4.1 Physicochemical characteristics for determining potability
In this study, a comprehensive examination of 11 groundwater quality variables was carried out on all groundwater samples. Table 4 and Fig. 3 provide a detailed summary of these parameters, including their range of minimum to maximum values, average values, instances of exceeding limits, and reference standards for drinking water according to WHO (2017) guidelines.
Table 4
Physicochemical data for evaluating the groundwater's suitability
Parameters | Ca2+ | Mg2+ | Na+ | K+ | Cl− | SO42− | HCO3− | NO3− | pH | TDS | EC | TH |
Mean | 150 | 105 | 342 | 26 | 504 | 607 | 219 | 81 | 7,20 | 1986 | 3103 | 816 |
Minimum | 97 | 82 | 245 | 20 | 400 | 310 | 94 | 49 | 6,57 | 1430 | 2380 | 626 |
Maximum | 264 | 154 | 440 | 40 | 660 | 1000 | 316 | 165 | 7,61 | 3040 | 4400 | 1301 |
Standard Deviation | 44 | 20 | 62 | 4 | 84 | 196 | 70 | 23 | 0,25 | 419 | 885 | 185 |
WHO (2017) | 75–200 | 50–150 | 200 | 12 | 200–600 | 200–400 | 300–500 | 50 | 6.5–8.5 | 500–1000 | 500–1500 | 100–500 |
% Exceeded PL | 12 | 0 | 100 | 100 | 16 | 84 | 0 | 96 | 0 | 100 | 100 | 100 |
The data on groundwater quality in the CI of Reggane show that the groundwater of the CI exhibited a pH varying from 6.57 to 7.61, indicating a predominantly alkaline characteristic (92%) and falling within the acceptable range of 6.5 to 8.5. The waters of the CI show significant mineralization, as indicated by the wide range of EC measurements (2380 to 4400 µS/cm). Water with conductivity exceeding 1500 µS/cm is considered unsuitable (WHO 2017). According to the WHO (2017) guidelines, all groundwater samples from the CI are deemed inappropriate for direct human use without prior treatment. The TDS was from 1430 to 3040 mg/L and mean value at 1986 mg/L. All exceeded the WHO-recommended TDS threshold. Excessive exposure to high levels of TDS can lead to cardiovascular diseases, kidney stones, digestive problems, etc. (Subba Rao et al. 2019; Sahu et al. 2018).
High concentrations of calcium can lead to gastric problems, while a deficiency of Ca2+ in water for drinking can contribute to the formation of kidney stones, strokes, colorectal cancer, hypertension, and other issues (Sengupta 2013). The average calcium content in the waters of the CI is 150.48 mg/l, ranging from 97 to 226 mg/l. The average sodium content in the waters of the CI is 342.12 mg/l, varying from 245 to 440 mg/l, with all samples recording levels above the permitted limit. High levels of sodium present a risk of cardiovascular and circulatory problems (Haritash et al. 2008). Potassium concentrations in all samples surpassed the recommended threshold of 12 mg/l, with an average value of about 26.54 mg/l and variations from 20 to 40 mg/l, which can potentially lead to neurological and gastrointestinal issues (Ramesh and Soorya 2012). Bicarbonate levels were found between 94 and 316 mg/l.
High amounts of chloride in water can lead to elevated blood pressure, having a concentration of chloride ions in the CI water ranging from 400 to 660 mg/l, and an average of 504.72 mg/l. Notably, 16% of the samples exceed the 600 mg/l limit. The dietary significance of chlorine, particularly in the form of sodium chloride (NaCl), is prominent, especially concerning chronic heart conditions (WHO 2017). SO42− in the CI water varied from 310 to 1000 mg/l, with a mean of 607.28 mg/l. About 84% of the examined samples were within the WHO's limit of 400 mg/l. High SO4 levels can induce laxative effects, impart an unpleasant bitter taste to water, and contribute to health concerns such as diarrhea, dehydration, and abdominal pain (Egbueri 2023; Karunanidhi et al., 2021).
Bicarbonate is formed during the decomposition of soil organic matter, involving root respiration and the degradation of humus, producing CO2, which reacts with rainwater to form bicarbonate ions. In water, bicarbonate primarily affects the pH and can lead to the formation of deposits in pipes (Alum et al. 2023). Generally, it does not pose a major health risk, but high levels can impact water quality and appliances. Within the area, the levels of bicarbonate in groundwater ranged from 94 to 316 mg/l, averaging at 219 mg/l. All tested samples were below the safe level recommended by WHO (2017).
High nitrate in groundwater are primarily due to excess agricultural fertilizers, waste from animals and humans, as well as plant debris (Egbueri 2023; Egbueri et al. 2023a, b). In drinking water, the WHO permissible concentration of NO3 is 50 mg/l. However, in the studied area, nitrate concentrations range from 49 to 165 mg/l, exceeding the limit recommended by the WHO (2017). This indicates contamination stemming from urban and agricultural sources. The geographic distribution reveals that the northern and central areas of the region exhibit particularly high concentrations of nitrate (Fig. 3).
