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Conceptual model of the Şavşat (Artvin/NE Turkey) Geothermal Field developed with hydrogeochemical, isotopic, and geophysical studies

Conceptual model of the Şavşat (Artvin/NE Turkey) Geothermal Field developed with... fatma@ktu.edu.tr Geological Engineering The Şavşat (Artvin, Turkey) Geothermal Field (ŞGF) is located on the northeastern Department, Karadeniz border of Turkey. This field is characterized by thermal and mineralized springs and Technical University, Trabzon, Turkey travertine. The temperature of the thermal water is 36 °C, whereas that of the mineral- Full list of author information ized spring in the area is approximately 11 °C. The Na–HCO –Cl-type thermal water has is available at the end of the a pH value of 6.83 and an EC value of 5731 µS/cm. The aim of this study is to character- article ize the geothermal system by using geological, geophysical, and hydrogeochemical data and to determine its hydrochemical properties. A conceptual hydrogeological model is developed for the hydrogeological flow system in the ŞGF. According to the hydrogeological conceptual model created by geological, geophysical, and hydrogeo- chemical studies, the reservoir comprises volcanogenic sandstone and volcanic rocks. The cap rock for the geothermal system is composed of turbiditic deposits consisting of mudstone–siltstone–sandstone alternations. An increase in the geothermal gradient is mainly due to Pleistocene volcanic activity in the field. The isotopic values of thermal 18 2 3 water (δ O, δ H, δ H) indicate a deeply circulating meteoric origin. The estimated reservoir temperature calculated by silica geothermometers is 100–150 °C, and the mixing rate of cold groundwater with geothermal waters is approximately 70%. It may be possible to obtain warmer fluids from a 300-m-deep borehole cutting through a fracture zone identified by geophysical studies. Heating by conduction via the geother - mal gradient resulting from young volcanic activity drives geothermal waters upwards along faults and fractures that act as hydrothermal pathways. The positive δ C VPDB value (+ 4.31‰) indicates a metamorphic origin for the thermal water. The S value CDT (~ 10‰) shows that the sulfur in the geothermal water is derived from volcanic sulfur (SO ). Keywords: Geothermal, Hydrogeochemistry, Hydrogeological conceptual model, Geophysical studies, Şavşat (Artvin-Turkey) © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Gültekin et al. Geotherm Energy (2019) 7:12 Page 2 of 26 Introduction The Şavşat Geothermal Field (ŞGF) is located in northeast Turkey on the border with Georgia (Fig. 1). According to meteorological data from the closest station to the study area in the town of Şavşat, the mean annual precipitation is 580  mm, and the mean annual temperature has been 9.9 °C for the last 10 years (MGM 2017). The older units of the study area are Late Cretaceous-age volcanic rocks (Güven 1993), Paleocene-early Eocene-age sedimentary rocks (Erendil et  al. 1989), and middle Eocene-age andesite and basalt-type volcanic rocks and volcanogenic sandstones (Güven 1993). Young units are Lutetian-age turbidites (Erendil et  al. 1989) and Oligo-Miocene-age sedimentary rocks, including sandstone, siltstone, and marl alternations (Karaköse et al. 1994; Konak et  al. 1998). Previous studies in the area (Kara 1997; Akkuş et  al. 2005) reported ther- mal springs with temperatures of nearly 36 °C. These springs have been used for many years in primitive facilities for balneological purposes by local people. During field stud - ies, thermal springs were observed at different locations along the Çermik Stream valley with temperatures ranging from 20 to 36 °C. In recent years, private undertakings that run mountain plateau tourism (or yayla) in the area have wanted to obtain warmer water to provide better service to the region. With this aim, drilling studies were performed in 2016, and water with temperatures of nearly 39  °C was obtained from 120  m depth. This well, drilled immediately beside the thermal spring, caused the spring to dry up. In this area, apart from some information included in the Geothermal Energy Inven- tory of the Turkish Geological Survey (MTA), there has not been any study to date that assesses the geothermal, hydrogeological, or hydrogeochemical properties. To define the geothermal system and its surroundings, it is important to determine the hydrogeo- logical, hydrochemical, and isotopic properties. Geological and geophysical studies and Fig. 1 Location map of the Şavşat (Artvin/Turkey) Geothermal Field Gültekin et al. Geotherm Energy (2019) 7:12 Page 3 of 26 hydrogeochemical and isotope techniques have been widely used elsewhere to deter- mine the hydrodynamic structure of geothermal systems in recent years (Tarcan et  al. 2005; Piscopo et al. 2006; Schaffer and Sass 2014; Yurteri and Şimşek 2017; Uzelli et al. 2017). In this geothermal field, there has not been any study that employs a conceptual approach based on hydrogeological and hydrogeochemical studies, such as hydrochemi- cal facies compositions, isotopic features, water–rock interactions, mixing processes, and reservoir temperature. This study focuses on understanding the mechanism of the geothermal system in the ŞGF for future use, determining the areas with higher tem- peratures and revealing the hydrogeochemical characteristics of the geothermal water. In accordance with this purpose, geological, geophysical, and hydrogeochemical studies were performed in the area to characterize components such as the reservoir, geother- mal fluids, and cap rock of the geothermal system. Establishing a hydrogeologic con - ceptual model can further help to determine flow paths, including recharge through flow–discharge processes, as well as mixing behavior. Methodology Field studies were completed in three different forms: geological studies, geophysical studies, and measurements and sampling on site. Geological studies were carried out on 1/25,000-scale topographic maps and 1/100,000-scale geological maps of the study area prepared previously by a variety of researchers. Geological units were observed in the field, and the geological maps made by different researchers were revised. Rock samples were taken for lithological identification. During field studies, observations and geologic-tectonic studies of the area were used to identify sample locations for ther- mal, cold, and surface waters. To determine the chemical content and variations of the geothermal and cold waters, samples were taken from each location to represent rainy, dry, and interval periods from May 2016 to October 2017, and measurements were per- formed in the field. In-situ measurements and sampling were performed at the geothermal well (ILICAS), in mineral water within the basin (GMS), in mineral water outside the basin (CDMS), in cold spring water (SSSK, GSK), and in the Çermik Stream before (CERDERY) and after (CERDERA) thermal water mixing. The coordinates of each sampling location were recorded by a handheld GPS and marked on a geological map. Measurements of temperature (T), electrical conductivity (EC), pH, total dissolved sol- ids (TDS), and dissolved oxygen (DO) at the sampling points were performed with a YSI- 556 multiparameter meter. The probes used during the measurements were preserved by washing them with pure water before and after each measurement and were used after daily calibrations with buffer solutions. All water samples were filtered through 0.45  μm membranes on site. To determine the major anion–cation and trace element contents of the waters, samples were taken with polyethylene (HDPE) bottles. For major anion–cation and trace element analysis, 500 mL and 50 mL bottles were used, respec- tively. Nitric acid (HNO ) was added for cation and trace element analysis to bring the pH to < 2. Major anion–cation, trace element, and tritium ( H) analyses were carried out at the Water Chemistry Laboratory at Hacettepe University (Turkey) using the follow- 2+ 2+ + + ing methods: major cation analysis (Ca, Mg, Na , and K ) was performed by atomic absorption spectrometry. Cl was analyzed using an AgNO titrimetric method. Sulfate 3 Gültekin et al. Geotherm Energy (2019) 7:12 Page 4 of 26 concentrations were determined by spectrophotometry together with alkalinity stand- ard titration methods, whereas B and SiO were analyzed with the spectrophotometric method. The major ion balance error of the analyses was less than 5%. Trace element analysis was performed with the inductively coupled plasma-mass spectrometry (ICP- MS) method. 18 2 13 3 Samples of 100  mL for δ O, δ H, and δ C analyses and samples of 500  mL for δ H analysis were collected. For the δ S analysis, 250 mL to 1000 mL samples were collected based on the amount of sulfate in the waters. The samples were kept cold until they were sent to the laboratories. A snow sample was taken by immersing a cylindrical tube into the snow. Then, this snow mass was poured into a polyethylene container and kept at 18 2 13 + 4  °C until it became liquid. δ O, δ H, and δ C isotopic analyses were performed at the Iso-Analytical Laboratory in the UK using the isotope ratio mass spectrometry 18 34 (IRMS) method. For geothermal water, the δ O and δ S analyses of the sulfate were performed by the Isotope Tracer Technologies Inc. (Canada) laboratory using the IRMS technique. Standard deviations of the stable isotope analyses were 0.11‰ for O and 0.90‰ for deuterium. The AquaChem 2012.1 program was used to evaluate the results of the chemical analy - ses, to prepare diagrams and to calculate the saturation indices (SI) of selected minerals. In recent years, geophysical methods have been commonly used in the investigation and development of geothermal resources, especially with the development of tech- nology, and these methods have provided successful results (Majumdar et  al. 2000; El- Quady 2006; Özürlan and Şahin 2006; Drahor and Berge 2006; Abiye and Haile 2008; Wu et  al. 2012). Although many geophysical methods are used in geothermal fields, electrical resistivity is one of the most preferred methods for investigating many geo- thermal fields. High temperatures and water flow in geothermal systems change electri - cal conductivity properties underground. Due to these changes, it is possible to obtain important information about the location, depth, and structure of geothermal resources by using the electrical resistivity method. Electrical resistivity is one of the most widely used geophysical methods in underground surveys (Telford et al. 1990; Reynolds 1997). In this method, the basic principle is to send a known current through two current elec- trodes into the ground and measure the voltage difference with two potential electrodes. From these measured potentials, the calculation of the resistivity and thickness of the underground layers is based on Ohm’s law. The resistivity distribution of the under - ground materials is measured by the resistivity method. Two-dimensional (2D) resistiv- ity images of the underground layers are obtained by using a large number of electrode pairs in the electrical resistivity tomography method (ERT). This method is a very effec - tive and widely used method for identifying lateral and vertical changes, presence and level of groundwater, hot springs, salinity, cavities, and weathered rocks (Reynolds 1997; Aizawa 2014). In the field, a pole-to-pole electrode array was used along the three profiles with a total profile length of 600 m, and 2D ERT data were acquired with an Advanced Geosciences, Inc. (AGI) SuperSting R8-IP (Figs.  2, 3). The power supply of this device consisted of a transmitter unit capable of producing a 2000  mA current with 400  W power and a special generator with 4 kWA power. The electrical resistivity measurements targeted a research depth of approximately 300 m. Gültekin et al. Geotherm Energy (2019) 7:12 Page 5 of 26 Fig. 2 Satellite image of 2D resistivity profiles in the study area Fig. 3 The appearance of 2D resistivity lines on topography map Results and discussion Geological and hydrogeological settings The Late Cretaceous-age Kızılkaya Formation comprises dacite, dacitic pyroclas- tics and volcanogenic siltstone, sandstone, and pebblestone (Güven 1993). The Paleocene-early Eocene-age Ardanuç Formation, which contains limestone, sand- stone, tuff, and claystone intercalations, overlies the Mesozoic units (Fig.  4). The thin- and medium-bedded unit has a total thickness of 200  m (Erendil et  al. 1989). Gültekin et al. Geotherm Energy (2019) 7:12 Page 6 of 26 Fig. 4 Geological map of the Şavşat (Artvin/Turkey) Geothermal Field (revised from Erendil et al. 1989 and Konak et al. 1998) The Lutetian (middle Eocene)-age Kabaköy Formation conformably overlies the Paleocene units (Güven 1993). The Kabaköy Formation, which outcrops widely in the study area, contains volcanogenic sandstone and pyroclastics at the lower lev- els. After a thick mudstone layer, it continues upward with basaltic and andesitic rocks. The thickness of the volcanic rocks, mostly composed of augite basalts, is 200  m. The total thickness of the Kabaköy Formation is over 800  m (Güven 1993). Gültekin et al. Geotherm Energy (2019) 7:12 Page 7 of 26 The Şavşat Formation, which outcrops as mudstone–siltstone–sandstone alterna- tions with a turbiditic character, is Lutetian in age (Erendil et  al. 1989). The thick- ness of the unit consisting of round, small pebbles at the bottom and sandstones, siltstone and marls of yellowish color in the upper part is 400  m. The Pınarlı For- mation (Karaköse et  al. 1994; Konak