Recharge Sources and Genetic Model of Geothermal Water in Tangquan, Nanjing, China

: This paper introduces a method to study the origin of geothermal water by analysis of hydrochemistry and isotopes. In addition, the genetic mechanism of geothermal water (GTW) is revealed. The study of the origin of geothermal water is useful for the sustainability of geothermal use. As an example, Tangquan is abundant in GTW resources. Elucidating the recharge sources and formation mechanism of the GTW in this area is vitally important for its scientiﬁc development. In this study, the GTW in Tangquan was systematically investigated using hydrochemical and isotopic geochemical analysis methods. The results show the following. The GTW and shallow cold water in the study area differ signiﬁcantly in their hydrochemical compositions. The geothermal reservoir has a temperature ranging from 63 to 75 ◦ C. The GTW circulates at depths of 1.8–2.3 km. The GTW is recharged by the inﬁltration of meteoric water at elevations of 321–539 m and has a circulation period of approximately 2046–6474 years. The GTW becomes mixed with the shallow cold karst water at a ratio of approximately 4–26% (cold water) during the upwelling process. In terms of the cause of its formation, the geothermal system in the study area is, according to analysis, of the low-medium-temperature convective type. This geothermal system is predominantly recharged by precipitation that falls in the outcropping carbonate area within the Laoshan complex anticline and is heated by the terrestrial heat ﬂow in the area. The geothermal reservoir is composed primarily of Upper Sinian dolomite formations, and its caprock is made up of Cambrian, Cretaceous, and Quaternary formations. Through deep circulation, the GTW migrates upward along channels formed from the convergence of northeast–east- and north–west-trending faults and is mixed with the shallow cold water, leading to geothermal anomalies in the area.


Introduction
Geothermal resources, consisting of heat, minerals and water, are types of green resources [1,2]. The development of geothermal has been popular all over the world. However, geothermal water extraction without scientific planning will destroy geothermal resources, which will cause a decline of the hydraulic head and the reservoir temperature [3].
One famous hot spring is located in Tangquan, Nanjing, which north of the Yangtze River. The geothermal water (GTW) is quite abundant. With a discharge rate of 4590 t per day and a maximum temperature of 47 • C, the GTW in Tangquan contains over 30 trace elements beneficial to the human body. The development of geothermal resources in Tangquan began in the 1960s. By 2010, a series of geothermal and geological surveys, detailed surveys, and explorations had been conducted in this area. These efforts yielded a relatively deep understanding of the formation conditions and geological structures in this area, determined its geothermal zone, and provided preliminary estimates of the quantity of GTW resources. However, the temperature of the geothermal reservoir (T R ) and the

Materials
The sampling campaign was implemented in April 2019; a total of 15 water samples were collected, including 6 GTW samples and 9 cold groundwater samples from civil wells. Figure 1 shows basic information about the sampling sites. All samples for hydrochemical analyses were filtered with a 0.45 µm membrane before bottling. Samples for cation analysis were acidified with ultra-purified HNO 3 to adjust the sample to pH < 2. A total of 60 mL of filtered water was collected into High Density Polyethylene(HDPE) vials for anion analyses. A total of 20 mL of filtered water was collected into screw-capped HDPE vials for stable δ 18 O and δ 2 H analyses, whereas 1000 mL of unfiltered water was collected into screwcapped HDPE bottles for δ 13 C and δ 14 C analyses, 200 mL of water for 87 Sr/ 86 Sr analyses, and 1000 mL of water for δ 34 S analyses. K + , Na + , Ca 2+ , Mg 2+ , Cl − , HCO 3 − , SO 4 2− , and H 2 SiO 3 were tested at the Nanjing Supervision and Testing Center for Mineral Resources. 2 H, 18 O, 18 O, 87 Sr, 86 Sr, and 14 C were tested at the Ministry of Land and Resources of China

