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Hydrochemical, isotopic, and reservoir characterization of the Pasinler (Erzurum) geothermal field, eastern Turkey

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Abstract

The reservoir temperature and conceptual model of the Pasinler geothermal area, which is one of the most important geothermal areas in Eastern Anatolia, are determined by considering its hydrogeochemical and isotope properties. The geothermal waters have a temperature of 51 °C in the geothermal wells and are of Na–Cl–HCO3 type. The isotope contents of geothermal waters indicate that they are of meteoric origin and that they recharge on higher elevations than cold waters. The geothermal waters are of immature water class and their reservoir temperatures are calculated as 122–155 °C, and their cold water mixture rate is calculated as 32%. According to the δ13CVPDB values, the carbon in the geothermal waters originated from the dissolved carbon in the groundwaters and mantle-based CO2 gases. According to the δ34SCDT values, the sources of sulfur in the geothermal waters are volcanic sulfur, oil and coal, and limestones. The sources of the major ions (Na+, Ca2+, Mg2+, Cl−, and HCO3−) in the geothermal waters are ion exchange and plagioclase and silicate weathering. It is determined that the volcanic rocks in the area have effects on the water chemistry and elements like Zn, Rb, Sr, and Ba originated from the rhyolite, rhyolitic tuff, and basalts. The rare earth element (REE) content of the geothermal waters is low, and according to the normalized REE diagrams, the light REE are getting depleted and heavy REE are getting enriched. The positive Eu and negative Ce anomalies of waters indicate oxygen-rich environments.
ORIGINAL PAPER
Hydrochemical, isotopic, and reservoir characterization of the Pasinler
(Erzurum) geothermal field, eastern Turkey
Esra Hatipoğlu Temizel
1
&Fatma Gültekin
1
Received: 23 September 2016 / Accepted: 12 December 2017
#Saudi Society for Geosciences 2017
Abstract
The reservoir temperature and conceptual model of the Pasinler geothermal area, which is one of the most important geothermal
areas in Eastern Anatolia, are determined by considering its hydrogeochemical and isotope properties. The geothermal waters
have a temperature of 51 °C in the geothermal wells and are of NaClHCO
3
type. The isotope contents of geothermal waters
indicate thatthey are of meteoric origin and that they recharge onhigher elevations than cold waters. The geothermal waters are of
immature water class and their reservoir temperatures are calculated as 122155 °C, and their cold water mixture rate is calculated
as 32%. According to the δ
13
C
VPDB
values, the carbon in the geothermal waters originated from the dissolved carbon in the
groundwaters and mantle-based CO
2
gases. According to the δ
34
S
CDT
values, the sources of sulfur in the geothermal waters are
volcanic sulfur, oil and coal, and limestones. The sources of the major ions (Na
+
,Ca
2+
,Mg
2+
,Cl
, and HCO
3
) in the geothermal
waters are ion exchange and plagioclase and silicate weathering. It is determined that the volcanic rocks in the area have effects on
the water chemistry and elements like Zn, Rb, Sr, and Ba originated from the rhyolite, rhyolitic tuff, and basalts. The rare earth
element (REE) content of the geothermal waters is low, and according to the normalized REE diagrams, the light REE are getting
depleted and heavy REE are getting enriched. The positive Eu and negative Ce anomalies of waters indicate oxygen-rich
environments.
Keywords Pasinler geothermal area .Hydrogeochemistry .Isotope geochemistry .Wat errock interaction .Conceptual model .
Erzurum, Turkey
Introduction
Turkey is located in volcanic regions and active earthquake
lines; thus, it is very rich in geothermal waters. The number of
thermal sources is more than 1500, but about 200 of them are
operated as spas. Despite the high potential of the sources, the
spa tourism in Turkey is only regional.
Geothermal areas in Turkey are investigated intensively
along the grabens at Western Anatolia (Filiz 1982; Simsek
1982;Gülec1988;Simsek1997;GemiciandFiliz2001;
Yılmazer 2001; Tarcan 2002; Tarcan and Gemici 2003;
Tarcan 2005; Magri et al. 2010; Karakus and Simsek 2013;
Bundschuh et al. 2013), the Northern Anatolian fault zone,
and Central (Gultekin et al. 2011; Baba and Sanliyuksel
2011; Pasvanoglu and Gultekin 2012). But, there are very
few geothermal studies in the Eastern Anatolian volcanic re-
gion (Pasvanoğlu 2013;YüceandTaskıran 2013; Firat Ersoy
and Calik Sönmez 2014).
Pasinler, located 40 km to the east of Erzurum City, is one
of the geothermal fields in Eastern Anatolia, which is a volca-
nic and neotectonic province of Turkey (Fig. 1). The study
area has a semiarid climate and annual mean air temperature
is 5.5 °C. The average annual precipitation is 474.6 mm, and
the mean evapotranspiration is 543.9 mm in the region. The
main surface waters in the study area are the Hasankale River
(HC) and the Hamam Stream (HD). Jandarma spring (JK) is
the most important cold spring in the basin (Fig. 2).
Many geothermal springs are present and seven geothermal
wells having depths of 200750 m have been drilled by MTA
(General Directorate of Mineral Research and Exploration of
Turkey) to develop the geothermal energy production in the
Pasinler geothermal field. Although the geothermal wells
(PS1-A, PS-2, PS-3, PS-4, PS-5, EHD-1, OZ) have a
*Esra Hatipoğlu Temizel
hatipogluesra@gmail.com
1
Department of Geological Engineering, Karadeniz Technical
University, 61080 Trabzon, Turkey
Arabian Journal of Geosciences (2018) 11:3
https://doi.org/10.1007/s12517-017-3349-6
temperature 22.5 to 51 °C, people have been using the thermal
waters in this area only for balneotherapy and bathing pur-
poses. Because of their temperature, however, most of these
waters can also be used for different purposes such as green-
house irrigation, barn and poultry heating, mushroom cultiva-
tion, soil heating, swimming pools, and fish farms (Lund et al.
2010).
Geochemical studies of geothermal systems provide a frame-
work to understand the physiochemical processes responsible for
their origin and evolution. In addition, hydrogeochemical studies
have a great importance in determining the intended use of water.
The aims of this study are to investigate the hydrochemical prop-
erties and the source of hot waters in the Pasinler geothermal
area, define the hot water transportation system, and determine
the reservoir temperatures. As can be seen from the above expla-
nations, in the Pasinler geothermal field, detailed hydrogeochem-
ical studies have not been done yet. These deficiencies will be
eliminated with this study.
Materials and methods
Between 2011 and 2012, fieldwork was carried out four times
in the surroundings of Pasinler (Erzurum) in order to collect
water samples from boreholes, springs, streams, and shallow
cold groundwater systems, for chemical and isotopic (δ
18
O,
Fig. 1 Location map of the
Pasinler geothermal field
(Erzurum, Turkey)
3 Page 2 of 20 Arab J Geosci (2018) 11:3
δ
2
H, tritium, δ
13
C, δ
34
S) analyses. Total dissolved solids
(TDS), electrical conductivity (EC), pH, and water tempera-
ture (T) were measured in situ by using a portable multipa-
rameter (YSI). Samples were collected in 250- and 100-mL
polyethylene bottles which had been rinsed with distilled wa-
ter twice before sampling for anioncation and trace element
analyses respectively. Double-capped polyethylene bottles
with 100 mL (δ
18
O, δ
2
H, δ
13
C) and 500 mL (
3
Handδ
34
S)
volume were used for isotope samples. Major anion and cation
compositions of the water samples were determined at the
Water Chemistry Laboratory at Hacettepe University
(Turkey), using the following methods: Major cations (Ca
2+
,
Mg
2+
,Na
+
,andK
+
) were analyzed by ion chromatography
system. Cl
was analyzed using an AgNO
3
titrimetric method.
Sulfate concentrations were determined by spectrophotometry
together with alkalinity standard titration methods, whereas B
and SiO
2
were analyzed with the spectrophotometric method.
The major ion balance error of analyses is less than 5%. Heavy
metal and rare earth element (REE) concentrations were ana-
lyzed at the ACME Laboratory (Canada) using inductively
coupled plasma mass spectrometry (ICP/MS). In this study,
the geothermal water analyses of the MTA (Akkus et al. 2005)
were utilized for water chemistry study. Water chemistry anal-
yses were applied according to the American Public Health
Association (APHA 1995), American Water Works
Association (AWWA 1995), and Water Pollution Control
Federation (WPCF 1995) standards. δ
18
O, δ
2
H, δ
13
C, and
δ
34
S isotopic analyses were done at the ISO Analytical
Laboratory in the UK using equilibration IRMS (isotope ratio
mass spectrometry) and acid-CF-IRMS methods. Results are
reported in per mille vs. V-SMOW (Vienna-Standard Mean
Ocean Water), V-PDB (ViennaPeedee Belemnite), and V-
CDT (Vienna Canyon Diablo Troilite) standards.
3
H was an-
alyzed by a liquid scintillation water chemistry laboratory at
Hacettepe University (Ankara). Tritium is reported in tritium
units (TU) with a total analytical error of 0.1 to 0.3 TU.
0700000 0710000 0720000 0730000 0740000 0750000
44500004440000443000044200004410000
LEGEND
Upper
Cretaceous
Eocene
Lower
Upper
Quaternary
Yaylasırtı Gabbro
Erzurum-Kars Plateau V
Normal fault
Stream/
Creek
Geothermal well
Cold water well
Petroleum well
Thrust fault
PS-2
PS-2
0 4 8 12 km
27
22
22
A
PET-1
PET-2
PET-2
PET-3
24
OBANDEDE
Hasanbaba M.