Ultimately, comparing the chemical characteristics of groundwater with the WHO guidelines reveals several concerns. All the samples examined exceed the WHO's permissible limits for EC, TDS, total hardness, sodium, and potassium, making them unsuitable for direct consumption. Further chemical analysis indicates that 12%, 16%, 84%, and 96% of wells exceed the permissible limit for calcium, chloride, sulfates, and nitrates, respectively. These findings highlight the necessity of treating groundwater from the CI aquifer before its use.
4.3 Assessment of nitrate pollution and associated risks
High concentrations of nitrates in groundwater and rivers, which are primary sources of drinking water, can create environmental, economic, social, and public health problems in cases of excessive levels. Nitrates (NO3−) in water can transform into nitrites (NO2−) in the human body through a simple chemical reduction process. Nitrites reduce the capacity of hemoglobin to carry oxygen to cells, potentially leading to methemoglobinemia, or "blue baby syndrome" (Obeidat et al. 2012; Adimalla and Qian 2019; Adimalla 2020). Although this condition is rare and primarily observed in newborns after ingesting water high in nitrates, the precautionary principle has led the WHO (2017) to recommend a maximum limit of 50 mg/l of nitrates.
The NPI stands as a reliable metric for evaluating the extent of nitrate contamination. Figure 5a illustrates the classification of nitrate levels in the CI groundwater. Within the area, NPI values range from 1.47 to 7.25. Approximately 12% of the sampled locations show moderate pollution, while 36% exhibit significant pollution, and 52% are classified as having very significant pollution (Obeidat et al. 2012; Egbueri et al. 2023a, b). The NPI distribution map illustrates that sites heavily polluted with nitrates are widespread in the studied region, with notable exceptions being wells F-21, F-19, and F-14.
In the study area, 96% of the measured nitrates exceeded the WHO standard, suggesting a high risk associated with the non-carcinogenicity of nitrates in this region. Comparing these data with land use in the region, it is observed that the increase in nitrate levels corresponds to the density of urban fabric and agricultural areas. The shallow depth of the aquifer and the predominantly sandy geological formations facilitate the infiltration of water rich in nitrogen compounds into the aquifer, due to the high solubility of nitrates. These nitrate-rich waters eventually get captured by wells intended for drinking water supply.
HQ values were determined for adults, children, and infants. Infants exhibited HQ values ranging from 3.13 to 10.46, with an average of 5.16. For children, HQ values ranged from 2.47 to 8.27, averaging at 4.08. Adults displayed HQ values varying from 1.61 to 5.38, with an average of 2.65. It was found that the groundwater intended for infants, children, and adults all had a HQ > 1, indicating a substantial risk to the inhabitants.
The spatial distribution of these health risk indices reveals that the entire studied area is exposed to the risks linked with the ingestion of groundwater high in nitrates, with HQ values above 1. Specifically, infants and children face severe health risks (HQ > 3) (Shukla et al. 2020). This increased risk is attributed to high nitrate concentrations, which are a consequence of dense population and intensive agricultural practices in the region. In contrast, adults are exposed to high risks (HQ between 1 and 3), largely due to their greater body mass.
These findings indicated the severity of health issues stemming from the consumption of water contaminated by nitrates and highlight the urgent need for intervention. The establishment of water treatment infrastructures to purify water before its distribution in the drinking water supply network is essential to mitigate these risks.
4.4 Assessment of water suitability for agricultural utilization
The mineral elements in irrigation water play a crucial role in influencing soil health and plant development; consequently, the characteristics of groundwater are significantly impacted by these elements. Among these minerals, salt is particularly detrimental to plants, impeding their metabolism and limiting water absorption, while potentially being toxic to crops. Additionally, the presence of salt can alter the soil's structure, aeration, and permeability, indirectly affecting plant growth. The water suitability analysis for irrigation hinges on various factors, encompassing water characteristics, soil type, soil drainage properties, local climate, and plant resistance to salt. Understanding these factors is critical for effectively managing agricultural practices and optimizing crop yields. Water properties, such as pH, salinity, and nutrient content, profoundly influence soil health and plant growth (Egbueri and Agbasi 2022). Soil drainage properties dictate the rate at which water moves through the soil profile, impacting both water availability to plants and the risk of waterlogging or salinization (Alum et. al. 2023; Singh et al. 2024). Soil type plays a pivotal role in determining water retention capacity, nutrient availability, and root penetration depth, all of which are essential for sustaining healthy crop growth. Local climate conditions, including temperature, precipitation, and evaporation rates, directly influence water requirements and crop water usage efficiency. Finally, plant resistance to salt is a crucial factor, particularly in areas with saline water sources, as it determines the tolerance of crops to elevated levels of salts present in irrigation water (Egbueri and Agbasi 2022).
