## 3 Results
### Single Pollution Index
#### 3.2.1 Single Pollution Index for Soils  
The single pollution index (_PI_) to describe the contamination risk of heavy metals for soil of agricultural land proposed by [PERSON] et al. [42] is  
\[PI=\begin{cases}0&C<C_{\text{RS}}\\ 1&C_{\text{RS}}\leq C<C_{\text{RI}}\\ 10&C_{\text{RI}}\leq C\end{cases} \tag{1}\]  
where \(C\) are the contents of heavy metals in soil samples, and \(C_{\text{RS}}\) and \(C_{\text{RI}}\) are their risk screening and intervention values for soil of agricultural land in GB15618-2018, respectively (Table A1).  
According to Equation (1), the _PI_ values of Hg, Pb, Cd, As, and Cr in 3312 surface soil samples in this study are calculated to draw geochemical maps of the single pollution index of heavy metals in the Xinping area (Figure 3). The geochemical maps of _PI_ can not only intuitively visualize the spatial distribution of heavy metal pollution in the area through different color scales [43], but also identify the pollution source for each heavy metal like multivariate statistical analysis and a new geochemical gene technique [44; 45; 46].  
For Hg, there are 3 samples with _PI_ values of 10 that belong to the intervention level, 3 samples with _PI_ values of 1 that belong to the screening level, and 3306 samples with _PI_ values of 0 that belong to the background level. For Pb, there are 2 samples with _PI_ values of 10 that belong to the intervention level, 83 samples with _PI_ values of 1 that belong to the screening level, and 3227 samples with _PI_ values of 0 that belong to the background level. For Cd, there are 6 samples with _PI_ values of 10 that belong to the intervention level, 384 samples with _PI_ values of 1 that belong to the screening level, and 2922 samples with _PI_ values of 0 that belong to the background level. These results indicate that most areas are classified as the background level except sporadic areas as the screening and intervention level of Hg, Pb, and Cd.  
For As, there are 30 samples with _PI_ values of 10 that belong to the intervention level, 399 samples with _PI_ values of 1 that belong to the screening level, and 2883 samples with _PI_ values of 0 that belong to the background level. The result indicates that most areas belong to the background level except small areas belonging to the screening level and sporadic areas belonging to the intervention level.  
For Cr, there are 112 samples with _PI_ values of 10 that belong to the intervention level, 577 samples with _PI_ values of 1 that belong to the screening level, and 2623 samples with _PI_ values of 0 that belong to the background level. The result indicates that most areas belong to the background level, while some areas belonging to the intervention level are mainly distributed in a planar shape within the area where the peridotite rock is located, and the areas belonging to the screening level are also mainly distributed within the peridotite rock and its surrounding areas in a planar shape. This indicates that the Cr pollution must result from the parent rock.  
Figure 3: Geochemical maps of the single pollution index of heavy metals in surface soil samples from the Xinping area along with its geological map.