Sampling work was conducted in December and January 2020, these months are regarded as the dry and cold seasons based on local meteorological records. The experimental samples of the three geothermal fluids (Well ZK1, ZK2, and JK1) studied in this paper were all from the carbonate karst water of the Lower Triassic Jialingjiang Formation (T₁ j) in the urban area of Jingmen. The remaining experimental samples were collected from surface water (lake), groundwater (cold spring, pore-fractured water), local rain, and snow in the field. The sampling information of the three geothermal wells is shown in Table 1.

The results of the water chemical analysis of the three geothermal wells, the ZK1, ZK2, and JK1, surface water (lake), and groundwater (cold spring, pore-fractured water) are shown in Table 2. According to the Piper three-line diagram (Figure 2), it can be seen that geothermal fluid is very different from surface water and groundwater. The water chemical type of geothermal fluid is Ca-SO₄ type. The cation is mainly Ca²⁺, accounting for more than 70% of the milliequivalent percentage of cations, followed by Mg²⁺, accounting for about 20%. The anion is dominated by SO₄ ^2-, accounting for about 80% of the milliequivalent percentage of anions, followed by HCO₃ ⁻, accounting for about 15%. The total dissolved solid (TDS) is greater than 1,500 mg/L. The water chemical type of surface water (lake) and groundwater (cold spring, pore-fractured water) is Ca-HCO₃ type. Like geothermal fluid, the cation is mainly Ca²⁺. The anion is dominated by HCO₃ ⁻, followed by SO₄ ^2-. The maximum TDS is 622.96 mg/L, which is significantly lower than that of geothermal fluid. Whether it is geothermal water, surface water, or groundwater, the pH value is basically greater than 7.0, which is weakly alkaline.

The Upper Cretaceous Paomagang Formation (K₂p) contains abundant gypsum, and SO₄ ^2- is dissolved in gypsum. The fault zone provides the space and passage for the groundwater deep circulation, and the leaching of groundwater creates a hydrologic geochemistry environment conducive to fluorine migration and accumulation. In addition, when the water contains a lot of sodium, it forms soluble sodium fluoride, which ensures the high stability of fluorine in solution and creates favorable conditions for the enrichment of fluorine in the geothermal fluid.

The soluble ions (Na⁺, Ca²⁺, Cl⁻, and HCO₃ ⁻) of groundwater can be sourced from a variety of natural processes, such as precipitation, water-rock interaction, and evaporation. The TDS versus Na⁺/(Na⁺ + Ca²⁺)and Cl⁻/(Cl⁻ + HCO₃ ⁻)diagrams can be used to evaluate the effects of these three different processes (Gibbs, 1970). The majority of the samples plotted in the field of rock weathering (Figures 3A,B), indicating water-rock interaction is the main factor controlling dissolved hydro chemical components of water samples.

From the experimental results of the trace element content in the geothermal fluids of the wells ZK1, ZK2, and JK1 (Table 3), there are as many as 18 trace elements in the geothermal fluids. In addition to the common Sr and F with high content, they also contain Fe, B, Ba, Li, Br, I, and As, and other elements. In accordance with the classification standard of Specification for Estimation and Evaluation of Geothermal Resources, the geothermal fluids of the three geothermal wells, the ZK1, ZK2, and JK1 are divided according to the content of trace elements, which can be named for “fluorine water” and “strontium water".

The quantitative judgment of the chemical equilibrium state between minerals and thermal fluids in geothermal systems is not only the basic premise for evaluating the applicability of the conventional chemical temperature scale methods but also an important basis and means for studying many hydrothermal-chemical processes (Wang et al., 1993). The Na-K-Mg triangular diagram method was proposed by Giggenbach and Goguel (1989) to study the equilibrium state and type division of underground thermal fluids. The coordinate formulas in the Na-K-Mg triangular diagram are as follows: S = C N a 1000 + C K 100 + C M g (2) N a   % = C N a 10 S (3) K   % = C K S (4) M g   % = 100 C M g S (5) Where C_Na, C_K, and C_Mg are the concentration of Na, K, and Mg ions in the water, respectively, mg/L.

The three vertices of the triangular diagram represent the maximum value of Na/1,000, K/100, and M g . Using this triangle, the chemical composition of the geothermal fluids can be divided into three types: “fully equilibrated waters”, “partially equilibrated waters” and “immature waters”. The concentrations of the Na, K, and Mg ions in the three geothermal wells, the ZK1, ZK2, and JK1 in Table 1 are converted into percentages using formulas (3) to (6). The calculation results are shown in Table 6. It can be found by projecting the calculation results into the Na-K-Mg triangle diagram that all the experimental data of the geothermal fluids in the urban area of Jingmen fall in the “immature waters” area (Figure 5), which indicates that the water-rock reaction of geothermal fluid in this area has not yet reached equilibrium.

The mixing effect among different fluids in geothermal systems exists extensively. Geothermal water is often mixed water, and deep hot water or steam mixes with other water on its way to the surface or to a geothermal well (Yin, 1988). This situation generally occurs in the upper part of geothermal systems. Cold water is somewhat denser than hot water and may submerge under sufficient pressure into shallow thermal aquifers.

