3.1.2. Hydrochemical Facies

Piper's trilinear diagram can be used to characterize the total chemical properties and major ionic composition changes of a water body [36,37]. As shown in Figure 2, the groundwater sample points were concentrated in zone 4 of the diamond-shaped domain of the Piper trilinear diagram, indicating that the chemistry of Sr-rich groundwater in the study area is dominated by alkaline earth metal elements (calcium and magnesium) rather than the alkali metal elements (sodium). On the other hand, the groundwater sample points were concentrated in the E region of the anion triangle of the Piper trilinear diagram, indicating the dominance of weak acidic anions (carbonic acid), whereas the observed cations were concentrated in the B region of the Piper trilinear diagram (cationic triangular domain), suggesting a non-dominant type. In general, the distribution of Sr-rich groundwater sampling sites in the study area was relatively concentrated, showing six main types, among which HCO<sup>3</sup> <sup>−</sup> Ca·Mg·Na was the main groundwater facies type, accounting for 70% of the total groundwater samples, followed by HCO<sup>3</sup> <sup>−</sup> Ca·Na·Mg and HCO<sup>3</sup> − Ca·Mg groundwater facies types, both accounting for 20% of the total groundwater samples, while HCO<sup>3</sup> <sup>−</sup> Na·Mg·Ca, HCO<sup>3</sup> <sup>−</sup> Mg·Ca·Na and HCO<sup>3</sup> <sup>−</sup> Mg·Na·Ca groundwater facies type were less abundant. In addition, TIS salinity diagram can further characterize the chemical composition of water [38]. As shown in Figure 3, (Cl<sup>−</sup> + HCO<sup>3</sup> −) content is 5.11~10.21 meq·L −1 , SO<sup>4</sup> <sup>2</sup><sup>−</sup> content is 1.4~3.6 meq·<sup>L</sup> −1 . Indeed, the total ionic salinity (TIS) of water ranges from 14.2 to 26.4 meq·L −1 . The strontium-rich groundwater has high TIS value, which indicates that the strontium-rich groundwater is the result of a long-term evolution of the groundwater system in the study area.

*Water* **2022**, *14*, x FOR PEER REVIEW 6 of 16

**Figure 2.** Piper trilinear diagram of Sr-rich groundwater. **Figure 2.** Piper trilinear diagram of Sr-rich groundwater. **Figure 2.** Piper trilinear diagram of Sr-rich groundwater.

**Figure 3.** TIS salinity diagram of Sr-rich groundwater. **Figure 3.** TIS salinity diagram of Sr-rich groundwater. **Figure 3.** TIS salinity diagram of Sr-rich groundwater.

### 3.1.3. Correlation Analysis of Hydrochemical Compositions 3.1.3. Correlation Analysis of Hydrochemical Compositions 3.1.3. Correlation Analysis of Hydrochemical Compositions

