Elsevier

Geothermics

Volume 39, Issue 3, September 2010, Pages 242-249
Geothermics

Hydrogeochemistry and geothermometry of Changal thermal springs, Zagros region, Iran

https://doi.org/10.1016/j.geothermics.2010.06.007Get rights and content

Abstract

This study addresses the hydrogeochemistry of thermal springs that emerge from the Asmari limestone in a gorge at Changal Anticline in the vicinity of the Salman-Farsi dam. The Changal thermal springs vary in temperature between 28 and 40 °C. Chemical and isotopic compositions of the thermal waters suggest two distinct hydrogeological systems: a deep, moderate-temperature (∼40 °C) geothermal system recharged by deeply circulating meteoric waters, and a shallow cold aquifer system related to local groundwater. The source geothermal fluid temperature was calculated using different geothermometers and mineral saturation indexes. Based on chemical and isotopic data, it is hypothesized that: (1) mixing occurs between the ascending geothermal water and shallow cold water; (2) the resulting thermal waters reaching surface are a mixture of 80% local, shallow meteoric water and 20% geothermal water; and (3) the circulation depth of the meteoric water is about 1500 m. The thermal reservoir temperature is estimated to be between 70 and 80 °C according to calculations using different geothermometers and computation of saturation indices for different solid phases.

Introduction

Iran is situated in a tectonically active part of the eastern Mediterranean basin, as part of the Alpine-Himalayan orogenic belt. Based on the distribution of tectonic, magmatic and sedimentary features, Iran's plateau can be divided into the following major geological units: (1) Zagros region; (2) Sanangaj-Sirjan; (3) Central Iran; (4) East and south east zones; and (5) Alborz (Stocklin, 1968, Alavi, 2004). The latter (Alborz) extends from north-western to north-eastern Iran and includes the most important geothermal provinces of the country (Yousefi et al., 2006). However, the Zagros region in southern Iran is also a potentially useful geothermal province as it hosts thermal springs with average temperatures of about 40 °C and a distribution closely controlled by major faults (Karimi and Moore, 2008). The geochemical characteristics and fluid–mineral interaction of these thermal waters have rarely been studied in detail (e.g., Mosavi, 1993, Kompani-Zare and Moore, 2001, Karimi and Moore, 2008). This study presents chemical and isotopic data from 10 thermal springs and seven piezometers from the vicinity of the Salman-Farsi dam, an arch-gravity dam situated on the northern flank of the Changal Anticline in the Zagros region. Nine of the springs in this study, referred to here as the Changal thermal springs, occur on the southern flank of the Changal Anticline in association with the Asmari limestone and one other, the Yarg spring, appears 10 km upstream from the dam site. This study provides a geochemical characterization of the Changal thermal springs and a hydrological model with emphasis on: (1) source temperature of the thermal fluid, based on chemical geothermometry and mineral saturation indices, (2) degree of mixing between the deep thermal fluid and the shallow cold water, based on chemical and isotopic data.

The study area is located in the Zagros Mountains Range, which comprises a series of subparallel, NW-SE trending anticlines and synclines formed by compressional tectonics during the Miocene. The stratigraphic and structural setting of the Zagros Mountain range is described in detail by Stocklin and Setudehnia (1977) and Alavi (2004). The Changal Anticline (Fig. 1) is one of the major NW-SE trending anticlines in the study area and it plunges steeply northward (Fazeli, 2007). Two main fault systems, J1 and J2, were reported in the northern flank of the Changal Anticline (Fig. 1, Fig. 2). These two fault systems are perpendicular or semi-perpendicular to the axis of the Changal Anticline. The average strike and dip are N30 and 80° and N336 and 80° for J1 and J2, respectively (Milanovic et al., 2002). The highest elevation in the study area is 2200 m on the Karbasi Anticline, while the elevation of the Changal Anticline varies from 1000 to 1500 m. The lithostratigraphic sequence at the Salman-Farsi dam site was reported by Milanovic et al. (2002) in the context of a study on water retention. The major geological formations in the study area, in increasing order of age, include Pabdeh-Gurpi formation (Paleocene/Oligocene), Asmari formation (Oligocene to early Miocene), Razak formation (Miocene), Gachsaran formation (Miocene-Eocene), Mishan formation (Middle to Late Miocene), Aghajari formation (late Miocene to Pliocene), Bakhtiari formation (late Pliocene-Pleistocene) and recent alluvium. The Pabdeh-Gurpi formation is a cherty, fossiliferous and conglomeratic limestone grading into shale and marly limestone. Lithologically, the Asmari formation (Fig. 2) can be divided into three sub units (Milanovic et al., 2002) Lower Asmari (regularly bedded brown limestone, cherty limestone and thin bedded limestone alternating with a number of thin marly beds), middle Asmari (marly limestone and intensively karstified and vuggy porous crystalline nummulitic limestone), and upper Asmari (shaley and marly limestone, alternated with marl and siltstone). Some karst channels and caverns have formed along geological discontinuities within the Asmari limestone formation (Fazeli, 2007). A huge cave, named Golshan cave, with a volume of 150,000 m3 was discovered during excavation and drilling at the dam site. The Gachsaran formation mainly includes evaporate minerals, such as halite and gypsum. The Mishan formation is limestone interbedded with gray marl and clay, the Aghajari is calcalreous sandstone and red marl with interbedded gypsum and the Bakhtiari formation is a conglomerate.

