Hydrogeochemical Characteristics and Evolutionary Mechanisms of the Nanping Geothermal Field, Southeastern Hainan Island, China

Abstract

The southeastern Hainan Island boasts abundant hydrothermal resources, most of which are exposed as thermal springs. Analyzing the hydrochemical characteristics, hydrochemical evolutionary mechanisms, and material transition of these resources is significant for their exploitation and utilization. This study investigated the Nanping geothermal field in southeastern Hainan Island, using five groups of geothermal water samples collected in 2022, as well as seven groups of geothermal water samples, one group of shallow groundwater samples, and one group of surface water samples taken in 2023. Specifically, this study examined water–rock interactions in the geothermal field using the Gibbs model, ion ratios, chloro-alkaline indices (CAIs), and the sodium adsorption ratio (SAR). Moreover, the mineral transfer process in groundwater was analyzed using inverse hydrogeochemical simulation. The results indicate that in the study area the geothermal water temperatures range from 64 °C to 80 °C, pH values from 8.32 to 8.64, and TDS concentrations from 431 mg/L to 623 mg/L. The primary hydrochemical types of geothermal water in the study area include Cl-Na and Cl·HCO₃-Na, suggesting low-temperature, slightly alkaline geothermal water. The hydrochemical components of geothermal water in the study area are primarily affected by water–rock interactions. Besides the dissolution of silicate minerals and halite, cation exchange reactions contribute greatly to the formation of Na⁺ and K⁺ in geothermal water. Geothermal water receives recharge from the atmospheric precipitation of the Diaoluo Shan area in the northwest of the study area, with the recharge elevation ranging from 967 to 1115 m. The inverse hydrogeochemical simulation results reveal that during the water–rock interactions, silicate minerals, clay minerals, gypsum, and halite dissolve, while quartz and carbonate minerals precipitate. Additionally, these processes are accompanied by cation exchange reactions dominated by the replacement of Na⁺ in surrounding rocks by Ca²⁺ in geothermal water. This study can provide a geological basis for the exploitation, utilization, and management of the Nanping geothermal field.

1. Introduction

Geothermal energy represents an all-weather clean energy resource that can be efficiently exploited and utilized [1,2]. Groundwater serves as the carrier of hydrothermal resources. The hydrogeochemical analyses of groundwater, including its major compositions and hydrochemical types, can reveal the formation mechanisms, occurrence environments, geothermal reservoir temperatures, and recharge sources of geothermal resources [3,4,5,6], thereby providing a scientific reference for the exploration, exploitation, and utilization of geothermal resources. During migrating, groundwater typically undergoes water–rock interactions with surrounding rocks along runoff paths, leading to changes in its hydrochemical characteristics. Particularly, during its deep circulation geothermal water tends to mix with shallow cold water due to its high temperatures, deep circulation depth, and prolonged retention. As a result, geothermal water exhibits more complex, diversified hydrochemical characteristics compared to shallow cold water [7,8,9,10]. The environment is locally rich in CO₂; consequently, Ca²⁺, Mg²⁺, HCO₃⁻, and SO₄²⁻ content in the water increases along the water flow [11].

Hainan Island, located at the intersection of the Tethyan and Pacific tectonic domains [12,13], exhibits a dome-shaped topography characterized by a higher central portion and lower peripheral areas (Figure 1a). This island contains numerous granite masses and well-developed fault structures. Along the fault zones are fissured geothermal fields distributed in a banded shape, boasting abundant geothermal resources [14,15] and a high thermal background [16,17,18,19]. Based on their occurrence conditions, geothermal resources in Hainan Island can be classified into two categories, namely uplifted mountain and sedimentary basin types [20], with the former predominating. The uplifted mountain-type geothermal resources are primarily distributed in the south-central part, with geothermal reservoirs dominated by the tectonic fissure type. Such geothermal resources are exposed under the control of NE- and EW-trending deep-seated faults, with a nationally high density [14]. A total of 36 tectonic fracture-type thermal springs are exposed across Hainan Island, with granite thermal springs exhibiting high flow velocities and temperatures. The southeastern Hainan Island contains thermal springs with the highest single-well flow rate and the highest wellhead temperature nationally—the Nanping (51.2 m³/h) and Qixianling (97 °C) thermal springs, respectively. The Gaofeng geothermal field in southeastern Hainan Island exhibits uplifted mountain-type geothermal resources. Based on the characteristics of the hydrochemical components, isotopes, and geothermal field of geothermal water, researchers have properly analyzed the formation mechanism of geothermal water in the Gaofeng geothermal field, revealing that the geothermal water originated from leaching-type continental sedimentary water [21]. For groundwater along the south coast of Hainan Island, its hydrogeochemical process is governed by evaporative concentration, and this is closely associated with seawater with high total dissolved solids (TDSs) concentration. The water–rock interactions of the groundwater are dominated by the weathering and dissolution of rocks containing sodium-bearing silicate minerals [22,23]. For geothermal water with high TDSs concentration in coastal areas of Hainan Island, besides potential seawater intrusion, the enhanced water–rock interactions caused by tectonic activity contribute significantly to the pronouncedly elevated Na⁺, Ca²⁺, and Cl⁻ concentrations in geothermal water at the intersections of coastal fault zones [24]. Therefore, hydrothermal water in southeastern Hainan Island features complex, diverse hydrochemical characteristics. Focusing on the Nanping geothermal field, the hydrochemical characteristics and temperature signatures of geothermal water exposed within a 1 km range show certain differences. Through the study of water–rock interaction processes, the different processes experienced and the degree of mixing can be indicated, which can indicate the runoff and circulation paths of geothermal water and provide guiding significance for the development and utilization of similar geothermal systems.

