Application of Water Quality Index to Assess the Potability of the Phreatic Aquifer in Ouargla (Algeria)

Groundwater is essential for sustainable development and drinking water supply in the Saharan regions. This work aims to assess the quality of water for domestic use and the hydrogeochemical characteristics of the phreatic aquifer of the Ouargla. Forty-two (42) wells capturing the phreatic aquifer are sampled and analyzed. The parameters analyzed are pH, EC, TDS, cations and anions. The water’s potability was evaluated using the Water Quality Index (WQI) method, using the main physicochemical parameters that have potential adverse effects on human health. The results obtained show three classes: the poor class (14.29%), the very poor class (19.04%), and the non-potable class (66.67%). The analysis of the Piper and the Gibbs diagrams, the Pearson correlation matrix and the diagrams of relationship between the major elements, as well as the indices of saturation of the main minerals, shows that the groundwater samples were arranged into two groups; Ca-Mg-SO 4 -Cl (38.5%) and Na-Cl (61.5%), the geochemical processes occurring in the aquifer mainly include the dissolution of halite, gypsum, the cation exchange between water and clay minerals and anthropogenic inputs.


INTRODUCTION
The people of the Ouargla basin have traditionally relied on the water table for drinking water and irrigation of palm trees. This aquifer is tapped by traditional wells, which provide enough water for a population of only a few thousand people. Nonetheless, the basin's water needs have increased due to the huge growth in population, necessitating a reliance on the basin's highly abundant deep water reserves, represented by the terminal complex aquifer and the intercalary continent aquifer. During the 1980s, significant flows have been drawn from the CT and CI aquifers, contributing to groundwater recharge and the region's sparse rainfall. Because there is no natural outlet, the piezometric level at the surface has risen throughout the years of deep aquifer mining. The rising phenomena has had major environmental implications, such as oases degrading and groundwater quality changing (Boussaada et al., 2023;Satouh et al., 2021;Bouselsal et al., 2015). This phenomena has harmed most lower Saharan regions' water tables (Kharroubi et al., 2022) (the Ouargla basin, the valley of Oued Souf, and the valley of Oued Righ), resulting in major economic losses for the rehabilitation of degraded areas. The current study intends to examine the consumption quality of phreatic groundwater in the Ouargla basin as well as to present the mechanisms of salinization of the waters of this aquifer.

DESCRIPTION OF THE STUDY AREA
The Ouargla basin is located in northeastern Algeria (Fig. 1) The Ouargla basin is part of the sedimentary basin of the northern Sahara extending over approximately one million km 2 , of which 60% is in Algeria (Zeddouri et al., 2010;Castany, 1982). This basin is a depression of SSW-NNE direction (Chellat, 2014;Busson, 1970;Cornet, 1964) filled with a sedimentary series of the Paleozoic and Meso-Cenozoic, with a thickness exceeding 5000 m.
In the study area, the interpretation of logs of water and oil drilling shows that the basement contains three aquifers (Boussaada et al., 2023;Kharroubi et al., 2022;ANRH, 2004;OSS, 2003;UNESCO, 1972) these are from top to bottom, phreatic aquifer, the aquifer of the terminal complex (CT), and the aquifer of the intercalary continental (CI). The phreatic aquifer is unconfined, Quaternary and Mio-Pliocene age, represented by sands, sandstones, and gravels with gypsum and tuff levels (calcareous con-

MATERIAL AND METHODS
A groundwater sample campaign was carried out in 42 wells to gather phreatic groundwater from Ouargla. The GPS locations of the sampled water sites were validated on the field. After sampling, the physicochemical parameters (pH, temperature, and conductivity) were determined on-site using a portable multi-parameter. Water samples were collected in plastic bottles according to Rodier (1996) sampling methodology for laboratory examination. The data was statistically analyzed using the SPSS version 19.0 software. To create the statistical research, the Pearson correlation matrix, and to compare the numerous factors evaluated with potability norms. The saturation indices for the major evaporite minerals may be calculated using software such as PHREEQC (Sracek et al., 2004;Mukherjee et al., 2008;Parkhurst and Appelo, 1999). DIAGRAMMES software (Simler, 2014) was used to display the samples on Piper diagrams and determine the chemical facies. Water potability was measured by comparing it to Algerian drinking water standards (JORADP, 2011) ( Table 1)

Chemical facies and salinity
We were able to determine the water types using hydro-chemical data from phreatic groundwater. These chemical data are shown on the Piper diagram (Piper, 1944) (Fig. 5). The graphic distinguishes four kinds of groundwater (Peiyue et al., 2019 ): (1) Ca-Mg-HCO 3 , (2) Ca-Mg-SO 4 -Cl, (3) Na-Cl, and (4) Na-HCO 3 . According to the Piper diagram, the measured waters are classified as Ca-Mg-SO 4 -Cl (38.5%) and Na-Cl (61.5%). This classification emphasizes the dominance of the evaporitic facies and the hydrogeochemical processes that regulate the high mineralization of phreatic aquifer of Ouargla, which varies from 2.3 g/l (P162) to 281.6 g/l (P059).

