Long-Lasting Boiling-Wells: Geochemical Windows into the Tectonic Activity of the Maodong Fault (China)

: The Maodong Fault (China) was mainly active during the Late Pleistocene. However, in the past century, numerous destructive earthquakes have occurred along the fault zone, indicating its continuing activity. Therefore, reﬁned monitoring of the tectonic activity along the fault is required. Boiling-Wells located in the Maodong Fault Zone were selected for this purpose. The parameters, including the rare earth elements (REE) and gas components, such as CO 2 , Rn, and Total Volatile Organic Compounds (TVOC), in the wells were analyzed. By combining ﬁeld observations with the analytical data, we constrained the relationships between the anomalies of the hydrochemical composition and the gas composition in the Boiling-Wells and the Maodong Fault: (1) CO 2 and TVOC in the Boiling-Wells originated from Cenozoic magmatism and associated intrusive rocks. High concentrations of Rn are closely linked to tectonic activities of the Maodong Fault. CO 2 , TVOC, and Rn are all transported to the Boiling-Wells along the Maodong Fault, with CO 2 acting as a carrier gas for Rn. (2) REE in the Boiling-Wells was mainly sourced from CO 2 ﬂuids that originated from deep-seated Cenozoic magmas and intrusive rocks. The concentrations of the REE and their distribution patterns were controlled by the input of CO 2 ﬂuids and by epigenetic processes. (3) The abnormally high contents of Ca 2+ , HCO 3 − , Pb 2+ , and Al 3+ in the Boiling-Wells are attributed to the migration of externally-derived (deep) CO 2 ﬂuids through the Maodong Fault. (4) The anomalies of the gaseous (Rn, CO 2 , and TVOC) and hydrochemical components (Ca 2+ , HCO 3 − , Pb 2+ , Al 3+ , ∑ REE, and REE patterns) in the Boiling-Wells are closely related to the tectonic activity of the Maodong Fault. Therefore, the long-lasting Boiling-Wells provide an excellent geochemical window into the evolution of the Maodong Fault. Our study documents that the contents and variations of speciﬁc hydrochemical and gaseous components of Boiling-Wells are well-suited geochemical tracers to identify and characterize the tectonic activity of the Maodong Fault. This method is also applicable for the monitoring of tectonic activities of major faults zones with comparable preconditions worldwide.


Introduction
Seismic activities along faults or fractures may disturb the original equilibrium state of the water-rock interaction in the crust, resulting in abnormal changes in the chemical composition of gas and water in the groundwater [1][2][3][4][5][6]. Long-term observations have shown that the composition of groundwater near the Earth's surface and the gas composition in the soil sensitively record the stress and strain state of the Earth's crust [7,8]. Fault zones are important locations in the continental crust for the release of gases [9][10][11][12]. Previous studies have shown that the concentrations of CO 2 , Rn, and other gases in the water and soil of the fault zones are often higher than those in the adjoining areas [9][10][11][12]. For instance, the contents of Cl − and SO 4 2− in nearby groundwater fluctuated and rapidly increased eight months before the onset of the Kobe earthquake (M7.2) in 1995 [3]. Before the Jingtai Earthquake (M6.2) in 1990, the contents of HCO 3 − , CO 3 2− , and Cl − in hot springs such as Zhongwei and Shaokou (China) increased by 40% abnormally [13]. Perrier et al. [9] observed CO 2 bubbles that emerged from the deep crust in the Syabru-Bensi hot spring (Nepal), located on the Main Central Thrust zone of Nepal. High contents (16,,000 Bq/m 3 ) of Rn were observed in the CO 2 bubbles collected in the water of Syabru-Bensi hot spring (Nepal), which were considered to result from degassing in the Himalayas. Meng et al. [14] recognized distinct corresponding relations between the concentrations of Rn and Hg and tectonic activities in the Haiyuan fault zone (China) by analyzing the gas content in the soil. The examples demonstrate that abnormal concentrations of specific gas components in groundwater and soil can be used as indicators for fault activity and as indexes for changes of stress in the crust.
