## 4 Results and Discussion
### Results of Comprehensive Vulnerability Assessment and Its Validation  
#### 4.2.1 Results of Groundwater Vulnerability  
Vulnerability mapping was conducted using the Geographic Information System (GIS) based on hydrogeological data of the study area and with reference to the DRASTIC model, as shown in Figure 4. The groundwater vulnerability index values for Tangshan City range from 85 to 173, classified into four levels according to the DRASTIC classification principle (refer to Table 3). Higher vulnerability assessment index values indicate a higher susceptibility to pollution, while lower values suggest a lower vulnerability and reduced susceptibility to pollution. Additionally, karst collapse areas and regions affected by mining subsidence are directly classified as the highest vulnerability areas.  
The vulnerability assessment map of shallow groundwater in Tangshan City reveals the following distribution results. Firstly, the shallow groundwater in Tangshan City exhibits high vulnerability, with the highest vulnerability area accounting for 4% of the total study area, the higher vulnerability area accounting for 53%, and the moderate vulnerability area accounting for 25%. Together, these three categories account for 82% of the area, while the low vulnerability area only accounts for 18%. This highlights the weak protection performance of shallow groundwater in the region, making it susceptible to pollution.  
Figure 4: Comprehensive evaluation of vulnerability of groundwater in Tangshan.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Groundwater Vulnerability** & **Vulnerability** & **Level** \\
**Composite Index Value** & & \\ \hline \((70,100)\) & Potentially contaminated & Low \\ \((100,120)\) & Easily contaminated & Moderate \\ \((120,150)\) & highly prone to be contaminated & Higher \\ \(>\)150 & Particularly vulnerable to be contaminated & Highest \\ \hline \hline \end{tabular}
\end{table}
Table 3: Classification of vulnerability of shallow groundwater in Tangshan City.  
Secondly, the spatial variability and causes of groundwater vulnerability in the study area are complex. The area around Luannan County has relatively shallow groundwater depth, significant vertical recharge, and a thick aquifer, leading to its classification as an area with high underground vulnerability. The valley area in the study region is also classified as a high vulnerability area due to its homogeneous lithology, simple rock structure, and strong permeability. These areas experience frequent agricultural activities and have well-developed aquaculture, resulting in a high pollution load and easy pollution of groundwater [46]. Similarly, highly vulnerable areas are also found in the central and southeastern regions of Tangshan, characterized by high vertical net recharge, relatively high permeability coefficients, good water permeability, and limited self-purification capacity. In addition, the karst subsidence area and the mined-out coastal area were directly classified as the highest vulnerable areas, covering 1.463 km\({}^{2}\). These factors contribute to weak anti-pollution capabilities of groundwater, making it prone to pollution. Additionally, these areas are marked by intense human activities, including urban industrial and domestic sewage discharge from Tangshan City, as well as excessive fertilization and sewage irrigation in agricultural regions. Therefore, they should be prioritized as key areas for groundwater protection.  
Thirdly, low-vulnerability areas are scarce and mainly concentrated in the northern part of Tangshan, such as the northern edge of Zunhua City, Qianxi County, and the northwest of Qian'an City. In these areas, groundwater depth is significant, the vadose zone exhibits fine lithology, and recharge amounts are limited, reducing vulnerability to external pollutants.