Hydrogeological modelling of geothermal waters in Pamukkale, western Anatolia, Turkey

Nevzat Özgüra, İlke Kıymazb, Emre Uzunb, Duygu Sengül Kutlub

a Department of Geological Engineering, Faculty of Engineering, Süleyman Demirel University, Isparta, Turkey

b Graduate School of Applied and Natural Sciences, Süleyman Demirel University, Isparta, Turkey


Abstract

The study area, located in the eastern part of the continental rift zone of the Büyük Menderes within the Menderes Massif, western Anatolia, is composed of Paleozoic metamorphic rocks, Mesozoic limestones and Eocene via Pliocene to Quaternary sediments. Paleozoic marbles, quartzites and carbonate schiata, Mesozoic limestone, Pliocene sediments and Quaternary alluvium and travertine serve as permeable rocks for the geothermal waters. The geothermal waters in Pamukkale and environs with outlet temperatures of about 35 °C and reservoir temperatures up to 250 °C can be considered as Ca-Mg-SO4-HCO3 type waters. The formation of the travertine in Pamukkale is one of the world’s wonders, directly connected with decreasing temperatures and CO2 partial pressures. The formation of travertine deposits depends upon the solubility of CaCO3 controlled principally by CO2 partial pressure, temperature and pH values, in which reaction equilibriums play an important role. Moreover, the travertine deposits, which show a U-series age of at least 400 ka form one of the important world wonders. The geothermal waters of Pamukkale, with its high sulphate contents up to 650 mg/l and Rn concentrations up to 20 Bq/l, were modelled hydrogeologically from schematical points.


Introduction

In Turkey, geothermal waters are located in large areas in connection with tectonic features and volcanism from the Middle Miocene to recent in age. The high-enthalpy geothermal waters form in the continental rift zones of the Menderes Massif, which suffered compression and later extension tectonics. The study area of Pamukkale is located 20 km NW of the provincial capital of Denizli (Figures 1 and 2) which is 360 m above sea level. The geothermal waters of Pamukkale are located in the southern shoulder of Çökelez Mountain, in the travertine platform and within the travertine mass forming an unrivalled example in the world. The area of travertine and the antique ruins of the Hierapolis City form an important centre due to its original natural structures and historical value. The study area of Pamukkale has an area of 44 km2 and is a Special Environment Protection Region with five residential areas: Develi, Karahayit, Pamukkale, Yeniköy and Akköy. The aim of this study is (i) to update geological mapping of the geothermal areas in Pamukkale and environs, (ii) to describe the water-rock interaction by mineralogical, petrographical and geochemical working methods, (iii) to investigate the formation and development of geothermal waters by hydrogeological, hydrogeochemical and isotope geochemical methods and (iv) to create an conceptual hydrogeological model of the Pamukkale geothermal field.


Figure 1: Geological map of the Menderes Massif and the location of the study area of Pamukkale. 1: Mercury deposit of Haliköy, 2: Antimony deposit of Emirli, 3: Arsenopyrite and gold deposit of Küre (modified from Özgür, 1998).


Figure 2: Geothermal waters in the rift zone of the Büyük Menderes and the location of Pamukkale (modified from Özgür, 1998).


Material and methods

In order to understand the hydrogeochemical features of the geothermal waters in Pamukkale and environs, 16 samples were collected (Kıymaz, 2011; Kutlu, 2015; Uzun, 2017) (Figure 3; Table 1). During this sampling campaign, in-situ measurements such as temperature, pH, Eh, electrical conductivity and alkalinity were taken (Table 1). The cations of Na+, Ca2+, Mg2+, K+, Si4+ and B3+ were analysed by ICP-OES methods, while the analyses of anions such as F, SO42-, Cl and NO3 were performed by IC methods. The values of HCO3 and CO32- have been calculated by the alkalinity measurements in the field. The evaluation of the hydrogeochemical data was carried out using Aquachem 3.7 (Calmbach, 1999).


Figure 3: Sample locations of the geothermal field of Pamukkale (modified from Kıymaz, 2011; Kutlu, 2015; Uzun, 2017).


Table 1: In-situ measurements and hydrogeochemical analyses of the geothermal waters in Pamukkale and environs (Kıymaz, 2011; Kutlu, 2015; Uzun, 2017).

