Hydrogeochemistry and Genesis Analysis of Thermal and Mineral Springs in Arxan, Northeastern China

water

Article Hydrogeochemistry and Genesis Analysis of Thermal and Mineral Springs in Arxan, Northeastern China

Xiaomin Gu *, Qiulan Zhang *, Yali Cui, Jingli Shao, Yong Xiao, Peng Zhang and Jinxing Liu School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China; [email protected] (Y.C.); [email protected] (J.S.); [email protected] (Y.X.); [email protected] (P.Z.); [email protected] (J.L.) * Correspondence: [email protected] (X.G.); [email protected] (Q.Z.); Tel.: +86-10-8232-2671 (X.G.); +86-10-8232-2671 (Q.Z.)

Academic Editor: David Polya Received: 26 September 2016; Accepted: 13 January 2017; Published: 19 January 2017

Abstract: In this work, the hydrogeochemistry and environmental isotopic compositions of thermal and mineral springs in Arxan, northeastern China, were used to assess the genesis of the thermal system hosted by deep-seated faults. The reservoir temperature was calculated using the mineral saturation index and geothermometers. According to isotopic analysis, the spring water was of meteoric origin. Sixteen springs in the Arxan geothermal system with outlet temperatures ranging from 10.9 to 41.0 ◦C were investigated. The water samples can be classiﬁed into four groups by using a Piper diagram. The aquifer in which the Group I and Group III samples were obtained was a shallow cold aquifer of the Jurassic system, which is related to the local groundwater system and contains HCO3–Ca·Na groundwater. The Group II and Group IV samples were recharged by deeply circulating meteoric water with HCO3–Na and HCO3·SO4–Na·Ca groundwater, respectively. The springs rise from the deep basement faults. The estimated thermal reservoir temperature is 50.9–68.8 ◦C, and the proportion of shallow cold water ranges from 54% to 87%. A conceptual ﬂow model based on hydrogeochemical results and hydrogeological features is given to describe the geothermal system of the Arxan springs.

Keywords: thermal spring; mineral spring; hydrochemistry; genesis; Arxan

1. Introduction The Arxan springs are located along the deep fault of Arxan, one of the richest geothermal resource regions in Inner Mongolia, Northeast China, and include both thermal and mineral springs. The thermal springs have temperatures ranging between 25 and 42 ◦C, and the springs with temperatures lower than 25 ◦C are deﬁned as mineral springs [1]. The thermal and mineral springs in the study area are characterized by different chemical compositions and have temperatures ranging from 22.3 to 41.0 ◦C. The area where these springs are found is small, and this type of area is very rare in China and globally. Numerous studies have demonstrated the hydrogeochemical characteristics and genesis of the thermal and/or mineral springs in the geothermal system [2,3]. For example, Zaher et al. established a conceptual model based on numerical simulations to determine the origins and sources of heat in thermal springs with varied temperatures in Egypt [4]. Mao et al. determined the chemical and isotopic compositions of thermal and mineral springs in the Dongguan Basin of southern China and assessed how the hydrothermal system was controlled by deep-seated faults [5]. The characteristics of the thermal and cold water and the formation of the geothermal system within a 16 km2 area in western Turkey were determined by Pasvano˘gluin 2012, who indicated that the heat source was likely related to volcanic activity [6]. Katsanou et al. classiﬁed the thermal springs and mineral springs in

Water 2017, 9, 61; doi:10.3390/w9010061 www.mdpi.com/journal/water Water 2017, 9, 61 2 of 17 northwest Greece into three groups based on different temperatures and chemical compositions and determined the functions and the origins of the springs [7]. However, few studies have reported the genesis of both thermal and mineral springs similar to those occurring in Arxan. The first studies of the Arxan thermal springs began as early as the 1970s [8,9]. In this study, the geothermal structure and hydrogeological conditions of the Arxan springs, including the features of the temperature field and the geothermal reservoir of the geothermal system, were investigated, and their geothermal and geophysical characteristics and heat sources were briefly described. In general, thermal springs are associated with various heat sources, such as magma intrusion [2,10], radioactivity and tectogenesis [4,11,12]. Controversy exists regarding the heat sources of the Arxan thermal springs. Some authors [13,14] believe that the thermal springs are of meteoric origin and are heated by magma chambers. In contrast, other authors [15] believe that precipitation flows along the fault and is heated by the terrestrial heat flow and that the formation of thermal springs with low–medium temperatures results from the mixing of deep thermal water and shallow cold water. However, these previous studies only partially describe the hydrochemical characteristics of the thermal springs and the formation of the Arxan geothermal system. The genesis of both the thermal and mineral springs and the relationship between the thermal springs and groundwater in this type of geothermal system have not been thoroughly investigated. The different mixing ratios of the thermal springs have also not been quantitatively analyzed. Thus, further research must be conducted to understand the genesis of the Arxan geothermal system. In this study, the chemical and isotopic features of the Arxan springs are characterized. The temperatures of the geothermal reservoir and the hot–cold water mixing ratios of the Arxan springs were estimated. Furthermore, a conceptual model of the local groundwater route was developed to better understand the occurrence of both thermal and mineral springs in this geothermal system. The research results offer guidance for using Arxan springs and provide new insights into the processes that control the hydrochemical features of the thermal and mineral springs and the genesis and flow patterns of this type of geothermal system.

2. Geological and Hydrogeological Setting The study area is located in Arxan, northeastern China (Figure1), and the main landforms are middle mountains and valleys. The elevation of the highest mountain in this study area is 1275 m, and the climate of the study area is mainly characterized as temperate monsoon, with a mean temperature of 1.0 ◦C. The rainy season occurs from June to August, with a mean annual precipitation of approximately 450 mm and a mean evaporation of 1116 mm/a. The Arxan River is the main body of surface water in the study area. A cluster of thermal and mineral springs occurs within an area of 0.035 km2. The total length of this area is approximately 700 m, and the total width of the area is between 40 and 60 m [14]. Paleozoic, Mesozoic and Cenozoic rocks are exposed in the study area (Figure1), and the basement rocks are magmatic rocks. Two broad stages of geological activity (i.e., the Late Ordovician–Mid-Devonian and the Mid-Jurassic–Early Cretaceous) had a signiﬁcant effect on the regional strata and geological structure [15]. Magma was found to intrude up until the Mid-Jurassic period. The body of igneous rock is elongated towards the northeast, and is 1.5 km wide and 1.7 km deep [8]. Granite is the most widely outcropping type of bedrock, accounting for more than 20% of the bedrock in Arxan. The study area is located in the tectonic unit of the Mongolian arcuate structure and the Neocathaysian uplift zone. The main deep active faults that control the geological tectonics are the Arxan–Gaole conjugate NNE (North-North-East)- and NNW (North-North-West)-trending faults. The deep-seated fault formed by a series of tectonic activities and from continuous stratigraphic deposition. The tectonic evolution in Arxan exhibits a latitudinal sequence, i.e., from an NW (North-West) system to a new Cathaysian system. Generally, the Arxan thermal springs are controlled by the NNW extensional fracture (F1) and NNE compressive fracture (F2), which are located in the central part of the caldera. The NNE fault is a volcanic conduit ﬁlled with acidic lava and can be Water 2017, 9, 61 3 of 17 Water 2017, 9, 61 3 of 17 considered as an impermeable wall. The faults extend for 10 km, with an inclination ranging from ◦ ◦ 7070° to 80° 80 [9].[9].

Figure 1. Map of the study area: 1, gravel, sand and clay (Quaternary); 2, tuff and breccia (Jurassic); Figure 1. Map of the study area: 1, gravel, sand and clay (Quaternary); 2, tuff and breccia (Jurassic); 3, conglomerate and killas (Devonian); 4, glutenite and argillaceous siltstone (Ordovician); 5, granite 3, conglomerate and killas (Devonian); 4, glutenite and argillaceous siltstone (Ordovician); 5, granite (Yanshanian); 6, quartz diorite (Yanshanian); 7, dioritic porphyrite (Yanshanian); 8, granite dyke; (Yanshanian); 6, quartz diorite (Yanshanian); 7, dioritic porphyrite (Yanshanian); 8, granite dyke; 9, 9, fault; 10, inferred fault; 11, volcanic conduit; 12, caldera; 13, rivers; 14, thermal springs; 15, mineral fault; 10, inferred fault; 11, volcanic conduit; 12, caldera; 13, rivers; 14, thermal springs; 15, mineral springs; 16, sampling points of the shallow (a) and deep (b) groundwater; and 17, study area. springs; 16, sampling points of the shallow (a) and deep (b) groundwater; and 17, study area.

