*3.2. Relationship between Depth of Wells and the Concentration of Physicochemical Parameters*

Figure 3 shows the scatter plot of F−, Na<sup>+</sup> , and NO<sup>2</sup> − concentrations with groundwater level depth. It shows that the water samples were mostly concentrated in the shallow depth (less than 30 m). Fluoride is present in lower concentrations in shallow groundwater than in deep groundwater. This is because the dissolution of F-containing minerals such as

fluorite is an important source of F− in groundwater of the study area, and the amount of fluorite is higher in the deep aquifer. The alkaline pH can influence CaF<sup>2</sup> activity and favors the mobilization of F− from soil and weathered rocks into groundwater. This assumption was also formulated by other researchers [64–66]. The enrichment of F− can also be influenced by the ratio between HCO<sup>3</sup> <sup>−</sup>, Na<sup>+</sup> and Ca2+ in groundwater, as confirmed by Saxena and Ahmed [67], Rango et al. [68], and Kimambo et al. [64]. Na<sup>+</sup> concentration is lower in the shallow aquifer, which also supports the phenomenon of low F− in shallow groundwater. ter than in deep groundwater. This is because the dissolution of F-containing minerals such as fluorite is an important source of F− in groundwater of the study area, and the amount of fluorite is higher in the deep aquifer. The alkaline pH can influence CaF2 activity and favors the mobilization of F− from soil and weathered rocks into groundwater. This assumption was also formulated by other researchers [64–66]. The enrichment of F− can also be influenced by the ratio between HCO3−, Na+ and Ca2+ in groundwater, as confirmed by Saxena and Ahmed [67], Rango et al. [68], and Kimambo et al. [64]. Na+ concentration is lower in the shallow aquifer, which also supports the phenomenon of low F− in shallow groundwater.

health effects such as methemoglobinemia and thyroid effects at a concentration below 50 mg/L of NO3−. This health risk can seriously affect bottle-fed infants when mathemoglobinemia is complicated by the presence of microbial contamination and subsequent gas-

Excessive boiling of water for microbiological safety purposes may increase the concentration of NO3−*.* Water for drinking should be heated until it reaches a rolling boil [42]. For NO3−, 45.8% of the samples exceeded the limits (20 mg/L) set by the Ministry of Health

Figure 3 shows the scatter plot of F−, Na+, and NO2− concentrations with groundwater level depth. It shows that the water samples were mostly concentrated in the shallow depth (less than 30 m). Fluoride is present in lower concentrations in shallow groundwa-

*3.2. Relationship between Depth of Wells and the Concentration of Physicochemical Parameters* 

*Water* **2021**, *13*, x FOR PEER REVIEW 10 of 21

trointestinal infection that manifests as diarrhea.

of the P.R. China [43].

**Figure 3.** Relationship between fluoride and depth (**a**), sodium and depth (**b**), nitrite and depth (**c**). **Figure 3.** Relationship between fluoride and depth (**a**), sodium and depth (**b**), nitrite and depth (**c**).

Samples with low concentration of NO2− are usually observed in the shallow aquifer than in the deep aquifer. This may be due to the oxidation environment in the shallow aquifer that favors the transformation of NO2− to NO3−. Numerous studies have shown that human activities such as agriculture, industry, domestic sewage, landfills, and household waste influences shallow groundwater quality [1,32,69]. Samples with low concentration of NO<sup>2</sup> − are usually observed in the shallow aquifer than in the deep aquifer. This may be due to the oxidation environment in the shallow aquifer that favors the transformation of NO<sup>2</sup> − to NO<sup>3</sup> −. Numerous studies have shown that human activities such as agriculture, industry, domestic sewage, landfills, and household waste influences shallow groundwater quality [1,32,69].

### *3.3. Hydrochemical Types of Groundwater 3.3. Hydrochemical Types of Groundwater*

The Durov diagram depicted in Figure 4b reveals that most of the samples are concentrated in the field of HCO3-Ca type and combined HCO3·SO4-Ca·Mg type. This situation may result from the dissolution of CO3− minerals and F− [68]. As also discussed by Ravikumar et al. [70] and Lloyd and Heathcote [71], the HCO3-Ca dominant frequently indicates that recharging waters in limestone and sandstone is associated with dolomite. To assess the water quality, a Piper diagram (Figure 4a) was used to characterize the hydrogeochemical facies of groundwater samples from the study area. The Piper plot shows that Ca2+, Na+, and Mg2+ are dominant cations in the region. Conversely, HCO3− and SO42<sup>−</sup> dominate the facies, while Cl− is quasi-inexistent. The general classification of all samples The Durov diagram depicted in Figure 4b reveals that most of the samples are concentrated in the field of HCO3-Ca type and combined HCO3·SO4-Ca·Mg type. This situation may result from the dissolution of CO<sup>3</sup> − minerals and F− [68]. As also discussed by Ravikumar et al. [70] and Lloyd and Heathcote [71], the HCO3-Ca dominant frequently indicates that recharging waters in limestone and sandstone is associated with dolomite. To assess the water quality, a Piper diagram (Figure 4a) was used to characterize the hydrogeochemical facies of groundwater samples from the study area. The Piper plot shows that Ca2+, Na<sup>+</sup> , and Mg2+ are dominant cations in the region. Conversely, HCO<sup>3</sup> − and SO<sup>4</sup> <sup>2</sup><sup>−</sup> dominate the facies, while Cl<sup>−</sup> is quasi-inexistent. The general classification of all samples shows 81.25% Ca·Mg-HCO3, 8% Ca·Mg-SO4·Cl, 4.1% Na-Cl and 6.25% Na-HCO<sup>3</sup> water type (Figure 4a). The dominant Ca·Mg-HCO<sup>3</sup> type may indicate that the influence of dissolution on groundwater chemistry is more considerable, and it signifies the dominance of alkaline earths over alkalis; weak acids exceed strong acids. This observation was also found by other researchers, notably Xu et al. [72], Ravikumar et al. [70] and Singh et al. [16]. [16].