Methodology
Quartz geothermometers were used in this study to calculate TR. The circulation depth (Z) value of the geothermal fluid at each sampling site can be calculated based on TR using the following equation [12]: where Z is the circulation depth (m), Z0 is the depth of the constant-temperature zone (m), TR is the temperature of the geothermal reservoir (°C), T0 is annual average the temperature of the constant-temperature zone (°C), and gradT is the geothermal gradient (°C/hm).
The following equation shows how to calculate the GW recharge elevation based on the elevation effect of the 18 O isotope in precipitation [13]: 18 18 where H is the GW recharge elevation (m), δ 18 OGTW is the isotopic composition of the sample collected at the GW sampling site, δ 18 Or is the isotopic composition of the sample of precipitation at the reference site, gradδ 18 O is the average gradient in the low-latitude southern region of China (−0.3‰/100 m) [14], and Hr is the ground elevation at the GW reference site (i.e., the Nanjing Observation Station of the GNIP) (26 m). The mixing relationship between the GTW and the cold GW in the study area was examined using a binary mixing model based on Sr and 87 Sr/ 86 Sr, as shown below [15][16][17][18] where (

Methodology
Quartz geothermometers were used in this study to calculate T R . The circulation depth (Z) value of the geothermal fluid at each sampling site can be calculated based on T R using the following equation [12]: (1) where Z is the circulation depth (m), Z 0 is the depth of the constant-temperature zone (m), T R is the temperature of the geothermal reservoir ( • C), T 0 is annual average the temperature of the constant-temperature zone ( • C), and gradT is the geothermal gradient ( • C/hm). The following equation shows how to calculate the GW recharge elevation based on the elevation effect of the 18 O isotope in precipitation [13]: where H is the GW recharge elevation (m), δ 18 O GTW is the isotopic composition of the sample collected at the GW sampling site, δ 18 O r is the isotopic composition of the sample of precipitation at the reference site, gradδ 18 O is the average gradient in the low-latitude southern region of China (−0.3‰/100 m) [14], and H r is the ground elevation at the GW reference site (i.e., the Nanjing Observation Station of the GNIP) (26 m). The mixing relationship between the GTW and the cold GW in the study area was examined using a binary mixing model based on Sr and 87 Sr/ 86 Sr, as shown below [15][16][17][18] contents of the mixed sample, end member 1, and end member 2, respectively; and f is the mixing ratio for end member 1.
The age of the GTW in Tangquan was determined by 14 C using the following equation [19]: where α 0 and α t are the initial 14 C activity and the 14 C activity after a time period t, respectively. However, the dissolution of carbonate minerals dilutes the initial 14 C activity of the dissolved inorganic carbon (DIC) in GW, resulting in an older 14 C age.

General Information of Tangquan
Tangquan is located to the west of the main urban area of Nanjing and within the Yangtze Platform Fold Belt of the Yangtze Paraplatform. In terms of geotectonic elements, Tangquan encompasses low mountains and hills. All the hot springs and geothermal wells in Tangquan are distributed at the junction of the Laoshan Uplift and the Liuhe-Quanjiao Sag (Figure 1), where the terrain is high in the southeast and low in the northwest and faults and fissure structures have developed. The Laoshan complex anticline on the south side is distributed along the east-northeast (ENE) direction and covers an area of approximately 100 km 2 . In addition, the highest altitude of the Laoshan complex anticline is 442 m at its main peak, Longdongshan (also known as Dalashan), and the altitudes of the remaining areas are mostly 180-390 m. The transition zone of the plain polder area sits at altitudes of 15-40 m. Outcrops of Upper Sinian dolomite and limy dolomite are present at the core of the Laoshan complex anticline and plunge to the piedmont hillocky and plain area. With developed karst fissures and a relatively large thickness, this series of carbonate formations is the main geothermal reservoir being developed and used in the study area, which is overlain by thin-bedded Upper Cambrian dolomite, Upper Cretaceous silty and fine sandstone, and Neogene and Quaternary loose sedimentary formations.
In the study area, two main groups of faults control the Mesozoic and Cenozoic sedimentary characteristics and are closely related to the geothermal system. (1) ENE-trending Tangquan-Fanji Fault (F1). Located on the northwest side of Mount Laoshan, the Tangquan-Fanji Fault is a large normal fault that dips northwestward, cuts deep into the ground, and is composed of multiple parallel faults. (2) Northwest (NW)-trending Tangquan-Lulang Fault (F21) and East Tangquan-Jiangning Township Fault (F22). Formed in a relatively later period, this group of faults is large in size, cuts deep into the ground, and offsets earlier-formed northeast (NE)-trending faults. Figure 2 shows their planar locations [20].  Table 1 summarizes some of the test results. The temperatures of the GTW samples ranged from 32 to 46 °C (the temperature of sample H3 was relatively low due to an insufficient water discharge time).

