## 3 Results and Discussion
### Risk Evaluation
#### 3.2.2 Comprehensive Evaluation of Soil Heavy Metal Pollution  
The single pollution index and Nemerow comprehensive index described above can only evaluate the degree of heavy metal pollution in each individual sampling site. Such analyses do not integrate experimental data with multivariate statistics, spatial analysis and geostatistical analysis. They do not reveal the interaction between the distribution differences of heavy metals and driving factors, the spatial proximity effect and spatial distribution characteristics of heavy metals at each sampling site. It is also not possible to achieve the division (zoning) of soil heavy metal pollution in the study area. In order to further describe its spatial variation characteristics in the entire study area, we adopted inverse distance weighted (IDW) and the spatial superposition methods to conduct spatial interpolation and comprehensive evaluation of soil heavy metal pollution. The vertical distribution pattern of heavy metals (Pb, As, Cu, Zn, Ni, Cr) in affected area of slag field was shown in Figure 2. The six heavy metals in the study area had an island-like (Pb), strip-like (Cu), and spot-like (As, Zn, Ni, Cr) distribution. Sites with the highest content of Pb was mainly distributed in the areas around the National Highway 108 (site #3, 22, 23, 24, 27, 28, 32, 38, 40, 44, 45, 50, 54, 55, 57, 60, 61, 62, 63 points, Figure 2) and around the slag field (site #34, 35, Figure 2). Pb are mainly distributed in the upper reaches and southeastern parts of the Lima River. The high As content areas (site #29, 31, 47, 49, 51, 58 points, Figure 2) are distributed in dry land and forest land in the northeast of the study area and the whole south. It is possible that these areas are located at the bottom of the slope, with serious soil erosion and low vegetation coverage. Under the influence of the dry and hot valley, heavy metals are accumulated by rainwater. Sites with low content of the overall Cu content was 0.94-27.27 mg\(\cdot\)kg\({}^{-1}\), and the pollution level was relatively low. The areas with high content of Cu (site #1, 3, 25, 34, 35 points, Figure 2) were distributed in strip-like form near the National Highway 108. Except for the lower reaches of Lima River with serious Zn pollution (site #14, 43 points, Figure 2), the remaining study areas had uniform distribution of Zn. The sites with low content of Ni were in the west of the study area, and the sites with high content of Ni (site #6, 16, 34, 43, 59, 61 points, Figure 2) was in the east of the study area, especially in the lower reaches of the Lima River. The areas with \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Evaluation** & **Index** & **First-Order** & **Second-Order** & **Tertiary** & **Four Pole** & **Five Grades** \\ \hline \multirow{8}{*}{Single-factor pollution index} & \(P_{i}\) & \(P_{i}\leq 1\) & 1 \(<P_{i}\leq 2\) & 2 \(<P_{i}\leq 3\) & \(P_{i}>3\) & \\  & Degree of pollution & No pollution & Mild pollution & Moderately & Severe & \\  & & & & polluted & pollution & \\  & Pb & 100 & 0.00 & 0.00 & 0.00 & \\  & As & 82.54 & 17.46 & 0.00 & 0.00 & \\  & Cu & 100 & 0.00 & 0.00 & 0.00 & \\  & Zn & 100 & 0.00 & 0.00 & 0.00 & \\  & Ni & 100 & 0.00 & 00 & 0.00 & \\  & Cr & 95.24 & 4.76 & 0.00 & 0.00 & \\  & \(P_{N}\) & \(P_{N}\leq 0.7\) & 0.7 \(<P_{N}\leq 1\) & 1 \(<P_{N}\leq 2\) & 2 \(<P_{N}\leq 3\) & \(P_{N}>3\) \\  & & & & & Moderate & Heavy \\  & & & & & pollution & pollution \\  & & & & & & pollution \\  & & & & & & \\  & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Statistics of the proportion of soil heavy metal element pollution grade evaluation samples in the study area. high content of Cr (site #30, 59, 60 points, Figure 2) showed a spot-like distribution in the northern woodland, garden land, dry land, and the lower reaches of the Lima River.