Elsevier

Applied Geochemistry

Volume 113, February 2020, 104492
Applied Geochemistry

Geothermometry and geochemistry of groundwater in the Continental Intercalaire aquifer, southeastern Algeria: Insights from cations, silica and SO4–H2O isotope geothermometers

https://doi.org/10.1016/j.apgeochem.2019.104492Get rights and content

Highlights

  • Water mineralization is derived from the leaching of halite, gypsum and anhydrite.

  • Quartz and CaSO4–H2O are the most reliable geothermometers.

  • CaSO4–H2O geothermometer provides reliable results even below 100 °C.

  • The CI reservoir shows an estimated temperature ranging from ~70 to ~93 °C.

  • This aquifer has a geothermal potential as a source of renewable energy.

Abstract

Characterization of geochemistry and geothermometry of the groundwater from the Continental Intercalaire (CI) aquifer, one of the largest aquifers in the world, stretching over one million km2 surface area shared between Algeria, Tunisia and Libya, was conducted in southeastern Algeria nearby the border with Tunisia. Thirty-two water samples were collected from boreholes to analyze the physicochemical parameters and stable isotopes (δ18O(H2O), δ18O(SO4), δ 34S(SO4)) and determine the origin of water mineralization of the CI aquifer. Water temperature was assessed using several geothermometers, such as cations, silica and SO4–H2O stable isotopes. The CI aquifer displayed a discharge temperature varying from 45 to 65.1 °C due to water circulation at deeper depths ranging from 1000 to 2200 m. The Results show that CI water contains high total dissolved solids (TDS) and neutral to slightly alkaline pH. The most frequent water types were Na–Cl–SO4 and Ca–SO4 indicating the geological formation nature of CI aquifer which is mainly composed of evaporites. The application of Na–K and Na–K–Ca geothermometers yielded unreliable temperatures. However, Na–Li geothermometer resulted in relatively plausible temperatures ranging from 74.69 to 147.83 °C. Quartz and CaSO4–H2O isotope geothermometer are the most suitable methods for temperature estimation, due to their attainment of equilibrium resulting in temperatures ranging from 62 to 93 °C, and validation by the multiple mineral equilibrium approach. The present study demonstrated a successful application of CaSO4–H2O isotope geothermometer in a low enthalpy (<100 °C) geothermal system. The results corroborate previous findings on the same CI aquifer at the Tunisian side and on carbonate-evaporite geothermal reservoirs. Furthermore, geothermal potential of the CI aquifer has been highlighted suggesting its use as a source of renewable energy which could be applied in heating greenhouses and/or generating electricity.

Introduction

Geothermal sources are considered as a renewable energy potential which originated from heat inside the earth globe. This energy is usually linked to several geological formations that contain water resources either with their geothermal gradient at high depth or with their occurrence nearby heat sources such as magma. In ancient times, the use of geothermal springs was mostly limited to bathing, whereas in modern times geothermal energy has become a valuable source for industry and agriculture. The application of geothermal energy differs depending on the reservoir temperature. For instance, a geothermal reservoir with a temperature of 80 °C could generate electricity, while with a temperature of 30 °C, it would rather be useful for heating greenhouses, warming soil, or improving fish farming (Lindal, 1973).

Deep aquifers may also represent a considerable source of geothermal energy such as many underground aquifers in Algeria. Occurrence of thermal water from aquifers in northeastern Sahara in Algeria, particularly in Ouargla and Touggourt areas, was formerly reported (Saibi, 2009; Chaib and Kherici, 2014). This thermal water is used for bathing, heating greenhouses and domestic usage purposes. Recently, a fish farming project has been successfully implemented using thermal water from The Albian CI aquifer in Ouargla, Algeria (Saibi, 2009). Although hydrogeological and hydrochemical aspects of the Albian CI aquifer, which is recognized as one of the largest aquifers in the world, has been largely studied (Guendouz and Michelot, 2006; Chkir et al., 2009; Moulla et al., 2012; Petersen et al., 2013, 2018; Petersen, 2014; Gonçalvès et al., 2015; Slimani et al., 2017), more details on the geothermal characterization need to be investigated, especially at a regional scale including several locations in the northeastern Sahara of Algeria. A natural extension of the CI aquifer within Algeria-Tunisia southern border has been considered as a geothermal reservoir (Ben Dhia and Meddeb, 1990; Kamel, 2012; Makni et al., 2013).