High EC harms plant water uptake, causing drought and water stress related to excess salt. The presence of sodium is also a crucial criterion in evaluating irrigation water, as its reaction with the soil can reduce permeability. High sodium rates can accumulate high Na+ in the soil for a long time, hindering water infiltration due to soil dispersion. Toxicity due to sodium (Na+) and chloride (Cl−) occurs when these elements accumulate in plants at high concentrations, damaging or reducing crop yield. Toxic effects can manifest as burns or death of leaf tissues. Furthermore, high concentrations of bicarbonates (HCO3−) in irrigation water can lead to the precipitation of calcium and magnesium, increasing SAR and leading to soil infertility.
The IWQI provided a comprehensive assessment of the irrigation water quality, focusing on the five key parameters: SAR, EC, Cl−, Na+, and HCO3−. This index offers a holistic approach to evaluating water suitability for irrigation, considering multiple factors that directly influence soil and crop health. Groundwater quality zoning maps were generated for each of the five parameters (Fig. 4). These parameters are then combined to plot the map of IWQI.
The EC is commonly employed as an indicator of the sum of ion concentration found in natural water sources. It is closely associated with the total sum of anions or cations, measured by chemical analysis, and thus shows a strong correlation with the TDS content. Salinity, resulting from high levels of salts, affects water absorption by plants. These salts increase the osmotic pressure in the soil, rendering it challenging for plant roots to absorb the necessary water. This leads to a kind of internal drought for the plants, where, despite moist soil appearance, plants can wilt due to insufficient absorption to counterbalance water loss through transpiration (Bouselsal and Saibi 2022; Ouarekh et al. 2022). The measured EC values in the CI water exhibit a range from 2380 to 4400 uS/cm. Consequently, 40% and 60% of the EC values for the studied wells fall into the low restriction and moderate restriction classes, respectively. The high EC could be a result of a reduction in plant osmotic activity, interfering with the absorption of water and nutrients from the soil (Aravinthasamy et al. 2020).
In groundwater, the SAR is crucial for assessing the sodium risk for irrigation, based on the absolute and relative concentrations of cations (Eq. 13). SAR, which considers the concentrations of sodium, calcium, and magnesium, is essential for determining the influence of irrigation water on the structure of soil. A high sodium concentration reduces soil permeability and alters its structure by causing the exchange of Na+ with Ca2+ and Mg2+, leading to the dispersion of clay particles and soil degradation. This process can cause salinization and harm crops. SAR values help anticipate and prevent soil degradation, thereby contributing to efficient agricultural production. This approach allows for the adaptation of irrigation practices to maintain soil and crop health. SAR values varied from 4.02 to 7.01 meq/l, having a mean value of 5.26 meq/l. As per FAO guidelines, the studied wells are classified under the Low restriction category, suggesting potential challenges associated with the utilization of the groundwater for irrigation (Ayers and Westcot, 1994).
High sodium content in water can significantly impair soil quality, with implications for agricultural productivity and environmental sustainability. The presence of excess sodium can induce a series of adverse effects, including the reduction of soil permeability, which can impede water movement through the soil profile, affecting root growth and nutrient uptake by plants. Additionally, elevated sodium levels can lead to water infiltration problems, exacerbating soil erosion and runoff issues. Moreover, there exists a risk of cation exchange between sodium ions in the water and calcium and magnesium ions in the soil, resulting in an increase in soil sodium content and compromising soil structure and fertility. Thus, it is crucial to address and mitigate high sodium levels in water sources to safeguard soil health and agricultural sustainability (Todd 1980). Additionally, high sodium levels can lead to leaf burn and necrosis along the outer edges of plant leaves (Ayers 1994). In the studied groundwater samples, sodium concentration ranged from 10.65 to 19.13 meq/l with a mean of 14.87 meq/l. All analyzed groundwater samples were classified in the High restriction category, indicating that their sodium content is high enough to potentially cause significant harm to both soil health and plant growth. This classification highlights the need for careful management or treatment of this water if it is to be used for irrigation purposes.
The chloride anion (Cl-), present in all waters, is characterized by its solubility and ability to easily leach into drainage water. While essential for plant growth, chloride can become inhibitory and even highly toxic to certain species at high concentrations. The concentration of chloride directly influences crop health (Bauder et al 2011; Ludwick et al 1990). Less than 2 meq/l is generally safe for all plants, but between 2 and 4 meq/l, sensitive plants may exhibit mild to moderate damage in response to high chloride concentrations. From 4 to 10 meq/l, even moderately tolerant plants can experience negative effects, and beyond 10 meq/l, chloride can cause severe problems, seriously affecting crop growth and yield. The range of groundwater chloride concentrations examined varies between 11.27 and 18.59 meq/l with an average value of 14.22 meq/l. All the analyzed groundwater samples were classified in the High restriction category, indicating that they possess chloride levels that can be harmful to a majority of crops. This necessitates careful consideration and potential treatment measures, if irrigation is to be done with the water.
Alkalinity, primarily due to carbonate and bicarbonate ions, measures water's ability to neutralize acids and can lead to high pH in water. High levels of bicarbonate indirectly increase the amount of sodium present in water, which can be detrimental to crops and soil. Therefore, monitoring the SAR for highly alkaline irrigation waters is important. Ideally, bicarbonate concentrations should be below 1.5 meq/l for irrigation (Makki et al. 2021; Ayers and Westcot 1994). The findings indicated that the concentration of bicarbonate ions (HCO3−) in the analyzed groundwater samples ranged between 1.54 and 5.18 meq/l. Consequently, for irrigation, 20% of the water samples fell into the moderate restriction group and 80% of the samples went into the low restriction category.
