**3. Results**

*3.1. Hydrochemical Characteristics Analysis*

The groundwater in the study area was weakly alkaline with low salinity overall. The pH values ranged from 7.38 to 8.50, with an average value of 7.82. The TDS values ranged

from 287.03 to 3426.54 mg/L, with an average of 744.56 mg/L. The SD value of TDS was 545.50 mg/L, and the *C<sup>v</sup>* value was 73.22%, meaning that the spatial dispersion degree of TDS was high. The concentrations of TH were higher, ranging from 362.83 to 1851.67 mg/L, with an average of 451.54 mg/L. The SD value of TH was as high as 1170.65 mg/L, and the *C<sup>v</sup>* value was 63.22%. Among the main anions in groundwater, the content of HCO<sup>3</sup> − was the highest, followed by Cl−, SO<sup>4</sup> <sup>2</sup><sup>−</sup> and NO<sup>3</sup> <sup>−</sup>. Among the main cations, Mg2+ and Ca2+ were the dominant ions, while the concentrations of Na<sup>+</sup> and K<sup>+</sup> were relatively lower. The concentrations of F− ranged from 0.31 to 2.39 mg/L, with an average of 0.91 mg/L. The NO3<sup>−</sup> concentrations fluctuated widely, ranging from 0.43 to 419.71 mg/L, with an average of 79.77 mg/L. The concentrations of CODMn ranged from 0.12 to 5.07 mg/L, with an average of 1.44 mg/L. According to Table 3, the SD values of Mg2+, Na<sup>+</sup> , HCO<sup>3</sup> −, SO<sup>4</sup> <sup>2</sup>−, Cl−, NO<sup>3</sup> <sup>−</sup>, TDS, TH and CODMn were high, and their *C<sup>v</sup>* values were correspondingly high, which indicated that the dispersion degrees of them were high. The probable reasons for the high *C<sup>v</sup>* values are discussed below.

**Table 3.** Statistics of hydrochemical parameters of groundwater (n = 38).


The units of all groundwater quality indices, except pH, are in mg/L. SD: mg/L. *Cv*: %.

Hydrochemical characteristics of groundwater may be influenced by natural factors and human activities. Some natural factors, such as earthquakes and volcanic eruptions, may lead to the remarkable increase in the ion contents (K<sup>+</sup> , Na<sup>+</sup> , HCO3−, and SO<sup>4</sup> <sup>2</sup>−), and the influence areas of these factors are large [50,51], while the influences caused by other natural factors are relatively slight. Loose Quaternary sediment is widely distributed in the study area without obvious changes in lithology, no obvious tectonic movement has occurred in recent years, and the extreme values of the analytic indices are distributed discontinuously. Hence, it was inferred that natural factors were not the main cause of the high dispersion and high *C<sup>v</sup>* values. According to previous studies [52], the hydrochemical indices, influenced by human activities, were characterized by high dispersion and high fluctuation, and their *C<sup>v</sup>* values were correspondingly high. Thus, the *C<sup>ν</sup>* value can reflect the influence of human activities to some extent, and the disturbance of human activities can also lead to a high value of *Cν*. In general, there is a variation if the *C<sup>ν</sup>* value is higher than 30%, and there is a great variation if the *C<sup>ν</sup>* value is higher than 60%. The greater *C<sup>ν</sup>* is, the greater the difference, and the greater the influence of external factors on the groundwater index. As shown in Table 3, the *C<sup>v</sup>* values of pH and H2SiO<sup>3</sup> were low, meaning that the distributions of the two indices were spatially steady, while the *C<sup>v</sup>* values of other indices were high, the *C<sup>v</sup>* values of Mg2+, Na<sup>+</sup> , Cl<sup>−</sup> and NO<sup>3</sup> − were even greater than 100%. The spatial variations were also reflected in Figure 3, the concentrations of these indices in some areas were much higher than those in other areas. In summary, the intensive variations were mainly influenced by human activities.