Sample Location T(oC) pH Eh
(mV)
EC

(µS/cm)

Na+
(mg/l)
K+
(mg/l)
Ca+2
(mg/l)
Mg+2
(mg/l)
B+3
(mg/l)
F
(mg/l)
SO4-2
(mg/l)
Cl
(mg/l)
Si+4
(mg/l)
NO3
(mg/l)
HCO3
(mg/l)
EU-1 Plütonyum spring 34.8 6.61 155 2420 42.10 5.52 442 94.10 0.80 1.82 706 14.10 30.10 1.20 1176.1
EU-2 Gelin Hamamı 34.7 6.69 157 2410 42.40 5.60 434 90.40 0.90 1.40 661 14.60 30.50 1.10 1125
EU-3 Beltes spring 34.1 6.91 144.3 2410 42.50 5.45 445 96.10 0.80 1.35 662 14.70 30.40 1.00 1147.3
EU-4 Jandarma spring 34.1 6.96 128.7 2410 42.5 5.45 325 95.50 0.80 1.34 661 12.90 30.50 0.90 1164.2
EU-5 Karahayıt spring 44 6.67 136.8 2540 117 24.3 367 118 1.60 1.85 915 38.8 29.70 5.60 1196.3
DK-1 Pamukkale spring 35 6.56 259.9 2430 44.2 5.45 99.4 401 1.0 0.8 649 13 15.8 1.56 1098
DK-2 Plütonyum spring 35.1 6.44 282.1 2400 40.1 5.08 95.4 479 0.9 1.8 642 14.1 15.2 1.88 1079.7
DK-3 Karahayıt spring 46.6 6.52 63 2680 107 21.1 124 414 1.6 2.2 905 31.8 26.8 0.65 927.2
AB-1 Özel İdare spring 35 6.22 210 2410 48.85 15.55 455.05 69.90 0.71 1.35 624.8 12.29 19.19 0.51 1128.5
AB-2 Jandarma spring 33 6.24 229 2420 42.95 3.10 449.90 71.25 0.46 1.39 611.9 12.64 19.12 0.58 1159
AB-3 Karahayıt spring 52 6.39 113 2790 131.65 21.80 528.5 123.15 0.96 1.88 872.3 27.23 28.94 0.01 1189.5
AB-4 Karahayıt Richmond Hotel 48 6.18 161 2810 124.3 17.25 440.75 95.30 1.60 2.21 879.7 51.61 21.32 0.05 1128.5
AB-5 Karahayıt groundwater 23.9 8.01 287 448 86.4 0.65 3.77 0.48 0.22 0.34 11.12 5.57 5.10 13.53 231.8
AB-6 Gölemezli well 1 67 6.89 194 2420 247.7 52.2 148.75 72 3.40 0.99 377.7 31.41 29.92 <0.01 1159
AB-7 Gölemezli well 2 69 6.69 144 3470 207.85 42.95 555.5 84.25 2.99 1.20 431.9 27.44 24.31 <0.01 2074
AB-8 Gölemezli Hamam 59 6.28 253 4460 431.6 45.05 464.15 109.5 5.74 2.45 1664 70.84 59.03 <0.01 1250.5

 


Results

Geologic setting

Denizli Basin is located in the Aegean Region, where the E-W trending continental rift zone of the Büyük Menderes – limited by active and normal faults – and the NW-SE trending continental rift zone of the Gediz incorporate (Figure 4). In the study area, Paleozoic metamorphic rocks of the Menderes Massif form the basement rocks which are overlain by Mesozoic limestones, Eocene to Pliocene sedimentary rocks and Quaternary alluvium and travertine. The travertine of Pamukkale precipitates on the falling block of the Pamukkale fault, which is located in the eastern part of the basin and constrains the basin in the North (Altunel, 1996). In the areas with intensive fissures of the main fault, intensive precipitations of travertine can be observed. Parallel to oblique fissures were generated in connection with main Pamukkale fault. In the study area, opening fissures were observed, and ridge type travertine has been observed in some of the fissures. With the exception of Pamukkale, the travertine of Denizli can be observed in localities such as Yeniköy, Küçükdereköy, Irlıganlı, Kocabaş, Koyunaliler and Karateke in the eastern direction. The factors affecting the precipitation of travertine are (i) the compositions, saturation indexes and partial CO2 pressures of geothermal and mineral waters, (ii) the temperatures, flow regimes and flow rates of the geothermal waters and (iii) the temperature of the geothermal waters during flow.


Figure 4: Geological map of the geothermal field of Pamukkale and environs (modified from Akkuş et al., 2005).