Hydrochemical Composition
The Total Dissolved Solids (TDS) of the GTW ranged from 776 to 2794 mg/L, with an average of 2070 mg/L. Hydrochemically, the GTW was primarily of the SO4-Ca type. The TDS of the cold GW was relatively low at 312-1610 mg/L, with an average value of 733 mg/L. The Piper diagram in Figure 3 shows the hydrochemistry of the GW in the study  Table 1 summarizes some of the test results. The temperatures of the GTW samples ranged from 32 to 46 • C (the temperature of sample H3 was relatively low due to an insufficient water discharge time).

Hydrochemical Composition
The Total Dissolved Solids (TDS) of the GTW ranged from 776 to 2794 mg/L, with an average of 2070 mg/L. Hydrochemically, the GTW was primarily of the SO 4 -Ca type. The TDS of the cold GW was relatively low at 312-1610 mg/L, with an average value of 733 mg/L. The Piper diagram in Figure 3 shows the hydrochemistry of the GW in the study area. Hydrochemically, the cold GW was primarily of the SO 4 -HCO 3 -Ca/HCO 3 -SO 4 -Ca-Na type. Sampling site C1 was close to GTW well H1, which reaches to depths where carbonate rocks are distributed. As a result of the dissolution of carbonate rocks, hydrochemically, the sample collected from sampling site C1 was of the SO 4 -Ca-Mg type.
area. Hydrochemically, the cold GW was primarily of the SO4-HCO3-Ca/HCO3-SO4-Ca-Na type. Sampling site C1 was close to GTW well H1, which reaches to depths where carbonate rocks are distributed. As a result of the dissolution of carbonate rocks, hydrochemically, the sample collected from sampling site C1 was of the SO4-Ca-Mg type. As demonstrated by the hydrochemistry represented by the Piper diagram, the cold-GW sampling C1-C9 points are scattered and distributed in areas different from those where the GTW H1-H6 are distributed. These differences in hydrochemical composition show that the GTW and shallow cold GW undergo different reactions with rocks, and that the formation of deep GTW is not closely hydraulically linked with the shallow cold GW.

Estimation of the Geothermal Reservoir Temperature (TR) and Depth (Z)
The concentrations of the major elements in the GTW in a geothermal reservoir depend heavily on the temperature, the variation of which significantly affects the cationic ratios and the concentration of water-soluble SiO2 in the GTW. Two main types of geothermometers are commonly used, namely cationic geothermometers (e.g., Na-K and Na-K-Ca geothermometers), which are obtained based on the relationship between the K-Ca-Na-Mg concentration ratio and the temperature [21][22][23], and SiO2 geothermometers [24], which depend on the solubility of minerals (e.g., quartz and chalcedony) that control water-soluble SiO2. All the GTW samples collected from the study area fall into the immature water area on the Na-K-Mg ternary diagram shown in Figure 4. Thus, the GTW in the study area has not reached equilibrium with feldspar silicate minerals such as Na, K, and Mg silicate materials. This finding suggests that general cationic geothermometers, such as Na-K, K-Mg and Na-K-Ca, are unsuitable for the study area. As demonstrated by the hydrochemistry represented by the Piper diagram, the cold-GW sampling C1-C9 points are scattered and distributed in areas different from those where the GTW H1-H6 are distributed. These differences in hydrochemical composition show that the GTW and shallow cold GW undergo different reactions with rocks, and that the formation of deep GTW is not closely hydraulically linked with the shallow cold GW.