et  al. 1998), comprising sandstone, siltstone, marl, lacustrine cherty limestone, and gypsum lenses, unconformably overlies the turbiditic unit (Erendil et  al. 1989). The thickness of the unit consisting mainly of sandstone and siltstone is approximately 300  m. Marl, lacustrine cherty limestone intercalations, and gypsum lenses are generally observed in these gray and medium- bedded rocks. The Pınarlı Formation is late Miocene in age (Karaköse et  al. 1994) and includes landslides of varying sizes. The Pliocene–Pleistocene Bülbülan Forma- tion (Erendil et al. 1989) overlies older units and consists of basalt, andesite, trachy- basalt, trachyandesite, and pyroclastics in the study area. Plagioclase microliths and a small number of augite microcrystals are observed in the groundmass consisting of volcanic glass in the trachybasalts. Calcite as a secondary mineral accompanies these minerals. Plagioclase, sanidine, hornblende, and opaque minerals are observed in the trachyandesites. In the trachyandesites, chlorite is observed as a secondary mineral. The Bülbülan Formation overlies the Şavşat and Pınarlı Formations, with an angular unconformity ranging between 400 and 1000 m (Konak et al. 1998). The youngest unit in the area is alluvium cropping out in narrow areas along valleys. Both thrust and normal faults are observed in the area. Thrust faults have NE–SW trends, whereas normal faults occur in several different directions. In addition, fold- ing is represented by anticlines and synclines with NE–SW axial trends. Clastic sedimentary rocks and volcanic rocks dominate the study area. Sedimen- tary rocks with high primary porosity have varying permeabilities depending on the degree of cementation. Volcanic rocks have a low primary porosity (3–5%), except where they are fractured by cooling and tectonic activities. Tuffs belonging to the late Cretaceous Kızılkaya Formation are permeable, whereas dacite and rhyodacite are impermeable. The volcanogenic sandstones of the Kabaköy Formation, which are extensively exposed in the study area, have good permeability characteristics. Pyroclastic rocks also have high permeability. The volcanic rocks in the Kabaköy Formation are permeable where they have been subjected to tectonic and cooling activities. The open fractures allow the deep circulation of water due to locally enhanced vertical permeabilities. Due to its thickness, extent, and permeabil- ity, this unit is considered to be the most important aquifer in the area. The Şavşat Formation, with mudstone–siltstone–sandstone alternations, has very low perme- ability; therefore, it is evaluated as semipermeable. Marl and gypsum layers of the Pınarlı Formation are impermeable. This unit, acting as an impermeable cap rock, prevents heat loss and maintains water pressure. Forming high hills in the study area, volcanic units of the Bülbülan Formation are permeable where they are fractured. Alluvium in the area is permeable, but it has insignificant groundwater storage capacities because its dimensions are small. The most prevalent stream in the area is the Akdamla Stream. It merges with the Meydancık Stream outside the study area. The thermal spring is located in the Çer- mik Stream valley, which is the branch of the Akdamla Stream. Gültekin et al. Geotherm Energy (2019) 7:12 Page 8 of 26 Geophysical studies Due to the physical changes, the electrical resistivity method is used to obtain signifi - cant information about the geothermal source location, its depth, and the discontinu- ous structures that are possibly faults and fractures/joints, which may be important for the geothermal system. With the aim of determining the locations and depths of structural elements providing channels for fluids to reach the surface in the ŞGF, the ERT method was applied to three profiles. Two of the profiles were oriented approxi - mately in the north–south direction, and the third was in the east–west direction. The locations of the electrical resistivity profiles were primarily determined based on the geology, topography, and ground conditions in the study area. The study area is quite mountainous, and flat areas are quite limited. For this reason, the measurement profiles were located as much as possible in the flattest area. Furthermore, an attempt was made to plan measurement profiles in such a way that they were perpendicu - lar to the faults indicated by previous geological studies (Erendil et  al. 1989; Güven 1993; Karaköse et al. 1994; Konak et al. 1998). Because the terrain conditions did not allow an overly long profile length, the pole–pole array was preferred to obtain maxi - mum depth information. The apparent resistivity data obtained from three profiles using electrical resistivity measurements were assessed with the Res2Dinv program (Loke 2010), and true resistivity values and depth information were obtained for the study area as a result of inverse solution processes. The RMS errors for the three pro - files were below 15%. The RES2DINV program is written to run as automatically and robustly as possible, requiring very few input parameters from the user (Loke 2000). The program uses the smoothness-constrained least-squares method inversion tech - nique (Geotomo Software; DeGroot-Hedlin and Constable 1990). As the topography varies in the measurement profiles in the study area, the eleva - tion data for each electrode point were measured, and topography calculations were included in the inverse solution process. 2D inversion was applied to the apparent resistivity data obtained in the study area to create 2D underground resistivity images. The information obtained regarding the underground structure from the 2D resistiv - ity images was interpreted by taking the high and low resistance values into account. Considering the high and low resistivity zones, fault zones and hot water regions were marked on the images (Fig.  5). Fault zones and low and high resistivity areas on the 2D resistivity images were compared with the lithology in an attempt to determine the structure of the geothermal source. Moreover, a 3D resistivity map (Fig.  6) was created from all 2D profiles, and a change in resistivity with depth was displayed. In the resistivity images, high resistivity, medium resistivity, and low resistivity values are shown with red, green, and blue colors, respectively. While the resistivity values range from 10 to 5000 Ω m, the maximum penetration depth is approximately 300 m. The study area is generally composed of volcanic rocks, and very high resistivity areas (red color) represent massive rocks. In contrast, very low resistivity areas (blue color) may show the presence of hot water. Moreover, 2D resistivity images (Lines 1 and 2) indicate a large fault zone. The sections obtained by matching these profiles with the lithologies indi - cate (Fig.  5) very promising hot water regions between 150 and 300  m of horizon- tal distance on Line 1, 250 and 350  m on Line 2, and 1250 and 1350  m in areas of Gültekin et al. Geotherm Energy (2019) 7:12 Page 9 of 26 Fig. 5 2D resistivity sections and lithological-structural relation map for Line 1, Line 2, Line 3 profiles Fig. 6 3D resistivity map obtained from 2D profiles topographic elevation. It appears that drilling to at least 300 m in this area would reach water with higher pressure and temperature than the present well and spring water. Gültekin et al. Geotherm Energy (2019) 7:12 Page 10 of 26 Hydrogeochemical properties Water chemistry Samples were taken from the thermal water output in the ŞGF to identify the chemi- cal and isotopic characteristics of mineral and cold water springs. The water descrip - tions together with the coordinates and elevations of the sample locations are given in Table  1. Some properties of water, such as pH, T, DO, EC, and TDS, were measured at the sample locations in the field (Table  2). The mean values of T, pH, and EC of the discharged waters from the artesian well (ILICAS) from 120 m depth were 37.5 °C, 6.83, and 5731 µS/cm, respectively. In thermal waters dominated by Na and HCO ions, SiO 3 2 was 97.65 mg/L, B was 44 mg/L, F was 2.35 mg/L, and Li and Br were < 1 mg/L (Table 2). Ciritdüzü mineral water (CDMS) dominated by Na and HCO ions had a pH value of 6.42, an EC of 3195  µS/cm, SiO of 88.21  mg/L, B of 63  mg/L, and Li, Br and F of < 1 mg/L. The GMS, which discharges into a swamp area, had an EC value of 522 µS/cm, a pH of 7.58, and SiO of 26.4 mg/L. Cold spring waters (SSSK, GSK) had pH values of nearly 7.9 and EC values of 181.5 and 274  µS/cm, respectively. These waters dominated by Ca and HCO ions contain SiO between 10.93 and 40.8 mg/L and B values below 1 mg/l (Table 2). The chemical characteristics of Çermik Stream water before (CERDERY) and after (CERDERA) mixing with geothermal water were different (Table  2). In the CERDERY water dominated by Ca and H CO ion pairs, pH and EC were 7.86 and 154  µS/cm, respectively. For CERDERA, the EC was 619 µS/cm and was dominated by the anion and cation of Na and HCO . The ionic difference is because Çermik Stream water is physi - cally and chemically affected by geothermal well water. According to the IAH (1979) classification, the ŞGF thermal waters are Na–HCO –Cl type, while the cold springs and surface waters are generally Ca–HCO type (Table  3). Ca and HCO ions are dominant in the rainy season in CERDERY, whereas Mg and Na ions are accompanied by Ca ions in the periods when the effect of precipitation is reduced. The Piper diagram (1944) was used to classify the geothermal, cold spring, and surface waters and to determine hydrogeochemical processes, and the Schoeller diagram (1962) was used to compare the chemical content of the waters. The thermal water in the ŞGF has higher alkali elements (Na + K) than earth alkali elements (Ca + Mg). The waters with low S O have similar Cl and H CO ion concentrations. The thermal water 4 3 and CDMS have similar chemical compositions. The cold springs and surface waters have more earth alkali elements (Ca + Mg) than alkali elements (Na + K), with more Table 1 Coordinates and elevations of sampling points in the study area Sample name Definitions Coordinates (UTM 37 T Elevation (m) WGS84) ILICAS Thermal well water 0282740–4586153 1495 CDMS Mineral water 0281351–4573470 1145 GMS Mineral water 0283639–4586548 1600 SSSK Cold spring water 0282628–4588250 1550 GSK Cold spring water 0283484–4586187 1712 CERDERY Surface water 0281280–4586418 1490 CERDERA Surface water 0282940–4585958 1496 Gültekin et al. Geotherm Energy (2019) 7:12 Page 11 of 26 Table 2 Physical and chemical properties of waters in Şavşat Geothermal Field (concentrations are in mg/L) +2 +2 + + − Sample name T (°C) pH EC (µS/cm) TDS DO (mg/L) Ca Mg Na K HCO ILICAS May 16 39.30 6.64 6207 3224 0.47 293.34 39.73 1611.70 39.35 2511.00 Oct 16 38.80 6.90 5734 3044 2.19 288.00 46.35 1323.00 35.14 2621.00 March 17 35.08 6.93 5414 2950 2.13 273.41 42.55 1432.70 36.36 2379.00 July 17 36.60 6.85 5570 3002 1.70 281.00 49.60 1501.71 38.37 2571.56 Mean 37.45 6.83 5731 3055 1.62 283.94 44.56 1467.28 37.31 2490.64 CDMS May 16 12.00 6.20 2741 2369 2.98 369.40 60.07 1003.50 22.28 2379.00 March 17 10.60 6.57 2859 2562 3.00 440.35 62.38 920.51 30.67 2562.00 July 17 11.40 6.50 3985 2589 2.00 461.85 65.62 1030.24 31.57 2691.17 Mean 11.33 6.42 3195 2507 2.66 423.87 62.69 984.75 28.17 2544.06 GMS July 17 13.00 7.58 522 341 1.50 74.71 41.75 62.68 2.79 550.19 SSSK May 16 9.22 7.59 155 144 9.94 55.75 8.07 11.46 0.08 191.30 Oct 16 9.00 8.14 171 156 10.50 54.72 8.77 11.63 0.14 195.00 March 17 8.00 8.14 174 165 8.80 52.81 8.41 13.11 0.12 189.10 July 17 11.30 8.00 226 146 10.00 56.09 9.02 13.29 0.15 221.27 Mean 9.38 7.97 181 153 9.81 54.84 8.57 12.37 0.12 199.17 GSK July 17 8.04 7.87 274 178 10.00 56.38 13.84 20.99 0.06 269.12 CERDERY May 16 8.64 7.92 122 116 10.20 43.15 6.55 13.40 0.42 155.40 Oct 16 6.90 8.20 155 150 10.96 48.37 9.60 17.93 0.71 214.00 March 17 2.60 7.13 111 126 13.00 38.91 7.38 14.84 0.43 140.30 July 17 21.00 8.18 228 148 8.45 51.04 9.85 20.36 0.63 191.37 Mean 9.79 7.86 154 135 10.65 45.37 8.35 16.63 0.55 175.27 CERDERA May 16 13.80 8.53 411 340 9.37 50.90 9.07 116.46 2.91 251.00 Oct 16 10.00 8.30 787 715 11.00 54.00 19.31 300.00 7.09 439.00 March 17 7.18 7.38 404 394 11.00 50.97 8.81 133.61 3.03 262.30 July 17 18.50 8.32 875 569 8.00 70.80 18.90 203.72 4.81 370.80 Mean 12.37 8.13 619 505 9.84 56.67 14.02 188.45 4.46 330.78 Gültekin et al. Geotherm Energy (2019) 7:12 Page 12 of 26 Table 2 (continued) −2 −2 − − − − Sample name CO SO Cl NO NO F Li Br SiO B 3 4 2 3 2 ILICAS May 16 0.00 190.00 1367.00 < 0.01 < 0.01 2.59 1.14 1.54 125.80 25.57 Oct 16 0.00 166.00 1052.00 0.00 0.00 2.10 0.91 0.66 87.80 16.32 March 17 0.00 179.16 1184.50 0.00 2.65 0.90 0.00 96.74 18.40 July 17 0.00 155.36 1127.34 0.00 0.78 2.06 0.94 1.24 80.27 115.70 Mean 0.00 172.63 1182.71 0.00 0.26 2.35 0.97 0.86 97.65 44.00 CDMS May 16 0.00 150.00 772.00 < 0.01 0.75 0.63 0.27 1.59 117.00 22.93 March 17 0.00 172.42 821.13 0.00 0.00 0.00 0.32 0.00 71.94 23.87 July 17 0.00 153.92 831.69 0.28 0.86 0.37 0.36 0.44 75.70 142.15 Mean 0.00 158.78 808.27 0.28 0.54 0.33 0.32 0.68 88.21 62.98 GMS July 17 0.00 7.35 3.19 0.02 0.19 0.98 0.02 0.02 26.40 < 1 SSSK May 16 0.00 4.79 1.25 < 0.01 0.98 0.06 < 0.01 < 0.01 8.91 < 0.5 Oct 16 12.00 7.59 1.56 0.00 1.07 0.06 0.00 0.00 12.30 0.69 March 17 12.00 11.34 2.38 0.00 1.11 0.09 0.00 0.00 11.15 < 1 July 17 0.00 5.16 0.89 0.00 0.94 0.05 0.00 0.01 11.35 < 1 Mean 6.00 7.22 1.52 0.00 1.03 0.07 0.00 0.00 10.93 0.69 GSK July 17 0.00 3.68 0.89 0.02 0.88 0.04 0.00 0.02 40.80 < 1 CERDERY May 16 0.00 4.93 1.13 < 0.01 0.22 0.10 < 0.01 < 0.01 16.27 < 0.5 Oct 16 0.00 5.67 1.62 0.00 0.01 0.21 0.00 0.00 20.33 < 1 March 17 18.00 5.59 1.03 0.00 0.43 0.12 0.00 0.00 19.20 < 1 July 17 23.53 4.04 0.83 0.00 0.01 0.18 0.00 0.00 22.15 < 1 Mean 10.38 5.06 1.15 0.00 0.17 0.15 0.00 0.00 19.49 < 1 CERDERA May 16 0.00 17.68 83.69 0.05 0.05 0.27 < 0.01 0.10 30.00 < 0.5 Oct 16 24.00 46.00 250.00 0.00 0.00 0.55 0.18 0.18 32.98 3.91 March 17 36.00 23.74 103.94 0.00 0.11 0.46 0.08 0.00 21.41 < 1 July 17 23.53 32.43 191.70 0.00 0.18 0.35 0.10 0.20 27.61 3.01 Mean 20.88 29.96 157.33 0.02 0.09 0.41 0.12 0.12 28.00 3.46 Gültekin et al. Geotherm Energy (2019) 7:12 Page 13 of 26 Table 3 Classification of the waters in Şavşat Geothermal Field according to IAH (1979) Sample name Definitions Sampling date Water type ILICAS Thermal well water May 16 Na–HCO –Cl October 16 Na–HCO –Cl March 17 Na–HCO –Cl July 17 Na–HCO –Cl CDMS Mineral water May 16 Na–Ca–HCO –Cl March 17 Na–Ca–HCO –Cl July 17 Na–Ca–HCO –Cl GMS Mineral water July 17 Ca–Mg–Na–HCO ŞSSK Spring water May 16 Ca–HCO October 16 Ca–HCO March 17 Ca–HCO July 17 Ca–HCO GSK Spring water July 17 Ca–Mg–HCO CERDERY Surface water May 16 Ca–HCO October 16 Ca–Mg–Na–HCO March 17 Ca–Na–HCO July 17 Ca–Na–HCO CERDERA Surface water May 16 Na–Ca–HCO –Cl October 16 Na–HCO –Cl March 17 Na–Ca–HCO July 17 Na–Ca–HCO weak acid compounds (CO + HCO ) than strong acid compounds (Cl + SO ) (Fig.  7). 