(2) Chloro–alkaline indices

According to Figure 7b, the CAI<sup>1</sup> and CAI<sup>2</sup> values of most groundwater samples were smaller than 0, indicating that Ca2+ and Mg2+ in groundwater were replaced by Na<sup>+</sup> , and the reactions occurred: Ca2+ + 2NaX 2Na<sup>+</sup> + CaX; Mg2+ + 2NaX 2Na<sup>+</sup> + MgX. This was mainly because the sorptive abilities of Ca2+ and Mg2+ are higher than that of Na<sup>+</sup> [31]. The CAI<sup>1</sup> and CAI<sup>2</sup> values of a few samples were greater than 0, which indicated that the Na<sup>+</sup> ions in groundwater in some areas were exchanged by Ca2+ or Mg2+ ions, and the reactions might occur: 2Na<sup>+</sup> + CaX Ca2+ + 2NaX; 2Na<sup>+</sup> + MgX Mg2+ + 2NaX. Previous studies [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<sup>+</sup> contents of the two samples were much higher than the average content, and the high Na<sup>+</sup> content might cause the Na<sup>+</sup> ions in groundwater to be exchanged with the Ca2+ or Mg2+ ions in aquifer media.

**Figure 7.** Coefficient diagrams of (Na+ − Cl<sup>−</sup>) vs. (Ca2+ + Mg2+ − SO42<sup>−</sup> − HCO3<sup>−</sup>) for analyzing cation exchange reaction (**a**) and distribution diagram of Chloro–alkaline indices (**b**). **Figure 7.** Coefficient diagrams of (Na<sup>+</sup> <sup>−</sup> Cl−) vs. (Ca2+ + Mg2+ <sup>−</sup> SO<sup>4</sup> <sup>2</sup><sup>−</sup> <sup>−</sup> HCO<sup>3</sup> −) for analyzing cation exchange reaction (**a**) and distribution diagram of Chloro–alkaline indices (**b**).

### *3.3. Groundwater Source 3.3. Groundwater Source*

The global meteoric water line (GMWL) was δD = 8δ18O + 10 [60]. Due to the lack of the precipitation isotopic data of the study area, the local meteoric water line (LMWL) of Hohhot, which was located in the same climate zone as the study area and near the study area, was chosen to reflect the D–18O isotopic composition of precipitation in the study area, and the LMWL equation was δD = 7.68δ18O − 0.72 (*R*2 = 0.8964) [61]. The study area is situated inland with less precipitation and intensive evaporation, and is comprehensively influenced by the East Asian summer monsoon and westerly circulation. The water vapor of the precipitation, which was caused by westerly circulation, originates from the North Atlantic and is transported from Xinjiang to inland China. It is characterized by low humidity and obvious secondary evaporation, which causes the temperature effect [61,62]. The water vapor brought by the East Asian summer monsoon has a high humidity and is slightly influenced by evaporation; along with transportation inland, the heavy isotopes are preferentially condensed, and δD and δ18O values in precipitation decrease with increasing of precipitation, which was called the rainfall effect [61,62]. For the above rea-The global meteoric water line (GMWL) was δD = 8δ <sup>18</sup>O + 10 [60]. Due to the lack of the precipitation isotopic data of the study area, the local meteoric water line (LMWL) of Hohhot, which was located in the same climate zone as the study area and near the study area, was chosen to reflect the D–18O isotopic composition of precipitation in the study area, and the LMWL equation was δD = 7.68δ <sup>18</sup><sup>O</sup> <sup>−</sup> 0.72 (*<sup>R</sup>* <sup>2</sup> = 0.8964) [61]. The study area is situated inland with less precipitation and intensive evaporation, and is comprehensively influenced by the East Asian summer monsoon and westerly circulation. The water vapor of the precipitation, which was caused by westerly circulation, originates from the North Atlantic and is transported from Xinjiang to inland China. It is characterized by low humidity and obvious secondary evaporation, which causes the temperature effect [61,62]. The water vapor brought by the East Asian summer monsoon has a high humidity and is slightly influenced by evaporation; along with transportation inland, the heavy isotopes are preferentially condensed, and δD and δ <sup>18</sup>O values in precipitation decrease with increasing of precipitation, which was called the rainfall effect [61,62]. For the above reasons, the slope of the LMWL was less than that of the GMWL (Figure 8).