Tımaryaylası
Y
Karabıyık
Uzunahmet
Büyük Tüy
Hanahmet
Karavelet
Yastıktepe Uzunahmet
Bulkasım
Hündül R.
Alvar
Küçük Tüy
1000
N
2000
3000
0
AS
A’
(m)
Düzyurdun R. Çaykara R. Kürdüdere
- +
- +
- +
- +
Ovaköy
A’
B
B’
Düzyurdun Creek
Tımar Creek
Büyük Creek
Çaykara Creek
Hamamdere
Komar Dere
Hasankale Stream
Sahveletdere
Kuru Creek
Fig. 2 Geological map of the Pasinler geothermal field (revised from Yılmaz et al. 1989) and geological cross section of the Pasinler geothermal field
Table 1 Lithostratigraphic relations of the geologic units and hydrogeological properties
Age Formation Lithology Hydrogeological properties
Quaternary Alluvium Permeable
PliocenePleistocene Horosan formation Conglomerate, sandstone, marl,
siltstone claystone
Semipermeable
Upper Miocene Erzurum-Kars plateau volcanics Andesite, basalt, dacite, rhyodacites, rhyolite,
and pyroclastics
Permeable (pyroclastics), semipermeable
(volcanic rocks)
Lower Miocene Haneşdüzü formation Reef limestones, sandstone, claystone conglomerate Semipermeable
Eocene Alibaba volcanics Andesite, basalt, pyroclastics Impermeable
Eocene Yaylasırtıgabbros Gabbros Impermeable
Eocene DervişHalit formation Shale, claystone, marl, sandstone, and sandy limestones Impermeable (shale, claystone, marl),
permeable (sandstone)
Upper Cretaceous Şahvelet ophiolites Serpentinite, peridotite, gabbro, and diabase Impermeable
Arab J Geosci (2018) 11:3 Page 3 of 20 3
Table 2 Results of physical and chemical analyses of the waters from the study area
Sample name Description Date of sampling T(°C) pH EC (μS/cm) TDS Ca Mg Na K HCO
3
SO
4
Cl SiO
2
Facies
PS1-A Borehole Aug. 1991
a
42 7.6 4060 2639 104 73 570 41 1202 16 628 170
PS-2 Borehole Aug. 1991
a
42 7.5 5641 3667 69 104 920 74 1530 5 1059 169
Oct. 1994
a
40 7.01 5030 3270 150 76 850 60 1410 5.7 1040
May 2011 37 5.3 6330 4115 219 93 890 83.7 1670 5.2 1155 172 NaClHCO
3
May 2012 38.4 5.2 5692 3700 200 100 910 86.8 1625 5.7 1174 170 NaClHCO
3
PS-3 Borehole May 2012 36.9 5.5 6516 4236 240 89 910 73.7 1830 3.04 1078 171 NaClHCO
3
Aug. 2012 36 6.5 6274 4392 245 106 927 72.2 1920 3.16 1060 173 NaHCO
3
Cl
Nov. 2012 37.6 6.5 6621 4304 241 105 895 71.2 1910 2.88 1074 172 NaClHCO
3
PS-4 Borehole Aug. 2000
a
42 6.7 6604 4293 185 75 1005 70 1930 3.9 1020
May 2011 36.1 5.1 5505 3637 201 83.3 1010 77.5 1910 4.2 1970 157 NaHCO
3
Cl
PS-5 Borehole Aug. 2000
a
39 6.1 5120 3584 163 81.9 700 81.2 1571 6.6 976
HDK Thermal spring May 2011 34.4 6.3 5243 3408 293 198 361 51.5 1925 1.1 578 190 MgNaCaHCO
3
Cl
May 2012 34.2 6.3 4844 3149 289 159 354 42.9 1817 0.51 475 181 NaCaMgHCO
3
Cl
Aug. 2012 34 6.5 5404 3513 286 161 405 42.1 1919 0.34 535 174 NaCaMgHCO
3
Cl
Nov. 2012 33.6 6.5 4810 3367 281 165 410 42.1 1921 0.33 540 NaCaMgHCO
3
Cl
OZ Borehole May 2011 23 6.2 1456 946 93.7 58.3 59.2 10.3 700 1.2 24 125 MgCaNaHCO
3
May 2012 23.1 6.2 1457 930 99 60.1 59.1 10 730 1.41 20 160 MgCaNaHCO
3
Aug. 2012 23 6.2 1506 1014 100 61.3 59.3 10 760 1.24 20 123 MgCaHCO
3
Nov. 2012 22.5 6.4 1529 990 101 61.6 57.5 9.63 731 5.04 21 MgCaHCO
3
JK Cold spring May 2012 13.3 7.8 420 304 33 23 11.3 2.66 221 7.8 2.02 MgCaHCO
3
Aug. 2012 14.2 7.9 470 308 35.8 22.1 11 2.33 223 7.3 2.18 MgCaHCO
3
Nov. 2012 13.2 7.9 504 328 38 22.5 13.8 3.43 225 7.27 11.1 CaMgHCO
3
HC Stream water May 2012 18.4 6.3 452 293 40 18 10 3.2 208 7.4 4 CaMgHCO
3
Aug. 2012 23.1 7.7 721 505 63 33 20 5.1 346 18 7.5 CaMgHCO
3
Nov. 2012 9.6 8.2 784 510 60 26 19.5 5.4 371 9.7 9.8 CaMgHCO
3
HD Stream water May 2012 18.7 6.3 1142 742 86 48 35.2 7.1 545 4.24 15 CaMgHCO
3
Aug. 2012 19.8 7.4 1122 785 85 47.2 40.6 7.2 582 3.76 17.9 CaMgHCO
3
Nov. 2012 11.9 6.9 1013 709 82 45.9 38.4 6.7 512 4.07 16.3 CaMgHCO
3
HDE Stream water May 2012 11.7 6.1 810 525 70 37.3 12.9 2.5 389 6.25 6.5 CaMgHCO
3
Aug. 2012 20.9 7.9 750 523 70.8 36.7 11.5 4.3 383 7.11 6.2 CaMgHCO
3
Nov. 2012 8.4 8.6 710 461 62.3 32.4 10.4 3.2 335 8.1 5.45 CaMgHCO
3
3205 Jun. 2011
b
14.8 8.02 475 310 12 8.4 60 2.34 189 0.48 36.2
Sept. 2012
b
13.1 7.9 394 275 24.4 17 52.2 7 129 0.48 30 NaMgCaHCO
3
11216 Jun. 2009
b
16.3 8.07 310 202 18 14.4 10.3 3.1 152 2.4 0.71
Oct. 2009
b
16 7.9 342 223 28 10.8 11.5 3.51 152 9.12 7.1
12571 Jun. 2011
b
15.5 7.9 273 180 15.2 10.3 13.1 3.9 128 2.4 7.1
Sept. 2012
b
14.1 8.01 308 200 27.4 18.3 14.7 1.95 110 9.6 6 MgCaNaHCO
3
27335 Jun. 2011
b
13.3 7.8 305 196 14 10.3 24.6 1.5 116 17.7 12.7
Sept. 2012
b
14 8.06 316 206 27.6 21.7 20.7 0.7 103 23 9.2 MgCaNaHCO
3
54324 Jun. 2011
b
13.9 7.3 300 195 18.4 10 14.2 7.8 134 1 8.5
Sept. 2012
b
12.9 8.1 317 206 38.6 22.6 6.4 3.1 117 4.8 3.9 MgCaHCO
3
Cation and anion concentrations are in milligrams per liter
a
Samples are results of analyses of MTA (Akkus et al. 2005)
b
Samples from DSI (state hydraulic works)
3 Page 4 of 20 Arab J Geosci (2018) 11:3
Saturation indexes of mineralswere calculated by using the
PHREEQC chemical equilibrium software WATEQ4F data-
base (Parkhurst and Appelo 1999). The software AquaChem
(Calmbach 1997) computer code was used to determine the
hydrochemical properties of thermal and cold waters. Silica
geothermometers were applied for reservoir temperature
estimations to Pasinler geothermal waters. These methods
are referred to Fournier (1977)andArnorsson(1983)
formulations.
Geological and hydrogeological settings
The dominant rock types in the Pasinler (Erzurum) geothermal
area are volcanics that are formed at different times. These
volcanics are overlain by young sediments and alluvium at
the center of the area (Fig. 2). The basement rocks of the study
area are Upper Cretaceous age Şahvelet ophiolites including
serpentinite, peridotite, gabbro, and diabase (Yılmaz et al.
1989).The Şahvelet ophiolites are unconformably overlain
by the DervişHalit formation and Tertiary units (Fig. 2).
The DervişHalit formation comprises shale, claystone, marl,
sandstone, and sandy limestones. Yaylasırtıgabbros of the
Eocene age (Sungurlu 1971) are massive and display a frac-
tured structure. Andesite, basalt, pyroclastics of Alibaba vol-
canics are (Sungurlu 1971) conformably overlain by Late
Oligocene Early Miocene units. The Haneşdüzü formation
of Lower Miocene age consists of reef limestones. Erzurum-
Kars plateau volcanics of Upper Miocene age is formed an-
desite, basalt, dacite, rhyodacites, rhyolite, and pyroclastics.
The Horosan formation (conglomerate, sandstone, marl, silt-
stone claystone) and Quaternary alluvium are the youngest
units in the field.
In the study area which is one of the most neotectonic
provinces of Turkey, strikeslip faults and intense fracture
networks formed by the compressional tectonic regime are
the basic structural elements (Şengör 1980). These structural
elements have enhanced the development of the geothermal
system and the circulation of the thermal waters.