As shown in Figure 2, the Arxan springs are located along the compressive plumose fracture As shown in Figure2, the Arxan springs are located along the compressive plumose fracture and are divided into six bands. The temperature of the springs ranges from 10.9 to 41.0 °C.◦ Two andhydrostratigraphic are divided into groups six bands. can be Theclassified temperature according of theto the springs different ranges types from of the 10.9 aquifer to 41.0 mediaC. Two and hydrostratigraphiclithology [14]. Group groups I is composed can be classified of sand and according gravel toof theQuaternary different strata, types ofis rich the aquiferin pore mediawater, and lithologyis distributed [14]. in Group the Arxan I is composed River valley. of sand The and specific gravel yield of Quaternary of Group strata, I is 55 is richm3/(h in∙m), pore and water, the 3 · andgroundwater is distributed depth in ranges the Arxan between River 0 valley.and 3 m. The Most specific mineral yield springs of Group (Group I is 55I) that m /(h existm), here and have the groundwaterwater temperatures depth lower ranges than between 10.0 °C. 0 and The 3 water m. Most temperature mineral springsfluctuates (Group seasonally, I) that and exist the here amount have ◦ waterof spring temperatures discharge lower is approximately than 10.0 C. 30 The m water3/day. temperatureSome wells fluctuatesin the deeper seasonally, part of and the the spring amount are 3 ofartesian spring wells, discharge and the is shallow approximately groundwater 30 m samples/day. Some were wells taken in from the this deeper aquifer part (Group of the III). spring Group are artesianII is the main wells, local and thehydrostratigraphic shallow groundwater group samples composed were of takentuff and from breccia this aquifer of Jurassic (Group strata, III). i.e., Group the IImost is the widely main outcropping local hydrostratigraphic rocks. The bedrock group composed fissure aquifer of tuff consists and breccia of a of weathered Jurassic strata, fissure i.e., pore the mostwater widely aquifer outcroppingwith a sedimentary rocks. thickness The bedrock ranging fissure from aquifer 3 to 8 m consists and a fault of a fissure weathered confined fissure aquifer. pore waterMost of aquifer the thermal with a springs sedimentary (Group thickness II) in this ranging aquifer from occur 3 to where 8 m and the joint a fault fissure fissure is confinedwell developed. aquifer. MostThe water of the temperature thermal springs of Group (Group II ranges II) in this from aquifer 22.3 to occur 41.0 °C, where and the the joint spring fissure discharge is well ranges developed. from ◦ The5 to water192 m3 temperature/day. A deep of groundwater Group II ranges sample from (Group 22.3 to 41.0IV) wasC, andalso thetaken spring from discharge this aquifer. ranges from 5 3 to 192Groundwater m /day. A deep recharge groundwater is generally sample considered (Group IV)to result was also from taken precipitation from this aquifer.and the leakage of waterGroundwater from springs recharge and the isspring generally‐fed river. considered Groundwater to result flows from from precipitation the southwest and the to leakage northeast of wateralong fromthe main springs flow and path the and spring-fed eventually river. discharges Groundwater into the flows Arxan from River. the southwest to northeast along the main flow path and eventually discharges into the Arxan River.

Water 2017, 9, 61 4 of 17 Water 2017, 9, 61 4 of 17

Figure 2. Location of the spring samples and the profile (A‐A’) of the study area. Figure 2. Location of the spring samples and the proﬁle (A-A’) of the study area. 3. Materials and Methods 3. Materials and Methods The spring samples were collected in the study area during October 2015. Twenty sets of Thesamples, spring including samples 16 were from collected the springs, in the 3 studyfrom the area shallow during groundwater October 2015. and Twenty 1 from sets the of deep samples, includinggroundwater, 16 from thewere springs, collected 3 fromin polyethylene the shallow bottles groundwater for hydrochemical and 1 from and the environmental deep groundwater, isotopic were collectedanalysis. in polyethylene The water temperature bottles for (T), hydrochemical pH and dissolved and oxygen environmental (DO) were isotopicmeasured analysis. in situ using The a water temperaturemulti‐parameter (T), pH instrument and dissolved (Multi oxygen 350i/SET, (DO) Munich, were Germany), measured and in situ the usingdissolved a multi-parameter solids and isotopic compositions were determined in the laboratory. The hydrochemical compositions (K+, Na+, instrument (Multi 350i/SET, Munich, Germany), and the dissolved solids and isotopic compositions Ca2+, Mg2+, Cl−, SO42−, HCO3−) were measured at the Fourth Institute of Hydrogeology and were determined in the laboratory. The hydrochemical compositions (K+, Na+, Ca2+, Mg2+, Cl−, Engineering Geology of Inner Mongolia based on China’s national standards for the examination of 2− − SO4 natural, HCO mineral3 ) were water measured for drinking at the [16] Fourth The major Institute cation of (K Hydrogeology+, Na+, Ca2+, Mg and2+) concentrations Engineering were Geology of Innerdetermined Mongolia by using based inductively on China’s coupled national plasma standards‐mass spectrometry for the examination (Agilent 7500ce of natural ICP‐MS, mineral Tokyo, water + + 2+ 2+ for drinkingJapan), and [16 ]the The major major anion cation contents (K , Na (Cl, Ca−, SO,42 Mg−) were) concentrations determined using were spectrophotometry determined by using inductively(PerkinElmer coupled Lambda plasma-mass 35, Waltham, spectrometry MA, USA). (Agilent The total 7500ce dissolved ICP-MS, solids (TDS) Tokyo, and Japan), HCO3− and contents the major − 2− anionwere contents determined (Cl , by SO using4 ) acid were‐gravimetric determined analysis using and spectrophotometry base titration, respectively. (PerkinElmer The differences Lambda 35, between the cation–anion balances of all samples were less than 5%.− Waltham, MA, USA). The total dissolved solids (TDS) and HCO3 contents were determined by using 18 2 acid-gravimetricδ O, δH analysis and tritium and analyses base titration, were conducted respectively. at the The Analytical differences Laboratory between of the the cation–anion Beijing Research Institute of Uranium Geology. The 18O and 2H compositions were determined using the balances of all samples were less than 5%. standard dioxide–water equilibrium method and the zinc reaction method, respectively. The stable δ18O, δ2H and tritium analyses were conducted at the Analytical Laboratory of the Beijing isotopic ratios were measured based on isotope ratio mass spectrometry by using a Finnigan MAT 18 2 Research253 (Thermo Institute Fisher of Uranium Scientific, Geology.Waltham, MA, The USA)O and with anH analytical compositions precision were of ±0.2‰ determined for δ18O using and the standard±2‰ dioxide–waterfor δ2H. The standard equilibrium is the methodVienna Standard and the zincMean reaction Ocean Water method, (VSMOW). respectively. The tritium The stable isotopicconcentrations ratios were in measured the samples based were on determined isotope ratio using mass electrolytic spectrometry enrichment by using and a Finniganthe liquid MAT 253 (Thermoscintillation Fisher technique Scientiﬁc, based Waltham,on China’s MA,national USA) standards with an for analytical the examination precision of tritium of ±0.2 [17]. Thefor δ18O and ±measurement2 for δ2H. accuracy The standard for 3H ranges is the between Vienna ±0.3 Standard and ±0.5 Mean tritium Ocean units Water (TU). (VSMOW). Theh tritium concentrationsh in the samples were determined using electrolytic enrichment and the liquid scintillation 4. Results and Discussion technique based on China’s national standards for the examination of tritium [17]. The measurement 3 accuracy4.1. Hydrochemical for H ranges Characteristics between ± 0.3 and ±0.5 tritium units (TU). 4. ResultsThe and samples Discussion from the thermal springs (Group II) were obtained near each other because the distance between any two springs was less than 10 m; however, the output temperatures of the 4.1. Hydrochemicalsprings ranged Characteristicsfrom 22.3 to 41.0 °C. The temperatures of the mineral springs (Group I) were relatively

The samples from the thermal springs (Group II) were obtained near each other because the distance between any two springs was less than 10 m; however, the output temperatures of the Water 2017, 9, 61 5 of 17 springs ranged from 22.3 to 41.0 ◦C. The temperatures of the mineral springs (Group I) were relatively consistent, ranging between 10.9 and 12.9 ◦C. The Group II samples were weakly alkaline, with pH values ranging from 7.2 to 7.9, and the Group I samples and most of the shallow groundwater samples (Group III) had low pH values. The electrical conductivities (EC) of the Group I and Group III samples ranged from 310 to 515 µS/cm and from 77 to 177 µS/cm, respectively, and were generally lower than the ECs of Group II (567–909 µS/cm) and Group IV (766 µS/cm) samples. The dissolved oxygen (DO) contentWater were 2017, 9, 61 relatively high in the mineral springs (73–144 mg/L) and shallow5 groundwater of 17

