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Hydrochemistry and geothermometry of an Albian aquifer from Oued Righ region in northeastern Algerian Sahara
Geothermal Energy volume 2, Article number: 3 (2014)
The area of Oued Righ is one of the important geothermal areas in northeastern Algerian Sahara. It is characterized by a hot arid climate with intense dryness and very high evaporation rates. These various climatic characteristics are reflected on the hydrography of the study area. The water requirements of the Oued Righ region are provided by groundwater resources contained in the two aquifers: the complex terminal and the deeper confined continental intercalaire aquifer.
Twenty-seven samples were taken in November 2010 and April 2012; Water samples were analyzed for major and minor dissolved chemical constituents.
Samples collected from the CI aquifer are characterized by high temperatures varying from 40°C to 60°C and conductivities of 2110 to 3370 µS/cm. The total dissolved solids (TDS) of the thermal waters range from 1488 to 5480 mg/l. The waters are generally of sodium and magnesium sulfated and sodium chlorinated types. The results of mineral equilibrium modeling indicate that the thermal waters of Oued Righ are undersaturated with respect to evaporite minerals and oversaturated or nearly in equilibrium with respect to dolomite, calcite, and aragonite. This paper presents ternary diagrams of Na-K-Mg1/2 and Cl-SO4-HCO3 and a calculation model, which allows location of geothermal water with the two diagrams, involving knowledge of their chemical composition.
Groundwater in the Albian aquifer of Oued Righ shows a change in its chemical properties between the two sampling periods, resulting from water-rock interactions and mixing processes.
The groundwater reservoir of the continental intercalaire (CI) is the deep reservoir of the septentrional Sahara. The continental intercalaire aquifer of North Africa is one of the largest confined aquifers in the world, comparable in scale to the great artesian basin of Australia and covers some 600,000 km2 on only Algerian and Tunisian territories with a potential reservoir thickness of between 120 and 1000 m (Castany ).
The water chemistry is of Na-SO4-Cl composition, and the total dissolved solids (TDS) of the thermal waters range from 1488 to 5480 mg/l. Its temperatures range from 40°C to 60°C. The current studies in the area are directed towards the geochemical evaluation of thermal waters in the Oued Righ field on the basis of chemical geothermometry and mineral equilibrium calculations.
The area is located in the northeastern Algerian Sahara. It is limited by large chotts and the piedmonts of Zab to the north, by the Mio-Pliocene and Turonien plateaus of the dorsal Mozabite and Daias plateaus to the west, by the sandy regs of the Ouarglie area to the south, and by the dune belt of the Grand Erg Oriental to the east. The study area is considered to be arid, the mean annual precipitation is less than 100 mm, and the mean annual temperature is around 22°C. The mean annual potential evapotranspiration is approximately 1165 mm.
Geology and hydrogeology
In the study area, there are two aquifer systems, separated by thick argillaceous and evaporitic series, base of the upper Cretaceous: the CI and complex terminal (CT) (Figure 1). The geology of the studied area has been investigated by several authors (Cornet ; Bishop ; Castany ). The CI aquifer is located within a complex succession of clastic sediments of Mesozoic age; the thickness and lithology of which show significant lateral variation (UNESCO ) (Figure 2).
The aquifer is, however, hydraulically continuous over the whole basin from north to south from the Saharan Atlas to the Tinrhet Plateau and further south to the Tassili Mountains of the Hoggar, and west to east from western Algeria to the Libyan border (Edmunds et al. ).
Twenty-seven samples were taken in November 2010 and April 2012; the geographical location of the sampling site is shown in Figure 3. The physicochemical parameters (temperature, pH, and electric conductivity) were measured in situ using a WTW multi-parameter (Weilheim, Germany). Water samples were analyzed for major and minor dissolved chemical constituents. Ca, Mg, Cl, and HCO3 were determined by the titration method. SO4 was determined using spectrophotometric method. Na and K were analyzed using a flame photometer. Mineral saturation indices for a number of common minerals potentially present in the studied localities were calculated at measured discharge temperatures using PHREEQC 2.7 (US Geological Survey, Denver, Colorado) (Parkhurst and Appelo ) interfaced with Diagrams 5.8.
Results and discussion
Hydrogeochemical properties of thermal waters
Samples collected from the CI aquifer are characterized by high temperatures varying from 47.5°C to 60.4°C in November 2010 and 40.5°C to 61.3°C in April 2012 (Table 1). The conductivity values in the CI groundwater range from 2130 to 3300 μS/cm (November 2010) and 2110 to 3370 μS/cm (April 2012), having average TDS values of 1488 to 5480 mg/l (April 2012) and 1563 to 2047 mg/l (November 2010).