The stratigraphic units are classified according to the
hydrogeological characteristics of the rocks. According to
the Bureau of Reclamation (1995) classification, alluviums
and Erzurum-Kars plateau volcanics with a hydraulic conduc-
tivity value of 10
5
m/s (Dilek, 1973; Freeze and Cherry,
1979) are permeable units. The semipermeable units are
Horosan formation and Haneşdüzü formation with 10
7
m/s
(Dilek, 1973) hydraulic conductivity. The Şahvelet ophiolites,
DervişHalit formation, and Alibaba volcanics with perme-
ability of about 10
10
m/s (Freeze and Cherry, 1979) are im-
permeable units (Table 1). A total of six geothermal wells
(OZ, PS-2, PS-3, PS-4, PS-5, EHD-1), one geothermal spring
(HDK), one cold water spring (JK), three surface waters (HD,
HDE, HC), and five groundwater wells (3205, 11216, 12571,
27335, 54324) are evaluated in this study.
Results and discussion
Hydrogeochemistry
In order to conduct chemical analyses, samples were gathered
from the geothermal wells and hot and cold water springs in
the area. Also, the results of the analyses that were carried out
by MTA on geothermal waters and by DSI (State Hydraulic
Works) on cold waters were also assessed. The physicochem-
ical characteristics and types of all waters are presented in
Table 2. In the study area, the water temperature of hot spring
80 60 40 20 20 40 60 80
20
40
60
80
20
40
60
80
20
40
60
80
20
40
60
80
Ca
Na+K
Mg
HCO
3
+CO
3
SO
4
a
Ca Mg Na Cl
Parameters
0.001
0.010
0.100
1.000
10.000
100.000
HCO
3
SO
4
b
Cl+SO
4
=>
<=Ca+Mg
Concentration (meq/l)
Cl
River waters
Geothermal spring
Geothermal wells
Cold water spring
Ground water wells
Fig. 3 Piper (a) and Schoeller (b) diagrams of the water samples
Arab J Geosci (2018) 11:3 Page 5 of 20 3
waters ranges from 22.5 to 51 °C, and the pH values range
from 5.7 to 7.6. The electrical conductivity (EC) values are in
the range of 9706233 μS/cm. The total dissolved solids
(TDS) are between 2538 and 4392 mg/L. In contrast, the water
temperatures of cold spring and river waters are ranging from
9.7 to 23.1 °C, and the pH values range from 6.15 to 8.6. Their
TDS concentrations between 123 and 785 mg/L and EC
ranged from 200 to 1142 μS/cm. Based on the International
Association of Hydrogeologists (IAH 1979) classification,
three different groundwater facies were determined
(Table 2). Generally, geothermal wells are of NaClHCO
3
facies, geothermal springs are of NaCaMgHCO
3
Cl fa-
cies, and cold waters are of MgCaHCO
3
facies. The main
chemical characteristics of the studied waters are summarized
in the Piper diagram in Fig. 3(Piper 1944). The cold water and
thermal well waters clearly plot in distinct fields. Hot spring
waters are located between them. This situation indicates that
the hot waters are mixed with shallow ground waters.
According to the piper diagram, in the study area CaMg
HCO
3
(stream waters, cold wells, cold spring and geothermal
spring) and NaClHCO
3
(geothermal wells) the dominant
water types were observed (Fig. 3a). The Schoeller diagram
Table 3 Trace element compositions of the waters from the study area (μg/L)
PS-2 PS-3 PS-4 HDK OZ JK HC HD HDE
Al 10 23 10 20 2 1 20 24 10
As 20 22 17 20 0.5 6.7 6.3 23 5
B 9998 10,364 9042 14,700 1075 50 110 14,707 40
Ba 899 889 832 520 9.14 44.5 81.4 527 20
Br 1799 1726 1646 1700 175 10 10 1729 10
Cd 0.05 0.05 0.05 < 0.05 0.05 0.05 0.13 0.2 < 0.05
Co 0.02 0.02 0.02 0.02 0.02 0.11 0.29
Cs 12.3 12.8 10.6 0.35 0.01 0.03 3.7
Cu1110.50.30.22.63.20.5
Fe 4081 4257 3837 4300 4845 <0.01 <0.01 4305 < 0.01
Ga 0.05 0.05 0.05 0.05 0.05 0.05 0.05
Ge 3 4.2 3 3 0.9 0.5 0.5 0.3 0.5
Hf 0.02 0.02 0.02 0.02 0.02 0.02 0.02
Hg <1 <1 <1 0.2 0.1 0.1 0.2
Li 1209 1227 1092 300 67.6 2 5 310 30
Mn 476 494 440 100 103 < 0.05 < 0.05 141 < 0.05
Mo <0.1 <0.1 <0.1 0.3 0.9 5.6 1.1
Nb 0.2 0.2 0.2 0.01 0.01 0.02 0.01
Ni 0.2 4 0.2 5 0.2 0.7 3.7 5 < 0.02
Pb11110.50.10.40.70.3
Rb 140 147 131 80 11.5 2.71 6.6 82.8 2
Sb 0.05 0.05 0.05 0.05 0.08 0.21 0.22
Sc 21 22 20 20 15 2 2 22 5
Se 6 5 5 0.7 0.5 1.2 5
Si 80,803 80,103 73,522 89,100 58,941 10,000 12,600 89,130 21,800
Sn 0.05 0.05 0.05 0.13 0.05 0.07 0.08
Sr 2366 2578 2370 2500 367 492 695 2520 300
Ta 0.02 0.02 0.02 0.02 0.02 0.02 0.02
Th 0.05 0.05 0.05 0.09 0.05 0.05 0.05
Tl 0.1 0.1 0.1 0.01 0.01 0.04 0.08
U 0.02 0.02 0.02 0.02 0.6 2.16 0.57
W 0.2 0.2 0.2 0.08 0.10 0.22 0.25
Y 0.6 0.6 0.6 0.02 0.01 0.03 0.3
Zn710581.9415.5101
Zr 1.7 2.2 2.9 0.2 0.03 0.02 0.04 0.05 0.2
3 Page 6 of 20 Arab J Geosci (2018) 11:3
shows the total concentration of major ions in semi log-scale
(Fig. 3b). As can be seen from this figure, thermal waters have
different chemical characteristics to cold waters. While Na +
K and Cl concentrations are high in the thermal waters, these
values are low in cold waters. But the SO
4
concentration is
low in all waters because of sulfate reduction. Sulfate is typ-
ically the first or second most abundant anion in natural waters
(Hem, 1970). Sulfate is the completely oxidized form of sul-
fur, which is stable under aerobic conditions. Sulfate reduction
occurs intensively in natural groundwater systems. In the stud-
ies on sulfate reduction, it has been stated that sulfate reduc-
tion makes a significant amount of H
+
consumption and HS
production (Miao et al., 2012). In addition, sulfate reduction
under natural conditions is conducted by prokaryotic bacteria
through chemical reactions in which organic carbon or H
2
is
oxidized while sulfate is reduced (Canfield 2001); sulfate-
reducing bacteria are effective on the subsurface sulfate reduc-
tion (Berner et al., 2002; Aravena and Mayer, 2009). The most
significant indicator of sulfate reduction is H
2
S production
(Miao et al., 2012). In the study area, an intense odor of rotten
eggs around geothermal wells and thermal springs is an indi-
cation of H
2
S production and sulfate reduction. The measured
low pH and relatively high Fe concentration in the thermal
water support this subject.
Trace element contents of waters
TraceelementslikeLi,B,Co,Ni,Ga,Rb,Cs,Sr,andBaremain
unaffected in the thermal waters due to secondary processes
(Giggenbach 1991) and hence play a significant role in
understating the evolution of the thermal waters. Plenty of trace
elements in geothermal waters indicates that thermal waters have
a greater reactivity leading to increased leaching of the minor
elements from the host rock during deep circulation (Ma et al.
2011).Trace element analyses have been carried out on the sam-
ples taken from geothermal wells, hotcold springs, and surface
waters. Trace element concentrations of the Pasinler waters are
presented in Table 3. The lithophile elements such as Li, Rb, and
Cs, which are useful in understanding the deep processes
(Giggenbach 1991), are plotted in the trilinear diagram (Fig. 4).
All of the samples indicate uptake of Cs in zeolites at tempera-
tures lower than 300 °C. The concentrations of these lithophile
elements were in the range of 67.61227 ppb for Li, 0.35
12.8 ppb for Cs, and 11.5147 ppb for Rb. Among the other
lithophile elements, the concentrations of Sr and Ba in thermal
waters are 3672578 and 9.14899 ppb, respectively. Sr, with
higher concentrations than the other trace elements in the thermal
water, reflects the interaction between the ascending thermal wa-
ters and volcanic rocks (Delalande et al. 2011). The boron con-
centration of the thermal waters varied from 1075 to 10,364 ppb.
While the B concentration values in cold and surface waters are
low (40100 ppb), in the hot springs (HDK) discharging along
the Hamam stream and stream water (HD), these values are high
(14,700 ppb). The maximum Al and Mn abundances are 23 and
494.3 ppb, respectively, in the geothermal waters (Table 3). The
chalcophile elements such as As, Cu, Pb, Se, and Zn are gener-
ally dominant in the sulfate waters. The concentration of these
elements ranges in the Pasinler waters from 0.5 to 23 ppb for As,
0.3 to 1 ppb for Cu, 0.5 to 1 ppb for Pb, 0.7 to 6 ppb for Se, and
1.9 to 10 ppb for Zn. In the presence of sulfur species, the solu-
bility of minerals containing chalcophile elements is lower in
reducing conditions than in oxidizing conditions (Hem 1970).
