential, as it provides insights into heat generation mechanisms, hydrothermal circulation patterns, and the role of geological formations in fluid distribution [22,23,24]. Furthermore, the frequent overlap of sedimentary basins with densely populated and industrialized regions underscores their strategic importance for sustainable energy development and fossil fuel reduction [25,26]. Advancing research in this field not only supports energy diversification but also enhances environmental sustainability and resilience against climate change.

Sichuan basin is the largest sedimentary basin in southwestern China. Amounts of medium–low temperature geothermal resources have been investigated so far [27,28]. However, the knowledge of scaling trend and environmental quality of geothermal resources remain unclear, hampering the development of geothermal exploitation. To address the above issues, this study aims to: (1) clarify the hydrochemical type and water–rock interactions dominating hydrochemical compositions, (2) analyze the mineral equilibrium status and scaling trends, and (3) evaluate the environmental water quality and affecting factors. The achievements of this study would provide a valuable information for geothermal exploitation in sedimentary basins worldwide.

2. Geological and Geothermal Setting

The study area is located in the northwest part of the Sichuan Basin (Figure 1). It starts from Guangyuan City in the north, extends to Leshan City in the south, and reaches Ya’an City in the west. The hot spring sites are mainly distributed at the junction of the Sichuan Western Plateau and the basin. The administrative divisions where the hot spring sites are located from north to south include Meishan, Ya’an and Leshan Cities. The study area belongs to the humid subtropical southeast monsoon climate. It is rich in hot water resources. Its main characteristics are warm climate and abundant precipitation, with four distinct seasons, long summers and short autumns, low frost and snow, and high humidity.

The Sichuan Basin covers an area of approximately 22 × 10⁴ km² and is a large self–flowing basin of significant size in China. The upper part is composed of red terrestrial sedimentary layers of the Jurassic and Cretaceous periods, with the main lithology being thick sandstone and mudstone, featuring numerous layers and intense facies changes. The underlying Triassic and Permian strata are mainly marine carbonate deposits, with relatively stable thickness and distribution, forming the main water–bearing strata of the Sichuan Basin. They are mainly exposed at the basin margins, partially in the Wudong Fold Belt, and mostly buried deep underground. Most areas within the Sichuan Basin lack the Devonian and Carboniferous strata, while the Silurian, Ordovician and Cambrian strata of the Lower Paleozoic are relatively well–developed, mainly consisting of carbonate and clastic rocks. These strata are exposed at the periphery of the basin and are deeply buried within it. The metamorphic rocks and various magmatic rocks of the pre–Devonian period are only exposed in the mountainous areas of the northern and western parts of the basin and in southwestern Sichuan. The strata of the Upper Triassic series in the basin ended marine deposition and under the humid and hot climate conditions, deposited the coal–bearing strata of the Xuzhahe Formation of the Triassic; under the arid climate, thick red clastic rocks of lacustrine facies of the Jurassic and Cretaceous were deposited. Therefore, it is called the “Red Basin of Sichuan”. The majority of the later basin areas were eroded, and the Paleogene and Neogene strata were largely absent in the Sichuan Basin, scattered and accumulated in the intermontane basins and foothills, with the main lithology being consolidated and semi–consolidated sandstone and shale. The Quaternary strata in the western part of the basin are the most developed, forming the western plains of the Sichuan Basin, while the rest are scattered and distributed in modern river valleys and their terraces on both sides.

3. Materials and Methods

A total of 8 sets of geothermal water samples were collected in this study, including the Ermeishan radon hot spring, Ermeishan Well 3, Well 5, and Meiziwan hot spring in the Ermeishan area; the Jiuliiping Huasheng hot spring, Jiuliiping Half Mountain hot spring, and Taoyuan hot spring in Hongya County; and Zhougongshan hot spring in Yucheng District. The sampling points were selected as the main spring outlets with the largest flow rate in the hot spring areas, aiming to represent the deep geothermal water samples with relatively weaker influence from the shallow or surface environment. The sampling depth ranges from G1 to G8 to 1652 m, 2110 m, 2434 m, 0 m, 0 m, 2516 m, 2528 m, 1539 m and 3475 m, respectively. The temperature and pH, which are easily variable water chemical parameters, were measured on-site using the German Multi3630IDS portable multi-parameter water quality detection analyzer. The water samples were stored in polyethylene sampling bottles that were rinsed at least three times to ensure that the water samples were filled and to avoid the generation of bubbles. The concentration of HCO₃⁻ was determined by titration with hydrochloric acid on-site. All the tested water samples were sent to Beijing Kehe Testing Technology Co., Ltd. (Beijing, China) The main cations (Na⁺, K⁺, Ca²⁺, Mg²⁺, Li, B, Sr) and H₂SiO₃ were determined by ICP-OES (ICAP6000, ThermoFisher Scientific, Waltham, MA, USA), and the anions were determined by IC (Dionex ICS600). The precision of the main cation and the anion were 8% and 5%, respectively, and the error was ±2‰ (