The Liyang M5.5 earthquake in 1974 and the Liyang M6.0 earthquake in 1979 occurred along the Maodong Fault Zone (Figure 1). In addition, earthquakes with a lower magnitude were very frequently recorded along the Maodong Fault Zone and adjoining areas (Figure 1), documenting that the potential of intermediate to strong earthquakes still persists [15][16][17][18][19]. The Maodong Fault Zone runs through highly populated areas in China with large and economically important cities such as Xuancheng, Nanjing, Changzhou, Zhenjiang, Danyang, and Yangzhou ( Figure 1). For the safety of the population and the maintenance of the economy, therefore, it is necessary to precisely monitor the activity of the Maodong Fault Zone.
shown that the composition of groundwater near the Earth's surface and the gas composition in the soil sensitively record the stress and strain state of the Earth's crust [7,8]. Fault zones are important locations in the continental crust for the release of gases [9][10][11][12]. Previous studies have shown that the concentrations of CO2, Rn, and other gases in the water and soil of the fault zones are often higher than those in the adjoining areas [9][10][11][12]. For instance, the contents of Cl − and SO4 2− in nearby groundwater fluctuated and rapidly increased eight months before the onset of the Kobe earthquake (M7.2) in 1995 [3]. Before the Jingtai Earthquake (M6.2) in 1990, the contents of HCO3 − , CO3 2− , and Cl − in hot springs such as Zhongwei and Shaokou (China) increased by 40% abnormally [13]. Perrier et al. [9] observed CO2 bubbles that emerged from the deep crust in the Syabru-Bensi hot spring (Nepal), located on the Main Central Thrust zone of Nepal. High contents (16,000 Bq/m 3 ) of Rn were observed in the CO2 bubbles collected in the water of Syabru-Bensi hot spring (Nepal), which were considered to result from degassing in the Himalayas. Meng et al. [14] recognized distinct corresponding relations between the concentrations of Rn and Hg and tectonic activities in the Haiyuan fault zone (China) by analyzing the gas content in the soil. The examples demonstrate that abnormal concentrations of specific gas components in groundwater and soil can be used as indicators for fault activity and as indexes for changes of stress in the crust.
The Liyang M5.5 earthquake in 1974 and the Liyang M6.0 earthquake in 1979 occurred along the Maodong Fault Zone (Figure 1). In addition, earthquakes with a lower magnitude were very frequently recorded along the Maodong Fault Zone and adjoining areas (Figure 1), documenting that the potential of intermediate to strong earthquakes still persists [15][16][17][18][19]. The Maodong Fault Zone runs through highly populated areas in China with large and economically important cities such as Xuancheng, Nanjing, Changzhou, Zhenjiang, Danyang, and Yangzhou ( Figure 1). For the safety of the population and the maintenance of the economy, therefore, it is necessary to precisely monitor the activity of the Maodong Fault Zone. to abnormally high concentrations of CO 2 , Rn, and other constituents degassing from the water [20], the water in the wells appears to boil constantly. Therefore, the six wells are called Boiling-Wells. The depths of Boiling-Wells are 3 to 4 m, and the water temperature is about 19 • C in summer. The six Boiling-Wells are concentrated in a nearly north-south direction following the strike of the fault (inset in Figure 1). The distance between the individual wells is 0.5 m to 10 m (Figure 1).
Due to the variable "boiling" conditions, the taste and color of the well water vary ( Figure 1). The water of the three wells in the north is limpid and tastes like beer, lemon, and sprite, whereas that of the three wells in the south is turbid to cloudy and tastes like rust, bitter, and spice. Based on geophysical studies, Jiang et al. [20] detected secondary faults in the Maodong Fault Zone as well as Cenozoic intrusions below the Boiling-Wells. They concluded that the boiling phenomenon is induced by gas that was dissolved in water, rises up along the Maodong Fault, and later degasses in the wells. The gases were dominated by CO 2 and mixed with minor amounts of Rn and total volatile organic compounds (TVOC). In addition, chemical components such as Ca 2+ , HCO 3 − , and rare earth elements (REE) were significantly enriched in the Boiling-wells water compared to water from surrounding shallow wells [20]. However, the origin of the exceptional composition of the boilingwell water (particularly the striking REE enrichment) and its link to the Maodong Fault were uncertain.
We studied the genesis of the geochemical anomalies, including the gas and hydrochemical components, and the REE contents, as well as their distribution pattern in the Boiling-Wells, to elucidate their relationship with the tectonic activity of the Maodong Fault by sampling and analyzing water from the Boiling-Well, surrounding shallow wells, and a nearby lake. The anions, cations, metals, REE, and gas components in water samples were analyzed, and the previously published data of natural shallow groundwater in other plain basins were also collected.

