## 4 Discussion  
### Comparison of Assessment Results Using Different Indices  
Comparing the pollution evaluation results illustrated in Figure 2, it can be obviously seen that soil heavy metal pollution in both the comprehensive results and each element descended from the industrial area, to city parks, to the vicinity of the landfill. Except Cr and Ni in city park soils and Hg and Cr in soils in the vicinity of the landfill, other heavy metals appeared with different enrichment degrees in surface soils of different function areas within Guangzhou (Figure 2). Furthermore, the pollution degree of each element in soils of city parks, the vicinity of the landfill, and the industrial area were decreased following the order Cd, Hg, Zn, As, Pb, and Cu; Cd, As, Zn, Ni, Pb, and Cu; and Cd, Zn, Cu, Pb, Hg, As, Ni, and Cr, respectively. Despite Cd being the most important heavy metal pollutant in soils collected from the three function areas, the percentages of different Cd pollution degrees were much different (Figure 2). Heavy metals Hg, As, and Zn were also important heavy metal pollutants for soils of city parks, the vicinity of the landfill, and the industrial area, respectively. A suitable method for analysis is the premise of regional soil pollution assessment. Compared the results of \(I_{MN}\), \(mC_{d}\), and \(CSI\) illustrated in Figure 2, it can be seen that comprehensive pollution degrees classified by \(I_{MN}\) and \(mC_{d}\) were similar for soil samples collected from city parks. However, pollution degrees classified by \(CSI\) were much weaker than those classified by \(I_{MN}\) and \(mC_{d}\) for the three function areas, indicating that heavy metal pollution assessed by \(CSI\) may be underestimated for urban surface soils. Results of \(I_{MN}\) indicated that percentages of measured soil samples under clean and slight pollution were 26.8% and 7.2% for samples collected from the vicinity of the landfill and the industrial area, respectively. Meanwhile, these percentages classified by calculated results of \(mC_{d}\) were 53.7% and 26.7% for the vicinity of the landfill and the industrial area, respectively (Figure 2b,c). Since obvious significant differences appeared between assessing the results of \(I_{MN}\) and \(mC_{d}\) (Figure 2), comprehensive assessment results by previous studies are summarized in Figure 3 to compare with the calculated results of \(I_{MN}\) and \(mC_{d}\) in the present study. As illustrated in Figure 3a, previous studies indicated that no more than 35% (about 34.4%) of measured soil samples suffered high or serious pollution of heavy metals. The calculated results of \(mC_{d}\) illustrated in Figure 2a are similar to the mentioned results of previous studies. However, the calculated \(I_{MN}\) results of the present study illustrated in Figure 2a indicated nearly half of the measured samples (about 47.7%) were under high or serious pollution. These results indicated that heavy metal pollution might be overesti  
Figure 3: Soil heavy metal pollution assessment results by previous studies for the city of Guangzhou. (**a**) City parks (revised from [13; 51; 56; 67; 72]), (**b**) Landfills (revised from [14; 55; 59; 73]), and (**c**) Industrial areas (revised from [13; 14; 47; 49; 53; 57; 60; 61]). mated for soils collected from city parks using \(I_{MN}\). As for soils collected from the vicinity of landfills, the percentage of slight and moderate pollution assessed by calculated results of \(I_{MN}\) was almost consistent with these results of previous studies (Figures 2b and 3b). Meanwhile, the percentages of unpolluted and moderately polluted samples assessed by the calculated \(mC_{d}\) were twice as much as and half of the results of previous studies, respectively (Figures 2b and 3b). From Figures 2c and 3c, the percentages of seriously polluted samples assessed by results of \(I_{MN}\) and \(mC_{d}\) for soils collected from the industrial area were consistent with and much lower than the results reported in previous studies, respectively.