**Figure 3.** Spatial distribution of hydrochemical indices (**a**–**j**). The unit of TDS is g/L and the units of other indices are mg/L. **Figure 3.** Spatial distribution of hydrochemical indices (**a**–**j**). The unit of TDS is g/L and the units of other indices are mg/L.

Based on the Piper diagram (Figure 4a), the anions were mainly distributed closer to the HCO3– side for the groundwater samples of the upstream aquifer (Q3*al+ pl*), while the cations were mainly distributed close to the Mg2+ and Ca2+ sides, indicating that the chemical facies upstream were mainly HCO3–Mg·Ca or HCO3–Ca·Mg. For the groundwater samples of the downstream aquifer(Q4*<sup>l</sup>* ), the cations were obviously biased toward the side of Na+, and the concentrations of anions (Cl– and SO42–) increased. Therefore, the anion types gradually transitioned from HCO3 upstream to HCO3·Cl (Cl·HCO3) and Cl·SO4 downstream, and the types of cations changed from Mg·Ca (Ca·Mg) to Mg·Ca·Na (Ca·Mg·Na), and even Na·Mg·Ca (Na·Mg·Ca) and Na type. Noticeably, the NO3– mole fractions of the C29 and C30 samples were 36.25% and 33.18%, respectively. To reflect nitrate contamination [39,40], the NO3– ion was taken into account when determining the As shown in Figure 3, the concentrations of TDS, Ca2+, Mg2+, Cl<sup>−</sup> and SO<sup>4</sup> <sup>2</sup><sup>−</sup> in groundwater were generally high in the central region of the study area, low in the east and west, low in the north and high in the south. The terrain of the study area gently slopes toward Huangqihai Lake, and the groundwater runs off slowly; the water – rock interactions were fully reacted, and groundwater ions accumulated from upstream to downstream. Furthermore, the groundwater depths in most areas downstream were shallower than the extreme evaporation depth, and the evaporation of groundwater was intensive. Due to the above factors, the contents of TDS, Ca2+, Mg2+, Cl<sup>−</sup> and SO<sup>4</sup> <sup>2</sup><sup>−</sup> were generally high in the downstream area. The concentration of F− was low in the middle and relatively high in the periphery. The concentrations of NO<sup>3</sup> <sup>−</sup> and CODMn were relatively low in most areas but high in area nearby the Huangqihai Lake.

hydrochemical facies. Therefore, two new hydrochemical types, except the 49 types, appeared in the C29 and C30 samples, namely, the NO3·HCO3–Ca·Mg and HCO3·NO3·Cl– Ca·Mg facies. According to studies by Apollaro et al. [53], the iso–ionic–salinity (TIS) lines were added in the correlation plot of (Na+ + K+) vs. (Ca2+ + Mg2+) to reflect the ionic salinity. As shown in Figure 4b, the content of Ca2+ and Mg2+ was higher than that of Na+ + K+ on the whole, and the groundwater had low ionic salinity which ranged from 10.43 to 56.21 meq/L. The TDS values were comparatively low with an average of 744.56 mg/L which were in the range of fresh water (TDS < 1000 mg/L). Based on the Piper diagram (Figure 4a), the anions were mainly distributed closer to the HCO<sup>3</sup> <sup>−</sup> side for the groundwater samples of the upstream aquifer (Q<sup>3</sup> *al+ pl*), while the cations were mainly distributed close to the Mg2+ and Ca2+ sides, indicating that the chemical facies upstream were mainly HCO3–Mg·Ca or HCO3–Ca·Mg. For the groundwater samples of the downstream aquifer(Q<sup>4</sup> *l* ), the cations were obviously biased toward the side of Na<sup>+</sup> , and the concentrations of anions (Cl<sup>−</sup> and SO<sup>4</sup> <sup>2</sup>−) increased. Therefore, the anion types gradually transitioned from HCO<sup>3</sup> upstream to HCO3·Cl (Cl·HCO3) and Cl·SO<sup>4</sup> downstream, and the types of cations changed from Mg·Ca (Ca·Mg) to Mg·Ca·Na