Study Area
The study area is located in the Maodong Fault Zone at the eastern margin of the Maoshan Mountain in Danyang City (Jiangsu Province, China; Figure 1). Quaternary sediments cover the Maodong Fault in the study area. The climate of the area is subtropical humid, with an annual average precipitation of 1032.6 mm and evaporation of 1517.7 mm. The shallow groundwater mainly occurs in Quaternary loose sediments that are composed of brownish-yellow silty clay and silty sand [21]. The groundwater is mainly supplied by atmospheric precipitation, and evaporation is the main discharge mode [21].
The Maodong Fault ( Figure 1) shows an overall NNE-strike with a dip angle of 35 to 85 • and an overall dip toward the southeast [16,19,22]. The fault comprises a series of subparallel normal faults that are staggered in steps. The Maodong Fault was mainly active in the Late Pleistocene, but several destructive earthquakes in the past century occurred along the fault zone (Figure 1), indicating that the fault is still active and that the monitoring of the tectonic activity of the fault needs to be strengthened [16].

Water Sampling
In addition to water samples of the six Boiling-Wells and of a nearby lake that exhibits a comparable boiling phenomenon, we collected samples from shallow groundwater within an area of 10 to 15 km around the Boiling-Wells as references. The distribution of sample locations was shown in Figure 2. All samples represent shallow groundwater that was hosted by loosely accumulated Quaternary silty clay. A total of 40 water samples were collected, including six samples from Boiling-Wells, one sample of the boiling lake, and 33 groundwater samples from less than 10 m depths. For conventional hydrochemical analysis, the sample volume used for the anion test was 1.5 L, which was stored airtight. The volume used for the cation test was 0.5 L, acidified with 5 ml nitric acid to pH < 2 and stored airtight. The volume of water samples used for REE element analysis was 0.5 L, which was packed in a polyethylene bottle that had been cleaned with acid. Subsequently, the polyethylene bottle was sealed with a sealing film and sent to the laboratory for analysis. was hosted by loosely accumulated Quaternary silty clay. A total of 40 water samples were collected, including six samples from Boiling-Wells, one sample of the boiling lake, and 33 groundwater samples from less than 10 m depths. For conventional hydrochemical analysis, the sample volume used for the anion test was 1.5 L, which was stored airtight. The volume used for the cation test was 0.5 L, acidified with 5 ml nitric acid to pH < 2 and stored airtight. The volume of water samples used for REE element analysis was 0.5 L, which was packed in a polyethylene bottle that had been cleaned with acid. Subsequently, the polyethylene bottle was sealed with a sealing film and sent to the laboratory for analysis.

CO 2 , TVOC, and Rn Anomalies of the Boiling-Wells
The analysis of gas components (Table 1) documents that the gas in the studied water samples of the six Boiling-Wells, the boiling lake, and the common shallow well is dominated by CO 2 , with subordinate Rn and TVOC. Note: "bdl": below the detection limit. 1 The contents of Rn and TVOC were measured on the surface of the wells and the lake. The content of TVOC represents the average value of continuous measurement for 2 h.
The content of free CO 2 in the studied water samples of the Boiling-Wells was extraordinarily high, ranging from 439 to 491 mg/L, with an average of 461.67 mg/L. In contrast, the content of free CO 2 in the groundwater of the common shallow well and the boiling lake was below the detection limit. The pH of Boiling-Wells water decreased with the increase of the free CO 2 content, indicating that the water of Boiling-Wells is carbonated.
Our investigations demonstrate that organic pollution sources are lacking around the Boiling-Wells, precluding an external pollution source of TVOC. TVOC was detected in the Boiling-Wells, with concentrations ranging from 0.013 to 0.021 µg/L, but it was absent in the surrounding air, boiling lake water, and shallow groundwater well water.
The concentration of Rn was relatively high around the Boiling-Wells (Table 1), but it was negligible in the surrounding air, lake water, and shallow groundwater well water. The Rn contents in the Boiling-wells water ranged from 962 pCi/L (Sample BW1) to 520 pCi/L (Sample BW5). BW1 showed the highest intensity, indicating that the Rn increased with rising intensity of the boiling ( Table 1). The data suggest that the well represented by Sample BW1 has a better connectivity with deep faults.

