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