2. Study Area

The study area, situated within Lingshui County, in the southeast of Hainan Island, geotectonically lies in the southeastern Wuzhishan fold belt. It features underdeveloped strata, with only Cambrian, Triassic, Cretaceous, and Quaternary strata exposed in small areas (Figure 1b). Fault structures are well developed in the study area. They predominantly exhibit EW, NE, and NW strikes, with nearly NS-trending structural traces occasionally visible. The most representative fault zones include the EW-trending Jiusuo-Lingshui and Jianfeng-Diaoluo fault zones, which control the spatial distributions of sedimentary suites, magmatic rocks, and thermal springs in the study area.

The intrusion exposed in the study area is the Liugong pluton (Figure 1b)—a part of the Baocheng pluton. The Liugong pluton consists primarily of coarse-to-medium-grained porphyritic biotite monzogranites (ηγmK₁^c). This pluton exhibits flesh red K-feldspar phenocrysts, which gradually decrease on the margin, and well-developed Carlsbad twins. Primary minerals in this pluton include quartz (21% to 30%), plagioclase (22% to 40%), K-feldspar (27% to 44%), biotite (3% to 7%), and hornblende (0% to 1%). During the 1:50,000 regional geological survey conducted by the Hainan Geological Survey Institute from 2007 to 2009, fresh samples were collected from the Liugong pluton, and their zircon U-Pb ages were determined using the high-precision laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method. As a result, the zircon U-Pb ages of the samples were determined at 107.6 Ma ± 1.9 Ma, suggesting that the intrusion occurred in the late Early Cretaceous [25]. The geothermal reservoirs in the Nanping geothermal field, under the control of fault structures, show thermal and hot springs exposed along the faults. Therefore, they are tectonic fracture-type banded geothermal reservoirs hosted by the Baocheng pluton. They exhibit complex lithologies, with areas at shallow burial depths containing well-developed fissures and fractured rocks.

3. Samples and Methods

3.1. Samples Collection and Test

Field surveys and samplings were conducted in thermal spring outcrops and boreholes within the Nanping geothermal field from July to November 2022, and in June 2023 (Figure 1c). Five groups of water samples were collected in 2022, with two groups taken from thermal spring outcrops (NP01 and NP04) and three groups from boreholes (NPZK01, NPZK03, and NPZK04). Water samples collected in 2023 comprised four groups from thermal spring outcrops (NP02, NP03, NP04-2, and NP08), three groups from boreholes (NPZK01-2, NPZK03-2, and NPZK04-2), one group from shallow groundwater (NP16), and one group from surface water (DB01). The coordinates and elevations of the sampling sites were determined using real-time kinematic (RTK) equipment during the field surveys. The temperatures, pH values, and electrical conductivity of the water samples were measured on-site using a WTW Multi 3400i (Munich, Germany) portable water quality analyzer.

The water samples were stored in brown glass bottles and colorless polyethylene bottles with added fixatives in the laboratory. During sampling, the sampler was rinsed three times using the water to be sampled. Then, water samples were transferred to bottles containing fixatives. Notably, the bottles must be filled up with water samples, with no space being left, and sealed with wax. Immediately after collection, the samples were transported to the Hainan Provincial Geological Testing and Research Center for tests and analyses using instruments including a spectrophotometer (UV-5200PC, Shanghai, China), an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP 6300 Duo, Waltham, MA, USA), and an ion chromatograph (Aquion 1200, Waltham, MA, USA). The concentrations of K⁺, Na⁺, Ca²⁺, and Mg²⁺ were measured using the ICP-OES (iCAP 6300 Duo, Waltham, MA, USA), with detection limits of 0.020 mg/L for K⁺, 0.005 mg/L for Na⁺, 0.011 mg/L for Ca²⁺, and 0.013 mg/L for Mg²⁺. The concentrations of Cl⁻, SO₄²⁻, and F⁻ were measured using the ion chromatograph (Aquion 1200, Waltham, MA, USA), with detection limits of 0.007 mg/L for Cl⁻, 0.018 mg/L for SO₄²⁻, and 0.006 mg/L for F⁻. The concentration of metasilicic acid was measured using the spectrophotometer (UV-5200PC, Shanghai, China), with a detection limit of 0.1 mg/L. The concentrations of CO₃²⁻ and HCO₃⁻ were determined by titration, and the total dissolved solids (TDSs) were calculated.