The study area's groundwater had computed IWQI values ranging from 20 to 40, with an average of 31. It was noticed that every well examined have severe restrictions for use in irrigation. This class indicates a significantly lower water quality, often characterized by high concentrations of salts or other contaminants that can be harmful both to soils and plant growth. In exceptional circumstances, this water can be used very limitedly, with specific precautions. For soils, water in this category often requires treatments such as the addition of gypsum to counterbalance high SAR levels and reduce salinity. The soils also need to have high permeability to allow effective drainage, thus avoiding the accumulation of harmful salts. Irrigation must be carefully managed by applying excess water to leach accumulated salts, which requires meticulous water planning and management. As for plants, only those with high salt tolerance should be cultivated with this water. Even in this case, special salinity management practices need to be implemented to minimize adverse effects. It is particularly important to avoid using this category of water for plants sensitive to salts or other contaminants present. The IWQI geographical distribution map for the research region demonstrates that IWQI values range from 20 to 30 in the southern portion of the study area, indicated in brown, and between 30 and 40 in the northern section, highlighted in red.
The SAR serves as a pivotal parameter in the classification of irrigation water, primarily based on its influence on soil permeability. Todd (1980)’s classification delineates irrigation water quality into five distinct categories, ranging from excellent (> 20%) to poor (< 80%), predicated on SAR values. In our study, SAR values exhibited a range from 40.94–58.64%, with an average of 49.94%. Notably, all groundwater samples were classified as "permissible" for irrigation, indicating a satisfactory level of suitability.
Kelly's Ratio (KR) is another essential parameter that evaluates the concentration of sodium relative to bivalent cations. According to Doneen (1964), groundwater with a KR less than 1 is deemed suitable for irrigation. In our study, KR values ranged from 0.66 to 1.37, with an average of 0.94. These values categorize the groundwater into two distinct groups: good (64%) and doubtful (36%) suitability for irrigation, reflecting varying degrees of sodium content.
Furthermore, the Magnesium Adsorption Ratio (MAR) is influential in assessing water quality, with values below 50 considered appropriate for irrigation, while values exceeding 50 pose concerns. In our investigation, MAR values spanned from 45.21 to 61.27, with an average of 54.18. These findings classify groundwater into two categories: suitable (20%) and unsuitable (80%) for irrigation, highlighting the variability in magnesium content.
Additionally, the Permeability Index (PI) provides a supplementary indicator of groundwater suitability for irrigation, with higher values indicating better permeability. PI rates in our study area ranged from 45.51 to 63.13%, with an average of 54.24%. Notably, all groundwater samples demonstrated suitability for irrigation based on PI results, reinforcing the overall favorable groundwater quality for agricultural use.
4.5 Hydrochemistry and mechanisms governing the chemistry and mineralization
4.5.1 Hydrochemical types of groundwater
The chemical attributes of an aquifer system manifest through hydrochemical facies, serving as indicators of the interaction between water and the minerals within the lithological context of the aquifer. These facies delineate fluctuations in the chemical composition across distinct groundwater masses within the aquifer, with their evolution influenced by various factors including aquifer lithology, dissolution kinetics, and flow dynamics. To gain deeper insights into the geochemical evolution processes of groundwater within our study area, the analyzed water samples are plotted on the Piper (1944) diagram. This graphical representation allows for the visualization of the dominant chemical compositions and the identification of hydrochemical facies present within the aquifer system, aiding in the interpretation of groundwater quality and geochemical processes. The analysis of the Piper diagram, presented in Fig. 8, clearly reveals that in the anion triangle, the waters of the CI aquifer show a predominance of chloride with a clear tendency towards sulfate. In the cation triangle, these waters are characterized by a predominance of sodium, with a marked tendency towards calcium. The diamond configuration highlights the classification of the samples into two distinct water types: the Ca-Mg-Cl type and the Na-Cl type.
The presence of these two water types indicates the complexity of the mineralization processes at work. It has been noticed that waters drawn from the CI aquifer undergo a type of water evolution depending on the flow direction, resulting from a sustained relationship between geological formations and groundwater, ion exchange, and the impact of evaporation on the water's saturation degree with respect to carbonated and sulfated minerals (Ismail et al. 2023). This evolution leads to a change in the upstream of the study region, where Na-Cl water type dominates, to downstream, where the Ca-Mg-Cl water type is predominant.
4.5.2 Gibbs diagram
The chemistry of groundwater and its evolutionary mechanisms in the study area are depicted in Fig. 9. According to the plotted Gibbs diagram, the tested waters were dispersed between the field dominated by rock-water interaction and the field dominated by evaporation processes (Egbueri et al. 2020; Bouselsal and Saibi 2022; Arfa et al. 2023). It has been noted that the research area's groundwater is greatly impacted by rock-water interactions, attributable to the limited water circulation within the geological formations. This limited circulation is a consequence of low precipitation levels typical of the arid climate, which results in an extended residence time of water through the pores. Additionally, the arid climate enhances water evaporation, leading to an increased concentration of ions in the groundwater.
