*2.1. Study Area*

The northern Huangqihai Basin (113◦2 0–113◦280 E, 40◦430–41◦3 0 N), is located in Right Chahaer County and the Jinning District of Wulanchabu city in Inner Mongolia, China (Figure 1a). The study area has a continental monsoon climate with an annual average temperature of 5.23 ◦C. The annual average rainfall is 359.30 mm and mainly concentrated in summer. The basin is surrounded by mountains on three sides; the terrain is generally high in the north and low in the south. Surface water resource is rare, the main rivers have dried up in their middle and downstream regions in recent years, and the other rivers are seasonal. The development of the regional social economy is highly dependent on groundwater, especially for agriculture.

The entire study area is covered by the unconsolidated Quaternary sediments and the main aquifer is the Quaternary phreatic aquifer. Based on the lithologic characteristics, the phreatic aquifer is further divided into two aquifers: the Quaternary Holocene lacustrine aquifer (Q<sup>4</sup> *l* ) and the Quaternary Upper Pleistocene alluvial–diluvial aquifer (Q<sup>3</sup> *al+pl*) (Figure 1b). The Q<sup>4</sup> *<sup>l</sup>* aquifer is distributed around Huangqihai Lake and consists of medium and fine sand, while the Q<sup>3</sup> *al+pl* aquifer is distributed around the Q<sup>4</sup> *<sup>l</sup>* aquifer and mainly composed of sandy gravel, pebbles and coarse sand. According to previous research [22], the dynamic type of the groundwater level is the rainfall infiltration–artificial exploitation type. In summer, rainfall is abundant, but the groundwater level does not immediately rise and even declines due to the high consumption of irrigation. After irrigation, the groundwater level recovers due to the hysteresis recharge of rainfall and reaches a high level in spring. The groundwater levels and depths may change over time, but the overall flow direction of the Quaternary phreatic water does not obviously change during a hydrological year, that is, the groundwater generally flows from the north to the south following the topography. As for the Quaternary phreatic water (Figure 1b), the Q<sup>3</sup> *al+pl* aquifer is located upstream of the hydraulic head field and the Q<sup>4</sup> *<sup>l</sup>* aquifer is located downstream of that. Influenced by the terrain, geomorphic type and hydrological conditions, the hydraulic gradient upstream is steeper than that downstream. As seen from Figure 1c, the groundwater depth changes from deep to shallow from north to south. The groundwater depths near Huangqihai Lake and rivers are usually shallower than 5 m, and the depths in other areas are deeper than 5 m. According to previous studies [22], the extreme evaporation depth of the groundwater is 5 m, in other words, the depths of groundwater in most areas exceed the extreme evaporation depth.

**Figure 1.** Location of the study area and water samples (**a**), hydraulic head field (**b**) and the burial depth of groundwater (**c**). **Figure 1.** Location of the study area and water samples (**a**), hydraulic head field (**b**) and the burial depth of groundwater (**c**).

### The entire study area is covered by the unconsolidated Quaternary sediments and *2.2. Data Preparation and Methods*

### the main aquifer is the Quaternary phreatic aquifer. Based on the lithologic characteristics, 2.2.1. Data Preparation