Correlation analysis can reveal the consistency and variability of the sources of groundwater hydrochemical parameters. As shown in Figure 4, Cl− showed a high positive correlation with Na+ and Ca2+ and moderate correlation with Mg2+, with correlation coefficients of 0.92, 0.94, and 0.59, respectively. HCO3− showed moderately positive correlations with Na+ and Ca2+, with correlation coefficients of 0.74 and 0.69, respectively, while SO42− and NO3− revealed negative correlations with all cations in groundwater. NO3<sup>−</sup> showed a moderate negative correlation with K+, with a correlation coefficient of −0.56. As for Cl−, it showed a moderate positive correlation with HCO3−, with a correlation coefficient of 0.59. Sr2+ showed a high correlation with Na+ and Cl−, with correlation coefficients of 0.85 and 0.84, respectively, suggesting that the source of Sr in groundwater is closely related to the effect of atmospheric precipitation. In addition, Sr2+ revealed moderate positive correlations with Ca2+ and HCO3−, with correlation coefficients of 0.73 and 0.59, respectively, indicating that the source of Sr may be the same as these two ions. The corre-Correlation analysis can reveal the consistency and variability of the sources of groundwater hydrochemical parameters. As shown in Figure 4, Cl− showed a high positive correlation with Na+ and Ca2+ and moderate correlation with Mg2+, with correlation coefficients of 0.92, 0.94, and 0.59, respectively. HCO3− showed moderately positive correlations with Na+ and Ca2+, with correlation coefficients of 0.74 and 0.69, respectively, while SO42− and NO3− revealed negative correlations with all cations in groundwater. NO3<sup>−</sup> showed a moderate negative correlation with K+, with a correlation coefficient of −0.56. As for Cl−, it showed a moderate positive correlation with HCO3−, with a correlation coefficient of 0.59. Sr2+ showed a high correlation with Na+ and Cl−, with correlation coefficients of 0.85 and 0.84, respectively, suggesting that the source of Sr in groundwater is closely related to the effect of atmospheric precipitation. In addition, Sr2+ revealed moderate positive correlations with Ca2+ and HCO3−, with correlation coefficients of 0.73 and 0.59, respectively, indicating that the source of Sr may be the same as these two ions. The corre-Correlation analysis can reveal the consistency and variability of the sources of groundwater hydrochemical parameters. As shown in Figure 4, Cl− showed a high positive correlation with Na<sup>+</sup> and Ca2+ and moderate correlation with Mg2+, with correlation coefficients of 0.92, 0.94, and 0.59, respectively. HCO<sup>3</sup> − showed moderately positive correlations with Na<sup>+</sup> and Ca2+, with correlation coefficients of 0.74 and 0.69, respectively, while SO<sup>4</sup> <sup>2</sup><sup>−</sup> and NO<sup>3</sup> <sup>−</sup> revealed negative correlations with all cations in groundwater. NO<sup>3</sup> <sup>−</sup> showed amoderate negative correlation with K<sup>+</sup> , with a correlation coefficient of −0.56. As for Cl−, it showed a moderate positive correlation with HCO<sup>3</sup> −, with a correlation coefficient of 0.59. Sr2+ showed a high correlation with Na<sup>+</sup> and Cl−, with correlation coefficients of 0.85 and 0.84, respectively, suggesting that the source of Sr in groundwater is closely related to the effect of atmospheric precipitation. In addition, Sr2+ revealed moderate positive correlations with Ca2+ and HCO<sup>3</sup> −, with correlation coefficients of 0.73 and 0.59, respectively, indicating that the source of Sr may be the same as these two ions. The correlation

lation between TDS and ions can better reflect the genesis of groundwater. Indeed, TDS

lation between TDS and ions can better reflect the genesis of groundwater. Indeed, TDS

between TDS and ions can better reflect the genesis of groundwater. Indeed, TDS showed positive correlation coefficients with Cl− and Na+ of 0.81 and 0.76, indicating high and moderate correlation, respectively, and an atmospheric precipitation source of groundwater. The correlation coefficients of TDS with Ca2+ and HCO<sup>3</sup> − were 0.84 and 0.60, showing high and moderate correlations, respectively, indicating that Ca2+ and HCO<sup>3</sup> − contribute significantly to the formation of groundwater chemistry types in the carbonate-rich rock study area. showed positive correlation coefficients with Cl− and Na+ of 0.81 and 0.76, indicating high and moderate correlation, respectively, and an atmospheric precipitation source of groundwater. The correlation coefficients of TDS with Ca2+ and HCO3− were 0.84 and 0.60, showing high and moderate correlations, respectively, indicating that Ca2+ and HCO3<sup>−</sup> contribute significantly to the formation of groundwater chemistry types in the carbonaterich rock study area.

**Figure 4.** Correlation coefficients between hydrochemical parameters of Sr-rich groundwater. **Figure 4.** Correlation coefficients between hydrochemical parameters of Sr-rich groundwater.

