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Article

Genesis of Geothermal Waters in Zhongshan City, China: Hydrochemical and H-O-C Isotopic Implications

by
Yanan Li
1,
Ximin Bai
2,
Changsheng Huang
3,
Wei Chen
4,5,*,
Chuanming Ma
1,
Wei Huang
6,
Gao Deng
7,
Xiangrong Qiu
7,
Shengnan Chen
7,
Yongjun Yang
7,
Ying Huang
7,
Xuefeng Wu
7 and
Hailong Ye
2
1
School of Environmental Studies, China University of Geosciences, Wuhan 430074, China
2
Hydrogeological Brigade of Jiangxi Geological Bureau, Nanchang 330095, China
3
Wuhan Center of Geological Survey CGS, Wuhan 430205, China
4
Fourth Geological Team of Hubei Geological Bureau, Xianning 437100, China
5
Hubei Key Laboratory of Resources and Eco-Environment Geology (Hubei Geological Bureau), Wuhan 430034, China
6
Guangdong Geological Survey Institute, Guangzhou 510080, China
7
Guangdong Geological and Technical Engineering Consulting Company, Guangzhou 510080, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(13), 1765; https://doi.org/10.3390/w16131765
Submission received: 23 April 2024 / Revised: 6 June 2024 / Accepted: 12 June 2024 / Published: 21 June 2024
Browse Figures
Versions Notes

Abstract

:
Investigations of the geochemical compositions of geothermal water, as well as their movements and geneses, are of great significance for the exploration and exploitation of hydrothermal resources. In Zhongshan City, a southern city in Guangdong Province, large amounts of geothermal heat have been discovered. The results of investigations show that the hydrochemical types of geothermal water in the study area are Cl-Na·Ca and Cl-Na. H-O isotopes are basically near the atmospheric precipitation line, and the calculated recharge elevation of geothermal water ranges from 716 to 822 m, which is close to the altitude of the North Peak Mountain in Taishan City. The deep thermal storage temperature ranges from 95.32 to 149.71 °C, and the depth of the thermal cycle ranges from 2638.57 m to 4581.07 m. The genetic model of the geothermal water in this area is that, at favorable structural positions with satisfied water storage conditions, the mixture of atmospheric precipitation and seawater that circulates deep in Earth is heated by terrestrial heat flow under actions such as deep heat exchange and water–rock reactions to leach the salt, finally forming the highly mineralized geothermal water that uplifts out of the surface along faults and crops. The formation of the genetic model of geothermal water will provide a geological basis and technical support for the efficient development and utilization of geothermal resources in Zhongshan City and the coastal area of Southeast China.

1. Introduction

Resources and the environment are the basis of human survival and development. Likewise, the problem of environmental pollution caused by traditional energy use has become a significant issue facing worldwide sustainable development [1]. In contrast with traditional fossil fuels, geothermal water resources are renewable [2,3,4,5,6,7] and destined to play a more critical role in the energy-diversified mode of supply due to their wide distribution, large storage capacity, environmental protection, good stability, and other features [8,9,10,11]. The development and utilization of geothermal water energy have become especially rapid in the last 50 years [12,13], indicating that increasing attention is paid to exploiting geothermal water resources. In order to develop geothermal water resources scientifically and reasonably, it is important to study the conceptual genetic model for geothermal water resources [14,15,16]. Currently, conceptual genetic model geothermal water resources research has been relatively mature, using isotope, hydrologic, earth, chemical, and other methods [17,18,19,20,21].
Zhongshan City, home to one of the most abundant geothermal water resources in the coastal area of Southeast China, has a long development history. This region has been affected by the comprehensive action of plate movement for a long time, and the development of fault structures in the territory is accompanied by crustal uprise and multi-stage strong magmatic activity, which creates favorable conditions for the formation and occurrence of geothermal resources. According to an estimation, the total recoverable amount of geothermal fluid is 1.76 × 106 m3/a, and the annual recoverable thermal energy is 3.42 × 1011 kJ/a, which can save 19,400 t/a coal3. After long-term development, the utilization of geothermal water resources in Zhongshan City has gradually formed a mode and pattern that mainly focus on bathing and has medical treatment and recuperation functions.
The geothermal water resources exploration of Zhongshan City began in the 1970s with the work of many geologists. After decades of development, the regional geological background; heat flow characteristics; distribution; scale; geological structure; development; and utilization of geothermal resources, as well as the strata, structure, and heat storage types of the geothermal field, have been preliminarily identified. However, the lack of in-depth research and understanding of the formation mechanism, occurrence environment, and migration law of geothermal water resources lead to many difficulties in the efficient development and utilization of geothermal water resources and the prevention and control of land subsidence caused by thermal reservoir compression.
Based on previous research, this study analyzed the water chemistry and isotopic characteristics of various water bodies in Zhongshan City to obtain the mode of geothermal water circulation and establish a conceptual genetic model for the city’s geothermal water. Thus, it provides a geological basis and technical support for the efficient development and utilization of geothermal resources in the coastal area of Southeast China.

2. Overview of the Study Area

The study area is located south of the South China orogenic belt (Figure 1), in the central and southern part of the Pearl River Delta and the core area of the Guangdong–Hong Kong–Macao Greater Bay Area. Its topography is based on the South China paratransit. After a long period of climate change and wind and rain erosion, it has mainly been formed by alluvial plains, with low hills and hilly platforms scattered throughout. The region is long and narrow in the north and south, short and narrow in the east and west, and is divided into the north, the south plain area, and the central mountain area, as well as beaches and water areas. The plain area is about 1242 square kilometers, accounting for 68% of the city’s area. The mountain area is about 400 square kilometers, accounting for 25%; the river accounts for 7%; and the beach area is about 150 square kilometers. The city’s coastline is about 57 km, near the Lingdingyang area of the Pearl River mouth, most of which is on the silt coast. The outcropped stratigraphic unit, dominated by the widely developed Cenozoic quaternary system, is similar to the southern Cathaysia landmass [22,23] (Figure 1). The northern, middle, and southern parts of the area are outcropped by the Palaeozoic and Mesozoic strata, and the old strata from the Proterozoic Sinian system are sparsely outcropped in the north.
Magmatic rock is widely developed in the study area (Figure 1), especially irruptive rock, but in comparison, volcanic rock is less developed. The majority of acidic intrusive rock and a small number of intermediate and intermediate–acidic rocks are distributed in the area. The lithology of the intrusive rock is mainly composed of monzonitic granite and some quartz diorite, granodiorite, and granite porphyry. The volcanic rock, primarily distributed in the Middle Jurassic Longtankeng Formation, mainly comprises dacitic breccia lava, dolomite tuff breccia, dacitic breccia-bearing rhyolitic crystal tuff, rhyolitic breccia-bearing lithic–crystal tuff, rhyolitic vitric tuff, and tuffaceous fine-grained quartz sandstone.
A fault structure (Figure 1) developed in the study area, but the fold structure was not formed. The main faults mostly strike in the North–East direction, including the primary faults of the Guzhen–Nansha, Northern Wugui Mountain, Southern Wugui Mountain, and Pingsha faults, as well as the secondary faults of the Dayong–Minzhong, Lanbian–Banfu, and Nanlang–Shenwan. Among them, the Northern Wugui Mountain, Dayong–Minzhong, Lanbian–Banfu, and Southern Wugui Mountain faults dip in the North–West direction, and the other faults dip in the South–East direction. In addition, some faults also strike in the North–West direction, including the primary faults of Xi River and Shunde–Yakou and the secondary faults of the Huanpu–Nanlang fault, all dipping in the South–West direction.

