*2.1. Study Area*

The study area is located in the Shimachuan River basin of Fugu County in the eastern part of the Loess Plateau in North Shaanxi in China, belonging to the typical semi-arid continental monsoon climate (Figure 1). The terrain of the study area is generally high in the west and low in the east, with elevations ranging from 940 m to 1100 m. The geomorphology of the study area mainly consists of Loess mountains and river valley terraces. Loess mountains are mainly distributed on both sides of Shimachuan valley, with an elevation of 1000~1100 m and a width of 100~250 m, and incline to both sides of the valley at a slope angle of 10~20◦ . The Loess peak is narrow, along the watershed there are large ups and downs, and the ground is very broken, forming a unique Loess landform. The Loess Mountain area has uniform lithology, good water permeability, small catchment area and serious soil erosion. Valley terraces are mainly distributed along the Shimachuan river, with an elevation of 960~1100 m. The valley terraces are long, sporadic and intermittent. The area of the valley terraces is small. The front edge of the valley terraces is 2~3 m higher than the river bed, 15~40 m wide, and most of the terraces tilt to the river bed at a slope angle of 3~5◦ . The overall surface water network is poorly developed in the study area. The Shimachuan River in the study area is a first-class tributary of the west bank of the Yellow

River. In addition, groundwater is the main water supply source in the study area, stored at the bottom of the Quaternary formation and in bedrock fissures. Sr-rich groundwater in the study area is mainly stored in the weathered and fractured aquifer (unconfined) of the Yanchang Formation. Its recharge mainly occurs from atmospheric precipitation and the unconfined water of the loose layer of the Quaternary formation and surface water, which eventually discharges at the downstream outlet of the Shimachuan River. The stratigraphy of the Yanchang formation is covered by loose sediments of the Quaternary formation, with outcrops in the river valley; the thickness of strata is 200~411 m, the maximum exposed thickness is 65 m, and the thickness of the weathering layer is 20~43.26 m. In addition, the lithology of the Yanchang formation consists of a thickly laminated light gray-green medium to fine-grained feldspathic sandstone with medium sorting and roundness. It has large inclined beddings, wedge-shaped beddings and massive beddings. The study area is located in the northeast of the secondary tectonic units of Ordos Basin, with stable geological structures and oscillatory rise caused by the neotectonic movement. There is no trace of magmatic activity in the area. The overall structure is monoclinal with a dip direction of 292◦ and a dip angle of 1~4◦ . stored at the bottom of the Quaternary formation and in bedrock fissures. Sr-rich groundwater in the study area is mainly stored in the weathered and fractured aquifer (unconfined) of the Yanchang Formation. Its recharge mainly occurs from atmospheric precipitation and the unconfined water of the loose layer of the Quaternary formation and surface water, which eventually discharges at the downstream outlet of the Shimachuan River. The stratigraphy of the Yanchang formation is covered by loose sediments of the Quaternary formation, with outcrops in the river valley; the thickness of strata is 200~411 m, the maximum exposed thickness is 65 m, and the thickness of the weathering layer is 20~43.26 m. In addition, the lithology of the Yanchang formation consists of a thickly laminated light gray-green medium to fine-grained feldspathic sandstone with medium sorting and roundness. It has large inclined beddings, wedge-shaped beddings and massive beddings. The study area is located in the northeast of the secondary tectonic units of Ordos Basin, with stable geological structures and oscillatory rise caused by the neotectonic movement. There is no trace of magmatic activity in the area. The overall structure is monoclinal with a dip direction of 292° and a dip angle of 1~4°.

than the river bed, 15~40 m wide, and most of the terraces tilt to the river bed at a slope angle of 3~5°. The overall surface water network is poorly developed in the study area. The Shimachuan River in the study area is a first-class tributary of the west bank of the Yellow River. In addition, groundwater is the main water supply source in the study area,

*Water* **2022**, *14*, x FOR PEER REVIEW 3 of 16

**Figure 1.** Regional geological map of Tianjiazhai East Ditch, Fugu County. The geographical location of the study area belongs to the 19th projection band of the Gaussian projection six-degree band, and the coordinates used in the figure are the Gaussian projection six-degree band coordinate sys-**Figure 1.** Regional geological map of Tianjiazhai East Ditch, Fugu County. The geographical location of the study area belongs to the 19th projection band of the Gaussian projection six-degree band, and the coordinates used in the figure are the Gaussian projection six-degree band coordinate system.

### tem. *2.2. Sampling*

*2.2. Sampling*  In this study, 30 groundwater samples were first collected from hydrogeological boreholes in the study area during the March 2018 to September 2019 period, according to the Technical Requirements for Water Sample Collection and Delivery issued by the China Geological Survey. These were then analyzed for groundwater quality, the test indexes including pH, total dissolved solids (TDS), total hardness, cation (K+, Na+, Ca2+ and Mg2+), anion (Cl−, HCO3−, SO42− and NO3−) and strontium elements. The rock type of the In this study, 30 groundwater samples were first collected from hydrogeological boreholes in the study area during the March 2018 to September 2019 period, according to the Technical Requirements for Water Sample Collection and Delivery issued by the China Geological Survey. These were then analyzed for groundwater quality, the test indexes including pH, total dissolved solids (TDS), total hardness, cation (K<sup>+</sup> , Na<sup>+</sup> , Ca2+ and Mg2+), anion (Cl−, HCO<sup>3</sup> −, SO<sup>4</sup> <sup>2</sup><sup>−</sup> and NO<sup>3</sup> −) and strontium elements. The rock type of the sampling point was sandstone of Yanchang Formation. In addition, 25 and 24 rock samples were collected at different depths from drilling G019 and G020 in the study area,

sampling point was sandstone of Yanchang Formation. In addition, 25 and 24 rock

respectively, while 22 rock samples were collected from a selected profile between drilling G020 and the Temple River Gully. All the rock samples were analyzed for Sr content.