3 3 4 Thermal waters have compositions similar to those of mineral waters, although hot water has higher ion concentrations (Fig. 8). The processes that affect the major ion concentrations of thermal water in the ŞGF were evaluated according to Hounslow (1995). The ratios of HCO /SiO and Mg/ 3 2 (Ca + Mg) are 67.4 and 0.22, respectively. In addition, the ratio of SiO /(Na + K–Cl) is 0.018, and the ratio of (Na + K–Cl)/(Na + K–Cl + Ca) is 0.72. According to these values, carbonate decomposition, cation exchange, and plagioclase decomposition processes control the major ion concentrations of the thermal water. Mineral saturation The change in the mineral saturation states in water helps to determine the stages of hydrochemical evolution and is important in terms of which chemical reactions have effects on water chemistry (Drever 1997; Langmuir 1997). Especially for thermal and mineral waters, the early estimation of scaling and corrosion properties is very impor- tant in terms of preventing residue that may occur during the use of the waters. Addi- tionally, chemical reactions occurring in groundwater provide an opportunity to interpret the hydrochemical environment. Primary and secondary minerals were microscopically determined in the rocks outcropping in the study area, and the saturation of all waters sampled in terms of these minerals was investigated (Table 4, Fig. 9). The primary minerals in the volcanic rocks exposed in the study area are silicate minerals such as plagioclase, K-feldspar, <=Ca + Mg Gültekin et al. Geotherm Energy (2019) 7:12 Page 14 of 26 ILICAS1-May16 ILICAS1-May16 ILICAS May16 ILICAS May16 80 80 ILICAS Oct1 ILICAS Oct16 6 ILICAS Mar17 ILICAS Mar17 60 60 ILICAS Jul17 ILICAS Jul17 SSSK-May16 SSSK-May16 SSSK-Oct16 SSSK-Oct16 40 40 SSSK-Mar17 SSSK-Mar17 SSSK-Jul17 SSSK-Jul17 20 20 GSK-Jul1 GSK-Jul17 7 CDMS-May16 CDMS-May16 CDMS-Mar17 CDMS-Mar17 Mg SO4 CDMS-Jul1 CDMS-Jul17 7 GMS-Jul1 GMS-Jul17 7 CERDERY-May16 CERDERY-May16 80 80 CERDERY-Oct1 CERDERY-Oct16 6 CERDERY-Mar17 CERDERY-Mar17 60 60 CERDERY-Jul17 CERDERY-Jul17 CERDERA-May16 CERDERA-May16 40 40 CERDERA-Oct1 CERDERA-Oct16 6 CERDERA-Mar17 CERDERA-Mar17 20 20 CERDERA-Jul17 CERDERA-Jul17 Ca Na+K HCO3+CO3 Cl Fig. 7 Piper diagram for thermal and cold waters in the study area 100.00 ILICAS1-May16 ILICAS1-May16 ILICAS May16 ILICAS May16 ILICAS Oct1 ILICAS Oct16 6 ILICAS Mar17 ILICAS Mar17 ILICAS Jul17 ILICAS Jul17 SSSK-May16 SSSK-May16 SSSK-Oct16 SSSK-Oct16 10.00 SSSK-Mar17 SSSK-Mar17 SSSK-Jul17 SSSK-Jul17 GSK-Jul17 GSK-Jul17 CDMS-May16 CDMS-May16 CDMS-Mar17 CDMS-Mar17 CDMS-Jul17 CDMS-Jul17 1.00 GMS-Jul17 GMS-Jul17 CERDERY-May1 CERDERY-May16 6 CERDERY-Oct1 CERDERY-Oct16 6 CERDERY-Mar1 CERDERY-Mar17 7 CERDERY-Jul1 CERDERY-Jul17 7 CERDERA-May1 CERDERA-May16 6 CERDERA-Oct1 CERDERA-Oct16 6 0.10 CERDERA-Mar1 CERDERA-Mar17 7 CERDERA-Jul1 CERDERA-Jul17 7 0.01 Ca Mg Na+K Cl SO4 HCO3 Parameters Fig. 8 Schoeller diagram indicating ionic concentrations of waters in the study area Cl + SO4=> Concentration (meq/l) Gültekin et al. Geotherm Energy (2019) 7:12 Page 15 of 26 Table 4 Mineral saturation indices (SI) for thermal waters in the Şavşat Geothermal Field Minerals Formula ILICAS CDMS SSSK CERDERY Albite NaAlSi O 1.75 1.48 − 3.01 − 1.90 3 8 Anhydrite CaSO − 1.47 − 1.47 − 3.15 − 3.22 Aragonite CaCO 0.69 0.00 − 0.20 − 0.07 Barite BaSO 0.41 0.82 – − 1.41 Ca-montmorillonite Ca0.165Al 33Si 67O (OH) 6.40 7.10 1.91 2.78 2 3 10 2 Calcite CaCO 0.83 0.16 − 0.04 0.09 Celestine SrSO − 1.94 − 3.49 − 4.17 − 4.07 Dolomite CaMg(CO ) 1.28 − 0.32 − 0.82 − 0.54 3 2 Fluorite CaF − 0.23 − 0.94 − 3.27 − 2.91 Gypsum CaSO :2H O − 1.33 − 1.21 − 2.90 − 2.96 4 2 Goethite FeOOH 6.24 3.38 7.54 8.10 Halite NaCl − 4.40 − 4.77 − 9.37 − 9.33 Hematite Fe O 14.55 8.70 17.01 18.13 2 3 Illite K0.6Mg0.25Al 3Si 5O (OH) 5.49 5.54 0.12 1.63 2 3 10 2 K-feldspar KAlSi O 2.31 2.33 − 2.62 − 0.85 3 8 K-mica KAl Si O (OH) 12.28 12.20 5.95 7.54 3 3 10 2 Kaolinite Al Si O (OH) 6.72 7.41 3.93 4.30 2 2 5 4 Amorphous silica SiO 0.78 1.15 0.07 0.34 Rhodocrosite MnCO − 1.32 − 2.51 – – Siderite FeCO − 0.60 − 1.37 − 0.88 − 1.04 Talc Mg Si O (OH) − 1.84 − 7.00 − 5.26 − 2.55 3 4 10 2 Witherite BaCO − 2.63 − 3.23 – − 3.80 -5 -10 -15 ILICAS CDMS SSSK CERDERY Fig. 9 Mineral saturation indices (SI) for selected minerals in the Şavşat Geothermal Field augite, hornblende, and quartz, while secondary minerals are calcite, quartz, and chlorite. The SI was calculated for minerals selected from the limestone, sandstone, gypsum lenses, and opaque minerals forming sedimentary units using the AquaChem chemical equilibrium software. Negative SI values in Table  4 indicate an undersatu- rated solution, and positive values indicate an oversaturated solution. SI values that are meaningless (e.g., − 40) were ignored. When the SI values were examined, all hot and cold waters were not saturated with respect to sulfate minerals such as anhydrite Saturaon Indi ces (SI) Albite Anhydrite Aragonite Barite Ca-Montmorillonite Calcite Celeste Do lomite Fluorite Gypsum Goethite Halite Hemate Illite K-feldsp ar K-mica Kaolinite Amorph ous Silica Rhodochrosite Siderite Talc With erite 20 Gültekin et al. Geotherm Energy (2019) 7:12 Page 16 of 26 Na/1000 ILICAS1-May16 ILICAS1-May16 ILICAS May16 ILICAS May16 ILICAS Oct1 ILICAS Oct16 6 ILICAS Mar17 ILICAS Mar17 ILICAS Jul1 ILICAS Jul17 7 Full Equilibrium PartiallyEquilibrated and MixedWaters Im mature Waters K/100 Sqr(Mg) Fig. 10 Na–K–Mg ternary diagram (Giggenbach 1988) for the thermal waters of the Şavşat Geothermal Field and gypsum and minerals such as fluorite and halite. The geothermal water (ILICAS) and Ciritdüzü mineral water (CDMS) were saturated with respect to aragonite, cal- cite, and dolomite, while the cold spring waters were undersaturated in these min- erals. The silicate mineral K-feldspar was oversaturated in the hot water and CDMS but undersaturated in the cold waters. Both thermal water and mineral water showed slight oversaturation in amorphous silica. All waters were oversaturated with respect to K-mica, clay minerals of kaolinite and illite, and iron minerals of goethite and hem- atite, but they were undersaturated with respect to talc. Geothermometers The estimation of the aquifer temperature in geothermal systems is very important in terms of the appropriate use of thermal and mineral waters. With the aim of defining the reservoir rock temperature in geothermal areas, geothermometry methods were developed based on chemical (Fournier 1977; Arnorsson et  al. 1983) and isotopic (Lloyd 1968; Mizutani and Rafter 1969) analyses of springs and wells. Another method used is the Na–K–Mg combined geothermometer developed by Giggenbach (1988) for estimating the aquifer temperature of thermal waters and determining the maturity of rocks in contact with the water (Fig.  10). This geother - mometer may be used to test the validity of cation geothermometers and rapidly interpret the reservoir rock temperatures of thermal waters. On the Giggenbach (1988) diagram, the thermal water from the Şavşat Geothermal Field is plotted in “the 80 Gültekin et al. Geotherm Energy (2019) 7:12 Page 17 of 26 Table 5 Estimated reservoir temperatures (°C) for  the  Şavşat thermal waters using silica geothermometers (SiO = 125.8 mg/L) Geothermometers Equations References Calculated temp. SiO (ά Cristobalite) t = 1000/(4.78 − logSiO ) − 273.15 Fournier (1977) 100 2 2 SiO (Quartz) t = 1309/(5.19 − logSiO ) − 273.15 Fournier (1977) 150 2 2 SiO (Quartz steam loss) t = 1522/(5.75 − logSiO ) − 273.15 Fournier (1977) 144 2 2 SiO (Quartz steam loss) t = 1264/(5.31 − logSiO ) − 273.15 Arnorsson et al. (1983) 121 2 2 SiO (Quartz steam loss) t = 1164/(4.9 − logSiO ) − 273.15 Arnorsson et al. (1983) 143 2 2 SiO (Chalcedony) t = 1032/(4.69 − logSiO ) − 273.15 Fournier (1977) 125 2 2 SiO (Chalcedony) t = 1112/(4.91 − logSiO ) − 273.15 Arnorsson et al. (1983) 122 2 2 partly matured water” area. Generally, Na/K geothermometers provide confirmatory results at temperatures between 180 and 350  °C and may provide erroneous results below 120 °C. At these low temperatures, Na and K ions are affected by clay minerals and do not control ion exchange reactions. Thus, the values above the actual aquifer temperatures were obtained by geothermometer calculations (Gemici 1999), and sil- ica geothermometers were used to calculate the reservoir temperatures in the studied geothermal field. Calculated reservoir temperatures close to or below the discharge temperature of the geothermal waters were ignored. The quartz geothermometer gave reservoir temperatures ranging from 121 to 150 °C for the ŞGF, while the chalcedony geothermometer yielded 122–125  °C reservoir temperatures (Table  5). At low tem- peratures, the dissolved silica concentration controls chalcedony; however, the quartz geothermometer provides better results at higher temperatures. Therefore, the esti - mated reservoir temperature of the ŞGF was 120–125 °C, according to the chalcedony geothermometer (Table 5). The reservoir temperatures calculated by the SO –H O oxygen isotope geother- 4 2 mometers proposed by Lloyd (1968) and Mizutani and Rafter (1969) were 60–70  °C for the ŞGF, which were lower than those calculated by the silica geothermometers. The reason for this is the change in δ O values due to the mixing of sulfate-poor shallow groundwater with geothermal waters. Enthalpy–silica mixture model The hot fluid component of the geothermal system mixes with cold groundwater at dif - ferent depths and rates while traveling to the surface. The Şavşat water is determined to be peripheral water based on the Cl–SO –HCO diagram (Fig. 11). A variety of mixing 4 3 models have been developed to determine the reservoir temperature and mixing ratio in geothermal systems (Fournier and Truesdell 1974; Şahinci 1991). The most commonly used models are the silica–enthalpy and enthalpy–chloride mixing models. The silica– enthalpy mixing model diagram may be used to estimate reservoir temperatures in situ- ations with no loss of steam and temperature before mixing and loss of steam before mixing (adiabatic cooling). This mixing model is used to determine the reservoir tem - perature of geothermal fields and evaluate the effects of the mixing processes (Truesdell and Fournier 1977). Figure 12 shows the silica–enthalpy mixing model based on quartz solubilities. In this model, two end member fluids are presented: a cold water sample Gültekin et al. Geotherm Energy (2019) 7:12 Page 18 of 26 CI Savsat geothermal water Steam Heated Waters HCO SO Fig. 11 Cl–SO –HCO ternary diagram for geothermal waters 4 3 Fig. 12 Silica–enthalpy mixture model for Şavşat Geothermal Water Gültekin et al. Geotherm Energy (2019) 7:12 Page 19 of 26 (SSSK sample; temperature: 9.38 °C) as one endmember and the thermal waters (ILICAS sample; temperature: 37.45 °C) as the other endmember. In this model, thermal waters form as the result of the mixing of thermal water with cold water, assuming maximum steam loss. The intersection point with the solubility curve for quartz (maximum steam loss) yields a reservoir temperature of 195  °C for the ŞGF thermal water. The mixing rate with cold groundwater is calculated to be 70% for the ŞGF. This value is higher than the reservoir temperature calculated by the silica geothermometers. The ŞGF thermal waters may lose some heat due to possible mixing with cold water along the fracture zones during its ascent to the surface. Therefore, the silica geothermometer appears to reflect reservoir temperatures more accurately than the other geothermometers. Isotope studies Oxygen-18 and deuterium stable isotopes were employed to determine the possible recharge areas of water, and tritium was employed to calculate the relative age of the waters and their residence times. Carbon-13 was used to determine the origin of car- bon as well as oxygen-18 and sulfur-34 isotopes in sulfate in the waters (Table 6). 