3. General Situation of Geothermal Resources in the Study Area

The geothermal water resources are distributed in uplifted mountains in six medium- and low-temperature geothermal fields (Figure 1c), with the fluid temperature varying between 29.5 °C and 99 °C. These include the geothermal fields of Huchiwei, the Eastern Huchiwei Lot, Zhangjiabian, Zoumadun Village, Banfu Town, and Yongmo. The geothermal fields of Huchiwei and the Eastern Huchiwei Lot are on the east coast of the study area. The geothermal fields of Zhangjiabian and Zoumadun Village are located in the middle of the study area, the geothermal fields of Banfu Town are located in the southwest of the study area, and the geothermal fields of Yongmo are located in the central–southern region of the study area (Figure 1c). Currently, the geothermal exploration and exploitation in this area mainly focus on the geothermal fields of Huchiwei, the Eastern Huchiwei Lot, Zhangjiabian, and Yongmo.
The reservoir of geothermal resources in the study area can be divided into two types: the belt-shaped type and the quasi-lamellar type. The belt-shaped geothermal reservoir, widely distributed in all geothermal fields, is mainly located in the faulted fracture zones of Yanshanian granite, quartz diorite, and metamorphic rock. The quasi-lamellar geothermal reservoir was only discovered in the geothermal fields of Yongmo and Zhangjiabian, where the geothermal water is stored in the Quaternary layers of coarse sand.
The caprock of the geothermal field in the study area is generally composed of Quaternary mud and clay or Yanshanian granite-weathered deposits of cohesive soil that are several meters to dozens of meters thick. Most of these reservoirs are thermally well-insulated.
As shown in Figure 1c, most geothermal fields are located along the NE-SW faults and near the junctions of faults, which have good extensibility, excellent connectivity, and high water permeability characteristics. In addition, they can often be found around granites, which shows that the formation of geothermal fields is associated with the lithostratigraphic features and structural conditions. The circulation of thermal water is closely related to fracture zones. Simultaneously, the repeated activities of deep-seated faults provide favorable conduits for groundwater migration [24].

4. Sampling and Methods

In order to investigate the hydrothermal and geochemical features of the study area, a field sampling campaign was conducted from April 2020 to March 2021. During the campaign, a total of 6 thermal water samples and 51 cold-water samples were collected from 47 locations in accordance with the Geologic Exploration Standard of Geothermal Resources and Examination Methods of Underground Water Quality, and the relevant sampling points are shown in Figure 1c. The thermal water samples were collected from the wells of the geothermal fields of Huchiwei, the Eastern Huchiwei Lot, Zhangjiabian, and Yongmo, which have been highly developed. All the samples were filtered and acidified. It is worth noting that, due to the large temperature range of hot water in the geothermal fields of Huweici, we collected three samples of geothermal water with different temperatures. The cold-water samples included twenty-seven well-water samples, three ordinary spring-water samples, two mineral-water samples, four river-water samples, thirteen rainwater samples, and one seawater sample. Water samples were taken when the pH and water temperature were stable. During sample taking, the geographic location and coordinates, geological structure, and other information were recorded, and the water temperature was measured using an infrared thermometer. The unstable indicators, including the temperature, pH, conductance ratio, dissolved oxygen, and total dissolved solids of the water samples, were detected with a multi-parameter water quality analyzer. Also, sampling bottles were rinsed three times with the water sample collected before the sample was taken. To avoid contamination, the sampling bottles were kept free of air bubbles and carefully sealed with 3M seal strips. Upon delivery to the laboratory, the water samples were kept in a refrigerator to maintain a temperature of about 4 °C.
Water chemical analysis was performed by the Guangdong Geological Experiment and Testing Center, during which the samples of geothermal water, well water, ordinary spring water, mineral water, river water, and seawater were tested, as shown in Table 1. K+, Na+, Ca2+, and Mg2+ in the water samples were analyzed using ICP–AES (ICAP6300, Thermo Fisher Scientific, Waltham, MA, USA), which has a limit of detection of 0.02 mg/L. HCO3−, CO32−, Cl−, F−, SO42−, and NO3− in the water samples were analyzed using an ion chromatograph (model NoSwandon ECO), which has a limit of detection of 0.01 mg/L. Hydrogen and oxygen isotope and Carbon-14 (14C) dating were tested and performed by the Third Institute of Oceanography, Ministry of Natural Resources. The samples used for hydrogen and oxygen isotope testing included geothermal water, well water, ordinary spring water, mineral water, river water, rainwater, and seawater. Representative samples were taken from geothermal water to conduct 14C dating, as shown in Table 2 and Table 3. The extraction and purification of CO2 gas were performed using the CO2-H2O equilibration method to test the oxygen isotope. The water volume for each sample was 2 mL, the equilibration temperature was 25 °C, and the equilibration time was 12 h. H2 was prepared at the reaction temperature of 400 °C via the zinc method for testing the hydrogen isotope. The purified CO2 and H2 were used to obtain the δ18O and δD isotope values with an MAT-253 EM-type mass spectrometer. The obtained values were compared with the standard mean ocean water (SMOW) and expressed by the isotope deviation between the sample and standard isotopes; the analytical error of δ18O was around 0.2‰, and that of δD was around 2‰. Combined with the wet oxidation method and the liquid scintillation counting method, Carbon-14 dating was performed using the accelerator mass spectrometry (AMS) 14C (i.e., AMS-Accelerator Mass Spectrometry) method, and the test precision was less than 5–10%.

