## 4 Lithological combination  
In aquifers, different lithologies have varying water-bearing capacities, and this is also true after weathering. Even among rocks of the same type, such as sandstones, different grain sizes can impact water-bearing properties differently. Generally, coarse-grained, medium-grained, and fine-grained sandstones have well-developed porosity and fractures, making them major water storage spaces in aquifers. In contrast, siltstones, sandy mudstones, and mudstones have tighter structures with less developed fractures, resulting in weaker permeability. For boreholes SK2 and J7 with the same weathering intensity, the water inflow per unit of SK2 is 0.1948 L/(s-m), while for J7, it is 0.0464 L/(s-m). The difference is because the lithology of SK2 is primarily medium-grained sandstone, while the lithology of J7 is dominated by fine-grained sandstone, with siltstone as a secondary component. Similarly, for boreholes K7 and BK49, which have the same elevation of the weathered bedrock surface and similar weathering degrees and core recovery rates, the water inflow per unit of K7 is 0.1380 L/(s-m), while for BK49, it is 0.0149 L/(s-m). The difference is attributed to the fact that the lithology of K7 is mainly medium-grained sandstone with coarse sandstone as a secondary component, whereas the lithology of BK49 consists mainly of medium-grained sandstone and siltstone, with sandy mudstone as a secondary component. Accordingly, different types of weathered bedrock lithologies in the study area are assigned values, and a quantitative index of the lithological combination \(r\) is constructed (Table 3):  
\[r=\frac{\sum r_{i}\cdot n_{i}}{\sum n_{i}} \tag{8}\]  
In this equation, \(r\) is an indicator of the lithological combination of the overall weathered bedrock; \(r_{i}\) represents each rock type in the weathered bedrock layer; and \(n_{i}\) describes the rock thickness corresponding to each rock type;  
5. Elevation of the weathered bedrock surface  
The elevation of the weathered bedrock surface, which represents paleotopographic factors, has a significant impact on the water-bearing properties of the weathered bedrock. Overall, the water-bearing properties of the low-lying areas located on the top of the weathered bedrock are greater than that of the uplifted areas; under other similar controlling factors, the water inflow per unit is generally greater in the lower regions of the weathered bedrock top than in the higher areas of the weathered bedrock top surface. For boreholes BK42, K3-1, and BK28, the water inflows per unit are 0.1051, 0.1220, and 0.0050 L/(s-m), respectively. The corresponding elevations of the weathered bedrock surface are 1180.40, 1218.32, and 1242.62 m. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Degree of Weathering** & **Strong** & **Moderate** & **Weak** \\ \hline Value & 3 & 2 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Weathering degree value assignment table. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Lithology** & **Mudstones** & **Sandy** & **Siltstone** & **Fine-** & **Medium-** & **Coarse-** & **Coarse-** & **Coarse-** \\  & **Mudstone** & **Sallstone** & **Sandstone** & **Grained** & **Grained** & **Grained** & **Conglomerate** \\ \hline value & 1 & 1.5 & 2 & 3 & 4 & 5 & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Lithological grade value assignment table. The sand-to-base ratio is the ratio between the weathered sandstone thickness and the total weathered bedrock thickness. Sandy mudstone and mudstone are prone to mudification phenomena in the process of weathering, and the mudstone and sandstone after mudification are relatively watertight compared with sandstone, which can weaken the hydraulic connection of the surrounding rock layers to a certain extent. Therefore, the sand-to-base ratio is a characterization index of the strength of vertical hydraulic connections in a weathered bedrock aquifer.