3.1.1. Descriptive Analysis of Hydrochemical Characteristics

As reported in Table 1, the Sr2+ content in Sr-rich groundwater in the study area ranged from 0.85 to 2.99 mg·L −1 , with a mean value of 1.45 mg·L −1 , which is 7.25 times higher than the national limit value of Sr2+ content in natural mineral water for drinking (0.20 mg·L −1 ). Compared with Sr2+ content in Sr-rich groundwater in other areas of China, the Sr2+ content in groundwater in the study area is close to that in Xintian County of Hunan Province (1.57 mg·L −1 ), and lower than that in Guanling area of Guizhou Province (2.43 mg·L −1 ). The Sr2+ content in groundwater in the study area is higher than that in Changchun, Jilin province (0.29 mg·L −1 ), Xianning, Hubei Province (0.63 mg·L −1 ), Yutian county, Xinjiang Province (1.1 mg·L −1 ) and Pingxiang city, Jiangxi Province (1.29 mg·L −1 ). Sr-rich groundwater pH range was 7.50~8.30, with an average value of 7.92, indicating a slightly alkaline Sr-rich groundwater. Part of the water samples in the study had a high PH value of 8.3, mainly because there was a little amount of CO<sup>3</sup> <sup>2</sup><sup>−</sup> in the groundwater. CO<sup>3</sup> 2− will combine with H<sup>+</sup> in water, leading to the decrease of hydrogen ion concentration in groundwater. The pH value of Sr-rich groundwater meets the requirements of the Chinese Standards (6.5–8.5) and the WHO Guidelines (6.5–8.5). TDS (total dissolved solids) ranged from 332 to 718 mg·L −1 , with a mean value of 492.57 mg·L −1 , showing low TDS levels in Sr-rich groundwater. The total hardness ranged from 205.00 to 825.00 mg·L −1 , with a mean value of 324.90 mg·L −1 , suggesting medium-hard and ultra-hard water. The TDS of Sr-rich groundwater meets the requirements of the Chinese Standards (1000 mg·L −1 ) and the WHO Guidelines (1000 mg·L −1 ). The dominant cations in Sr-rich groundwater were Ca2+, Na<sup>+</sup> , and Mg2+, with average contents of 71.09, 60.57, and 31.77 mg·<sup>L</sup> −1 , accounting for 42.73, 36.41, and 19.10% of total cation contents, respectively, whereas K<sup>+</sup> showed the lowest content in Sr-rich groundwater, with an average value of 2.92 mg·L −1 , accounting for only 1.76% of total cation contents. The Na<sup>+</sup> content of Sr-rich groundwater is lower than the maximum Na<sup>+</sup> content required by the Chinese Standards (200 mg·L −1 ) and the WHO Guidelines (200 mg·L −1 ). On the other hand, HCO3<sup>−</sup> was the dominant anion in

Sr-rich groundwater, with mean values of 348.80 mg·L −1 , accounting for 73.52% of total anion contents, whereas NO<sup>3</sup> − showed the lowest content in Sr-rich groundwater, with an average value of 7.76 mg·L −1 , accounting for only 1.63% of total anion contents. The Cl− content of Sr-rich groundwater is lower than the maximum Cl− content required by the Chinese Standards (250 mg·L −1 ) and the WHO Guidelines (250 mg·L −1 ). The SO<sup>4</sup> 2− content of Sr-rich groundwater is lower than the maximum SO<sup>4</sup> <sup>2</sup><sup>−</sup> content required by the Chinese Standards (250 mg·L −1 ) and the WHO Guidelines (500 mg·L −1 ). The NO<sup>3</sup> − content of Sr-rich groundwater is lower than the maximum NO<sup>3</sup> − content required by the WHO Guidelines (50 mg·L −1 ) and higher than the maximum NO<sup>3</sup> − content required by the Chinese Standards (20 mg·L −1 ). This is due to the agricultural activities of local residents in the study area, resulting in the increase of NO<sup>3</sup> − content. The coefficients of variation of the hydrochemical parameters of Sr-rich groundwater were all less than 1, indicating relatively stable contents of these hydrochemical components in Sr-rich groundwater. This finding may be due primarily to the fact that Sr-rich groundwater in the study area is strongly controlled by geological conditions and less influenced by human activities.

**Table 1.** Statistics on the hydrochemical characteristics of Sr-rich groundwater.


\* TH is the total hardness, <sup>1</sup> CS is the Chinese Standards, <sup>2</sup> WG is the WHO Guidelines.