3.2. Research Methods

Through a detailed analysis of the hydrochemical component characteristics of geothermal water in the study area, a hydrochemical component Schoeller diagram and a Piper trilinear diagram were constructed to systematically investigate the principal ion component and hydrochemical types of the geothermal water. To further investigate the genesis and evolution of hydrochemical components in this region, water–rock interactions occurring in geothermal water was analyzed using methods such as the Gibbs model, ion ratio coefficient, and cation exchange. Simultaneously, the hydrochemical simulation software PHREEQC 3.7.3 was employed to inverse hydrogeochemical modeling to quantitatively analyze the phenomena of dissolution and precipitation of minerals in the geothermal water.

3.3. Test Results

The results of the hydrochemical tests are shown in

Table 1. Statistical table of hydrochemical parameters in Nanping geothermal field.

Group	ID	Time	T/°C	pH	K+	Na+	Ca2+	Mg2+	Cl−	SO42−	CO32−	HCO3−	F−	TDSs	H2SiO3	IBE
					mg·L−1
A	NP01	2022	75	8.32	4.9	144	17.8	0.2	153	80.6	3.9	80	6.3	573	105	−0.49
A	NP02	2023	74	8.32	5.4	169	20	0.15	219	24.5	4.6	89.2	0.43	623	118	1.68
A	NP03	2023	64	8.35	5.4	165	17.7	0.26	188	24.8	9.2	79.8	6.4	589	119	6.10
A	NPZK04	2022	80	8.32	6.1	162	15.7	0.19	183	76.7	2	82	6.2	620	111	−0.80
A	NPZK04-2	2023	80	8.33	5.7	165	15.1	<0.02	201	25.4	18	61	6.4	588	117	3.76
B	NPZK01	2022	65	8.64	4	105	9.2	0.23	71	68.8	10	74	7.8	441	118	3.35
B	NPZK01-2	2023	65	8.51	3.7	114	7.5	<0.02	86.3	20.8	18	75.1	7.8	431	127	10.51
B	NPZK03	2022	68	8.32	4.4	113	12.6	0.28	71.7	67.6	5.9	112	7.2	488	121	2.87
B	NPZK03-2	2023	68	8.38	4	112	13.3	0.31	94.5	19.9	4.6	113	7.2	463	122	6.16
B	NP04	2022	68	8.32	4	111	14	0.27	81.2	58.9	6.3	113	6.8	479	109	1.61
B	NP04-2	2023	68	8.39	4	112	13	0.27	86.3	19.6	14	103	6.9	454	123	8.49
B	NP08	2023	64	8.37	3.6	106	9.1	0.2	72.6	19.5	9.2	108	7.8	435	129	8.35
C	NP16	2023	24	6.09	5.2	15.4	16.5	5.3	21.5	4.1	0	61	0.29	216	56.4	10.07
C	DB01	2023	25	7.1	2.3	6.4	7.4	2	6.9	1.3	0	42	0.22	103	36.2	−2.02

. Based on these results, the Schoeller diagram (Figure 2), correlation PC load diagram (Figure 3), and Piper diagram (Figure 4) were drawn in the groundwater.

Ions in the geothermal water exhibit different concentrations. Generally, cations in the geothermal water are dominated by Na⁺ and Ca²⁺ (Table 1 and Figure 2). Both cations exhibit concentrations ranging from 105 mg/L to 169 mg/L and from 7.5 mg/L to 20.0 mg/L, respectively, collectively accounting for 96.7% of the total. The concentrations of cations in the geothermal water generally decrease in the order of Na⁺, Ca²⁺, K⁺, and Mg²⁺. In contrast, cations in the surface water and shallow groundwater are dominated by Ca²⁺ and Na⁺, which collectively represent 72.8% of the total. Their concentrations decrease in the order of Ca²⁺, Na⁺, K⁺, and Mg²⁺. Primary anions in the geothermal water include Cl⁻ and HCO₃⁻ (Figure 2), with concentrations ranging from 71 mg/L to 219 mg/L and from 61 mg/L to 113 mg/L, respectively, jointly representing 62.5% to 91.2% of the total. Anions in the geothermal water from NP01, NP02, NP03, and NPZK04 are dominated by Cl⁻, while primary anions in other geothermal water decrease include Cl⁻, HCO₃⁻, and SO₄²⁻ (in a descending order in terms of concentrations). In contrast, anions in the surface water and shallow groundwater are dominated by HCO₃⁻. Overall, the concentrations of all conventional ions (except for Ca²⁺ and Mg²⁺) in various water decrease in the order of geothermal water, shallow groundwater, and surface water. The TDS concentrations of the various water also follow the same decreasing order. These findings indicate that elevated temperatures and prolonged runoff processes correspond to significant leaching, reflecting the impacts of water–rock interactions on the hydrogeochemical environment.