Hydrochemical Anomalies of the Boiling-Wells
The hydrochemical analyses show that the contents of Pb 2+ , Al 3+ , Fe 2+ , Mn 2+ , K + , Ca 2+ , HCO 3 − , total hardness, and TDS (Total Dissolved Solid) in the Boiling-Wells were significantly higher than those in surrounding shallow wells ( Table 2). Their average values were 12,145%, 4088%, 3493%, 581%, 393%, 276%, 248%, 199%, and 105% higher than those in the surrounding shallow groundwater. Moreover, the contents of H 2 SiO 3 , As, Zn 2+ , Mg 2+ , Cl − , SO 4 2− , and F − were slightly higher in the Boiling-Wells than in the shallow wells. The contents of COD Mn , pH, and Na + were slightly lower than in the shallow wells. Their mean values were 20.3%,16.6% and 31.0% lower than those in the surrounding shallow groundwater. The average content of NO 3 − in the Boiling-Wells was 97% lower than that in surrounding shallow wells. The Gibbs diagram that was based on the chemical data of the groundwater shows the relationship between TDS and the ratios of cations and anions ( Figure 3) and is a powerful tool to analyze the chemical origin of groundwater [24][25][26]. It is widely used to explain the mechanism of atmospheric precipitation, rock weathering (water-rock interaction), evaporation, concentration, and other processes that control the chemistry of groundwater [27]. An increase of evaporation and concentration will increase the amount of Na + and Cl − in the water body and will decrease the concentration of Ca 2+ and HCO 3 − . However, the Boiling-Wells samples clearly differ from the other shallow wells samples ( Figure 3). The deviation of Boiling-Wells in the Gibbs diagram indicates that it has undergone different chemical processes from common shallow water.

REE Anomalies of the Boiling-Wells
The REE contents of the studied samples are summarized in Table 3. The ∑REE contents in the Boiling-Wells (Sample BW1 to BW6) ranged from 3.5800 to 314.5880 μg/L. The ∑REE contents in the lake water (Sample BL7) and shallow well (Sample BSG8) were

REE Anomalies of the Boiling-Wells
The REE contents of the studied samples are summarized in Table 3. The ∑REE contents in the Boiling-Wells (Sample BW1 to BW6) ranged from 3.5800 to 314.5880 µg/L. The ∑REE contents in the lake water (Sample BL7) and shallow well (Sample BSG8) were 44.0220 and 1.5150 µg/L, respectively. We collected REE data of shallow groundwater in typical plain basins of China (Jianghan Plain [28], Datong Basin [29], Suzhou Plain [23]) for a comparative analysis. All the data was normalized to North American shale (NASC) [30,31], and the normalized REE distribution patterns are shown in Figure 4. The normalized REE values of the Boiling-Wells ranged from 10 −4 to 10 −3 orders of magnitude, and they revealed double-peak patterns with positive Eu and Tb anomalies. The normalized REE values of shallow groundwater for the Jianghan Plain, Datong Basin, and Suzhou Plain in northern Anhui Province were more depleted and ranged from 10 −7 to 10 −5 orders of magnitude, and they revealed single-peak patterns with a strong positive Eu anomaly.
In Figure 5, we compare the (La/Yb) N and δEu data between the Boiling-Wells with other plain basins of China. The δEu values of the Boiling-Wells water ranged from 1.5472 to 14.5780, with a lower average value of 6.9605 compared with other plains. The (La/Yb) N values of the Boiling-Wells ranged from 1.1312 to 1.5853, with an average of 1.3592, showing a depletion of the HREE. However, the average values of (La/Yb) N of shallow groundwater in other plain basins of China were significantly lower than that of the Boiling-Wells and lower than 1, indicating that general shallow groundwater is enriched in HREE.
Through the data comparison, the REE patterns, ∑REE, and (La/Yb) N were significantly different from those in general shallow groundwater, indicating that its genesis and sources are different from those of general shallow groundwater.
the data was normalized to North American shale (NASC) [30,31], and the normalized REE distribution patterns are shown in Figure 4. The normalized REE values of the Boiling-Wells ranged from 10 −4 to 10 −3 orders of magnitude, and they revealed double-peak patterns with positive Eu and Tb anomalies. The normalized REE values of shallow groundwater for the Jianghan Plain, Datong Basin, and Suzhou Plain in northern Anhui Province were more depleted and ranged from 10 −7 to 10 −5 orders of magnitude, and they revealed single-peak patterns with a strong positive Eu anomaly. Through the data comparison, the REE patterns, ∑REE, and (La/Yb)N were significantly different from those in general shallow groundwater, indicating that its genesis and sources are different from those of general shallow groundwater.