Table 1. Sample water chemistry basic information table.

Sampling Number	pH	TDS	Na+	K+	Ca2+	Mg2+	Li	Br
G1	7.7	300.5	11	5.5	34.07	18.85	0.09	0.15
G2	7.2	3443.8	98	26.5	611.2	197.6	0.21	1.57
G3	7.2	4503.6	520	51	586.2	167.2	1.48	6.46
G4	7.3	2973.5	265	4.3	460.9	152	0.05	0.59
G5	6.5	368.8	12.5	1.5	52.1	30.4	0.02	0.15
G6	7.2	938.6	46	8	184.4	36.48	0.05	0.15
G7	7.2	3218.7	15	12	633.3	198.3	0.02	0.15
G8	7.4	17,223.6	5300	310	621.2	142.9	11.7	115
Sampling Number	Sr	NH4+	HCO3−	SO42−	Cl−	F−	SiO2	B
G1	0.76	0.17	177	41.6	0.35	1.18	12.28	0.68
G2	12.81	0.02	305.1	1976	200.4	2.4	26.98	3.75
G3	13.02	0.03	274.6	2208	654.1	2.51	40.41	4.30
G4	8.02	0.02	152.5	1472	531.9	0.39	10.77	0.60
G5	0.05	0.02	131.2	778	1.42	1.37	23.39	0.21
G6	2.92	0.13	131.2	590	2.84	0.98	3.21	0.23
G7	14.75	0.02	164.8	2152	13.12	1.85	23.9	1.91
G8	21	2.48	262.4	2980	7516	5.6	66.7	43.18

).

4. Hydrochemical Results

Based on the data processing results, the pH values of the water samples collected in the study area are 6.5 to 7.7. And the TDS values varied from 300.5 to 17,223.6 mg/L. Except for the water sample G8 whose main cation is Na⁺ (5300 mg/L), the main cations of the other water samples are all Ca²⁺, among which the Ca²⁺ content of Taoyuan Hot Spring is the highest (633.3 mg/L). The main anion of G1 is HCO₃⁻, the main anions of G2–G7 are SO₄²⁻, and the main anion of G8 is Cl⁻.

These results can be clearly observed in Figure 2. G1 is a Ca–HCO₃ type, G2–G7 are Ca–SO₄ type, and G8 is Na–Cl-type water. This indicates that there are Ca–SO₄-type geothermal waters with high salinity in the study area.

5. Discussion

5.1. Main Processes Affecting Hydrochemistry

The geochemical properties of Cl⁻ are relatively stable, and it is not prone to interact with the surrounding rocks during the migration of geothermal water [29]. Therefore, it can be used to indicate the water–rock reactions of geothermal water. In Figure 3, Na⁺ and K⁺ in geothermal water show a certain correlation with Cl⁻, thus preliminarily revealing the presence of deep components mixed in the geothermal water in the study area [30]. It was also observed that there is a linear relationship between Ca, Mg, and SO₄²⁻ and Cl⁻ (Figure 3c–e), but the squared regression coefficients (0.64, 0.66, and 0.60) are significantly lower. The reason for this result may be that these components may also be obtained from mixture or affected by water–rock interactions and ion exchange [31]. Figure 3g shows that Cl⁻ in geothermal water also has a good correlation with Sr (linear regression coefficient of 0.55). Since Cl⁻ mainly originates from the deep part, in addition to the dissolution of related minerals during the water–rock interaction process, Sr may also come from deep geothermal fluids [32]. B and Li are considered conservative elements [30] and are closely related to deep fluids. Both of them have a certain correlation with Cl⁻ (Figure 3h,i). It is speculated that deep circulating geothermal water exists in the water samples in the study area and is mixed with deep fluids to obtain some B and Li ions. Furthermore, SiO₂ has a poor correlation with Cl⁻ (Figure 3f), and the points are scattered randomly, indicating that both SiO₂ and Cl⁻ have multiple sources and interact with each other among different sources.