18 2 18 2 δ O and  δ H relationships The δ O and δ H values in the waters were evaluated according to the Global Meteoric Water Line (GMWL) (Craig 1961) and the Eastern Black Table 6 Isotope analysis results for  water samples collected in  the  Şavşat Geothermal Field 18 13 34 18 Sample name Date Definitions δD δ O T (TU) δ C δS (SO ) δO (SO ) 4 4 V-SMOW V-SMOW V-PDB VCDT V-SMOW ILICAS May 16 Geothermal − 100.38 − 13.03 0.74 4.31 10.6 9.6 water October 16 − 97.99 − 12.84 2.49 March 17 − 97.14 − 13.24 0.90 7.70 10.9 11.6 July 17 − 100.36 − 13.03 1.11 CDMS May 16 Mineral water − 94.81 − 12.4 3.59 7.65 7.7 6.7 March 17 − 95.78 − 12.45 3.13 6.4 6.0 July 17 − 94.88 − 12.48 1.95 9.55 GMS July 17 Mineral water − 96.29 − 13.39 SSSK May 16 Cold spring − 88.19 − 12.95 4.67 − 15.4 3.4 0.0 water October 16 − 89.96 − 12.67 4.37 March 17 − 86.58 − 12.33 4.35 − 12.36 4.3 1.7 July 17 − 88.38 − 12.59 5.70 GSK July 17 Cold spring − 88.17 − 12.60 water CERDERY May 16 Surface water − 86.37 − 12.21 6.0 − 13.82 5.2 − 0.7 October 16 − 82.32 − 11.78 4.29 March 17 − 90.60 − 12.50 6.35 − 7.79 3.5 0.8 July 17 − 88.15 − 12.40 5.25 CERDERA October 16 Surface water − 85.46 − 11.95 5.97 March 17 − 90.59 − 12.65 3.81 July 17 − 88.30 − 12.31 4.08 SAVSTKAR March 17 Snow − 120.97 − 17.56 5.77 Gültekin et al. Geotherm Energy (2019) 7:12 Page 20 of 26 -80 -80 -90 -85 -100 -90 -110 -95 -100 -120 -105 -130 -13.5 -13.1 -12.7 -12.3 -11.9 -11.5 -18.0 -17.0 -16.0 -15.0 -14.0 -13.0 -12.0 Oxygen-18 (‰) Oxygen-18 (‰) ILICAS1-May16 SSSK-May16 CDMS-May16 CERDERY-May16 CERDERA-May16 SNOW-Mar17 ILICAS May16 SSSK-Oct16 CDMS-Mar17 CERDERY-Oct16 GMWL CERDERA-Oct16 ILICAS Oct16 DKMWL SSSK-Mar17 CERDERY-Mar17 CDMS-Jul17 CERDERA-Mar17 ILICAS Mar17 SSSK-Jul17 GMS-Jul17 CERDERY-Jul17 EVPL CERDERA-Jul17 ILICAS Jul17 GSK-Jul17 18 2 Fig. 13 δ O–δ H diagram for waters in the Şavşat Geothermal Field (GMWL Global Meteoric Water Line, DKMWL Eastern Black Sea Meteoric Water Line, EVPL Evaporation Line) 2 18 Sea Meteoric Water Line (DKMWL: δ H = 8 δ O + 16) (Ekmekçi and Gültekin 2015). 18 2 The cold spring and surface waters in the Şavşat field have δ O and δ H values between the GMWL and DKMWL (Fig.  13). The surface waters, spring waters, and geothermal 18 2 waters plot in different areas. The δ O and δ H values of the snow sample are very dif- ferent from those of the waters. It is determined that the cold spring and surface waters are recharged by precipitation falling at much lower elevations in the basin compared to the snow. The DKMWL is considered, and the δ O value of the geothermal water shifts to more positive values. This is due to water–rock interactions. The cold spring water is recharged from higher elevations compared to the surface waters. The CDMS has a more positive δ O compared to the values of the surface waters, which indicates a longer dura- tion of interaction with rocks (Fig. 14). Carbon isotope ( C) Water filtering underground dissolves CO in soil and differen - − −2 tiates it into HCO and CO species. The distribution of dissolved inorganic carbon 3 3 (DIC) species varies with respect to pH. The variations in DIC and δ C also stem VPDB from changes in pH values (Clark and Fritz 1997). The dissolved inorganic carbon C DIC and δ C of groundwater develop as a result of differentiation reactions in aquifers VPDB or soil. The δ C isotopes were used to determine the source of carbon in the samples. 13 13 Analyses were carried out on DIC for δ C (Table 6). The δ C value of geothermal VPDB water in the study area was determined to be 4.31‰ (Table 6). Dissolved inorganic carbon in geothermal water in the study area originates from freshwater carbonates and meta- morphic CO (Clark and Fritz 1997). For CDMS, the δ C value is similar to that of 2 VPDB geothermal water, while other cold waters have negative values. The source of carbon in cold waters might be a mixture of freshwater carbonates, groundwater DIC, and soil C O (Clark and Fritz 1997). Deuterium (‰) Deuterium (‰) Gültekin et al. Geotherm Energy (2019) 7:12 Page 21 of 26 3 3 − 3 Fig. 14 H–EC, H–Cl , and H–T (°C) relations for waters in the study area 34 −2 Sulfur isotope ( S) Sulfur is found in the crust as dissolved SO, SO, H S, and S O 4 2 2 2 species. Organic sulfur is found in humic materials, kerogen, and hydrocarbons. Sul- fur-34 is partitioned into many different sulfur compounds. Similarly, the oxygen-18 con - tent of sulfate is an important tool for tracing the sulfur cycle (Clark and Fritz 1997). Val- ues exceeding − 20‰ are related to limestones and evaporites. The sulfur-34 ratio in the oxidation of juvenile sulfur is generally between − 5‰ and + 5‰ (Clark and Fritz 1997). Negative sulfur-34 values occur in diagenetic environments where typically reduced sul- −2 34 fur compounds are present (Krouse 1980). The results of the dissolved SO ion S iso- tope analysis for the study area are given in Table 6. The S for the geothermal water CDT in the Şavşat Geothermal Field is nearly 10‰. In cold waters, the S value varies from CDT 10 to 3‰. According to Krouse (1980), these values show that the sulfur in the geother- mal water is derived from volcanic sulfur (S O ) and from Cenozoic-age C aSO , while the 2 4 sulfur in the cold water is derived from magmatic rocks. Tritium content The tritium values for the geothermal waters vary from 0.74 to 2.59 TU (Table 6). In mineral water (CDMS), it is nearly 3 TU, while in cold spring and 3 3 − 3 surface waters, it varies from 3 to 6 TU. The H–Eİ, H–Cl , and H-temperature cor- Gültekin et al. Geotherm Energy (2019) 7:12 Page 22 of 26 relations for the water samples are given in Fig. 14. For the geothermal water, mineral water, cold spring water, and surface water, there is a significant negative correlation between the tritium and EC values. Geothermal waters with low tritium values are deeply circulated waters, and the residence time is longer than those of mineral water and cold spring water. Şavşat (Artvin‑Turkey) Geothermal System A hydrogeological conceptual model was developed based on geological, hydro- chemical, isotopic, and geophysical studies in the study area (Fig.  15). The ŞGF is a liquid-dominated geothermal system. The ŞGF is recharged by infiltrating meteoric waters from the Kabaköy Formation and Bülbülan Formation in the north-northeast. The northern bounding normal fault (F2) and formation boundaries in the northeast (Görizil Hill) might be conduits for water flow. In addition, the NE–SW trending anti - cline axis is also suitable for the circulation of water. The old thermal springs emerged along the anticline axis and faults. Drilling and hydrochemical studies indicate that the reservoir rock is volcanogenic sandstone and augite basalt-type volcanic rocks. The porosity of the volcanogenic sandstone (Kabaköy Formation) is 7–10%, and that of the augite basalts is 3–5%. Additionally, volcanogenic sandstones have gained secondary porosity along bed- ding planes, fractures, and joints in the sandstone and augite basalts. This feature has allowed the unit to gain properties conducive to the storage and circulation of water. Volcanogenic sandstones with reservoir features widely outcrop in the field. As a result, there is no cap rock fully enclosing the system. However, in areas where the Şavşat Formation displays turbiditic features composed of mudstone–siltstone–sand- stone alternations, it forms a cap rock for the geothermal system. Şavşat thermal waters are controlled by both the regional and the local flow sys - tems, and their chemical and isotopic compositions are attributed to mixing with cold 18 2 shallow groundwater during their ascent to the surface. The δ O and δ H isotopes of the thermal water show more negative values compared to those of the cold waters and more positive values compared to those of the snow. The δ O values of the ther- mal water show a slightly positive shift as a result of water–rock isotope exchange. The tritium values of the thermal water are much lower than those of both the cold water and the snow samples. According to these data, geothermal waters form when meteoric water falls as rain into the basin and is transmitted underground, stored in volcanogenic sandstones, and heated up by the geothermal gradient. The heated water reaches the surface by rising along a fracture zone determined in geophysical stud- ies as being oriented parallel to the Çermik Stream valley (Fig.  15). According to the δ C values, the carbon in the thermal water has a metamorphic origin, whereas the carbon in the cold water is derived from the dissolution of Oligocene–Miocene lacus- trine carbonate deposits in the region or from CO gas that accumulated in pores. The δ S values show a volcanic origin for sulfur in the geothermal water. CDMS is mineralized cold water outside the hydrological basin of the ILICAS thermal spring; it is very similar to thermal water in terms of its chemical and isotopic compositions. However, due to lower EC and TDS values, it can be defined as a mixing water. Gültekin et al. Geotherm Energy (2019) 7:12 Page 23 of 26 Fig. 15 Hydrogeological conceptual model for Şavşat (Artvin/Turkey) Geothermal Field Gültekin et al. Geotherm Energy (2019) 7:12 Page 24 of 26 According to Türkecan (2017), volcanic rocks outcropping in wide areas in Kars and Ardahan are late Pliocene–early Pleistocene (Duru and Keskin 2014) andesite, dacite, and rhyolite. Andesitic and dacitic lava flows form ridges and domes, but dacitic rocks also occasionally occur as lava flows. These volcanic rocks, named the Ardahan andesite (Karaköse et  al. 1994), Ulgartepe andesite (Karaköse et  al. 1994), or Dumanlıdağ volcanic rocks (Aktimur et  al. 1982), have been dated at 1.6–2.7 mil- lion years with a variety of methods (Innocenti et al. 1982; Karaköse et al. 1994). The volcanic rocks represented by trachyandesite, trachybasalt, hornblende andesite, and pyroclastics outcrop as ridges and domes in the Şavşat Geothermal Field and can be correlated with the Pliocene–Pleistocene volcanic rocks based upon their petro- graphic properties (Fig. 15). The young volcanic activity in the area caused an increase in the geothermal gradient. Therefore, the heat source for the geothermal system is considered to be this young volcanic activity. The δ S values of the geothermal waters also support this idea. The reservoir temperature for the ŞGF was calculated as 100–150  °C by the silica geothermometer. According to the calculated reservoir temperature, the field is clas - sified as a low-moderate enthalpy field (Muffler and Cataldi 1978; Benderitter and Cormy 1990; Hochstein 1990; Haenel et  al. 1988). Magmatic activity feeding young volcanic rocks in the field was interpreted as a source of heat. Deeply heated water associated with this magmatic activity was driven upward along the faults and fractures. Conclusions The Şavşat (Artvin-Turkey) Geothermal Field contains outcrops of volcanic, volcano sedimentary, and sedimentary units formed during the late Cretaceous to the Pliocene– Pleistocene time periods. The artesian water recharging from a 120-m-depth borehole in the ŞGF has a temperature of 38  °C, an EC value of 5731  µS/cm, and a pH of 6.83. The Na–HCO –Cl-type geothermal water is oversaturated with respect to aragonite, calcite, dolomite, amorphous silica, K-mica, kaolinite, talc, and hematite minerals and undersaturated with respect to minerals such as gypsum, anhydrite, halite, fluorite, rho - dochrosite, and siderite. The reservoir temperature is estimated at 100–150  °C using a silica geothermometer. The mixing rate of geothermal waters with cold groundwater is calculated to be 70%. In deeply circulated geothermal waters, carbon is derived from metamorphic CO , and sulfur is of volcanic origin. The cold waters indicate an origin of groundwater DIC. The reservoir rocks of the ŞGF are volcanogenic sandstone and late Cretaceous vol - canic rocks of andesite, basalt, and pyroclastics. The heat source is the geothermal gradi - ent arising from Quaternary volcanic activity. The late Eocene turbiditic unit comprising mudstone, siltstone, and marl forms a partial cap rock. Geophysical studies in the field identified a potential thermal water reservoir at elevations of 1250–1350 m. At borehole of at least 300 m may intersect an area of fluids with more pressure and higher tempera - tures than the present well and spring water. Gültekin et al. Geotherm Energy (2019) 7:12 Page 25 of 26 Abbreviations ŞGF: Şavşat Geothermal Field; T: temperature; EC: electrical conductivity; TDS: total dissolved substance; DO: dissolved oxygen; GMWL: Global Meteoric Water Line; DKMWL: Eastern Black Sea Meteoric Water Line; DIC: dissolved inorganic carbon. Authors’ contributions FG conducted hydrogeological studies and contributed significantly to the writing of the paper. EHT conducted hydro - geochemical studies. AEB conducted and commented on geophysical studies. MZK conducted geological studies and described the rocks type. AFE participated in hydrogeological studies. BMS participated in geological mapping studies and sampling. All authors read and approved the final manuscript. Author details 1 2 Geological Engineering Department, Karadeniz Technical University, Trabzon, Turkey. Geophysical Engineering Depart- ment, Karadeniz Technical University, Trabzon, Turkey. Acknowledgements This study was supported by The Scientific and Technical Research Council of Turkey ( TUBITAK Project Number: 115Y142). The authors thank TUBİTAK for their financial support. The authors also thank Prof. Dr. Necati TÜYSÜZ from the KTÜ for his help with the English of the final text. We wish to thank the Executive Editor Prof. Dr. Ernst Huenges, three anonymous reviewers. Competing interests The authors declare that they have no competing interests. Availability of data and materials Not applicable. Funding All of this study was prepared by data of TÜBİTAK funded project numbered 115Y142. Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. 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Conceptual model of the Şavşat (Artvin/NE Turkey) Geothermal Field developed with hydrogeochemical, isotopic, and geophysical studies