(65–242 mg/L)consistent, when ranging compared between 10.9 with and the 12.9 content °C. The Group in the II thermalsamples were springs weakly (5.8–87 alkaline, mg/L) with pH and deep groundwatervalues (192 ranging mg/L). from 7.2 to 7.9, and the Group I samples and most of the shallow groundwater ( − − 2− The mainsamples anion Group in allIII) ofhad the low water pH values. samples The electrical was HCO conductivities3 , and high(EC) of Cl the Groupand SO I and4 Groupcontents were observed inIII the samples Group ranged II and from Group 310 to 515 IV μ samples.S/cm and from The 77 main to 177 cations μS/cm, respectively, were Na+ andand were Ca 2+generally, and additional 2+ lower than the ECs of Group II (567–909 μS/cm) and Group IV (766 μS/cm) samples. The dissolved Mg wasoxygen detected (DO) in content the Group were Irelatively and Group high IIIin samplesthe mineral (Table springs1). (73–144 According mg/L) toand Figure shallow3, Group I mainly consistedgroundwater of HCO(65–2423–Ca mg/L)·Na when water, compared whereas with the Group content II in mainly the thermal consisted springs (5.8–87 of HCO mg/L)3–Na water. The shallowand groundwater deep groundwater samples (192 mg/L). (Group III) had chemical characteristics that were similar to those The main anion in all of the water samples was HCO3−, and high Cl− and SO42− contents were of Group I, i.e., Group III samples mainly consisted of HCO3–Ca·Na and HCO3·SO4–Ca·Na water observed in the Group II and Group IV samples. The main cations were Na+ and Ca2+, and additional accordingMg to the2+ was Piper detected diagram in the Group (Figure I and3). Group III samples (Table 1). According to Figure 3, Group I The relativelymainly consisted high concentrations of HCO3–Ca∙Na water, of DO whereas in the Group Group II Imainly samples consisted were of related HCO3–Na to shallowwater. The circulation and a rapidshallow renewal groundwater rate. The samples Group (Group II thermal III) had waterchemical samples characteristics had athat distinct were similar outlet to temperature those of and EC, whichGroup were higherI, i.e., Group in the III Group samples II mainly samples consisted than in of the HCO Group3–Ca∙Na I samples.and HCO3 This∙SO4–Ca ﬁnding∙Na water showed that according to the Piper diagram (Figure 3). the Group II thermalThe relatively water high samples concentrations had a of slightly DO in longerthe Group residence I samples time were thanrelated the to Groupshallow I thermal water samples.circulation The and Ca a and rapid Mg renewal concentrations rate. The Group of GroupII thermal II water ranged samples from had 8.2 a to distinct 19 mg/L outlet and from 0.01 to 0.87temperature mg/L, respectively. and EC, which The were relatively higher in the low Group Ca II and samples Mg concentrationsthan in the Group I mainly samples. resulted This from cation exchangefinding showed because that the the Ca Group and II Mgthermal supplied water samples from calcitehad a slightly dissolution longer residence were removed time than from the the Group I thermal water samples. The Ca and Mg concentrations of Group II ranged from 8.2 to + solution while19 mg/L Na and remained from 0.01 in to excess.0.87 mg/L, The respectively. Group II The samples relatively were low Ca also and characterized Mg concentrations by high Na − − (114–183 mg/L),mainly resulted HCO3 from(206–371 cation exchange mg/L) because and Cl the Ca(17–31 and Mg mg/L) supplied concentrations, from calcite dissolution and the were DO content was betweenremoved 5.8 and from 87.1 the mg/L solution and while was lowerNa remained than that in excess. observed The for Group the GroupII samples I samples, were also which were inﬂuencedcharacterized by the deep by circulation high Na+ of(114–183 groundwater mg/L), HCO in the3− (206–371 aquifer. mg/L) Group and III, Cl which− (17–31 had mg/L) a higher DO concentrations, and the DO content was between 5.8 and 87.1 mg/L and was lower than that observed content (65–242for the Group mg/L), I samples, was mainly which were recharged influenced by by shallow the deep circulation groundwater of groundwater rich in in atmospheric the aquifer. oxygen and had TDSGroup and III, ECs which ranging had a from higher 85 DO to 175content mg/L (65–242 and mg/L), from 77 was to mainly 177 µS/cm, recharged respectively, by shallow indicating a short circulationgroundwater time. rich in The atmospheric deep groundwater oxygen and had (Group TDS and IV) ECs consisted ranging from of 85 HCO to 1753· SOmg/L4–Na and· Ca water. The high TDSfrom content77 to 177 and μS/cm, low respectively, DO concentration indicating a implied short circulation relatively time. longer The deep circulation groundwater and resident (Group IV) consisted of HCO3∙SO4–Na∙Ca water. The high TDS content and low DO concentration times, similarimplied to the relatively Group longer II samples. circulation and resident times, similar to the Group II samples.

Figure 3. Piper diagram showing the hydrochemical compositions of the samples. Figure 3. Piper diagram showing the hydrochemical compositions of the samples.

Water 2017, 9, 61 6 of 17

Table 1. Results of the chemical analysis of the samples. n.a., not analyzed; T, water temperature; TDS, total dissolved solids; EC, electrical conductivities; DO, dissolved oxygen.

TDS EC DO Na K Ca Mg Cl SO HCO SiO δ18O δ2H Tritium No. T (◦C) pH 4 3 2 (mg/L) (µS/cm) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) ( SMOW) ( SMOW) (TU) Group I h h S015 10.9 7.2 295 493 73 68 5.8 28 2.79 12 54 190 29 −14.2 −111 9.6 ± 0.5 S016 11.9 7.0 294 515 94 72 2.9 27 2.43 14 50 193 29 −15.1 −113.6 12.0 ± 0.5 S018 12.9 7.1 382 310 144 86 1.8 34 4.72 21 60 209 32 −14.6 −111.5 11.5 ± 0.5 S025 12.9 7.0 279 488 76 67 2.7 25 1.97 16 50 168 32 −14.5 −106.2 11.2 ± 0.5 Group II S033 22.0 7.3 345 567 56 114 2.4 8.2 0.1 17 49 206 35 −15.0 −115.0 8.2 ± 0.5 S039 23.0 7.2 380 606 58 114 3.2 18 0.9 21 70 216 32 −14.4 −112.1 7.1 ± 0.5 S040 22.3 7.2 403 598 65 116 3.0 19 0.8 21 68 242 32 −15.4 −113.2 6.4 ± 0.4 S050 23.8 7.5 572 857 83 183 3.6 19 0.87 28 80 371 39 −14.6 −111.6 9.8 ± 0.5 S055 24.0 7.9 505 858 87 169 3.7 11 0.14 28 100 299 44 n.a. n.a. n.a. S029 24.0 7.4 536 775 31 182 2.7 10 0.01 28 112 308 38 −15.0 −113.3 5.0 ± 0.4 S052 28.2 7.4 505 823 45 164 3.3 11 0.09 26 70 302 47 −14.7 −113.6 3.7 ± 0.4 S049 29.5 7.5 548 843 38 176 3.2 15 0.00 31 95 313 38 −14.4 −114.2 3.4 ± 0.4 S044 30.1 7.4 446 744 40 151 2.7 8.7 0.03 21 66 280 41 −13.7 −112.8 4.4 ± 0.4 S048 34.0 7.5 463 760 25 155 3.2 9.2 0.03 24 75 302 45 n.a. n.a. n.a. S032 35.0 7.4 556 909 5.9 182 3.4 11 0.23 30 97 336 45 −14.9 −115.4 3.5 ± 0.4 S046 41.0 7.4 530 807 5.8 183 3.0 10 0.01 26 98 321 40 −14.7 −114.2 1.3 ± 0.4 Group III Z001 6.0 6.8 175 177 92 25 0.6 20 1.09 6.9 53 55 21 n.a. n.a. n.a. Z002 15.9 6.6 173 173 65 12 1.1 19 1.93 6.9 13 72 24 n.a. n.a. n.a. Z003 8.9 8.2 136 136 242 7 1.9 16 2.70 8.7 8 56 18 n.a. n.a. n.a. Z004 8.7 7.3 85 77 219 4.5 1.6 11 1.48 0.1 10 44 17 n.a. n.a. n.a. Group IV Z005 9.7 7.8 768 766 192 60 2.5 55 10.3 33 110 125 30 n.a. n.a. n.a. Water 2017, 9, 61 7 of 17

Water 2017, 9, 61 7 of 17 Chloride displays conservative behavior and can be used as a tracer in geothermal systems [18]. Chloride displays conservative behavior and can be used as a tracer in geothermal systems [18]. Some major ions and ionic ratios plotted against the Cl content are shown in Figure4. Strong positive Some major ions and ionic ratios plotted against the Cl content are shown in Figure 4. Strong positive correlationscorrelations are are observed observed in in the the EC EC vs. vs. ClCl (R-value(R‐value of of 0.9) 0.9) and and T Tvs. vs. Cl Cl(R‐value (R-value of 0.79) of 0.79) plots plots (Figure(Figure4a,b), 4a,b), implying implying the mixing the mixing of thermal of thermal and mineraland mineral water. water. Also, Also, good good correlations correlations exist exist between Na andbetween Cl (R Na = 0.88) and Cl and (R between= 0.88) and K between and Cl K (R and = 0.7) Cl (R (Figure = 0.7) (Figure4c,d), suggesting 4c,d), suggesting the progressive the progressive reaction of waterreaction with of feldspar water with in the feldspar geothermal in the geothermal system, and system, the ion and contents the ion are contents considered are considered as independent as indicatorsindependent of the residenceindicators timeof the [ 19residence]. Thus, time the [19]. residence Thus, the time residence of the samples time of the increased samples from increased Group III to Groupfrom I,Group Group III IIto andGroup Group I, Group IV. TheII and strong Group correlations IV. The strong of correlations HCO3 vs. Clof HCO (R =3 0.85) vs. Cl and (R = SO 0.85)4 vs. Cl (R = 0.84)and SO indicate4 vs. Cl that (R the= 0.84) thermal indicate water that was the diluted thermal by water shallow was colddiluted water by (Figureshallow4 e,f)cold [ 20water]. (Figure 4e,f) [20].

FigureFigure 4. Plots 4. Plots of ion of ion contents contents vs. vs. chloride chloride contents contents in in the the thermal thermal and and mineral mineral water water in Arxan in Arxan (a) EC (a ) EC vs. Cl; (b) T vs. Cl; (c) Na vs. Cl; (d) K vs. Cl; (e) HCO3 vs. Cl; and (f) SO4 vs. Cl. vs. Cl; (b) T vs. Cl; (c) Na vs. Cl; (d) K vs. Cl; (e) HCO3 vs. Cl; and (f) SO4 vs. Cl.