When the ratio of (K⁺ + Na⁺)/Cl⁻ approaches 1, it indicates that the Na⁺ and K⁺ in the water mainly originate from the dissolution of halite. As shown in Figure 4a, all the geothermal water sample points in the study area, except for G8, are near the y = x line, suggesting that the Na⁺ and K⁺ in the water are derived from the dissolution of halite [33]. Considering that there are sandstone and slate and other rocks distributed in the study area, the enriched Na⁺ and K⁺ in the geothermal water of G8 might be caused by the leaching of silicate minerals [9]. To reveal the weathering of silicate minerals, the thermodynamic activity diagrams were drawn for analysis (Equations (1) and (2)), and the phase boundaries were plotted as 100 °C (black) and 200 °C (red) [34]. It can be seen that almost all the geothermal water draws near the junction of kaolinite and muscovite (Figure 4b,c), indicating that the enriched Na⁺ and K⁺ in the geothermal water are mainly provided by the weathering of the above silicate minerals.

When the molar ratio of Ca²⁺ + Mg²⁺/HCO₃⁻ in groundwater is close to 0.5 (Equations (3) and (4)), these ions mainly originate from the dissolution of calcite or dolomite [34,35,36]. Figure 4d shows that the water samples in the study area are basically distributed near the dissolution lines of dolomite and calcite, indicating that the dissolution of calcite and some dolomite has occurred in the geothermal water. This is consistent with the occurrence of limestone and dolomite in the study area. In Figure 4e, the water samples collected in the study area are all on or near the 1:1 line, suggesting the dissolution of gypsum in these water samples.

As can be seen in Figure 4f, the fitted slope of the geothermal water samples is −0.82, and the linear regression coefficient is 0.75. This indicates that the cation exchange effect is an important factor influencing the water chemical formation process of geothermal water [33,35,36]. Meanwhile, except for G5, all the other water samples are located around the straight line with a slope of 1, suggesting that the water–rock interaction in G5 is relatively weak. Its low TDS also confirms this.

Furthermore, Chloro-Alkaline Indices (CAI) are indicators used to evaluate the ion exchange process and hydrochemical characteristics in water and mainly reflect the relationship between chloride ions and alkalinity in water. CAI-I and CAI-II (Equations (5) and (6)) can confirm the type of cation exchange [34]. The calculation results are shown in

Table 2. CAI-I and CAI-II indices for water samples in the study area.

Serial Number	CAI-I (No Unit)	CAI-II (No Unit)
G1	−61.70942	−0.161724
G2	0.1260492	0.0154428
G3	−0.296539	−0.108407
G4	0.2244371	0.1015928
G5	−13.53145	−0.02954
G6	−26.52959	−0.147242
G7	−1.592181	−0.012404
G8	−0.124738	−0.398618

Note: CAI-I and CAI-II index have no unit.

. It can be observed that the chloro-alkaline indices of G1, G3, G5, G6, G7, and G8 are all negative, indicating that positive cation exchange has occurred. This means that Ca²⁺ in geothermal water has been replaced by Na⁺ in the surrounding rock. It is consistent with the presence of sandstone and limestone in the formation of G1 and G3. Meanwhile, this result provides evidence that G8 water samples are Na-Cl type water. For G2 and G4, the chloro-alkaline indices of all sample points are positive, indicating that reverse cation exchange has occurred. This means that Na⁺ in geothermal water has been replaced by Ca²⁺ in the surrounding rock [37].