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Abstract

fatma@ktu.edu.tr Geological Engineering The Şavşat (Artvin, Turkey) Geothermal Field (ŞGF) is located on the northeastern Department, Karadeniz border of Turkey. This field is characterized by thermal and mineralized springs and Technical University, Trabzon, Turkey travertine. The temperature of the thermal water is 36 °C, whereas that of the mineral- Full list of author information ized spring in the area is approximately 11 °C. The Na–HCO –Cl-type thermal water has is available at the end of the a pH value of 6.83 and an EC value of 5731 µS/cm. The aim of this study is to character- article ize the geothermal system by using geological, geophysical, and hydrogeochemical data and to determine its hydrochemical properties. A conceptual hydrogeological model is developed for the hydrogeological flow system in the ŞGF. According to the hydrogeological conceptual model created by geological, geophysical, and hydrogeo- chemical studies, the reservoir comprises volcanogenic sandstone and volcanic rocks. The cap rock for the geothermal system is composed of turbiditic deposits consisting of mudstone–siltstone–sandstone alternations. An increase in the geothermal gradient is mainly due to Pleistocene volcanic activity in the field. The isotopic values of thermal 18 2 3 water (δ O, δ H, δ H) indicate a deeply circulating meteoric origin. The estimated reservoir temperature calculated by silica geothermometers is 100–150 °C, and the mixing rate of cold groundwater with geothermal waters is approximately 70%. It may be possible to obtain warmer fluids from a 300-m-deep borehole cutting through a fracture zone identified by geophysical studies. Heating by conduction via the geother - mal gradient resulting from young volcanic activity drives geothermal waters upwards along faults and fractures that act as hydrothermal pathways. The positive δ C VPDB value (+ 4.31‰) indicates a metamorphic origin for the thermal water. The S value CDT (~ 10‰) shows that the sulfur in the geothermal water is derived from volcanic sulfur (SO ). Keywords: Geothermal, Hydrogeochemistry, Hydrogeological conceptual model, Geophysical studies, Şavşat (Artvin-Turkey) © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Gültekin et al. Geotherm Energy (2019) 7:12 Page 2 of 26 Introduction The Şavşat Geothermal Field (ŞGF) is located in northeast Turkey on the border with Georgia (Fig. 1). According to meteorological data from the closest station to the study area in the town of Şavşat, the mean annual precipitation is 580  mm, and the mean annual temperature has been 9.9 °C for the last 10 years (MGM 2017). The older units of the study area are Late Cretaceous-age volcanic rocks (Güven 1993), Paleocene-early Eocene-age sedimentary rocks (Erendil et  al. 1989), and middle Eocene-age andesite and basalt-type volcanic rocks and volcanogenic sandstones (Güven 1993). Young units are Lutetian-age turbidites (Erendil et  al. 1989) and Oligo-Miocene-age sedimentary rocks, including sandstone, siltstone, and marl alternations (Karaköse et al. 1994; Konak et  al. 1998). Previous studies in the area (Kara 1997; Akkuş et  al. 2005) reported ther- mal springs with temperatures of nearly 36 °C. These springs have been used for many years in primitive facilities for balneological purposes by local people. During field stud - ies, thermal springs were observed at different locations along the Çermik Stream valley with temperatures ranging from 20 to 36 °C. In recent years, private undertakings that run mountain plateau tourism (or yayla) in the area have wanted to obtain warmer water to provide better service to the region. With this aim, drilling studies were performed in 2016, and water with temperatures of nearly 39  °C was obtained from 120  m depth. This well, drilled immediately beside the thermal spring, caused the spring to dry up. In this area, apart from some information included in the Geothermal Energy Inven- tory of the Turkish Geological Survey (MTA), there has not been any study to date that assesses the geothermal, hydrogeological, or hydrogeochemical properties. To define the geothermal system and its surroundings, it is important to determine the hydrogeo- logical, hydrochemical, and isotopic properties. Geological and geophysical studies and Fig. 1 Location map of the Şavşat (Artvin/Turkey) Geothermal Field Gültekin et al. Geotherm Energy (2019) 7:12 Page 3 of 26 hydrogeochemical and isotope techniques have been widely used elsewhere to deter- mine the hydrodynamic structure of geothermal systems in recent years (Tarcan et  al. 2005; Piscopo et al. 2006; Schaffer and Sass 2014; Yurteri and Şimşek 2017; Uzelli et al. 2017). In this geothermal field, there has not been any study that employs a conceptual approach based on hydrogeological and hydrogeochemical studies, such as hydrochemi- cal facies compositions, isotopic features, water–rock interactions, mixing processes, and reservoir temperature. This study focuses on understanding the mechanism of the geothermal system in the ŞGF for future use, determining the areas with higher tem- peratures and revealing the hydrogeochemical characteristics of the geothermal water. In accordance with this purpose, geological, geophysical, and hydrogeochemical studies were performed in the area to characterize components such as the reservoir, geother- mal fluids, and cap rock of the geothermal system. Establishing a hydrogeologic con - ceptual model can further help to determine flow paths, including recharge through flow–discharge processes, as well as mixing behavior. Methodology Field studies were completed in three different forms: geological studies, geophysical studies, and measurements and sampling on site. Geological studies were carried out on 1/25,000-scale topographic maps and 1/100,000-scale geological maps of the study area prepared previously by a variety of researchers. Geological units were observed in the field, and the geological maps made by different researchers were revised. Rock samples were taken for lithological identification. During field studies, observations and geologic-tectonic studies of the area were used to identify sample locations for ther- mal, cold, and surface waters. To determine the chemical content and variations of the geothermal and cold waters, samples were taken from each location to represent rainy, dry, and interval periods from May 2016 to October 2017, and measurements were per- formed in the field. In-situ measurements and sampling were performed at the geothermal well (ILICAS), in mineral water within the basin (GMS), in mineral water outside the basin (CDMS), in cold spring water (SSSK, GSK), and in the Çermik Stream before (CERDERY) and after (CERDERA) thermal water mixing. The coordinates of each sampling location were recorded by a handheld GPS and marked on a geological map. Measurements of temperature (T), electrical conductivity (EC), pH, total dissolved sol- ids (TDS), and dissolved oxygen (DO) at the sampling points were performed with a YSI- 556 multiparameter meter. The probes used during the measurements were preserved by washing them with pure water before and after each measurement and were used after daily calibrations with buffer solutions. All water samples were filtered through 0.45  μm membranes on site. To determine the major anion–cation and trace element contents of the waters, samples were taken with polyethylene (HDPE) bottles. For major anion–cation and trace element analysis, 500 mL and 50 mL bottles were used, respec- tively. Nitric acid (HNO ) was added for cation and trace element analysis to bring the pH to < 2. Major anion–cation, trace element, and tritium ( H) analyses were carried out at the Water Chemistry Laboratory at Hacettepe University (Turkey) using the follow- 2+ 2+ + + ing methods: major cation analysis (Ca, Mg, Na , and K ) was performed by atomic absorption spectrometry. Cl was analyzed using an AgNO titrimetric method. Sulfate 3 Gültekin et al. Geotherm Energy (2019) 7:12 Page 4 of 26 concentrations were determined by spectrophotometry together with alkalinity stand- ard titration methods, whereas B and SiO were analyzed with the spectrophotometric method. The major ion balance error of the analyses was less than 5%. Trace element analysis was performed with the inductively coupled plasma-mass spectrometry (ICP- MS) method. 18 2 13 3 Samples of 100  mL for δ O, δ H, and δ C analyses and samples of 500  mL for δ H analysis were collected. For the δ S analysis, 250 mL to 1000 mL samples were collected based on the amount of sulfate in the waters. The samples were kept cold until they were sent to the laboratories. A snow sample was taken by immersing a cylindrical tube into the snow. Then, this snow mass was poured into a polyethylene container and kept at 18 2 13 + 4  °C until it became liquid. δ O, δ H, and δ C isotopic analyses were performed at the Iso-Analytical Laboratory in the UK using the isotope ratio mass spectrometry 18 34 (IRMS) method. For geothermal water, the δ O and δ S analyses of the sulfate were performed by the Isotope Tracer Technologies Inc. (Canada) laboratory using the IRMS technique. Standard deviations of the stable isotope analyses were 0.11‰ for O and 0.90‰ for deuterium. The AquaChem 2012.1 program was used to evaluate the results of the chemical analy - ses, to prepare diagrams and to calculate the saturation indices (SI) of selected minerals. In recent years, geophysical methods have been commonly used in the investigation and development of geothermal resources, especially with the development of tech- nology, and these methods have provided successful results (Majumdar et  al. 2000; El- Quady 2006; Özürlan and Şahin 2006; Drahor and Berge 2006; Abiye and Haile 2008; Wu et  al. 2012). Although many geophysical methods are used in geothermal fields, electrical resistivity is one of the most preferred methods for investigating many geo- thermal fields. High temperatures and water flow in geothermal systems change electri - cal conductivity properties underground. Due to these changes, it is possible to obtain important information about the location, depth, and structure of geothermal resources by using the electrical resistivity method. Electrical resistivity is one of the most widely used geophysical methods in underground surveys (Telford et al. 1990; Reynolds 1997). In this method, the basic principle is to send a known current through two current elec- trodes into the ground and measure the voltage difference with two potential electrodes. From these measured potentials, the calculation of the resistivity and thickness of the underground layers is based on Ohm’s law. The resistivity distribution of the under - ground materials is measured by the resistivity method. Two-dimensional (2D) resistiv- ity images of the underground layers are obtained by using a large number of electrode pairs in the electrical resistivity tomography method (ERT). This method is a very effec - tive and widely used method for identifying lateral and vertical changes, presence and level of groundwater, hot springs, salinity, cavities, and weathered rocks (Reynolds 1997; Aizawa 2014). In the field, a pole-to-pole electrode array was used along the three profiles with a total profile length of 600 m, and 2D ERT data were acquired with an Advanced Geosciences, Inc. (AGI) SuperSting R8-IP (Figs.  2, 3). The power supply of this device consisted of a transmitter unit capable of producing a 2000  mA current with 400  W power and a special generator with 4 kWA power. The electrical resistivity measurements targeted a research depth of approximately 300 m. Gültekin et al. Geotherm Energy (2019) 7:12 Page 5 of 26 Fig. 2 Satellite image of 2D resistivity profiles in the study area Fig. 3 The appearance of 2D resistivity lines on topography map Results and discussion Geological and hydrogeological settings The Late Cretaceous-age Kızılkaya Formation comprises dacite, dacitic pyroclas- tics and volcanogenic siltstone, sandstone, and pebblestone (Güven 1993). The Paleocene-early Eocene-age Ardanuç Formation, which contains limestone, sand- stone, tuff, and claystone intercalations, overlies the Mesozoic units (Fig.  4). The thin- and medium-bedded unit has a total thickness of 200  m (Erendil et  al. 1989). Gültekin et al. Geotherm Energy (2019) 7:12 Page 6 of 26 Fig. 4 Geological map of the Şavşat (Artvin/Turkey) Geothermal Field (revised from Erendil et al. 1989 and Konak et al. 1998) The Lutetian (middle Eocene)-age Kabaköy Formation conformably overlies the Paleocene units (Güven 1993). The Kabaköy Formation, which outcrops widely in the study area, contains volcanogenic sandstone and pyroclastics at the lower lev- els. After a thick mudstone layer, it continues upward with basaltic and andesitic rocks. The thickness of the volcanic rocks, mostly composed of augite basalts, is 200  m. The total thickness of the Kabaköy Formation is over 800  m (Güven 1993). Gültekin et al. Geotherm Energy (2019) 7:12 Page 7 of 26 The Şavşat Formation, which outcrops as mudstone–siltstone–sandstone alterna- tions with a turbiditic character, is Lutetian in age (Erendil et  al. 1989). The thick- ness of the unit consisting of round, small pebbles at the bottom and sandstones, siltstone and marls of yellowish color in the upper part is 400  m. The Pınarlı For- mation (Karaköse et  al. 1994; Konak et  al. 1998), comprising sandstone, siltstone, marl, lacustrine cherty