*4.2. Evolution of the Spatial Distribution of Hydrochemical Parameters among Different Types of Lakes*

Topographic factors clearly control the distribution of slightly brackish and saline lakes, resulting in these lakes showing zonal distributions (Figure 7). Saline lakes tend to be found in relatively low-lying areas, whereas slightly brackish lakes are found in relatively high-lying areas. The larger lakes in the northern part of the desert and the lakes in the southeastern part of the desert are separated by a topographic fold and uplift, forming different groundwater recharge systems [42]. Therefore, there are significant differences in the hydrochemical types of the lakes in the region, despite their proximity. For example, the distance between Badain East Lake and Badain West Lake in the southeastern edge of the desert is less than 200 m, with the TDS of the latter lake exceeding that of the former by a factor of 100. This difference can be attributed to the higher water level of East Lake, resulting in the release of salt, whereas salt accumulates in the West Lake [43].

In our investigation, we found that the lakes on the southeast side of the desert were small and shallow, while the lakes on the north side were large and deep. The freshwater and slightly brackish lakes were mainly distributed in the piedmont area at a high altitude (1200–1300 m), whereas saline lakes were mainly distributed in the desert hinterland at a low altitude (1100–1200 m). As Figure 5 shows, Saline lakes are widely distributed in the region, while slightly brackish lakes are concentrated, mainly in the southeast of Yabuli Mountain. Among the 80 lake water samples, 21 located at the southern edge of the Badain Jaran Desert are freshwater and slightly brackish, with TDS ranging from 0.88 g L−<sup>1</sup> to 20.21 g L−<sup>1</sup> . 35 hyposaline lakes located in the southern-central research area show TDS values from 39.44 g L−<sup>1</sup> to 199.26 g L−<sup>1</sup> . Four mesosaline located in the west research area show TDS value from 279.48 g L−<sup>1</sup> to 452.73 g L−<sup>1</sup> . The other 10 investigated lakes are all hypersaline, located in the northwest with TDS values varying between 467.01 g L−<sup>1</sup> and 619.74 g L−<sup>1</sup> .Besides for Ca2+, Mg2+ and PH, the change trend of other anions and cations is consistent with TDS. There is a roughly increasing trend of ions from the southeast to northwest, and the West Nuoertu Lake located on the northwest edge had the highest salinity while the Dundejilin Lake located on the southern edge had the lowest salinity.

**Figure 7.** The spatial distribution of ion contents in the lakes of the Badain Jaran Desert.

Freshwater and slightly brackish lakes were concentrated within 30–45 km from the Yabulai Mountain. Saline lakes were widely distributed over a range of between 20–135 km. This lake distribution shown a close relationship between lake water and precipitation in Yabuli Mountain [44,45]. Due to the concentrated distribution of slightly brackish water lake, the variation trend of hydrochemical from Yabulai Mountain is not significant, we focus on the analysis of the relationship between the hydrochemical changes of saline lake and the distance from Yabulai Mountain (Figure 8). Significant changes in Cl−, SO<sup>4</sup> <sup>2</sup>−, and Na<sup>+</sup> occurred. Cl<sup>−</sup> increased from 6.07 g·<sup>L</sup> −1 to 169.83 g·L −1 , SO<sup>4</sup> <sup>2</sup><sup>−</sup> increased from 1.33 g·L −1 to 110.05 g·L −1 , whereas Na<sup>+</sup> increased from 12.79 g·L −1 to 295.48 g·L −1 . The values of TDS, Cl−, SO<sup>4</sup> <sup>2</sup>−, HCO<sup>3</sup> −, CO<sup>3</sup> <sup>2</sup><sup>−</sup> and Na<sup>+</sup> in most saline lakes increase with the distance from the Yabulai Mountains. Nine lakes, about 100 km from the Yabulai Mountain, have exceptionally high sodium and potassium levels. Along the terrain from the southeast to the northwest, the farther away from the Yabulai Mountains, the higher the ion content. The freshwater lakes and slightly brackish lakes in the Badain Jaran Desert are recharged by mountain precipitation and lateral runoff. The rapid rate of lake water renewal leads to a relatively low lake water TDS. The saline lakes are positioned far away from the mountain area and receive groundwater recharge. The long recharge path and complex geological conditions result in a slow lake water regeneration rate and intense evaporation, leading to relatively high TDS. This conclusion is consistent with that of Yang [46]. It is believed that the shallow groundwater is recharged by recent local precipitation infiltration near Yabulai Mountain in the southeastern margin of the desert, whereas deep groundwater is recharged by precipitation during humid periods. This assertion is consistent with the analyses by Hofmann [47] which showed the presence of two discontinuous hydraulic flow systems in this area. One is located north of the geological anticline, ~40 km from the southern edge of the desert. This flow system is recharged by precipitation. The other is located south of the geological anticline and is recharged by water from outside the dune area.