Hydrogeology

In the study area, Paleozoic marbles, Mesozoic limestones, Pliocene sediments and Quaternary alluviums and travertine occur as permeable rocks in general. Paleozoic marbles can be observed between Karahayıt and Pamukkale and in the NE part of the Pamukkale main springs, whereas Mesozoic limestones occur to the north of Pamukkale main springs. Pliocene sediments are found in the environs of Pamukkale main springs and in the upper part of Yenice horst between Pamukkale and Karahayıt. The Kolonkaya and Tosunlar formations on the first shallow reservoir rock form an intercalation of claystones, marls and sandstones and are good cap rocks for the first reservoir rocks. These cap rocks have a thickness of 350– 600 m.

Hydrogeochemistry

The geothermal waters of Pamukkale and environs can be considered Ca-Mg-(SO4)-HCO3 type waters in the Piper diagram (Figure 5). Hydrogeochemically, the geothermal waters in the study area display the dominant cations Ca>Mg>Na+K and dominant anions HCO3>SO4>Cl.  These show an environmental and shallow origin according to the Cl-SO4-HCO3 ternary diagram (Kutlu, 2015). Accordingly, the geothermal waters have high sulphate contents, which might be attributed to gypsum and pyrite mineral phases in impermeable cap rocks. The waters are immature waters according to the ternary diagram of Na/1000-K/100-ÖMg  (Figure 6; Giggenbach, 1988). Moreover, geochemical thermometers of Na-K and Na-K-Ca show calculated temperatures ranging from 200 to 280 °C in the study area.


Figure 5: Piper diagram of the geothermal waters in Pamukkale and environs.


Figure 6: Na/1000-K-100-ÖMg ternary diagram of the geothermal waters of Pamukkale and environs.


Figure 7: Sinter terrace of the travertine deposits in the study area of Pamukkale.


The abundant travertine deposits of Pamukkale are related to the unusual geological, tectonic and geomorphological setting in the study area (El Desouky et al., 2015; Figure 7). The study area is characterised by abundant carbonate successions in its substratum, which provide the necessary parent carbonate sources for the formation of travertine deposits. According to El Desouky et al. (2015), Miocene to Pliocene subvolcanic activities in the area probably play a major role in the formation of travertine deposits by (1) acting as a heat source for the geothermal fluids, (2) enhancing decarbonation processes in the deep subsurface and (3) contributing to the CO2 source via mantle degassing. The extensional tectonic features associated with the development of the study area cause a network of faults and fissures that enhance circulation of geothermal waters. In the area, the rain fall rates of up to 600 mm and the presence of high mountains (1500–2000 m) ensure the necessary meteoric waters and hydraulic head for the travertine-precipitating geothermal waters. The formation of travertine deposits depends upon the solubility of CaCO3 controlled principally by CO2 partial pressure, temperature and pH values in which reaction equilibriums play an important role:

(1) H2O + COÛ  H2CO3

CO2 is dissolved in waters as H2CO3

(2) CaCO3 + H2COÛ Ca2+ + 2HCO3

The process that increases the CO2 proportion enhances the die solution of CaCO3, whereas each reduction in CO2 proportion preludes the precipitation of CaCO3. Under lower pH values, in which the most carbonates are solved as H2CO3, the reaction proceeds to the right side; under higher pH values, the reaction proceeds to the left side due to the precipitation of CaCO3. CO2 is less soluble in hot waters than in cold waters. The solubility of CaCO3 decreases slightly with increasing temperatures.

(3) CaCO3 + H+ Û Ca2+ + HCO3

HCO3 ions are derived from the reaction of H+ with the carbonates.

For the formation of travertine deposits in Pamukkale and environs, the temperature and CO2 partial pressure are two rival parameters. In Pamukkale, the important parameter is the decrease of CO2 partial pressure, and probably the temperature plays a secondary role. The strong temperature decrease in the ascending geothermal waters increases CaCO3 solubility in waters. Moreover, the pressure release due to escape of CO2 at the surface encourage carbonate precipitation.  In volcanic activities in depth, i.e. volcanic rocks in Denizli (Semiz, 2003), the partial pressure of CO2 in which CaCO3 is solved is high. With the additional carbonate dissolution in the reservoir, CO2 is consumed. However, the CO2 partial pressure decreases insignificantly. Moreover, the geothermal waters are supersaturated due to CaCO3 if the waters reach the reservoir. The carbonates precipitate if temperature equilibrium by the fast ascending geothermal waters does not take place in the same proportion as the pressure decrease at outflow. In addition, it is well known that blue-green seaweeds are involved in carbonate precipitation: the seaweeds extract CO2 from the system in the microenvironment and thus encourage carbonate precipitation as aragonite (Ramon, 1983).