2NaAlSi₃O₈ + 3H₂O + CO₂ → Al₂ (Si₂O₅)(OH)₄ + 4SiO₂ + 2Na⁺ + 2HCO₃⁻,

(1)

2KAlSi₃O₈ + 3H₂O + CO₂ → Al₂(Si₂O₅)(OH)₄ + 4SiO₂ + 2K⁺ + 2HCO₃⁻,

(2)

CaMg(CO₃)₂ + 2CO₂ + 2H₂O → Ca²⁺ + Mg²⁺ + 4HCO₃⁻

(3)

CaCO₃ + CO₂ + H₂O → Ca²⁺ + 2HCO₃⁻

(4)

CAI-I = (Cl⁻ − (Na⁺ + K⁺))/Cl⁻

(5)

CAI-II = (Cl⁻ − (Na⁺ + K⁺))/(HCO₃⁻ + CO₃²⁻ + SO₄²⁻ + NO₃⁻

(6)

5.2. Mineral Equilibrium Status of Geothermal Waters

The mineral saturation index (SI) is of great significance for evaluating the balance and reactivity between minerals and groundwater [12]. During the water–rock interaction process, the mineral balance can reflect the thermodynamic processes in natural water systems [35]. In this study, the SI values of the main minerals in the study area were calculated and evaluated using the PHREEQC 3.0 software [38]. The selected parameters in this study were temperature, pH, Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻, F, NH₄⁺, HCO₃⁻, Si, Sr, B, Li and Br, and the conditional temperature and pH were 25 °C and 7 °C, respectively.

The calculation results are shown in Figure 5. Mineral equilibrium status of geothermal waters has not reached over-saturation. The SI values of calcite, aragonite and dolomite in most of the geothermal water are all greater than 0, indicating supersaturation conforming to the extensive distribution characteristic of carbonate rocks in the study area. The SI values of quartz and chalcedony are close to 0, indicating a basically saturated state. However, the SI values of the remaining minerals in the geothermal water are all less than 0, indicating an unsaturated state. This is related to the lack of gypsum, rock salt, and other sulfate or silicate minerals in the strata of the study area.

5.3. Mineral Scaling Trends of Geothermal Waters

Corrosion is one of the main problems in the utilization and production process of geothermal fluids [39]. When the scale-forming phase is present in the geothermal circulation, it will precipitate and form scale when the solubility decreases and becomes supersaturated due to gas precipitation or changes in temperature and/or pressure, as well as the mixing of water solutions with different chemical compositions [12]. When the scale formation is excessive, it will lead to the formation of a thick scale layer on the wellbore, thereby slowing down the flow rate of geothermal fluids, reducing the flow volume, and even blocking the pipelines [40], seriously affecting the development and utilization of geothermal resources. Therefore, this study explores the scaling trend in the study area through index analysis and saturation index discrimination.

5.3.1. Index Analysis Method

(1) Trend of carbonate scaling in geothermal fluids

The carbonate scaling substances in geothermal fluids are mainly calcium carbonate. According to previous studies, the methods currently most frequently used to qualitatively determine the trend of calcium carbonate scaling in geothermal water are the Leshin Index (LI) and the Ryzner Index (RI) [41]. Among the other indices, the Puckorius Scaling Index (PSI) is suitable for water bodies with high alkalinity or large pH fluctuations, and the results are more stable. The LSI (Larson-Skold Index) is suitable for corrosion risk assessment but weak for scale assessment. The SDI (Stiff-Davis Index) is suitable for complex water bodies, with complex calculations but more comprehensive results. However, the advantages of the LI and RI index are that they are simple, fast, low-cost, directly reflect the scaling tendency, and are widely used in practical engineering. Although they are not a complete substitute for complex hydrochemical analysis, they are of great value in initial assessment and rapid judgment in the field. In geothermal water, when the content of Cl⁻ ions is high (milligram equivalent percentage > 25%), the Leshin Index method is more reasonable for determining the trend of calcium carbonate scaling; when the Cl⁻ content is low (milligram equivalent percentage < 25%), the Ryzner Index method can be used to determine the trend of calcium carbonate scaling [42]. The formulas are shown in

Table 3. Analysis of the relevant equations for the trend characteristics of geothermal water scaling. [Cl⁻], [SO₄²⁻], [ALK] represent Cl⁻, SO₄²⁻ and HCO₃⁻ in units of meq/L; ppmCa²⁺, ppmSO₄²⁻ represent Ca²⁺ and SO₄²⁻ in units of ppm; SiO₂ in units of mg/L.