5. Results

5.1. Water’s Chemical Characteristics

A Piper trilinear diagram [25] (Figure 2), a Schoeller diagram (Figure 3), and a Box plot (Figure 4) of the main water chemical indicators in the study area were drawn based on test data, which can visually reflect the main ionic composition and hydrochemical types of water samples [25].
The well and spring waters, which represent shallow underground water, are mostly shown in the middle part of the Piper trilinear diagram. Their total dissolved solids (TDSs), positive ions (including Na+, Ca2+, and K+), and negative ions (including Cl−, SO42−, HCO3−, and F−) have a similar concentration and are homogenetic. The concentration of NO3− in some water samples is high, which may be caused by long-term organic pollution. The pH value variation is large, between 4.55 and 7.88. The chemical types of well water and spring water are the most complex, including HCO3-Ca, HCO3-Na·Ca, HCO3-Ca·Na, HCO3·SO4-Ca, Cl·HCO3-Na, Cl·HCO3-Ca·Na, NO3·Cl-Na·Ca, HCO3·Cl-Ca·Na, and HCO3·ClNO3-Ca types of water.
The HCO3−-Ca and HCO3·Cl-Ca types of mineral water, as shown in the lower-left area of the Piper trilinear diagram, have a similar chemical composition to some well water and spring water. However, they contain a small number of negative ions (mainly HCO3−, followed by Cl− and SO42−) belonging to a special shallow underground water, which is rich in minerals in the study area.
River water belongs to the HCO3-Ca and HCO3-Ca·Na types of water, as shown in the middle part of the Piper trilinear diagram, which is close to the shallow underground water in the area in terms of the total dissolved solids (TDSs), pH value, and the concentrations of positive (e.g., Na+, Ca2+, and K+) and negative ions (e.g., Cl−, SO42−, HCO3−, and F−).
As shown in the right part of the Piper trilinear diagram, seawater is considered the Cl-Na type, whose dissolved solids (TDSs), positive ions (such as Na+, Ca2+, and K+), and negative ions (such as Cl−, SO42−, HCO3−, and F−) are between geothermal water and well water, spring water, and river water. However, the concentrations of Mg2+ and CO32− are the highest among all the water bodies.
Geothermal water is categorized as Cl-Na·Ca and Cl-Na types of mineral water, as shown in the right part of the Piper trilinear diagram (Figure 2). Geothermal water’s chemical characteristics are similar to seawater, indicating that geothermal water may be subjected to the mixing action of seawater. The total dissolved solids (TDSs) in geothermal water are the highest in other water bodies, reaching 4.2 to 13.3 g/L; so, geothermal water is saline water. Geothermal water mainly contains the positive ions of Na+, Ca2+, Mg2+, and K+ (Na+ > Ca2+ > Mg2+ > K+) and the negative ions of Cl−, SO42−, HCO3−, F−, and NO3− (Cl− > SO42− > HCO3− > F− > NO3−). It is weakly alkaline, with a pH value of 6.78–7.41. In geothermal water, Na+, Ca2+, K+, Cl−, and F− concentrations are higher than in other water bodies; the concentration of Mg2+ is between that in seawater and that in well water, spring water, mineral water, and river water. The concentration of HCO3− is close to that in other water bodies. The concentration of NO3− is lower than that in other water bodies. The formation process of geothermal water differs from that of well water, spring water, mineral water, and river water. It is worth noting that the F− content in geothermal water is much higher than that of cold water in the study area. The possible reasons for this include the basement of the study area being granite, which contains biotite and amphibole components. Furthermore, geothermal water undergoes deep circulation through continuous heating and long-term leaching of fluorine-rich biotite, amphibole, and other minerals.
Another useful index for water classification is ionic salinity or total ionic salinity (TIS) [26], which shows the sum of anion and cation total contents (expressed in meq/L). Iso-TIS lines are reported in Figure 5 (SO42− vs. HCO3− + Cl−), where geothermal waters and seawater fall in the 40 and 200 meq/L range of iso-TIS lines, and well water, spring water, mineral water, and river water have the lowest TIS (from 0 to 6 meq/L). The results above are consistent. Furthermore, the formation process of geothermal water differs from that of well water, spring water, mineral water, and river water.

5.2. Water–Rock Equilibrium

To judge the extent of the water–rock equilibrium, each water body’s chemical data were input into a Na-K-Mg triangular diagram [27,28,29] (Figure 6). As shown in Figure 6, the three geothermal water samples of the geothermal fields of Huchiwe and the water samples of other water bodies are all in the immature area (at the Mg end). The geothermal water samples of the other geothermal fields are in partial equilibrium areas. The above results show that three geothermal water samples of the geothermal fields of Huchiwe are in the immature area, which indicates that they did not reach the ionic equilibrium in the geothermal reservoir or that the ion concentration in geothermal water decreased due to cold- and hot-water mixing when the water rose upwards [30]. In addition, a sufficient water–rock exchange reaction occurred between three geothermal water samples of the other geothermal fields in partial equilibrium areas and their surrounding rocks. Water maturity is also high, with chemical water characteristics similar to those in the geothermal reservoir.

5.3. Geothermal Water Source

5.3.1. Langlier–Ludwig Diagram

The Langlier–Ludwig diagram (Figure 7) is generally used to analyze the geothermal water source [31,32]. All the water bodies in the study area lie in a mixing area of shallow cold water and atmospheric precipitation. The geothermal water and seawater samples near the seawater or connate water area show that the geothermal water may be a mixture of seawater and atmospheric precipitation.

5.3.2. δD-δ18O Isotopic Analysis

D and 18O are relatively stable isotopes in water and are widely used to study the source of water [33,34,35]. In this study, the rainfall measurement data of Zhongshan City were used to obtain the local precipitation line equation (δD = 7.069δ18O − 6.77) as a reference for the local meteoric water line.
A δD-δ18O isotopic analysis performed on all the water bodies and atmospheric precipitation in the study area showed that the δ18O and δD values vary with different water bodies. As shown in the δD-δ18O relation diagram (Figure 8), all the water bodies in the study area are near the local meteoric water line, which indicates that atmospheric precipitation is the basic source of replenishment for the water bodies. As shown in the figure, the geothermal water line is shifted to the right of the atmospheric precipitation line, meaning a little “oxygen shift” occurred. The isotope value is relatively low, showing that the deep geothermal water is laterally replenished by faraway groundwater runoff over a long period instead of being directly replenished by local atmospheric precipitation [36]. It also indicates that deep geothermal water in the study area is relatively enclosed and has a weak hydraulic connection with the shallow underground water.
The deuterium excess parameter d, where d = Δd − 8δ18O, is an indicator of oxygen isotope exchange in the water–rock reaction [37], and its evolution is mainly controlled by the surrounding rock, oxygen-containing components, lithology, enclosure conditions of aquifer, retention time of water bodies, and physical and chemical properties of water bodies. The larger the absolute value of d in the replenishment area, the longer water is retained in the aquifer and the slower the underground water runoff. Meanwhile, the gradient variation in the d value also reflects the direction of underground water [38]. The d value of geothermal water in the study area is −6.5–11.5‰, which indicates that the geothermal water runoff in the area is slow and retained in the aquifer for a long time, corresponding to the enrichment phase of 18O [22].