(Ca·Mg·Na), and even Na·Mg·Ca (Na·Mg·Ca) and Na type. Noticeably, the NO<sup>3</sup> − mole fractions of the C29 and C30 samples were 36.25% and 33.18%, respectively. To reflect nitrate contamination [39,40], the NO<sup>3</sup> − ion was taken into account when determining the hydrochemical facies. Therefore, two new hydrochemical types, except the 49 types, appeared in the C29 and C30 samples, namely, the NO3·HCO3–Ca·Mg and HCO3·NO3·Cl– Ca·Mg facies. According to studies by Apollaro et al. [53], the iso–ionic–salinity (TIS) lines were added in the correlation plot of (Na<sup>+</sup> + K<sup>+</sup> ) vs. (Ca2+ + Mg2+) to reflect the ionic salinity. As shown in Figure 4b, the content of Ca2+ and Mg2+ was higher than that of Na<sup>+</sup> + K<sup>+</sup> on the whole, and the groundwater had low ionic salinity which ranged from 10.43 to 56.21 meq/L. The TDS values were comparatively low with an average of 744.56 mg/L which were in the range of fresh water (TDS < 1000 mg/L). *Water* **2022**, *14*, 3168 12 of 26

**Figure 4.** Piper diagram of groundwater (**a**) and correlation plot of (Na+ + K+) vs. (Ca2+ + Mg2+), also showing TIS salinity diagram for reference (**b**). **Figure 4.** Piper diagram of groundwater (**a**) and correlation plot of (Na<sup>+</sup> + K<sup>+</sup> ) vs. (Ca2+ + Mg2+), also showing TIS salinity diagram for reference (**b**).

In this study, the Pearson correlations of groundwater indices were analyzed to reflect the possible sources and chemical reactions related to the hydrochemical indices (Table 4). In this study, the Pearson correlations of groundwater indices were analyzed to reflect the possible sources and chemical reactions related to the hydrochemical indices (Table 4).


**Table 4.** Correlation matrices of hydrochemical indices (n = 38). **Table 4.** Correlation matrices of hydrochemical indices (n = 38).

H2SiO3 −0.188 0.000 0.596 \*\* 0.477 \*\* 0.528 \*\* 0.205 0.565 \*\* 0.079 1 TDS −0.224 0.352 \* 0.949 \*\* 0.962 \*\* 0.917 \*\* 0.825 \*\* 0.961 \*\* 0.455 \*\* 0.524 \*\* 1 \* and \*\* represent significant levels of 0.05 and 0.01, respectively.

TH −0.239 0.392 \* 0.965 \*\* 0.892 \*\* 0.910 \*\* 0.768 \*\* 0.929 \*\* 0.450 \*\* 0.553 \*\* 0.976 \*\* 1 CODMn −0.040 0.086 0.671 \*\* 0.711 \*\* 0.708 \*\* 0.584 \*\* 0.679 \*\* 0.331 \* 0.345 \* 0.699 \*\* 0.646 \*\* 1 \* and \*\* represent significant levels of 0.05 and 0.01, respectively. According to Table 4, the correlation coefficient (*r*) of TDS and TH was 0.976, indicating that there was a good positive correlation between them. The two indices had good positive correlations with Mg2+, Na+, Ca2+, Cl–, HCO3– and SO42–, reflecting the significant contribution of these elements in mineralization of groundwater. *r* (Ca2+ vs. Mg2+) did not According to Table 4, the correlation coefficient (*r*) of TDS and TH was 0.976, indicating that there was a good positive correlation between them. The two indices had good positive correlations with Mg2+, Na<sup>+</sup> , Ca2+, Cl−, HCO<sup>3</sup> <sup>−</sup> and SO<sup>4</sup> <sup>2</sup>−, reflecting the significant contribution of these elements in mineralization of groundwater. *r* (Ca2+ vs. Mg2+) did not reach a significance level of 0.05, meaning that there is no obvious relationship between Ca2+ and Mg2+. This was mainly because the sources of Ca2+ and Mg2+ or the reactions related to the two ions in groundwater were different. Both Ca2+ and Mg2+ had a positive

