## 2 Studied Site
### Hydrogeological Conditions
(2) Types of aquifers  
The groundwater types in the study area are mainly pore water in loose accumulation layer, fractured water, and fractured karst groundwater. Pore water mainly distributes in shallow surfaces and gullies. The permeability of fractured and karst groundwater is different due to the lithological difference of each aquifer. Karst is widely distributed in carbonate area. Waterfalls and sinkholes can be seen everywhere. Therefore, there are many springs in reservoir areas, and groundwater resources are abundant. However, in mudstone and shale areas, springs are less exposed and karst is not developed. The permeability of each formation is shown in Table 3.  
The groundwater level in the dam site is greatly affected by rainfall. If the rainfall is large, the groundwater level is high, and the level ranges from 3 m to 20 m. According to the relationship between the groundwater level of boreholes and the river level, the groundwater hydraulic gradient is calculated. On the left bank, hydraulic gradient is 0.33 for borehole ZK-10, 0.16 for borehole ZK-23, and 0.18 for borehole ZK-28. On the right bank, hydraulic gradient is 0.47 for borehole ZK-16, 0.33 for borehole ZK-22, and 0.27 for borehole ZK-27. So, the gradient is smaller on the valley slopes, but larger on the bank slopes. In the riverbed, the levels are influenced by deep circulation conditions of groundwater, and confined water appears in many places. For example, groundwater tables are 982.75 m for ZK-18, 954.96 m for ZK-19, 956.15 m for ZK-20, and 982.45 m for ZK-24. They are 2.96-30.75 m above the river level. Therefore, the groundwater levels on both banks are higher than that of the river, and groundwater recharges to the river water. Field investigation shows that most springs have larger discharge in rainy season and smaller discharge in dry season. It shows that groundwater is mainly recharged by rainfall. According to the groundwater levels on the both banks, there exists groundwater watershed in the dam site. The outcrop of all springs is higher than the river level. Therefore, the groundwater is discharged to the river in the study area under the natural conditions (Figure 3). The karst development is controlled by lithology and structure in the study area. In all pure dolomite and limestone, karst development is relatively strong. For example, sinkholes \(K_{17}\), \(K_{18}\), \(K_{19}\), and Longdang karst pipes on the left bank are mainly developed in thick massive limestone T\({}_{2}\)g\({}^{3-6}\). However, the karst development is weak in the heavier \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Strategraphic** & **Strategraphic** & **Strategraphic** & **Lithology** & **Permeability of Rock Mass** \\
**Age** & **Symbol** & **Thickness (m)** & & \\ \hline Paleogene & E\({}_{2}\)1 & \(>\)220 & Conglomerate and sandstone & Moderate \\ \hline \multirow{7}{*}{Triassic} & T\({}_{2}\)g\({}^{4}_{3}\) & \(>\)150 & Dolomitic limestone & Strong \\  & T\({}_{2}\)g\({}^{3}\) & 570 & Marl and argillaceous dolomite & Moderate \\  & T\({}_{2}\)g\({}^{2}\) & 120 & Mudstone and dolomite interbedding & Relative impermeability \\  & T\({}_{2}\)g\({}^{1-2}\) & 80 & Argillaceous dolomite & Weak karst permeable layer \\  & T\({}_{2}\)g\({}^{1-1}\) & 240 & Limestone and dolomitic limestone & Strong karst permeable layer \\  & T\({}_{1}\)yr\({}^{2}\) & 50 & Mudstone with limestone & Relative impermeability \\  & T\({}_{1}\)yr\({}^{1}\) & 284 & Muddy limestone & Strong \\  & T\({}_{1}\)f & 625 & Marlite & Impermeable layer \\ \hline \multirow{3}{*}{Permian} & P\({}_{2}\)c & 2.5–20 & Limestone & Strong \\  & P\({}_{2}\)\({}^{12}\) & 234 & Sandstone with clay & Impermeable layer \\ \cline{1-1}  & P\({}_{2}\)\({}^{11}\) & 38 & Bioclastic limestone & Strong \\ \cline{1-1}  & P\({}_{1}\)m & 318–544 & Bioclastic limestone & Strong \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Permeability statistics of different strata. argillaceous layers of T\({}_{2}\)B\({}^{3-3}\) and T\({}_{2}\)g\({}^{3-2}\).