Estimation of the Geothermal Reservoir Temperature (TR) and Depth (Z)
The concentrations of the major elements in the GTW in a geothermal reservoir depend heavily on the temperature, the variation of which significantly affects the cationic ratios and the concentration of water-soluble SiO 2 in the GTW. Two main types of geothermometers are commonly used, namely cationic geothermometers (e.g., Na-K and Na-K-Ca geothermometers), which are obtained based on the relationship between the K-Ca-Na-Mg concentration ratio and the temperature [21][22][23], and SiO 2 geothermometers [24], which depend on the solubility of minerals (e.g., quartz and chalcedony) that control water-soluble SiO 2 . All the GTW samples collected from the study area fall into the immature water area on the Na-K-Mg ternary diagram shown in Figure 4. Thus, the GTW in the study area has not reached equilibrium with feldspar silicate minerals such as Na, K, and Mg silicate materials. This finding suggests that general cationic geothermometers, such as Na-K, K-Mg and Na-K-Ca, are unsuitable for the study area.
Thus, quartz geothermometers were used in this study to calculate T R . Table 2 summarizes the calculation results. The T R values calculated using the amorphous silicon, α-cristobalite, and β-cristobalite geothermometers are lower than those measured in the field. Some of these calculated T R values are even negative and are therefore inconsistent with real-world conditions. The T R values calculated using the chalcedony geothermometer are slightly higher than those at the mouths of the wells and are consistent with real-world conditions and thus are the most reliable. T R in Tangquan, Nanjing, ranges from 63 to 75 • C, suggesting the presence of low-medium geothermal resources in normal geothermal background conditions. Well H3 was undisturbed prior to sampling in this study. The field sampling conditions precluded extraction of water for a protracted period of time. As a consequence, samples of hot water with a relatively low temperature (approximately Thus, quartz geothermometers were used in this study to calculate TR. Table 2 summarizes the calculation results. The TR values calculated using the amorphous silicon, αcristobalite, and β-cristobalite geothermometers are lower than those measured in the field. Some of these calculated TR values are even negative and are therefore inconsistent with real-world conditions. The TR values calculated using the chalcedony geothermometer are slightly higher than those at the mouths of the wells and are consistent with realworld conditions and thus are the most reliable. TR in Tangquan, Nanjing, ranges from 63 to 75 °C, suggesting the presence of low-medium geothermal resources in normal geothermal background conditions. Well H3 was undisturbed prior to sampling in this study. The field sampling conditions precluded extraction of water for a protracted period of time. As a consequence, samples of hot water with a relatively low temperature (approximately 20 °C), which reached temperature equilibrium with the shallow water, instead of the in situ GTW, were collected. Thus, the TR values measured in the field and those calculated using the chalcedony geothermometer are relatively low.   According to the heat-flow data for the study area, gradT is 2.4 • C/hm, Z 0 is 12-20 m, and the annual average temperature is 17 • C [25] Thus, the GTW in the study area circulates at depths of approximately 1.8-2.3 km. Table 3 summarizes the calculation results.

