| Summary: | Hydrochemical research and identification of nitrate contamination are of great significant for the endorheic basin, and the Northern Huangqihai Basin (a typical endorheic basin) was comprehensively researched. The results showed that the main hydrochemical facies were HCO<sub>3</sub>–Mg·Ca and HCO<sub>3</sub>–Ca·Mg. Spatial variation coefficients of most indices were greater than 60%, which was probably caused by human activities. The hydrochemical evolution was mainly affected by rock weathering and also by cation exchange. The D–<sup>18</sup>O relationship of groundwater was δD = 5.93δ<sup>18</sup>O − 19.18, and the <i>d</i>–<i>excess</i> range was −1.60–+6.01‰, indicating that groundwater was mainly derived from precipitation and that contaminants were very likely to enter groundwater along with precipitation infiltration. The NO<sub>3</sub>(N) contents in groundwater exceeded the standard. Hydrochemical analyses indicated that precipitation, industrial activities and synthetic NO<sub>3</sub> were unlikely to be the main sources of nitrate contamination in the study area. No obvious denitrification occurred in the transformation process of nitrate. The δ<sup>15</sup>N(NO<sub>3</sub>) values ranged from +0.29‰ to +14.39‰, and the δ<sup>18</sup>O(NO<sub>3</sub>) values ranged from −6.47‰ to +1.24‰. Based on the δ<sup>15</sup>N(NO<sub>3</sub>) – δ<sup>18</sup>O(NO<sub>3</sub>) dual isotope technique and hydrochemical methods, manure, sewage and NH<sub>4</sub> fertilizers were identified to be the main sources of nitrate contamination. This study highlights the effectiveness of the integration of hydrochemical and isotopic data for nitrate source identification, and is significant for fully understanding groundwater hydrochemistry in endorheic basins and scientifically managing and protecting groundwater.
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