reach a significance level of 0.05, meaning that there is no obvious relationship between

relationship with SO42–, which indicated that sulfates rich in calcium and magnesium were dissolved in groundwater. *r* (Mg2+ vs. HCO3–) was as high as 0.968; that is, the dissolution of carbonate rich in Mg2+ in groundwater, such as dolomite, was the main source of Mg2+. Meanwhile, *r* (Ca2+ vs. HCO3–) was small and did not reach the significance level of 0.05, indicating that the dissolution of carbonate was not the main source of Ca2+ in groundwater or that the relationship between Ca2+ and HCO3– was weaker due to other reactions, such as cation exchange and crystallization. *r* (Na+ vs. Cl–) was as high as 0.951, indicating that the dissolution of salt rocks in groundwater was the main source of Na+ and Cl–. According to *r* (Na+ vs. HCO3–) and *r* (Na+ vs. SO42–), both of which were higher than 0.8, it

relationship with SO<sup>4</sup> <sup>2</sup>−, which indicated that sulfates rich in calcium and magnesium were dissolved in groundwater. *r* (Mg2+ vs. HCO<sup>3</sup> −) was as high as 0.968; that is, the dissolution of carbonate rich in Mg2+ in groundwater, such as dolomite, was the main source of Mg2+ . Meanwhile, *r* (Ca2+ vs. HCO<sup>3</sup> −) was small and did not reach the significance level of 0.05, indicating that the dissolution of carbonate was not the main source of Ca2+ in groundwater or that the relationship between Ca2+ and HCO<sup>3</sup> − was weaker due to other reactions, such as cation exchange and crystallization. *r* (Na<sup>+</sup> vs. Cl−) was as high as 0.951, indicating that the dissolution of salt rocks in groundwater was the main source of Na<sup>+</sup> and Cl−. According to *r* (Na<sup>+</sup> vs. HCO<sup>3</sup> <sup>−</sup>) and *r* (Na<sup>+</sup> vs. SO<sup>4</sup> <sup>2</sup>−), both of which were higher than 0.8, it was inferred that there were other sources of Na<sup>+</sup> in addition to the dissolution of salt rock. H2SiO<sup>3</sup> was positively correlated with Mg2+ and Na<sup>+</sup> , illustrating the dissolution of silicate containing Mg2+ and Na<sup>+</sup> in groundwater. F− had a relatively good correlation with pH, HCO<sup>3</sup> <sup>−</sup>, Na<sup>+</sup> and Cl−, that is, high F<sup>−</sup> groundwater was generally accompanied by a distinctive hydrochemical characteristic: Ca–poor and Na–rich with alkaline conditions and high HCO<sup>3</sup> − concentration. These results were consistent with previous studies [16,54,55]. The dissolution of fluorite and some silicate minerals, such as micas, was the main source of F− in groundwater [3]. *Water* **2022**, *14*, 3168 13 of 26 was inferred that there were other sources of Na+ in addition to the dissolution of salt rock. H2SiO3 was positively correlated with Mg2+ and Na+, illustrating the dissolution of silicate containing Mg2+ and Na+ in groundwater. F– had a relatively good correlation with pH, HCO3–, Na+ and Cl–, that is, high F– groundwater was generally accompanied by a distinctive hydrochemical characteristic: Ca–poor and Na–rich with alkaline conditions and high HCO3– concentration. These results were consistent with previous studies [16,54,55]. The dissolution of fluorite and some silicate minerals, such as micas, was the main source of F– in groundwater [3].