Characteristics of Stable D and 18 O Isotopes
Fifteen isotope samples of D and 18 O were collected from the GW in the study area. The test data for the D and 18 O isotopes were plotted to show the δD-δ 18 O relationship ( Figure 5). The local meteoric water line (LMWL) was produced by linear regression of the isotope data for monthly precipitation (Lu et al. 2018) provided by the Nanjing Observation Station of the Global Network for Isotopes in Precipitation (GNIP). As demonstrated in Figure 5, the δD (‰) and δ 18 O (‰) values for both the GTW and the cold GW fall near the global meteoric water line and the LMWL for Nanjing. This suggests that the GTW and the cold GW are primarily recharged by the infiltration of atmospheric precipitation.  Figure 5). The local meteoric water line (LMWL) was produced by linear regression of the isotope data for monthly precipitation (Lu et al. 2018) provided by the Nanjing Observation Station of the Global Network for Isotopes in Precipitation (GNIP). As demonstrated in Figure 5, the δD (‰) and δ 18 O (‰) values for both the GTW and the cold GW fall near the global meteoric water line and the LMWL for Nanjing. This suggests that the GTW and the cold GW are primarily recharged by the infiltration of atmospheric precipitation. The δD and δ 18 O values for the GTW range from -42.89‰ to -53.21‰ and from -7.11‰ to -8.40‰, respectively. The projected points are located in the lower left section in Figure 5. This result suggests that the GTW is recharged by the infiltration of atmospheric precipitation at relatively high elevations and with more depleted δD and δ 18 O values and is tenuously linked with the shallow cold GW. Table 4 summarizes the calculated GW recharge elevations. The elevation calculated for the sample collected from well H3 is relatively low due to the mixing of cold GW and is unable to reflect the real-world recharge elevation. The recharge elevations calculated for the remaining samples range from 321 to 539 m, which are close to the elevation of the main body of Mount Laoshan. The δD and δ 18 O values for the GTW range from -42.89‰ to -53.21‰ and from -7.11‰ to -8.40‰, respectively. The projected points are located in the lower left section in Figure 5. This result suggests that the GTW is recharged by the infiltration of atmospheric precipitation at relatively high elevations and with more depleted δD and δ 18 O values and is tenuously linked with the shallow cold GW. Table 4 summarizes the calculated GW recharge elevations. The elevation calculated for the sample collected from well H3 is relatively low due to the mixing of cold GW and is unable to reflect the real-world recharge elevation. The recharge elevations calculated for the remaining samples range from 321 to 539 m, which are close to the elevation of the main body of Mount Laoshan.  [26]. The aforementioned results reveal a certain mixing effect between the GTW sample collected from well H3 and the cold GW.
Karst GTW is insubstantially linked and highly unlikely to mix with fissure water in clastic rocks. However, it is possible that karst GTW becomes mixed with shallow cold karst water during the upwelling process. Here, the sample ( 87 Sr/ 86 Sr = 0.709417, Sr = 1060 µg/L) collected from sampling site C2 near the GTW is treated as a cold-water end member. In addition, the sample with the highest temperature and the highest Sr content of all the GTW samples (i.e., the sample collected from well H2) is treated as a hot-water end member. The above mixing model was used to calculate the mixing ratios for the other GTW samples. Table 5 summarizes the calculation results. The cold-water mixing ratio (87%) for the sample collected from well H3 is the highest. This result agrees with the aforementioned analysis. The cold-water mixing ratios for the other samples are relatively low (<30%).  Figure 6 shows the δ 14 C-δ 13 C relationship of the GTW in the study area. Overall, there is a negative correlation between δ 14 C and δ 13 C, demonstrating the dilution effect of the dissolution of carbonate minerals. In this study, the Pearson model, which takes into consideration the isotopic fractionation of CO 2 and DIC in soils [27], was employed to correct the 14 C age. Table 6 summarizes the calculation results. As demonstrated in Table 6, the GTW in the study area is relatively old, with an age varying from 2046 to 6474 aBP. As a result of its mixing with cold water, the age of the sample collected from well H3 is relatively young. Based on the calculated T R as well as the locations of the wells, the age of the sample collected from well H3 should be similar to that of the sample collected from well H1 [28,29].

Analysis of the Heat Source
The terrestrial heat flow rate in Tangquan is approximately 58-62 mW/m 2 , which is close to the arithmetic mean (61.5 ± 13.9 mW/m 2 ) of heat flow rate measurements for Mainland China [30]. The geothermal gradient in Tangquan is approximately 24 °C/km. According to the temperature measurements obtained by drilling in adjacent areas, in the study area, Z0 = 12-20 m and T0 = 17 °C.
In the following section, radioactive heat sources are discussed. Let us assume that radioactive elements are uniformly distributed in the topmost 10 km of the Earth's crust. The upper limit A of the average heat generation rate of the Earth's crust in the study area is 1.6 μW/m 3 [31]. Thus, the upper limit of the heat generated by radioactive decay within the topmost 10 km of the Earth's crust is as follows: △q = A△z = 16 mW/m 2 [11]. The average heat generated by the decay of radioactive heat sources will be even lower. Thus, radioactive heat sources do not constitute special heat sources of the geothermal system in the study area.
In addition, there are no young magmatic intrusions within the shallow crust in the study area. Thus, there are no additional heat sources, such as magmatic heat. The contribution of the mechanical frictional heat generated by active faults and earthquakes to the terrestrial heat flow is extremely low and can be considered negligible. Thus, the