5.4. Water–Rock Interactions

5.4.1. Gibbs Diagram

The Gibbs diagram is an important tool used to qualitatively determine the influence of the formation of rocks and minerals, atmospheric precipitation, and the evaporative concentration action on the chemical composition of water bodies [39]. As shown in Figure 9, the geothermal water and seawater samples are in the area near the seawater, which suggests that the chemical materials in geothermal water may be derived from the mixing action of seawater. The formation of geothermal water is significantly different from other water bodies. The chemical materials in spring water, most river water, and some well water are mainly controlled by meteoric water. In contrast, those in mineral water and the rest of the well water are influenced by rock weathering. It is noteworthy that some well-water and spring-water samples are located in the upper part of the rock-weathering control area and beyond the diagram frame, indicating that, in addition to the influence of rock weathering and a little evaporative concentration action, the underground cold water in the study area is also influenced to some extent by the mixing action of deep geothermal water.

5.4.2. Ionic Ratio Diagram

The water–rock action in underground water results in the dissolution or precipitation of minerals in which the molecular weight ratio of each ion is relatively constant. Therefore, the ionic ratio method can be adopted to determine the main minerals in which the reaction occurs, thereby further analyzing the main source of each ion [40].
The ratio of γCl−/γ (Na+ + K+) can be used to judge whether the dissolution action of silicate minerals influences underground water [41]. The γCl−/γ (Na+ + K+) of geothermal water in the study area is above 1 (Figure 10a), and that of other water bodies is less than 1. The shallow underground water, such as minerals, wells, and spring water, is slightly influenced by oceanic and atmospheric precipitation. The dissolution action of silicate minerals (e.g., potassium feldspar and soda feldspar) is the main source of excessive Na+ and K+ in shallow underground water, and positive ion exchange may also play a part in the formation of Na+. In contrast, the action has a reduced influence on underground geothermal water.
The ratio of γHCO3−/γ (Cl− + SO42−) can indicate the dissolution status of evaporite rock and silicate minerals [41,42]. The γHCO3−/γ (Cl− + SO42−) of geothermal water and seawater in the study area is above 1 (Figure 10b), which indicates that geothermal water is slightly influenced by the dissolution of evaporite rock and silicate minerals, and the γHCO3−/γ (Cl− + SO42−) of most the shallow underground water samples, such as the samples of mineral water, well water, and spring water, which may be jointly influenced by the dissolution of evaporite rock and silicate minerals, is near the 1.1 isometric line; some samples far from the 1.1 isometric line may largely influence by the dissolution of silicate minerals.
The ratios of γ (Ca2+ + Mg2+)/γHCO3− and γ (Ca2+ + Mg2+)/γ (HCO3− + SO42−) can further reflect the source of Ca2+, Mg2+, HCO3−, and SO42− in underground water [43]. Regarding the geothermal water and seawater samples in the study area, the ratios of γ (Ca2+ + Mg2+)/γHCO3− and γ (Ca2+ + Mg2+)/γ (HCO3− + SO42−) are above 1 (Figure 10c,d), proving that the geothermal water is slightly influenced by the dissolution of silicate and carbonate minerals. Regarding shallow underground water, such as mineral water, well water, and spring water, the ratios of γ (Ca2+ + Mg2+)/γHCO3− > 1 (Figure 10c) and γ (Ca2+ + Mg2+)/γ (HCO3− + SO42−) are near the 1.1 isometric line (Figure 10c,d), which indicates that the chemical characteristics may be jointly influenced by the dissolution of silicate and carbonate minerals.

5.5. Mixing Fraction of Shallow Groundwater

The silicon–enthalpy model can determine the cold-water mixing ratio in geothermal water. This model involves two silicon and enthalpy equations [44]:
Hcold·X + Hhot (1 − X) = Hspg.
Sicold·X + Sihot (1 − X) = Sispg.
where Hcold and Sicold denote, respectively, the content of end-member enthalpy (cal/g) and SiO2 (mg/L) in cold water before mixing, which is determined in this study by referring to the mean values (Hcold ≈ 25.3 cal/g, Si ≈ 13 mg/L) in cold spring at the yearly average temperature; Hhot and Sihot denote the content of end-member enthalpy (cal/g) and SiO2 (mg/L) in geothermal water before mixing, respectively. Hspg and Sispg denote the content of final enthalpy and SiO2, respectively, whose reference values vary at different temperatures (generally lower than the 100 °C hot spring, when the enthalpy value is close to spring-water temperature) [44,45]. X is the mixing ratio of underground cold water. Using Equations (1) and (2), a mixing ratio (X) − temperature (T) curve (Figure 9) was drawn. The Y value at the intersection of the enthalpy curve and the silicon curve represents the proportion of cold water in geothermal water [44]. As seen in Figure 11, the geothermal water of the geothermal fields of the Eastern Huchiwei Lot, Zhangjiabian, and Yongmo is nearly mixed with the shallow underground cold water [28]. Unlike other geothermal fields, the relatively low-temperature geothermal water in the Huweichi geothermal field lacks the mixing of shallow underground cold water. In contrast, the relatively high-temperature geothermal water has 60% shallow groundwater mixing.

5.6. Replenishment Elevation of Geothermal Water

The δD and δ18O of atmospheric precipitation have an elevation effect, so the replenishment elevation of geothermal water in the replenishment area can be determined using the calculation formula below [46]:
H = (δG − δG)/K + h
where h is the sample elevation (m); δG is the δ18O value of sample (‰); δG is the δ18O value of atmospheric precipitation (‰); K is the δ18O elevation gradient of atmospheric precipitation (‰/100 m); and H is the replenishment elevation (m) [47].
Previous research shows that the atmospheric precipitation is δD = −62.5‰ and the δ18O elevation gradient is −3.3‰/hm [48]. Based on our calculation, the replenishment elevation in the study area is 716 to 822 m. This is consistent with the elevation of the North Peak Mountain in Taishan City. These mountains have thin quaternary coverage, bare bedrock in most areas, and developed joints and fissures, which provide favorable conditions for the deep infiltration of atmospheric precipitation. According to the topographic characteristics and the recharge elevation of the study area, it can be inferred that the source of the geothermal water supply in the study area is piedmont water mixed with atmospheric precipitation [49,50].
In conclusion, the shallow underground water in the study area is mainly replenished by atmospheric precipitation, and its chemical materials are mainly from the dissolution of silicate and carbonate minerals. In contrast, the formation of geothermal water is less influenced by the dissolution of silicate and carbonate minerals, which is related to the mixing action between the deeply circulated atmospheric precipitation and seawater. This further demonstrates that seawater replenishment contributes to the formation of geothermal water. Since atmospheric precipitation and surface are the main replenishment sources of geothermal water, and the detention time of geothermal water is long due to its slow runoff, a strong water–rock reaction cannot be avoided.