### *3.2. Hydrochemical Evolution Mechanism 3.2. Hydrochemical Evolution Mechanism*

### 3.2.1. Hydrochemical Process 3.2.1. Hydrochemical Process

According to the Gibbs diagram, the main evolution mechanism of groundwater was classified into three types: evaporation, rock weathering and atmospheric precipitation [5,33,41]. As shown in Figure 5, the ratio of Na+/(Na<sup>+</sup> + Ca2+) ranged from 0.14 to 0.59, and the ratio of Cl−/(Cl<sup>−</sup> + HCO<sup>3</sup> −) ranged between 0.05 and 0.59, indicating that the ions' concentrations of groundwater in the study area were mainly affected by rock weathering. The Na+/(Na<sup>+</sup> + Ca2+) and Cl−/(Cl<sup>−</sup> + HCO<sup>3</sup> −) ratios of the downstream aquifer (Q<sup>4</sup> *l* ) were larger than those of the upstream aquifer (Q<sup>3</sup> <sup>a</sup>*l+pl*). The burial depth of groundwater gradually became shallow from upstream to downstream, evaporation strengthened, some Ca2+ ions were precipitated as CaCO3, and the contents of Na<sup>+</sup> and Cl− were further concentrated. According to the Gibbs diagram, the main evolution mechanism of groundwater was classified into three types: evaporation, rock weathering and atmospheric precipitation [5,33,41]. As shown in Figure 5, the ratio of Na+/(Na+ + Ca2+) ranged from 0.14 to 0.59, and the ratio of Cl−/(Cl− + HCO3–) ranged between 0.05 and 0.59, indicating that the ions' concentrations of groundwater in the study area were mainly affected by rock weathering. The Na+/(Na+ + Ca2+) and Cl–/(Cl− + HCO3–) ratios of the downstream aquifer (Q4*<sup>l</sup>* ) were larger than those of the upstream aquifer (Q3a*l+pl*). The burial depth of groundwater gradually became shallow from upstream to downstream, evaporation strengthened, some Ca2+ ions were precipitated as CaCO3, and the contents of Na+ and Cl– were further concentrated.

**Figure 5.** Gibbs diagram of groundwater. **Figure 5.** Gibbs diagram of groundwater.

### 3.2.2. Analysis of the Main Dissolution and Migration 3.2.2. Analysis of the Main Dissolution and Migration

Based on the above analysis, the probable dissolutions and migrations were further judged by using the ion proportional coefficients method [16,56]. Based on the above analysis, the probable dissolutions and migrations were further judged by using the ion proportional coefficients method [16,56].

(1) (Ca2+ + Mg2+)/(HCO3– + SO42–) (1) (Ca2+ + Mg2+)/(HCO<sup>3</sup> − + SO<sup>4</sup> <sup>2</sup>−)

(2) Ca2+/Mg2+

In general, Ca2+ and Mg2+ in groundwater mainly come from the dissolution of carbonate, silicate and evaporite, so the (Ca2+ + Mg2+)/(HCO3– + SO42–) ratio was used to determine the main sources of Ca2+ and Mg2+ [16]. As shown in Figure 6a, the samples were mostly located above the 1:1 line, led by the groundwater samples in the upstream aquifer (Q3*al+pl*). Combined with Table 4, *r* (Ca2+ vs. SO42–) and *r* (Mg2+ vs. SO42–) were high, which In general, Ca2+ and Mg2+ in groundwater mainly come from the dissolution of carbonate, silicate and evaporite, so the (Ca2+ + Mg2+)/(HCO<sup>3</sup> <sup>−</sup> + SO<sup>4</sup> <sup>2</sup>−) ratio was used to determine the main sources of Ca2+ and Mg2+ [16]. As shown in Figure 6a, the samples were mostly located above the 1:1 line, led by the groundwater samples in the upstream aquifer (Q<sup>3</sup> *al+pl*). Combined with Table 4, *r* (Ca2+ vs. SO<sup>4</sup> <sup>2</sup>−) and *r* (Mg2+ vs. SO<sup>4</sup> <sup>2</sup>−) were

indicated that Ca2+ and Mg2+ in the groundwater were derived not only from the dissolu-

The ratio of Ca2+ to Mg2+ was used to reflect the dissolution of calcite and dolomite [41]. As shown in Figure 6b, the samples were generally distributed near the 1:1 line. The

1:1, which may be caused by carbonate precipitation and cation exchange.