4.5.3 Rock-water interaction
Gaillardet et al. (1999) proposed normalized molar ratio models for Na to assist in identifying the main alteration and dissolution mechanisms influencing groundwater chemistry. These models consist of graphs plotting Ca normalized to Na against HCO3 and Ca normalized to Na against Mg. Waters draining carbonates have the following Na-normalized ratios: Ca/Na = 50, Mg/Na = 10, and HCO3/Na = 120. The Na-normalized ratios of waters draining silicates are as follows: Mg/Na = 0.24 ± 0.2, Ca/Na = 0.35 ± 0.15, and HCO3/Na = 2 ± 1. The HCO3 in the waters draining evaporites is reduced in comparison to Mg and Ca, causing a downward divergence from the silicate-carbonate line (Gaillardet et al., 1999). The calculated ratios for the analyzed water samples vary as follows: Mg/Na = 0.6 ± 0.12, Ca/Na = 0.51 ± 0.11, and HCO3/Na = 0.25 ± 0.10. These ratios are then plotted on graphs to specify the origin of chemical ions in the groundwater. The graphs in Fig. 6 show that the field of silicate is where the groundwater points towards evaporite alteration. This implies that silicates and evaporites have contributed to the groundwater chemistry.
4.5.4 Sources of major ions
The hydrochemical characteristics are affected by factors including the makeup of aquifers, climate, water residence time, water-rock interactions, and human activities. Particularly in arid regions like Reggane, high evaporation rates and low precipitation inputs can increase groundwater salinity. Additionally, the distance traveled by groundwater and its residence time also influence its salinity and hydrochemical composition.
The exploration of various mechanisms contributing to the chemistry of the CI aquifer has been shown by the use of binary diagrams, which correlate the main major elements. This technique, recognized for its relevance in the field of hydrogeology, is crucial for studying the origins, both geogenic and anthropogenic, of the chemical constituents of groundwater. This approach is supported by a wealth of noteworthy scientific studies (Luo et al. 2021; Zhang et al. 2021; Bouselsal and Saibi 2022; Hao et al. 2020; Yang et al. 2016; Subramani et al. 2010; Hounslow 1995), collectively emphasizing its significance in facilitating a comprehensive understanding of the influences and interactions within aquifer systems. These research endeavors have shed light on the importance of employing this approach to unravel the complexities of groundwater dynamics, thereby contributing to informed decision-making and sustainable management practices.
4.5.5 Dissolution of evaporites
There are several possible reasons/origins for the groundwater Cl− and Na+ ions. It is natural, either due to inputs from meteoric rains enriched in Cl− and Na+ from seawater, especially in coastal regions. In such cases, the Cl−/Na+ ion ratio should be similar to that of seawater, approaching 1.16. If the Cl− and Na+ ions originate from the dissolution of halite (Eq. 16) present in the surrounding geological formations, the ratio of Na+ to Cl− ought to be one. It is important to note that sodium can also have other sources, such as the dissolution of albite-plagioclase or the alteration of silicate rocks (Hounslow 1995). The analysis of the Cl− vs Na+ diagram reveals that most of the sites are plotted beneath the equilibrium line [Na+] = [Cl−] (Fig. 11a), indicating an excess of sodium compared to chlorides. This observation could be explained by the way rocks and groundwater interact. The excess of sodium ions seems to be compensated by a deficit in calcium ions, resulting from the disintegration of existing silicate rocks and the mechanism of cation exchange on clays, which fix calcium and magnesium while releasing sodium.
NaCl → Na+ + Cl− (Eq. 16)
The SO42− vs Ca2+ graph helps to identify points affected by gypsum dissolution, in accordance with Eq. 17 (Zhang et al. 2021; Arfa et al. 2022; Kebili et al. 2021). Points aligned along the 1:1 line indicate that sulfates and calcium originate from gypsum's dissolution. In our current study (Fig. 11b), the analyzed water points deviate from the 1:1 line, indicating an excess of sulfate. This situation can be explained by the cation exchange process occurring through clay minerals, whereby Na+ ions are released into the water in exchange for the fixation of Ca2+ and/or Mg2+ ions.
CaSO4⋅2H2O ⇄ Ca2+ + SO42−+ 2H2O (Eq. 17)
4.5.6 Silicate weathering
One of the main geochemical processes affecting the chemical makeup of groundwater is the weathering of silicates. The evaluation of this process has been conducted through the analysis of two ratios: (HCO3− + SO42−) vs. (Ca2+ + Mg2+) and Na + K vs. total cations. The ratio between (HCO3− + SO42−) and (Ca2+ + Mg2+) serves as a key indicator for discerning between carbonate and silicate alteration processes (Zhang et al. 2021; Datta and Tyagi 1996). Analysis of the samples relative to the 1:1 line depicted in Fig. 11c yields the following interpretations:
-
Samples aligned along the 1:1 line suggest mineralization resulting from the dissolution of calcite, dolomite, and gypsum.