5.7. Occurrence Environment

The characteristic component ratio of geothermal water is an effective indicator reflecting the spatial–temporal evolution, environmental occurrence, and change process of underground water [51,52]. The common ionic ratio coefficients include γNa+/γCl− and γSO42−/γCl−.
The metamorphic coefficient, γNa+/γCl−, can be used to explore the stratigraphic enclosure. In addition, the smaller γNa+/γCl− is, the more underground water is enclosed, concentrated, and metamorphic, indicating that the water environment is relatively reduced [53]. The mean value of γNa+/γCl− for the standard seawater and marine sedimentary water systems is 0.85, and the lixiviation value in rock salt-containing strata is 1 [54]. The range of variation for γNa+/γCl− in geothermal water is 0.67–0.85 (0.74 on average), which is similar to that of sedimentary water whose aquifer is relatively enclosed.
The desulfuration coefficient, γSO42−/γCl−, can reflect the oxidation-reduction environment of underground water: The smaller γSO42−/γCl− is, the more the underground water environment is reduced [55]. The range of variation for γSO42−/γCl− in geothermal water is 0.019 to 0.051 (0.029 on average, much less than the mean value (0.1) for seawater) [56], which indicates that geothermal water is in a highly reduced environment.

5.8. Geothermal Water Age

The geothermal water age generally reflects the retention time of replenishment water in the aquifer [57]. Carbon-14 dating is one of the most reliable research methods used to determine geothermal water age in the range of 1–30 ka [58,59]. The dating data of geothermal water in this area are shown in Table 3.
There are three groups of old geothermal fields whose underground water is circulated from deep-seated faults. This reveals that the source of replenishment is wide and the circulation time is long (about 20,000 years). Among them, the Yongmo geothermal field is fairly young due to the replenishment of a small amount of shallow underground water.

5.9. Geothermometer

Geothermal reservoirs are principally characterized by the infiltration into the hot aquifer of external cold water, which flows through the heating zone at the bottom of the reservoirs and is finally heated upon the strong and continuous replenishment of conductive heat flow. The research on the geothermal reservoir temperature is critically important for establishing a conceptual model and a formation mechanism of geothermal reservoirs in geothermal fields. At present, the geothermometers used to estimate the geothermal reservoir temperature mainly include a cation geothermometer, SiO2 geothermometer, gas geothermometer, and isotope geothermometer [50,51,52,53,54,55,56,57,58,59,60,61,62,63] These geothermometers have different application conditions, so it is necessary to select a suitable geothermometer according to the specific conditions of a geothermal system to obtain optimal estimates of the geothermal reservoir temperature [64]. In this study, a cation geothermometer and a SiO2 geothermometer were used to calculate the geothermal reservoir temperature in the study area.
Cation geothermometers usually include Na+/K+, Na+-K+-Ca2+, and K+/Mg2+ [49]. The Na+/K+ geothermometer applies to high-temperature underground water in calcium-poor regions, where the underground water temperature is generally 150 °C [65]. The geothermal water in the study area is rich in calcium and lies in the complete equilibrium area in the Na+-K+-Mg2+ triangular diagram, so the Na+/K+ geothermometer is not applicable. The Na+-K+-Ca2+ geothermometer applies to calcium-rich, chlorine-poor, and slightly mineralized medium-to-low-temperature geothermal water or shallow geothermal water [66,67]. When the geothermal water is not mixed with cold water, the K+/Mg2+ geothermometer can effectively estimate the geothermal reservoir temperature in medium-to-low-temperature geothermal fields [68].
Considering the uncertainty of a single thermometer, it is necessary to validate it with other types of thermometers. When the geothermal water temperature is less than 300 °C, SiO2 dissolved in water is generally not influenced by other ions or complex compounds. Therefore, the SiO2 geothermometer can be used to estimate the geothermal reservoir temperature in the study area.
The above analysis shows that the SiO2 and K+/Mg2+ geothermometers can accurately calculate the geothermal reservoir temperature in the study area (Table 4). Furthermore, the SiO2 and K+/Mg2+ geothermometers obtained similar temperature results and therefore proved to be adaptable. The mean geothermal reservoir temperature (Table 4) was obtained from the above two geothermometers, and the range of the geothermal reservoir temperature was 95.32 °C to 149.71 °C.

5.10. Circulation Depth of Geothermal Water

The circulation depth of geothermal water can be calculated according to the following formula, which contains the geothermal reservoir temperature as a parameter:
H = (t − t0)/g + h
where g is the geothermal gradient, calculated based on the average bedrock geothermal gradient, 2.8 °C/100 m; t is the geothermal reservoir temperature, °C; t0 is the average temperature (using 22 °C); and h is the depth of the constant temperature zone (using 20 m) [48]. The circulation depth of geothermal water calculated based on geothermal reservoir temperature data in various geothermal fields is shown in Table 5. As shown, the range of calculated depth for the geothermal fields is 2638.57 m to 4581.07 m.

6. Discussion

6.1. Regional Geological Structure Background

The study area is located in the South China plate, which was formed by splicing the southern Cathaysia landmass and the Yangzi landmass along the Jiangnan orogenic belt in the Neoproterozoic Era. From the Neoproterozoic to the Late Mesozoic, the South China lithosphere experienced multi-phase and episodic growth, with lateral accretion (block assemblage) and vertical growth (magmatic invasion). Finally, on the background of the late Mesozoic Paleo–Pacific plate subduction and intracontinental extension, the unique South China Basin ridge structure was formed [69,70,71]. In the early Cenozoic era, due to the westward subduction of the Pacific plate or the collision between the Indian plate and the Eurasian plate, the South China continent was generally in the extensional environment: there was a series of effusive rocks (dominated by extensional fault-related alkali basalt and basalt) under the joint action of intracontinental volcanos and rifts; in the Late Cenozoic era, influenced by the uninterrupted northward drifting of the Philippine Sea plate and the Australian plate, the structural environment on the southeastern South China plate was gradually subjected to left-lateral torsional stress, and the periphery of the South China plate was gradually enclosed or underwent structural inversion [72]. With the subduction of the Pacific plate, the partial melting of the mantle and the upwelling of the asthenosphere lead to extensive lithospheric thinning [72].

6.2. Underground Structural Features

The underground structural features can be obtained based on the S-wave velocity structure of the crust (Figure 12) and the geologic and geophysical data. As shown in the figure, the upper-crust S-wave velocity in the study area is low. Within the burial depth of 9–15 km, sedimentary rock and magma intrusions are widely distributed, and the wave velocity of the magma intrusions is generally more than 3 km/s. The middle-crust S-wave velocity is 3–3.6 km/s. The boundary line between the middle and lower crusts is at a depth of 20 km, where the lithology is dominated by salic shallow metasedimentary rock. The lower-crust S-wave velocity is 3.6–4 km/s, and the dominant lithology there is basalt or gabbro. The burial depth of Mohorovicic discontinuity is about 28 km and the lithosphere thickness is small. Some parts of the lower crust have low-velocity and high-conduction zones, where the upper mantle intrudes into the crust in the extensional setting. For example, the cutting depth of the southern and northern faults in Wugui Mountain is 15–20 km, penetrating the upper and middle crusts in the study area [73].