Method	Equation	No.	Reference
Larson Index (LI)	LI = ([Cl−] + [SO42−])/[ALK]	1	[42,43]
Ryzner Index (RI)	RI = 2pHs − pHa	2	[42,43]
	pHs = log[Ca2+] − log[ALK] + Kc	3
	pHs = log[Ca2+] − log[ALK] + Ke	4
	T(°F) = 32 + 9/5 × T	5
Relative saturation of gypsum (R.S.)	R.S. = 10^(logppmCa2+ + logppmSO42− −logKgypsum)	6	[44]
Relative saturation of silica (R.S.)	R.S. = SiO2/(2.446 × 10,000 × e^( −1553/Tk))	7

. The judgment criteria of the Leshin Index are: when LI > 0.5, it indicates no scaling but corrosive; when LI < 0.5, it indicates possible scaling and no corrosiveness. The Kc of the Ryzner Index is a complex constant including the equilibrium constant and activity coefficient related to temperature, which is usually estimated by graphical method [43], and Ke is a constant. The judgment criteria of the Ryzner Index are: when RI < 4.0, the scaling is very severe; when 4.0 < RI < 5.0, the scaling is severe; when 5.0 < RI < 6.0, the scaling is moderate; when 6.0 < RI < 7.0, the scaling is slight; and when RI > 7.0, no scaling occurs.

The calculation results of the trend of calcium carbonate scaling in geothermal water in the study area using the Larson Index and Ryzner Index are shown in

Table 4. Prediction results of CaCO₃ scaling trend in the study area.

Number	LI	Trend of Scaling Up	RI1	Trend of Scaling Up	RI2	Trend of Scaling Up	Cl− (%)
G1	–	–	5.67	Medium	7.08	No scaling or deposition	0.26
G2	–	–	2.91	Very serious	4.10	Serious	10.91
G3	14.31	No scaling or deposition	3.04	–	4.23	–	26.77
G4	18.26	No scaling or deposition	4.30	–	4.94	–	31.16
G5	–	–	5.35	Medium	6.97	Slight	0.22
G6	–	–	4.65	Serious	5.87	Medium	0.55
G7	–	–	3.45	Very serious	4.60	Serious	0.77
G8	63.65	No scaling or deposition	1.69	–	4.21	–	76.17

and Figure 6a. Since the Cl⁻ milligram equivalent percentages of G3, G4, and G8 (26.77%, 31.16%, 76.17%) exceed 25%, the estimation results using the Larson Index for these three geothermal waters are LI = 14.31, 18.26, and 63.65, respectively, which are greater than 0.5, indicating that G3, G4, and G8 geothermal waters will not experience calcium carbonate scaling and have corrosiveness. The average Cl⁻ milligram equivalent percentages of the other five geothermal areas are all less than 25%, and the estimation results using the Ryzner Index are as follows: the calcium carbonate scaling trend of G1 is no scaling to slight; the calcium carbonate scaling trend of G2 is severe to very severe; the calcium carbonate scaling trend of G5 is slight to moderate; the calcium carbonate scaling trend of G6 is moderate to severe; and the calcium carbonate scaling trend of G7 is severe to very severe.

(2) Trend of sulfate mineral scaling

The sulfate scaling substances in geothermal fluids mainly include gypsum, celestite and barite, among which gypsum scale [44] is the most influential. When the temperature of geothermal fluids is less than 100 °C, CaSO₄ mainly precipitates in the form of gypsum (CaSO₄·2H₂O).

The criterion for judging the relative saturation of gypsum R.S. is: when R.S. < 1, it indicates that it is unsaturated and gypsum scaling will not occur; when R.S. > 1, it indicates that it is saturated and gypsum scaling may occur. The estimation results are shown in Figure 6b, and it is concluded that the R.S. of G2, G3, G4, G6, and G8 is greater than 1, indicating that these geothermal waters will undergo gypsum scaling. This is consistent with the high SO₄²⁻ content in these geothermal waters and the main water–rock reactions analyzed earlier, which are the results of gypsum dissolution. The R.S. of G1, G5, and G7 is less than 1, indicating that gypsum is unsaturated in these geothermal waters and gypsum scaling will not occur.