The Schoeller plot method is used to illustrate changes in water chemistry over time for water samples at different locations and is often used to evaluate the mixing of hot and cold water. The method is to connect the logarithm of the component concentrations of many water samples into a curve. Since the logarithmic coordinates are used for the concentrations, the concentrations in a large range can be represented in the figure. The effect of mixing with dilution water has the effect of shifting the curve vertically without changing its shape. By virtue of the experimental data of water chemistry analysis, it can be seen in the semi-logarithmic Schoeller plot that most of the components of the geothermal water (such as K⁺, Mg²⁺, Ca²⁺, SiO₄ ^2-, Cl⁻, and F⁻) follow the same trend as the bedrock underground cold water samples except for some minor differences (Figure 6), which indicates that subsurface hot water mixes with cold water.

Meanwhile, according to the T isotope detection results of the three geothermal wells, the ZK1, ZK2, and JK1, cold spring water, rainwater, and snow water, the T content of the geothermal fluid is relatively close to that of cold spring water and rainwater samples. Combined with the results of the field hydrogeological survey, the possible reason is the mixing of shallow cold water. After the 1990s, due to the frequent activities of urban construction and mountain quarrying in the recharge area of the northern part of the geothermal field, the infiltration intensity and infiltration volume of surface water increased, and geothermal water was mixed with shallow cold water in the process of rising along the Jingmen fracture and its fracture zone, and the low T value in hot water (T value in ancient hot water is 0) mixed with the high T value in the shallow cold water, resulting in a significant increase of T content in hot water. Based on the analysis of the Na-K-Mg triangle diagram, the experimental data of the geothermal fluid in the urban area of Jingmen area all fall in the “immature waters” area, and they are all located near the apex of the lower right corner of the Mg ion, which indicates that the geothermal water in this area is in the primary stage of the water-rock reaction. It is mainly caused by the mixing of a large amount of shallow cold water.

Based on the analysis results of the Schoeller diagram method, T isotope and Na-K-Mg triangle diagram, it is jointly defined that there is a mixing of underground hot water and shallow cold water in the urban area of Jingmen.

To evaluate the mixing ratios of cold water, the field temperature and silica concentration of geothermal fluid and shallow cold water has been carried out, based on silicon and enthalpy content. Fournier and Truesdell (1974), Fournier (1977) proposed a graphical way. The intersection of silica and enthalpy content represents the maxing portion of the geothermal fluid and shallow cold water and can estimate the reservoir temperature (Zhang et al., 2017) (Figure 7) We can infer the mixing ratio and reservoir temperature are 0.56–0.78 and 112–175°C, respectively.

A silica-enthalpy graphic method can be used to estimate the reservoir temperature of the mixing water (Giggenbach, 1988; Gupta and Roy, 2007; Sun, 2015), and the relationship between geothermal fluid temperature and silica and enthalpy content (Table 7) (Fournier and Truesdell, 1974; Gupta and Roy, 2007; Maria et al., 2012). This method has two assumptions: ① No further solutions or deposition of silica exist before or after mixing. ② The mineral that controls the silica concentration is quartz. Using a straight line to connect the silica-enthalpy points of cold water and thermal water, and make an extension line to intersect the quartz solubility curve (No steam loss) (Figure 8) (Fournier and Truesdell, 1974). The mixing ratio of cold water is calculated by dividing the distance between the silicon-enthalpy points of cold water and the geothermal fluid by the distance between the silicon-enthalpy points of cold water and the intersection of the above extension line. When no steam loss occurs, the mixing ratio of cold-water ranges from 30%–40%, while the reservoir temperature is about 105–154°C, suggesting that a large supply of cold water is in a surficial place.

The preliminary analysis of the genetic model of the underground hot water in the geothermal field is as follows: the groundwater is recharged by meteoric precipitation in the exposed carbonate rock area of the middle and low mountains in the Xiaoyan-Lixi area in northwest Jingmen, and recharge altitude is 395–435 m. Geothermal water is modern water (less than 5–10 years). The red sandstone and mudstone of the Upper Cretaceous Paomagang Formation (K₂ p) distributed in the area are good caprocks of the geothermal field. Such rocks, with high mud content, extremely weak water permeability, and poor conductivity, are good thermal insulation materials, and heat energy is not easily dissipated from them. The thermal reservoirs are mainly calcareous dolomite and dolomite of the Lower Triassic Jialingjiang Formation (T₁ j). Jingmen Fault is the thermal and water-conducting structure of the geothermal field. According to the drilling exposure, the carbonate core near the fracture zone is more broken, dense with small fissures generally 2–5 mm wide, and some measuring 1–3 cm wide. Generally speaking, there are several and sometimes even 10 to 18 fissures distributed per 10 cm. The fissures are open, mostly filled with calcite veins, and obvious water erosion traces can be found on the fracture surface, indicating obvious groundwater activity. Jingmen Fault has an extension of 170 km and a cutting depth of more than 1,000 m. Some studies believe that the fracture is a fracture of the silicon-magnesium layer, which has cut through the Moho surface. Jingmen Fault enables the exchange of heat between groundwater and the deep part of the Earth’s crust, and the groundwater migrates to the site with favorable geological conditions, upwelling along the fractured zone, forming a relatively high-temperature center, and a part of thermal water laterally “seeps” to the shallow pores in the aquifer or pore and fissure aquifer to mix with the cold water in it and form medium and low temperature underground hot water around the center of the geothermal area. During the Cretaceous-Paleogene period, Jingmen fault depression sag on the east side of Jingmen Fault subsided significantly and received thicker Cretaceous-Paleogene deposits, while the west side of Jingmen Fault was relatively uplifted, so the thermal reservoir was buried relatively shallowly. According to the above genetic conditions, the genetic model diagram of the geothermal system in the urban area of Jingmen has been initially constructed (Figure 9).