Analysis of the Heat Source
The terrestrial heat flow rate in Tangquan is approximately 58-62 mW/m 2 , which is close to the arithmetic mean (61.5 ± 13.9 mW/m 2 ) of heat flow rate measurements for Mainland China [30]. The geothermal gradient in Tangquan is approximately 24 • C/km. According to the temperature measurements obtained by drilling in adjacent areas, in the study area, Z 0 = 12-20 m and T 0 = 17 • C.
In the following section, radioactive heat sources are discussed. Let us assume that radioactive elements are uniformly distributed in the topmost 10 km of the Earth's crust. The upper limit A of the average heat generation rate of the Earth's crust in the study area is 1.6 µW/m 3 [31]. Thus, the upper limit of the heat generated by radioactive decay within the topmost 10 km of the Earth's crust is as follows: q = A z = 16 mW/m 2 [11]. The average heat generated by the decay of radioactive heat sources will be even lower. Thus, radioactive heat sources do not constitute special heat sources of the geothermal system in the study area.
In addition, there are no young magmatic intrusions within the shallow crust in the study area. Thus, there are no additional heat sources, such as magmatic heat. The contribution of the mechanical frictional heat generated by active faults and earthquakes to the terrestrial heat flow is extremely low and can be considered negligible. Thus, the geothermal system in Tangquan is of the nonvolcanic type and is heated by normal terrestrial heat flow. As a result, within the accessible depth (3 km), the conditions required for forming high-temperature geothermal resources are lacking, and the geothermal resources are low-temperature resources (<90 • C).

Geothermal Reservois and Caprock Conditions
The Sr isotope ratio of the GTW in Tangquan, Nanjing, ranges from 0.708876 to 0.709299, with an average of 0.709005. This result suggests that the GTW primarily flows through carbonate (Sr isotope ratio = 0.708-0.710) formations [32]. As the main constituent of the main body of Mount Laoshan, Upper Sinian dolomite, which is rich in carbonate minerals, outcrops in the middle of Mount Laoshan and becomes concealed in the piedmont zone at a depth that gradually increases northwestward from the piedmont zone. From the perspective of geological development, during the Late Sinian, the sedimentary dolomite of the Dengying Formation was elevated and underwent regression as a result of the Tongwan Movement and was subsequently subject to weathering and erosion. In addition, the Earth's crust was intensely active during the Mesozoic. This series of carbonate formations underwent multiple episodes of fault activity, uplifting, erosion, and karstification during the Yanshanian period [33]. This process led to the development of karst fissures, which are favorable spaces for storing fluids.
At the northern foot of Mount Laoshan, a series of ENE-trending, NW-dipping tectonic fault zones with the downthrown side to the north have developed. In this area, sandstone sediments of the Cretaceous Chishan and Pukou Formations have been deposited, and their thickness increases northwestward. The abovementioned clastic assemblage and Quaternary loose sedimentary formations have a low permeability and a low thermal conductivity (generally 0.71-0.92 W/m • C), which is far lower than that (2.01 W/m • C) of limestone (dolomite), and can form good caprocks for geothermal reservoir aquifers [20,28].

Tectonic Control
Tangquan is located on the southeastern margin of the Liuhe-Quanjiao Sag within the Mesozoic Fault Depression. The mountains south of Tangquan are a complex anticline structure composed of the Sinian formations of the Laoshan Uplift. The ENE-trending faults that control the distribution of the geothermal water wells in Tangquan are relatively large in size and are offset by later-formed NW-trending faults. These faults jointly form a tectonic framework that stores and diverts water and heat. The fracture zone is formed from the convergence and cutting of the nearly EW-trending Tangquang-Fanji Fault Zone (F1) and the NW-trending Tangquan-Lulang Fault (F21); the East Tangquan-Jiangning Township Fault (F22) is the main channel for the GTW and home to the majority of the existing geothermal water in Tangquan.
In addition, the GTW in the middle of Mount Laoshan is recharged at relatively high elevations (321-539 m), which differ relatively considerably (by approximately 40 m) from those (281-499 m) of the hot spring discharge zone in Tangquan. The resulting hydraulic pressure difference causes subsequent forced circulation of the infiltrated water.

