## 3 Results and Discussion  
### The Spatial Distribution Patterns of Heavy Metals in the Twelve Cities  
The average values of Cu, Pb, Cd, Cr, Ni and Zn were 24.13, 31.77, 0.16, 130.63, 22.13 and 71.19 mg-kg\({}^{-1}\), respectively, suggesting that most cropland soils were suffering from the contamination of Cr, Pb and Cd. Concentrations of Cr ranged from 31.2 to 335.0 mg-kg\({}^{-1}\); that indicated the significantly high spatial disparity in these cities. Contents of Cr in Liaocheng and Dezhou were below the background value, and significantly lower than that in other cities (Figure 2). Moreover, cluster analysis of Cr among the twelves cities showed that three coastal cities (Weifang, Weihai and Yantai) and one inland city (Zibo) were gathered into one cluster, with the significantly high concentrations of \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Soil** & **Cu** & **Pb** & **Cd** & **Cr** & **Ni** & **Zn** & **Related Cities** \\
**Types** & & & & & & & & \\ \hline Brown & 22.4 & 25.1 & 0.092 & 64.5 & 26.5 & 68.5 & Qingdao, Weihai, \\ soil & 22.4 & 25.1 & 0.092 & 64.5 & 26.5 & 68.5 & Yantai, Dongying \\  & & & & & & & \\ Cinnamon & 24.3 & 21.3 & 0.1 & 64.8 & 30.7 & 74.1 & Zaozhuang, \\ soil & 24.3 & 21.3 & 0.1 & 64.8 & 30.7 & 74.1 & Weifang, Zibo, \\  & & & & & & & & \\ Fluvo-aquic & 22.1 & 21.9 & 0.103 & 66.6 & 29.6 & 71.1 & Dezhou, Jinan, \\ soil & & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 1: The background values of the metals in different cities of Shandong Province (mg-kg\({}^{-1}\)). 181.13, 177.8, 161.62 and 172.78 mg\(\cdot\)kg\({}^{-1}\), respectively (Figures 2 and 3). Similarly, the gross values of industrial output of Weifang, Yantai and Zibo were 230.75, 330.94 and 218.31 billon in 2017, which were also significantly higher than other cities, except Qingdao (tourism dominated the gross product). The higher accumulation of Cr and higher gross value of industrial output in these cities than in other cities may indicate that industrial producing can aggravate the accumulation of Cr in the agricultural soils, and the specific relations need further research. Both [PERSON] et al. [26] and [PERSON] et al. [27] showed that certain metals can migrate from the inland to the coast, especially Cr and Ni, and in the areas along the flowing rivers. They showed that concentrations of Cr was gradually higher from inland to coast in this study, while the spatial distribution of Ni was not significant (Figure 2). This trend of Cr in this study may attribute to the impact of water flows and the high mobility of Cr [28]. With a slight fluctuation from 8.1 to 40.6 mg\(\cdot\)kg\({}^{-1}\), average concentrations of Ni were lower than the background values, indicating the relatively low and stable accumulation of Ni in agricultural soils of the studied cities. Though the contents of Ni in Zibo and Heze (28.65 and 27.8 mg\(\cdot\)kg\({}^{-1}\)) were approaching the background values, the contamination of Ni was the slightest compared to other metals. Previous reports showed that the contamination of Ni is not serious in cropland, which may attribute to the relative scarcity of Ni in China and the low human input [29; 30]. The content of Cu in Weihai (42.87 mg\(\cdot\)kg\({}^{-1}\)) was significantly higher than in other cities, and was followed by that in Zibo (33.98 mg\(\cdot\)kg\({}^{-1}\)), suggesting that agricultural soils in the two cities faced high input of Cu. It was deduced that the primary input of Cu was from the industrial activities, as Weihai and Zibo are cities of heavy industry [31; 32]. In addition, contents of Cu in Heze and Zaozhuang also exceeded the background values. The mean concentration of Zn was 71.19 mg\(\cdot\)kg\({}^{-1}\), and half of the cities showed higher contents of Zn than the background value. Moreover, Weihai and Zibo showed significantly higher accumulation of Zn than other cities, which was similar with the spatial distribution of Cu. The finding further indicated that Zn and Cu had similar industrial input [31].