## 3 Results
### Speciation Characteristics of Heavy Metals  
The speciation characteristics of heavy metal in the soil around the mining area are presented in Table 4 and Figure 2. Cr is mainly in residual form, with an average content of 58.67 mg/kg, accounting for 82.04% of the 7 forms, followed by humic acid combined form of 9.52%. Arsenic is mainly in residual form, with an average content of 172.02 mg/kg, accounting for 69.15% of the 7 forms. Ni, Pb, and Zn are mainly in residual form, accounting for 59.85%, 57.34%, and 61.11%, respectively, followed by iron-manganese oxide combined form, of which Pb accounts for the highest proportion, reaching 22.86%. Some studies show that Pb in air dust is mainly in iron-manganese oxide form [32]. Cu is mainly in residue form and humic acid combined form, followed by iron-manganese oxide combined form and strong organic combined form. Cd is mainly in ion exchange form, with an average content of 26.72 mg/kg, accounting for 24.12%, and the proportions of other forms are in the following order: strong organic combined form \(>\) humic acid combined form \(>\) iron-manganese oxide combined form \(>\) residual form \(>\) carbonate combined form \(>\) water-soluble form.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Maximum** & **Minimum** & **Average** & \begin{tabular}{c} **Reference** \\ **Background** \({}^{\ast}\) \\ \end{tabular} & \
\begin{tabular}{c} **Risk Screening** \\ **Values** \\ \end{tabular} &
\begin{tabular}{c} **Risk Intervention** \\ **Values** \\ \end{tabular} \\ \hline pH & 7.7 & 3.7 & 5.9 & & 5.5 \(<\) pH \(\leq\) 6.5 & 5.5 \(<\) pH \(\leq\) 6.5 \\ organic matter & 10.5 & 0.5 & 4.3 & & & \\ Cr & 169.2 & 32.7 & 66.9 & 82.1 & 150 & 850 \\ Ni & 151.7 & 9.1 & 42.7 & 26.6 & 70 & \\ Cu & 328.7 & 13.4 & 86.5 & 27.8 & 50 & \\ Pb & 7104.5 & 1.8 & 730 & 24.0 & 90 & 500 \\ Zn & 20,075 & 16.8 & 2422 & 75.6 & 200 & \\ As & 4097.9 & 1.2 & 307.1 & 20.5 & 40 & 150 \\ Cd & 255.9 & 0.2 & 16.4 & 0.27 & 0.3 & 2.0 \\ \hline \end{tabular}  
* * Refers to China National Environmental Monitoring Center [33].  
\end{table}
Table 3: Statistical parameters of pH value, organic matter, and heavy metal content of the farmland soil around Dachang mining area.  
Figure 3 shows that the migration ability of each heavy metal element in farmland soil with the change in environmental conditions is in the following order: Cd > As > Ni > Zn > Cr > Cu > Pb; the order of the biological availability of each heavy metal element with the change in environmental conditions is: Cd > Zn > As > Ni > Cu > Cr > Pb. Compared with other elements, the absolute content of water-soluble, ion exchange, and carbonate combined forms of Cd is higher. Cd is the most active and easily absorbed and utilized by plants, thus accumulating toxicity in organisms, followed by As, Ni, and Zn. The residual form of Cr, Cu, and Pb elements are higher, and the biological activity and mobility of these heavy metals are poor.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & **Water-Soluble** & **Ion Exchange** & **Carbonate** & **Humic Acid** & **Iron-Manganese** & **Strong Organic** & **Residual** \\  & **Form** & **Form** & **Combined Form** & **Combined Form** & **Oxide** & **Combined Form** & **Combined Form** & **Form** \\ \hline Cr * & 0.16 & 0.41 & 0.56 & 6.81 & 1.35 & 3.55 & 58.67 \\ Ni * & 0.36 & 0.44 & 1.44 & 2.36 & 6.02 & 5.01 & 23.30 \\ Cu * & 0.40 & 0.49 & 2.85 & 62.38 & 25.93 & 20.29 & 76.84 \\ Pb * & 4.26 & 0.60 & 61.50 & 625.74 & 1017.02 & 188.54 & 2551.00 \\ Zn * & 115.21 & 93.73 & 387.57 & 546.96 & 1253.83 & 959.23 & 5273.57 \\ As * & 7.01 & 0.97 & 6.43 & 43.51 & 7.27 & 11.55 & 172.02 \\ Cd * & 4.72 & 26.72 & 10.53 & 17.64 & 16.08 & 20.03 & 15.04 \\ \hline \hline \end{tabular}  
* * represents the arithmetic mean contents of different speciation.  
\end{table}
Table 4: Contents of different forms of heavy metals in soil around Dachang mining area.  
Figure 2: The proportion chart of different forms of heavy metals in the farmland soil around the Dachang mining area.