The CI aquifer where the present study was conducted is enclosed in a stable, non volcanic area with a typical geothermal gradient of 2–3 °C/100m (Takherist and Lesquer, 1989) and a depth ranging from 1000 to 2200 m (Edmunds et al., 2003), thus making it a focus of attention as a geothermal reservoir.

Assessment of the subsurface temperature is usually challenging because of limitations in direct measurement methods. For instance, bottom hole surveys provide accurate results, however; this direct method is highly costly due to the drilling requirements. Thus, alternative indirect methods such as geochemistry are used. These economic alternatives use geothermometers that have been developed for the main purpose of estimating reservoir temperatures using water chemical or isotope composition (D'Amore and Arnórsson, 2000). These techniques are commonly applied to volcanic geothermal systems under various conditions (Fournier, 1977), whereas with low temperature and non-volcanic systems more caution is required (Kharaka and Mariner, 1989). Although geothermometry of carbonate-evaporite geothermal reservoirs has been extensively studied (Kharaka and Mariner, 1989; Ben Dhia and Meddeb, 1990; López-Chicano et al., 2001; Mohammadi et al., 2010; Makni et al., 2013; Wang et al., 2015; Pasvanoğlu, 2015; Belhai et al., 2016; Yang et al., 2017; Blasco et al., 2017, 2018), geothermometers are neither designed for nor applied to such environment due to erroneous results they might provide. A previous study on the CI aquifer in Ouargla, southern Algeria, using some geothermometry compounds such as Na–K, Na–K–Ca and K–Mg (Abdelali et al., 2017) showed promising results, especially with K–Mg geothermometer.

The goal of the present study was to highlight the importance of geothermometry and geochemistry application to the CI aquifer using several geothermometers to cover a larger study area southern Algeria including Touggourt, Djamâa, Meghaier and El Oued. Moreover, recently drilled wells in CI aquifer that had not been studied previously (Edmunds et al., 2003) have been herein included. The first objective was to conduct a geochemical study to identify the chemistry and the origin of water mineralization, thus the type of water that CI aquifer encloses. The second objective was to apply geothermometry to assess the CI reservoir temperatures based on chemical and isotope geothermometers including δ18O (SO4–H2O) isotope, and mineral saturation index.

Section snippets

Geological and hydrogeological context

The North Western Sahara Aquifer System (NWSAS) is one of the largest aquifer systems in the world with more than one million km2 surface area (UNESCO, 1972) stretching over large areas of three countries: Algeria, Tunisia and Libya. In addition to the quaternary water table, which fluctuates approximately at 2 m depth from the soil surface, the NWSAS includes two overlaying large aquifers referred to as 1) the Complex Terminal (CT) aquifer; and 2) the Continental Intercalaire (CI) aquifer,

Analytical techniques

From January 25th to February 14th, 2017, thirty-two water samples were collected from wells drilled in CI aquifer at different locations denoted as follows: Ouargla (23, 24, 25, 26, Kh1, G2 and M3), Touggourt (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11), Djamâa (12, 13, 14, 16, 17, 18), Meghaier (19, 20, 21, 22) and El Oued (27, 28, 29, 30). Chemical characterization of the water samples was performed considering previously established standards (Rodier, 2016). Some physico-chemical parameters such as

Hydrochemistry properties

The physicochemical analysis of the water samples (n = 32) is reported in Table 1S (Supplementary Material). The physical properties showed neutral to slightly alkaline pH ranging from 6.74 to 8.35 (average = 7.21, σ = 0.37), electrical conductivity (EC) varies from 1900 to 3830 μs/cm (average of 2806 μs/cm, σ = 0.36 μs/cm), high discharge temperatures varying from 44.4 to 65.1 °C (average = 56.59 °C, σ = 4.38 °C) with an increasing trend towards the NNE direction reaching 65.1 °C in the wells