The ionic ratios of the water samples were calculated as shown in Table 2. The mean Na/Cl ratio Theincreased ionic from ratios Group of the III water to Group samples IV, Group were I calculated and Group as II. shown The mean in Table K/Cl 2ratios. The of mean the Groups Na/Cl I ratio increasedand III from samples Group were III larger to Group than those IV, Group of the IGroups and Group II and II. IV The samples. mean The K/Cl mean ratios Ca/Mg of theratios Groups of I and IIIthe samples Group II weresamples larger were than greater those than of those the Groups of the other II and groups. IV samples. The mean The SO4 mean/Cl ratios Ca/Mg generally ratios of the Groupincreased II samples from Group were III, greater to Group than II, those Group of IV the and other Group groups. I, while The the mean HCO SO3/Cl4/Cl ratio ratios generally generally increased as follows: Group IV < Group III < Group II < Group I. increased from Group III, to Group II, Group IV and Group I, while the HCO3/Cl ratio generally increasedDue as follows: to the water–rock Group IV interactions < Group III(dissolution < Group of II Na < Group‐feldspar I. and K‐feldspar, respectively), the high Na/Cl and K/Cl ratios indicated a longer and deeper circulation path. The Ca/Mg ratio of the Due to the water–rock interactions (dissolution of Na-feldspar and K-feldspar, respectively), the thermal water was higher than that of the cold water, indicating Mg depletion due to the occurrence high Na/Cl and K/Cl ratios indicated a longer and deeper circulation path. The Ca/Mg ratio of the of clay minerals [21]. The SO4/Cl ratios were lower in the thermal water than in the cold water, which thermal water was higher than that of the cold water, indicating Mg depletion due to the occurrence of clay minerals [21]. The SO4/Cl ratios were lower in the thermal water than in the cold water, which was related to the loss of S through the precipitation of sulﬁdes or sulfates [20]. The HCO3/Cl ratios Water 2017, 9, 61 8 of 17

Water 2017, 9, 61 8 of 17 was related to the loss of S through the precipitation of sulfides or sulfates [20]. The HCO3/Cl ratios in the thermal water were higher than those in the cold water, indicating mixing between ascending inthermal the thermal water water and wereshallow higher cold than water those [10] in the as coldwell water, as a indicatingshorter circulation mixing between path and ascending faster thermalgroundwater water cycle and shallow [19]. cold water [10] as well as a shorter circulation path and faster groundwater cycle [19]. Table 2. Mean ionic ratios of the water samples. Table 2. Mean ionic ratios of the water samples. Na/Cl K/Cl Ca/Mg SO4/Cl HCO3/Cl Samples (Mean) (Mean) (Mean) (Mean) (Mean) Na/Cl K/Cl Ca/Mg SO /Cl HCO /Cl Samples 4 3 Mineral springs (Group I) (Mean)4.8 (Mean)0.2 (Mean)10.3 (Mean)3.5 (Mean)12.5 ThermalMineral springs (Group (Group I) II) 4.86.3 0.20.1 10.395.1 3.53.3 12.511.6 ShallowThermal groundwater springs (Group (Group II) III)) 6.31.7 0.10.2 95.110.3 3.33.0 11.67.7 DeepShallow groundwater groundwater (Group(Group III) IV) 1.71.8 0.20.1 10.35.3 3.03.4 7.73.8 Deep groundwater (Group IV) 1.8 0.1 5.3 3.4 3.8 4.2. Source of Recharge 4.2. Source of Recharge The linear relationship between the 2H and 18O contents of meteoric water can be used to identify 2 18 the originsThe linear of spring relationship water [22] between and estimate the H and the rechargeO contents elevation. of meteoric According water can to data be used obtained to identify from theprevious origins studies, of spring the water recharge [22] andelevation estimate of the the spring recharge samples elevation. is approximately According to data1012 obtained m. The mean from 18 previousδ18O value studies, of precipitation the recharge is − elevation7.1‰, and of the springelevation samples gradient is approximately of δ18O for meteoric 1012 m. water The mean is −4.95‰δ O 18 valueper hundred of precipitation meters [23]. is − The7.1 isotope, and the data elevation ranges from gradient −118.3‰ of δ toO − for111.5‰ meteoric for water2H and is from−4.95 −16.0‰per 2 hundredto −14.2‰ meters for 18 [O23 ].with The respect isotopeh datato the ranges VSMOW from (Table−118.3 1). toThe−111.5 mean δfor18O Hvalue and fromof precipitation−16.0h to 18 18 −is14.2 −7.1‰.for O with respect to the VSMOW (Table1). The meanh δ O valueh of precipitation is −7.1h . 18 2 TheTheh relationshiprelationship betweenbetween thethe δδ 18O and δδ2H of the springs is shown in Figure5 5 with with the the Global Globalh MeteoricMeteoric WaterWater Line Line (GMWL) (GMWL) [24 ][24] and and Local Local Meteoric Meteoric Water Water Line (LMWL) Line (LMWL) [25]. As [25]. shown As in shown Figure 5in, allFigure the water 5, all samplesthe water fall samples near the fall GMWL near the and GMWL the LMWL, and implyingthe LMWL, that implying the spring that water the isspring of meteoric water 18 origin.is of meteoric A signiﬁcant origin. OA shiftsignificant occurred 18O in shift the thermaloccurred springs in the inthermal the study springs area, in indicating the study partial area, 18 isotopicindicating equilibrium partial isotopic under equilibrium a high reservoir under temperature. a high reservoir The temperature.O value of the The thermal 18O value springs of the is higher,thermal which springs results is higher, from which dissolution results from from the dissolution surrounding from rocks the [surrounding26,27]. rocks [26,27].

Figure 5. δ2H vs δ18O plot for water samples and the Global Meteoric Water Line (GMWL) and Local Figure 5. δ2H vs δ18O plot for water samples and the Global Meteoric Water Line (GMWL) and Local MeteoricMeteoric WaterWater LineLine (LMWL). (LMWL).

Water 2017, 9, 61 9 of 17

Water 2017, 9, 61 9 of 17 The isotopic composition of the thermal water was affected by various factors, including latitude, The isotopic composition of the thermal water was affected by various factors, including temperature and elevation. According to previous studies, the equation proposed by Dansgaard for latitude, temperature and elevation. According to previous studies, the equation proposed by the relationship between surface air temperature and δ18O in northern hemisphere and high latitude Dansgaard for the relationship between surface air temperature and δ18O in northern hemisphere and zone is as follows [28]: high latitude zone is as follows [28]: δ18O = 0.69t − 13.6 (1) δ18O = 0.69t − 13.6 (1) where t is the mean air temperature in the recharge area (◦C). whereWhile t is the the mean relationship air temperature between elevationin the recharge and δ 18areaO was (°C). analyzed using the following equation: While the relationship between elevation and δ18O was analyzed using the following equation: H = (δG − δP)/K + h (2) H = (δG − δP)/K + h (2) δ wherewhere H is the elevation of of the the recharge recharge area area (m), (m), h h is is the the elevation elevation of of the the water water samples samples (m), (m), δG Gis δ18 δ δ18 isthe the δ18O Ovalue value of ofthe the samples samples (‰), ( δ), P Pisis the the mean mean δ18OO value value of of precipitation precipitation (‰), ( ), and and K is thethe 18 elevation gradientgradient ofof δδ 18O for meteoric h water (‰/100 ( /100 m) m) [23]. [23]. Consequently, Consequently, the mean h air temperature ◦ inin thethe rechargerecharge areaarea waswas approximatelyapproximately −−1.7 °C.hC. The The mean mean recharge elevation was calculated to vary between 1160 and 1186 m, and the surroundingsurrounding middle mountains were all at thethe inferredinferred elevationelevation forfor recharge.recharge. The groundwatergroundwater residence residence time time and and mixing mixing of waterof water in geothermal in geothermal systems systems were analyzedwere analyzed using tritiumusing tritium values values [23]. As[23]. shown As shown in Table in Table1, the 1, tritium the tritium values values of Group of Group I ranged I ranged between between 9.6 9.6± ± 0.5 and 11.511.5 ±± 0.50.5 TU, and the tritium values ofof GroupGroup IIII rangedranged fromfrom 1.3 1.3± ± 0.4 toto 9.89.8 ±± 0.5 TU. Tritium contents below 1 TU suggestsuggest an ancientancient (older(older thanthan 6060 yearsyears ofof age)age) groundwatergroundwater source,source, whereaswhereas valuesvalues aboveabove 1 TU represent a modern (younger than 60 years of age) groundwater source [[29].29]. Thus, thethe tritiumtritium values varying from 1.3 toto 11.511.5 TUTU indicatedindicated a mixturemixture ofof modernmodern andand oldold groundwatergroundwater whenwhen ascendingascending towardstowards thethe surfacesurface [[30,31].30,31]. The EC–tritium graph (Figure6 6a)a) shows shows the the existence existence of of water water from from different different origins. origins. Group Group I I shows high tritiumtritium and low EC values,values, indicatingindicating shallow circulating water, whereas Group II hashas relativelyrelatively lowlow tritiumtritium andand highhigh ECEC valuesvalues andand isis consideredconsidered deepdeep circulatingcirculating water. water.

(a) (b)

Figure 6.6. Plots of the tritiumtritium contentscontents forfor waterwater samplessamples againstagainst thethe hydrochemicalhydrochemical parametersparameters of ((aa)) ECEC ((µμS/cm)S/cm) and and ( b)) Cl (mg/L). (mg/L).