The concentrations of chalcophile elements are low in the inves-
tigated water due to sulfate reduction. Among the siderophile
elements, the concentrations of Fe, Ni, and Ge in the waters are
38374845, 0.24, and 0.94.2 ppb, respectively. The concen-
trations of Hg and Mo ranged from below detection limit to
1 ppb Hg and 0.1 ppb Mo (except sample OZ). It was observed
that thermal water had a good correlation between lithophile
elements and chloride concentrations. Correlation coefficients
computed for ClLi, ClRb, ClCs, ClBr, and ClBpairsare
0.939, 0.997, 0.961, 0.763, and 0.343, respectively (Fig. 5). A
positive correlation was also observed between Rb, Li, and Cs
themselves. Correlation coefficients calculated for these elements
are 0.910 for RbLi, 0.939 for RbCs, and 0.994 for LiCs.
Strong correlations observed between K and RbLiCs (KRb
0.999, KCs 0.934, and KLi 0.903) may indicate that these
elements substitute for potassium in clay minerals (Mutlu 2007).
Geothermometers
Various chemical geothermometers have been developed to esti-
mate the reservoir temperatures in the geothermal system
(Arnorsson 1983; Fournier 1977,1979; Giggenbach 1988;
Kharaka and Mariner 1989; Truesdell and Fournier 1977;
Verma and Santoyo 1997). Among these, the cation (NaK,
NaLi, NaKCa, etc.) and silica (quartz, chalcedony,
Cs*10
Rb*4
Li
20
20
20
40
60
80
80
80
60
60 40
40
Csuptakein
zeolites<300C
Outflow
Upflow
Li uptake in
quartz
Rock Dissolution
Basalt
Rhyolite
Rbuptakein
Illites>300C
Fig. 4 LiRbCs diagram (symbol is as in Fig. 3b)
Arab J Geosci (2018) 11:3 Page 7 of 20 3
amorphous silica, etc.) geothermometers are the most widely
used. In this study, silica geothermometers were applied to cal-
culate the reservoir temperatures of the thermal waters. The re-
sults of geothermometric calculations are given in Table 4,and
the calculated reservoir temperature varies from 46 to 176.7 °C.
Geothermometer results give a wide range for reservoir temper-
ature. The reservoir temperatures of about 4060 °C, calculated
with amorphous silica and cristobalite thermometers, do not re-
flect the reality. Quartz geothermometers are not suitable for low
temperatures (Fournier 1977). Therefore, the most reliable reser-
voir temperature for the Pasinler geothermal field is the temper-
atures of approximately 122155 °C calculated by a chalcedony
geothermometer. According to the NaKMg diagram
(Giggenbach 1988), Pasinler waters are located in the Bimmature
0 300 600 900 1200
Cl ( mg /L)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Li (mg/ L)
R2=0.939
0 300 600 900 1200
Cl (mg/ L)
0
0.04
0.08
0.12
0.16
Rb (mg /L)
R2=0.997
0 300 600 900 1200
Cl (mg/L)
0
0.004
0.008
0.012
0.016
Cs (mg/L)
R2=0.961
0 300 600 900 1200
Cl (mg/L)
0
0.4
0.8
1.2
1.6
2
Br (mg/L)
R2=0.763
0 300 600 900 1200
Cl (mg/L)
0
4
8
12
16
B(mg/L)
R2=0.343
0 0.04 0 .08 0 .12 0.1 6
Rb (mg/L)
0
0.4
0.8
1.2
1.6
Li (mg/L)
R2=0.910
0 0.04 0 .08 0 .12 0.16
Rb (mg/ L)
0
0.004
0.008
0.012
0.016
Cs (m g/ L)
R2=0.939
0 0 .4 0.8 1.2 1 .6
Li (mg/L )
0
0.004
0.008
0.012
0.016
Cs (mg/L)
R2=0.994
020406080100
K(mg/L)
0
0.04
0.08
0.12
0.16
Rb (mg/L)
R2=0.999
0 20406080100
K
(
m
g
/L
)
0
0.004
0.008
0.012
0.016
R2=0.934
0 20406080100
K
(
m
g
/L
)
0
0.4
0.8
1.2
1.6
Li (mg/L)
R2=0.903
Cs (mg/L)
Fig. 5 Plots of Cl vs. Li, Cl vs. Rb, Cl vs. Cs, Cl vs. Br, Cl vs. B, Rb vs. Li, Rb vs. Cs, Li vs. Cs, K vs. Rb, K vs. Cs, and K vs. Li of allgeothermal water
samples (symbol is as in Fig. 3b)
3 Page 8 of 20 Arab J Geosci (2018) 11:3
waters^which indicates that these waters are shallow or mixed
and, thus, have not yet reached the water rock equilibrium in the
region (Fig. 6). For this reason, cation geothermometers give
higher results and according to Giggenbach (1988) cannot be
applied reliably.
Enthalpychloride mixing model
The enthalpychloride mixing model of Fournier (1977)was
used to predict the underground temperature for the Pasinler
mixed geothermal water. This mixing model takes into account
both mixing and boiling processes (Magana 1999). Its applica-
tion basically involves relating analyzed chloride levels to water
enthalpy, which can be derived from measured discharge tem-
perature, geothermometry temperature, and silicaenthalpy
mixing model temperature (Magana 1999). Enthalpychloride
and silicaenthalpy mixture models were applied to the
Pasinler geothermal area, and reservoir temperatures are deter-
mined as 160235 and 220250 °C, respectively. But, when the
geological properties of the basin are taken into account, it is
considered that the temperature values that were calculated using
the silicaenthalpy model are unrealistic. Thus, the reservoir tem-
peratures that were calculated via the enthalpychloride model
are accepted (Fig. 7). Moreover, the mixing ratio of hot water to
the cold water supply was calculated as 32%. The thermal waters
Table 4 Silica geothermometry
temperatures for Pasinler thermal
waters (°C)
Geothermometers PS1A PS-2 PS-3 PS-4 HDK OZ
Surface temperature (°C) 42 37 36.9 36.1 34.4 23.03
1. SiO
2
(amorphous silica)
a
46.1 47.1 46.6 41.5 53.2 28.9
2. SiO
2
(άKristobalit)
a
119.1 120.2 119.6 113.9 126.8 100
3. SiO
2
(βKristobalit)
a
69.5 70.5 70 64.4 77.1 50.9
4. SiO
2
(chalcedony)
a
146.4 147.7 147 140.7 155 125.3
5. SiO
2
(quartz)
a
169.1 170.2 169.6 164.1 176.7 150.5
6. SiO
2
(quartz steam loss)
a
159.3 160.2 159.7 155.2 165.5 143.8
7. SiO
2
(chalcedony conductive cooling)
b
141.8 142.9 142.4 136.6 149.6 122.6
8. SiO
2
(quartz steam loss)
b
137.3 138.2 137.7 132.8 144 120.6
9. SiO
2
(quartz steam loss)
b
142 143.2 142.5 136.3 150.5 121.1
10. SiO
2
(quartz steam loss)
b
162.9 164 163.4 157.4 171.1 142.6
11. SiO
2
(quartz steam loss)
b
158.6 159.5 159 154.4 164.8 143
a
Fournier (1977)
b
Arnorsson et al. (1983)
Fig. 7 Enthalpy-chloride mixing model for the Pasinler thermal waters
80 60 40 20
20
40
60
80 20
40
60
80
Sqr
(
M
g)
K/100
Na/1000
40
80
120
160
200
240
280
320
Full equilibrium
Partially equilibrium
or mature waters
Immature waters
Fig. 6 Na, K, Mg trilinear equilibrium diagram (based on Giggenbach,
1988) for the geothermal water samples (symbols are as in Fig. 3b)
Arab J Geosci (2018) 11:3 Page 9 of 20 3
in the vicinity and the cold spring (JK) are close to the tritium
values (0.030.57 TU). Therefore, the mixture is not with surface
waters but with cold waters which are partly deep circulation.
Saturation indices
By using the saturation index approach, it is possible to
predict reactive minerals in the subsurface from the
groundwater chemical data without examining samples
of the solid phases (Deutsch 1997).
Table 5shows the saturation indices (SI) of the geothermal
water from Pasinler calculated with the software PHREEQC
Interactive 2.8 computer code WATEQ4F database (Parkhurst
and Appelo 1999) on the basis of outlet temperature and pH.
Results indicate that the studied geothermal wells (except the
PS1-A) are supersaturated (SI > 0) with respect to quartz while
they are undersaturated (SI< 0) with respect to anhydrite, ara-
gonite, barite, calcite, dolomite, gypsum, halite, magnesite,
and talc. The HDK hot spring is supersaturated with aragonite,
calcite, dolomite, and quartz whereas it is undersaturated with
anhydrite, barite, gypsum, halite, and talc (Fig. 8).