Link between CO2, Rn, and TVOC Anomalies in Boiling-Wells and the Maodong Fault
Cenozoic basalts are intermittently distributed along the Maodong Fault [16,32]. The basalts contain xenoliths of upper mantle herzolite. A depth of >50 km is constrained for

Link between CO 2 , Rn, and TVOC Anomalies in Boiling-Wells and the Maodong Fault
Cenozoic basalts are intermittently distributed along the Maodong Fault [16,32]. The basalts contain xenoliths of upper mantle herzolite. A depth of >50 km is constrained for the basaltic melt, indicating that the Maodong Fault may have reached the upper mantle [15][16][17]33,34]. Previous studies have demonstrated that CO 2 gas reservoirs in eastern China are temporally and spatially related to Cenozoic basalts and probably originated from the upper mantle [35][36][37][38][39]. On a global scale, the distribution of areas that are characterized by the release of mantle-derived CO 2 is often related to seismically active fault zones, indicating that the release of the CO 2 -rich mantle fluids is controlled by deep seated faults [37,40,41].
Compared with common well water, the most obvious abnormality in Boiling-wells is the boiling phenomenon caused by the continuous emergence of CO 2 . The studied Boiling-Wells of the Maodong Fault Zone and the underlying strata were intruded by dolerites that are related to the basalt emplacement. In addition, bubbles also appear in lakes, ponds, and ditches along the strike of the Maodong Fault. Therefore, we propose that the CO 2 in the Boiling-Wells was sourced from the Cenozoic mantle-derived basaltic melt and associated intrusive rocks and was introduced into the Boiling-Wells along the Maodong Fault.
Previous studies have documented that the gas in the deep mantle degassing zone contains hydrocarbons (a kind of TVOC) [42,43]. Moreover, gas of the Huangqiao CO 2 gas field that occurs 100 km to the northeast of the Maodong Fault contains trace amounts of mantle-derived TVOC [44][45][46]. Based on data of field tests, TVOC has been documented in the Boiling-Wells, but it was absent in the nearby shallow groundwater wells. Figure 6 shows the positive correlation of the TVOC concentration and the free CO 2 in the Boiling-Wells. Therefore, we propose that the CO 2 in the Boiling-Wells originated from mantlederived magmas, similar to the Huangqiao Gas Field. This observation further corroborates that both TVOC and CO 2 in the studied Boiling-Wells were sourced from mantle degassing and subsequently migrated upward along the Maodong Fault. shows the positive correlation of the TVOC concentration and the free CO2 in the Boiling-Wells. Therefore, we propose that the CO2 in the Boiling-Wells originated from mantlederived magmas, similar to the Huangqiao Gas Field. This observation further corroborates that both TVOC and CO2 in the studied Boiling-Wells were sourced from mantle degassing and subsequently migrated upward along the Maodong Fault. Radon commonly associates fracture zones and migrates to the surface along faults and associated fracture zones [1]. The content of Rn in the atmosphere is generally much lower than 3 pCi/L [47] in areas without a fault. Previous studies documented a striking correlation between the distribution of high Rn contents in the atmosphere and the distribution of active faults and earthquakes in many cases [48][49][50]. With the accumulation of stress and strain along active faults, an exceptionally high concentration of Rn may occur. Therefore, Rn is often used to trace the location and intensity of faults [48,[51][52][53][54].
The striking Rn enrichment in the Boiling-Wells ( Table 1) further confirms that the Radon commonly associates fracture zones and migrates to the surface along faults and associated fracture zones [1]. The content of Rn in the atmosphere is generally much lower than 3 pCi/L [47] in areas without a fault. Previous studies documented a striking correlation between the distribution of high Rn contents in the atmosphere and the distri-bution of active faults and earthquakes in many cases [48][49][50]. With the accumulation of stress and strain along active faults, an exceptionally high concentration of Rn may occur. Therefore, Rn is often used to trace the location and intensity of faults [48,[51][52][53][54].