The geothermal water in the study area exhibits pH values ranging from 8.32 to 8.64, with an average of 8.38, suggesting slightly alkaline water. In contrast, the shallow groundwater and surface water have pH values varying from 6.09 to 7.1, indicating that they are slightly acidic to neutral water. The geothermal water has temperatures ranging from 64 °C to 80 °C (average: 70 °C). Furthermore, it exhibits TDS concentrations from 431 mg/L to 623 mg/L (average: 515 mg/L), which significantly exceed those of shallow groundwater and surface water, suggesting freshwater thermal springs.

Three distinct groups of parameters (i.e., A, B, and C; Figure 3) were derived from a quantitative analysis of the relationships between hydrochemical components of groundwater and correlations among the water samples. Group A represents geothermal water distributed in the eastern part of the study area. It consists of water from higher-temperature thermal springs NP01 (2022), NP02 (2023), and NP03 (2023) and boreholes NPZK04 (2022) and NPZK04-2 (2023), with a hydrochemical type of Cl-Na (Figure 4). Group B represents geothermal water distributed in the western part of the study area. It consists of water from lower-temperature thermal springs NP04 (2022), NP04-2 (2023), and NP08 (2023) and boreholes NPZK01 (2022), NPZK03 (2022), NPZK01-2 (2023), and NPZK03-2 (2023). Group C represents cold water (Figure 4) consisting of shallow groundwater from NP16 (2023) and surface water from DB01 (2023), with hydrochemical types of HCO₃·Cl-Ca·Na and HCO₃₋Ca·Na.

The formation of bicarbonate-type geothermal water is typically linked to the Caledonian and Yanshanian intermediate-acid granite and diorite intrusions or other rock masses. Specifically, the lithologies of geothermal reservoirs generally govern various components in geothermal water, and the high-temperature and high-pressure environment in the deep part is a key factor affecting the water quality of geothermal water, and its role is mainly reflected in the following aspects: Firstly, the increase in temperature will significantly enhance the water–rock interaction and promote the dissolution of silicate and carbonate minerals. Secondly, temperature directly affects the dissociation equilibrium of weak acids, thereby changing the pH value and ionic form of the water body. Furthermore, the increase in temperature will also affect the polymerization state of silicon dioxide. Under alkaline high-temperature conditions, various siloxy anions (such as H₃SiO₄⁻) may be formed. These morphological changes will all pose challenges to the calculation of ion equilibrium [26]. As a result, the concentrations of Na⁺, K⁺, and trace elements increase, leading to the formation of geothermal water with a hydrochemical type of sodium bicarbonate.

4. Discussion

4.1. Analysis of Hydrochemical Evolution

4.1.1. Gibbs Diagram-Based Analysis

Gibbs (1970) designed a semi-log graph to determine factors governing hydrochemical components (e.g., rock weathering, meteoric water, and evaporative concentration) by identifying the hydrochemical components in various water bodies [27].

The characteristic values of the TDSs concentration of the geothermal water samples fall within the central part of the Gibbs diagram. Deep geothermal reservoirs in the study area are primarily composed of granites, with primary minerals including plagioclase, K-feldspar, quartz, and biotite. Furthermore, the leaching of these silicates renders the Na⁺/(Na⁺ + Ca²⁺) ratios of geothermal water slightly higher than those associated with water–rock interactions, and the projected points of these samples show a trend slightly inclining toward the upper right. These results suggest that the hydrochemical components of geothermal water in the study area are primarily governed by water–rock interactions, along with some influence from evaporative concentration. The Cl⁻/(Cl⁻+ HCO₃⁻) ratios principally fall within the range related to water–rock interactions, confirming the primary influence of water–rock interactions on the hydrochemical components of geothermal water. In contrast, the characteristic values of TDS concentrations of the cold water samples are slightly lower than those of geothermal water samples, falling in the central part of the Gibbs diagram and tilting towards the meteoric water-controlled range. This result indicates that the hydrochemical components of the cold water are principally controlled by meteoric water (Figure 5).

4.1.2. Ion Ratios

During deep circulation and migration, geothermal water undergoes exchange reactions with components in surrounding rocks. As a result, stable components accumulate in water, while unstable ones remain in surrounding rocks. The continuous ion accumulation results in an ionic equilibrium between geothermal water and surrounding rocks. Given that the mixing of geothermal water with other water exerts minimal influence on the proportions of some elements, the correlations between elements, combined with the proportions of elements in minerals, can accurately reflect the initial hydrochemical characteristics of geothermal water. This approach can be used to investigate water–rock interactions during water flow and qualitatively determine element sources, thereby revealing the migration processes and hydrochemical origins of hydrochemical components in groundwater. Compared to simple hydrochemical types, ion ratios allow for a deeper analysis of the spatiotemporal evolution of water quality.

