3.2.3. Hydrochemical Conditions

The Sr-rich groundwater in Tianjiazhai is mainly stored in the weathered fractured unconfined aquifer of the Yanchang Formation. Indeed, the rocks in the study area have a large weathering thickness and fracture development, mostly in the form of lines and veins and locally in the form of a network. Atmospheric precipitation recharges groundwater through fissure infiltration, resulting in CO<sup>2</sup> seepage into the ground with precipitation during the infiltration process (the free carbon dioxide in groundwater is determined by the volumetric method of standard base solution). Under certain temperature and pressure conditions, Sr-bearing minerals (rocks) undergo hydrolysis and dissolution filtration to form Sr-rich groundwater. The thick layer of weathered rocks developed in the Tianjiazhai area, endowed with rich groundwater resources, provides good water conductivity channels and reaction space for groundwater circulation and water–rock interaction. ductivity channels and reaction space for groundwater circulation and water–rock interaction. The Sr content in the upper and lower parts of Borehole G019 (Figure 8a) ranged from

the Tianjiazhai area, endowed with rich groundwater resources, provides good water con-

*Water* **2022**, *14*, x FOR PEER REVIEW 12 of 16

The Sr content in the upper and lower parts of Borehole G019 (Figure 8a) ranged from 0.14 to 0.20‰ and from 0.33 to 0.39‰, respectively. The Sr content in the upper and lower parts of borehole G020 (Figure 8b) ranged from 0.11 to 0.40‰ and from 0.51 to 0.62‰, respectively. The Sr contents of rock samples at different depths in the hydrogeological borehole were generally lower in the upper strata than in the lower strata. This result may be due to the higher weathering degree of the rocks in the upper strata than in the lower strata, resulting in high Sr element concentration, with high content, entering the groundwater. As can be seen from Table 3, Sr was prevalently present in the strata of the study area, ranging from 0.0741 to 0.147‰. By comparing the Sr content in the borehole and profile, lower Sr contents were observed in the profile than those in the borehole, which may be due to the longer leaching time in the area where the profile is located, allowing Sr to easily dissociate from rocks and enter groundwater. Therefore, the Sr enrichment in groundwater in Tianjiazhai is influenced by the degree of weathering and water–rock reaction time. The higher the degree of weathering and the longer the water–rock reaction time, the more easily Sr is leached from the rock into the groundwater system. 0.14 to 0.20‰ and from 0.33 to 0.39‰, respectively. The Sr content in the upper and lower parts of borehole G020 (Figure 8b) ranged from 0.11 to 0.40‰ and from 0.51 to 0.62‰, respectively. The Sr contents of rock samples at different depths in the hydrogeological borehole were generally lower in the upper strata than in the lower strata. This result may be due to the higher weathering degree of the rocks in the upper strata than in the lower strata, resulting in high Sr element concentration, with high content, entering the groundwater. As can be seen from Table 3, Sr was prevalently present in the strata of the study area, ranging from 0.0741 to 0.147‰. By comparing the Sr content in the borehole and profile, lower Sr contents were observed in the profile than those in the borehole, which may be due to the longer leaching time in the area where the profile is located, allowing Sr to easily dissociate from rocks and enter groundwater. Therefore, the Sr enrichment in groundwater in Tianjiazhai is influenced by the degree of weathering and water–rock reaction time. The higher the degree of weathering and the longer the water–rock reaction time, the more easily Sr is leached from the rock into the groundwater system.

**Figure 8.** Sr content of strata in hydrogeological boreholes G019 (**a**) and G020 (**b**). **Figure 8.** Sr content of strata in hydrogeological boreholes G019 (**a**) and G020 (**b**).

**Table 3.** Sr content in rock sample profiles. **Table 3.** Sr content in rock sample profiles.


pm-11 Fine-grained sandstone 0.116 pm-22 Loess 0.266 Strontium enrichment in groundwater is related to groundwater circulation and transport characteristics. In groundwater flow systems, the farther the runoff distance, the longer the circulation path, the longer the retention time of groundwater in the aquifer, and the more favorable the dissolution and enrichment of strontium in groundwater [49– 51]. The Sr-rich groundwater in Tianjiazhai is mainly stored in the weathered fractured aquifer of the Yanchang Formation, with a single water-bearing lithology and good Strontium enrichment in groundwater is related to groundwater circulation and transport characteristics. In groundwater flow systems, the farther the runoff distance, the longer the circulation path, the longer the retention time of groundwater in the aquifer, and the more favorable the dissolution and enrichment of strontium in groundwater [49–51]. The Sr-rich groundwater in Tianjiazhai is mainly stored in the weathered fractured aquifer of the Yanchang Formation, with a single water-bearing lithology and good groundwater circulation conditions. The Sr contents in the altered bedrock fracture water are relatively stable. In addition, the clastic rock layer is less permeable and provides a semi-closed/semi-

groundwater circulation conditions. The Sr contents in the altered bedrock fracture water

open groundwater environment, with slow groundwater runoff, long retention time, and sufficient water–rock interaction, resulting in dissolution and enrichment of Sr in the groundwater. Long-term contact between groundwater and surrounding Sr-bearing minerals (rocks) is the fundamental condition for the formation of Sr-rich groundwater. The longer the contact time between groundwater and Sr minerals (rocks), the higher the Sr content in the groundwater.
