*2.3. Methods*

The pH value was determined by water quality tester (SD150), the total dissolved solids (TDS) was determined by gravimetric method (GR), the total hardness was determined by ethylene diamine tetraacetic acid disodium titration, and the contents of cations K<sup>+</sup> , Na<sup>+</sup> , Ca2+ and Mg2+ were determined by atomic absorption spectrometer (ICP-AES) [27–29]. The contents of anions Cl−, SO<sup>4</sup> <sup>2</sup><sup>−</sup> and NO<sup>3</sup> − were determined by ion chromatograph (IC-90), and HCO3<sup>−</sup> was determined by hydrochloric acid titration [30,31]. Sr2+ was determined by plasma mass spectrometry (ICP-MS). The content of strontium (Sr) in rock was determined by EDTA volumetric method [32,33]. The free carbon dioxide in groundwater was determined by the volumetric method of standard alkali solution [34].

In this study, the descriptive statistics of physicochemical parameters of groundwater were performed using SPSS 22.0 software. Mineral saturation indexes (SI) were calculated by hydrochemical simulation software Phreeqc, and the database was the phreeqc.dat thermodynamic database [35]. Pearson correlation analysis was performed to evaluate the relationship between physicochemical parameters of groundwater. In addition, groundwater chemical types were classified using the Shukarev classification method and Piper trilinear diagram, while the main controlling factors of groundwater chemistry in the study area were determined using the Gibbs diagram. The main ion sources of Sr-rich groundwater were studied using ionic ratio diagrams. The formation mechanism of Sr in groundwater was investigated based on the geological and hydrogeological conditions as well as hydrogeochemical theory.
