3.1.5. Qualitative Analysis of Ion Sources

Water bodies are constantly interacting with the natural environment through various hydrogeochemical processes. In the process of groundwater circulation, hydrochemical ratios have a certain regularity, which can be used to assess the water chemistry and its main sources [42].

The Na+/Cl<sup>−</sup> ratio can characterize the degree of Na<sup>+</sup> enrichment in groundwater [43]. According to the correlation analysis (Figure 4), a high correlation was observed between Na<sup>+</sup> and Cl<sup>−</sup> (r = 0.92). However, most of the water samples fall above the rock salt dissolution line (Figure 7a), with a Na+/Cl<sup>−</sup> ratio range of 0.96–4.58, with an average value of 2.09. The result showed a higher Na<sup>+</sup> enrichment compared to Cl−, suggesting that Na<sup>+</sup> does not originate from the dissolution of rock salt only. Indeed, besides cation exchange that may increase the Na<sup>+</sup> content in Sr-rich groundwater in Tianjiazhai, the dissolution of silicate minerals (e.g., sodium feldspar) can contribute to the increase in Na<sup>+</sup> concentrations in groundwater.

Ca2+/Mg2+, (Ca2+ + Mg2+)/HCO<sup>3</sup> <sup>−</sup>, and (Ca2++Mg2+)/(HCO<sup>3</sup> − + SO<sup>4</sup> <sup>2</sup>−) ratios can be used to determine the sources of Ca2+ and Mg2+ in groundwater [44,45]. From Figure 7b, the results showed a Ca2+/Mg2+ ratio range of 0.33–1.05, with a mean value of 0.67. Except for a few samples located on the 1:1 line, most groundwater samples fell below the 1:1 line, indicating that the sources of Ca2+ and Mg2+ in the study area are calcite and dolomite dissolution; this is mainly due to the fact that calcite reached saturation faster than dolomite. As shown in Figure 7c, all groundwater samples fall above the 1:1 line of the Ca2++Mg2+/HCO<sup>3</sup> <sup>−</sup> ratio. In addition, the results showed a Ca2++Mg2+/HCO<sup>3</sup> − ratio range of 1.18–1.99, with an average value of 1.55, indicating that Ca2+ and Mg2+ were more enriched than HCO<sup>3</sup> <sup>−</sup>. This finding suggests that Ca2+ and Mg2+ were derived from the dissolution of carbonate rocks as well as from other sources, such as the dissolution of gypsum. In addition, most groundwater samples fall at the 1:1 line of the [(Ca2++Mg2+)/(HCO<sup>3</sup> <sup>−</sup>+SO<sup>4</sup> <sup>2</sup>−)] ratio (Figure 7d), which indicates equilibrium states of these ions, while chloro-alkaline indices indicated the prevalence of cation exchange. Moreover, this result demonstrates that Ca2+ and Mg2+ were not only from the dissolution of calcite, dolomite, and gypsum but also from silicate dissolution. (Ca2++Mg2+) and (HCO<sup>3</sup> <sup>−</sup>+SO<sup>4</sup> <sup>2</sup>−) were in equilibrium under the combined influence of carbonate rock, silicate rock, and cation exchange.

Most of the groundwater samples fell near the 1:1 line of the Ca2+/SO<sup>4</sup> <sup>2</sup><sup>−</sup> ratio (Figure 7e), while only a few samples fell below the 1:1 line, showing that most of the groundwater samples have relatively balanced Ca2+ and SO<sup>4</sup> <sup>2</sup>−, while a few samples are depleted in Ca2+ or enriched in SO<sup>4</sup> <sup>2</sup>−. This result may be due to the presence of a certain degree of cation exchange and sedimentation of carbonate rocks during the evolution of groundwater.

The relationship between the HCO<sup>3</sup> <sup>−</sup> and (SO<sup>4</sup> <sup>2</sup>−+Cl−) can further reflect the dissolved carbonate rock in groundwater. As shown in Figure 7f, the water samples fall above

the 1:1 line of the HCO<sup>3</sup> −/(SO<sup>4</sup> <sup>2</sup>−+Cl−) ratio. Moreover, the HCO<sup>3</sup> −/(SO<sup>4</sup> <sup>2</sup>−+Cl−) ratios range from 1.02 to 3.53, with an average value of 1.45. The HCO<sup>3</sup> − enrichment compared to (SO<sup>4</sup> <sup>2</sup>−+Cl−) indicated the dominance of HCO<sup>3</sup> − in Sr-rich groundwater in Tianjiazhai, suggesting that the water chemistry is mainly influenced by carbonate rock dissolution, which is consistent with the groundwater facies type. 1.02 to 3.53, with an average value of 1.45. The HCO3− enrichment compared to (SO42−+Cl−) indicated the dominance of HCO3− in Sr-rich groundwater in Tianjiazhai, suggesting that the water chemistry is mainly influenced by carbonate rock dissolution, which is consistent with the groundwater facies type.