**Figure 4.** Piper (**a**) and Durov (**b**) diagrams showing the samples classifications. **Figure 4.** Piper (**a**) and Durov (**b**) diagrams showing the samples classifications.

### *3.4. Hydrochemical Correlation Analysis of Water Quality 3.4. Hydrochemical Correlation Analysis of Water Quality*

To better understand the major hydrogeochemical processes that control the chemical characteristics, it is necessary to carry out a Pearson's correlation analysis that shows the relationship between each pair of physicochemical indices [39,73]. Table 3 gives the correlation values of physicochemical parameters of water samples. To better understand the major hydrogeochemical processes that control the chemical characteristics, it is necessary to carry out a Pearson's correlation analysis that shows the relationship between each pair of physicochemical indices [39,73]. Table 3 gives the correlation values of physicochemical parameters of water samples.

shows 81.25% Ca·Mg-HCO3, 8% Ca·Mg-SO4·Cl, 4.1% Na-Cl and 6.25% Na-HCO3 water type (Figure 4a). The dominant Ca·Mg-HCO3 type may indicate that the influence of dissolution on groundwater chemistry is more considerable, and it signifies the dominance of alkaline earths over alkalis; weak acids exceed strong acids. This observation was also found by other researchers, notably Xu et al. [72], Ravikumar et al. [70] and Singh et al.

**Table 3.** Pearson correlation matrix between physicochemical parameters of water samples. **Table 3.** Pearson correlation matrix between physicochemical parameters of water samples.


pH 1 **0.598** −0.148 **0.379** F− 1 0.209 **0.703** Bold number indicates that the correlation is significant at the 0.05 level (two-tailed). Italic number indicates that the correlation is significant at the 0.01 level (two-tailed).

EC 1 0.10 Cr6+ 1 Bold number indicates that the correlation is significant at the 0.05 level (two-tailed). Italic number indicates that the correlation is significant at the 0.01 level (two-tailed). As shown in Table 3, there is a strong correlation, which is explained by ions exchange between TDS and EC with *r* = 0.980 at the level of *p* > 0.01, Ca2+ content and TH As shown in Table 3, there is a strong correlation, which is explained by ions exchange between TDS and EC with *r* = 0.980 at the level of *p* > 0.01, Ca2+ content and TH with *r* = 0.916 at the level of *p* > 0.01, Cl- and TDS with *r* = 0.860 at the level of *p* > 0.01, Cl<sup>−</sup> and EC with *r* = 0.857 at the level of *p* > 0.01, and SO<sup>4</sup> <sup>2</sup><sup>−</sup> correlates with TDS and EC with both *r* = 0.804 at the level of *p* > 0.01. In addition, a strong correlation exists between Na<sup>+</sup> and TDS, SO<sup>4</sup> <sup>2</sup>−, EC, and Cl<sup>−</sup> with *r* = 0.765, 0.745, 0.742, and 0.727, respectively. Furthermore, there is a strong relationship between NO<sup>3</sup> <sup>−</sup> and TH with *<sup>r</sup>* = 0.662 at the level of *<sup>p</sup>* > 0.01,Cl<sup>−</sup> and SO<sup>4</sup> <sup>2</sup><sup>−</sup> with *r* = 0.623 at the level of *p* > 0.01, TH and EC with *r* = 0.607 at the level of *p* > 0.01, and Na<sup>+</sup> and F<sup>−</sup> with *r* = 0.602 at the level of *p* > 0.01.

> Although all the aforementioned correlations between parameters are positive, there is a strong negative correlation between Ca2+ and pH with *<sup>r</sup>* <sup>=</sup> <sup>−</sup>0.721 at the level of *<sup>p</sup>* > 0.01. Ca2+ and Mg2+ are significantly correlated to TH because they contribute to the water hardness.

A strong correlation between Cr6+ and F<sup>−</sup> with *r* = 0.703 at level *p* > 0.05, which may be due to the oxidation mechanism of Cr3+ to Cr6+ in the presence of F<sup>−</sup> in groundwater, was observed. Finally, a significant correlation between Cr6+ and both Na<sup>+</sup> and pH was also noticeable. All these parameters may have triggered the mobilization of Cr in the groundwater system [29].