### 3.1.4. Controlling Factors and Natural Processes 3.1.4. Controlling Factors and Natural Processes

A Gibbs diagram is an important tool for analyzing the chemical genesis of water and can be used to analyze the mechanism of ion formation in water bodies [39,40]. In the Gibbs diagram, the ion control mechanism is classified into three main effects, namely rock weathering, evaporation-concentration, and atmospheric precipitation. The variation of Cl−/(Cl− + HCO3−) and Na+/(Na+ + Ca2+) ratios in Sr-rich groundwater samples in the study area varied slightly from 0.04 to 0.25 and from 0.42 to 0.56, with mean values of 0.13 and 0.56, respectively. In addition, TDS values were at a moderate level, varying from 332 to 718 mg·L−1, with a mean value of 493 mg·L−1. As shown in Figure 5, the Sr-rich groundwater samples were mainly located in the rock weathering area of the Gibbs diagram, indicating that rock weathering was the most influential mechanism in the genesis of Srrich groundwater in the study area. In addition, some Sr-rich groundwater samples were close to the evaporation-concentration zone, indicating the slight influential effect of the evaporation-concentration process on the hydrochemical characteristics of Sr-rich groundwater in the study area. However, the groundwater samples were all far away from the atmospheric precipitation zone, suggesting the lack of any significant effect of A Gibbs diagram is an important tool for analyzing the chemical genesis of water and can be used to analyze the mechanism of ion formation in water bodies [39,40]. In the Gibbs diagram, the ion control mechanism is classified into three main effects, namely rock weathering, evaporation-concentration, and atmospheric precipitation. The variation of Cl−/(Cl<sup>−</sup> + HCO<sup>3</sup> <sup>−</sup>) and Na+/(Na<sup>+</sup> + Ca2+) ratios in Sr-rich groundwater samples in the study area varied slightly from 0.04 to 0.25 and from 0.42 to 0.56, with mean values of 0.13 and 0.56, respectively. In addition, TDS values were at a moderate level, varying from 332 to 718 mg·L −1 , with a mean value of 493 mg·L −1 . As shown in Figure 5, the Sr-rich groundwater samples were mainly located in the rock weathering area of the Gibbs diagram, indicating that rock weathering was the most influential mechanism in the genesis of Sr-rich groundwater in the study area. In addition, some Sr-rich groundwater samples were close to the evaporation-concentration zone, indicating the slight influential effect of the evaporation-concentration process on the hydrochemical characteristics of Sr-rich groundwater in the study area. However, the groundwater samples were all far away from the atmospheric precipitation zone, suggesting the lack of any significant effect of atmospheric precipitation on the water chemistry of Sr-rich groundwater.

atmospheric precipitation on the water chemistry of Sr-rich groundwater.

**Figure 5.** Gibbs plot of Sr-rich groundwater. **Figure 5.** Gibbs plot of Sr-rich groundwater. **Figure 5.** Gibbs plot of Sr-rich groundwater.

In this study, the direction and strength of the cation exchange interaction were assessed using the chloro-alkaline indices [41], namely CAI-I and CAI-II. These indices were calculated using the following formulas: In this study, the direction and strength of the cation exchange interaction were assessed using the chloro-alkaline indices [41], namely CAI-I and CAI-II. These indices were calculated using the following formulas: In this study, the direction and strength of the cation exchange interaction were assessed using the chloro-alkaline indices [41], namely CAI-I and CAI-II. These indices were calculated using the following formulas:

$$\text{CAI} - \text{I} = \frac{\text{Cl}^- - \left(\text{Na}^+ + \text{K}^+\right)}{\text{Cl}^-} \tag{1}$$

$$\text{CAI} - \text{II} = \frac{\text{Cl}^- - \left(\text{Na}^+ + \text{K}^+\right)}{\text{HCO}\_3^- + \text{SO}\_4^{2-} + \text{CO}\_3^{2-} + \text{NO}\_3^-} \tag{2}$$

When both CAI-I and CAI-II values are less than 0, it indicates that Ca2+ or Mg2+ in groundwater exchanged ions with Na+ in aquifer minerals. When CAI-I and CAI-II values are greater than 0, it indicates that Na+ in the groundwater exchanged ions with Ca2+ or Mg2+ in the aquifer minerals. In addition, the higher the absolute values of CAI-I and CAI-II the chloride index, the higher the degree of ion exchange. The results showed that CAI-I and CAI-II varied from −0.23 to 0.01 and from −3.71 to When both CAI-I and CAI-II values are less than 0, it indicates that Ca2+ or Mg2+ in groundwater exchanged ions with Na<sup>+</sup> in aquifer minerals. When CAI-I and CAI-II values are greater than 0, it indicates that Na<sup>+</sup> in the groundwater exchanged ions with Ca2+ or Mg2+ in the aquifer minerals. In addition, the higher the absolute values of CAI-I and CAI-II the chloride index, the higher the degree of ion exchange. When both CAI-I and CAI-II values are less than 0, it indicates that Ca2+ or Mg2+ in groundwater exchanged ions with Na+ in aquifer minerals. When CAI-I and CAI-II values are greater than 0, it indicates that Na+ in the groundwater exchanged ions with Ca2+ or Mg2+ in the aquifer minerals. In addition, the higher the absolute values of CAI-I and CAI-II the chloride index, the higher the degree of ion exchange.

