**1. Introduction**

As an important source of water supply for human beings and an important component of the hydrological cycle, groundwater is of great importance for the sustainable development of the socio-economy and the protection of ecological stability [1,2]. Groundwater hydrochemical characteristics are controlled by natural and anthropogenic factors and are the result of long-term interactions between groundwater and the environment. Therefore, hydrochemical analysis of groundwater can reveal the hydrochemical formation mechanisms [3,4]. In natural conditions, the hydrochemical characteristics of groundwater are mainly controlled by dissolution, concentration, desulfation/carbonation, cation exchange, and mixing processes [5,6]. However, various changes in the hydrochemical characteristics of groundwater may occur under the influence of anthropogenic activities [7,8]. Under natural conditions, elements in rock layers are enriched into groundwater by hydrogeochemical processes, which can lead to mineralization and contamination of groundwater [9]. For example, under natural conditions, large amounts of dissolution of soluble rocks (carbonate rocks) can lead to mineralization of groundwater. In addition, trace elements (arsenic and fluorine) may be enriched to levels threatening human health under natural hydrogeochemistry [10,11]. A series of groundwater pollution problems have emerged under the influence of human activities, such as nitrate pollution of groundwater.

**Citation:** Liang, C.; Wang, W.; Ke, X.; Ou, A.; Wang, D. Hydrochemical Characteristics and Formation Mechanism of Strontium-Rich Groundwater in Tianjiazhai, Fugu, China. *Water* **2022**, *14*, 1874. https://doi.org/10.3390/w14121874

Academic Editors: Thomas Meixner and Elias Dimitriou

Received: 4 May 2022 Accepted: 8 June 2022 Published: 10 June 2022

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Multivariate statistical analysis and hydrogeochemistry are often used to investigate the hydrochemical characteristics of groundwater and their determining factors [12]. Recent studies on groundwater hydrochemistry have used a combination of several methods (mathematical statistics, water chemistry, hydrogeochemical simulation, etc.) to assess the hydrochemical characteristics of groundwater [13–15].

Strontium (Sr) is one of the essential trace elements for human health, which maintains human physiological functions [16,17]. Strontium in groundwater is derived from Sr in the surrounding rock and enters the groundwater through a leaching process via infiltration of atmospheric precipitation [18,19]. Suitable hydrogeological conditions are the basic conditions for the formation of strontium-rich groundwater. In fact, the extent of Srenrichment in the groundwater depends on the hydrogeochemical processes occurring in aquifers [20]. The amount of Sr in the surrounding rock entering the groundwater system is positively correlated with the degree of rock weathering. In addition, CO<sup>2</sup> content, temperature, and redox conditions are the important factors affecting the Sr enrichment in groundwater [21]. The Sr abundance in the surrounding rocks determines the Sr content in groundwater. Long-distance runoff and long-path circulation of groundwater increase the water–rock reaction time, thus promoting strontium enrichment [22]. The formation of Sr-rich groundwater is controlled by tectonic magmatic activity, Sr content in surrounding rocks, and hydrogeochemical conditions [23]. In recent years, numerous researchers have devoted considerable attention to the formation mechanism and enrichment factors of Sr in groundwater [24–26].

Fugu County is located in the Loess Plateau area in Northwest China, where surface water resources are limited. Indeed, groundwater has long been the source of local water supply in the area. However, in recent years, with the exploitation of coal resources and the development of the industrial economy, groundwater resource in Fugu County has become overexploited and inefficient. In the Shimachuan River basin in the Tianjiazhai area of Fugu County, Sr-rich groundwater exists in the underlying carbonate strata. The population of Shimachuan River Basin is 10,647 people, and the main water source for the production and living of local residents is groundwater. However, there are few studies on the hydrochemical characteristics and formation mechanism of Sr-rich groundwater in this area. Therefore, in this study, the hydrochemical characteristics of Sr-rich groundwater and its formation mechanism were assessed using descriptive analyses, Piper trilinear diagrams, Gibbs plots, ionic ratios, and correlation analysis. In addition, the formation conditions of Sr in groundwater were analyzed by combining geological and hydrogeological conditions as well as hydrogeochemical theory to reveal the formation mechanism of Sr in groundwater.