6.3. Distribution Characteristics of Geothermal Water Resources

The distribution of geothermal water resources in the study area is mainly influenced by the regional fault structure, normally at the intersection or on the periphery of the deep-seated northeastern fault and the northwestern fault. In the study area, frequent, intense tectonic movement and multi-stage magma intrusion resulted in secondary faults or broken rocks and extensional joint fissures in the fault zone and its surroundings. Under the early extrusion stress, on both sides of the fault, compression-shear fractures that strike along the fault are easily formed, which are characterized by local horizontal water blocking and vertical water guiding; under the superimposed two-stage tectonic stress of “squeezing and extending”, water conductivity in the facture zones near the fault is greatly improved [75], which provides the space and pathway for the storage and transportation of geothermal water. Meanwhile, the tectonic activity of deep-seated faults helps form the space for geothermal reservoirs, and the tectonic fissures connect deep geothermal reservoirs and shallow geothermal water, creating an important channel for the thermal transmission of geothermal water.

6.4. Geothermal Water Heat Resources

The geothermal anomaly in Zhongshan is abundant, and the research on its heat source has been going on for a long time. Geothermal energy is generally created from the heat generated by radioactive crust elements and the upper mantle; under different thermal backgrounds, the geothermal energy’s source and convergence mode in the geothermal system determines the distribution of thermal anomalies [76]. Regarding the surface heat flow, a direct reflection of deep heat, 40–60% is from the Earth’s mantle, about 40% is from crust radiation, and 50% is from other parts [77]. The terrestrial heat flow in the study area is about 75 mW/m2, which is higher than that on the Yangtze Plate (64.2 mW/m2) [78]; this means the study area has favorable conditions for forming an excellent geothermal field.
One reason for this result is that the sharp thinning of the lithosphere due to the subduction of the West Pacific plate helps the study area to form a high-temperature and thin Mohorovicic discontinuity, with a temperature of about 550 °C [79], and the high-conductivity layer below the study area, compared with the low-conductivity layer, makes it easier to transmit the deep heat to the shallow parts and form a high-temperature potential there, which causes the formation of local thermal anomalies on the surface in the study area. In addition, the thicker granite body in the study area has higher thermal conductivity, which further conducts the heat from deeper depths to the near surface [80] and promotes geothermal formation in the study area.
On the other hand, heat in the widely distributed igneous intrusions, such as granite from the Formative age, which is Mesozoic [81], has been totally lost and should not be considered as a heat source, but the radioactive decay of their rich heat-producing elements [81], including 238U, 232Th, and 40K, generated a large amount of heat in the study area and increased the surface heat flow.
The temperature–depth curve measured in the geothermal gradient hot well in the study area shows that the temperature rises with the depth, which is a linear temperature rise and a typical thermal transmission mode [50]. Spatially, the geothermal gradient is characterized by a low temperature in the northern rock zone, a high temperature in the middle rock zone, and a low temperature in the southwestern rock zone. Among them, the geothermal gradient in the middle rock zone is about 30.0 °C/km, higher than the mean value (25.0 °C/km) in the southern area, but the geothermal gradient in the northern and southwestern rock zones is about 20.0 °C/km, generally lower than the mean value. Therefore, the zone with a large geothermal gradient is concentrated in the middle fracture development zone and the intrusive rock zone south of Wugui Mountain. This demonstrates that the fractures in these zones are favorable channels for heat transmission and help increase the surface heat flow in the study area. Based on the above analysis, the geothermal field in the study area is a non-volcanic geothermal system heated by normal terrestrial heat flow.

6.5. Mode of Geothermal Water Circulation

The geothermal water in the study area is considered the Cl-Na·Ca and Cl-Na types of geothermal mineral water, and its degree of mineralization is higher than other water bodies (even seawater) in the study area. In addition, its D-δ18O isotope characteristics and replenishment elevation show that the geothermal water originates in the lateral replenishment of underground runoff of atmospheric precipitation in the middle–low mountainous areas near North Peak Mountain in Taishan City. The metamorphic coefficient and the desulfuration coefficient of geothermal water reflect that the geothermal reservoir is in an enclosed and strongly reduced environment, and the circulation rate and duration of geothermal water in the geothermal reservoir are low and long, which is consistent with the old age and deep circulation of geothermal water. The maturity of most of the geothermal water samples is high, which indicates that, upon a strong water–rock reaction, a chemical equilibrium has been reached between geothermal water and the surrounding rock of the geothermal reservoir, but the ionic ratio diagram does not support the conclusion that the water–rock reaction occurred between geothermal water and silicate rock and carbonatite. In addition, the Langlier–Ludwig diagram and the Gibbs diagram show that geothermal water has chemical characteristics similar to seawater or connate water. Therefore, according to this research, the mode of the geothermal water circulation in the study area is atmospheric precipitation, which infiltrates the deep-seated fault structure and enters the deep geothermal reservoir through lateral runoff. Upon long-term transportation and underground water runoff circulation, it mixes with seawater. It slowly passes through the bedrock geothermal reservoir, where it is heated by the rock temperature of the deep geothermal reservoir. It forms highly mineralized deep geothermal reservoir resources upon metasomatism, leaching, and hydrothermal alteration with bedrock to leach the salt. When transported to the fracture zone of the geothermal reservoir at the intersection of deep-seated faults, geothermal water quickly upwells under hydrostatic pressure and buoyant force to form a hot spring (Figure 13).
The above analysis proposes a conceptual genetic model for the geothermal water resources in the study area (Figure 13). According to the model, under the subduction of the Pacific plate, geothermal water is developed in deep-seated fault zones in the study area, which, together with the crust uplift and multi-stage strong magma movement, provide favorable conditions for the occurrence of geothermal resources. Based on a mixture of seawater, deep circulation, and the heating effect of terrestrial heat flow (geothermal warming) and heat production from rocks, the atmospheric precipitation undergoes a water–rock reaction to leach the salt and uplifts along fractures until cropping out of the surface; thus, highly mineralized geothermal fields are formed.