-
Samples positioned above the 1:1 line indicate an abundance of Ca2+ + Mg2+ compared to SO42− + HCO3−, implying mineralization from carbonate alteration, gypsum dissolution, or reverse ion exchange reactions (Rajmohan and Elango 2004).
-
Samples located below the 1:1 line exhibit an excess of SO42− + HCO3− over Ca2+ + Mg2+, indicating mineralization driven by direct ion exchange processes and the alteration of silicate minerals.
This investigation revealed that 48% of groundwater samples demonstrated an excess of Ca2+ + Mg2+, while 24% exhibited an excess of HCO3− + SO42−. The remaining samples fell along the 1:1 line, suggesting a significant contribution from carbonate and gypsum alterations, as well as reverse ion exchange reactions, in enriching groundwater with calcium, magnesium, and bicarbonate ions, beyond mineral dissolution.
These findings align with the inferences made from the examination of the ratio of Na + K to total cations (Fig. 11d). About 60% of the samples in the Na + K versus total cations graph are over the 1:2 line, suggesting that a significant source of these cations is silicate weathering. However, 40% of the points are between the 1:1 and 1:2 lines, indicating that sulfate and carbonate mineral dissolution occurs in addition to the silicate weathering, which might also be contributing to the enrichment of Ca2+, Mg2+, and HCO3− in groundwater (Hao et al. 2020).
4.5.7 Cation exchange
The interaction between rock and water through cation exchange is a common phenomenon in aquifers, particularly those characterized by fine-grained environments. As previously discussed in the lithological description of the aquifer, the presence of clays is notable, whether as thin layers in the reservoir, as cement in the sandstones, or mixed with sands and gravels. Under these conditions, clays can potentially act as sources of ions for cation exchange processes. To determine if these cation exchange processes are occurring and contributing to the hydrochemical compositions of groundwater, chloro-alkaline indices (Schoeller 1965) such as CAI-1 (Eq. 18) and CAI-2 (Eq. 19) have been introduced. The CAI-1 and CAI-2 values of groundwater samples undergoing cation exchange are less than zero, but the CAI-1 and CAI-2 values of samples created via reverse cation exchange are positive (Luo et al. 2021; Zhang et al. 2021).
\(\text{C}\text{A}\text{I}-1 =\frac{{\text{C}\text{l}}^{-}-{(\text{N}\text{a}}^{+}{+ \text{K}}^{+})}{{\text{C}\text{l}}^{-}}\) (Eq. 18)
\(\text{C}\text{A}\text{I}-2 =\frac{{\text{C}\text{l}}^{-}-{(\text{N}\text{a}}^{+}{+ \text{K}}^{+})}{{\text{H}\text{C}{\text{O}}_{3}}^{-}+{\text{S}{\text{O}}_{4}}^{2-}+ {\text{C}{\text{O}}_{3}}^{-}{+\text{N}{\text{O}}_{3}}^{-}}\) (Eq. 19)
The CAI-1 vs. CAI-2 diagram (Fig. 12a) illustrates that 88% of the samples display negative values for both CAI-1 and CAI-2, while the remaining 12% exhibit positive values for both indices (Fig. 12a). This observation suggests that the predominant portion, accounting for 88% of the water samples, undergoes a cation exchange process, wherein Ca2+ and Mg2+ ions are replaced by Na+ and K+ ions sourced from the aquifer materials. Conversely, 12% of the water samples experience a reverse process, characterized by the replacement of Na+ and K+ ions with Ca2+ and Mg2+ ions originating from the aquifer materials (Appelo and Postma 2005; Zhang et al. 2021).
The (Ca + Mg − HCO3 − SO4) versus (Na + K − Cl) ratio, as depicted in Fig. 12b, played a crucial role in validating the involvement of cation exchange processes in groundwater mineralization within the study area (Hammed et al. 2023; Ouarekh et al. 2022; Bouselsal and Zoari 2022; Zhang et al. 2021). Analysis of the graph reveals that 22 samples are situated along the positive x-axis, indicating an enrichment of sodium and potassium ions in comparison to calcium and magnesium ions, thus suggesting the release of sodium and potassium. Conversely, 3 samples are positioned towards the negative x-axis, signaling an enrichment in Ca2+ and Mg2+ relative to Na+ and K+ ions. This enrichment is attributed to the fixation of sodium and potassium, resulting in the release of calcium and magnesium by the aquifer matrix.
4.5.8 Water saturation
In arid regions, like Reggane, the notable impact of evaporation combined with limited recharge due to precipitation can cause the groundwater to become more salinized. The amount of chemical ions in water increases due to evaporation, potentially leading to saturation and the subsequent precipitation of dissolved salts. Under such circumstances, the hydrochemical characteristics of groundwater can undergo changes, thus influencing the types of groundwater present.