The (Na⁺ + K⁺)/Cl⁻ ratio can reflect the influence of halite dissolution on geothermal water. A (Na⁺ + K⁺)/Cl⁻ ratio of 1 implies that Na⁺ originates from halite dissolution, whereas (Na⁺ + K⁺)/Cl⁻ ratios exceeding 1 might indicate that Na⁺ is derived from the dissolution of other minerals [28]. Figure 6a illustrates that the geothermal water samples from the study area consistently fall above the line of [Na⁺ + K⁺] = [Cl⁻]. In contrast, the groundwater and surface water samples fluctuate around the line. These distribution characteristics suggest that besides halite dissolution, Na⁺ and K⁺ in water samples might result from the dissolution of aluminosilicate minerals (e.g., albite and K-feldspar) and cation exchange with Ca²⁺. Additionally, the figure indicates that geothermal water of the same type exhibits similar Na⁺ + K⁺ concentrations.

The Na⁺ and K⁺ concentrations are associated with temperature and fluid–mineral equilibrium, and the Na⁺/K⁺ ratio can indicate the degree of geothermal water reactions. Specifically, Na⁺/K⁺ ratios above 15 suggest that geothermal water has experienced conductive cooling, lateral flow, and near-surface reactions. Na⁺/K⁺ ratios of less than 15 indicate that geothermal water ascends to the surface rapidly, undergoing limited interactions with surrounding rocks and a small mixing ratio with cold water. Figure 6b shows that the geothermal water samples from the study area have Na⁺/K⁺ ratios higher or far higher than 15. It can be inferred that geothermal water in the study area experiences intense water–rock interactions and mixing with a high proportion of shallow cold water.

The (Ca²⁺ + Mg²⁺) vs. HCO₃⁻ diagram (Figure 6c) indicates that the cold water samples fall above the line of (Ca²⁺ + Mg²⁺)/HCO₃⁻ = 1, signifying that the Ca²⁺, Mg²⁺, and HCO₃⁻ ions originate primarily from the dissolution of calcite and dolomite [29]. In contrast, the geothermal water samples are distributed around the line, indicating that Ca²⁺ and Mg²⁺ have additional sources besides the dissolution of calcite and dolomite. Since geothermal water in the study area is exposed in granitic layers, calcium- and magnesium-bearing feldspar may serve as an additional source of Ca²⁺ and Mg²⁺. Moreover, the geothermal water, experiencing prolonged and deeper circulation, exhibits higher HCO₃⁻ concentration compared to the cold water. This occurs possibly because more CO₂ is dissolved in the geothermal water in the runoff process.

4.1.3. Cation Exchange Reactions

Besides silicate weathering and the input of meteoric water, cation concentrations in water bodies are also influenced by cation exchange and adsorption between minerals and solutions. CAIs and the SAR can reflect the intensity of cation exchange reactions.

The calculational formulas of CAI1 and CAI2 are as follows:

$$\mathit{CAI}1={\displaystyle \frac{{\mathit{Cl}}^{-}-\left({\mathit{Na}}^{+}+{\mathit{K}}^{+}\right)}{{\mathit{Cl}}^{-}}}$$

$$\mathit{CAI}2={\displaystyle \frac{{\mathit{Cl}}^{-}-\left({\mathit{Na}}^{+}+{\mathit{K}}^{+}\right)}{{\mathit{HCO}}_3^{-}+{\mathit{SO}}_4^{2-}+\mathit{C}{\mathit{O}}_3^{2-}+\mathit{N}{\mathit{O}}_3^{-}}}$$

The calculational formula of SAR is as follows:

$$\mathit{S}\mathit{A}\mathit{R}={\displaystyle \frac{\mathit{N}{\mathit{a}}^{+}}{\sqrt{\left(\mathit{C}{\mathit{a}}^{2+}+\mathit{M}{\mathit{g}}^{2+}\right)/2}}}$$

In the case of positive CAI1 and CAI2 values, Na⁺ and K⁺ in the water replace Ca²⁺ and Mg²⁺ from rock minerals, while the opposite trend is true when both indices are negative. Moreover, higher absolute values of CAI1 and CAI2 correspond to a higher incidence of cation exchange reactions [30]. Figure 7a shows that most water samples exhibited negative CAI1 and CAI2 values, indicating that cation exchanges show a tendency to the replacement of Na⁺ and K⁺ in rocks and soils by Ca²⁺ and Mg²⁺ in water.

The SAR vs. TDS diagram (Figure 7b) reveals relatively high SARs overall, which tend to increase with the TDSs concentration. A higher SAR is associated with more pronounced cation exchange reactions and more significant interactions between water and the surrounding rocks.