Assessment of the potability of water
The purpose of water potability evaluation is to educate the general public, government agencies, and decision-makers about the status of water resources and to determine the most advantageous action to safeguard human health. Water quality was assessed for potability using Algerian drinking water standards (JORADP, 2011) ( Table 1). According to the results, the phreatic aquifer in Ouargla is not drinkable. Chemical ingredient concentrations do not satisfy Algerian norms. The groundwater under investigation is salty, and key element concentrations (Table 1) are typically greater than acceptable levels. The electrical conductivity ranges from 2790 µS/cm (P059) to 169600 µS/cm (P064), with a mean of 48380 µS/ cm. These values are much higher than the Algerian norm for potability (2800 µS/cm).
The high calcium and magnesium contents in the water lead to an increase in overall hardness (TH). in the region investigated, the TH varies between 789 and 7953.6 mg/L CaCO3, in wells P422 and P12, respectively, suggesting extremely hard water (Freeze and Cherry, 1979). These findings greatly outperform the Algerian norm of 500 mg/L CaCO 3  The approach is frequently utilized by numerous academics and environmentalists worldwide to characterize the level of water quality. This approach has the benefit of quantitatively combining a number of water quality factors into a single value (Kebili et al., 2021). What facilitates communication with the different water field operators. In this research, the Algerian potability requirements were employed to calculate the GWQI ( Table 2). The GWQI computation is broken down into three phases, as shown in equation 1 (Touahri et al., 2022;Sadeghi et al., 2021). The first step is to give each of the eleven characteristics a weight (wp) depending on their value in terms of total drinking water quality. These parameters are shown in Table 2. The relative weight (Wp) of each parameter is derived in the second step by dividing the total of the weights  (wp). In the third phase, the rating (qi) of each parameter is derived by dividing its concentration in the chemical parameter by the Algerian standard concentration. The resulting number is then multiplied by 100.

Mechanism of salt charge acquisition
To establish the mechanisms regulating the mineralization of Ouargla phreatic groundwater, the Gibbs diagram (Gibbs, 1970; Venkatramanan  This graphic displays three realms to which different forms of water belong (Fig. 4). Water-rock interaction mechanisms have an impact on the first domain. The second region illustrates the evaporation-affected waterways. Precipitation-influenced water is the third domain. Evaporation dominates the waters of Ouargla phreatic groundwater, according to the projection of the studied water samples. Field data confirms the obtained conclusion, where the static level of the water table under the agglomerations (city of Ouargla) and agricultural lands is at minus 3 meters, encouraging water evaporation, especially in summer.
The Pearson correlation matrix of chemical parameters in groundwater of phreatic aquifer (Table 3) reveals the strong positive correlations  between Ca 2+ and SO 4 2-, Na + and Cl -, K + and Cl -, Ca 2+ and Cl -, K + and SO The connection between Na + and Cl - (Fig.  5a) is nicely aligned on the halite dissolution 1:1 line, demonstrating that groundwater is the consequence of halite dissolving (Bouselsal and Belksier., 2018). The Ca 2+ and SO 4 2association (Fig. 5b) reveals that sulfate and calcium develop in proportion. This connection shows the solubility of anhydrite and gypsum. The bulk of the locations, however, are below the gypsum dissolving line. The existence of SO 4 2sources other than anhydrite and gypsum causes this sulfate overload. The association between Na + and SO 4 2- (Fig. 5c) reveals that thenardite (Na 2 SO 4 ) dissolution causes a corresponding rise in sulfate and sodium (Kraiem et al., 2014;Garrett, 2001). The excess of sulfate over sodium, on the other hand, implies another source of sulfate, which has previously been mentioned (dissolution of anhydrite and gypsum). The association between Na + and Ca 2+ (Fig. 5d) distinguishes two groups of water locations. A group 1 in which the increase of Na + and Ca 2+ occurs simultaneously as a result of evaporite dissolution. The second group consists of places where we see a drop in Ca 2+ but Na + continues in evolution, which may be explained by cation exchange or precipitation of less soluble sulfates than halite. The graph SO 4 2and Clshows the link between these two components (Fig. 5e). The graph displays SO 4 2-proportionate with Clsignifying the common saliferous origin. Sulfates and chlorides are abundant in other regions. The lithological variety in the research region is reflected in this. The cation exchange between groundwater and the sandy-clay formations of the aquifer is calculated by evaluating the bivariate diagram in Figure 5f (Bouselsal and Zouari, 2022;Qichen et al., 2020) between [(Mg 2+ +Ca 2+ ) -(SO 4 2-+ HCO 3 -)] and (Na + +K + -Cl -). The graphic shows two groups of points: group (1) represents the lack of the cation exchange phenomena, and group (2) represents Ca 2+ and Mg 2+ enrichment relative to Na + as a consequence of Na + fixation and Ca 2+ and Mg 2+ release by the aquifer matrix. Consequently, the cation exchange phenomenon, when combined with the evaporite dissolving process, regulates the salinization of phreatic aquifer.  The saturation index findings obtained with the PHREEQC program (Appelo and Postma, 1994). are shown in Figure 6. Groundwater, on the other hand, is calcite, aragonite, and dolomite saturated. Sulfate minerals have a lower saturation level than carbonate minerals. Anhydrite's saturation index varies from +3.35 to +1.69. The saturation index of gypsum varies from -0.78 to +2.26 (45.23% of samples are saturated). Ultimately, halite's saturation index varies from -5.51 to -0.24. (so it is undersaturated).

CONCLUSION
The phreatic aquifer of Ouargla is composed of Quaternary sands, sandstones, and gravels. The water table flows south to north most of the time. The hydrogeochemical investigation finds that evaporitic facies (Ca-Mg-SO4-Cl (38.5%) and Na-Cl (61.5%) predominate, with electrical conductivities of 2790 µS/cm (P059), 48380 µS/cm (P064), and 169600 µS/cm (P064), respectively. Salinity levels vary from 2.3 g/l (P162) to 281 g/l (P059). The groundwater analyzed is non-potable according to Algerian potability criteria. These groundwaters are saline and highly hard, with concentrations of most key elements above permissible values. The water quality index (WQI) approach finds that the WQI indices vary from 116 to 12271. Water quality was graded as bad (14.29%), extremely poor (19.04%), or non-potable (66.67%). The salinity of phreatic aquifer is connected to gypsum and halite dissolution, cation exchange, and pollution, according to a statistical and geochemical research.