(3) Trend of silicate mineral scaling

The scale formation of silicate minerals is rather complex [16]. The saturation R.S. related to amorphous silica can be used to judge it. The criterion for the relative saturation R.S. of amorphous silica is as follows: when R.S. < 1, no silicate scale will form; when R.S. > 1, silicate scale may form. The estimation results are shown in Figure 6c. It is concluded that no silicate minerals will form scale in the studied area.

5.3.2. Hydrogeochemical Modeling Using PHREEQC

During the generation process of geothermal fluids, potential scaling problems may occur during the ascent of geothermal fluids [7,16]. Therefore, it is necessary to use relevant simulation software to analyze a series of reactions that occur during the ascent of geothermal fluids and predict possible scaling. This study mainly used the PHREEQC 3.0 software to simulate and calculate the pH, outcrop temperature and thermal reservoir temperature conditions of geothermal water when it emerges, and to calculate the mineral saturation index (SI) of various minerals under these conditions. This is performed to determine the dissolution and precipitation states of different minerals [45], thereby predicting the scaling trend.

Typical carbonate minerals in geothermal fluids: aragonite, calcite, dolomite; sulfate minerals: hard gypsum and gypsum; silicate minerals: jasper, quartz and amorphous silica, were selected. The equilibrium states of various minerals under temperatures ranging from the outcrop temperature to 200 °C and pH values ranging from 4 to 11 were simulated. At the same time, one geothermal water sample was selected from each of the three regions in the study area for analysis. The results are shown in Figure 7.

It can be seen (Figure 7a–c) that with the increase in temperature, the saturation index SI of carbonate minerals G3 and G8 is always greater than 0, remaining in an oversaturated state. While for carbonate minerals G5, the saturation index SI shows a transition from an unsaturated state to an oversaturated state gradually, and there are also cases where it remains unsaturated. This result is consistent with the slight to moderate carbonate rock scaling phenomenon of G5 obtained in the previous text. It indicates that geothermal water in the study area may undergo carbonate scaling with the increase in temperature and depth.

With the increase in pH (Figure 7d–f), carbonate minerals in the geothermal water of the study area all show a transition from an unsaturated state to an oversaturated state. It is noteworthy that the pH values at which carbonate minerals reach equilibrium states vary among different geothermal areas (taking calcite as an example). Therefore, pH has a significant impact on the saturation of carbonate minerals, and an increase in pH will cause carbonate minerals to become oversaturated and precipitate. When geothermal fluids flow from the deep (high temperature, high pressure) to the shallow (low temperature, low pressure), CO₂ is desorbed and the geothermal water boils, which will lead to an increase in pH and promote CaCO₃ precipitation [16]. Since amorphous silica is always in an unsaturated state during the pH increase process, silicate scaling will not occur.

In conclusion, from the deep formation to the emergence at the surface, it is difficult to form sulfate and silicate scaling in the geothermal water of this area. Carbonate scaling is the main problem in this area, which is basically consistent with the conclusion obtained from the index analysis method in the section above.

5.3.3. Descaling and Prevention

The problem of CaCO₃ scale often causes the phenomenon of well plugging in geothermal wells, which hinders the smooth development and utilization of geothermal resources. Therefore, CaCO₃ scaling needs to be treated. The treatment methods of CaCO₃ scaling are mainly removal and prevention. The main methods are mechanical removal, controlling the partial pressure of CO₂, controlling the pH value of the solution and using chemical additives (scale inhibitors). For the medium–low temperature geothermal system, it cannot meet the demand for power generation, mainly to provide geothermal heating, hot spring bathing and breeding. Therefore, the measures that can be taken are mechanical demolition. Because the mechanical device of hot spring bathing is relatively simple, the machinery can be dismantled regularly to clean the scale. Depending on the actual scaling rate and thickness, it can be performed once or twice a year. The geothermal heat pump can be placed below the boiling point to cool the reservoir fluid by injecting cold water or injecting acid or CO₂ so as to control the pH in the geothermal water to inhibit calcite scaling. For high-temperature geothermal systems, to meet the needs of power generation, there are many methods that can be taken, such as mechanical dismantling, controlling the partial pressure of CO₂, controlling the pH value of the solution and using chemical additives. Among them, chemical additives are the most used and most effective method. For example, for CaCO₃ scaling, Yangbajing geothermal power station in China uses hydrochloric acid as a cleaning agent, alkyl pyridine as a corrosion inhibitor, and finally, 2% concentration of trisodium phosphate as a passivating agent to inhibit CaCO₃ scaling has a good effect. In addition, nano-Ca–DTPMP mineral inhibitors can also be added to reduce the concentration rate of Ca²⁺ ion reduction and prolong the existence time of Ca²⁺ ions in the solution, thus slowing down the precipitation rate of CaCO₃.