The γ (Na⁺ + K⁺ − Cl⁻)/γ (Ca²⁺ + Mg²⁺ − HCO₃⁻ − SO₄²⁻) ratio can reflect cation exchange and adsorption. A ratio approaching −1 suggests the occurrence of cation exchange and adsorption [31]. Figure 7c shows that most water samples from the study area fall near the slope line of −1, indicating the presence of cation exchange in groundwater. The geothermal water samples are concentrated in the lower right corner of the diagram, suggesting the more pronounced cation exchange in the geothermal water compared to that in the cold water. This finding, validating the results of CAIs, demonstrates that cation exchange reactions act as a significant factor in the formation of hydrochemical components in the study area.

4.2. Characteristics of Hydrogen and Oxygen Isotopes

4.2.1. Recharge Sources of Geothermal Water

Isotope geochemistry is typically used to study the origin, genesis, and circulation characteristics of groundwater. Under the background of medium- and low-temperature water–rock interactions, δ¹⁸O and δD in geothermal water remain relatively stable during the water cycle. The relationship can be used to indicate the recharge source of hot water between the distribution characteristics of δ¹⁸O and δD and the precipitation line. Additionally, it can reflect the degree of interaction between hot water and the geothermal reservoir surrounding the rocks and be used to calculate the recharge elevation.

Through the study of hydrogen and oxygen isotopes in atmospheric precipitation around the world an obvious linear relationship has been observed between δ¹⁸O and δD in atmospheric precipitation. Craig (1961) first proposed the Global Meteoric Water Line (GMWL) equation as δD = 8δ¹⁸O + 10 [32]. Based on the atmospheric rainfall data in Hainan, the Local Meteoric Water Line (LMWL) equation of Hainan Island had been derived as δD = 7.37δ¹⁸O + 11.07. The recharge sources of geothermal water can be determined based on the above equations.

The range of δ¹⁸O is −8.72‰ to −6.69‰, and the range of δD is −51.0‰ to −47.3‰ in the geothermal water of the study area, with, respectively, average values of −7.57‰ and −49.3‰ (

Table 2. Hydrogen and oxygen isotopes and recharge elevation.

ID	δD/‰	δ18O/‰	Sampling Point Elevation/m	Recharge Elevation/m
NPZK01-2	−50.4	−8.72	17	1093
NPZK03-2	−48.8	−7.90	18	1030
NPZK04-2	−48.1	−7.35	15	999
NP02	−48.3	−7.10	15	1007
NP03	−47.3	−6.69	15	967
NP08	−49.6	−7.46	18	1062
NP04	−51.0	−7.80	13	1113
NP01	−49.0	−7.50	11	1031
NPZK04	−51.0	−7.60	15	1115

). Figure 8 shows that the geothermal water points are located near both the Global Meteoric Water Line and the Local Meteoric Water Line, indicating that the recharge of geothermal water directly or indirectly originates from the infiltration recharge of atmospheric precipitation.

4.2.2. Recharge Elevation of Geothermal Water

The δD and δ¹⁸O values of atmospheric precipitation decrease with an increase in elevation, a phenomenon known as the elevation effect. Due to the elevation effect of hydrogen and oxygen isotopes in precipitation, it is possible to further calculate the recharge elevation of geothermal water based on the study of its recharge sources. The calculation formula is as follows:

$$\mathit{H}=\left({\mathit{D}}_{\mathit{G}}-{\mathit{D}}_{\mathit{P}}\right)/\mathit{k}+\mathit{h}$$

where H is the recharge elevation (m); h is the elevation of sampling point (m); D_G is the δD value in geothermal water (‰); D_P is the δD value of rainwater around the study area (‰), taken as −23.5 ‰; and k is the elevation gradient of δD in atmospheric precipitation (‰/100 m), with a value of −2.5 ‰/100 m.

The results show (Table 2) that the recharge elevation of geothermal water in the study area is 967–1115 m. According to the elevation range, the recharge area is roughly located in the Diaoluo Shan area in the northwest of the study area.

4.3. Inverse Hydrogeochemical Simulation

The hydrochemical components of individual water samples can be used solely for analyzing the concentrations of chemical elements at specific sampling sites and the results of corresponding water–rock interactions, allowing for a qualitative analysis of hydrochemical evolutionary characteristics and their controlling factors. However, water–rock interactions occur throughout the groundwater runoff path, complicating the hydrochemical evolutionary process. Inverse hydrogeochemical simulation enables the quantitative analysis of water–rock interactions along the groundwater runoff path.

Along the reaction pathway, groundwater undergoes runoff from the starting point, culminating with discharge at the end point. Based on the changes in the hydrogeochemical components of groundwater at the initial and end points, as well as the principle of mass balance (i.e., hydrogeochemical components of water at the starting point + “reaction phases” = hydrogeochemical components of water at the end point + “product phases”), the inverse hydrogeochemical simulation can infer the potential hydrogeochemical processes during the groundwater runoff process and, accordingly, quantitatively analyze the mass transfer of hydrochemical components in the process [33].

