## 4 Results and Discussion
### Total Trace Element Concentrations  
Results of the total concentrations of PHE analyzed in the amended and control (no amendment) soil samples are listed together in Table 2, and displayed in Figure 4 to graphically show the variation of concentration with depth. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Sample**} & \multicolumn{6}{c}{**Electrochemical**} & \multicolumn{6}{c}{**Mineral Composition (wt \%)**} \\ \cline{3-13}  & \multicolumn{2}{c}{**Depth**} & \multicolumn{2}{c}{**Parameters**} & \multicolumn{2}{c}{**Clay**} & \multicolumn{2}{c}{**Jerosite**} & \multicolumn{2}{c}{**Gypsum**} & \multicolumn{2}{c}{**Hematite**} & \multicolumn{2}{c}{**Calcite**} \\ \cline{3-13}  & \multicolumn{2}{c}{**(cm)**} & \multicolumn{2}{c}{pH} & \multicolumn{2}{c}{**Eh**} & \multicolumn{2}{c}{**EC**} & \multicolumn{2}{c}{**Quartz**} & \multicolumn{2}{c}{**Minerals**} * & \multicolumn{2}{c}{**Feldspars**} & \multicolumn{2}{c}{**Jerosite**} & \multicolumn{2}{c}{**Gypsum**} & \multicolumn{2}{c}{**Hematite**} & \multicolumn{2}{c}{**Calcite**} \\ \hline \multirow{6}{*}{**Amended**} & T10 & 0\(-\)10 & 6.3 & 343 & 3.32 & 10\(-\)15 & 5\(-\)10 & \(<\)5 & 10\(-\)15 & 10\(-\)15 & \(<\)5 & 55\(-\)60 \\  & T20 & 10\(-\)20 & 6.4 & 458 & 3.01 & 30\(-\)35 & 25\(-\)30 & \(<\)5 & 10\(-\)15 & 5\(-\)10 & \(<\)5 & 25\(-\)30 \\  & T30 & 20\(-\)30 & 6.3 & 421 & 2.85 & 40\(-\)45 & 30\(-\)35 & \(<\)5 & 15\(-\)20 & \(<\)5 & \(<\)5 & n.d. \\  & T40 & 30\(-\)40 & 5.8 & 472 & 1.72 & 40\(-\)45 & 30\(-\)35 & \(<\)5 & 15\(-\)20 & \(<\)5 & \(<\)5 & n.d. \\  & T50 & 40\(-\)50 & 6.1 & 502 & 1.95 & 35\(-\)40 & 30\(-\)35 & \(<\)5 & 15\(-\)20 & \(<\)5 & \(<\)5 & n.d. \\  & T60 & 50\(-\)60 & 6.3 & 407 & 2.90 & 35\(-\)40 & 35\(-\)40 & \(<\)5 & 15\(-\)20 & \(<\)5 & \(<\)5 & n.d. \\  & T70 & 60\(-\)70 & 6.2 & 525 & 2.61 & 40\(-\)45 & 30\(-\)35 & \(<\)5 & 15\(-\)20 & \(<\)5 & \(<\)5 & n.d. \\ \hline \multirow{6}{*}{Control soil} & ST10 & 0\(-\)10 & 3.2 & 619 & 4.02 & 40\(-\)45 & 35\(-\)40 & 5\(-\)10 & 10\(-\)15 & \(<\)5 & \(<\)5 & n.d. \\  & ST20 & 10\(-\)20 & 3.3 & 647 & 3.10 & 35\(-\)40 & 25\(-\)30 & \(<\)5 & 15\(-\)20 & 10\(-\)15 & \(<\)5 & \(<\)5 & n.d. \\  & ST30 & 20\(-\)30 & 3.1 & 644 & 3.07 & 40\(-\)45 & 25\(-\)30 & 5\(-\)10 &Total concentrations of As, Cr, and Pb increased significantly (\(p\) < 0.01) with depth in the amended soil up to a maximum of 1860 mg kg\({}^{-1}\) As at 40-50 cm depth, 72 mg kg\({}^{-1}\) Cr (50-60 cm), and 2660 mg kg\({}^{-1}\) Pb (40-50 cm). Compared to the control, the concentrations of As, Pb, and Cr in the amended soil were noticeably low at the upper sampling layers (up to 20 cm depth), and then increased with depth to levels comparable to even higher than baseline concentration. Such vertical distribution pattern may be related to a dilution effect caused by the incorporation of MWS to the plough layer. It is nevertheless noteworthy that As showed a contrasted soil profile distribution, with increasing downward concentrations suggesting leaching from the upper horizons and subsequent re-deposition. The levels of Cu, Cd, Ni, and Zn in the surface horizon were significantly (\(p\) < 0.01) higher than in the lower sampling depths. Notwithstanding the diluting effect due to MWS addition, the amounts of Cu (1210 mg kg\({}^{-1}\)), Cd (1.4 mg kg\({}^{-1}\)), Ni (38 mg kg\({}^{-1}\)), and Zn (851 mg kg\({}^{-1}\)) found in the topsoil (0-10 cm) of the amended mini plot were higher by a factor of 5.4 for Zn, 4.7 for Cd, 1.9 for Ni, and 1.6 for Cu relative to control soil (Figure 5). This increased upward trend is clearly indicative of metal enrichment in the amended surface layer. It was further found that the vertical distribution of such PHE in the two sampling sections (i.e., MWS-amended and control) was rather similar, with the concentration factor values ranging from 0.5 to 1.5, except for Cu that seemed to be more depleted in the 30-50 cm depth layer. Figure 4: Vertical distribution of trace elements along the two sampling sections (amended and control) of the soil profile.