The striking Rn enrichment in the Boiling-Wells (Table 1) further confirms that the formation of the Boiling-Wells was closely related to the Maodong Fault. However, as the half-life of Rn is only 3.85 days, and the migration rate is low, the migration distance of Rn is very limited. Therefore, a corresponding gas is usually required as a carrier of Rn to migrate [55]. Previous studies have documented that CO 2 can effectively act as a carrier gas for Rn [56,57], indicating the potential of Rn to migrate to the surface along faults. Therefore, the coupled CO 2 and Rn enrichment in the studied Boiling-Wells can be explained by the following model: Mantle-sourced CO 2 carried Rn to the surface along the Maodong Fault, migrated to the Boiling-Wells, and was finally released to the atmosphere.
Based on our new geochemical data, we propose that (1) CO 2 and TVOC in the studied Boiling-Wells were both derived from mantle-derived Cenozoic magmatism and that the origin of Rn is closely related to the tectonic activities in the Maodong Fault; (2) CO 2 , TVOC, and Rn all migrated to the Boiling-Wells along the Maodong Fault. Therefore, the Boiling-Wells are excellent "windows" to detect geogas anomalies caused by the Maodong Fault.

Geochemical Significance of REE Anomalies in Boiling-Wells
REE usually show a consistent geochemical behavior in water and are widely used to trace hydrogeological processes [58][59][60]. REE concentrations and fractionation patterns of groundwater are usually suitable to constrain the evolution of the aquatic system during the flow in the aquifer rocks [58][59][60]. During the supergene evolution, the LREE (Light REE) mainly existed in the form of LnCO 3 + in the groundwater, whereas the HREE mainly existed in the form of Ln(CO 3 ) 2 − . The LREE are more easily adsorbed by aqueous media, whereas the HREE (Heavy REE) tend to remain in the water. Therefore, supergene processes often cause an enrichment of the HREE in the groundwater [60]. The normalized REE data in the studied Boiling-Wells clearly differ from that of common groundwater. Mainly, the positive Eu and Tb anomalies of the REE patterns and the marked LREE enrichment (Table 3, Figures 4 and 5) indicate that the source of the REE in the Boiling-Wells is different from common shallow groundwater. Furthermore, supergene processes can be excluded for the REE enrichment in the Boiling-Wells based on our data. HCO 3 − can form a stable complex with the REE, and therefore, high concentrations of HCO 3 − may cause an increase of the REE concentration in water [29,58,61]. However, the contents of LREE and HREE both decreased with an increasing HCO 3 − concentration in the Boiling-Wells samples. While the decrease of LREE content was steep, the decrease of HREE content was comparably low (Figure 7). The data further prove that the main sources of the REE in the Boiling-Wells were different from those in common groundwater, that were mainly derived from supergene processes, and confirm our model that the REE in the Boiling-Wells were mainly sourced from mantle-derived CO 2 fluids related to the Cenozoic magmatism. With the emission of CO 2 , REE were induced into the Boiling-Wells on the Earth's surface. However, due to supergene processes, the concentration of REE in the water was reduced, finally resembling that of common shallow groundwater. Moreover, the original enrichment of the LREE gradually disappeared, the REE distribution curve was flattened, and the HREE became slightly enriched.
Loess of the Xiashu Formation is the dominant lithology of the water-bearing medium in the Boiling-Wells. Underlying rocks comprise Cretaceous siltstones and mudstones that are locally intruded by Cenozoic basalts (dolerites). Previous studies have documented the presence of CO 2 -rich fluid inclusions in the Cenozoic basalts and dolerites of the study area and in other magmatic rocks from eastern China [62]. CO 2 -rich fluids from mantlederived magmas may transport REE and may cause an enrichment of the LREE through fractionation processes [63][64][65].
To further refine the source of the REE in the Boiling-Wells, their REE contents were compared to those of Cenozoic basalts [66,67] and loess of the Xiashu Formation [68].
After normalization to the NASC, the Cenozoic basalts displayed enrichment of the LREE, with a moderate positive Eu anomaly and slightly negative Ce anomaly (Figure 8). The LREE patterns of the Boiling-Wells samples generally resembled those of the Cenozoic basalts but were clearly distinct from those of the loess of the Xiashu Formation. In contrast, the HREE characteristics of the Boiling-Wells resembled those of the loess but were different from those of the basalts. The data indicate that the REE characteristics of the Boiling-Wells were related to the input of mantle-derived CO 2 fluid but also to sorption and desorption processes related to the hosting loess of the Xiashu Formation during the supergene evolution. richment (Table 3, Figures 4 and 5) indicate that the source of the REE in the Boiling-Wells is different from common shallow groundwater. Furthermore, supergene processes can be excluded for the REE enrichment in the Boiling-Wells based on our data.
