## 4 Results and Discussion
### Chemical Partitioning  
Trace elements were unevenly distributed among the four operationally defined soil fractions, as determined by sequential chemical extraction (Table 3), which has been proven to be a useful procedure for assessing the long-term mobility of PHE [37]. Figure 5: Soil profile distribution of the concentration factor (calculated by dividing the total content of each trace element in the amended soil by that in the unamended soil) for: (**a**) Cd, Cu, Ni, and Zn; and (**b**) As, Cr, Sb, and Pb. Figure 6: Quotient between total concentration (C) of trace elements in the amended surface soil 12 years after amendment application (this study) and before treatment [24]. Fraction F1 represents the labile fraction of PHE that would be released into the environment when soil conditions become acidic, including water-soluble, exchangeable (non-specifically sorbed), and weak acid soluble (bound to carbonates) pools. Trace element concentrations in the fraction F1 of the amended soil appeared insignificant (generally less than 1 mg kg\({}^{-1}\)), except for Cu and Zn which reached 23.2 mg kg\({}^{-1}\) and 116 mg kg\({}^{-1}\), respectively. Despite the low level of Cd (0.54 mg kg\({}^{-1}\)) measured in this readily available fraction, it accounted for a relatively high extractability (28.9%). Fraction F2 includes PHE bound to Fe and Mn oxyhydroxoxides, which would be easily released into the solution as a result of reductive dissolution. Copper (554 mg kg\({}^{-1}\)) and Zn (436 mg kg\({}^{-1}\)) were again the most extractable PHE in the reducible fraction of the amended soil, with an extraction yield of 44.1% and 47.7%, respectively. In addition, Cd and Ni were solubilized in the second step of the sequential extraction (F2) to a considerable extent in terms of percent extraction, accounting for 47.1% and 39.9% of their respective total concentration in the surface soil. The results showed once again that Cu (282 mg kg\({}^{-1}\)) and Zn (158 mg kg\({}^{-1}\)) were the PHE removed at highest grade from the oxidizable fraction (F3) of the amended soil, which theoretically involves PHE bound to sulfides and/or organic matter. These released concentrations represent 22.4% and 17.3% of their respective total contents in soil and they scored the highest proportion of extractable PHE together with the extraction yield of Ni (21.7%). The amounts of PHE solubilized in the amended soil samples after multi-acid digestion ranged widely depending on the element studied, with Pb (639 mg kg\({}^{-1}\)), Cu (398 mg kg\({}^{-1}\)). As (263 mg kg\({}^{-1}\)), and Zn (203 mg kg\({}^{-1}\)) being the most abundant contaminants in the extracts. This residual fraction (F4) was the dominant pool for Pb (99.8%), As (96.0%), Sb (93.4%), and Tl (86.2%). It must be emphasized that this residual fraction comprises not only the PHE tightly bound to the crystal structure of silicates but also those associated with secondary sulfates, such as jarosite-group minerals, which have not been effectively solubilized during the first three steps of the BCR sequential extraction procedure. It is generally agreed that jarosite-group minerals can scavenge Pb, As, Sb, and Tl in AMD-impacted areas through structural incorporation and/or adsorption mechanisms [2; 42; 43].