## 4 Results and Discussion
### Risk Assessment of Soil's Heavy Metals
#### 4.4.2 Evaluation Results of the Potential Ecological Risk Index Method  
In this study, the reference value of soil heavy metal elements in the study area was referred to the background value of soil elements in the Guangxi Zhuang Autonomous Region [18], was used to calculate the potential ecological risk index (\(E_{r}{}^{i}\)) and comprehensive risk index (RI) for individual heavy metals in the study area using Formula (2). The evaluation results are shown in Table 8 and Figure 6. The results of the evaluation showed that the average value of \(E_{r}{}^{i}\) was in the order of Hg \(>\)Cd\(>\) As\(>\) Pb\(>\) Cu\(>\) Ni\(>\) Zn. Among them, 67.65% and 41.18% of the sampling sites of Hg and Cd were very high ecological risk, respectively; 47.06% of the sampling sites of As were medium ecological risk; and Pb, Cu, Ni, Cr, and Zn were slight ecological risk. The average value of the potential ecological risk index (RI) of soil heavy metals in the study area was 716.69, which is a very high ecological risk. Overall, the potential ecological risk of the soil in the study area was relatively significant, with Hg at a very high ecological risk level, Cd at a very high ecological risk level, As at a medium ecological risk level, and the combined ecological risk of the soil was very high.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Item**} & \multirow{2}{*}{**Cr**} & \multirow{2}{*}{**Ni**} & \multirow{2}{*}{**Cu**} & \multirow{2}{*}{**Zn**} & \multirow{2}{*}{**As**} & \multirow{2}{*}{**Cd**} & \multirow{2}{*}{**Pb**} & \multirow{2}{*}{**Hg**} \\ \cline{2-2} \cline{6-9}  & & & & & & & & & & \\ \hline Minimum & 0.10 & \(-\)0.47 & \(-\)0.07 & \(-\)0.06 & 0.37 & 1.36 & 0.39 & \(-\)0.17 \\ Maximum & 1.28 & 0.41 & 1.33 & 0.97 & 3.51 & 3.62 & 1.91 & 3.13 \\ Average & 0.63 & \(-\)0.08 & 0.32 & 0.35 & 1.74 & 2.38 & 0.91 & 2.46 \\ Standard deviation & 0.28 & 0.20 & 0.30 & 0.28 & 0.85 & 0.70 & 0.32 & 0.65 \\ contamination degree & \multirow{2}{*}{Slight} & \multirow{2}{*}{None} & \multirow{2}{*}{Slight} & \multirow{2}{*}{Slight} & \multirow{2}{*}{Moderate} & Moderate & Moderate & Moderate & Moderate & Moderate \\  & & & & & & & \(-\)High & & \(-\)High \\ \hline \hline \end{tabular}
\end{table}
Table 7: The evaluation results of the geo-accumulation index method of heavy metals in the study area.  
Figure 5: Evaluation results of the geo-accumulation index method.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Item**} & \multirow{2}{*}{**Cr**} & \multirow{2}{*}{**Ni**} & \multirow{2}{*}{**Cu**} & \multirow{2}{*}{**Zn**} & \multirow{2}{*}{**As**} & \multirow{2}{*}{**Cd**} & \multirow{2}{*}{**Pb**} & \multirow{2}{*}{**Hg**} \\ \cline{2-2} \cline{6-9}  & & & & & & & & & \\ \hline Minimum & 3.22 & 5.42 & 7.16 & 1.44 & 19.3 & 115.65 & 9.85 & 53.33 & 344.42 \\ Maximum & 7.27 & 9.98 & 18.91 & 2.93 & 170.89 & 552.31 & 28.01 & 524.21 & 1109.18 \\ Average & 4.73 & 7.17 & 9.58 & 1.95 & 59.68 & 263.18 & 14.46 & 355.94 & 716.69 \\ Hazard degree & \multirow{2}{*}{Slight} & \multirow{2}{*}{Slight} & \multirow{2}{*}{Slight} & \multirow{2}{*}{Slight} & \multirow{2}{*}{Mederate} & \multirow{2}{*}{Very high} & \multirow{2}{*}{Slight} & \multirow{2}{*}{Extremely high} & \multirow{2}{*}{Very high} \\ \cline{2-2} \cline{6-9}  & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 8: The evaluation results of the potential ecological risk index method of heavy metals in the study area.