An initial investigation including general geology, hydrogeology and hydrochemistry of fresh and thermal springs in and around the study area was done by Mosavi (1993). Prior to the construction of the Salman-Farsi dam, 40 springs were known to discharge at river level from outcrops of the Asmari Formation in the vicinity of the dam site (Milanovic et al., 2002). The water temperature and water chemistry of sampled springs (predominantly from the left bank of the river) suggest two different groundwater systems, a shallow phreatic system and a deeper thermal system. The shallow phreatic aquifer averages 28 °C and the deeper thermal system averages ∼42 °C (Milanovic et al., 2002). During borings and excavation of the diversion tunnel, thermal waters discharged directly into the tunnel and boreholes and emerged downstream of the dam site as thermal springs. All thermal springs are in contact with the Pabdeh-Gurpi formation because it contains impermeable marly layers (Fig. 2).

Water samples were collected from nine thermal springs, seven piezometers and a cold spring during two surveys in May and July 2008. The cold spring was selected to constrain the local meteoric composition and for comparison with the thermal waters (Fig. 1). One additional sample was taken from the Ghareh-Aghaj River close to the reservoir area of the Salman-Farsi dam (Fig. 2). The nine thermal springs (SPl to SP9 in Fig. 2) and seven piezometers (L2, L3, L4, L9, L20, RY2-9 and U11) are located close to the Salman-Farsi Dam site on the Changal Anticline (Fig. 2). Piezometers L2, L3, L4, L9 and L20 are located on the left side of the dam site and are artesian. The Yarg spring (Fig. 1) is located 1200 m above mean sea level, has a mean flow of 30 L/s and emerges from the Karbasi Anticline some 10 km upstream of the dam.

Water samples were collected in 150 mL dark glass bottles. Water temperature, pH, and electrical conductivity (EC) were measured in the field by portable instruments (EC meter and pH meter) made by ELE International (Table 1). Calcium and magnesium (measured by titration with EDTA), sodium and potassium (determined by flame photometry), chloride and sulphate (determined by turbidimetric methods), and bicarbonate (determined by titration with HCl) were analyzed in the Geochemistry Laboratory of the Department of Earth Sciences, Shiraz University. The quality of the analyses was evaluated using the ion balance (IB) equationIB=(sumofcationssumofanions)(sumofcations+sumofanions)×100where cations and anions are expressed in mequiv./L. All analyses reported in this study have IB < 5% (Table 1). The CO2 content and saturation indices of the water samples were calculated for different minerals using the PHREEQC code (Parkhurst and Appelo, 1999). The saturation index (SI) is defined as:SI=logIAPKTwhere IAP is the ion activity product and KT is the equilibrium constant of the mineral at the discharge temperature of the sample. For SI = 0, there is equilibrium between the mineral and the solution; SI < 0 and SI > 0 reflects subsaturation and supersaturation, respectively.

To investigate the isotopic composition of the sampled springs, six 20 mL grab samples were collected in dark bottles in July 2007 and analyzed for δ18O and δ2H in the Laboratory of Stable Isotope Ratio facility for Environmental Research, Utah University, USA. The isotope analyses were reported in ‰ relative to the VSMOW standard (Table 1).

Section snippets

Water chemistry

The chemical composition of the thermal waters is given in Table 1. Electrical conductivity values were 1253–3190 μS/cm. Water temperature ranged from 23 °C in the selected fresh cold water to 40 °C in spring SPL5 (Fig. 1, Fig. 2). All water samples exhibited a pH greater than 8. A weak H2S odour emanates from the thermal waters. However, our field testing using the iodometric method (Giggenbach et al., 1988) showed that the amount of dissolved H2S is below detection limits. Sulphate contents of

Conclusion

Although only a limited number of thermal water samples were studied, the samples indicate different chemical and isotopic compositions related to different hydrogeological systems. The thermal waters originate from meteoric water with a circulation depth of about 1500 m, primarily through the Asmari limestone. Chemically, the thermal waters are of NaCl and CaSO4 types, which confirms the influence of the geological formations dominated by carbonate and evaporate rocks. The high concentration of

Acknowledgments

We thank Dr. Malcolm Field of the National Center for Environmental Assessment of the U.S. Environmental Protection Agency for his review and constructive comments on the manuscript. The authors thank the Research Council of Shiraz University for financial support. We also thank Dr. Fabian Sepulveda and an anonymous reviewer for their detailed and constructive comments on the manuscript.

References (26)

  • R.O. Fournier

    A revised equation for the Na/K geothermometer

    Geotherm. Resour. Council Trans.

    (1979)
  • H. Gupta et al.

    Geothermal Energy: An Alternative Resource for the 21st Century

    (2007)
  • C.A. Hill

    Geology of Carlsbad Caverns and other caves of the Guadalupe Mountains, New Mexico and Texas

    New Mexico Bureau Mines Miner. Resour. Bull.

    (1987)
  • Cited by (0)

    View full text