0.004, with a mean value of −0.128 and −1.155, respectively (Figure 6a). In addition, almost groundwater samples revealed negative values of chloro-alkaline indices, except for a few points. This result suggests that the cation exchange process generally occurs in groundwater in the study area, more specifically replacement of Na+ with Ca2+ and Mg2+ from aquifer minerals to the groundwater. Except for a few samples, relatively high absolute values of the chloro-alkaline indices were observed in Sr-rich groundwater samples, suggesting a strong degree of cation exchange. A very small number of water samples do not undergo alternate cation adsorption, which may be due to the influence of some other sources. In addition, the relationship between Na+ + K+ and Cl− can also be investigated to assess cation exchange. As shown in Figure 6b, the water samples were distributed above the [(Na+ + K+) − Cl−] ratio 1:1 line, indicating a significant enrichment of groundwater by (Na+ + K+) compared to Cl−. Indeed, except for rock salt and potassium salt dissolution, there are also other sources of Na+ and K+, mainly due to the presence of a certain degree of cation exchange to increase the Na+ content. The results showed that CAI-I and CAI-II varied from −0.23 to 0.01 and from −3.71 to 0.004, with a mean value of −0.128 and −1.155, respectively (Figure 6a). In addition, almost groundwater samples revealed negative values of chloro-alkaline indices, except for a few points. This result suggests that the cation exchange process generally occurs in groundwater in the study area, more specifically replacement of Na<sup>+</sup> with Ca2+ and Mg2+ from aquifer minerals to the groundwater. Except for a few samples, relatively high absolute values of the chloro-alkaline indices were observed in Sr-rich groundwater samples, suggesting a strong degree of cation exchange. A very small number of water samples do not undergo alternate cation adsorption, which may be due to the influence of some other sources. In addition, the relationship between Na<sup>+</sup> + K<sup>+</sup> and Cl<sup>−</sup> can also be investigated to assess cation exchange. As shown in Figure 6b, the water samples were distributed above the [(Na<sup>+</sup> + K<sup>+</sup> ) − Cl−] ratio 1:1 line, indicating a significant enrichment of groundwater by (Na<sup>+</sup> + K<sup>+</sup> ) compared to Cl−. Indeed, except for rock salt and potassium salt dissolution, there are also other sources of Na<sup>+</sup> and K<sup>+</sup> , mainly due to the presence of a certain degree of cation exchange to increase the Na<sup>+</sup> content. The results showed that CAI-I and CAI-II varied from −0.23 to 0.01 and from −3.71 to 0.004, with a mean value of −0.128 and −1.155, respectively (Figure 6a). In addition, almost groundwater samples revealed negative values of chloro-alkaline indices, except for a few points. This result suggests that the cation exchange process generally occurs in groundwater in the study area, more specifically replacement of Na+ with Ca2+ and Mg2+ from aquifer minerals to the groundwater. Except for a few samples, relatively high absolute values of the chloro-alkaline indices were observed in Sr-rich groundwater samples, suggesting a strong degree of cation exchange. A very small number of water samples do not undergo alternate cation adsorption, which may be due to the influence of some other sources. In addition, the relationship between Na+ + K+ and Cl− can also be investigated to assess cation exchange. As shown in Figure 6b, the water samples were distributed above the [(Na+ + K+) − Cl−] ratio 1:1 line, indicating a significant enrichment of groundwater by (Na+ + K+) compared to Cl−. Indeed, except for rock salt and potassium salt dissolution, there are also other sources of Na+ and K+, mainly due to the presence of a certain degree of cation exchange to increase the Na+ content.

**Figure 6.** Chloro-alkaline indices (**a**) and the relationship between Na+ + K+ and Cl<sup>−</sup> (**b**) in Sr-rich groundwater. **Figure 6.** Chloro-alkaline indices (**a**) and the relationship between Na+ + K+ and Cl<sup>−</sup> (**b**) in Sr-rich groundwater. **Figure 6.** Chloro-alkaline indices (**a**) and the relationship between Na<sup>+</sup> + K<sup>+</sup> and Cl<sup>−</sup> (**b**) in Sr-rich groundwater.

The mineral saturation index (SI) can be used to indicate the dissolved equilibrium state of groundwater components. The results showed positive values of saturation indices of dolomite, gypsum, and calcite (Table 2), indicating that all three minerals were in supersaturated states and were in sedimentation states during groundwater evolution. However, the saturation index range of rock salt in the study area was negative, indicating that the rock salt was in a dissolved state during groundwater evolution.


**Table 2.** Mineral saturation index in Sr-rich groundwater.
