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 HCO₃–Mg·Ca and HCO₃–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–¹⁸O relationship of groundwater was δD = 5.93δ¹⁸O − 19.18, and the d–excess 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 NO₃(N) contents in groundwater exceeded the standard. Hydrochemical analyses indicated that precipitation, industrial activities and synthetic NO₃ 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 δ¹⁵N(NO₃) values ranged from +0.29‰ to +14.39‰, and the δ¹⁸O(NO₃) values ranged from −6.47‰ to +1.24‰. Based on the δ¹⁵N(NO₃) – δ¹⁸O(NO₃) dual isotope technique and hydrochemical methods, manure, sewage and NH₄ 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.

The Tabu catchment, a typical-desertified steppe in China, was selected as the study area to qualitatively analyze the interaction between surface water (SW) and groundwater (GW), and an integration of hydrochemical analysis and isotopic techniques was applied. The results show that the ion contents in SW and GW increased from upstream to downstream, and the hydrochemical evolutions were both controlled by rock weathering and influenced by evaporation. The δD–δ¹⁸O lines of SW and GW were δD = 5.14δ¹⁸O − 24.68 and δD = 6.89δ¹⁸O − 5.81, respectively. Along the I–I′ profile, the contents of most indices, δD and δ¹⁸O in SW and GW both showed increasing tendencies. All of the similarities in the hydrochemical characteristics and isotopic techniques indicated that SW was recharged by GW. The δD–δ¹⁸O inconsistency in SW and GW samples from midstream and downstream areas indicated that SW did not recharge to GW in these areas and was consumed by evaporation or replenished the moisture in the vadose zones. The runoff decreased, which was mainly caused by excessive exploitation of GW and a decline in the GW level. This study deepens the understanding of the hydrological cycle and provides guidance for the optimal combined utilization of SW and GW.

Groundwater is the main clean water resource in northern China, and its quality is critical for both human health and social sustainable development. Due to complex anthropogenic and/or geogenic processes, the sources of groundwater contaminants are not easy to determine. The Tabu River Basin, located in northern China, is an agriculture and pasture interlaced area in which phreatic groundwater is the predominant water resource for domestic and agricultural purposes. Groundwater with abnormally high levels of NO₃⁻, F⁻, and TDS was observed here based on 87 groundwater samples collected from the phreatic aquifer in 2022. In this study, hydrogeochemical and isotopic methods were used to trace groundwater contaminants in the phreatic aquifer, and a risk assessment was conducted to analyze their threat to human health. The results indicated that NO₃⁻ in the phreatic groundwater primarily originated from manure, the high concentration of TDS was highly associated with irrigation, and the enrichment of F⁻ was mainly controlled by geogenic factors, including alkaline condition, competitive adsorption, the dissolution of fluorine-bearing minerals, and cation exchange. A principal component analysis (PCA) showed that both anthropogenic (PC1, 50.7%) and geogenic (PC2, 19.9%) factors determined the quality of the phreatic groundwater in the study area. The human health risk assessment demonstrated that 98.9%, 92.0%, and 80.5% of the groundwater samples exceeded the permissible limit of the total noncarcinogenic risk for children, adult females, and adult males, respectively. The monitoring results from 2022 to 2023 suggested that phreatic groundwater contamination could not be mitigated through natural attenuation under the existing external pressures. Measures need to be taken to decrease the contamination of phreatic groundwater and enhance the groundwater sustainability in the Tabu River Basin. The findings of this study can provide a reference for sustainable groundwater development in the Tabu River Basin and other arid and semi-arid regions worldwide.

Based on the hydrogeological perspective, the variation in the groundwater level (W) was selected as the analysis factor, and the rainfall changes and groundwater exploitation (QGE) were considered to be the influencing factors. The influence of the two factors on W in the northern Huangqihai Basin was analyzed qualitatively and quantitatively. Results showed that both rainfall and QGE showed increasing trends from 1996 to 2020, while the W values decreased. The correlation between QGE and W was weaker in wet years than in normal years but was the highest in dry years. The mutation time was determined to be 2004. Based on the slope-changing ratio of the cumulative quantity method and the elastic coefficient method, it was determined that the contribution of rainfall changes to W was 22.08% and that of QGE was 77.92%. This study highlights the effectiveness of the integration of qualitative analyses and quantitative evaluation for identifying the influence of external factors on groundwater and well documents that the elastic coefficient method can be used in this field. More attention should be focused on the scientific management of QGE to minimize its negative influence on the sustainable development of groundwater.

The variation in the groundwater level, which is an indicative factor that can reflect changes in both groundwater level and groundwater quantity, was selected as the factor to be analyzed. Rainfall and groundwater exploitation were chosen as the representative factors of climate change and anthropogenic activities, respectively. By applying the elastic coefficient method and other mathematical statistical methods, the influence of climate change and anthropogenic activities on groundwater was analyzed qualitatively and quantitatively. The northern Huangqihai basin was chosen as the study area. The rainfall in the study area showed an increasing trend from 1996–2020, while most variations in groundwater level were negative. This result indicated that the positive influence of rainfall on the variation in the groundwater level in the study area was not enough to offset the negative influence of groundwater exploitation. The correlation between anthropogenic activities and variations in the groundwater level was lower in wet years than in normal years but the highest in dry years. Based on the comparative method of the slope changing ratio of cumulative quantity and the elastic coefficient method, it was determined that the contribution rate of climate change to the variation in groundwater level was 22.08% and that of human activities was 77.92%. Thus, the sustainable development of the groundwater environment can be realized by positively guiding anthropogenic activities and minimizing negative influences.

Despite the ecological and socioeconomic importance of agro-pastoral ecotones, changes in land use and land cover (LULC) and their driving mechanisms are not comprehensively understood. In this study, a systematic framework for LULC assessment covering comprehensive timeframes was constructed for the Tabu watershed. Results demonstrated that a new process of LULC changes began in 1998, with a significant increase in farmland and decrease in grassland. The increase in dynamic degrees and structural variation coefficients indicated intensive and frequent changes in LULC. Conversion ratios between grassland and farmland exceeded 95%, and construction land encroached upon grassland. Grassland changes were driven mainly by natural factors based on the random forest regression, as well as changes in farmland and construction land. The influence of anthropogenic drivers on LULC became significant. Under the sustainable development scenario, the increase in grassland with a high fractional vegetation cover in 2034 was the most significant, the area of bare land decreased, the area of construction land steadily increased, and the reduction in farmland area was under control. Under this scenario, both socioeconomic development and ecosystem stability can be achieved. This study provides insights into regional land dynamics and provides systematic guidance for sustainable land management.