high, which indicated that Ca2+ and Mg2+ in the groundwater were derived not only from the dissolution of carbonate and silicate minerals, but also from the dissolution of sulfate minerals. The ratios of (Ca2+ + Mg2+)/(HCO<sup>3</sup> <sup>−</sup> + SO<sup>4</sup> <sup>2</sup>−) in some groundwater samples were less than 1:1, which may be caused by carbonate precipitation and cation exchange. *Water* **2022**, *14*, 3168 15 of 26

**Figure 6.** Ion proportional coefficient diagrams: (Ca2+ + Mg2+) vs. (HCO3– + SO42<sup>−</sup>) (**a**), Ca2+ vs. Mg2+ (**b**), Na+ vs. Cl<sup>−</sup> (**c**), Na+ vs. HCO3<sup>−</sup> (**d**), and (SO42– + Cl–) vs. HCO3<sup>−</sup> (**e**). **Figure 6.** Ion proportional coefficient diagrams: (Ca2+ + Mg2+) vs. (HCO<sup>3</sup> <sup>−</sup> + SO<sup>4</sup> <sup>2</sup>−) (**a**), Ca2+ vs. Mg2+ (**b**), Na<sup>+</sup> vs. Cl<sup>−</sup> (**c**), Na<sup>+</sup> vs. HCO<sup>3</sup> − (**d**), and (SO<sup>4</sup> <sup>2</sup><sup>−</sup> + Cl−) vs. HCO<sup>3</sup> − (**e**).

3.2.3. Cation Exchange (2) Ca2+/Mg2+

Cation exchange influenced the main cations concentration, and it was judged by using the binary phase diagram of (Na+ − Cl–) vs. (Ca2+ + Mg2+ − SO42– − HCO3–) and Chlor– Alkali indices [4,16]. (1) (Na+ − Cl−) vs. (Ca2+ + Mg2+ − SO42<sup>−</sup> – HCO3−) If cation exchange was the dominant process influencing the contents of Na+, Ca2+ and Mg2+, the relationship between the two parameters was negative linear, with a slope of −1.0. As shown in Figure 7a, there was a certain linear relationship between (Na+ – Cl–) and (Ca2+ + Mg2+ − SO42– − HCO3–) in groundwater, but the correlation coefficient was low. It was indicated that cation exchange existed in the hydrogeochemical process, but it did not play a dominant role in the changes in the contents of Na+, Ca2+ and Mg2+. (2) Chloro–alkaline indices The ratio of Ca2+ to Mg2+ was used to reflect the dissolution of calcite and dolomite [41]. As shown in Figure 6b, the samples were generally distributed near the 1:1 line. The samples in the upstream aquifer (Q<sup>3</sup> *al+pl*) were mostly distributed above the 1:1 line; that is, the Ca2+ content was higher than the Mg2+ content, indicating that the main dissolved carbonate in the groundwater was calcite. Meanwhile, the samples in the downstream aquifer (Q<sup>4</sup> *l* ) were mostly distributed below 1:1; that is, the Ca2+ content was lower than the Mg2+ content in the downstream aquifer (Q<sup>4</sup> *l* ). Dolomite dissolution produces 1:1 Ca2+/Mg2+ , and calcite only produces Ca2+. If calcite or dolomite was dissolved in groundwater, the content of Ca2+ should be higher than that of Mg2+. In fact, the Ca2+ contents were lower than the Mg2+ contents, indicating that cation exchange may occur and that the Ca2+ ions in groundwater were adsorbed on the surface particles of the aquifer.