Isotopic characteristics
Isotope compositions of the waters have become important
tools in hydrogeology and have been widely used as natural
-16
-14
-12
-10
-8
-6
-4
-2
0
2
4
Sam
p
le name
Anhydrite
Aragonite
Barite
Calcite
Dolomite
Gypsum
Halite
Magnesite
Quartz
Talc
Saturation Index (SI)
Fig. 8 Mineral equilibrium
diagram for Pasinler thermal
spring
Table 5 Saturation index (SI) values of the waters in the study area
PS1-A PS-2 PS-3 PS-4 HDK OZ JK
Anhydrite 2.75 3.07 3.28 3.21 3.72 3.77 3.19
Aragonite 1.06 0.89 0.62 1.01 0.31 0.68 2.49
Barite 0.64 0.95 0.75 1.59 2.68 0.80
Calcite 1.20 0.75 0.49 0.87 0.45 0.53 2.34
Dolomite 2.75 1.40 0.92 1.65 1.19 0.94 4.65
Gypsum 2.62 2.92 3.12 3.05 3.55 3.55 2.94
Halite 5.14 4.71 4.73 4.69 5.38 7.45 9.17
Magnesite 0.96 1.24 1.03 1.38 0.15 0.99 2.85
Quartz 0.95 0.95 0.93 1.03 1.00 0.41
Talc 8.21 7.11 9.41 1.02 4.85 14.90
HD HDE HC 3205 12,571 27,335 54,324
Anhydrite 3.22 3.06 3.12 5.17 3.14 2.71 3.32
Aragonite 0.74 1.25 2.45 0.13 0.24 0.15
d
0
Barite 1.65 1.27 1.05 –– – –
Calcite 0.59 1.09 2.30 0.28 0.08 0.30 0.15
Dolomite 1.17 2.31 4.69 0.62 0.15 0.70 0.24
Gypsum 2.98 2.81 2.88 4.92 2.89 2.46 3.07
Halite 7.86 8.62 8.94 7.30 8.60 8.22 9.13
Magnesite 1.14 1.75 2.95 0.21 0.61 0.15 0.45
Quartz 0.79 0.75 0.40 –– – –
Talc 5.92 8.84 14.89 –– – –
3 Page 10 of 20 Arab J Geosci (2018) 11:3
Table 6 Isotopic analysis results
of the water samples in the
Pasinler geothermal area
Sample Sampling
date
δD
V-
SMOW
()
δ
18
O
V-
SMOW
()
T
(TU)
δ
13
C
V-PDB
()
δ
18
O(SO
4
)
V-
SMOW
()
δ
34
S(SO
4
)
V-
CDT
()
OZ May 11 94.68 13.54 0.18
OZ May 12 92.15 13.32 0.0 9.18
OZ Aug. 12 92.16 13.35 0.61 9.34 2.7 36.18
OZ Nov. 12 90.12 13.24 0.0
PS-2 May 11 94.2 13.37 0.33
PS-3 May 11 92.16 13.4 0.0
PS-3 May 12 94.71 13.3 0.57 8.8
PS-3 Aug. 12 93.44 13.34 0.27 9.54 1.9 18.64
PS-3 Nov. 12 96.05 13.1 0.39
HDK May 12 92.94 13.01 0.28 8.13
HDK Aug. 12 99.68 12.95 0.75 9 6.1 31.35
HDK Nov. 12 91.2 12.57 0.17
JK May 12 84.87 12.55 0.03 9.99
JK Aug. 12 86.71 12.03 0.0 7.14 5.4 1.94
JK Nov. 12 83.84 12.19 0.0
HD May 12 87.71 12.65 2.49 3.22
HD Aug. 12 94.1 12.89 1.59 6.19 8 14.4
HD Nov. 12 88.95 12.67 1.43
HDE May 12 86.66 12.34 3.76 9.61
HDE Aug. 12 90.73 12.52 4.64 7.93 3.8 13.44
HDE Nov. 12 87.15 12.6 2.84
HC Aug. 12 73.6 11.55 5.46 7.52
HC Nov. 12 81.9 11.64 6.83
-105
-100
-95
-90
-85
-80
-75
-70
(‰ SMOW)
OZ
OZ
OZ
OZ
PS-2
PS-3
PS-3
PS-3
PS-3
HDK
HDK
HDK
JK
JK
JK
HD
HD
HD
HDE
HDE
HDE
HC
HC
-14 -13.5 -13 -12.5 -12 -11.5 -11 -10.5 -10
18
O(
‰ SMOW)
Mediterranean Meteoric
Water Line
Geothermal
waters
Cold waters
OZ
May-2011
May-2012
August-2012
November-2012
May-2011
May-2011
May-2012
August-2012
November-2012
May-2012
August-2012
November-2012
May-2012
August-2012
November-2012
May-2012
August-2012
November-2012
May-2012
August-2012
November-2012
August-2012
November-2012
PS-2
PS-3
HDK
JK
HD
HDE
HC
Global Meteoric
Water Line
Van (Eastern Turkey)
Meteoric Water Line
Fig. 9 Plot of δDvs.δ
18
Oofall
water samples in the study area.
The Mediterranean meteoric
water line and the Van (Eastern
Turkey) meteoric line are
calculated from Gat and Carmi
(1970)andAydın et al. (2009),
respectively
Arab J Geosci (2018) 11:3 Page 11 of 20 3
tracers (Clark and Fritz 1997). All water samples were ana-
lyzed for δ
18
O, δ
2
H, and
3
H. Moreover, δ
13
C, δ
34
S, and
δ
18
O
SO4
in DIC and dissolved sulfate have been performed
on selected samples (Table 6). For thermal waters (spring and
boreholes), the values of δ
18
O range from 13.54 to
12.57and those of δ
2
H from 99.68 to 91.2.The
δ
2
Handδ
18
O values of cold springs and river water samples
vary from 90.73 to 73.6and 12.89 to 11.55 ,
respectively, and are similar to those for hot spring water sam-
ples. The stable isotopic composition (δ
2
Hversusδ
18
O) of the
waters, both cold and thermal, is shown in Fig. 9.According
to this diagram, the isotopic composition all of the waters in
the study area is located between the Global Meteoric Water
Line (GMWL) (Craig 1961) and the Mediterranean Meteoric
02468
Tritium (TU)
5
10
15
20
25
30
35
40
45
Temperature ( C)
OZ
OZ OZ
OZ
PS-2
PS-3
PS-3
PS-3
PS-3
HDKHDK
HDK
JK
JK
JK
HD
HD
HD HDE
HDE
HDE
HC
HC
0 1 000 2000 3000 4000 5000 6000 7000
EC
(
µ
S/cm
)
0
1
2
3
4
5
6
7
8
Tritium (TU)
OZ
OZ
OZ
OZ
PS-2
PS-3
PS-3
PS-3
PS-3
HDK
HDK
HDK
JK
JKJK
HD
HD
HD
HDE
HDE
HDE
HC
HC
Geothermal
waters
Cold waters
ab
C
Fig. 11 Tritiumelectrical conductivity (a) and tritiumtemperature (b) relations for the thermal and cold waters (symbols are as in Fig. 9)
510152025303540
Temperature
(
C
)
-13.6
-13.2
-12.8
-12.4
-12
-11.6
-11.2
18
OZ
OZ
OZ
OZ
PS-2
PS-3
PS-3
PS-3
PS-3
HDK
HDK
HDK
JK
JK
JK
HD
HD
HD
HDE
HDE
HDE
HC
HC
Deep Cycle
Shallow Cycle
O, SMOW (
‰)
Fig. 10 Oxygen-18 (δ
18
O)temperature diagram of the waters in the
study area (symbols are as in Fig. 9)
-12 -8 -4 0 4 8 12
C
‰VPDB
0
500
1000
1500
2000
2500
HCO
3
(mg/l)
OZ
OZ
PS-3
PS-3
HDK
HDK
JK JK
HD HD
HDE HDE
HC
Fig. 12 HCO
3
δ
13
C relation for the thermal and cold waters (symbols are
as in Fig. 9)
3 Page 12 of 20 Arab J Geosci (2018) 11:3
Wat er Line (Ga t and C armi 1970),andontheVan(East
Turkey) Meteoric Line (δ
2
H=8δ
18
O + 16.4; Aydın et al.
2009). This result indicates that the precipitations which feed
the water sources are occurring in a more arid area than the
world average (Fontes 1980). According to the Oxygen-18 vs.
temperature diagram, thermal water samples were recharged
at the same elevation in the basin (Fig. 10). Thermal waters in
the field have more negative δ
2
Handδ
18
O values than cold
waters. These low values indicate that thermal waters
recharged at the higher altitude than cold waters.
Tritium contents of thermal waters and cold waters vary
from 0.01 ± 0.50 TU to 6.83 ± 0.45 TU (Table 6). The amount
of tritium in water can be used to qualitatively determine
whether groundwater is modern or not (Clark and Fritz 1997;
Fig. 13
34
S
CDT
values of sulfur in
different material and
environment (Krouse, 1980)
??
?
?
?
PS-2
25416
PET-2
?
?
Geothermal water
Cold water
+ -
+ -
+ -
LEGEND
Upper
Cretaceous
Eocene
Yaylasırtı Gabbro
Normal fault
Stream
Geothermal well
Cold water well
Petroleum well
Lower
Upper
Quaternary
Erzurum-Kars Plateau V
0 1 2 4 km
Fig. 14 Conceptual model of the Pasinler (Erzurum) geothermal field
Arab J Geosci (2018) 11:3 Page 13 of 20 3
Zouari et al. 2003; Goff and McMurtry 2010). Tritium values
equal to or greater than 1 TU are acceptable as modern water;
moreover, tritium concentrations below 1 TU show that
groundwater is at least 50 years old. The tritium values below
0.8 TU indicate the system was recharged before the 1950s. In
Fig. 11a, b, the tritium concentrations for all waters from
Pasinler are plotted against EC and Tvalues, respectively.
The low TU high EC and T contents of the thermal waters
indicate that these thermal waters have deeper circulating and
longer residence times than the cold waters (except for JK cold
spring). The low tritium value of the JK cold spring can be
explainedbydeepcirculation.
To determine the source of carbon and sulfur (SO
4
)inthe
Pasinler waters, all water samples were analyzed for their
δ
13
C
VPDB
(Versus Pee Dee Belemnite) and δ
34
S
CDT
(Canyon
Diablo Troilite). The δ
13
C (DIC) contents for Pasinler waters
range from 8.13 to 9.54for thermal waters, from 7.14 to
9.99for cold spring, and from 9.61 to +6.19for surface
waters (Table 6). These results indicate that carbon in the thermal
waters originates from mainly groundwater DIC. Also, young
volcanic rocks outcropping around the study area suggest that
carbon in the waters originates from volcanic (mantle) CO
2
.