The δD values of groundwater ranged from −74.93 to −61.01‰, the δ18O values of groundwater ranged from −10.08 to −7.39‰, and the averages of δD and δ18O were 69.14 and 9.07‰, respectively. Samples were mostly distributed near the LMWL (Figure 8), and the linear equation of δD and δ18O of groundwater was δD = 5.93δ18O − 19.18 (*R*2 = 0.9067), indicating that local atmospheric precipitation was the main source of groundwater. Contaminants were very likely to enter groundwater along with the precipitation infiltration.

sons, the slope of the LMWL was less than that of the GMWL (Figure 8).

ment uncertainty should be taken into consideration.

**Figure 8.** Relationship between δD and δ18O of groundwater in the study area. Sampling sites: Upstream aquifer (Q3*al+pl*) and downstream aquifer (Q4*<sup>l</sup>* ); sampling times: September 2021 and May 2022. **Figure 8.** Relationship between δD and δ <sup>18</sup>O of groundwater in the study area. Sampling sites: Upstream aquifer (Q<sup>3</sup> *al+pl*) and downstream aquifer (Q<sup>4</sup> *l* ); sampling times: September 2021 and May 2022.

The deuterium excess values (*d–excess* = δD − 8δ18O) were used to analyze the inten-

The linear slope of δD and δ18O of groundwater was lower than that of GMWL (8) and LMWL (7.68), which meant that heavy isotopes were further enriched by evaporation during the runoff process of groundwater. The isotopic composition (δD and δ18O) was influenced to some extent by the regional climate and local processes (evaporation, vegetation distribution, anthropogenic activities) [48]; thus, the δD and δ18O values of groundwater may be different in different areas and different times. According to Table 5, the averages of δD and δ18O downstream were higher than those upstream which was caused by the intensification of evaporation in the shallow groundwater depth area. Comparing the δD and δ18O values in different seasons (Table 5), the δ18O averages in May 2022 were slightly higher than those in September 2021 regardless of whether the groundwater samples were distributed upstream and downstream; the δD average downstream in May 2022 was just 0.30‰ higher than that in September 2021, and the average upstream in May 2022 was −0.55‰ lower than that in September 2021. The reasons for the difference between different times need further research based on monthly isotopic data over years, and measure-

sity of groundwater evaporation. The stronger the evaporation was, the more negative the *d–excess* value was [63]. The *d–excess* values of groundwater ranged from −1.60 to 6.01‰, with an average value of 3.38‰. The average of *d–excess* in the study area was positive but smaller than the *d–excess* average of the global meteoric water (10‰) [60]. It was indicated that the ion contents of groundwater in the study area were controlled by water– rock interactions and influenced by evaporation. Due to intensive evaporation, the contents of ions are generally concentrated in the groundwater. The solubilities of some salts (such as NaCl) are high, and the contents of their ions in groundwater (such as Na+ and Cl–) can increase to a high level. On the other hand, the solubilities of some salts (such as CaCO3) are low. Taking CaCO3 as an example, the content of Ca2+ increases due to evaporation, but once its content is saturated, the Ca2+ ion in groundwater precipitates in the form of CaCO3, Ca2+ + CO3– = CaCO3, and the Ca2+ content in the groundwater decreases. This may change the ion compositions of groundwater, and further influence the hydrochemical facies. In general, intensive evaporation in the shallow aquifer can cause an increase in the TDS and TH contents [64]. By comparing the *d–excess* values of groundwater in different seasons (Table 5), the *d–excess* average in May (3.42‰) was slightly higher than that in September (3.40‰). Due to the lack of time series data of isotopes, the possible reasons were preliminarily inferred The δD values of groundwater ranged from −74.93 to −61.01‰, the δ <sup>18</sup>O values of groundwater ranged from −10.08 to −7.39‰, and the averages of δD and δ <sup>18</sup>O were 69.14 and 9.07‰, respectively. Samples were mostly distributed near the LMWL (Figure 8), and the linear equation of δD and δ <sup>18</sup>O of groundwater was δD = 5.93δ <sup>18</sup><sup>O</sup> <sup>−</sup> 19.18 (*<sup>R</sup>* 2 = 0.9067), indicating that local atmospheric precipitation was the main source of groundwater. Contaminants were very likely to enter groundwater along with the precipitation infiltration. The linear slope of δD and δ <sup>18</sup>O of groundwater was lower than that of GMWL (8) and LMWL (7.68), which meant that heavy isotopes were further enriched by evaporation during the runoff process of groundwater. The isotopic composition (δD and δ <sup>18</sup>O) was influenced to some extent by the regional climate and local processes (evaporation, vegetation distribution, anthropogenic activities) [48]; thus, the δD and δ <sup>18</sup>O values of groundwater may be different in different areas and different times. According to Table 5, the averages of δD and δ <sup>18</sup>O downstream were higher than those upstream which was caused by the intensification of evaporation in the shallow groundwater depth area. Comparing the δD and δ <sup>18</sup>O values in different seasons (Table 5), the δ <sup>18</sup>O averages in May 2022 were slightly higher than those in September 2021 regardless of whether the groundwater samples were distributed upstream and downstream; the δD average downstream in May 2022 was just 0.30‰ higher than that in September 2021, and the average upstream in May 2022 was −0.55‰ lower than that in September 2021. The reasons for the difference between different times need further research based on monthly isotopic data over years, and measurement uncertainty should be taken into consideration.