Differences were observed between the two groups due to the mixing of shallow groundwater Differences were observed between the two groups due to the mixing of shallow groundwater and deep circulating water at different ratios [32]. Meanwhile, the long residence time of and deep circulating water at different ratios [32]. Meanwhile, the long residence time of groundwater groundwater was also related to low permeability or deep circulation. The Cl–tritium relationship was also related to low permeability or deep circulation. The Cl–tritium relationship can also be used can also be used to distinguish between shallow and deep circulating water. The Group I water to distinguish between shallow and deep circulating water. The Group I water samples consisted of samples consisted of shallow circulating water according to the high tritium and low chloride values shallow circulating water according to the high tritium and low chloride values (Figure6b), whereas (Figure 6b), whereas the Group II water samples belonged to deep circulating water [33]. the Group II water samples belonged to deep circulating water [33]. Consequently, the shallow circulation of groundwater probably results in a short residence time, Consequently, the shallow circulation of groundwater probably results in a short residence time, whereas long residence times may be related to deep groundwater circulation [34]. Thus, we can infer whereas long residence times may be related to deep groundwater circulation [34]. Thus, we can infer that the Group I samples occur in the shallow cold aquifer related to the local groundwater system, whereas the thermal springs of Group II rising from the deep groundwater system are recharged by deeply circulating meteoric water and are heated by the geothermal reservoir. Meanwhile, the

Water 2017, 9, 61 10 of 17

Water 2017, 9, 61 10 of 17 that the Group I samples occur in the shallow cold aquifer related to the local groundwater system, whereasreservoir the temperature thermal springs of the ofthermal Group springs II rising is from higher the than deep the groundwater discharge systemtemperature, are recharged and the byreservoir deeply temperatures circulating meteoric of the thermal water andsprings are can heated be estimated by the geothermal using geothermometers. reservoir. Meanwhile, the reservoir temperature of the thermal springs is higher than the discharge temperature, and the reservoir temperatures4.3. Hydrogeochemical of the thermal Thermometry springs can be estimated using geothermometers.

4.3. HydrogeochemicalSilica geothermometers, Thermometry cation geothermometers, multimineral equilibrium geothermometers, etc., are widely used to estimate the temperatures of thermal reservoirs related to thermal Silica geothermometers, cation geothermometers, multimineral equilibrium geothermometers, springs [35–37]. The reservoir temperatures of the Arxan thermal springs were calculated by using etc., are widely used to estimate the temperatures of thermal reservoirs related to thermal Na–K, K–Mg, Chalcedony and Quartz geothermometers (Table 3). springs [35–37]. The reservoir temperatures of the Arxan thermal springs were calculated by using Na–K, K–Mg, Chalcedony and Quartz geothermometers (Table3). Table 3. Estimated reservoir temperature (°C) using chemical geothermometers.

Table 3. Estimated reservoir temperature (◦C) using chemical geothermometers.Quartz Quartz‐Max, Sample Temperature (°C) Na–K K–Mg Chalcedony No‐Steam Loss Steam Loss S033 Temperature22.0 107.4 87.6 54.9 85.9Quartz Quartz-Max,88.7 Sample ◦ Na–K K–Mg Chalcedony S039 23.0( C) 102.1 68.8 50.9 No-Steam82.1 Loss Steam85.4 Loss S040S033 22.3 22.0 116.7 107.4 68.9 87.6 50.9 54.982.1 85.9 85.4 88.7 S050S039 23.8 23.0 103.4 102.1 71.7 68.8 59.9 50.990.6 82.1 92.8 85.4 S055S040 24.0 22.3 108.1 116.7 95.2 68.9 65.6 50.995.9 82.1 97.5 85.4 S050 23.8 103.4 71.7 59.9 90.6 92.8 S029S055 24.0 24.0 108.191.9 100.1 95.2 58.7 65.689.4 95.9 91.8 97.5 S032S029 35.0 24.0 101.0 91.9 86.5 100.1 66.7 58.797.0 89.4 98.4 91.8 S044S032 30.1 35.0 101.0100.0 108.4 86.5 62.2 66.792.8 97.0 94.7 98.4 S046S044 41.0 30.1 100.095.5 127.0 108.4 61.1 62.291.7 92.8 93.8 94.7 S048S046 34.0 41.0 106.2 95.5 113.2 127.0 66.7 61.197.0 91.7 98.4 93.8 S048 34.0 106.2 113.2 66.7 97.0 98.4 S049 29.5 100.9 130.0 58.7 89.4 91.8 S049 29.5 100.9 130.0 58.7 89.4 91.8 S052S052 28.2 28.2 104.6 98.2 98.2 68.8 68.898.9 98.9 100.1 100.1

The results from the different methods show little agreement, with uncertainty mainly resulting The results from the different methods show little agreement, with uncertainty mainly resulting from the mixing of thermal water with shallow cold water and the lack of full equilibrium. from the mixing of thermal water with shallow cold water and the lack of full equilibrium. Meanwhile, Meanwhile, because each geothermometer has its own limitations [38–40], it is necessary to attain because each geothermometer has its own limitations [38–40], it is necessary to attain water–rock water–rock equilibrium in the geothermal reservoir and to select an appropriate estimation method equilibrium in the geothermal reservoir and to select an appropriate estimation method according to the according to the chemical characteristics of the thermal springs determined by the Na–K–Mg1/2 chemical characteristics of the thermal springs determined by the Na–K–Mg1/2 diagram (Figure7)[ 10]. diagram (Figure 7) [10].

1/2 FigureFigure 7. 7.Triangular Triangular diagram diagram of of Na–K–Mg Na–K–Mg1/2.

As shown in Figure 7, the mineral springs contain immature water, and the data points of the thermal springs fall in the partially equilibrated water field. The data suggest the occurrence of processes such as re‐equilibrium and mixing with cold water, implying that the geothermal water

Water 2017, 9, 61 11 of 17

As shown in Figure7, the mineral springs contain immature water, and the data points of the thermal springs fall in the partially equilibrated water field. The data suggest the occurrence of processes such as re-equilibrium and mixing with cold water, implying that the geothermal water temperatures changed to a greater extent during their re-equilibrium processes [10,41]. Thus, the most suitable geothermometer was the silica geothermometer [10]. The solubility of most minerals in geothermal water varies with temperature. To further estimate the temperature of a reservoir, the mineral that controls the geothermal fluid system must be identified. Mineral equilibrium calculations are used to estimate mineral reactivity in geothermal systems. A saturated index (SI) of zero shows a saturated solution relative to the solid, whereas a positive SI indicates an oversaturated solution, and a negative SI indicates an under-saturated solution. The saturation indices for many common minerals and for many of the measured outlet temperatures were calculated using the program PHREEQC [42,43] under the assumption that no conductive cooling occurred (Table4). The equilibrium state between minerals and water is temperature-dependent. The saturation indices were recalculated to estimate the equilibrium states of the hydrothermal minerals under different temperatures (Figure8).

Table 4. Saturation index of the water samples.

No. Anhydrite Aragonite Calcite Chalcedony Dolomite Gypsum Halite Quartz SiO2(a) Group I S015 −2.41 −0.88 −0.73 0.41 −2.29 −2.15 −7.61 0.88 −0.48 S016 −2.45 −1.04 −0.89 0.39 −2.67 −2.19 −7.53 0.87 −0.49 S018 −2.31 −0.89 −0.74 0.42 −2.16 −2.05 −7.29 0.89 −0.46 S025 −2.47 −1.13 −0.98 0.42 −2.89 −2.22 −7.50 0.89 −0.46 Group II S033 −2.96 −1.15 −1.01 0.35 −3.62 −2.73 −7.26 0.79 −0.50 S039 −2.49 −0.87 −0.72 0.30 −2.43 −2.26 −7.19 0.74 −0.55 S040 −2.49 −0.82 −0.67 0.31 −2.41 −2.26 −7.18 0.75 −0.54 S050 −2.47 −0.32 −0.17 0.38 −1.36 −2.24 −6.88 0.81 −0.47 S055 −2.60 −0.30 −0.15 0.42 −1.87 −2.37 −6.91 0.86 −0.42 S029 −2.59 −0.73 −0.59 0.36 −3.85 −2.37 −6.88 0.80 −0.48 S032 −2.54 −0.53 −0.39 0.31 −0.88 −2.37 −7.21 0.71 −0.49 S044 −2.83 −0.77 −0.63 0.33 −3.32 −2.62 −7.07 0.74 −0.5 S046 −2.55 −0.52 −0.39 0.2 −0.84 −2.41 −6.91 0.58 −0.59 S048 −2.64 −0.57 −0.43 0.32 −2.92 −2.56 −7.02 0.73 −0.49 S049 −2.48 −0.42 −0.28 0.3 −4.35 −2.28 −6.84 0.72 −0.52 S052 −2.74 −0.68 −0.54 0.41 −2.78 −2.53 −6.94 0.83 −0.42 Group III Z001 −2.48 −1.93 −1.77 0.33 −4.75 −2.22 −8.27 0.82 −0.58 Z002 −3.08 −1.93 −1.78 0.26 −4.31 −2.84 −8.61 0.72 −0.61 Z003 −3.34 −0.61 −0.46 0.22 −1.58 −3.09 −8.71 0.70 −0.68 Z004 −3.35 −1.75 −1.59 0.20 −3.97 −3.10 −10.84 0.69 −0.70 Group IV Z005 −1.88 −0.24 −0.08 0.44 −0.78 −1.63 −6.97 0.91 −0.46

