**1. Introduction**

Water quantity affects the extent of water exploitation, while water quality determines the value of water use [1]. In the natural environment, the specific hydrochemical characteristics of groundwater are formed over time in response to the comprehensive influence of climate, topography, aquifer lithology and other factors. The groundwater in some areas is low salinity freshwater and is rich in trace elements that are good for human health (such as Sr, Li and H2SiO3). This kind of water has very high value for use. However, in some areas, the groundwater is naturally inferior, characterized by high levels of salinity, fluorine and arsenic [2,3], which may aggravate water shortages due to poor water quality, especially for the endorheic basins that water resources are rare. Due to the intensification of human activities, the hydraulic head field and hydrochemical evolution process have been disturbed to a certain extent [4,5]. Variation in the vadose zone thickness changes the oxidation–reduction environment of the dissolved minerals during

**Citation:** Jin, J.; Wang, Z.; Zhao, Y.; Ding, H.; Zhang, J. Delineation of Hydrochemical Characteristics and Tracing Nitrate Contamination of Groundwater Based on Hydrochemical Methods and Isotope Techniques in the Northern Huangqihai Basin, China. *Water* **2022**, *14*, 3168. https://doi.org/10.3390/ w14193168

Academic Editor: Andrea G. Capodaglio

Received: 1 September 2022 Accepted: 2 October 2022 Published: 8 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the leaching process. Coupled with the input of artificial contaminants, the hydrochemical characteristics may change. Some activities may improve groundwater quality, while others cause groundwater pollution, such as nitrate contamination [6–9]. A high content of nitrate in water leads to eutrophication of water bodies and degradation of ecosystems. Drinking groundwater with a high nitrate content for a long time can cause serious diseases, such as methemoglobinemia, blue babies and gastric cancer [9]. At present, nitrogen contamination in water has become an international problem and needs to be solved in order to ensure the safety of drinking water and sustain the ecological health.

Approximately 20% of the Earth's land is covered by endorheic basins, but the basins account for only 2.3% of the total worldwide annual river runoff, and the hydrochemical research on them has not received enough attention [10]. Dowling et al. [11] studied the arsenic releasing mechanisms in the Bengal Basin based on the statistical methods and correlation analysis. Kawawa et al. [12] used the hydrochemical methods and isotopic techniques to study the mechanism of salinity changes and hydrochemical evolution of groundwater in the Machile–Zambezi Basin, and concluded that high groundwater salinity was associated with pre–Holocene environmental changes and was restricted to a stagnant saline zone. Nipada et al. [13] took the Western Lampang Basin as the study area and researched the arsenic contamination in groundwater based on the PHREEQC software. Endorheic basins are widely distributed in China and nitrate contamination occurs. Many studies have been performed by domestic scholars [5,14,15], but these studies were mainly focused on special indices, such as arsenic and fluorine [16,17]. As regards nitrate concentration, some scholars [6,8,18] pointed out that the nitrate concentrations in their studied basin increased due to the human activities. Mukherjee et al. [19] indicated that the ingestion of untreated nitrate contaminated groundwater in the lower Ganga Basin caused a risk of methemoglobinemia. Avilés et al. [20] concluded that the nitrate content in the Titicaca Basin was influenced by manure piles, synthetic N fertilizers, and sewage collector pipes based on the δ <sup>15</sup>N(NO3)–δ <sup>18</sup>O(NO3) isotopic technique. One major limitation of these studies is that the hydrochemical characteristics and identification of nitrate sources were separately researched in general. In fact, the comprehensive analysis of hydrochemical characteristics is conducive to revealing the variability of nitrate and identifying its source, and the two parts should be combined and comprehensively researched.

The northern Huangqihai Basin, located in the northern China, is a typical endorheic basin and plays an important role in the Beijing–Tianjin–Hebei region. Huangqihai Lake is one of the eight well–known lakes in Inner Mongolia, but its area shrank in the past two decades [21]. The groundwater level obviously declines [22]. Nitrate and other indices in the groundwater in some areas exceed the standard and are not suitable for drinking. Excessive exploitation and groundwater quality deterioration aggravate the contradiction between the supply and demand of groundwater resources. What was worse, the ecosystem reliant on groundwater resources has become increasingly fragile. Previous research on the northern Huangqihai Basin mainly focused on ecology [23–25], and pointed out that the wetland degeneration and the ecological deterioration were mainly controlled by a series of human activities, such as the unreasonable exploitation of groundwater, river closure and the increase of the building land. Regarding water quality, a few scholars [26,27] evaluated the trophic level of the surface water. However, few studies on the hydrochemistry and nitrate source of groundwater in the Huangqihai Basin have been reported. The northern Huangqihai Basin is an endorheic basin typical in arid and semiarid regions, and the hydrochemical research on it is expected to well develop the research system of endorheic basin. The study objective was the Quaternary phreatic water which often constitutes the most important source of drinking water in semiarid and arid regions but easily influenced by external factors, human health is closely associated with its hydrochemical evolution. In addition, the Huangqihai Basin plays an ecological significant role in the Beijing–Tianjin– Hebei region. Therefore, research on the hydrochemistry and tracing nitrate contamination in the northern Huangqihai Basin is not only significant for developing the theoretical

research on endorheic basin, but also has great practical meanings of the sustainable utilization of regional groundwater and ecological protection.

Currently, there are few reports that have systematically and comprehensively analyzed the hydrochemical characteristics and nitrate contamination of groundwater, especially in endorheic basins. This paper intends to comprehensively analyze the hydrochemical characteristics and seeks to highlight the effectiveness of the combined use of hydrochemical and isotopic data for tracing nitrate source. Therefore, the main objectives of this study are: (1) to analyze the hydrochemical characteristics of groundwater and the spatial distributions of the main indices; (2) to reveal the hydrochemical evolution of groundwater; (3) to identify the source of groundwater and the way through which artificial contamination may enter groundwater; and (4) to integrate the dual isotope technique and hydrochemical analyses to identify nitrate contamination. It is expected that this research can enrich hydrogeochemical research on endorheic basins in arid and semiarid regions and provide an effective way to identify the nitrate source.