**Figure 8.** The variability of lake ions distance from the Yabulai Mountains.

## *4.3. Analysis of Mechanisms Contributing to the Hydrochemistry of Different Types of Lakes*

In general, the major sources of dissolved ions in lakes include input from precipitation, weathering rocks, regional geologic, and anthropogenic input [48]. Evidenced by Gibbs diagram in Figure 4, indicated that the evaporation-crystallization reactions are the dominant in the study region. Moreover, some saline mineral deposits, including gypsum, hailte, anhydrite, epsomite, mirabilite, and thenardite, are extensive in these regions caused by intense evaporation-crystallization. Intense evaporation is a vital driving force in the formation of lakes and also affects their hydrochemical components and water balance. The higher HCO<sup>3</sup> <sup>−</sup>, SO<sup>4</sup> <sup>2</sup>−, Ca2+, and Mg2+ concentration in lakes from the freshwaterlake demonstrate that those regions are enriched in minerals like anhydrite, aragonite, calcite, dolomite and gupsum. In comparison to this lake, the slightly brackish and saline lakes have lower Ca2+. Calcium consumption led to the progressive enrichment of Na<sup>+</sup> , SO<sup>4</sup> <sup>2</sup>−, and Cl<sup>−</sup> in the lake brines where halite, thenardite, epsomiteand, and Mirabilite precipitated. The samples from lake were far below saturation with chloride minerals such as halite, showed by the negative SI values ranging between −11.36 and 3.22, which confirms that the soluble component Na<sup>+</sup> and Cl<sup>−</sup> concentration was not limited by mineral equilibrium in the sediments of the study area. Table 2 showed the saturation index of main minerals in lakes water, which were used to understand the hydrogeochemical evolution processes during flow paths of the groundwater from the recharge zone to the discharge zone. Based on mineral SI result, Ca2+/Na<sup>+</sup> ratio and the geological condition of the study area, the hydrogeochemical processes mainly included the dissolution of anhydrite, gypsum, halite, epsomite, mirabilite, and thenardite, and the precipitation of calcite, dolomite, magnesite, and aragonite from the recharge zone to the discharge zone.

#### **5. Conclusions**

The Badan Jaran desert is a vast arid region endowed with perennial lakes with varying salinities (0.88–619.74 g L−<sup>1</sup> ) and high pH values (7.54–10.52) due to the abundance of Na<sup>+</sup> , K<sup>+</sup> , Cl− and SO<sup>4</sup> <sup>2</sup>−. The results according to TDS of the water samples from 80 lakes indicated that freshwater, slightly brackish, brackish, and saline water lakes accounted for 1.25%, 25%, 0.0%, and 73.75%, respectively. The Piper termary diagram indicate that the lake hydrochemistry changed from the SO<sup>4</sup> <sup>2</sup>−-Cl−-HCO<sup>3</sup> <sup>−</sup> type to the SO<sup>4</sup> <sup>2</sup>−-Cl<sup>−</sup> type and lakes transitioned from freshwater to saline. Enrichment of Na<sup>+</sup> in the lake water of the Badain Jaran Desert exceeds that of Cl−, suggesting strong evaporative concentration and cation exchange. The spatial distribution of ion contents in the lakes indicated that freshwater and slightly brackish lakes were mainly distributed in the piedmont area at a high altitude (1200–1300 m) and were concentrated within 30–45 km from the Yabulai Mountain, whereas saline lakes were mainly distributed in the desert hinterland at a low altitude (1100–1200 m) and were widely distributed over a range of between 20–135 km, there is a roughly increasing trend of ions from the Yabulai Mountains. Evidenced by Gibbs diagram indicated that the evaporation-crystallization reactions are the dominant in the study region. Moreover, some saline mineral deposits, including gypsum, hailte, anhydrite, epsomite, mirabilite, and thenardite, are extensive in these regions caused by intense evaporation-crystallization.

**Author Contributions:** Conceptualization, B.J. and J.S.; methodology, B.J., H.X.; formal analysis, B.J.; investigation, B.J. and J.Q.; writing-original draft preparation, B.J.; writing-review and editing, B.J., J.S.; funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Major Science and Technology Project in Inner Mongolia Autonomous Region of China (No. zdzx2018057) and the Innovation Cross Team Project of Chinese Academy of Sciences, CAS (No. JCTD-2019-19), Transformation Projects of Scientific and Techological Achievements in Inner Mongolia Autonomous region of China (No. 2021CG0046).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author on reasonal request.

**Acknowledgments:** We thank all the participants in the Alashan Desert Eco-hydrological Experimental research Station, Northwest Institute of Eco-Environment and Resources, Chinese Academu of Sciences. We greatly appreciate suggestions from anonymous referees for the improvement of our paper. Thanks also to the editorial staff.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