All water samples were under-saturated with respect to anhydrite, aragonite, calcite, dolomite, gypsum, halite and amorphous silica but oversaturated with respect to chalcedony and quartz. Consequently, chalcedony and quartz have greater solubility in water at the pH values and output water temperatures measured in situ. Atmospheric aqueous CO2 resulted in the release of calcite and aragonite during tuff formation, which is the dominant host rock. The SI of calcite increased from Group III (mean value at −1.40) to Group I (mean value at −0.84), Group II (mean value at −0.50) and Group IV(mean value at −0.08), indicating that the circulation depth and residence time increased. If the equilibrium lines of a series of estimated minerals converge, the temperature most likely corresponds to the reservoir temperature [44]. As shown in Figure8, the SIs with respect to chalcedony, calcite, aragonite and quartz minerals tended to converge to zero; therefore, the Water 2017, 9, 61 12 of 17 convergence temperature that reached the equilibrium state ranged from 60 to 80 ◦C. The large convergence temperature was related to the mixing of shallow cold water and deep thermal water [45]. The equilibrium curves of chalcedony converge between the convergences ranges implied that the equilibrium between thermal water and silica was controlled by chalcedony [30]. Consequently, the estimated reservoir temperatures of the thermal springs by chalcedony geothermometer were reliable , and ranged from 50.9 to 68.8 ◦C (Table3)[46]. However, the ascending thermal springs from the deep geothermal reservoir may be cooled by Water 2017, 9, 61 12 of 17 conductive heat loss when travelling through cooler rocks and become mixed with shallow cold water. BasedConsequently, on the above the analysis,estimated the reservoir difference temperatures between the outletof the temperatures thermal springs of the thermalby chalcedony springs mainlygeothermometer resulted from were the reliable different , and mixing ranged ratios from of 50.9 shallow to 68.8 cold °C water.(Table 3) [46].

(a) (b)

Figure 8. Mineral equilibrium diagram of the sampling points (a) S032 and (b) S046 as representatives of thermal water.water.

4.4. MixingHowever, of Cold the and ascending Thermal thermal Water springs from the deep geothermal reservoir may be cooled by conductive heat loss when travelling through cooler rocks and become mixed with shallow cold water.The Based silica-enthalpy on the above method analysis, [47] the can difference be used to between evaluate the the outlet shallow temperatures cold water componentsof the thermal of mixedsprings water mainly and resulted the reservoir from the temperatures. different mixing The ratios mean of temperature shallow cold of thewater. shallow cold water was ◦ assumed to equal the annual mean temperature (1 C), and the SiO2 concentration was 3 mg/L. Using ◦ S0464.4. Mixing as an example,of Cold and the Thermal temperature Water was 41.0 C and the SiO2 concentration was 40 mg/L. Based on the steam table [48], the enthalpies of the thermal water and cold water were 171.7 and 4.2 kJ/kg, The silica‐enthalpy method [47] can be used to evaluate the shallow cold water components of respectively. The intersection point of the mixing line with the solubility curve for chalcedony resulted mixed water and the reservoir temperatures. The mean temperature of the shallow cold water was in an enthalpy of 239 kJ/kg, and the silica content was 62 mg/L in the deep thermal groundwater; assumed to equal the annual mean temperature (1 °C), and the SiO2 concentration was 3 mg/L. Using thus, the calculated reservoir temperature of the springs was 82–165 ◦C, which was higher than the S046 as an example, the temperature was 41.0 °C and the SiO2 concentration was 40 mg/L. Based on temperatures calculated by using the chalcedony geothermometers (50.9~68.8 ◦C). The overestimated the steam table [48], the enthalpies of the thermal water and cold water were 171.7 and 4.2 kJ/kg, temperatures probably resulted from the occurrence of conductive cooling. respectively. The intersection point of the mixing line with the solubility curve for chalcedony The thermal water temperature and the mixing ratio of cold and thermal water can be estimated resulted in an enthalpy of 239 kJ/kg, and the silica content was 62 mg/L in the deep thermal based on the chemical contents of the samples. To determine the two unknowns, the two following groundwater; thus, the calculated reservoir temperature of the springs was 82–165 °C, which was equations can be solved [49]: higher than the temperatures calculated by using the chalcedony geothermometers (50.9~68.8 °C). The overestimated temperatures( probably resulted from the occurrence of conductive cooling. HcX + HT(1 − X) = HS The thermal water temperature and the mixing ratio of cold and thermal water can be estimated(3) SiO2CX + SiO2T(1 − X) = SiO2S based on the chemical contents of the samples. To determine the two unknowns, the two following equations can be solved [49]: where HC is the enthalpy of the cold water (J), HT is the enthalpy of the deep thermal water (J), HS is the enthalpy of the spring water (J), SiO2C is the SiO- 2 concentration of the cold groundwater (mg/L), HXcT H(1 X) H S SiO2T is the SiO2 concentration of the deep thermal water (mg/L), SiO 2S is the SiO2 concentration(3) of SiO X SiO (1- X) SiO the springs (mg/L), and X is the mixing 2C ratio of cold 2T water with the 2S deep thermal water.

where HC is the enthalpy of the cold water (J), HT is the enthalpy of the deep thermal water (J), HS is the enthalpy of the spring water (J), SiO2C is the SiO2 concentration of the cold groundwater (mg/L), SiO2T is the SiO2 concentration of the deep thermal water (mg/L), SiO2S is the SiO2 concentration of the springs (mg/L), and X is the mixing ratio of cold water with the deep thermal water. The enthalpy is a function of temperature [50]; thus, X1 can be calculated at different temperatures as follows:

HH- X  TS 1 - HHTC (4) SiO- SiO X  2T 2S 2 - SiO2T SiO 2C

Water 2017, 9, 61 13 of 17

The enthalpy is a function of temperature [50]; thus, X1 can be calculated at different temperatures as follows: HT−HS X1 = H −H T C (4) SiO −SiO Water 2017, 9, 61 X = 2T 2S 13 of 17 2 SiO2T−SiO2C This equation can be solved by using the graphical method, in which S032 and S046 are employed This equation can be solved by using the graphical method, in which S032 and S046 are as the representative samples. In Figure9,X 1 is the curve labelled enthalpy, and X2 is the curve labelled employed as the representative samples. In Figure 9, X1 is the curve labelled enthalpy, and X2 is the silica. The point of intersection of the two curves provides the fraction of cold water and the estimated curve labelled silica. The point of intersection of the two curves provides the fraction of cold water temperature of the deep thermal water component. The results showed that the proportions of cold and the estimated temperature of the deep thermal water component. The results showed that the water for S032 and S046 were 0.78 and 0.65, respectively. proportions of cold water for S032 and S046 were 0.78 and 0.65, respectively.

(a) (b)

FigureFigure 9. Relationship 9. Relationship between between the temperature the temperature and the and mixing the mixing ratio ratioof cold of water cold water for (a) for S032 (a) and S032 (b and) S046. (b) S046. The ratios of cold water mixing and reservoir temperature are shown in Table 5. The reservoir temperatures of the geothermal springs ranged from 82.0 to 165.0 °C, and the ratios of cold water The ratios of cold water mixing and reservoir temperature are shown in Table5. The reservoir mixing ranged from 0.54 to 0.87. Although some springs had high reservoir temperatures, i.e., up to temperatures of the geothermal springs ranged from 82.0 to 165.0 ◦C, and the ratios of cold water 165.0 °C, the outlet temperatures were relatively low due to different amounts of cold water mixing. mixing ranged from 0.54 to 0.87. Although some springs had high reservoir temperatures, i.e., up to 165.0 ◦C, the outlet temperatures were relatively low due to different amounts of cold water mixing. Table 5. Estimated reservoir temperature and cold water mixing ratio.

No. Outlet TableTemperature 5. Estimated (°C) reservoirReservoir temperature Temperature and (°C) cold waterMixing mixingRatio ratio. of Cold Water S046 41.0 82.0 0.54 S033No. Outlet22.0 Temperature (◦C) Reservoir114.0 Temperature (◦C) Mixing Ratio0.83 of Cold Water S040S046 22.3 41.093.0 82.00.79 0.54 S039S033 23.0 22.087.0 114.00.76 0.83 S050S040 23.8 22.3136.0 93.00.86 0.79 S029S039 24.0 23.0122.0 87.00.83 0.76 S055S050 24.0 23.8165.0 136.00.87 0.86 S029 24.0 122.0 0.83 S052 28.2 150.0 0.84 S055 24.0 165.0 0.87 S049S052 29.5 28.2103.0 150.00.74 0.84 S044S049 30.1 29.5118.0 103.00.78 0.74 S048S044 34.0 30.1122.0 118.00.76 0.78 S032S048 35.0 34.0116 122.00.72 0.76 S032 35.0 116 0.72 4.5. Conceptual Model of the Geothermal System 4.5. Conceptual Model of the Geothermal System A conceptual model for the circulation of meteoric water along deep fractures based on the hydrogeologicalA conceptual features model of for the the Arxan circulation geothermal of meteoric system is water shown along in Figure deep 10. fractures based on the hydrogeologicalThe Arxan springs features are of therecharged Arxan geothermal by infiltrating system meteoric is shown water, in Figure and the 10 .northern extensional faultsThe might Arxan be the springs main areconduits recharged for the by geothermal inﬁltrating water meteoric flow. water, The thermal and the water northern circulates extensional along thefaults NNW might‐SSE be (South the main‐South conduits‐East) for fracture the geothermal and the fault water system. ﬂow. The The thermal groundwater water circulates emerges alongfrom thermalthe NNW-SSE springs (South-South-East) along the geothermal fracture system. and After the being fault heated system. at greater The groundwater depths, the emergesthermal water from rises through the major bounding faults and plumose fractures and then flows towards the center of the Arxan graben and mixes with shallow cold water as it ascends towards the surface. The vertical transport between aquifers is promoted by the NNW trending faults and a series of plumose fractures (Figures 2 and 10), which form the graben structures of the basin. The occurrence of both thermal and mineral springs is probably related to the inferred volcanic conduit. The aquifer in which the mineral springs occur is a shallow groundwater system, and the thermal springs occur in the confined aquifer