In Pamukkale, the formation of travertine deposits was generated in five phases (Eşder and Yılmazer, 1991):

(1) formation of the Çökelez fault, which strikes in NW-SE direction. The outflow of the geothermal waters is controlled by the faults directly. Today, the travertine deposits of this phase are noticeable in high elevation spheres. For these travertine deposits of the first phase, a U-series age of 400 ka can be accepted (Altunel, 1996).

(2) The formation of travertine deposits is of modern origin. An option of an outflow of hot spring was developed by the formation of the Karahayıt fault. The travertine of the second phase is widespread in accordance with foothill slope.

(3) There is a stairway fault in the area. The travertine of this phase is observed in SE part of Pamukkale and was utilised in the construction of the ancient city Hierapolis.

(4) In this phase, the stairway faults were generated widespread.

(5) The last phase shows the landscape as it is nowadays. Great parts of the formation of travertine deposits in higher elevation areas have been eroded. Recent travertine forms modern carbonate precipitations as travertine consisting particularly of aragonite.

Isotope geochemistry

In the study area, the stable isotope compositions (d18O and d2H) in the geothermal waters are shown in Figure 8 (Kıymaz, 2011; Kutlu, 2015; Uzun, 2017: Yaman, 2005). δ2H values in geothermal waters vary between -61.9 and -51.8 ‰, whereas d18O values range from -9.23 to -5.84 ‰. The tritium contents of the geothermal waters vary between 0.7 and 3.3 TU. The samples of the geothermal waters of Pamukkale lie along the global meteoric water line (GMWL), whereas the samples of the high temperature geothermal waters in Kızıldere (Özgür, 1998; Yaman, 2005) deviate from GMWL, indicating intense water-fluid interaction under high temperature conditions. 3H values up to 3.3 TU show an atmospheric or anthropogenic origin. Therefore, there is a mixing process between fresh groundwaters and geothermal waters in the area of Pamukkale.


Figure 8: Plot of d18O versus d2H of the geothermal waters in Pamukkale and environs. For the data of stable isotopes see Özgür (1998), Yaman (2005) and Kıymaz (2011).


Discussion

The geothermal waters of Pamukkale and environs were modelled hydrogeologically (Figure 9). The meteoric waters in the drainage area percolate at fault and fracture zones and through permeable clastic sediments into the reaction zone of the roof area of a magma chamber. The chamber is located at a probable depth of 4-5 km, where meteoric fluids are heated by the cooling magmatic melt and ascend to the surface due to their lower density caused by convection cells. The volatile components of CO2, SO2, HCl, H2S, HB, HF, and He from the magma reach the geothermal water reservoir, where an equilibrium forms between altered rocks, gas components, and fluids. Thus, the geothermal waters ascend along the tectonic zones of weakness at the continental rift zones of the Menderes Massif in the form of hot springs, gases, and steams. These fluids are characterised by high to medium CO2, H2S and NaCl contents. It is very important to note that the fluids indicate a reduced pH-neutral environment after equilibrium adjustment with hard rocks in the reaction zone, namely in the roof area of magma chamber (Giggenbach, 1992).


Figure 9. Hydrogeological modeling of the geothermal waters of Pamukkale and environs (Pl4: Tosunlar formation; Pl3: Kolonkaya formation; Pl2: Sazak formation; Pl1: Kızılburun formation) (modified from Dilsiz et al., 2004).


The formation of travertine deposits depends upon the solubility of CaCO3, controlled principally by CO2 partial pressure, temperature and pH values, in which reaction equilibriums play an important role. Recent travertine deposits form the modern carbonate precipitations consisting of aragonite. In the study area, the travertine can be considered as terrace, ridge and channel type travertine (Altunel, 1996). Additionally, the geothermal waters of Pamukkale have high sulphate contents of up to 650 mg/l (Özgür et al., 2004) and Rn concentrations of up to 20 Bq/l (Kulalı, 2016), in which the high sulphate contents are associated with sulphide minerals such as pyrite as well as gypsum minerals in reservoir and cap rocks and the high Rn concentrations. These features are connected with the decay of uranium minerals in the metamorphic rocks of the Menderes Massif.

Acknowledgements

This study has been funded by the Scientific Research Coordination Office of the Suleyman Demirel University under contract numbers 4137-YL1-14 and 4494-YL1-15. We thank Mrs. Eda Aydemir, Mr. Ümit Memiş and Mr. Mehmet Arıcı (Süleyman Demirel Üniversitesi, Isparta, Turkey) for completion of figures of this paper. We would also like to thank Dr. Ali Gökgöz (Pamukkale University, Denizli, Turkey) for kindly examining and correcting the manuscript.


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This article has been published in European Geologist Journal 43 – Geothermal – The Energy of the Future

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