## 2 Project Overview and Geological Context  
Located in the lower reaches of the Jinsha River, Yunnan Province in Southwest China, the Xiluodu hydropower project is China's second-largest and the world's third-largest hydropower project, with the total capacity up to \(1.267\times 10^{10}\) m\({}^{3}\). It is a double curvature arch dam that is 285.5-m high with a 200-m radius. The absolute altitude of the dam's foundation is 324.5 m. By contrast, the designed normal pool level of the reservoir is 600 m and the corresponding valley width is approximately 500 m.  
Thirty-one monolithts are planted across the river. The transversal section of the dam is curve-shaped, 14 m wide at the crest, and 60 m wide at the footing. As a typical project for China's 300 m-high arch dam, it began casting concrete in March 2009. The reservoir began its initial impoundment in March 2012 when the dam was under construction [27]. In September 2014, the reservoir reaches the designed storage water level.  
The reservoir area of the dam project is located in the center of a synclinal basin surrounded by limestone outcropped areas as shown in Figure 1. The natural water level of the river varies from 370 m to 390 m above sea level. Geological exploration shows that the river valley is the main outlet of the subsurface runoff.  
As shown in Figure 2, the reservoir banks are steep and the valley section is nearly U-shaped. Both sides of the valley slope and the river bedrock at the dam site are mainly composed of Permian basalt strata, which are hard, intact, and suitable for arch dam construction. On top of the basalt strata is Permian sandy shale. Approximately 500 m thick Permian limestone stratum lies under the Permian basalt strata with a shale sediment layer between them.  
Geological exploration before the construction of the dam showed signs of hot springs and a relatively high geothermal field at the dam site. Temperature measurements in the boreholes revealed that the ground temperature in limestone could be up to 49.6 \({}^{\circ}\)C as shown in Figure 3. The average ground temperature in basalt was around 35 \({}^{\circ}\)C while the water temperature in the Jinsha River varied from 8 \({}^{\circ}\)C to 22 \({}^{\circ}\)C.  
Researchers have tried to explain the high geothermal phenomenon in the aspect of rock permeability [25], groundwater circulation [24], and chemical water-rock interaction [26].  
Figure 1: _Cont._  
Figure 1: Overview of the Xiloudu Project: **(a)** regional location of Xiluoudu Project; **(b)** outline of the limestone outcrops.  
Figure 2: _Cont._  
According to the previous studies, the river valley is the main outlet of three different strands of groundwater. The sand shale (P\({}_{2\text{x}}\)) has a low vertical permeability, which means the surface water can only run down along the hillside without infiltrating into the basalt. Thus, the initial groundwater level in basalt is lower than 400 m and mainly controlled by the variation of the river level. The shale sediment layer (P\({}_{2}\beta_{n}\)) is a fine aquifers, causing hydraulic head discontinuity between basalt and limestone. The supply region of groundwater in underlying limestone is its stratum outcrops around the basin where the altitude is above 2000 m. The subsurface runoff is heated up by geothermal flux, which causes geothermal springs and relatively high ground temperature at the dam site. The schematic diagram of groundwater circulation is shown in Figure 4.  
Figure 3: Measurement during pre-project exploration, the high geothermal phenomenon is revealed near the dam site.  
Figure 2: Profiles of the Xliuudu dam site: (**a**) transverse section/2a section in Figure 1; (**b**) river-along section/2b section in Figure 1.