Water 2017, 9, 61 14 of 17 thermal springs along the geothermal system. After being heated at greater depths, the thermal water rises through the major bounding faults and plumose fractures and then flows towards the center of the Arxan graben and mixes with shallow cold water as it ascends towards the surface. The vertical transport between aquifers is promoted by the NNW trending faults and a series of plumose fractures (Figures2 and 10), which form the graben structures of the basin. The occurrence of both thermal and mineral springs is probably related to the inferred volcanic conduit. The aquifer in which the mineral Water 2017, 9, 61 14 of 17 springs occur is a shallow groundwater system, and the thermal springs occur in the confined aquifer ofof thethe JurassicJurassic system.system. The springs rise from thethe deepdeep basementbasement faultsfaults as hothot fluid,fluid, andand thethe differentdifferent temperaturestemperatures ofof thethe thermalthermal springssprings areare relatedrelated to to the the different different mixing mixing ratios ratios of of shallow shallow cold cold water. water.

Figure 10. Conceptual model of the Arxan geothermal system: 1, gravel, sand and clay (Quaternary); Figure 10. Conceptual model of the Arxan geothermal system: 1, gravel, sand and clay (Quaternary); 2, rhyolitic tuff (Jurassic); 3, tuffaceous conglomerate (Jurassic); 4, tuffaceous breccia (Jurassic); 5, 2, rhyolitic tuff (Jurassic); 3, tuffaceous conglomerate (Jurassic); 4, tuffaceous breccia (Jurassic); 5, granite granite (Yanshanian); 6, volcanic conduit; 7, plumose fractures; 8, inferred volcanic conduit; 9, caldera; (Yanshanian); 6, volcanic conduit; 7, plumose fractures; 8, inferred volcanic conduit; 9, caldera; 10, precipitation infiltration direction; 11, cold water flow direction; 12, geothermal fluid flow 10, precipitation infiltration direction; 11, cold water flow direction; 12, geothermal fluid flow direction; direction; 13, heat flux; 14, thermal springs; and 15, mineral springs. 13, heat flux; 14, thermal springs; and 15, mineral springs. 5. Conclusions 5. Conclusions The hydrogeochemical characteristics and genesis of the thermal and mineral springs hosted by The hydrogeochemical characteristics and genesis of the thermal and mineral springs hosted deep‐seated faults were determined. Different hydrogeochemical and environmental isotopic by deep-seated faults were determined. Different hydrogeochemical and environmental isotopic compositions are related to different hydrogeological systems. Twelve thermal springs exist with compositions are related to different hydrogeological systems. Twelve thermal springs exist with temperatures ranging between 22.3 and 41.0◦ °C, and the main type of water in the springs is temperatures ranging between 22.3 and 41.0 C, and the main type of water in the springs is HCO3-Na. HCO3‐Na. The mineral springs contain HCO3–Ca∙Na water and have temperatures ranging from 10.9 The mineral springs contain HCO –Ca·Na water and have temperatures ranging from 10.9 to 12.9 ◦C. to 12.9 °C. 3 Strong correlations between some ionic ratios in the thermal water implied that the water–rock Strong correlations between some ionic ratios in the thermal water implied that the water–rock interactions were enhanced. The thermal water was oversaturated with respect to silica minerals interactions were enhanced. The thermal water was oversaturated with respect to silica minerals (chalcedony and quartz) and under-saturated with respect to carbonate (aragonite and calcite) at (chalcedony and quartz) and under‐saturated with respect to carbonate (aragonite and calcite) at the the outlet temperatures. Based on the Na–K–Mg1/2 diagram, the mineral springs contain immature outlet temperatures. Based on the Na–K–Mg1/2 diagram, the mineral springs contain immature water water and the thermal springs contain partially equilibrated water. Considering the uncertainties and the thermal springs contain partially equilibrated water. Considering the uncertainties associated associated with using chemical geothermometers, the estimated reservoir temperatures varied from with using chemical geothermometers, the estimated reservoir temperatures varied from 50.9 to 50.9 to 68.8 ◦C according to the chalcedony geothermometer and the saturation index method. 68.8 °C according to the chalcedony geothermometer and the saturation index method. The hydrochemical and isotopic compositions of the water were used to establish a conceptual flow model. The δ18O, δ2H and tritium values implied that the thermal springs were of deep circulating meteoric origin, whereas the mineral springs were derived from shallow circulating water. The water was considered to originate from the infiltration of precipitation through faults and fracture zones to the deep hot reservoir. The thermal water ascended along hydrothermal pathways, and the different outlet temperatures of the thermal springs were caused by conductive cooling and different mixing ratios of shallow cold water, which was estimated to have contributions of approximately 54%–87%. These results can be combined with a hydrogeological model to reveal the characteristics of the geothermal system in Arxan and provide new insights into the processes that

Water 2017, 9, 61 15 of 17

The hydrochemical and isotopic compositions of the water were used to establish a conceptual flow model. The δ18O, δ2H and tritium values implied that the thermal springs were of deep circulating meteoric origin, whereas the mineral springs were derived from shallow circulating water. The water was considered to originate from the infiltration of precipitation through faults and fracture zones to the deep hot reservoir. The thermal water ascended along hydrothermal pathways, and the different outlet temperatures of the thermal springs were caused by conductive cooling and different mixing ratios of shallow cold water, which was estimated to have contributions of approximately 54%–87%. These results can be combined with a hydrogeological model to reveal the characteristics of the geothermal system in Arxan and provide new insights into the processes that control the hydrochemical properties of the thermal and mineral springs and the genesis and flow patterns of the geothermal system.

Acknowledgments: This research was supported by the Fundamental Research Funds for the Central Universities (No. 2652016020) and the project “Hydrogeology survey and spring water resources evaluation in Arxan Geopark” from the Arxan Geopark Authority. Special thanks are extended to Zhi Liu, Hongzhi Yu of the Arxan Geopark Authority, Mingzhong Tian and Lulin Wang of China University of Geosciences (Beijing), and Wei Zhao, Hanmo Wang of the Fourth Institute of Hydrogeology and Engineering Geology of Inner Mongolia for their help in the field survey and sampling. We also thank the Analytical Laboratory of Fourth Institute of Hydrogeology and Engineering Geology of Inner Mongolia for the chemical analysis, and the Analytical Laboratory of Beijing Research Institute of Uranium Geology for the isotope analysis. Author Contributions: Qiulan Zhang, Yali Cui and Jingli Shao conceived and designed the research direction; Yong Xiao, Pengzhang and Jingxing Liu did the field work and collected the water samples; Xiaomin Gu analyzed the data and wrote the paper. All authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflicts of interest.

References

1. Zhou, X. Groundwater Science Monographs; Geology Publishing House: Beijing, China, 2010; pp. 104–105. (In Chinese) 2. Minissale, A.; Mattash, M.A.; Vaselli, O.; Tassi, F.; Al-Ganad, I.N.; Selmo, E.; Shawki, N.M.; Tedesco, D.; Poreda, R.; Ad-Dukhain, A.M.; et al. Thermal springs, fumaroles and gas vents of continental Yemen: Their relation with active tectonics, regional hydrology and the country’s geothermal potential. Appl. Geochem. 2007, 22, 799–820. [CrossRef] 3. De Filippis, L.; Billi, A. Morphotectonics of ﬁssure ridge travertines from geothermal areas of Mammoth Hot Springs (Wyoming) and Bridgeport (California). Tectonophysics 2012, 548–549, 34–48. [CrossRef] 4. Zaher, M.A.; Saibi, H.; Nishijima, J.; Fujimitsu, Y.; Mesbah, H.; Ehara, S. Exploration and assessment of the geothermal resources in the Hammam Faraun hot spring, Sinai Peninsula, Egypt. J. Asian Earth Sci. 2012, 45, 256–267. [CrossRef] 5. Mao, X.; Wang, Y.; Zhan, H.; Feng, L. Geochemical and isotopic characteristics of geothermal springs hosted by deep-seated faults in Dongguan Basin, Southern China. J. Geochem. Explor. 2015, 158, 112–121. [CrossRef] 6. Pasvano˘glu,S. Hydrogeochemical study of the thermal and mineralized waters of the Banaz (Hamambo˘gazi) area, western Anatolia, Turkey. Environ. Earth Sci. 2012, 65, 741–752. [CrossRef] 7. Katsanou, K.; Siavalas, G.; Lambrakis, N. The thermal and mineral springs of Aitoloakarnania Prefecture: Function mechanism and origin of groundwater. Environ. Earth Sci. 2012, 65, 2351–2364. [CrossRef] 8. Cai, J.; Chu, Y.; Guo, H. The distribution and charactistics of Arxan geothermal sturcture. Heilongjiang Sci. Technol. Water Conserv. 2008, 36, 61–62. (In Chinese) 9. Qi, F.; Zhang, F.; Lu, S.; Sun, Y. The geothermal geological features of Arxan mountains. Jilin Geol. 2012, 31, 109–112. (In Chinese) 10. Giggenbach, W.F. Geothermal solute equilibria-derivation of Na-K-Mg-Ca geoindicators. Geochim. Cosmochim. Acta 1988, 52, 2749–2765. [CrossRef] 11. Guo, F.; Fan, W.; Li, C.; Zhao, L.; Li, H.; Yang, J. Multi-stage crust–mantle interaction in SE China: Temporal, thermal and compositional constraints from the Mesozoic felsic volcanic rocks in eastern Guangdong–Fujian provinces. Lithos 2012, 150, 62–84. [CrossRef] Water 2017, 9, 61 16 of 17