*Water* **2022**, *14*, x FOR PEER REVIEW 10 of 16

The Cl−/Na<sup>+</sup> and NO<sup>3</sup> <sup>−</sup>/Na<sup>+</sup> ratios in groundwater are generally higher when the groundwater is affected by anthropogenic activities [46]. The variation in the Cl−/Na<sup>+</sup> and NO<sup>3</sup> <sup>−</sup>/Na<sup>+</sup> ratios in the Sr-rich groundwater samples in Tianjiazhai ranged from 0.22 to 1.04 and from 0 to 0.22, with mean values of 0.56 and 0.05, respectively. The low Cl−/Na<sup>+</sup> and NO<sup>3</sup> <sup>−</sup>/Na<sup>+</sup> ratios obtained suggested that Sr-rich groundwater in Tianjiazhai is less affected by anthropogenic activities. The Cl−/Na+ and NO3−/Na+ ratios in groundwater are generally higher when the groundwater is affected by anthropogenic activities [46]. The variation in the Cl−/Na+ and NO3−/Na+ ratios in the Sr-rich groundwater samples in Tianjiazhai ranged from 0.22 to 1.04 and from 0 to 0.22, with mean values of 0.56 and 0.05, respectively. The low Cl−/Na+ and NO3−/Na+ ratios obtained suggested that Sr-rich groundwater in Tianjiazhai is less affected by anthropogenic activities.

**Figure 7.** Relationship between the main ion ratios of Srrich groundwater. *3.2. Formation Mechanism of Sr in Groundwater*  **Figure 7.** Relationship between the main ion ratios of Srrich groundwater. (**a**) Cl<sup>−</sup> and Na<sup>+</sup> ; (**b**) Mg2+ and Ca2+; (**c**) HCO<sup>3</sup> <sup>−</sup> and (Ca2+ + Mg2+); (**d**) (HCO<sup>3</sup> <sup>−</sup> + SO<sup>4</sup> <sup>2</sup>−) and (Ca2+ + Mg2+); (**e**) SO<sup>4</sup> <sup>2</sup><sup>−</sup> and Ca2+; (**f**) (SO<sup>4</sup> <sup>2</sup><sup>−</sup> + Cl−) and HCO<sup>3</sup> −.

### The upper part of the lithosphere is rich in trace elements, such as Sr, which is abun-*3.2. Formation Mechanism of Sr in Groundwater*

### dantly contained in most rocks [47]. Indeed, Sr is mostly distributed in rock-forming min-3.2.1. Source Conditions

3.2.1. Source Conditions

erals and is relatively concentrated in amphibolites, granites, and carbonates [48]. The Sr content in the clastic rocks of the Yanchang Formation in the study area is high. In addition, the lithology of this formation is a thick-bedded feldspathic sandstone of light graygreen medium to fine-grained feldspathic sandstone containing a large amount of potas-The upper part of the lithosphere is rich in trace elements, such as Sr, which is abundantly contained in most rocks [47]. Indeed, Sr is mostly distributed in rock-forming minerals and is relatively concentrated in amphibolites, granites, and carbonates [48]. The Sr content in the clastic rocks of the Yanchang Formation in the study area is high. In addition,

sium feldspar, which is rich in Sr. Strontium in groundwater is mainly derived from the

the lithology of this formation is a thick-bedded feldspathic sandstone of light gray-green medium to fine-grained feldspathic sandstone containing a large amount of potassium feldspar, which is rich in Sr. Strontium in groundwater is mainly derived from the dissolution of strontium in the sandstones of the Yanchang Formation. The Sr abundance in the surrounding rocks can reflect the Sr content in groundwater. Indeed, the presence of Sr-bearing minerals is the material basis for the formation of Sr-rich groundwater.