the phreatic aquifer is further divided into two aquifers: the Quaternary Holocene lacustrine aquifer (Q4*<sup>l</sup>* ) and the Quaternary Upper Pleistocene alluvial–diluvial aquifer (Q3*al+pl*) (Figure 1b). The Q4*<sup>l</sup>* aquifer is distributed around Huangqihai Lake and consists of medium and fine sand, while the Q3*al+pl* aquifer is distributed around the Q4*<sup>l</sup>* aquifer and mainly composed of sandy gravel, pebbles and coarse sand. According to previous research [22], the dynamic type of the groundwater level is the rainfall infiltration–artificial exploitation type. In summer, rainfall is abundant, but the groundwater level does not immediately rise and even declines due to the high consumption of irrigation. After irrigation, the groundwater level recovers due to the hysteresis recharge of rainfall and reaches a high level in spring. The groundwater levels and depths may change over time, but the overall flow direction of the Quaternary phreatic water does not obviously change during a hydrological year, that is, the groundwater generally flows from the north to the south following the topography. As for the Quaternary phreatic water (Figure 1b), the The groundwater level and depth data were measured in late September 2021. Thirty– eight groundwater samples were collected according to the Groundwater Quality Standard (GB/T 14848-2017) [28]. Before sampling, the wells were pumped for thirty minutes to obtain fresh groundwater. All groundwater samples were collected from the Quaternary phreatic aquifer and evenly distributed in different hydrogeological units (Figure 1a). Groundwater samples were sealed and stored in 5 L PVC bottles that were carefully cleaned before sampling. After collection, the samples were kept at 4 ◦C and later sent to the Inner Mongolia Mineral Resources Experimental Research Institute for analyzing. Hydrochemical indices were analyzed by using the standard methods as suggested by Analysis Methods of Groundwater quality (DZ/T 0064.1–2021) [29]. pH was analyzed by an ion meter (PXJ–1B, Jiangsu Electric Analysis Instrument Factory, Jiangyan, China). The concentrations of cations (Mg2+, Ca2+, Na<sup>+</sup> and K<sup>+</sup> ) and some trace elements (Fe, Mn, Cu, Pb, Cd, etc.) were analyzed by a PerkinElmer Optima 8300 with a detection accuracy of 0.001 mg/L. The concentrations of anions (SO<sup>4</sup> <sup>2</sup>−, Cl−, F<sup>−</sup> and NO<sup>3</sup> −) were measured by ion chromatography (IC850). The concentrations of NH<sup>4</sup> + , NO<sup>2</sup> − and H2SiO<sup>3</sup>

were analyzed by a visible spectrophotometer (7200, Tianmei Scientific Instument Co., LTD, Shanghai, China). The concentrations of HCO<sup>3</sup> −, total hardness (TH) and chemical oxygen demand of manganese (CODMn) were analyzed by the titration method. The total dissolved solids (TDS) were determined by the weighing method (Electronica scales JA31001). The accuracy of the testing results was checked using an ionic error equilibrium, and the relative error was controlled below 3%, which meant that the analyzing results were reliable [30]. Twenty–five chemical indices of the groundwater samples were analyzed. The concentrations of some indices, such as Cu, Cd, Hg, and Cr6+, were low, even below the detection limit. Based on the previous study and the real conditions of the study area, the analysis was focused on the main ions and the overstandard indices.

The samples for testing δD, δ <sup>18</sup>O, δ <sup>15</sup>N(NO3) and δ <sup>18</sup>O(NO3) were collected in late September 2021 and early May 2022 based on the Handbook of Hydrogeology [30], and twelve D–18O isotope samples and twelve <sup>15</sup>N–18O(NO3) isotope samples were collected in each phase. These samples were collected form the Quaternary phreatic aquifer, and numbered H1–H6 and H8–H13. D–18O isotope samples were sealed in 10 mL EP plastic tubes, and <sup>15</sup>N–18O(NO3) isotope samples were sealed in 50 mL EP plastic tubes after filtering with a 0.45 µm filter, and then stored at a low temperature. All isotope samples were analyzed by the LICA United Technology Limited. The D–18O isotope test machine was a Liquid water isotope analyzer (912–0050, Los Gatos Research, Inc., San Jose, CA, USA), and the <sup>15</sup>N–18O(NO3) isotope test machine was a Thermo Fisher MT253 and Flash 2000HT. Three parallel samples (H100 , H110 and H120 ) for analyzing those isotopes were collected and sent to another testing organization (Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences), and the relative errors between the results testing from the two testing organizations were all less than 3%.

The main ions (Mg2+, Ca2+, Na<sup>+</sup> , K<sup>+</sup> , HCO<sup>3</sup> <sup>−</sup>, SO<sup>4</sup> <sup>2</sup>−, Cl−) and other indices (NO<sup>3</sup> −, NO<sup>2</sup> <sup>−</sup>, NH<sup>4</sup> + , TDS, TH, CODMn and H2SiO3) were used to reflect the hydrochemical characteristics, reveal the hydrochemical evolution and evaluate groundwater quality. The D and <sup>18</sup>O isotopes were conducted to analyze the source of groundwater. The relationship analysis among NO<sup>3</sup> −, SO<sup>4</sup> <sup>2</sup>−, Cl−, Na<sup>+</sup> and K<sup>+</sup> and the <sup>15</sup>N(NO3) and <sup>18</sup>O(NO3) isotopes were combined to accurately identify nitrate contamination.
