3.2.2. Hydrodynamic Conditions

The Sr-rich groundwater in Tianjiazhai is mainly stored in the fractured unconfined aquifer of the Yanchang Formation. The natural geographical, geological, and hydrogeological conditions in the area determine the alternating characteristics of the overall circulation of Sr-rich groundwater. During the rainy season, the main sources of recharge for weathered fractured unconfined water in the Yanchang Formation are atmospheric precipitation, Quaternary loose layer unconfined water, and surface water, while during the dry period, the weathered fractured unconfined water of the Yanchang Formation is mainly recharged from atmospheric precipitation and through transient runoff. Indeed, discharge occurs from high to low and from West to East, recharging surface water and pore unconfined water in the loose layer of the Quaternary, thus locally recharging the Shimachuan area through runoff, from upstream to downstream. The weathered fissure groundwater in the Yanchang Formation is located in different geomorphological units, resulting in different recharge modes, runoff directions, and discharge conditions. The Yanchang Formation of weathered fissure unconfined water, which is distributed in the Loess Mountains, mainly receives infiltration recharge from the overlying Quaternary aquifer and then flows from the Loess to the surrounding lowlands and discharges to the surface in the lowlands of the topographically cut gullies to recharge the weathered fissure groundwater of the Yanchang Formation in the river valley area. The weathered fractured groundwater of the Yanchang Formation, distributed in the valley area, mainly receives the recharge from the pore water of the overlying loose layer and the lateral recharge of the same aquifer. Besides the low recharge rates from the lower confined water, the exposed section of the aquifer is recharged directly from the atmospheric precipitation when the local section of the valley area is in the favorable part of the geomorphology before flowing along the river valley downstream from high to low elevations and from upstream to downstream areas. The discharge of the aquifer occurs in the form of submerged flow discharge in the riverbed, followed by downstream discharge. In addition, discharge by evaporation and artificial mining is also the main discharge process. There is no aquifuge between aquifers of the Quaternary loose layer pore unconfined water and the weathered fissure unconfined water of the Yanchang Formation, indicating a relatively close hydraulic connection. Surface water bodies and the Quaternary-Holocene alluvial unconfined water have a complementary relationship, resulting in the close relationship between weathered fissure unconfined water in the Yanchang Formation and surface water. According to the results of the dynamic surface water observation and groundwater level data observed from observation wells in Shimachuan, the dynamic changes of groundwater are closely related to atmospheric precipitation with the characteristics of synchronous changes. All this provides hydrodynamic conditions for the Sr enrichment in groundwater.