Hydrogeochemical characteristics

The collected water samples may be ranged within carbonate-evaporite geothermal systems with high sulfate content. These geothermal systems have been extensively studied (Park et al., 2006; Goldscheider et al., 2010; Mohammadi et al., 2010; Boschetti, 2013; Wang et al., 2015; Yang et al., 2017; Blasco et al., 2017, 2018). The high mineralization of water indicates a long residence time accompanied by high temperatures as suggested by Edmunds et al. (2003), Petersen (2014) and Petersen et al.

Conclusion

The present research focused on geochemical and geothermometrical characterization of groundwater from the Continental Intercalaire (CI) aquifer at a regional scale in the northeastern Sahara of Algeria, using chemical and isotope data. The results showed that The CI waters are neutral to slightly alkaline, with a high salinity. Water chemistry is governed by calcium, sodium and very high chloride and sulfate concentration levels. Water types are of Na–Cl–SO4 and Ca–SO4 composition, which

Declaration of competing interest

The authors declare that there is no conflict of interest.

Acknowledgment

We would like to thank Dr. David Dettman from the Environmental Isotope Laboratory at the University of Arizona for his help with isotope analysis, and Amistadi Mary Kay for providing trace element analysis at the ALEC Laboratory, University of Arizona. Many thanks to the staff from the National Agency of Hydraulic Resources (ANRH) for the valuable help they provided during the field work. This article is a part of a national research project (PRFU) under the number: E04N01UN300120180002.

References (101)

  • S. Chatterjee et al.

    Characterization of subsurface processes estimation of reservoir temperature in Tural Rajwadi geothermal fields, Maharashtra, India

    Geothermics

    (2016)
  • H. Chiba et al.

    Oxygen isotope exchange rate between dissolved sulfate and water at hydrothermal temperatures

    Geochem. Cosmochim. Acta

    (1985)
  • N. Chkir et al.

    Uranium isotopes in groundwater from the continental intercalaire aquifer in Algerian Tunisian Sahara (Northern Africa)

    J. Environ. Radioact.

    (2009)
  • G.E. Claypool et al.

    The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation

    Chem. Geol.

    (1980)
  • E. Dotsika

    Isotope and hydrochemical assessment of the Samothraki Island geothermal area, Greece

    J. Volcanol. Geotherm. Res.

    (2012)
  • E. Dotsika

    H–O–C–S isotope and geochemical assessment of the geothermal area of Central Greece

    J. Geochem. Explor.

    (2015)
  • F. D'Amore et al.

    Observations on the application of chemical geothermometers to some hydrothermal systems in Sardinia

    Geothermics

    (1987)
  • W.M. Edmunds et al.

    Groundwater evolution in the Continental Intercalaire aquifer of southern Algeria and Tunisia: trace element and isotopic indicators

    Appl. Geochem.

    (2003)
  • R.O. Fournier

    Chemical geothermometers and mixing models for geothermal systems

    Geothermics

    (1977)
  • R.O. Fournier et al.

    An empirical Na-K-Ca geothermometer for natural waters

    Geochem. Cosmochim. Acta

    (1973)
  • C. Fu et al.

    A hydrochemistry and multi-isotopic study of groundwater origin and hydrochemical evolution in the middle reaches of the Kuye River basin

    Appl. Geochem.

    (2018)
  • W.F. Giggenbach

    Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators

    Geochem. Cosmochim. Acta

    (1988)
  • W.F. Giggenbach

    Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin

    Earth Planet. Sci. Lett.

    (1992)
  • J. Gonçalvès et al.

    Declining water budget in a deep regional aquifer assessed by geostatistical simulations of stable isotopes: case study of the Saharan “Continental Intercalaire

    J. Hydrol

    (2015)
  • A. Guendouz et al.

    Chlorine-36 dating of deep groundwater from northern Sahara

    J. Hydrol

    (2006)
  • N. Güleç

    Geochemistry of thermal waters and its relation to the volcanism in the Kizilcahamam (Ankara) area, Turkey

    J. Volcanol. Geotherm. Res.

    (1994)
  • I. Gunnarsson et al.

