## 3 Methodology
### Description of the GIS-Based DRASTIC Model Parameters  
The seven parameters in Equation (1) that affect and control groundwater flow and pollutant transport are adopted in DRASTIC to constitute the factor system for vulnerability assessment. Based on the \"Delineation of Priority Areas for Groundwater Pollution Prevention and Control Technical Guideline\" published by the China Geological Survey in 2022 [36], different parameter weights have been assigned by investigation experience and field experiment. According to the influence of each parameter on groundwater vulnerability, different weight values are assigned to them in Table 1.  
Based on the aforementioned seven parameters, an evaluation level is assigned to each parameter. As the local groundwater depth increases, vertical net recharge decreases, aquifer thickness increases, soil and vadose zone cutoff particles become finer, topographic slope increases, and aquifer permeability coefficient decreases, the scores for each parameter will decrease. A lower score indicates a smaller contribution of that parameter to the groundwater vulnerability assessment index, suggesting a reduced susceptibility to pollution. Conversely, higher scores indicate a greater contribution of the parameter to the groundwater vulnerability assessment index, indicating a higher vulnerability to pollution. The scores for each indicator range from 1 to 10, with specific classifications provided in Table 2.  
A step wise flow diagram representing the data processing, preparations of layers, and assigning of weights has been demonstrated below in Figure 2. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Description** & **Relative Weight** \\ \hline Depth of groundwater & Depth to groundwater table is the distance from the surface to the submerged & 5 \\ (D) & Surface; unit is m. & 4 \\ Vertical net recharge & Approximate using precipitation infiltration recharge instead of vertical net & 4 \\ (R) & Aquire thickness can be analyzed from borehole data; refers to the saturated & zone material properties, which \\ (A) & controls the pollutant attenuation processes; unit is m. & 3 \\ Soil media & The soil media is a weathered layer with a thickness of 2 m or less at the surface, & 2 \\ (S) & which controls the amount of recharge that can infiltrate downward. & 1 \\ Topography & Slope values can be automatically generated in GIS after DEM extraction from & 1 \\ (T) & 1:50,000 or 1:10,000 topographic maps. & 5 \\ Impact of Vadose Zone & The unsaturated zone material: it controls the passage and attenuation of the & 5 \\ (I) & contaminated material to the saturated zone. & 3 \\ Hydraulic conductivity & Indicates the ability of the aquifer to transmit water, and hence determines the & 3 \\ (C) & rate of flow of contaminant material within the groundwater system. & \\ \hline \end{tabular}
\end{table} 
Table 1: Description of DRASTIC model parameters and its weights. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Parameter** & & & & & & & & & & \\ \cline{2-10}  & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** \\ \hline D (m) & \textgreater{}30 & (25, 30] & (20, 25] & (15, 20] & (10, 15] & (8, 10] & (6, 8] & (4, 6] & (2, 4] & \(\leq\)2 \\ R (mm/a) & 0 & (0, 51] & (51, 71] & (71, 92] & (92, 117] & (117, 147] & (147, 178] & (178, 216] & (216, 235] & \(>\)235 \\ A (m) & \textgreater{}50 & (45, 50] & (40, 45] & (35, 40] & (30, 35] & (25, 30] & (20, 25] & (15, 20] & (10, 15] & \\ S & rock & clay & silt foam & foam & sandy & swelling or & & & & \\  & loam & & loam & & & & & & \\  & & & & & & & & & \\ T(\%) & \textgreater{}10 & (9, 10] & (8, 9] & (7, 8] & (6, 7] & (5, 6] & (4, 5] & (3, 4] & (2, 3] & \(\leq\)2 \\ I & clay & loam & sandy & silt- & silty, fine & & medium & & & \\  & & loam soil & sand & sand & sand & fine sand & & coarse sand & & gravel & cobble \\ C (m/d) & [0, \(\leq\)4] & (4, 12] & (12, 20] & (20, 30] & (30, 35] & (35, 40] & (40, 60] & (60, 80] & (80, 100] & \textgreater{}100] \\ \hline \end{tabular}
\end{table}