12. Mottl, M.J.; Seewald, J.S.; Wheat, C.G.; Tivey, M.K.; Michael, P.J.; Proskurowski, G.; McCollom, T.M.; Reeves, E.; Sharkey, J.; You, C.F.; et al. Chemistry of hot springs along the Eastern Lau Spreading Center. Geochim. Cosmochim. Acta 2011, 75, 1013–1038. [CrossRef] 13. Li, J.; Guo, H. The Geological Characteristics and Formation of Arxan Geothermal system. Heilongjiang Land Resour. 2012, 8, 47. (In Chinese) 14. Tang, S. Characteristics of the Late Jurassic Volcanic System in Arxan and the Formation Mechanism of Arxan Hot Springs. Jilin Geol. 1984, 3, 54–64. (In Chinese) 15. Han, X.; Jin, X.; Sun, C. Geothermal Structure of Aershan Hot Springs, Inner Mongolia. AC TA Geosci. Tia Sin. 2001, 22, 259–264. (In Chinese) 16. GB/T8538-2008. Drinking Natural Mineral Water; China Standards Press: Beijing, China, 2008. 17. GB12375-1990. Analytical Method of Tritium in Water; China Standards Press: Beijing, China, 1990. 18. Michard, G. Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr, Fe, Mn, W, F) in deep hot waters from granitic areas. Chem. Geol. 1990, 89, 117–134. [CrossRef] 19. Han, D.M.; Liang, X.; Jin, M.G.; Currell, M.J.; Song, X.F.; Liu, C.M. Evaluation of groundwater hydrochemical characteristics and mixing behavior in the Daying and Qicun geothermal systems, Xinzhou Basin. J. Volcanol. Geotherm. Res. 2010, 189, 92–104. [CrossRef] 20. Shakeri, A.; Moore, F.; Kompani-Zare, M. Geochemistry of the thermal springs of Mount Taftan, southeastern Iran. J. Volcanol. Geotherm. Res. 2008, 178, 829–836. [CrossRef] 21. Müller, G. Diagenesis in Argillaceous Sediments. Dev. Sedimentol. 1967, 8, 127–177. 22. Schäffer, R.; Sass, I. The thermal springs of Jordan. Environ. Earth Sci. 2013, 72, 171–187. [CrossRef] 23. Wang, H. An Introduction to Isotopic Hydrogeology; Geological Publishing House: Beijing, China, 1991; p. 191. 24. Craig, H. Isotopic Variation in Meteoric Waters. Science 1961, 133, 1702–1703. [CrossRef][PubMed] 25. Chen, J.S.; Sun, X.X.; Gu, W.Z.; Tan, H.B.; Rao, W.B.; Dong, H.Z.; Liu, X.Y.; Su, Z.G. Isotopic and hydrochemical data to restrict the origin of the groundwater in the Badain Jaran Desert, Northern China. Geochem. Int. 2012, 50, 455–465. [CrossRef] 26. Ma, Z.; Yu, J.; Su, Y.; Xie, J.; Jia, X.; Hu, Y. δ18O shifts of geothermal waters in the central of Weihe Basin, NW China. Environ. Earth Sci. 2009, 59, 995–1008. [CrossRef] 27. Capasso, G.; Dongarrà, G.; Favara, R.; Hauser, S.; Valenza, M. Isotope composition of rain water, well water and fumarole steam on the island of Vulcano, and their implications for volcanic surveillance. J. Volcanol. Geotherm. Res. 1992, 49, 147–155. [CrossRef] 28. Dansgaard, W. Stable isotopes in precipitation. Tellus 1964, 16, 436–468. [CrossRef] 29. Thomas, J.; Rose, T. Environmental Isotopes in Hydrogeology; Boca Raton Fla Lewis Publishers: Boca Raton, FL, USA, 1997; Volume 80, p. 532. 30. Fournier, R.O. Chemical geothermometers and mixing models for geothermal systems. Geothermics 1977, 5, 41–50. [CrossRef] 31. Ravikumar, P.; Somashekar, R.K. Environmental Tritium (3 H) and hydrochemical investigations to evaluate groundwater in Varahi and Markandeya river basins, Karnataka, India. J. Environ. Radioact. 2011, 102, 153–162. [CrossRef][PubMed] 32. Günay, G. Hydrology and hydrogeology of Sakaryaba¸sıKarstic springs, Çifteler, Turkey. Environ. Geol. 2006, 51, 229–240. [CrossRef] 33. Çelmen, O.; Çelik, M. Hydrochemistry and environmental isotope study of the geothermal water around Beypazarı granitoids, Ankara, Turkey. Environ. Geol. 2009, 58, 1689–1701. [CrossRef] 34. Gultekin, F.; Hatipoglu, E.; Ersoy, A.F. Hydrogeochemistry, environmental isotopes and the origin of the Hamamayagi-Ladik thermal spring (Samsun, Turkey). Environ. Earth Sci. 2011, 62, 1351–1360. [CrossRef] 35. Stanley, W.D. Geothermal systems, principles and case histories. Earth-Sci. Rev. 1982, 18, 91–92. [CrossRef] 36. Zhang, X.; Guo, Q.; Li, J.; Liu, M.; Wang, Y.; Yang, Y. Estimation of reservoir temperature using silica and cationic solutes geothermometers: A case study in the Tengchong geothermal area. Chin. J. Geochem. 2015, 34, 233–240. [CrossRef] 37. Spycher, N.; Peiffer, L.; Sonnenthal, E.L.; Saldi, G.; Reed, M.H.; Kennedy, B.M. Integrated multicomponent solute geothermometry. Geothermics 2014, 51, 113–123. [CrossRef] 38. Verma, M. Silica solubility geothermometers for hydrothermal systems. In Proceedings of the 10th International Symposium on Water–Rock Interaction, WRI-10, Villasimius, Italy, 10–15 July 2001. Water 2017, 9, 61 17 of 17

39. Dulanya, Z.; Morales-Simfors, N.; Sivertun, Å. Comparative study of the silica and cation geothermometry of the Malawi hot springs: Potential alternative energy source. J. Afr. Earth Sci. 2010, 57, 321–327. [CrossRef] 40. Pope, L.A.; Hajash, A.; Popp, R.K. An experimental investigation of the quartz, Na-K, Na-K-Ca geothermometers and the effects of fluid composition. J. Volcanol. Geotherm. Res. 1987, 31, 151–161. [CrossRef] 41. Baioumy, H.; Nawawi, M.; Wagner, K.; Arifin, M.H. Geochemistry and geothermometry of non-volcanic hot springs in West Malaysia. J. Volcanol. Geotherm. Res. 2015, 290, 12–22. [CrossRef] 42. PHREEQC; version 2.12.5; (Computer File) Water Chemistry, Computer Programs Geochemistry; U.S. Geological Survey: Denver, CO, USA, 1999. 43. Surveys, G. User’s Guide to PHREEQC—A Computer Program for Speciation, Reaction-Path, Advective-Transport, and Inverse Geochemical Calculations; U.S. Geological Survey: Denver, CO, USA, 1995. 44. Tole, M.P.; Ármannsson, H.; Zhong-He, P.; Arnórsson, S. Fluid/mineral equilibrium calculations for geothermal fluids and chemical geothermometry. Geothermics 1993, 22, 17–37. [CrossRef] 45. Liu, Y.; Zhou, X.; Deng, Z.; Fang, B.; Tsutomu, Y.; Zhao, J.; Wang, X. Hydrochemical characteristics and genesis analysis of the Jifei hot spring in Yunnan, southwestern China. Geothermics 2015, 53, 38–45. [CrossRef] 46. Arnórsson, S. The use of mixing models and chemical geothermometers for estimating underground temperatures in geothermal systems. J. Volcanol. Geotherm. Res. 1985, 23, 299–335. [CrossRef] 47. Truesdell, A.; Fournier, R. Procedure for estimating temperature of a hot-water component in a mixed water by using a plot of dissolved silica versus enthalpy. J. Res. U.S. Geol. Surv. 1977, 5, 49–52. 48. Keenan, J.H.; Keyes, F.G.; Hill, P.H.; Moore, J.G. Steam tables. J. Appl. Mech. 1978, 37, 567. [CrossRef] 49. Gupta, H.K.; Roy, S. Geothermal Energy: An Alternative Resource for the 21st Century; Elsevier Science & Technology: Amsterdam, The Netherlands, 2006. 50. Fournier,R.O.; Truesdell, A.H. An empirical Na-K-Ca geothermometer for natural waters. Geochim. Cosmochim. Acta 1973, 37, 1543–1550. [CrossRef]

© 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).