    Amorphous silica solubility and the thermodynamic properties of H4SiO°4 in the range of 0° to 350°C at Psat

    Geochem. Cosmochim. Acta

    (2000)
  • S. Kamel

    Application of selected geothermometers to Continental Intercalaire thermal water in southern Tunisia

    Geothermics

    (2012)
  • R. Laouar et al.

    Stable isotope study of the igneous, metamorphic and mineralized rocks of the Edough complex, Annaba, Northeast Algeria

    J. Afr. Earth Sci.

    (2002)
  • R. Laouar et al.

    Petrology, geochemistry and stable isotope studies of the Miocene igneous rocks and related sulphide mineralisation of Oued Amizour (NE Algeria)

    Ore Geol. Rev.

    (2018)
  • M. Liu et al.

    Sulfur isotope geochemistry indicating the source of dissolved sulfate in gonghe geothermal waters, northwestern China

    Procedia Earth Planet. Sci.

    (2017)
  • M. López-Chicano et al.

    Geochemistry of thermal springs, Alhama de Granada (southern Spain)

    Appl. Geochem.

    (2001)
  • Z. Mohammadi et al.

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

    Geothermics

    (2010)
  • A.S. Moulla et al.

    Updated geochemical and isotopic data from the Continental Intercalaire aquifer in the Great Occidental Erg sub-basin (south-western Algeria)

    Quat. Int.

    (2012)
  • J.L. Palandri et al.

    Reconstruction of in situ composition of sedimentary formation waters

    Geochem. Cosmochim. Acta

    (2001)
  • Z.-H. Pang et al.

    Theoretical chemical thermometry on geothermal waters: problems and methods

    Geochem. Cosmochim. Acta

    (1998)
  • C. Panichi et al.

    Environmental isotopes in geothermal studies

    Geothermics

    (1977)
  • S.-S. Park et al.

    Temperature evaluation of the Bugok geothermal system, South Korea

    Geothermics

    (2006)
  • A. Paytan et al.

    Sulfur isotope stratigraphy. A geological time scale

  • J.O. Petersen et al.

    Water-rock interaction and residence time of groundwater inferred by 234U/238U disequilibria in the Tunisian continental intercalaire aquifer system. procedia earth planet

  • J.O. Petersen et al.

    Groundwater flowpaths and residence times inferred by 14C, 36Cl and 4He isotopes in the Continental Intercalaire aquifer (North-Western Africa)

    J. Hydrol

    (2018)
  • M. Reed et al.

    Calculation of pH and mineral equilibria in hydrothermal waters with application to geothermometry and studies of boiling and dilution

    Geochem. Cosmochim. Acta

    (1984)
  • H. Saibi

    Geothermal resources in Algeria

    Renew. Sustain. Energy Rev.

    (2009)
  • B. Sanjuan et al.

    Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments

    Chem. Geol.

    (2014)
  • M.P. Verma

    Qrtzgeotherm: an ActiveX component for the quartz solubility geothermometer

    Comput. Geosci.

    (2008)
  • M. Voigt et al.

    Evaluation and refinement of thermodynamic databases for mineral carbonation

    Energy Procedia

    (2018)
  • J. Wang et al.

    Hydrochemical characteristics and geothermometry applications of thermal groundwater in northern Jinan, Shandong, China

    Geothermics

    (2015)
  • P. Yang et al.

    Hydrogeochemistry and geothermometry of deep thermal water in the carbonate formation in the main urban area of Chongqing, China

    J. Hydrol

    (2017)
  • S. Arnórsson

    Application of the silica geothermometer in low temperature hydrothermal areas in Iceland

    Am J Sci U. S.

    (1975)
  • I.S. Begley et al.

    High-precision δ2H and δ18O measurement for water and volatile organic compounds by continuous-flow pyrolysis isotope ratio mass spectrometry

    Anal. Chem.

    (1997)
  • Cited by (0)

    View full text