limestone, and gypsum lenses, unconformably overlies the turbiditic unit (Erendil et  al. 1989). The thickness of the unit consisting mainly of sandstone and siltstone is approximately 300  m. Marl, lacustrine cherty limestone intercalations, and gypsum lenses are generally observed in these gray and medium- bedded rocks. The Pınarlı Formation is late Miocene in age (Karaköse et  al. 1994) and includes landslides of varying sizes. The Pliocene–Pleistocene Bülbülan Forma- tion (Erendil et al. 1989) overlies older units and consists of basalt, andesite, trachy- basalt, trachyandesite, and pyroclastics in the study area. Plagioclase microliths and a small number of augite microcrystals are observed in the groundmass consisting of volcanic glass in the trachybasalts. Calcite as a secondary mineral accompanies these minerals. Plagioclase, sanidine, hornblende, and opaque minerals are observed in the trachyandesites. In the trachyandesites, chlorite is observed as a secondary mineral. The Bülbülan Formation overlies the Şavşat and Pınarlı Formations, with an angular unconformity ranging between 400 and 1000 m (Konak et al. 1998). The youngest unit in the area is alluvium cropping out in narrow areas along valleys. Both thrust and normal faults are observed in the area. Thrust faults have NE–SW trends, whereas normal faults occur in several different directions. In addition, fold- ing is represented by anticlines and synclines with NE–SW axial trends. Clastic sedimentary rocks and volcanic rocks dominate the study area. Sedimen- tary rocks with high primary porosity have varying permeabilities depending on the degree of cementation. Volcanic rocks have a low primary porosity (3–5%), except where they are fractured by cooling and tectonic activities. Tuffs belonging to the late Cretaceous Kızılkaya Formation are permeable, whereas dacite and rhyodacite are impermeable. The volcanogenic sandstones of the Kabaköy Formation, which are extensively exposed in the study area, have good permeability characteristics. Pyroclastic rocks also have high permeability. The volcanic rocks in the Kabaköy Formation are permeable where they have been subjected to tectonic and cooling activities. The open fractures allow the deep circulation of water due to locally enhanced vertical permeabilities. Due to its thickness, extent, and permeabil- ity, this unit is considered to be the most important aquifer in the area. The Şavşat Formation, with mudstone–siltstone–sandstone alternations, has very low perme- ability; therefore, it is evaluated as semipermeable. Marl and gypsum layers of the Pınarlı Formation are impermeable. This unit, acting as an impermeable cap rock, prevents heat loss and maintains water pressure. Forming high hills in the study area, volcanic units of the Bülbülan Formation are permeable where they are fractured. Alluvium in the area is permeable, but it has insignificant groundwater storage capacities because its dimensions are small. The most prevalent stream in the area is the Akdamla Stream. It merges with the Meydancık Stream outside the study area. The thermal spring is located in the Çer- mik Stream valley, which is the branch of the Akdamla Stream. Gültekin et al. Geotherm Energy (2019) 7:12 Page 8 of 26 Geophysical studies Due to the physical changes, the electrical resistivity method is used to obtain signifi - cant information about the geothermal source location, its depth, and the discontinu- ous structures that are possibly faults and fractures/joints, which may be important for the geothermal system. With the aim of determining the locations and depths of structural elements providing channels for fluids to reach the surface in the ŞGF, the ERT method was applied to three profiles. Two of the profiles were oriented approxi - mately in the north–south direction, and the third was in the east–west direction. The locations of the electrical resistivity profiles were primarily determined based on the geology, topography, and ground conditions in the study area. The study area is quite mountainous, and flat areas are quite limited. For this reason, the measurement profiles were located as much as possible in the flattest area. Furthermore, an attempt was made to plan measurement profiles in such a way that they were perpendicu - lar to the faults indicated by previous geological studies (Erendil et  al. 1989; Güven 1993; Karaköse et al. 1994; Konak et al. 1998). Because the terrain conditions did not allow an overly long profile length, the pole–pole array was preferred to obtain maxi - mum depth information. The apparent resistivity data obtained from three profiles using electrical resistivity measurements were assessed with the Res2Dinv program (Loke 2010), and true resistivity values and depth information were obtained for the study area as a result of inverse solution processes. The RMS errors for the three pro - files were below 15%. The RES2DINV program is written to run as automatically and robustly as possible, requiring very few input parameters from the user (Loke 2000). The program uses the smoothness-constrained least-squares method inversion tech - nique (Geotomo Software; DeGroot-Hedlin and Constable 1990). As the topography varies in the measurement profiles in the study area, the eleva - tion data for each electrode point were measured, and topography calculations were included in the inverse solution process. 2D inversion was applied to the apparent resistivity data obtained in the study area to create 2D underground resistivity images. The information obtained regarding the underground structure from the 2D resistiv - ity images was interpreted by taking the high and low resistance values into account. Considering the high and low resistivity zones, fault zones and hot water regions were marked on the images (Fig.  5). Fault zones and low and high resistivity areas on the 2D resistivity images were compared with the lithology in an attempt to determine the structure of the geothermal source. Moreover, a 3D resistivity map (Fig.  6) was created from all 2D profiles, and a change in resistivity with depth was displayed. In the resistivity images, high resistivity, medium resistivity, and low resistivity values are shown with red, green, and blue colors, respectively. While the resistivity values range from 10 to 5000 Ω m, the maximum penetration depth is approximately 300 m. The study area is generally composed of volcanic rocks, and very high resistivity areas (red color) represent massive rocks. In contrast, very low resistivity areas (blue color) may show the presence of hot water. Moreover, 2D resistivity images (Lines 1 and 2) indicate a large fault zone. The sections obtained by matching these profiles with the lithologies indi - cate (Fig.  5) very promising hot water regions between 150 and 300  m of horizon- tal distance on Line 1, 250 and 350  m on Line 2, and 1250 and 1350  m in areas of Gültekin et al. Geotherm Energy (2019) 7:12 Page 9 of 26 Fig. 5 2D resistivity sections and lithological-structural relation map for Line 1, Line 2, Line 3 profiles Fig. 6 3D resistivity map obtained from 2D profiles topographic elevation. It appears that drilling to at least 300 m in this area would reach water with higher pressure and temperature than the present well and spring water. Gültekin et al. Geotherm Energy (2019) 7:12 Page 10 of 26 Hydrogeochemical properties Water chemistry Samples were taken from the thermal water output in the ŞGF to identify the chemi- cal and isotopic characteristics of mineral and cold water springs. The water descrip - tions together with the coordinates and elevations of the sample locations are given in Table  1. Some properties of water, such as pH, T, DO, EC, and TDS, were measured at the sample locations in the field (Table  2). The mean values of T, pH, and EC of the discharged waters from the artesian well (ILICAS) from 120 m depth were 37.5 °C, 6.83, and 5731 µS/cm, respectively. In thermal waters dominated by Na and HCO ions, SiO 3 2 was 97.65 mg/L, B was 44 mg/L, F was 2.35 mg/L, and Li and Br were < 1 mg/L (Table 2). Ciritdüzü mineral water (CDMS) dominated by Na and HCO ions had a pH value of 6.42, an EC of 3195  µS/cm, SiO of 88.21  mg/L, B of 63  mg/L, and Li, Br and F of < 1 mg/L. The GMS, which discharges into a swamp area, had an EC value of 522 µS/cm, a pH of 7.58, and SiO of 26.4 mg/L. Cold spring waters (SSSK, GSK) had pH values of nearly 7.9 and EC values of 181.5 and 274  µS/cm, respectively. These waters dominated by Ca and HCO ions contain SiO between 10.93 and 40.8 mg/L and B values below 1 mg/l (Table 2). The chemical characteristics of Çermik Stream water before (CERDERY) and after (CERDERA) mixing with geothermal water were different (Table  2). In the CERDERY water dominated by Ca and H CO ion pairs, pH and EC were 7.86 and 154  µS/cm, respectively. For CERDERA, the EC was 619 µS/cm and was dominated by the anion and cation of Na and HCO . The ionic difference is because Çermik Stream water is physi - cally and chemically affected by geothermal well water. According to the IAH (1979) classification, the ŞGF thermal waters are Na–HCO –Cl type, while the cold springs and surface waters are generally Ca–HCO type (Table  3). Ca and HCO ions are dominant in the rainy season in CERDERY, whereas Mg and Na ions are accompanied by Ca ions in the periods when the effect of precipitation is reduced. The Piper diagram (1944) was used to classify the geothermal, cold spring, and surface waters and to determine hydrogeochemical processes, and the Schoeller diagram (1962) was used to compare the chemical content of the waters. The thermal water in the ŞGF has higher alkali elements (Na + K) than earth alkali elements (Ca + Mg). The waters with low S O have similar Cl and H CO ion concentrations. The thermal water 4 3 and CDMS have similar chemical compositions. The cold springs and surface waters have more earth alkali elements (Ca + Mg) than alkali elements (Na + K), with more Table 1 Coordinates and elevations of sampling points in the study area Sample name Definitions Coordinates (UTM 37 T Elevation (m) WGS84) ILICAS Thermal well water 0282740–4586153 1495 CDMS Mineral water 0281351–4573470 1145 GMS Mineral water 0283639–4586548 1600 SSSK Cold spring water 0282628–4588250 1550 GSK Cold spring water 0283484–4586187 1712 CERDERY Surface water 0281280–4586418 1490 CERDERA Surface water 0282940–4585958 1496 Gültekin et al. Geotherm Energy (2019) 7:12 Page 11 of 26 Table 2 Physical and chemical properties of waters in Şavşat Geothermal Field (concentrations are in mg/L) +2 +2 + + − Sample name T (°C) pH EC (µS/cm) TDS DO (mg/L) Ca Mg Na K HCO ILICAS May 16 39.30 6.64 6207 3224 0.47 293.34 39.73 1611.70 39.35 2511.00 Oct 16 38.80 6.90 5734 3044 2.19 288.00 46.35 1323.00 35.14 2621.00 March 17 35.08 6.93 5414 2950 2.13 273.41 42.55 1432.70 36.36 2379.00 July 17 36.60 6.85 5570 3002 1.70 281.00 49.60 1501.71 38.37 2571.56 Mean 37.45 6.83 5731 3055 1.62 283.94 44.56 1467.28 37.31 2490.64 CDMS May 16 12.00 6.20 2741 2369 2.98 369.40 60.07 1003.50 22.28 2379.00 March 17 10.60 6.57 2859 2562 3.00 440.35 62.38 920.51 30.67 2562.00 July 17 11.40 6.50 3985 2589 2.00 461.85 65.62 1030.24 31.57 2691.17 Mean 11.33 6.42 3195 2507 2.66 423.87 62.69 984.75 28.17 2544.06 GMS July 17 13.00 7.58 522 341 1.50 74.71 41.75 62.68 2.79 550.19 SSSK May 16 9.22 7.59 155 144 9.94 55.75 8.07 11.46 0.08 191.30 Oct 16 9.00 8.14 171 156 10.50 54.72 8.77 11.63 0.14 195.00 March 17 8.00 8.14 174 165 8.80 52.81 8.41 13.11 0.12 189.10 July 17 11.30 8.00 226 146 10.00 56.09 9.02 13.29 0.15 221.27 Mean 9.38 7.97 181 153 9.81 54.84 8.57 12.37 0.12 199.17 GSK July 17 8.04 7.87 274 178 10.00 56.38 13.84 20.99 0.06 269.12 CERDERY May 16 8.64 7.92 122 116 10.20 43.15 6.55 13.40 0.42 155.40 Oct 16 6.90 8.20 155 150 10.96 48.37 9.60 17.93 0.71 214.00 March 17 2.60 7.13 111 126 13.00 38.91 7.38 14.84 0.43 140.30 July 17 21.00 8.18 228 148 8.45 51.04 9.85 20.36 0.63 191.37 Mean 9.79 7.86 154 135 10.65 45.37 8.35 16.63 0.55 175.27 CERDERA May 16 13.80 8.53 411 340 9.37 50.90 9.07 116.46 2.91 251.00 Oct 16 10.00 8.30 787 715 11.00 54.00 19.31 300.00 7.09 439.00 March 17 7.18 7.38 404 394 11.00 50.97 8.81 133.61 3.03 262.30 July 17 18.50 8.32 875 569 8.00 70.80 18.90 203.72 4.81 370.80 Mean 12.37 8.13 619 505 9.84 56.67 14.02 188.45 4.46 330.78 Gültekin et al. Geotherm Energy (2019) 7:12 Page 12 of 26 Table 2 (continued) −2 −2 − − − − Sample name CO SO Cl NO NO F Li Br SiO B 3 4 2 3 2 ILICAS May 16 0.00 190.00 1367.00 < 0.01 < 0.01 2.59 1.14 1.54 125.80 25.57 Oct 16 0.00 166.00 1052.00 0.00 0.00 2.10 0.91 0.66 87.80 16.32 March 17 0.00 179.16 1184.50 0.00 2.65 0.90 0.00 96.74 18.40 July 17 0.00 155.36 1127.34 0.00 0.78 2.06 0.94 1.24 80.27 115.70 Mean 0.00 172.63 1182.71 0.00 0.26 2.35 0.97 0.86 97.65 44.00 CDMS May 16 0.00 150.00 772.00 < 0.01 0.75 0.63 0.27 1.59 117.00 22.93 March 17 0.00 172.42 821.13 0.00 0.00 0.00 0.32 0.00 71.94 23.87 July 17 0.00 153.92 831.69 0.28 0.86 0.37 0.36 0.44 75.70 142.15 Mean 0.00 158.78 808.27 0.28 0.54 0.33 0.32 0.68 88.21 62.98 GMS July 17 0.00 7.35 3.19 0.02 0.19 0.98 0.02 0.02 26.40 < 1 SSSK May 16 0.00 4.79 1.25 < 0.01 0.98 0.06 < 0.01 < 0.01 8.91 < 0.5 Oct 16 12.00 7.59 1.56 0.00 1.07 0.06 0.00 0.00 12.30 0.69 March 17 12.00 11.34 2.38 0.00 1.11 0.09 0.00 0.00 11.15 < 1 July 17 0.00 5.16 0.89 0.00 0.94 0.05 0.00 0.01 11.35 < 1 Mean 6.00 7.22 1.52 0.00 1.03 0.07 0.00 0.00 10.93 0.69 GSK July 17 0.00 3.68 0.89 0.02 0.88 0.04 0.00 0.02 40.80 < 1 CERDERY May 16 0.00 4.93 1.13 < 0.01 0.22 0.10 < 0.01 < 0.01 16.27 < 0.5 Oct 16 0.00 5.67 1.62 0.00 0.01 0.21 0.00 0.00 20.33 < 1 March 17 18.00 5.59 1.03 0.00 0.43 0.12 0.00 0.00 19.20 < 1 July 17 23.53 4.04 0.83 0.00 0.01 0.18 0.00 0.00 22.15 < 1 Mean 10.38 5.06 1.15 0.00 0.17 0.15 0.00 0.00 19.49 < 1 CERDERA May 16 0.00 17.68 83.69 0.05 0.05 0.27 < 0.01 0.10 30.00 < 0.5 Oct 16 24.00 46.00 250.00 0.00 0.00 0.55 0.18 0.18 32.98 3.91 March 17 36.00 23.74 103.94 0.00 0.11 0.46 0.08 0.00 21.41 < 1 July 17 23.53 32.43 191.70 0.00 0.18 0.35 0.10 0.20 27.61 3.01 Mean 20.88 29.96 157.33 0.02 0.09 0.41 0.12 0.12 28.00 3.46 Gültekin et al. Geotherm Energy (2019) 7:12 Page 13 of 26 Table 3 Classification of the waters in Şavşat Geothermal Field according to IAH (1979) Sample name Definitions Sampling date Water type ILICAS Thermal well water May 16 Na–HCO –Cl October 16 Na–HCO –Cl March 17 Na–HCO –Cl July 17 Na–HCO –Cl CDMS Mineral water May 16 Na–Ca–HCO –Cl March 17 Na–Ca–HCO –Cl July 17 Na–Ca–HCO –Cl GMS Mineral water July 17 Ca–Mg–Na–HCO ŞSSK Spring water May 16 Ca–HCO October 16 Ca–HCO March 17 Ca–HCO July 17 Ca–HCO GSK Spring water July 17 Ca–Mg–HCO CERDERY Surface water May 16 Ca–HCO October 16 Ca–Mg–Na–HCO March 17 Ca–Na–HCO July 17 Ca–Na–HCO CERDERA Surface water May 16 Na–Ca–HCO –Cl October 16 Na–HCO –Cl March 17 Na–Ca–HCO July 17 Na–Ca–HCO weak acid compounds (CO + HCO ) than strong acid compounds (Cl + SO ) (Fig.  7). 