HCO3 − can form a stable complex with the REE, and therefore, high concentrations of HCO3 − may cause an increase of the REE concentration in water [29,58,61]. However, the contents of LREE and HREE both decreased with an increasing HCO3 − concentration in the Boiling-Wells samples. While the decrease of LREE content was steep, the decrease of HREE content was comparably low (Figure 7). The data further prove that the main sources of the REE in the Boiling-Wells were different from those in common groundwater, that were mainly derived from supergene processes, and confirm our model that the REE in the Boiling-Wells were mainly sourced from mantle-derived CO2 fluids related to the Cenozoic magmatism. With the emission of CO2, REE were induced into the Boiling-Wells on the Earth's surface. However, due to supergene processes, the concentration of REE in the water was reduced, finally resembling that of common shallow groundwater. Moreover, the original enrichment of the LREE gradually disappeared, the REE distribution curve was flattened, and the HREE became slightly enriched. Loess of the Xiashu Formation is the dominant lithology of the water-bearing medium in the Boiling-Wells. Underlying rocks comprise Cretaceous siltstones and mudstones that are locally intruded by Cenozoic basalts (dolerites). Previous studies have documented the presence of CO2-rich fluid inclusions in the Cenozoic basalts and dolerites of the study area and in other magmatic rocks from eastern China [62]. CO2-rich fluids from mantle-derived magmas may transport REE and may cause an enrichment of the LREE through fractionation processes [63][64][65].
To further refine the source of the REE in the Boiling-Wells, their REE contents were compared to those of Cenozoic basalts [66,67] and loess of the Xiashu Formation [68]. After normalization to the NASC, the Cenozoic basalts displayed enrichment of the LREE, with a moderate positive Eu anomaly and slightly negative Ce anomaly ( Figure 8). The LREE patterns of the Boiling-Wells samples generally resembled those of the Cenozoic basalts but were clearly distinct from those of the loess of the Xiashu Formation. In contrast, the HREE characteristics of the Boiling-Wells resembled those of the loess but were different from those of the basalts. The data indicate that the REE characteristics of the Boiling-Wells were related to the input of mantle-derived CO2 fluid but also to sorption and desorption processes related to the hosting loess of the Xiashu Formation during the supergene evolution. The REE anomalies in the Boiling-Wells document a minor impact of supergene processes. The REE predominantly originated from REE-rich CO2 fluids that were related to mantle-derived Cenozoic magmas. The REE anomalies in the Boiling-Wells document a minor impact of supergene processes. The REE predominantly originated from REE-rich CO 2 fluids that were related to mantle-derived Cenozoic magmas.

Relationship between Water Chemistry Anomaly of the Boiling-Wells and the Activity of the Maodong Fault
The composition of groundwater affected by tectonic activity in fault zones or by earthquakes will be exceptional, and the injection of CO 2 is proposed as a main mechanism for the generation of the abnormalities [1][2][3][4][5][6].