Genetic Model
The geothermal resources currently being developed and used in Tangquan, Nanjing, are from a low-medium-temperature convective geothermal system. There are no local special heat sources in the shallow crust. Under the normal geothermal background of the area, the GW brings deep heat to the discharge zone on the north side of Mount Laoshan through deep circulation.
Atmosphere precipitation infiltrates in the outcropping carbonate area of Mount Laoshan and subsequently recharges the GW. The infiltrated water penetrates deep into the solution fissures in the Upper Sinian dolomite and tectonic fractures as it continuously absorbs heat from the rocks and gradually becomes heated. At a depth of 2.3 km, the infiltrated water reaches a temperature of approximately 75 • C, forming a deep geothermal reservoir. In addition, the infiltrated water reaches chemical equilibrium with the deep surrounding rocks during the prolonged seepage process. Eventually, GTW of the SO 4 -Ca type is formed. The ENE-and NW-trending tectonic faults on the north side of Mount Laoshan and their combinations form upwelling channels for the GTW. As a result of the hydraulic pressure difference, the GTW rises and becomes mixed with the shallow cold karst water in the discharge zone where the faults meet. The ENE-and NW-trending tectonic faults on the north side of Mount Laoshan and their combinations form upwelling channels for the GTW. As a result of the hydraulic pressure difference, the GTW rises and becomes mixed with the shallow cold karst water in the discharge zone where the faults meet. The mixed water forms a geothermal reservoir due to obstructions by upper low-permeability formations (e.g., Cretaceous and Quaternary formations) or rises to the surface and forms hot springs at locations where the caprock is discontinuous. Based on the above analysis, a genetic model is proposed for the geothermal system in Tangquan, Nanjing, as shown in Figure 7.
hydraulic pressure difference, the GTW rises and becomes mixed with the shallow cold karst water in the discharge zone where the faults meet. The ENE-and NW-trending tectonic faults on the north side of Mount Laoshan and their combinations form upwelling channels for the GTW. As a result of the hydraulic pressure difference, the GTW rises and becomes mixed with the shallow cold karst water in the discharge zone where the faults meet. The mixed water forms a geothermal reservoir due to obstructions by upper lowpermeability formations (e.g., Cretaceous and Quaternary formations) or rises to the surface and forms hot springs at locations where the caprock is discontinuous. Based on the above analysis, a genetic model is proposed for the geothermal system in Tangquan, Nanjing, as shown in Figure 7.

Conclusions
(1) The temperature of the GTW in Tangquan, Nanjing, ranges from 32 to 46 °C. Thus, the GTW in this area is low-temperature hot water. Hydrochemically, the GTW in Tangquan is of the SO4-Ca type. TR ranges from 63 to 75 °C. The GTW circulates at depths of 1.8-2.3 km. (2) The GTW in the study area originates from meteoric water, has a depleted D and O isotopic composition compared to the cold GW, and is recharged at elevations of 321-539 m, close to the elevation of the main body of Mount Laoshan. During the upwelling process, the GTW becomes mixed with the shallow cold karst water at a ratio of approximately 4-26% (cold water). At a relatively low cold-water mixing ratio, the temperature of the GTW is relatively high, and vice versa. The age of the GTW in the study area is approximately 2046-6474 a.

Conclusions
(1) The temperature of the GTW in Tangquan, Nanjing, ranges from 32 to 46 • C. Thus, the GTW in this area is low-temperature hot water. Hydrochemically, the GTW in Tangquan is of the SO 4 -Ca type. T R ranges from 63 to 75 • C. The GTW circulates at depths of 1.8-2.3 km. (2) The GTW in the study area originates from meteoric water, has a depleted D and O isotopic composition compared to the cold GW, and is recharged at elevations of 321-539 m, close to the elevation of the main body of Mount Laoshan. During the upwelling process, the GTW becomes mixed with the shallow cold karst water at a ratio of approximately 4-26% (cold water). At a relatively low cold-water mixing ratio, the temperature of the GTW is relatively high, and vice versa. The age of the GTW in the study area is approximately 2046-6474 a. (3) The geothermal system in the study area is of the low-medium-temperature type.
Atmospheric precipitation infiltrates in the high-elevation outcropping carbonate area of the Laoshan complex anticline and flows mainly through Upper Sinian dolomite formations. Under the background of the normal terrestrial heat flow, the infiltrated water is gradually heated by the surrounding rocks. Through long-term deep circulation, the GTW rises in the relatively low-lying area on the northwest side of Mount Laoshan, where the ENE-and NW-trending faults meet. During the upwelling process, the GTW becomes mixed with some shallow cold karst water. The mixed water forms a geothermal reservoir overlain by Cretaceous and Quaternary formations or rises to the surface at locations where the caprock is discontinuous and forms geothermal anomalies in the study area. (4) In order to ensure the sustainable development and utilization of GTW, it is suggested to further strengthen the dynamic monitoring of groundwater and the control of extraction amounts. A comprehensive plan for cascading development and utilization should be established-mainly for medical treatment, tourism, and bathing-and the management mode of GTW should be improved.