Carbon in the surface waters and cold spring (except for HD)
has a negative carbon content. HD (surface water), which has
thermal water discharges, has positive carbon values which are
controlled by CO
2
in the soil and groundwater DIC (Clark and
Fritz 1997). The δ
13
C (DIC) contents are plotted vs. alkalinity for
all water samples in Fig. 12. As a result, HCO
3
values of the
geothermal waters show an enriched value of δ
13
C with respect
to the cold water spring.
δ
34
SVCDT values of SO
4
are in range 18.64 to 36.18in
geothermal waters (OZ, PS3 and HDK), 1.94in cold water
spring (JK), and 13.44 to 14.4in surface waters (HD and
HDE). These results are evaluated according to the diagram
(Krouse 1980). The sources of the sulfur are volcanic origin
sulfur, oil, coal, and limestone according to the data obtained
from the PS3 geothermal well (Fig. 13). The economically im-
portant coal in the lower levels of the Horosan formation and oil
that seeps around the geothermal springs and Erzurum-Kars pla-
teau volcanics outcropping in a vast area also confirm the source
of the sulfur in the Pasinler basin. According to the diagram of
sulfur isotope distribution in nature (Krouse 1980), the sources of
the sulfur are limestone, volcanic rocks, oil-coal, and cold and
surface waters (JK, HD, HDE) too.
Conceptual model of the Pasinler geothermal field
The conceptual model of the Pasinler geothermal system has
been evaluated using hydrogeology, hydrochemistry, and envi-
ronmental isotope studies together with regional geological
structure (Fig. 14). According to the deep drills (Pelin 1970,
1981), it is clear that there are metamorphic rocks at the base-
ment, which are overlain by Upper Cretaceous sedimentary
rocks (DervişHalit formation). Sedimentary rocks at some areas
are cut off by Upper Miocene young volcanic rocks (Erzurum-
Kars plateau volcanics). Conglomerate and sandstone intercalat-
ed with marl (Horosan formation) and alluvium form the cap
rock in the basin. The compression regime along the NS direc-
tion (Yılmaz et al. 1989) and strikeslip faults have caused vol-
canic activities north of the basin (Aynalıand Bulut 2002). At the
north of the basin, dome structures of different rock types and
fissure-type volcanism have occurred (Keskin et al. 1998). These
volcanic rocks in the basin are basalt, andesite, dacite,
rhyodacite, rhyolite, and ignimbrite. The geothermal boreholes
drilled by MTA indicate that the reservoir rock is basalt and
basaltic tuff at deep, and rhyolite and rhyolitic tuff at upper
levels. Geothermometer calculations and isotope geochemistry
studies indicate that a low temperature and meteoric origin
Table 7 Chemistry of rocks in the study area (values are given as %)
Sample no. R1 R2 ZB1 BB1 RT1
Lithology Rhyolite Rhyolite Basalt Basalt Rhyolitic tuff
Formation name Erz.-Kars Plt. Volc. Erz.-Kars Plt. Volc. Erz.-Kars Plt. Volc Erz.-Kars Plt. Volc Erz.-Kars Plt. Volc
Na
2
O 5.35 5.32 3.53 3.44 4.71
MgO 0.23 0.09 5.57 5.70 0.41
Al
2
O
3
15.21 15.18 16.98 18.01 15.38
SiO
2
69.65 70.59 54.17 52.49 67.25
P
2
O
5
0.06 0.03 0.16 0.19 0.07
K
2
O 5.01 5.04 1.52 0.99 4.92
CaO 0.62 0.45 8.33 8.65 1.52
TiO
2
0.24 0.15 1.07 1.12 0.40
Cr
2
O
3
< 0.002 < 0.002 0.020 0.021 < 0.002
MnO 0.04 0.04 0.13 0.15 0.07
Fe
2
O
3
2.58 2.09 7.40 7.59 1.91
LOI 0.8 0.8 0.8 1.3 3.1
3 Page 14 of 20 Arab J Geosci (2018) 11:3
geothermal system. Precipitation in the basin is filtered to depth
along the faults and fractures and it is heated in the temperature
anomaly field. The magma that formed the young volcanics
(Erzurum-Kars plateau volcanics) is the main heat source of
the geothermal system in the Pasinler geothermal field.
Hydrochemical and isotope contents in the geothermal waters
show different degrees of mixing with cold groundwater before
rising to the surface. Silty and marl levels of the Horosan forma-
tion and clayey levels of alluvium are the cap rock of the system.
Table 8 Trace element compositions of the rocks from the study area (μg/L)
Elements Co Ni Cu Zn Ga As Se Rb Sr Y Zr Nb Mo Cd Sn Sb Cs Ba La Ce Hf Ta W Hg Tl Pb Th U
R1 1.0 1.8 3.5 64 20.7 0.5 0.5 112.1 41.6 44.2 427.2 28.1 3.2 0.1 7 0.1 3.7 870 68.0 114.7 9.6 1.5 7.9 0.01 0.1 4.0 16.5 5.0
R2 0.5 0.6 3.3 83 20.4 1.0 1.0 120.3 18.7 43.4 367.0 31.5 3.2 0.1 5 0.1 2.5 498 82.1 133.8 9.6 2 1.3 0.01 0.1 5.5 19.6 5.6
ZB1 31.9 50.4 29.2 36 18.3 0.5 0.5 37.9 433.2 22.3 188.5 11.7 1.3 0.1 2 0.1 1.0 362 26.2 49.8 3.7 0.6 0.7 0.01 0.1 0.9 7.4 1.7
BB1 31.2 69.2 22.0 41 19 0.5 0.5 19.4 458.0 25.5 187.7 11.1 1.0 0.1 2 0.1 0.6 549 35.0 56.1 4.3 0.7 0.7 0.01 0.1 4.0 7.3 1.6
RT1 1.3 3.6 2.8 64 17.5 0.5 0.5 119.4 149.6 28.7 468.0 24.3 0.9 0.1 3 0.1 3.5 884 54.8 93.2 10.5 1.6 2.4 0.01 0.1 1.2 23.4 6.1
Fig. 15 Distribution of trace element for rocks and waters (a),
chondrite-normalized REE patterns of rocks (Haskin et al., 1968)
(b), and waters (Evens et al., 1978) from the Pasinler (Erzurum)
geothermal field in Turkey (c)
Arab J Geosci (2018) 11:3 Page 15 of 20 3
Rock geochemistry
The different rock types (basalt, rhyolite, and rhyolitic
tuff) outcropping in the Pasinler geothermal area were
analyzed to compare with the water and rock geochem-
istry (Table 7). The chemistry of the surrounding rocks
shows that the most abundant oxides are SiO
2
and
Al
2
O
3
, respectively. The trace element concentrations of
the sampled rocks (basalt, rhyolite, and rhyolitic tuff) in
the study area are given in Table 8. According to the
analysis results, while trace elements such as Ba, Sr,
Rb, Zr, Ce, Zn, and Ga have high concentrations, the
elements such as Se, Cd, Sn, Sb, and Hg have low con-
centrations. The samples from the water and rock are
similar in their Sr, Ba, Rb, and Zn content. Since mag-
maticvolcanic rocks have clearly the highest Ba and Sr
elements (Hem 1970), the high concentrations of Ba and
Sr in the water indicate that this element originated from
rhyolite, rhyolitic tuff and basalt and waterrock interac-
tion in the study area (Fig. 15a).
Rare earth elements and waterrock interaction
In addition to the major elements in the geothermal wa-
ters, the rare earth elements (REE) can be used in the
investigation of waterrock interaction and exploration
for geothermal resources (Smedley 1991; Lewis et al.
1997,1998; Wood and Shannon, 2003; Sanada et al.
2006; Gammons et al. 2005, Göb et al. 2013,Shakeri
et al. 2015). REE content in the volcanic rocks and all
water samples were analyzed in the Pasinler basin and
arereportedinTable9. The chondrite-normalized
(Haskin et al. 1968) REE distribution of the rocks in
the study area shows variable enrichment in light REE
(LREE) compared to heavy REE (HREE) and negative
Eu anomalies (Fig. 15b). This is typical of the upper
continental crust (McLennan 1989). The low concentra-
tionsofREEwereobservedinthewaters(Fig.15c).The
chondrite-normalized (Evens et al. 1978) REE patterns of
the waters are different from those of the rocks; LREE
are slightly depleted relative to the HREE and have neg-
ative Ce anomalies and positive Eu anomalies which in-
dicate oxygen-rich environments (Constantopoulos 1988).
Wat errock interaction was evaluated according to
Hounslow (1995) and was simulated in the computer pro-
gram Aquachem 2012.2. The different ionic comparisons
of Hounslow 1995 and the calculated results in this study
are shown in Table 10. According to these results, the
(Na
+
+K
+
Cl
)/(Na
+
+K
+
Cl
+Ca
2+
)ratiosofall
the water samples are > 0.2 and < 0.8, indicating plagio-
clase weathering is possible. The ratio of Na
+
/(Na
+
+Cl
)
is > 0.5 for all waters (except for HDK) and 0.5 for HDK,
indicating a sodium source other than halitalbite, ion
exchange, and halite solution, respectively (Hounslow
1995). The ratio of Mg
2+
/(Ca
2+
+Mg
2+
) is < 0.5 for PS-
2, PS-3, PS-4, HC, HD, and HDE; 0.5 for OZ; and > 0.5
for HDK, and JK, while the situation of HCO
3
/SiO
2
>10
is indicating limestonedolomite weathering, dolomite
weathering and dolomite dissolution, and calcite precipi-
tation, respectively. The Ca
2+
/(Ca
2+
+SO
42
)ratioswere
found to be > 0.5, showing a calcium source other than
gypsum, carbonate, or silicate. The SiO
2
/(Na
+
+K
+
+Cl
)
ratiois<1intheexaminedwaters,indicatingcationex-
change. The source of Cl
ions is weathering of the rocks
(Hounslow 1995). The HCO
3
/ anion ratio is < 0.8 for
HDK, PS-2, PS-3, and PS-4 and > 0.8 for OZ, JK, HC,
HD, and HDE, showing seawater and brine, and silicate
or carbonate weathering, respectively (Hounslow 1995).
