**4. Conclusions**

Hydrochemical characteristics and evolution process of groundwater in the northern Huangqihai Basin were comprehensively analyzed by using multiple hydrochemical methods. The groundwater in the study area was generally weakly alkaline with low salinity. The relative anionic abundance of groundwater samples was in the order of HCO<sup>3</sup> −> Cl− > SO<sup>4</sup> <sup>2</sup><sup>−</sup> > NO<sup>3</sup> <sup>−</sup>, whereas the cationic abundance was Mg2+ > Ca2+ > Na<sup>+</sup> > K<sup>+</sup> . The distributions of Mg2+, Na<sup>+</sup> , Cl<sup>−</sup> and NO<sup>3</sup> − showed significant spatial variations (*C<sup>v</sup>* > 100%), indicating the influence of human activities on groundwater. The main chemical facies of groundwater were HCO3–Mg·Ca and HCO3–Ca·Mg. The mole fractions of NO<sup>3</sup> − in C29 and C30 samples were higher than 25%, and two new hydrochemical facies (NO3·HCO3– Ca·Mg and HCO3·NO3·Cl–Ca·Mg) appeared based on the improved Shukalev classification method. The hydrochemical evolution of groundwater was predominantly affected by rock weathering and also by cation exchange.

The main source of groundwater was precipitation by means of the D–18O isotope technique, and the relationship between δD and δ <sup>18</sup>O of groundwater was δD = 5.93δ <sup>18</sup>O − 19.18 (*R* <sup>2</sup> = 0.9067). The *d–excess* range was <sup>−</sup>1.60 to +6.01‰ with an average value of 3.38‰ (<10‰), indicated that groundwater was controlled by water–rock interactions and influenced by evaporation. Contaminants were very likely to enter groundwater along with precipitation infiltration, resulting in excessive nitrogen in groundwater.

The NO3(N) contents in some parts of the study area were far exceeded the level III standard. The NO<sup>3</sup> <sup>−</sup>/Na<sup>+</sup> ratios ranged from 0.00 to 3.69 with an average of 0.49, and the *C<sup>v</sup>* was 1.34 (>1). The ratios of SO<sup>4</sup> <sup>2</sup>−/Na<sup>+</sup> were smaller than those of NO<sup>3</sup> <sup>−</sup>/Na<sup>+</sup> overall, indicating that the nitrate content of groundwater was mainly affected by agricultural activities. The Cl<sup>−</sup> contents were generally more than 1 mmol/L, and the NO<sup>3</sup> −/Cl− ratios ranged between 0.1 and 10 with an average value of 0.58. The correlation between NO<sup>3</sup> <sup>−</sup> and K<sup>+</sup> was weak. The hydrochemical analysis showed that precipitation, industrial activities and synthetic NO<sup>3</sup> were unlikely to be the main sources of nitrate contamination.

The relationship between NO<sup>3</sup> − and δ <sup>15</sup>N(NO3) was not negative, and the slope of δ <sup>15</sup>N(NO3) vs. δ <sup>18</sup>O(NO3) in groundwater was <sup>−</sup>0.09, far lower than 0.48 and 0.77, respectively. The δ <sup>18</sup>O(NO3) values fell within the range (−10 to +10‰). These analyses indicated that no obvious denitrification occurred, and that nitrification was the main process during nitrogen transformation in the study area. The δ <sup>15</sup>N(NO3) values ranged from +0.29 to +14.39‰, and the δ <sup>18</sup>O(NO3) values ranged from <sup>−</sup>6.47 to +1.24‰. The

δ <sup>15</sup>N(NO3) and δ <sup>18</sup>O(NO3) data further affirmed the hydrochemical interpretation that precipitation, industrial sewage and synthetic NO<sup>3</sup> were unlikely sources of NO3, and the main sources were manure, sewage and NH<sup>4</sup> fertilizers.

The integration of hydrochemical analysis and dual isotope technique provides a further insight into the identification of nitrate contamination from multiple perspectives: hydrochemical characteristics, evolution mechanism, probable pathway of contaminants entering groundwater, bivariate relationships between NO<sup>3</sup> − and other indices, and isotope composition. It is recommended that extensive and sustained monitoring of the D–18O isotopes in groundwater and the <sup>15</sup>N(NO3) and <sup>18</sup>O(NO3) isotopes in the potential nitrate sources (fertilizers, manure and sewage) should be performed in further studies.

**Author Contributions:** Conceptualization, J.J.; methodology, J.J. and Z.W.; investigation, H.D. and J.Z.; data curation, Z.W. and Y.Z.; writing—original draft preparation, J.J.; writing—review and editing, J.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Basic Scientific Research Foundation Special Project of the China Institute of Water Resources and Hydropower Research (No. MK2021J07 and MK2020J10), Project of Collaborative Innovation Center for Grassland Ecological Security (Ecohydrological Characteristics and Ecosystem Services Assessment in Tabu River Watershed, No. MK0143A032021) and Science and Technology Planning Project of Inner Mongolia (No. MK0143A012022).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are grateful to all the editors and anonymous reviewers for their helpful comments that greatly improved the quality of the manuscript.

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