3 3 4 Thermal waters have compositions similar to those of mineral waters, although hot water has higher ion concentrations (Fig. 8). The processes that affect the major ion concentrations of thermal water in the ŞGF were evaluated according to Hounslow (1995). The ratios of HCO /SiO and Mg/ 3 2 (Ca + Mg) are 67.4 and 0.22, respectively. In addition, the ratio of SiO /(Na + K–Cl) is 0.018, and the ratio of (Na + K–Cl)/(Na + K–Cl + Ca) is 0.72. According to these values, carbonate decomposition, cation exchange, and plagioclase decomposition processes control the major ion concentrations of the thermal water. Mineral saturation The change in the mineral saturation states in water helps to determine the stages of hydrochemical evolution and is important in terms of which chemical reactions have effects on water chemistry (Drever 1997; Langmuir 1997). Especially for thermal and mineral waters, the early estimation of scaling and corrosion properties is very impor- tant in terms of preventing residue that may occur during the use of the waters. Addi- tionally, chemical reactions occurring in groundwater provide an opportunity to interpret the hydrochemical environment. Primary and secondary minerals were microscopically determined in the rocks outcropping in the study area, and the saturation of all waters sampled in terms of these minerals was investigated (Table 4, Fig. 9). The primary minerals in the volcanic rocks exposed in the study area are silicate minerals such as plagioclase, K-feldspar, <=Ca + Mg Gültekin et al. Geotherm Energy (2019) 7:12 Page 14 of 26 ILICAS1-May16 ILICAS1-May16 ILICAS May16 ILICAS May16 80 80 ILICAS Oct1 ILICAS Oct16 6 ILICAS Mar17 ILICAS Mar17 60 60 ILICAS Jul17 ILICAS Jul17 SSSK-May16 SSSK-May16 SSSK-Oct16 SSSK-Oct16 40 40 SSSK-Mar17 SSSK-Mar17 SSSK-Jul17 SSSK-Jul17 20 20 GSK-Jul1 GSK-Jul17 7 CDMS-May16 CDMS-May16 CDMS-Mar17 CDMS-Mar17 Mg SO4 CDMS-Jul1 CDMS-Jul17 7 GMS-Jul1 GMS-Jul17 7 CERDERY-May16 CERDERY-May16 80 80 CERDERY-Oct1 CERDERY-Oct16 6 CERDERY-Mar17 CERDERY-Mar17 60 60 CERDERY-Jul17 CERDERY-Jul17 CERDERA-May16 CERDERA-May16 40 40 CERDERA-Oct1 CERDERA-Oct16 6 CERDERA-Mar17 CERDERA-Mar17 20 20 CERDERA-Jul17 CERDERA-Jul17 Ca Na+K HCO3+CO3 Cl Fig. 7 Piper diagram for thermal and cold waters in the study area 100.00 ILICAS1-May16 ILICAS1-May16 ILICAS May16 ILICAS May16 ILICAS Oct1 ILICAS Oct16 6 ILICAS Mar17 ILICAS Mar17 ILICAS Jul17 ILICAS Jul17 SSSK-May16 SSSK-May16 SSSK-Oct16 SSSK-Oct16 10.00 SSSK-Mar17 SSSK-Mar17 SSSK-Jul17 SSSK-Jul17 GSK-Jul17 GSK-Jul17 CDMS-May16 CDMS-May16 CDMS-Mar17 CDMS-Mar17 CDMS-Jul17 CDMS-Jul17 1.00 GMS-Jul17 GMS-Jul17 CERDERY-May1 CERDERY-May16 6 CERDERY-Oct1 CERDERY-Oct16 6 CERDERY-Mar1 CERDERY-Mar17 7 CERDERY-Jul1 CERDERY-Jul17 7 CERDERA-May1 CERDERA-May16 6 CERDERA-Oct1 CERDERA-Oct16 6 0.10 CERDERA-Mar1 CERDERA-Mar17 7 CERDERA-Jul1 CERDERA-Jul17 7 0.01 Ca Mg Na+K Cl SO4 HCO3 Parameters Fig. 8 Schoeller diagram indicating ionic concentrations of waters in the study area Cl + SO4=> Concentration (meq/l) Gültekin et al. Geotherm Energy (2019) 7:12 Page 15 of 26 Table 4 Mineral saturation indices (SI) for thermal waters in the Şavşat Geothermal Field Minerals Formula ILICAS CDMS SSSK CERDERY Albite NaAlSi O 1.75 1.48 − 3.01 − 1.90 3 8 Anhydrite CaSO − 1.47 − 1.47 − 3.15 − 3.22 Aragonite CaCO 0.69 0.00 − 0.20 − 0.07 Barite BaSO 0.41 0.82 – − 1.41 Ca-montmorillonite Ca0.165Al 33Si 67O (OH) 6.40 7.10 1.91 2.78 2 3 10 2 Calcite CaCO 0.83 0.16 − 0.04 0.09 Celestine SrSO − 1.94 − 3.49 − 4.17 − 4.07 Dolomite CaMg(CO ) 1.28 − 0.32 − 0.82 − 0.54 3 2 Fluorite CaF − 0.23 − 0.94 − 3.27 − 2.91 Gypsum CaSO :2H O − 1.33 − 1.21 − 2.90 − 2.96 4 2 Goethite FeOOH 6.24 3.38 7.54 8.10 Halite NaCl − 4.40 − 4.77 − 9.37 − 9.33 Hematite Fe O 14.55 8.70 17.01 18.13 2 3 Illite K0.6Mg0.25Al 3Si 5O (OH) 5.49 5.54 0.12 1.63 2 3 10 2 K-feldspar KAlSi O 2.31 2.33 − 2.62 − 0.85 3 8 K-mica KAl Si O (OH) 12.28 12.20 5.95 7.54 3 3 10 2 Kaolinite Al Si O (OH) 6.72 7.41 3.93 4.30 2 2 5 4 Amorphous silica SiO 0.78 1.15 0.07 0.34 Rhodocrosite MnCO − 1.32 − 2.51 – – Siderite FeCO − 0.60 − 1.37 − 0.88 − 1.04 Talc Mg Si O (OH) − 1.84 − 7.00 − 5.26 − 2.55 3 4 10 2 Witherite BaCO − 2.63 − 3.23 – − 3.80 -5 -10 -15 ILICAS CDMS SSSK CERDERY Fig. 9 Mineral saturation indices (SI) for selected minerals in the Şavşat Geothermal Field augite, hornblende, and quartz, while secondary minerals are calcite, quartz, and chlorite. The SI was calculated for minerals selected from the limestone, sandstone, gypsum lenses, and opaque minerals forming sedimentary units using the AquaChem chemical equilibrium software. Negative SI values in Table  4 indicate an undersatu- rated solution, and positive values indicate an oversaturated solution. SI values that are meaningless (e.g., − 40) were ignored. When the SI values were examined, all hot and cold waters were not saturated with respect to sulfate minerals such as anhydrite Saturaon Indi ces (SI) Albite Anhydrite Aragonite Barite Ca-Montmorillonite Calcite Celeste Do lomite Fluorite Gypsum Goethite Halite Hemate Illite K-feldsp ar K-mica Kaolinite Amorph ous Silica Rhodochrosite Siderite Talc With erite 20 Gültekin et al. Geotherm Energy (2019) 7:12 Page 16 of 26 Na/1000 ILICAS1-May16 ILICAS1-May16 ILICAS May16 ILICAS May16 ILICAS Oct1 ILICAS Oct16 6 ILICAS Mar17 ILICAS Mar17 ILICAS Jul1 ILICAS Jul17 7 Full Equilibrium PartiallyEquilibrated and MixedWaters Im mature Waters K/100 Sqr(Mg) Fig. 10 Na–K–Mg ternary diagram (Giggenbach 1988) for the thermal waters of the Şavşat Geothermal Field and gypsum and minerals such as fluorite and halite. The geothermal water (ILICAS) and Ciritdüzü mineral water (CDMS) were saturated with respect to aragonite, cal- cite, and dolomite, while the cold spring waters were undersaturated in these min- erals. The silicate mineral K-feldspar was oversaturated in the hot water and CDMS but undersaturated in the cold waters. Both thermal water and mineral water showed slight oversaturation in amorphous silica. All waters were oversaturated with respect to K-mica, clay minerals of kaolinite and illite, and iron minerals of goethite and hem- atite, but they were undersaturated with respect to talc. Geothermometers The estimation of the aquifer temperature in geothermal systems is very important in terms of the appropriate use of thermal and mineral waters. With the aim of defining the reservoir rock temperature in geothermal areas, geothermometry methods were developed based on chemical (Fournier 1977; Arnorsson et  al. 1983) and isotopic (Lloyd 1968; Mizutani and Rafter 1969) analyses of springs and wells. Another method used is the Na–K–Mg combined geothermometer developed by Giggenbach (1988) for estimating the aquifer temperature of thermal waters and determining the maturity of rocks in contact with the water (Fig.  10). This geother - mometer may be used to test the validity of cation geothermometers and rapidly interpret the reservoir rock temperatures of thermal waters. On the Giggenbach (1988) diagram, the thermal water from the Şavşat Geothermal Field is plotted in “the 80 Gültekin et al. Geotherm Energy (2019) 7:12 Page 17 of 26 Table 5 Estimated reservoir temperatures (°C) for  the  Şavşat thermal waters using silica geothermometers (SiO = 125.8 mg/L) Geothermometers Equations References Calculated temp. SiO (ά Cristobalite) t = 1000/(4.78 − logSiO ) − 273.15 Fournier (1977) 100 2 2 SiO (Quartz) t = 1309/(5.19 − logSiO ) − 273.15 Fournier (1977) 150 2 2 SiO (Quartz steam loss) t = 1522/(5.75 − logSiO ) − 273.15 Fournier (1977) 144 2 2 SiO (Quartz steam loss) t = 1264/(5.31 − logSiO ) − 273.15 Arnorsson et al. (1983) 121 2 2 SiO (Quartz steam loss) t = 1164/(4.9 − logSiO ) − 273.15 Arnorsson et al. (1983) 143 2 2 SiO (Chalcedony) t = 1032/(4.69 − logSiO ) − 273.15 Fournier (1977) 125 2 2 SiO (Chalcedony) t = 1112/(4.91 − logSiO ) − 273.15 Arnorsson et al. (1983) 122 2 2 partly matured water” area. Generally, Na/K geothermometers provide confirmatory results at temperatures between 180 and 350  °C and may provide erroneous results below 120 °C. At these low temperatures, Na and K ions are affected by clay minerals and do not control ion exchange reactions. Thus, the values above the actual aquifer temperatures were obtained by geothermometer calculations (Gemici 1999), and sil- ica geothermometers were used to calculate the reservoir temperatures in the studied geothermal field. Calculated reservoir temperatures close to or below the discharge temperature of the geothermal waters were ignored. The quartz geothermometer gave reservoir temperatures ranging from 121 to 150 °C for the ŞGF, while the chalcedony geothermometer yielded 122–125  °C reservoir temperatures (Table  5). At low tem- peratures, the dissolved silica concentration controls chalcedony; however, the quartz geothermometer provides better results at higher temperatures. Therefore, the esti - mated reservoir temperature of the ŞGF was 120–125 °C, according to the chalcedony geothermometer (Table 5). The reservoir temperatures calculated by the SO –H O oxygen isotope geother- 4 2 mometers proposed by Lloyd (1968) and Mizutani and Rafter (1969) were 60–70  °C for the ŞGF, which were lower than those calculated by the silica geothermometers. The reason for this is the change in δ O values due to the mixing of sulfate-poor shallow groundwater with geothermal waters. Enthalpy–silica mixture model The hot fluid component of the geothermal system mixes with cold groundwater at dif - ferent depths and rates while traveling to the surface. The Şavşat water is determined to be peripheral water based on the Cl–SO –HCO diagram (Fig. 11). A variety of mixing 4 3 models have been developed to determine the reservoir temperature and mixing ratio in geothermal systems (Fournier and Truesdell 1974; Şahinci 1991). The most commonly used models are the silica–enthalpy and enthalpy–chloride mixing models. The silica– enthalpy mixing model diagram may be used to estimate reservoir temperatures in situ- ations with no loss of steam and temperature before mixing and loss of steam before mixing (adiabatic cooling). This mixing model is used to determine the reservoir tem - perature of geothermal fields and evaluate the effects of the mixing processes (Truesdell and Fournier 1977). Figure 12 shows the silica–enthalpy mixing model based on quartz solubilities. In this model, two end member fluids are presented: a cold water sample Gültekin et al. Geotherm Energy (2019) 7:12 Page 18 of 26 CI Savsat geothermal water Steam Heated Waters HCO SO Fig. 11 Cl–SO –HCO ternary diagram for geothermal waters 4 3 Fig. 12 Silica–enthalpy mixture model for Şavşat Geothermal Water Gültekin et al. Geotherm Energy (2019) 7:12 Page 19 of 26 (SSSK sample; temperature: 9.38 °C) as one endmember and the thermal waters (ILICAS sample; temperature: 37.45 °C) as the other endmember. In this model, thermal waters form as the result of the mixing of thermal water with cold water, assuming maximum steam loss. The intersection point with the solubility curve for quartz (maximum steam loss) yields a reservoir temperature of 195  °C for the ŞGF thermal water. The mixing rate with cold groundwater is calculated to be 70% for the ŞGF. This value is higher than the reservoir temperature calculated by the silica geothermometers. The ŞGF thermal waters may lose some heat due to possible mixing with cold water along the fracture zones during its ascent to the surface. Therefore, the silica geothermometer appears to reflect reservoir temperatures more accurately than the other geothermometers. Isotope studies Oxygen-18 and deuterium stable isotopes were employed to determine the possible recharge areas of water, and tritium was employed to calculate the relative age of the waters and their residence times. Carbon-13 was used to determine the origin of car- bon as well as oxygen-18 and sulfur-34 isotopes in sulfate in the waters (Table 6). 