4.3.1. Determination of Reaction Pathways and Possible Mineral Phases

Inverse hydrogeochemical simulation requires that the initial and end points of a reaction pathway should fall on the same flow path. The characteristics and origins of thermal springs on Hainan Island [34] reveal that geothermal water in the study area is recharged by meteoric water, which forms geothermal water against the deep thermal background after circulation in reservoirs. Hence, this study utilized meteoric water as the starting point and geothermal water of groups A and B, represented by groundwater in boreholes NPZK04 and NPZK03, respectively, as the end points of the reaction pathways, with water–rock interactions during circulation being accounted for. Accordingly, reaction pathways I and II were obtained.

The preceding hydrogeochemical analysis reveals that the ionic components of geothermal water in the study area originate principally from the interactions between geothermal water and surrounding rocks. Accordingly, determining possible minerals involved in water–rock interactions is the key to inverse hydrogeochemical simulation. The mineral composition of surrounding rocks is primarily contributed to by the minerals in parent rocks, as well as relevant chemical weathering and hydrothermal alteration. Typically, their mineral composition can be comprehensively determined using the following methods: (1) directly measuring; (2) indirectly selecting potential minerals based on the evolution of hydrochemical components along flow paths; and (3) calculating the mineral composition based on the saturation states of minerals.

As confirmed by the regional geologic data of Hainan Island and the analysis of rock minerals in granite geothermal reservoirs within the Lingshui area, primary minerals in the surrounding rocks of geothermal reservoirs in the study area include quartz, K-feldspar, albite, anorthite, clay minerals, calcite and dolomite, and dark minerals dominated by biotite, with the dark minerals susceptible to chlorite alteration. Among them, the content of clay minerals is 2.3–19.6%, the content of calcite is 0.3–2.4%, and the content of dolomite is 2.2–3.9% [35]. In addition, the hydrogeochemical analysis results indicate that halite, kaolinite, fluorite, calcite, gypsum, and CO₂ (g) also act as potential reactive phases, and groundwater migration is accompanied by cation exchange and adsorption. Therefore, NaX and CaX₂ were considered alternate terms. Nine elements, i.e., Na, K, Ca, Mg, Cl, HCO₃, SO₄, F, and Si, were selected as bound variables. Given that K-feldspar and albite, as minerals of a high-temperature origin, precipitate only under high-temperature conditions, it is assumed that feldspar minerals undergo merely dissolution reactions in the inverse hydrogeochemical simulation [36,37]. Possible mineral-phase dissolution chemical reaction equations are shown in

Table 3. Dissolution chemical reaction equation of minerals.

Mineral	Dissolution Chemical Reaction Equation
Albitite	NaAlSi3O8 + 8H2O = Na+ + Al(OH)4− + 3H4SiO4
K-Feldspar	KAlSi3O8 + 8H2O = K+ + Al(OH)4− + 3H4SiO4
Calcite	CaCO3 + H2O = Ca2+ + HCO3− + OH−
Dolomite	CaMg(CO3)2 + 2H2O = Ca2+ + Mg2+ + 2HCO3− + 2OH−
Gypsum	CaSO4·2H2O = Ca2+ + SO42− + 2H2O
Halite	NaCl = Na+ + Cl−
Cation exchange	Ca2+ + 2NaX = 2Na+ + CaX2

.

4.3.2. Analysis of the Simulation Results

(1)

Saturation index (SI) analysis

Prior to inverse hydrogeochemical simulation, the dissolution–precipitation trends of minerals in the solution, as well as intrinsic hydrochemical and hydrodynamic conditions, can be revealed using the SIs of mineral phases at the starting and end points of reaction pathways. Reaction pathways I and II shared the same starting point (meteoric water), with groundwater in boreholes NPZK04 and NPZK03 as end points, respectively. The SIs of various minerals along the reaction pathways are shown in

Table 4. Saturation index of endpoints of inverse hydrogeochemical modeling path.

Mineral	SI
	Initial Precipitation	End of Path I	End of Path Ⅱ
Albitite	-	−0.6	−0.73
K-Feldspar	-	−0.01	−0.01
Quartz	-	1.46	1.60
Chlorite	-	−5.01	−0.92
Calcite	−1.65	1.77	1.97
Dolomite	−3.52	2.23	3.90
Illite	-	−6.96	−6.94
Gypsum	−4.18	−1.33	−1.48
CO2 (g)	−2.38	−2.23	−1.92
Kaolinite		−8.24	−8.44
Halite	−7.30	−4.25	−4.71
Fluorite	-

.