Table 9 REE analytical results of spring and rock samples (concentration in ppb for springs and ppm for rock samples)
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
R1 68.0 114.7 11.95 44.0 7.70 0.98 7.12 1.23 7.46 1.55 4.94 0.77 4.89 0.85
Rock R2 82.1 133.8 14.83 53.3 9.20 0.67 8.20 1.42 7.61 1.74 5.43 0.84 5.60 0.84
ZB1 26.2 49.8 5.13 21.7 4.12 1.29 4.27 0.73 4.10 0.87 2.68 0.38 2.35 0.38
BB1 35.0 56.1 6.27 23.5 4.87 1.49 5.02 0.83 4.96 0.95 3.00 0.41 2.32 0.41
RT1 54.8 93.2 8.58 31.6 5.13 1.21 5.03 0.82 5.00 1.08 3.08 0.53 3.37 0.54
OZ 0.01 0.02 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
Waters HD 0.1 0.1 0.1 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.01 0.1 0.1 0.1
HDE 0.03 0.04 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
PS-2 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
PS-3 0.1 0.01 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
PS-4 0.1 0.01 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
HDK 0.01 0.01 0.01 0.2 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
HC 0.02 0.02 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
3 Page 16 of 20 Arab J Geosci (2018) 11:3
Table 10 Wate rrock interaction processes in the investigated waters (Hounslow, 1995)
Parameter HDK PS-2 PS-3 PS-4 OZ
(Na
+
+K
+
Cl
)/
(Na
+
+K
+
Cl
+Ca
2+
)
0.014 0.39 0.46 0.60 0.29
Plagioclase weathering unlikely Plagioclase weathering possible Plagioclase weathering possible Plagioclase weathering possible Plagioclase weathering possible
Na
+
/(Na
+
+Cl
) 0.50 0.54 0.56 0.59 0.79
Halite solution Sodium source other than
halitalbite, ion exchange
Sodium source other than
halitalbite, ion exchange
Sodium source other than
halitalbite, ion exchange
Sodium source other than
halitalbite, ion exchange
Mg
2+
/(Ca
2+
+Mg
2+
) 0.52 0.41 0.37 0.40 0.50
Dolomite weathering Limestonedolomite weathering Limestonedolomite weathering Limestonedolomite weathering Dolomite dissolution, calcite
precipitation, or seawater
Ca
2+
/(Ca
2+
+SO
42
) 0.99 0.99 0.99 0.99 0.99
Calcium source other than
gypsum-carbonates or silicates
Calcium source other than
gypsum-carbonates or silicates
Calcium source other than
gypsum-carbonates or silicates
Calcium source other than
gypsum-carbonates or silicates
Calcium source other than
gypsum-carbonates or silicates
SiO
2
/(Na + K + Cl) 7.4 0.18 0.13 0.08 0.49
Cation exchange Cation exchange Cation exchange Cation exchange Cation exchange
Cl
/ anions 0.30 0.53 0.49 0.47 0.05
Rock weathering Rock weathering Rock weathering Rock weathering Rock weathering
HCO
3
/ anions 0.64 0.44 0.48 0.51 0.93
Seawater or brine Seawater or brine Seawater or brine Seawater or brine Silicate or carbonate weathering
Calcite SI 0.45 0.7 0.48 0.8 0.53
Oversaturated calcite Undersaturated calcite Undersaturated calcite Undersaturated calcite Undersaturated with respect to
calcite
Parameter JK HC HD HDE
(Na
+
+K
+
Cl
)/
(Na
+
+K
+
Cl
+Ca
2+
)
0.21 0.14 0.21 0.10
Plagioclase weathering possible Plagioclase weathering possible Plagioclase weathering possible Plagioclase weathering possible
Na
+
/(Na
+
+Cl
)0.890.790.780.75
Sodium source other than halitalbite, ion
exchange
Sodium source other than halitalbite, ion
exchange
Sodium source other than halitalbite, ion
exchange
Sodium source other than halitalbite, ion
exchange
Mg
2+
/(Ca
2+
+Mg
2+
)0.530.420.470.46
Dolomite dissolution, calcite
precipitation, or seawater
Limestonedolomite weathering Limestonedolomite weathering Limestone-Dolomite weathering
Ca
2+
/(Ca
2+
+SO
42
)0.910.920.980.96
Calcium source other than
gypsum-carbonates or silicates
Calcium source other than
gypsum-carbonates or silicates
Calcium source other than
gypsum-carbonates or silicates
Calcium source other than
gypsum-carbonates or silicates
SiO
2
/(Na + K + Cl) 0.39 0.63 0.45 0.92
Cation exchange Cation exchange Cation exchange Cation exchange
Cl
/ anions 0.01 0.03 0.04 0.02
Rock weathering Rock weathering Rock weathering Rock weathering
HCO
3
/ anions 0.94 0.93 0.93 0.95
Silicate or carbonate weathering Silicate or carbonate weathering Silicate or carbonate weathering Silicate or carbonate weathering
Calcite SI 2.33 2.3 0.59 1.09
Undersaturated with respect to calcite Undersaturated with respect to calcite Undersaturated with respect to calcite Undersaturated with respect to calcite
Arab J Geosci (2018) 11:3 Page 17 of 20 3
The decomposition reactions of kaolinite decomposition
of silicates such as albite, anorthite, and K-feldspar are
givenbyEqs.(13) (Appelo and Postma, 1994)andde-
composition reactions of carbonates are given by Eq. (4).
2NaAlSi3O8þ2Hþþ9H2OAl2Si2O5OHðÞ
4þ2Naþþ4H
4SiO4
Albite Kaolinite
ð1Þ
2CaAl2Si2O8þ2HþþH2OAl2Si2O5OHðÞ
4þ2Ca2þ
Anorthite ð2Þ
2KAlSi3O8þ2Hþþ9H2OAl2Si2O5OHðÞ
4þ2Kþþ4H4SiO4
Kfeldspar ð3Þ
CaCO3þH2CO3Ca2þþ2HCO3
Calcite ð4Þ
Conclusions
The Pasinler basin has been formed because of the compression
regime along NS direction caused by sinistral strikeslip faults
at EW direction. It is accepted that the volcanic domes at the
north and south of the basin have been formed because of this
regime. The geothermal system is associated with the Erzurum-
Kars Plate volcanic rocks. NaClHCO
3
-type geothermal liq-
uid is of meteoric origin based on isotope composition. The
heat transfer in the system occurs with convective transporta-
tion. Both hydrogeochemical properties and isotopic composi-
tion of the waters indicate that the hot waters rising from the
geothermal reservoir via faults mix with ground waters coming
from the shallow cold water aquifer before surfacing. The tem-
peratures measured at the geothermal borehole (51 °C) and the
temperatures calculated according to silica geothermometers
show that the geothermal system has a low enthalpy. The triti-
um values below 0.8 TU in geothermal waters indicate the
system was recharged before the 1950s.
Based on the trace element concentration, it is seen that the
water chemistry is affected by the chemistry of the volcanic
rocks which form the geothermal reservoir. The major ions
(Ca, K, Na) in the geothermal water originated from the
weathering of plagioclase and cation exchange.
Acknowledgements We are grateful to Assistant Prof. Arzu Fırat Ersoy
for assistances in the fieldwork. The authors also thank Assistant Prof.
Adam Milewski from the University of Georgia (USA) for his help with
the English of the final text.
Funding information This research was supported by the Karadeniz
Technical University Research Project Fund (Project Number: 1063).
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... Given that only a limited number of hydrogeochemical surveys were carried out in this region (e.g. Akkuş et al., 2005;Firat Ersoy and Çalik Sönmez, 2014;Pasvanoğlu, 2014;Hatipoğlu Temizel and Gültekin, 2018), the results of this study will greatly contribute not only to establishment of a comprehensive geochemicalisotopic database but also assessment of fields with promising geothermal energy potential. In line with this objective, the chemical and isotopic compositions of thermal and cold waters were used to examine the role of lithological controls on water type, and to estimate reservoir temperatures using chemical and isotopic geothermometers. ...
... δ 18 O and δ 2 H range from −14.5 to −12.5‰ V-SMOW and − 104 to −93‰ V-SMOW in the northern province, while they change from −13.8 to −10.1‰ V-SMOW and − 98 to −84‰ V-SMOW in the central province, and − 14 to −3.4‰ V-SMOW and − 96 to −65‰ V-SMOW in the southern province Mutlu et al., 2013), respectively. The results of oxygen-hydrogen isotope compositions are consistent with previous studies (Aydın et al., 2009;Firat Ersoy and Çalik Sönmez, 2014;Pasvanoğlu, 2014;Hatipoğlu Temizel and Gültekin, 2018). Tritium contents of eastern Anatolian geothermal waters are between 0 and 6.29 TU and those of cold waters vary from 4.01 to 8.97 TU (Table 3). ...