18 2 18 2 δ O and  δ H relationships The δ O and δ H values in the waters were evaluated according to the Global Meteoric Water Line (GMWL) (Craig 1961) and the Eastern Black Table 6 Isotope analysis results for  water samples collected in  the  Şavşat Geothermal Field 18 13 34 18 Sample name Date Definitions δD δ O T (TU) δ C δS (SO ) δO (SO ) 4 4 V-SMOW V-SMOW V-PDB VCDT V-SMOW ILICAS May 16 Geothermal − 100.38 − 13.03 0.74 4.31 10.6 9.6 water October 16 − 97.99 − 12.84 2.49 March 17 − 97.14 − 13.24 0.90 7.70 10.9 11.6 July 17 − 100.36 − 13.03 1.11 CDMS May 16 Mineral water − 94.81 − 12.4 3.59 7.65 7.7 6.7 March 17 − 95.78 − 12.45 3.13 6.4 6.0 July 17 − 94.88 − 12.48 1.95 9.55 GMS July 17 Mineral water − 96.29 − 13.39 SSSK May 16 Cold spring − 88.19 − 12.95 4.67 − 15.4 3.4 0.0 water October 16 − 89.96 − 12.67 4.37 March 17 − 86.58 − 12.33 4.35 − 12.36 4.3 1.7 July 17 − 88.38 − 12.59 5.70 GSK July 17 Cold spring − 88.17 − 12.60 water CERDERY May 16 Surface water − 86.37 − 12.21 6.0 − 13.82 5.2 − 0.7 October 16 − 82.32 − 11.78 4.29 March 17 − 90.60 − 12.50 6.35 − 7.79 3.5 0.8 July 17 − 88.15 − 12.40 5.25 CERDERA October 16 Surface water − 85.46 − 11.95 5.97 March 17 − 90.59 − 12.65 3.81 July 17 − 88.30 − 12.31 4.08 SAVSTKAR March 17 Snow − 120.97 − 17.56 5.77 Gültekin et al. Geotherm Energy (2019) 7:12 Page 20 of 26 -80 -80 -90 -85 -100 -90 -110 -95 -100 -120 -105 -130 -13.5 -13.1 -12.7 -12.3 -11.9 -11.5 -18.0 -17.0 -16.0 -15.0 -14.0 -13.0 -12.0 Oxygen-18 (‰) Oxygen-18 (‰) ILICAS1-May16 SSSK-May16 CDMS-May16 CERDERY-May16 CERDERA-May16 SNOW-Mar17 ILICAS May16 SSSK-Oct16 CDMS-Mar17 CERDERY-Oct16 GMWL CERDERA-Oct16 ILICAS Oct16 DKMWL SSSK-Mar17 CERDERY-Mar17 CDMS-Jul17 CERDERA-Mar17 ILICAS Mar17 SSSK-Jul17 GMS-Jul17 CERDERY-Jul17 EVPL CERDERA-Jul17 ILICAS Jul17 GSK-Jul17 18 2 Fig. 13 δ O–δ H diagram for waters in the Şavşat Geothermal Field (GMWL Global Meteoric Water Line, DKMWL Eastern Black Sea Meteoric Water Line, EVPL Evaporation Line) 2 18 Sea Meteoric Water Line (DKMWL: δ H = 8 δ O + 16) (Ekmekçi and Gültekin 2015). 18 2 The cold spring and surface waters in the Şavşat field have δ O and δ H values between the GMWL and DKMWL (Fig.  13). The surface waters, spring waters, and geothermal 18 2 waters plot in different areas. The δ O and δ H values of the snow sample are very dif- ferent from those of the waters. It is determined that the cold spring and surface waters are recharged by precipitation falling at much lower elevations in the basin compared to the snow. The DKMWL is considered, and the δ O value of the geothermal water shifts to more positive values. This is due to water–rock interactions. The cold spring water is recharged from higher elevations compared to the surface waters. The CDMS has a more positive δ O compared to the values of the surface waters, which indicates a longer dura- tion of interaction with rocks (Fig. 14). Carbon isotope ( C) Water filtering underground dissolves CO in soil and differen - − −2 tiates it into HCO and CO species. The distribution of dissolved inorganic carbon 3 3 (DIC) species varies with respect to pH. The variations in DIC and δ C also stem VPDB from changes in pH values (Clark and Fritz 1997). The dissolved inorganic carbon C DIC and δ C of groundwater develop as a result of differentiation reactions in aquifers VPDB or soil. The δ C isotopes were used to determine the source of carbon in the samples. 13 13 Analyses were carried out on DIC for δ C (Table 6). The δ C value of geothermal VPDB water in the study area was determined to be 4.31‰ (Table 6). Dissolved inorganic carbon in geothermal water in the study area originates from freshwater carbonates and meta- morphic CO (Clark and Fritz 1997). For CDMS, the δ C value is similar to that of 2 VPDB geothermal water, while other cold waters have negative values. The source of carbon in cold waters might be a mixture of freshwater carbonates, groundwater DIC, and soil C O (Clark and Fritz 1997). Deuterium (‰) Deuterium (‰) Gültekin et al. Geotherm Energy (2019) 7:12 Page 21 of 26 3 3 − 3 Fig. 14 H–EC, H–Cl , and H–T (°C) relations for waters in the study area 34 −2 Sulfur isotope ( S) Sulfur is found in the crust as dissolved SO, SO, H S, and S O 4 2 2 2 species. Organic sulfur is found in humic materials, kerogen, and hydrocarbons. Sul- fur-34 is partitioned into many different sulfur compounds. Similarly, the oxygen-18 con - tent of sulfate is an important tool for tracing the sulfur cycle (Clark and Fritz 1997). Val- ues exceeding − 20‰ are related to limestones and evaporites. The sulfur-34 ratio in the oxidation of juvenile sulfur is generally between − 5‰ and + 5‰ (Clark and Fritz 1997). Negative sulfur-34 values occur in diagenetic environments where typically reduced sul- −2 34 fur compounds are present (Krouse 1980). The results of the dissolved SO ion S iso- tope analysis for the study area are given in Table 6. The S for the geothermal water CDT in the Şavşat Geothermal Field is nearly 10‰. In cold waters, the S value varies from CDT 10 to 3‰. According to Krouse (1980), these values show that the sulfur in the geother- mal water is derived from volcanic sulfur (S O ) and from Cenozoic-age C aSO , while the 2 4 sulfur in the cold water is derived from magmatic rocks. Tritium content The tritium values for the geothermal waters vary from 0.74 to 2.59 TU (Table 6). In mineral water (CDMS), it is nearly 3 TU, while in cold spring and 3 3 − 3 surface waters, it varies from 3 to 6 TU. The H–Eİ, H–Cl , and H-temperature cor- Gültekin et al. Geotherm Energy (2019) 7:12 Page 22 of 26 relations for the water samples are given in Fig. 14. For the geothermal water, mineral water, cold spring water, and surface water, there is a significant negative correlation between the tritium and EC values. Geothermal waters with low tritium values are deeply circulated waters, and the residence time is longer than those of mineral water and cold spring water. Şavşat (Artvin‑Turkey) Geothermal System A hydrogeological conceptual model was developed based on geological, hydro- chemical, isotopic, and geophysical studies in the study area (Fig.  15). The ŞGF is a liquid-dominated geothermal system. The ŞGF is recharged by infiltrating meteoric waters from the Kabaköy Formation and Bülbülan Formation in the north-northeast. The northern bounding normal fault (F2) and formation boundaries in the northeast (Görizil Hill) might be conduits for water flow. In addition, the NE–SW trending anti - cline axis is also suitable for the circulation of water. The old thermal springs emerged along the anticline axis and faults. Drilling and hydrochemical studies indicate that the reservoir rock is volcanogenic sandstone and augite basalt-type volcanic rocks. The porosity of the volcanogenic sandstone (Kabaköy Formation) is 7–10%, and that of the augite basalts is 3–5%. Additionally, volcanogenic sandstones have gained secondary porosity along bed- ding planes, fractures, and joints in the sandstone and augite basalts. This feature has allowed the unit to gain properties conducive to the storage and circulation of water. Volcanogenic sandstones with reservoir features widely outcrop in the field. As a result, there is no cap rock fully enclosing the system. However, in areas where the Şavşat Formation displays turbiditic features composed of mudstone–siltstone–sand- stone alternations, it forms a cap rock for the geothermal system. Şavşat thermal waters are controlled by both the regional and the local flow sys - tems, and their chemical and isotopic compositions are attributed to mixing with cold 18 2 shallow groundwater during their ascent to the surface. The δ O and δ H isotopes of the thermal water show more negative values compared to those of the cold waters and more positive values compared to those of the snow. The δ O values of the ther- mal water show a slightly positive shift as a result of water–rock isotope exchange. The tritium values of the thermal water are much lower than those of both the cold water and the snow samples. According to these data, geothermal waters form when meteoric water falls as rain into the basin and is transmitted underground, stored in volcanogenic sandstones, and heated up by the geothermal gradient. The heated water reaches the surface by rising along a fracture zone determined in geophysical stud- ies as being oriented parallel to the Çermik Stream valley (Fig.  15). According to the δ C values, the carbon in the thermal water has a metamorphic origin, whereas the carbon in the cold water is derived from the dissolution of Oligocene–Miocene lacus- trine carbonate deposits in the region or from CO gas that accumulated in pores. The δ S values show a volcanic origin for sulfur in the geothermal water. CDMS is mineralized cold water outside the hydrological basin of the ILICAS thermal spring; it is very similar to thermal water in terms of its chemical and isotopic compositions. However, due to lower EC and TDS values, it can be defined as a mixing water. Gültekin et al. Geotherm Energy (2019) 7:12 Page 23 of 26 Fig. 15 Hydrogeological conceptual model for Şavşat (Artvin/Turkey) Geothermal Field Gültekin et al. Geotherm Energy (2019) 7:12 Page 24 of 26 According to Türkecan (2017), volcanic rocks outcropping in wide areas in Kars and Ardahan are late Pliocene–early Pleistocene (Duru and Keskin 2014) andesite, dacite, and rhyolite. Andesitic and dacitic lava flows form ridges and domes, but dacitic rocks also occasionally occur as lava flows. These volcanic rocks, named the Ardahan andesite (Karaköse et  al. 1994), Ulgartepe andesite (Karaköse et  al. 1994), or Dumanlıdağ volcanic rocks (Aktimur et  al. 1982), have been dated at 1.6–2.7 mil- lion years with a variety of methods (Innocenti et al. 1982; Karaköse et al. 1994). The volcanic rocks represented by trachyandesite, trachybasalt, hornblende andesite, and pyroclastics outcrop as ridges and domes in the Şavşat Geothermal Field and can be correlated with the Pliocene–Pleistocene volcanic rocks based upon their petro- graphic properties (Fig. 15). The young volcanic activity in the area caused an increase in the geothermal gradient. Therefore, the heat source for the geothermal system is considered to be this young volcanic activity. The δ S values of the geothermal waters also support this idea. The reservoir temperature for the ŞGF was calculated as 100–150  °C by the silica geothermometer. According to the calculated reservoir temperature, the field is clas - sified as a low-moderate enthalpy field (Muffler and Cataldi 1978; Benderitter and Cormy 1990; Hochstein 1990; Haenel et  al. 1988). Magmatic activity feeding young volcanic rocks in the field was interpreted as a source of heat. Deeply heated water associated with this magmatic activity was driven upward along the faults and fractures. Conclusions The Şavşat (Artvin-Turkey) Geothermal Field contains outcrops of volcanic, volcano sedimentary, and sedimentary units formed during the late Cretaceous to the Pliocene– Pleistocene time periods. The artesian water recharging from a 120-m-depth borehole in the ŞGF has a temperature of 38  °C, an EC value of 5731  µS/cm, and a pH of 6.83. The Na–HCO –Cl-type geothermal water is oversaturated with respect to aragonite, calcite, dolomite, amorphous silica, K-mica, kaolinite, talc, and hematite minerals and undersaturated with respect to minerals such as gypsum, anhydrite, halite, fluorite, rho - dochrosite, and siderite. The reservoir temperature is estimated at 100–150  °C using a silica geothermometer. The mixing rate of geothermal waters with cold groundwater is calculated to be 70%. In deeply circulated geothermal waters, carbon is derived from metamorphic CO , and sulfur is of volcanic origin. The cold waters indicate an origin of groundwater DIC. The reservoir rocks of the ŞGF are volcanogenic sandstone and late Cretaceous vol - canic rocks of andesite, basalt, and pyroclastics. The heat source is the geothermal gradi - ent arising from Quaternary volcanic activity. The late Eocene turbiditic unit comprising mudstone, siltstone, and marl forms a partial cap rock. Geophysical studies in the field identified a potential thermal water reservoir at elevations of 1250–1350 m. At borehole of at least 300 m may intersect an area of fluids with more pressure and higher tempera - tures than the present well and spring water. Gültekin et al. Geotherm Energy (2019) 7:12 Page 25 of 26 Abbreviations ŞGF: Şavşat Geothermal Field; T: temperature; EC: electrical conductivity; TDS: total dissolved substance; DO: dissolved oxygen; GMWL: Global Meteoric Water Line; DKMWL: Eastern Black Sea Meteoric Water Line; DIC: dissolved inorganic carbon. Authors’ contributions FG conducted hydrogeological studies and contributed significantly to the writing of the paper. EHT conducted hydro - geochemical studies. AEB conducted and commented on geophysical studies. MZK conducted geological studies and described the rocks type. AFE participated in hydrogeological studies. BMS participated in geological mapping studies and sampling. All authors read and approved the final manuscript. Author details 1 2 Geological Engineering Department, Karadeniz Technical University, Trabzon, Turkey. Geophysical Engineering Depart- ment, Karadeniz Technical University, Trabzon, Turkey. Acknowledgements This study was supported by The Scientific and Technical Research Council of Turkey ( TUBITAK Project Number: 115Y142). The authors thank TUBİTAK for their financial support. The authors also thank Prof. Dr. Necati TÜYSÜZ from the KTÜ for his help with the English of the final text. We wish to thank the Executive Editor Prof. Dr. Ernst Huenges, three anonymous reviewers. Competing interests The authors declare that they have no competing interests. Availability of data and materials Not applicable. Funding All of this study was prepared by data of TÜBİTAK funded project numbered 115Y142. Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. 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Published: May 6, 2019

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