7. Conclusions

Based on the hydrochemical characteristics, the test results of stable isotopes, and the geological setting, this study investigated the hydrogeochemical evolution mechanisms and a conceptual genetic model of the geothermal water resources of the geothermal system through H and O isotope analysis and hydrochemical analysis in Zhongshan City. The findings allow for the following conclusions:
(1)
The analysis of the chemical and isotopic characteristics of water samples in the study area at different periods shows that geothermal water has a relatively enclosed occurrence condition, and its main replenishment source is a mixture of atmospheric precipitation and seawater.
(2)
The westward subduction of the Pacific plate, the significant thinning of the crust, and the development of deep-seated faults promote the transmission of mantle-derived heat to shallow sections. Moreover, the radioactive decay of rich heat-producing elements, including 238U, 232Th, and 40K, supplies a large amount of heat to the study area.
(3)
At favorable structural positions with good water storage conditions, the mixture of atmospheric precipitation and seawater that circulates deep in the Earth is heated by terrestrial heat flow (geothermal warming) and rock-derived heat; undergoes a water–rock reaction to leach the salt; and uplifts along the faults until cropping out of the surface, thereby forming highly mineralized geothermal fields.
(4)
In this study, a conceptual genetic model of the geothermal water resources in the study area was established, but the driving mechanism of geothermal water was not found and will be emphatically studied in the future.

Author Contributions

Y.L., C.H., W.C., X.B. and C.M. conceived the investigations and led the interpretation; Y.L. conceived preliminary investigations and contributed experimental infrastructure and contextual data; Y.L., C.H., W.H. and G.D. installed the instrumentation; Y.L. and X.Q. led the groundwater sampling and analysis; C.H. managed the data acquisition; S.C. and Y.Y. led the data management; Y.H., H.Y. and X.W. designed the modeling scenarios; Y.L. performed the modeling and contributed to the interpretation; Y.L. drafted the manuscript. All authors reviewed the manuscript. Y.L. and W.C. are co-first authors and have contributed equally to this work. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Project of Jiangxi Geological Exploration Fund Project [No. 20160007 and 20190006], Wuhan Multi factor Urban Geological Survey demonstration Project [No. WHDYS-2021-005], China Geological Survey [No. DD20211391], Jiangxi Province Key R&D Project [No. 2023BBG74005] and Nanchang Key Laboratory of Hydrogeology and High-Quality Groundwater Resources Exploitation and Utilization [No. 20232B21].

Data Availability Statement

The datasets used or analyzed during the current study are available from the corresponding authors upon reasonable request.