Table 4 indicates that carbonate minerals calcite and dolomite exhibit SI values exceeding zero, indicating that they are saturated in geothermal water, with a tendency to precipitate. This demonstrates that substantial carbonate minerals are dissolved during the geothermal water migration, gradually reaching saturation and precipitating in the discharge area. This finding is consistent with the calcite sediments observed in borehole cores. In contrast, silicate minerals K-feldspar, albite, and anorthite, along with gypsum and halite, all have SI values of less than zero, which trend upward along the reaction pathways. This result suggests that these minerals continuously dissolve during the runoff process, leading to a gradual increase in the Na, Ca, Cl, and SO₄ concentrations. Quartz has an SI value of greater than zero, indicating that it is saturated in geothermal water. Additionally, clay minerals kaolinite, montmorillonite, and illite exhibit SI values below zero, suggesting that these clay minerals are undersaturated in geothermal water and undergo continuous leaching. This occurs primarily due to the diagenetic transformation of these minerals.

(2)

Analysis of the reaction pathway simulation results

Based on the analysis of the SIs of minerals, this study conducted simulations of reaction pathways I and II using the PHREEQC software. Since the simulations involved many minerals and chemical reactions, the calculation results exhibited a multiplicity of solutions. Therefore, it is necessary to adjust the input uncertainties of various points to minimize the number of output results. Furthermore, there is a need to comprehensively analyze the variation trends in the hydrochemical components of water samples and the mineral composition characteristics of rock samples to determine the most proper results.

For the mass transfer calculation in inverse hydrogeochemical simulation, positive mass transfer indicates that corresponding minerals dissolve into groundwater, while negative mass transfer suggests that corresponding minerals precipitate from groundwater. The calculation results (

Table 5. The result of mineral transfer amount in inverse hydrogeochemical modeling.

Mineral	Path I	Path II
Albitite	2.05 × 10−3	1.62 × 10−3
Anorthite	2.20 × 10−3	2.05 × 10−4
K-Feldspar	9.73 × 10−5	4.35 × 10−5
Quartz	−3.00 × 10−3	−3.50 × 10−3
Calcite	−2.24 × 10−3	−2.53 × 10−3
Dolomite	−1.22 × 10−4	−7.79 × 10−5
Calcium Montmorillonite	−2.88 × 10−3	−3.52 × 10−3
Chlorite	4.96 × 10−4	4.81 × 10−4
Illite	1.62 × 10−4	7.25 × 10−5
Kaolinite	3.75 × 10−3	2.61 × 10−3
CO2 (g)	6.87 × 10−4	1.26 × 10−3
Gypsum	7.85 × 10−4	6.00 × 10−4
Halite	4.99 × 10−3	2.12 × 10−3
NaX	8.30 × 10−4	3.88 × 10−4
CaX2	−4.15 × 10−4	−1.94 × 10−4

) show that during runoff along reaction pathway I, silicate minerals (i.e., K-feldspar, albite, and anorthite), clay minerals (i.e., kaolinite, montmorillonite, and illite), gypsum, and halite dissolve, while quartz and carbonate minerals (i.e., calcite and dolomite) precipitate. These processes are accompanied by cation exchange reactions dominated by the replacement of Na⁺ in surrounding rocks by Ca²⁺ in geothermal water.

The mass transfer results of minerals along both reaction pathways reveal that compared to reaction pathway II, reaction pathway I exhibits generally higher mass transfer of dissolved minerals and significantly higher cation exchange. This finding suggests that reaction pathway I features deeper geothermal water circulation and more intense water–rock interactions.

5. Conclusions

(1)

Geothermal water in the study area exhibits temperatures ranging from 64 °C to 80 °C, pH values from 8.32 to 8.64, and TDS concentrations from 431 mg/L to 623 mg/L (higher than those of cold water), suggesting slightly alkaline freshwater and low-temperature geothermal water. The concentrations of conventional components in various water bodies in the study area decrease in the order of geothermal water, shallow groundwater, and surface water. In geothermal water, the concentrations of cations decrease in the order of Na⁺, Ca²⁺, K⁺, and Mg²⁺, while the concentrations of anions decrease in the order of Cl⁻, HCO₃⁻, and SO₄²⁻. Primary hydrochemical types of geothermal water in the study area include Cl-Na and Cl·HCO₃-Na.

(2)

Analyses using the Gibbs model, ion ratios, CAIs, and SARs reveal that the hydrochemical components of geothermal water in the study area were formed primarily due to intense water–rock interactions. Besides the dissolution of silicate minerals and halite, cation exchange reactions also serve as an important factor in the formation of Na⁺ and K⁺ in geothermal water.

(3)

Geothermal water in the study area is mainly derived from atmospheric precipitation recharge, with a recharge elevation ranging from 967 to 1115 m. The author speculates that the main recharge area is located in the Diaoluo Shan area of the northwest of the study area.

(4)

The inverse hydrogeochemical simulation results indicate that during the water–rock interactions, silicate minerals, clay minerals, gypsum, and halite dissolve, whilst quartz and carbonate minerals precipitate. Additionally, these processes are accompanied by cation exchange reactions dominated by the replacement of Na⁺ in surrounding rocks by Ca²⁺ in geothermal water.