*2.1. Study Area*

Linfen Basin (35◦230–36◦570 N, 110◦220–112◦340 E) is situated in the southwest of Shanxi Province and includes Huozhou City, Hongtong County, Yaodu District, Quwo County, Xiangfen County, Yicheng County, and Houma City (Figure 1). It covers an

area of ~4686 km<sup>2</sup> . It is surrounded by the Hanhou Mountains to the north, the Emei platform to the south, the Taiyue and Zhongtiao Mountains to the east, and the Luoyun Mountains to the west. The area has been subjected to semi-arid and semi-humid monsoon climatic conditions, with mean annual precipitation of 420 to 550 mm, and mean annual temperatures of 10 ◦C [51]. The study area is not only an important irrigated agricultural area in the Loess Plateau but also the main supply center of energy sources in China. The area is rich in mineral resources, of which coal is the largest mineral resource. The main rivers in the study area are the Fenhe River, Xinshuihe River, Qinhe River, Huihe River, Ehe River, and Qingshuihe River. The total amount of regional water resources is 1.52 billion m<sup>3</sup> , of which the river runoff is 1.32 billion m<sup>3</sup> (including 0.48 billion m<sup>3</sup> of spring water), and the groundwater resource is 1.026 billion m<sup>3</sup> . The water resource in this area is scarce, with the per capita water resource occupancy being only 350 m<sup>3</sup> [52]. to the south, the Taiyue and Zhongtiao Mountains to the east, and the Luoyun Mountains to the west. The area has been subjected to semi-arid and semi-humid monsoon climatic conditions, with mean annual precipitation of 420 to 550 mm, and mean annual temperatures of 10 °C [51]. The study area is not only an important irrigated agricultural area in the Loess Plateau but also the main supply center of energy sources in China. The area is rich in mineral resources, of which coal is the largest mineral resource. The main rivers in the study area are the Fenhe River, Xinshuihe River, Qinhe River, Huihe River, Ehe River, and Qingshuihe River. The total amount of regional water resources is 1.52 billion m3, of which the river runoff is 1.32 billion m3 (including 0.48 billion m3 of spring water), and the groundwater resource is 1.026 billion m3. The water resource in this area is scarce, with the per capita water resource occupancy being only 350 m3 [52].

Linfen Basin (35°23′–36°57′ N, 110°22′–112°34′ E) is situated in the southwest of Shanxi Province and includes Huozhou City, Hongtong County, Yaodu District, Quwo County, Xiangfen County, Yicheng County, and Houma City (Figure 1). It covers an area of ~4686 km2. It is surrounded by the Hanhou Mountains to the north, the Emei platform

*Water* **2022**, *14*, x FOR PEER REVIEW 3 of 19

**2. Materials and Methods** 

*2.1. Study Area* 

**Figure 1.** (**a**) Spatial distribution of groundwater sampling sites and groundwater depths and (**b**) Regional hydrogeological map of the study area. **Figure 1.** (**a**) Spatial distribution of groundwater sampling sites and groundwater depths and (**b**) Regional hydrogeological map of the study area.

The interior of the basin is dominated by Quaternary strata. The Lower Pleistocene is mainly yellowish-brown and grayish-yellow silty sand and sandy clay. This layer is widespread in the basin and is about 200 m thick in the middle of the basin. The Middle Pleistocene is a set of sand, sandy soil, and loam interbedded sediment, which has a thickness of ~150 m. The Upper Pleistocene in the piedmont inclined plain area is sand gravel mixed with sandy soil. Near the river valley, it is mostly sandy soil and loam deposited in river and lake facies, and the thickness of this layer is 30–50 m. The stratum lithology of Holocene is sandy soil, loam, sand, and gravel, which is mainly distributed in the Fenhe terrace. The exposed strata in the mountain area include gneiss, limestone, shale, sandstone, mudstone, sandy conglomerate, and loess [53]. The fault structure in the study area is complex, mostly being hidden faults, and the intersection of large faults is a favorable part of modern hot springs and mineralization [54].

According to the burial depth and hydraulic characteristics of the aquifers in the study area, the pore water of loose rocks in the study area can be divided into phreatic water, middle-layer confined water, and deep-layer confined water. Phreatic aquifers are mostly distributed in the middle of the basin, loess tableland, and piedmont inclined plain in a belt shape, and the aquifers are mainly medium and fine sand. Compared with the eastern piedmont and the central part of the basin, the middle-layer confined water aquifer in the western piedmont has a large thickness and coarse particles and has good water storage conditions. The distribution characteristics of deep-layer confined water are consistent with that of middle-layer water, and the aquifer is mainly sand and sand gravel. Groundwater recharge mainly includes lateral runoff, surface water seepage, and precipitation infiltration. The discharge of groundwater mainly depends on evaporation and artificial mining [54,55]. In the slow flow season, groundwater mainly belongs to the (SO<sup>4</sup> <sup>2</sup>−-Ca2+) type, the (HCO<sup>3</sup> <sup>−</sup>-Ca2+-Na<sup>+</sup> ) type, and the (HCO<sup>3</sup> <sup>−</sup>-Mg2+-Na<sup>+</sup> ) type. In the quick flow season, groundwater is dominated by the (HCO<sup>3</sup> <sup>−</sup>-Ca2+-Na<sup>+</sup> ) type and the (HCO<sup>3</sup> <sup>−</sup>-SO<sup>4</sup> <sup>2</sup>−-Ca2+-Na<sup>+</sup> ) type [4]. In general, the groundwater depth in the study area shows a trend of high in the east and low in the west. The buried depth of groundwater in Yaodu and Xiangfen is generally deeper than that in other areas at 35–45 m (Figure 1).