According to Figure 7b, the CAI1 and CAI2 values of most groundwater samples were (3) Na+/Cl<sup>−</sup> and Na+/HCO<sup>3</sup> −

smaller than 0, indicating that Ca2+ and Mg2+ in groundwater were replaced by Na+, and the reactions occurred: Ca2+ + 2NaX ⇄ 2Na+ + CaX; Mg2+ + 2NaX ⇄ 2Na+ + MgX. This was mainly because the sorptive abilities of Ca2+ and Mg2+ are higher than that of Na+ [31]. The CAI1 and CAI2 values of a few samples were greater than 0, which indicated that the Na+ ions in groundwater in some areas were exchanged by Ca2+ or Mg2+ ions, and the reactions might occur: 2Na+ + CaX ⇄ Ca2+ + 2NaX; 2Na+ + MgX ⇄ Mg2+ + 2NaX. Previous studies The Na+/Cl<sup>−</sup> ratio can indicate the dissolution of salt rocks and silicates in groundwater [16]. Most of the samples were located above the 1:1 line (Figure 6c). In the process of hydrochemical evolution, the content of Cl− was steady and participated less in the reaction, and its main source was the dissolution of the salt rocks. The Na<sup>+</sup> content was higher than the Cl− content, which may be due to silicate dissolution as well as salt rock dissolution during the flow process of groundwater [57].

[31,58,59] have shown that cation exchange is also influenced by other factors, such as the sediment granularity in the aquifer, pH and concentrations of the ions. Taking C1 and C7 as examples, the Na+ contents of the two samples were much higher than the average content, and the high Na+ content might cause the Na+ ions in groundwater to be exchanged with the Ca2+ or Mg2+ ions in aquifer media. Plagioclase exists in the study area [22]. Plagioclase minerals generally include albite, labradorite (intermediate), anorthite and so on. Groundwater in the study area was in a slightly alkaline environment, in which plagioclase dissolved. The dissolution reactions are listed below. According to (5) and (6), the albite released 1:1 Na+/HCO<sup>3</sup> <sup>−</sup>and 1:3 Na+/SiO2, and the labradorite produced 1:3 Na+/HCO<sup>3</sup> <sup>−</sup> and 1:3 Na+/SiO<sup>2</sup> [4]. Figure 6d shows that the ratio of Na+/HCO<sup>3</sup> − mostly ranged between 1:3 and 1:1, which meant that the content of HCO<sup>3</sup> <sup>−</sup> was higher than that of Na<sup>+</sup> . Combined with Table 4, *r* (Na<sup>+</sup> vs. HCO<sup>3</sup> −) and

*r* (Na<sup>+</sup> vs. H2SiO3) were positive, indicating the dissolution of albites and labradorites in groundwater. If the dissolution degrees of albite and labradorite were the same, both would release 1:3 Na+/SiO2. However, *r* (Na<sup>+</sup> vs. HCO<sup>3</sup> <sup>−</sup>) was higher than *r* (Na<sup>+</sup> vs. H2SiO3), meaning that the reaction degree of labradorites was more intensive than that of albites. Furthermore, cation exchange also affected the content of Na<sup>+</sup> .

$$\text{Albite:}\ \mathrm{NaAlSi\_3O\_3} + \mathrm{CO\_2} + 2\mathrm{H\_2O} = \mathrm{Na^+} + 3\mathrm{SiO\_2} + \mathrm{Al(OH)\_3} + \mathrm{HCO\_3}^-\tag{5}$$

### Labradorites: 2NaCaAl3Si5O<sup>16</sup> + 3CO<sup>2</sup> + 9H2O = 2Na<sup>+</sup> + 2 Ca2+ + 2SiO<sup>2</sup> + 3Al2Si2O5(OH)<sup>4</sup> +6HCO<sup>3</sup> − (6)

(4) (SO<sup>4</sup> <sup>2</sup><sup>−</sup> + Cl−)/HCO<sup>3</sup> −

The dissolution of carbonate and silicate was the main source of HCO<sup>3</sup> − in the study area, while the weathering and dissolution of salt rocks and the oxidation of sulfide minerals were the main sources of Cl<sup>−</sup> and SO<sup>4</sup> <sup>2</sup>−. Most of the samples were distributed below the 1:1 line (Figure 6e), which meant that the content of HCO<sup>3</sup> − was higher than that of SO<sup>4</sup> <sup>2</sup><sup>−</sup> and Cl−. This result indicated that the dissolution of carbonate and silicate minerals played a dominant role in the hydrochemical process, while the dissolution of salt rock and oxidation of sulfur minerals was relatively weak.