The groundwater system's mineral balance (equilibrium) is represented by the Saturation Index (SI) (Zhang et al. 2021). The SI of the primary minerals vary as follows in this study: aragonite (-0.76 to 0.12 with an average of -0.19), calcite (-0.61 to 0.27 with an average of -0.05), dolomite (-1.02 to 0.72 with an average of 0.09), anhydrite (-1.43 to -0.77 with an average of -1.13), gypsum (-1.21 to -0.55 with an average of -0.91), and halite (-5.61 to -5.20 with an average of -5.40). Furthermore, plotting the SI against the TDS values was done (Fig. 12c). The groundwater did not include saturating amounts of the anhydrite, gypsum, and halite, while the carbonate minerals (aragonite, dolomite, and calcite) were distributed close to the zero line, reaching saturation (Zhang et al. 2023a). Thus, one of the main continuing processes is the breakdown of halite (NaCl) and sulfates, contributing to the increased concentrations of Na+, Cl−, Ca2+, and SO42−.
The assessment of evaporation's influence on the groundwater system can be conducted by analyzing the correlation between Cl− concentrations and TDS (Luo et al. 2021; Li et al. 2021; Yang et al. 2016). In this study, Cl− primarily originated from the dissolution of halite, a process unaffected by cation exchange phenomena. Figure 12d depicts a noticeable elevation in Cl− concentration corresponding to increasing TDS (R = 0.93), indicating the significant impact of evaporation on the groundwater within the region.
4.5.9 Sources of nitrate
Human activity has a significant impact on the chemistry of groundwater. Indeed, the chemical composition is altered by the infiltration of municipal wastewater under urban centers and agricultural drainage water under cultivated lands. In the studied area, the primary contaminant affecting the quality of groundwater is nitrate. The measured concentrations of NO3− vary between 49 and 165 mg/l, averaging at 81 mg/l, thus exceeding the acceptable limit of 45 mg/L set by the WHO in 2017 for all analyzed samples. In natural water, unaffected by human intervention, usually, the concentration of NO3− does not surpass 20 mg/L (Spalding and Exner 1993; Egbueri et al. 2023a). Any value above this limit indicates contamination of anthropogenic origin. Due to its high solubility and significant mobility, nitrate present in groundwater is a useful tool for determining how intense human activity is (Egbueri 2023). The main sources of NO3− in these waters are connected to domestic wastewater outputs and agricultural operations, explaining the high concentrations observed. A thorough analysis was conducted to analyze the mechanisms regulating the groundwater's hydrogeochemistry enriched in nitrates.
The molar ratios graph of Cl−/Na+ vs. NO3−/Na+ (Fig. 13a) serves as a widely recognized tool for evaluating the sources of nitrate in groundwater (Zhang et al. 2023b; Amiri et al. 2022). As depicted in Fig. 13a, the Cl−/Na+ and NO3−/Na+ ratios observed in groundwater samples span from 0.75 to 1.12 and from 0.05 to 0.21, with mean values of 0.96 and 0.09 respectively. These ratios provide insights into the relative abundance of chloride, nitrate, and sodium ions in the groundwater. Stoichiometric processes governing the relationships among these ions are essential for understanding the sources of nitrate. For instance, the presence of high Cl−/Na+ ratios indicates potential contamination from sources rich in chloride ions, such as sewage or industrial effluents. Meanwhile, the NO3−/Na+ ratios shed light on the contribution of NO3, with higher values suggestive of nitrate inputs from agricultural fertilizers or organic waste (Agbasi et al. 2023). Furthermore, the position of the samples on the graph suggests that agricultural activities and domestic discharges are the primary contributors to the water pollution in the area. This inference aligns with the observed Cl−/Na+ and NO3−/Na+ ratios, indicating potential contamination from agricultural runoff and domestic sewage discharge, both of which are known sources of nitrate pollution in groundwater (Zhang et al. 2023b; Amiri et al. 2022).
In order to exclude the impact of groundwater content or dilution on identifying the source of NO3−, the NO3−/Cl− ratio was applied (Zhang et al. 2023b; Amiri et al. 2022). Generally, waters with high salinity have a low NO3−/Cl− ratio under natural conditions, indicating an anthropogenic influence such as inputs from agricultural, domestic, and municipal pollutants (Luo et al. 2018). The sampled groundwater is predominantly linked to a predominance of residential inputs, as Fig. 13b illustrates. This indicates that domestic wastewaters account for the majority of the nitrate in groundwater, with very little coming from agricultural operations. In other words, the study region's nitrate-enriched groundwater is primarily found in the residential regions as opposed to agricultural zones, which is confirming that the hydrochemistry of the waters is substantially influenced by the domestic effluents from residential areas.
4.6 Multivariate statistical analysis
4.6.1 Analysis of hydrochemical parameters via Pearson's correlation matrix
In hydrogeological studies, correlation analysis explores the links between the physicochemical properties of groundwater, using a correlation coefficient ranging from − 1 to + 1. This coefficient illustrates the nature and intensity of the relationships between different variables: a coefficient nearing + 1 indicates a strong positive correlation, while one nearing − 1 signifies a negative correlation. A coefficient around zero implies a lack of significant correlation. The correlation matrix for 11 variables (TDS, pH, Cl−, SO42−, HCO3−, Na+, NO3−, K+, Ca2+, and Mg2+) in this study, presented in Table 5, highlights high correlations, indicating notable hydrochemical interactions.