Conflicts of Interest

Author Gao Deng, Xiangrong Qiu, Shengnan Chen, Yongjun Yang, Ying Huang, Xuefeng Wu are employed by the company Guangdong Geological and Technical Engineering Consulting Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Water 16 01765 g001
Figure 1. (a) Location map of South China in China. (b) Geological map of South China, location; (modified from the literature [23]). (c) Geological map and water sample location map of Zhongshan City.
Figure 1. (a) Location map of South China in China. (b) Geological map of South China, location; (modified from the literature [23]). (c) Geological map and water sample location map of Zhongshan City.
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Water 16 01765 g002
Figure 2. Piper three−line diagram of water chemistry in major water bodies.
Figure 2. Piper three−line diagram of water chemistry in major water bodies.
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Water 16 01765 g003
Figure 3. Schoeller diagram of water chemistry of main water bodies.
Figure 3. Schoeller diagram of water chemistry of main water bodies.
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Water 16 01765 g004
Figure 4. Box plot of the major chemical index of water samples.
Figure 4. Box plot of the major chemical index of water samples.
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Water 16 01765 g005
Figure 5. SO42− vs. Cl− + HCO3−. Data are in meq/L. Iso-salinity lines are drawn for reference.
Figure 5. SO42− vs. Cl− + HCO3−. Data are in meq/L. Iso-salinity lines are drawn for reference.
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Water 16 01765 g006
Figure 6. Na−K−Mg triangle diagram [27,28,29].
Figure 6. Na−K−Mg triangle diagram [27,28,29].
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Water 16 01765 g007
Figure 7. Langlier−Ludwig diagrams of various water samples in the study area [32,33].
Figure 7. Langlier−Ludwig diagrams of various water samples in the study area [32,33].
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Water 16 01765 g008
Figure 8. δD and δ18O maps of various water samples in the study area.
Figure 8. δD and δ18O maps of various water samples in the study area.
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Water 16 01765 g009
Figure 9. Gibbs diagrams of various water samples in the study area [39].
Figure 9. Gibbs diagrams of various water samples in the study area [39].
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Water 16 01765 g010
Figure 10. (a) γ (Na+ + K+) versus γCl− map of various water samples in the study area; (b) γHCO3− versus γ (Cl− + SO42−) map of various water samples in the study area; (c) γ (Ca2+ + Mg2+) versus γHCO3− map of various water samples in the study area; (d) γ (Ca2+ + Mg2+) versus γ (HCO3− + SO42−) map of various water samples in the study area.
Figure 10. (a) γ (Na+ + K+) versus γCl− map of various water samples in the study area; (b) γHCO3− versus γ (Cl− + SO42−) map of various water samples in the study area; (c) γ (Ca2+ + Mg2+) versus γHCO3− map of various water samples in the study area; (d) γ (Ca2+ + Mg2+) versus γ (HCO3− + SO42−) map of various water samples in the study area.
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Water 16 01765 g011
Figure 11. Illustration of the silicon–enthalpy model of geothermal water in the study area. (a) The sample of D18. (b) The sample of D20. (c) The sample of D40. (d) The sample of D26. (e) The sample of D41. (f) The sample of D42.
Figure 11. Illustration of the silicon–enthalpy model of geothermal water in the study area. (a) The sample of D18. (b) The sample of D20. (c) The sample of D40. (d) The sample of D26. (e) The sample of D41. (f) The sample of D42.
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Water 16 01765 g012
Figure 12. The crustal S-wave velocity structure and its interpretation and inference (modified from literature [74]).
Figure 12. The crustal S-wave velocity structure and its interpretation and inference (modified from literature [74]).
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Water 16 01765 g013
Figure 13. Conceptual genetic model of geothermal water resevoir.
Figure 13. Conceptual genetic model of geothermal water resevoir.
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Table 1. Concentrations of main ions in each water body in the study area (mg/L).
Table 1. Concentrations of main ions in each water body in the study area (mg/L).
Type of WaterGeothermal FieldSampleTDSPHK+Na+Ca2+Mg2+Cl−SO42−HCO3−NO3−F−
Geothermal waterEastern Huchiwei geothermal fieldD1813,0337.40208.23230.001091.9017.206556.00206.9062.791.052.89
Geothermal waterYongmo
geothermal field		D2042847.4170.081034.2226.0815.601888.10142.50116.673.332.12
Geothermal waterZhangjiabian
geothermal field		D4010,4996.9283.812532.00117151.625770.9206.2373.570.751.19
Geothermal waterHuweichi
geothermal field		D2697456.7871.132487.71078.5075.565644.50162.0778.450.800.75
Geothermal waterHuweichi
geothermal field		D4110,3007.44229.003000.001050.0015.005680.00208.0066.50<0.12.22
Geothermal waterHuweichi
geothermal field		D4210,0007.55222.003030.001060.0014.305340.00205.0070.60<0.12.42
Well water/D012216.4814.0414.0931.134.2614.7836.6568.7743.343.3
Well water/D021886.3914.1017.0723.327.4320.0322.62107.7222.390.21
Well water/D03475.633.675.512.160.346.350.7611.967.870.21
Well water/D051015.324.7813.2410.461.2515.8110.2211.9633.710.12
Well water D06426.723.133.841.470.305.282.065.985.370.15
Well water/D07225.630.642.673.700.166.350.798.971.050.07
Well water/D081554.554.0513.7914.331.4521.099.792.9953.240.15
Well water/D09535.673.033.251.930.816.351.408.974.620.12
Well water/D10264.06.8618.8511.2146.123.5015.8120.91140.5935.590.21
Well water/D11445.882.793.752.130.225.282.128.972.775.88
Well water/D121957.0626.7711.8527.811.9016.7723.5983.7825.790.09
Well water/D131516.504.569.2322.424.867.3416.968.7721.330.19
Well water/D14885.743.566.079.790.895.258.8529.909.480.10
Well water/D15515.761.641.858.280.433.1610.0114.955.680.05
Well water/D16646.043.813.434.210.306.311.4811.965.890.05
Well water/D212555.9111.7229.6428.924.5444.0620.7550.8362.320.16
Well water/D24565.571.313.304.411.535.250.0720.935.330.12
Well water/D251415.477.6115.907.762.7117.8311.0514.9534.970.12
Well water/D273097.462.5521.7341.761.047.394.75171.440.883.25
Well water/D313836.9418.8627.0176.137.0031.4631.79273.145.160.21
Well water/D321717.781.675.8444.604.6512.5918.57130.767.190.14
Well water/D332726.329.4819.7819.783.1818.8862.7375.5549.160.17
Well water/D353737.409.3022.3183.135.3230.4234.95232.4635.440.09
Well water/D363726.6316.7828.0766.024.5351.3935.09104.6189.110.12
Well water/D371526.717.3119.7616.811.7929.3711.5755.2111.430.23
Well water/D382346.7811.2011.7944.202.0814.6833.80104.6132.110.21
Mineral water/D191146.372.865.6911.302.307.346.3735.886.890.61
Mineral water/D173466.6918.5710.4673.754.9611.5226.91206.3746.510.15
Spring water/D04435.562.403.980.910.165.280.735.983.550.12
Spring water/D22356.720.742.691.391.285.250.808.974.500.06
Spring water/D231816.118.0431.087.615.1543.0021.0517.9429.260.16
River water/D2971.37.233.035.656.860.675.284.6029.064.350.19
River water/D3086.56.682.955.246.361.327.395.5229.064.450.19
River water/D3944.57.062.163.361.450.305.281.5714.531.930.21
River water/D341837.471.315.3445.844.7610.4915.59139.477.120.17
Sea water/D2824028.4330.19617.6073.83102.231192.2193.31122.049.030.23
Table 2. Isotopic data of each water body in the study area.
Table 2. Isotopic data of each water body in the study area.
Type of WaterGeothermal FieldSampleDV-SMOW‰18OV-SMOW‰
Atmospheric precipitation/S01−56.5−7.70
Atmospheric precipitation/S04−83.6−10.87
Atmospheric precipitation/S05−90.8−11.31
Atmospheric precipitation/S06−91.5−11.42
Atmospheric precipitation/S07−92.0−12.45
Atmospheric precipitation/S08−97.4−11.82
Atmospheric precipitation/S09−72.9−11.00
Atmospheric precipitation/S10−86.2−10.30
Atmospheric precipitation/S11−68.6−8.80
River water/D29−48.9−8.01
River water/D30−52.2−7.75
River water/D39−46.02−5.13
River water/D34−46.51−4.80
Geothermal waterHuweichiD26−25.7−2.40
Geothermal waterYongmoD20−41.1−3.70
Geothermal waterZhangjiabianD40−36.08−3.65
Well water D12−65.5−9.10
Well water/D13−58.2−7.30
Well water/D22−39.33−5.22
Well water/D23−38.38−5.12
Well water/D24−40.26−5.10
Well water/D25−49.78−5.12
Mineral water/D25−38.34−5.03
Spring water/D04−42.0−9.1
Spring water/D22−35.6−8.2
Sea water/D28−45.86−4.88
Table 3. Summary of 14C age isotope data.
Table 3. Summary of 14C age isotope data.
Geothermal FieldHuweichi Geothermal FieldYongmo Geothermal FieldEastern Huchiwei Geothermal Field
14C age19,600751522,880
Table 4. Calculation results of thermal storage temperature of geothermal water in the study area.
Table 4. Calculation results of thermal storage temperature of geothermal water in the study area.
Type of WaterSampleK+/Mg2+ GeothermometersSiO2 GeothermometersAverage Temperature
Geothermal waterD18144.92154.49149.71
Geothermal waterD20111.94154.26133.1
Geothermal waterD40100.0790.5795.32
Geothermal waterD2690.59100.8495.72
Geothermal waterD41148.14 145.65 146.89
Geothermal waterD42146.48 142.58 144.53
Table 5. Results of geothermal water circulation depth.
Table 5. Results of geothermal water circulation depth.
SampleD18D20D40D26D41D42
Thermal storage temperature (°C)149.71133.195.3295.72148.14146.48
Water circulation depth (m)4581.073987.862638.572652.8645254465.71
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Li, Y.; Bai, X.; Huang, C.; Chen, W.; Ma, C.; Huang, W.; Deng, G.; Qiu, X.; Chen, S.; Yang, Y.; et al. Genesis of Geothermal Waters in Zhongshan City, China: Hydrochemical and H-O-C Isotopic Implications. Water 2024, 16, 1765. https://doi.org/10.3390/w16131765

AMA Style

Li Y, Bai X, Huang C, Chen W, Ma C, Huang W, Deng G, Qiu X, Chen S, Yang Y, et al. Genesis of Geothermal Waters in Zhongshan City, China: Hydrochemical and H-O-C Isotopic Implications. Water. 2024; 16(13):1765. https://doi.org/10.3390/w16131765

Chicago/Turabian Style

Li, Yanan, Ximin Bai, Changsheng Huang, Wei Chen, Chuanming Ma, Wei Huang, Gao Deng, Xiangrong Qiu, Shengnan Chen, Yongjun Yang, and et al. 2024. "Genesis of Geothermal Waters in Zhongshan City, China: Hydrochemical and H-O-C Isotopic Implications" Water 16, no. 13: 1765. https://doi.org/10.3390/w16131765

APA Style

Li, Y., Bai, X., Huang, C., Chen, W., Ma, C., Huang, W., Deng, G., Qiu, X., Chen, S., Yang, Y., Huang, Y., Wu, X., & Ye, H. (2024). Genesis of Geothermal Waters in Zhongshan City, China: Hydrochemical and H-O-C Isotopic Implications. Water, 16(13), 1765. https://doi.org/10.3390/w16131765

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