5.4. Drinking Water Quality Evaluation

The weights of each parameter and the results of EWQI were presented in

Table 5. Relative weights of each hydrochemical parameter.

Parameter	pH	TDS	Na+	Ca2+	Mg2+	NH4+	Cl−	SO42−	F−
Weight	0.112	0.095	0.201	0.044	0.048	0.219	0.197	0.033	0.051

and Figure 8. NH₄⁺ emerged as the most influential factor, with a weight of 0.219, closely followed by Na⁺ (0.201), Cl⁻ (0.197), pH (0.112), TDS (0.095), F⁻ (0.051), Mg²⁺ (0.048), Ca²⁺ (0.044) and SO₄²⁻ (0.033). The drinking water quality was divided into five ranks: extremely excellent (0–25), good (25–50), medium (50–100), poor (100–200), and extremely poor (>200). The drinking water quality geothermal water ranged from 25.42 to 1489.90, with a mean value of 305.25. Geothermal water G1, G5 and G6 could serve as an excellent water resource for domestic drinking. One water sample (G7) was classified at a good rank, while G2 and G4 were categorized as medium for drinking. Notably, G3 and G8 were considered as extremely poor for drinking with values of 214.27 and 1489.90, respectively. The results of the water–rock interaction showed that the leaching of silicate minerals has contributed to enrichment of Na⁺ and K⁺ concentrations in the geothermal water of G8, resulting in the extremely poor drinking water quality.

Moreover, a sensitivity analysis was carried out in this study to quantitatively disclose the dominant factors influencing drinking water quality [36]. Among these factors, NH₄⁺ (3.803) exerted the most prominent impact on the EWQI scores, followed by Cl⁻ (2.823), Na⁺ (2.677), pH (2.224), Ca²⁺ (1.506), SO₄²⁻ (1.169), F⁻ (1.127), Mg²⁺ (0.850) and TDS (0.808). The organic matter within coal seams can decompose under the high-temperature conditions of geothermal water, thereby releasing ammonia or ammonium ions. Additionally, Cl⁻ and Na⁺ are the outcomes of the leaching of silicate minerals. In conclusion, the sensitivity of each parameter was consistent with the specific geological conditions, thus verifying the accuracy of the EWQI in evaluating drinking water quality.

6. Conclusions

In this study, eight groups of the geothermal waters from the Sichuan sedimentary basin were sampled for hydrochemical analysis. The water–rock interaction, mineral equilibrium status, mineral scaling trend and environmental water quality were analyzed by hydrochemical compositions. The main conclusions drawn are as follows:

(1) The anions and cations were dominated by SO₄²⁻ and Ca²⁺, respectively. The main hydrochemical type of geothermal waters belongs to Ca–SO₄. The hydrochemical compositions of geothermal waters were dominated by the dissolution of calcite and gypsum, silicate weathering and positive cation exchange.

(2) Mineral equilibrium status of geothermal waters has not reached over-saturation. The SI values of calcite, aragonite and dolomite in most of the geothermal water were all greater than 0, indicating supersaturation. The remaining minerals were below saturation with the SI values lower than zero. Carbonate scaling would be the main problem during geothermal exploitation.

(3) The drinking water quality geothermal water ranged from 25.42 to 1489.90, with a mean value of 305.25. Most of the geothermal water samples, except G3 and G8, were suitable for drinking purposes. The poor water quality of the G3 and G8 samples were attributed to elevated Na⁺ and K⁺ concentrations. The weights of affecting factors followed the order of NH₄⁺ (3.803) > Cl⁻ (2.823) > Na⁺ (2.677) > pH (2.224) > Ca²⁺ (1.506) > SO₄²⁻ (1.169) > F⁻ (1.127) > Mg²⁺ (0.850) > TDS (0.808).