# Evolution of the Residual Pollution in Soils after Bioremediation Treatments
## 3 Results and Discussion
### Pollutant Concentration in Soils  
The concentration of PHEs in the soils twenty years after the accident and completed all remediation measures, indicated significant differences in depth and between sectors depending on the considered element (Table 2). The less mobile elements (Pb and As) have the highest concentration in S1, and especially in the uppermost 10 cm of the soils; meanwhile, the elements with higher mobility in soils (Zn and Cu) showed fewer differences between sectors and trend to increase their concentration in depth, mainly from 10 to 50 cm. In the acidic sector (S1), all elements (except Cu) trend to decrease in depth. For As and Pb, this reduction is gradual, and significant differences were only observed by comparing samples from 0-10 cm with 30-50 cm depth; while for Zn, this reduction was significant at 10 cm depth. This behavior was previously described and related to the soil properties and the climate, indicating that under acidic conditions and the high evapotranspiration in the area [28], the PHEs tend to concentrate in the uppermost part of the soils [29]. In the neutral-alkaline sector (S2), elements behave differently. In \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{\
\begin{tabular}{c} **Depth** \\ **(cm)** \\ \end{tabular} } & \multicolumn{2}{c}{**Pb**} & \multicolumn{2}{c}{**As**} & \multicolumn{2}{c}{**Zn**} & \multicolumn{2}{c}{**Cu**} \\ \cline{2-10}  & **S1** & **S2** & **S1** & **S2** & **S1** & **S2** & **S1** & **S2** \\ \hline \multirow{2}{*}{0–10} & 393.92 aA & 101.75 aB & 188.20 aA & 51.42 aB & 364.71 aA & 392.94 aA & 119.72 aA & 89.41 aB \\  & (369.89) & (75.76) & (135.67) & (39.79) & (246.05) & (210.18) & (73.91) & (46.44) \\ \hline \multirow{2}{*}{10–30} & 254.22 & \multirow{2}{*}{148.15 aB} & 143.91 & \multirow{2}{*}{75.20 aB} & 212.58 & \multirow{2}{*}{447.10 aB} & 212.58 & \multirow{2}{*}{447.10 aB} & 105.66 aA & 114.28 \\  & abA & (157.38) & (94.34) & (82.44) & (128.71) & (260.50) & (73.80) & abA \\ \hline \multirow{2}{*}{30–50} & 166.04 & \multirow{2}{*}{130.77 aA} & \multirow{2}{*}{90.40 bA} & 154.93 & \multirow{2}{*}{422.94 aB} & 210.14 & \multirow{2}{*}{92.48 aA} & 120.14 \\  & bA & (100.84) & (71.96) & (43.21) & (105.99) & (301.69) & (70.72) & bA \\ \hline \hline \end{tabular}
\begin{tabular}{c} Lowercase letters indicate significant differences among depths and capital letters between sectors (Tukey HDS test \\ \(p<0.05\)). \\ \end{tabular}
\end{table} 
Table 2: Mean (standard deviation) of total concentrations (mg kg\({}^{-1}\)) of potentially harmful elements (PHEs) in the soils twenty years after the accident, at different depths and in the two studied sectors (S1 and S2). Figure 2: Organic carbon (OC) content in all sampling plots and depths for the two studied soil sectors (S1 and S2). these soils, the elements infiltrated in the soil matrix due to the pollution and the acidic oxidation of the tailings, but the presence of high concentration of carbonates in these soils produced the precipitation of most elements in depth and preventing their rise to the surface despite the intense evapotranspiration in the area. The comparison of the concentration in PHEs measured in uncontaminated soils (UCS) in the area [30], indicated a significant increase in the uppermost 10 cm of the soils in most cases (Table 3). These concentrations indicated that the recovered soils twenty years after the accident, currently converted into the GGC, retained concentration of PHEs above the background levels in the area despite the remediation measures. The soils in the S1 sector have significantly higher concentrations of As and Pb that should be monitored [6], with values exceeding around 10 times the background concentrations in the uncontaminated soils in the area. Zn and Cu have higher concentration than UCS, but with no significant differences between sectors.