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.

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.

The Tabu River Basin (TRB) is one of the most ecologically fragile areas in the arid regions of northern China; it is a key component of the desert steppe north of the Yinshan Mountains. The fractional vegetation coverage (FVC) represents a vital indicator of ecological health in the TRB. In this study, we explored the impacts of climate change and human activities on vegetation growth and utilized Landsat data (30 m) from the Google Earth Engine to generate a long-term FVC dataset (1986–2023) in the TRB. Furthermore, we established a framework for quantitatively identifying the effects of climate change and anthropogenic activities on the FVC in desert steppe regions. The results revealed that: (1) the FVC exhibits considerable spatial heterogeneity, with higher values observed in the southeastern and southwestern areas and lower values in the northern part; (2) over the past 38 years, the annual average FVC has shown fluctuations, with a slight declining trend, while the Hurst exponent indicates a reverse persistence pattern in the FVC across the TRB; and (3) the correlation between the FVC and the temperature is marginally stronger than that with precipitation, and the influence of climate change on promoting the FVC outweighs the role of human activities. These results offer valuable insights for ecological restoration and sustainable development efforts and provide scientific support for monitoring vegetation in the region.

Groundwater is the main water supply source especially in arid/semiarid regions, whose quality has been widely concerned. To clarify the spatial distribution, occurrence and the impacts of fluoride (F⁻) and nitrate (NO₃⁻) in groundwater, 26 groundwater samples were collected from domestic water wells and agricultural irrigation wells in Xilinhot city, a famous tourist city on the Xilin Gol grassland of China. The integration of statistical analysis and hydrochemical diagrams and the model of health risk assessment recommended by the USEPA were carried out in the study. The analysis reveals that the groundwater is weakly alkaline, and the hydrochemical facies are mainly HCO₃‐Ca·Mg and SO₄·Cl‐ Na + K, which are controlled by natural processes and anthropogenic factors in the study area. Groundwater with high fluorine is chiefly located in the west of the study area, and high‐nitrate groundwater is chiefly located in the middle and southern parts of the study area. Fluoride in groundwater is predominantly derived by the weathering and dissolution of fluoride‐bearing minerals and ion exchange. The values of hazard index (HI) of 65.38% groundwater samples were greater than the recommended limit for noncarcinogenic risk (HI = 1) for children, followed by 38.46% for adult females and 30.77% for adult males. The water quality of the urban water supply should be treated. At the same time, contaminant filters and other instruments should be supplied to residents in pastoral area to reduce fluoride and nitrate concentration in drinking water, which will mitigate health risks and ensure the safety of drinking water in the study area.

The Yellow River irrigation area in Tumochuan Plain is one of the primary grain production areas in the middle and upper reaches of the Yellow River Basin (YRB). The groundwater in the area is bitter and salty, which significantly influences drinking water safety of residents. To investigate its chemical characteristics and material sources, we collected 12 groundwater samplings, 3 irrigation water samplings, and 1 precipitation sample during the winter irrigation period (WIP) in November 2016 and the spring irrigation period (SIP) in April 2017, respectively. We then analysed the hydrogen and oxygen stable isotopes and hydrochemical characteristics of the shallow groundwater in the study area and investigated their affecting factors by using environmental isotopes, Gibbs diagrams, Schoeller diagrams, and ion proportionality coefficient analysis. Next, we qualitatively analysed the material sources. The results show the followings: (1) The concentrations of major ions in groundwater in the SIP are generally higher than in the WIP, which may be recharged by snow melting water in the spring. (2) The average values of δD and δ¹⁸O for the groundwater are -78.0‰ and -10.3‰, respectively, in the WIP and -77.4‰ and -10.3‰, respectively, in the SIP. However, the characteristics of hydrogen and oxygen isotopes in the groundwater are almost identical in the WIP and SIP. (3) The formation of groundwater chemical constituent are controlled by evaporation concentration and lixiviation, and dissolution of evaporite and mirabilite is the most important factor. Synchronously, the formation of groundwater chemical constituent is also influenced ion exchange and human activities. The study provides effective guidance for groundwater resource development in arid and semi-arid regions.

The desert steppe on the northern foothills of the Yinshan Mountains serves as a critical ecological barrier and an agro-pastoral ecotone in northern China. Sustainable utilization of groundwater resources is essential for safeguarding regional ecological security and supporting socio-economic development. In the region, groundwater not only sustains fragile ecosystems but also constitutes the sole source of drinking water for local. However, the groundwater quality exhibits spatial heterogeneity due to the complex geological settings and anthropogenic activities. The study focused on the Tabu River Basin, a representative area of the desert steppe, where 107 groundwater samples were collected. By integrating conventional hydrochemical analysis, self-organizing map (SOM), explainable artificial intelligence (XAI) methods, and health risk assessment coupled with Monte Carlo simulation, we systematically characterized groundwater chemistry, evaluated its suitability for drinking purposes, and identified the dominant factors controlling water quality variations. The results showed that the self-organizing map classified three groundwater clusters, and hydrochemical facies were primarily identified as HCO₃⁻–Ca²⁺, Cl⁻–Na⁺, and mixed HCO₃⁻–Ca²⁺·Na⁺ types, primarily governed by cation exchange and human activities. The entropy-weighted water quality index (EWQI) showed that 23.3% of the samples were classified as excellent, 36.5% as moderate, while 15.9% and 24.3% fell into the poor and very poor categories, respectively. Further analysis employing the XGBoost model combined with SHAP (Shapley Additive Explanations) interpretability techniques identified nitrate (NO₃⁻) and total dissolved solids (TDS) as the key drivers of water quality deterioration. Health risk assessment results indicated that 98.9%, 92.0%, and 80.5% of groundwater samples exceeded the acceptable threshold for total non-carcinogenic health risks for children, adult females, and adult males, respectively. By synergistically combining traditional hydrochemical approaches with unsupervised machine learning (SOM) and interpretable machine learning (XGBoost+SHAP), the study establishes a multidimensional and highly interpretable analytical framework, which not only advances the understanding of groundwater evolution mechanisms in arid and semi-arid inland basins but also provides robust scientific support for the sustainable management and utilization of regional groundwater resources.