**Table 5.** Statistics of δD, δ <sup>18</sup>O and *d–excess* in different seasons (Unit: ‰).

The deuterium excess values (*d–excess* = δD − 8δ <sup>18</sup>O) were used to analyze the intensity of groundwater evaporation. The stronger the evaporation was, the more negative the *d–excess* value was [63]. The *d–excess* values of groundwater ranged from −1.60 to 6.01‰, with an average value of 3.38‰. The average of *d–excess* in the study area was positive but smaller than the *d–excess* average of the global meteoric water (10‰) [60]. It was indicated that the ion contents of groundwater in the study area were controlled by water–rock interactions and influenced by evaporation. Due to intensive evaporation, the contents of ions are generally concentrated in the groundwater. The solubilities of some salts (such as NaCl) are high, and the contents of their ions in groundwater (such as Na<sup>+</sup> and Cl−) can increase to a high level. On the other hand, the solubilities of some salts (such as CaCO3) are low. Taking CaCO<sup>3</sup> as an example, the content of Ca2+ increases due to evaporation, but once its content is saturated, the Ca2+ ion in groundwater precipitates in the form of CaCO3, Ca2+ + CO<sup>3</sup> <sup>−</sup> = CaCO3↓, and the Ca2+ content in the groundwater decreases. This may change the ion compositions of groundwater, and further influence the hydrochemical facies. In general, intensive evaporation in the shallow aquifer can cause an increase in the TDS and TH contents [64].

By comparing the *d–excess* values of groundwater in different seasons (Table 5), the *d–excess* average in May (3.42‰) was slightly higher than that in September (3.40‰). Due to the lack of time series data of isotopes, the possible reasons were preliminarily inferred based on previous studies [61,65]: (1) the difference in the isotopic composition of precipitation (the main source of groundwater) between different seasons and (2) the different influences of evaporation on groundwater between different seasons. The measurement uncertainty should be considered. Continuous measurements of stable isotopes both in precipitation and groundwater and further research are needed. From the spatial distribution, the *d–excess* values decreased from the upstream aquifer (Q<sup>3</sup> *al+pl*) to the downstream aquifer (Q<sup>4</sup> *l* ). The main reason was that the water–rock interactions continuously proceeded along the direction of groundwater runoff, and an oxygen shift occurred. The burial depth of groundwater became shallow from north to south, and evaporation intensified; thus, the heavy isotopes were enriched. From the time perspective, the content of Cl− would be enriched due to evaporation. According to the Pearson correlation analysis, *r* (Cl− vs. δ <sup>18</sup>O) was 0.611, but it did not reach the significance level of 0.05, indicating that evaporation slightly affected the isotopic compositions.