According to our investigation, the aquifer medium and discharge path of the Boiling-Wells are the same as those of the surrounding shallow wells. However, water in the Boiling-Wells had an exceptional high concentration of Ca 2+ and HCO 3 − (Table 2). Choi et al. [69] studied carbonated groundwater in Korea and documented the input of mantle-derived CO 2 into the carbonated groundwater. The carbonation process can increase the contents of specific ions in the groundwater, particularly of HCO 3 − , Ca 2+ , and TDS. Koh et al. [70] reported the input of magmatic CO 2 into the basalt aquifer of Jeju Island (South Korea), which caused an increase of dissolved inorganic carbon (DIC, including free CO 2 , H 2 CO 3 , HCO 3 − , and CO 3 2− ) in the water, attaining up to 10 mM/L. High anomalies of HCO 3 − and Ca 2+ in the studied Boiling-Wells are consistent with the findings of Choi et al. [69] who proposed that the hydrochemical anomalies in the Boiling-Wells could be related to the input of CO 2 -rich fluids. Enrichment of Pb 2+ and Al 3+ in the studied Boiling-Wells (Table 2), which are rather immobile during epigenetic geochemical processes, indicates that these components originated from different sources. An abnormal increase of B was observed in the Deacontr Spring (located in the central Apennines, Italy) prior to the Collelongo earthquake (M4.1) in 2019. The B enrichment was attributed to the desorption of B from aquifer media, which was caused by deep CO 2 degassing [71]. Therefore, we provisionally propose that migration of externally derived (deep) CO 2 could trigger the desorption of additional Pb 2+ and Al 3+ as well as other elements in the studied Boiling-Wells. The variations of the Pb 2+ and Al 3+ contents are very consistent and generally correlate with an increase of the Rn content ( Figure 9). Therefore, we suggest that the enrichment of Pb 2+ and Al 3+ in the Boiling-Wells is closely related to the activity of the Maodong Fault. who proposed that the hydrochemical anomalies in the Boiling-Wells could be related to the input of CO2-rich fluids. Enrichment of Pb 2+ and Al 3+ in the studied Boiling-Wells (Table 2), which are rather immobile during epigenetic geochemical processes, indicates that these components originated from different sources. An abnormal increase of B was observed in the Deacontr Spring (located in the central Apennines, Italy) prior to the Collelongo earthquake (M4.1) in 2019. The B enrichment was attributed to the desorption of B from aquifer media, which was caused by deep CO2 degassing [71]. Therefore, we provisionally propose that migration of externally derived (deep) CO2 could trigger the desorption of additional Pb 2+ and Al 3+ as well as other elements in the studied Boiling-Wells. The variations of the Pb 2+ and Al 3+ contents are very consistent and generally correlate with an increase of the Rn content ( Figure 9). Therefore, we suggest that the enrichment of Pb 2+ and Al 3+ in the Boiling-Wells is closely related to the activity of the Maodong Fault.

Conclusions
The Maodong Fault (eastern China) was mainly active during the Late Pleistocene. However, in the past century, earthquakes of magnitude 4.5 or higher have occurred along the fault, indicating that the fault is still active. Therefore, it is necessary to strengthen the monitoring of the tectonic activities of the fault [15,[72][73][74]. Water of the long-lasting Boiling-Wells in Jiuli Village, Danyang City, Jiangsu Province (China) was investigated to

Conclusions
The Maodong Fault (eastern China) was mainly active during the Late Pleistocene. However, in the past century, earthquakes of magnitude 4.5 or higher have occurred along the fault, indicating that the fault is still active. Therefore, it is necessary to strengthen the monitoring of the tectonic activities of the fault [15,[72][73][74]. Water of the long-lasting Boiling-Wells in Jiuli Village, Danyang City, Jiangsu Province (China) was investigated to study the potential of the wells for tracing seismic activity along fault zones. The hydrochemical and gas components, such as CO 2 , Rn, TVOC, and REE, of the water in the Boiling-Wells and in surrounding shallow wells were analyzed. Water in the six Boiling-Wells had high concentrations of CO 2 , Rn, TVOC, Ca 2+ , HCO 3 − , Pb 2+ , Al 3+ , and ∑REE, as well as REE distribution patterns that significantly differ from common shallow groundwater. Through a comparison of our field investigations and our new analytical data with published data for similar areas, we obtained the following conclusions: (1) CO 2 and TVOC in the Boiling-Wells originated from Cenozoic magmatism and associated intrusive rocks. High concentrations of Rn are closely linked to tectonic activities of the Maodong Fault. CO 2 , TVOC, and Rn are all transported to the Boiling-Wells along the Maodong Fault, with CO 2 acting as a carrier gas for Rn. (2) REE in the Boiling-Wells was mainly sourced from CO 2 fluids that originated from deep-seated Cenozoic magmas and intrusive rocks. The concentrations of the REE and their distribution patterns were controlled by the input of CO 2 fluids and by epigenetic processes. Although the anomalies of CO 2 , Rn, TVOC, Ca 2+ , HCO 3 − , Pb 2+ , Al 3+ , and ∑REE, as well as the REE distribution patterns in the studied boiling wells, are closely related to the activity of Maodong Fault, the impact of time on the variation of the chemical compositions and the related activities of Maodong Fault (especially seismic activity) remains uncertain, so far. Therefore, we will conduct monthly frequency geochemical monitoring of the variation of suitable geochemical tracers that will characterize the tectonic activities of the Maodong Fault, and we will correlate the monitoring data with the overall geostress field in a future project.