... Regarding the spatial distribution of δ 18 O SO4 , the southern part usually has the lowest values (Table 3). In a previous study by Hatipoğlu Temizel and Gültekin (2018), the average δ 34 S SO4 and δ 18 O SO4 values of waters in the Pasinler (Erzurum) geothermal area are reported 19.3‰ V-CDT and 3.8‰ V-SMOW which are consistent with values obtained in this work (21.8‰ V-CDT and 5.2‰ V-SMOW). ...
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The hydrogeochemistry of geothermal waters from 31 geothermal fields in eastern Turkey is investigated with regard to major ion compositions, stable (δ 18 O-δ 2 H-δ 34 S) and tritium (3 H) isotope systematics. Discharge temperature of studied waters varies from 24 to 65 °C. Four different geochemical processes were found to control the major ion concentrations of waters which include dissolution of carbonates, fluid-mineral interaction, oxidation of sulfur-bearing minerals, and chloride enrichment. The northern, central and southern provinces are represented by different local meteoric water lines (LMWL) with deuterium excess of 15.0, 13.9 and 16.5‰ V-SMOW, respectively. The stable isotope values of the thermal waters are close to LMWLs and indicate a meteoric origin. The enrichment in oxygen isotope composition (0.6 to 7.7‰ V-SMOW) in some thermal waters resulted from water-rock interaction process and 18 O-exchange process between CO 2 and H 2 O. The δ 34 S and δ 18 O contents in dissolved sulfate cover a wide range from 6.2 to 32‰ V-CDT and from −2.5 to 14.8‰ V-SMOW, respectively , indicating that the sulfate isotope systematics of the majority of waters is governed by dissolution of terrestrial sulfate and marine evaporites. The reservoir temperatures estimated by chemical and isotopic geothermometers of K\ \Mg (27-127 °C), silica (29-179 °C) and 18 O SO4-H2O (51-196 °C), and by the silica-enthalpy mixing model (130 to 235 °C) yielded inconsistent results. The geological factors (e.g., relatively thick crust, low surface heat flux, absence of ideal cover units) in eastern Turkey have resulted in the development of low-or moderate-temperature geothermal systems.
... Various chemical geothermometers are used to estimate reservoir temperatures of geothermal systems and determine effective use of geothermal waters (Fournier, 1977;Sanliyuksel and Baba, 2011;Temizel and Gultekin, 2018). These geothermometers assume that the equilibrium of chemical compositions attained in the geothermal reservoir is maintained during the ascent of geothermal waters from a deep reservoir to the surface (Karingithi, 2009;Mao et al., 2015;Hsu and Yeh, 2020). ...
... The observed variability for the cold waters from 0.5 to 4.91 TU for 3 H reflects seasonal variations affecting the meteoric waters. Baba and Ertekin (2007) reported that 3 in water can be used to qualitatively determine whether the groundwater is modern or not (Barbier et al., 1983;Clark and Fritz, 1997;Temizel and Gultekin, 2018). The 3 H values equal to or greater than 1 TU are accepted as modern water; moreover, 3 H concentrations below 1 TU show that groundwater was recharged prior to the period of atmospheric testing of thermonuclear weapons (Ravikumar and Somashekar, 2011). ...
... Bölge içerisinde 23 adet termal su kaynağı yer almaktadır (MTA, 2019). Geçmişte, farklı alanlardaki bu termal suların hidro-jeokimyasal özellikleri ile ilgili araştırmalar yapılmış ve buna bağlı olarak ta Doğu Anadolu Bölgesi'nde yer alan jeotermal potansiyel alanlar belirlenmiştir (Akkuş ve ark., 2005;Baba ve ark., 2010;Fırat Ersoy & Çalık Sönmez, 2014;Pasvanoğlu, 2014Pasvanoğlu, ve 2020Temizel & Gültekin, 2018; Uzelli ve ark., 2021). Şekil 1. Türkiye jeotermal kaynakları ve sıcaklık dağılımı (MTA, 2019). ...
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Doğu Anadolu geçmişten günümüze aktif tektonizma etkisi altında kalarak gelişimini sürdürmekte ve bu gelişime bağlı olarak aktif bir volkanizmanın ürünlerini sergilemektedir. Aktif tektonizma ve volkanizmanın sonucu olarak bölge yüksek jeotermal potansiyeline sahiptir. Son yıllarda, jeotermal potansiyeli tanımlanmış Batı Anadolu jeotermal kaynakları; enerji üretimi, konut/şehir ısıtma, termal turizm, seracılık vb. gibi geniş bir yelpazede kullanım olanağı sağlamaktadır. Ancak Doğu Anadolu (DA) jeotermal kaynakları yerel ölçekte sadece termal turizm amacı ve küçük ölçekli seracılık çalışmalarında kullanılmaktadır. Doğu Anadolu Bölgesi’nde günümüze kadar yaklaşık 23 adet jeotermal kaynak belirlenmiştir. Bugüne kadar yapılan çalışmalar sıcak su kaynaklarının çıkış noktaları ile fay hatları arasında bir ilişkinin olduğunu göstermektedir. Sıcak su noktaları bu kırık hatları boyunca yüzeye ulaşmakta ve fay hatlarına paralellik göstermektedir. KD-GB uzanımlı Erciş-Zilan-Ilıca vadisi boyunca yaklaşık 11 adet sıcak su noktası bulunmaktadır ve Doğu Anadolu bölgesi için mevcut potansiyelin % 40’na karşılık gelmektedir. Bu vadiler boyunca gözlenen sıcak su noktalarının çıkışları belirlendiğinde bir hat boyunca devam ettikleri anlaşılmaktadır. Bu çizgisellik KD-GB doğrultusunda olup, yaklaşık olarak vadi uzanımına paraleldir. Bu alanda yer alan sıcak su noktalarının Zilan bölgesinin deformasyonunu denetleyen Zilan Fayı ile ilişkisi bu çalışmada arazi çalışmaları ile ortaya konulmaya çalışılmıştır. Yapılan bu çalışma ile sıcak su noktalarının çıkış merkezlerinin hassas RTK-GPS ile belirlenmiş ve vadi boyunca arazi çalışmaları yapılarak faylanma verisi toplanmıştır. Elde edilen bu verilerin ışığında Erciş bölgesini ve civarını etkileyebilecek Zilan Fayı’nın KD-GB uzanımlı sol yanal doğrultu atımlı aktif bir fay olduğu ve bölgede yer alan Zilan Jeotermal alanının deformasyonunu denetlediği görülmektedir.
... The circulating hot waters in the studied areas may be under the effect of cold groundwaters as they have mixed, since cation geothermometers may then give higher results in terms of geothermal reservoir temperature. As indicated by the hydrogeochemical analyses of the samples, Na + , K + and Cl − values are high, possibly a result of the volcanic units (Can et al. 1986), weathering of the plagioclase (Temizel and Gültekin 2017) or induced from evaporates present as gypsum in the media. Thus, silica geothermometers have been used to estimate the reservoir temperatures of the studied areas and the calculated temperatures range from 36 to 159 °C (Table 6). ...
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Geothermal water sources located within The Erzurum province were identified and hot water samples were taken from four different geothermal areas. The results of in situ and hydrogeochemical analyses of these hot water samples were interpreted and the properties of hot water, water–rock associations, estimated reservoir temperature and hot water usage areas were determined. The temperatures of the samples collected from the study area vary between 26.2 and 57.7 °C, while pH values change from 6.09 to 7.33, EC values obtained from in situ measurements are between 1829 and 9480 µS/cm and Eh values are (− 190) to (26.3) mV. Total dissolved solids of the hot waters have a range from 838.7 to 3914.1 mg/l. The maximum estimated reservoir temperature is calculated as 250 °C by applying chemical geothermometers. However, considering the actual temperatures of Pasinler, Köprüköy, Horasan and Ilıca thermal waters and wells, the most reliable temperature range depending on the applied geothermometers’ results indicate minimum and maximum reservoir temperatures 85–158.9 °C, respectively, taking in account the errors. According to the isotope analysis, the waters circulating within the geothermal system are of meteoric origin and modern waters. In addition, two samples taken from clayey levels observed in the field were analyzed and the mineralogy of the clays was evaluated.
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To understand the characteristics of geothermal resources of the Dazhuang geothermal reservoir in the Minle Basin, regional geothermal geological data of the Minle Basin were collected. Combined with the Dazhuang geothermal reservoir survey, drilling and logging data, the main thermal control factors of Dazhuang geothermal reservoir “source-reservoir-pass-cover” are analysed, and a geothermal model of Dazhuang geothermal reservoir is established. The Dazhuang geothermal reservoir is based on the geothermal flow of the pull-apart basin formed on the basis of a right-handed strike-slip fault. It uses atmospheric precipitation as well as melted ice and snow in the southern Qilian Mountains as replenishment sources and flows into the thermal reservoir of the basin through the Neogene pore strata. The heat flow generated by the asthenosphere in the longitudinal direction of the Earth’s crust normally increases to form a medium–low-temperature conduction geothermal system. The Dazhuang geothermal reservoir has the characteristics of large caprock thickness, many thermal reservoir sections and rapid replenishment. The thermal reservoir caprock is mainly Quaternary and Neogene Pliocene, with a thickness of 1798 m. The thermal reservoir is composed of sandstone in the Quanzi member of the Baiyanghe Formation in the Miocene, and the cumulative thickness of the aquifer can reach 390m. The resource evaluation of the Dazhuang geothermal reservoir research area is performed using the heat storage method. The total geothermal resources in the study area are 2.08 × 108 GJ, equivalent to 612.1 × 104 t of standard coal, which has a significant development value.
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