Table 5
Illustration of the relationships between physicochemical parameters using Pearson’s correlation matrix
Variables | Ca2+ | Mg2+ | Na+ | K+ | Cl− | SO42− | HCO3− | NO3− | TDS | EC | pH |
Ca2+ | 1 | | | | | | | | | | |
Mg2+ | 0.784 | 1 | | | | | | | | | |
Na+ | 0.631 | 0.352 | 1 | | | | | | | | |
K+ | 0.760 | 0.641 | 0.540 | 1 | | | | | | | |
Cl− | 0.618 | 0.505 | 0.837 | 0.543 | 1 | | | | | | |
SO42− | 0.878 | 0.720 | 0.731 | 0.726 | 0.551 | 1 | | | | | |
HCO3− | 0.164 | 0.237 | -0.165 | 0.013 | -0.085 | -0.146 | 1 | | | | |
NO3− | 0.027 | -0.062 | -0.128 | -0.096 | -0.047 | -0.116 | -0.192 | 1 | | | |
TDS | 0.937 | 0.740 | 0.778 | 0.858 | 0.734 | 0.902 | 0.038 | -0.068 | 1 | | |
EC | 0.421 | 0.463 | 0.287 | 0.476 | 0.258 | 0.458 | 0.087 | -0.238 | 0.472 | 1 | |
pH | -0.322 | -0.195 | -0.328 | -0.035 | -0.359 | -0.168 | -0.295 | -0.171 | -0.235 | -0.138 | 1 |
Major ions such as chloride, sulfate, potassium, sodium, calcium, and magnesium are identified as principal contributors to the increase in salinity in the CI aquifer. Strong correlations are observed, for example, among calcium and sulfate (r = 0.87), sulfate and magnesium (r = 0.72), sulfate and potassium (r = 0.73), chloride and sodium (r = 0.83), and between chloride and calcium (r = 0.61) (Bouselsal and Saibi 2022). These relationships suggest that the dissolution of evaporite rocks containing sulfate and chloride in the aquifer leads to an increase in water mineralization. This mineralization, in turn, promotes evaporitic minerals to dissolve, such as gypsum (CaSO4·2H2O) and halite (NaCl) (Kebili et al. 2021; Hammed et al. 2023), resulting in the dissolution of salts and the enrichment of water in chloride, sulfate, potassium, sodium, magnesium, and calcium.
4.6.2 Analysis of hydrochemical parameters via principal component loadings
The principal component analysis was performed on eleven analyzed variables (TDS, EC, pH, Ca, Mg, Na, K, SO4, Cl, HCO3, and NO3). The PCA results, depicted in Fig. 14a, indicate that two principal components collectively account for 63.62% of the total variance in the parameters. This suggests that the hydrochemical evolution of groundwater is influenced by two distinct processes.
The first extracted principal component (F1) explains 51.03% of the total variance and is characterized by high loadings of TDS, EC, Cl, SO4, K, Na, Mg, and Ca, along with a moderate loading of HCO3. Factor 1 signifies the salinization factor, encompassing natural processes like the dissolution of evaporites (halite and/or gypsum), precipitation/dissolution of carbonates, and non-conservative transport phenomena (such as cation exchange with clay minerals).
On the other hand, the second factor (F2) represents 12.59% of the total variation and is primarily influenced by NO3 and pH. This suggests the contribution of anthropogenic inputs to groundwater mineralization, indicating the impact of human activities on groundwater quality (Zhang et al. 2023b; Amiri et al. 2022).
4.6.3 Analysis of hydrochemical parameters via hierarchical clusters
Figure 14b illustrates a dendrogram resulting from the hierarchical cluster analysis applied to the hydrochemical data from the CI aquifer, revealing their distribution into three distinct groups based on salinity levels. The first group, representing 60% of the samples, is characterized by low mineralization, with electrical conductivity not exceeding 3200 µS/cm in the majority. These samples, primarily of the NaCl type, originate from the upstream part of the aquifer. The second group, comprising 32% of the samples, is distinguished by moderate mineralization. The waters' electrical conductivity ranged from 3200 to 4200 µS/cm, and they present a combination of NaCl and Ca-Mg-Cl types. These samples generally come from agricultural areas, where irrigation and intense evaporation under the arid climate lead to a change in water type from NaCl to Ca-Mg-Cl downstream of the aquifer, due to the return of irrigation waters enriched in salts and cation exchange processes. Finally, the third group, which includes only two boreholes, is characterized by high mineralization, with electrical conductivity exceeding 4200 µS/cm, indicating a Ca-Mg-Cl water type. These boreholes are located south of the Reggane airport, where evaporation and infiltration of wastewater from nearby residences significantly impact the groundwater's chemical makeup.