Dominant cations are mainly sodium and calcium and range, respectively, from 211.26 to 370.20 mg/l and from 108 to 208 mg/l (November 2010) (Table 2). The second period (April 2012) of the CI aquifer has sodium ranging from 117.5 to 298.75 mg/l and calcium varying between 132 and 216.8 mg/l (Table 3). Dominant anions are sulfate (266.18 to 4450.91 mg/l) and chloride (305.30 to 683.37 mg/l) in April 2012. The water samples for the period November 2012 are characterized by sulfate (410.10 to 714.54 mg/l) and chloride (312.40 to 695.80 mg/l). Variable water types may indicate different hydrogeochemical processes such as mixing and water-rock interaction (El-Fiky ). A piper trilinear diagram (Figure 4) shows that all the thermal waters are characterized by the dominance of Cl + SO4 over HCO3 and Na + K over Ca + Mg for the period November 2010 and SO4 + Cl over HCO3 and Ca + (Na + K) over Mg for the period April 2012. The sodium sulfated type is present in 53.85% of the samples for the period November 2010 and 7.14% for the period April 2012; some samples are rich in chloride and sodium, showing sodium chlorinated type which represents 46.15% (November 2010) and 7.14% (April 2012). The calcium sulfated type is present in 57.14% only for the period April 2012. Finally, the magnesium sulfated type represents 28.57% for the period April 2012. This is due to the combination of mixing with cold groundwater and water-rock interaction processes in the thermal aquifers (Tarcan ).
Mineral saturation status
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 ).
In this study, the calculation of saturation indices of carbonate (calcite, aragonite, and dolomite) and evaporite (gypsum, anhydrite, and halite) minerals with respect to the calculated water composition was performed using the PHREEQC program (Parkhurst and Appelo ) which uses the WATEQ Debye-Hückel equation. Values of saturation index greater than, equal to, and less than zero represent oversaturation, equilibrium, and undersaturation, respectively. All thermal waters in the study area are undersaturated with respect to evaporite minerals (gypsum, anhydrite, and halite) for the period November 2010 and for the period April 2012, indicating that these minerals are undergoing dissolution (Figures 5 and 6), and explaining the high concentration of evaporite minerals in the reservoir. We may assume that the SI values falling within the range of ±0.5 units from 0 indicate the equilibrium state (Pulmmer et al. ). Most are oversaturated or nearly in equilibrium with respect to dolomite, calcite and aragonite for the period November 2010 and for the period April 2012, indicating that these carbonate minerals occur in the groundwater.
A geothermometric technique proposed by Giggenbach () discriminates between immature waters and fully equilibrated waters originating in deep reservoirs. A total of 27 completed and analyzed groundwater samples from the CI were used in the Giggenbach diagram (Figure 7). The Na-K-Mg1/2 ternary diagram (Giggenbach ) is used for evaluating equilibrium between the hot waters and rocks at depth and to estimate reservoir temperature. Figure 7 shows a Na-K-Mg1/2 triangular diagram for thermal water samples from wells reaching the CI aquifer in Oued Righ data points plot adjacent to the Mg1/2 corner, which is typical of ‘immature waters’ that do not attain equilibrium with their associated rocks or mixing with superficial waters.
All the samples collected in the CI aquifer are plotted in the Cl-SO4-HCO3 ternary diagram (Figure 8). It is shown that the waters of Oued Righ plot between the Cl and SO4 fields of volcanic waters, but they never attain maturity.
Water quality for irrigation
The suitability of groundwater for irrigation is dependent on the mineral constituents of water on the both the plant and soil (Maoui et al. ). In order to determine the suitability of groundwater for irrigation use, the Wilcox classification diagram (1955) in Figure 9 has been used. This graph is based on electrical conductivity (EC) and on sodium adsorption ratio (SAR). The SAR is of particular importance because a high Na content in irrigation water may increase soil hardness and reduce its permeability (Tijani ). High SAR can disperse soil aggregates, which in turn reduces the number of large pores in soil (Grattan ). Plotting of SAR on the Wilcox diagram (Figure 9) illustrates that most of the groundwater samples fall in the two fields (C3S1 and C4S1), indicating a very high salinity and low alkalinity hazard. This can be suitable for plants having good salt tolerance.
Groundwater in the Albian aquifer of Oued Righ shows a change in its chemical properties between the two sampling periods, resulting from water-rock interactions and mixing processes. The waters are generally of sodium and magnesium sulfated and sodium chlorinated types. The saturation indices of the study area show that evaporite minerals are undersaturated and carbonate minerals are oversaturated or nearly in equilibrium. The geothermal waters from the Oued Righ are immature waters, as indicated by the ternary diagram Na-k-Mg1/2. Wilcox classification shows that most groundwater samples fall in to two fields (C3S1 and C4S1), indicating a very high salinity and low alkalinity hazard.
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The authors declare that they have no competing interests.
This research is a part of the phd thesis. NK supervisor of the phd thesis. WC carried out the sample analysis and drafted the manuscript. Both authors read and approved the final manuscript.
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Chaib, W., Kherici, N. Hydrochemistry and geothermometry of an Albian aquifer from Oued Righ region in northeastern Algerian Sahara. Geotherm Energy 2, 3 (2014). https://doi.org/10.1186/s40517-014-0003-3
- Continental intercalaire
- Thermal waters
- Oued Righ