# Evolution of the Residual Pollution in Soils after Bioremediation Treatments
## 3 Results and Discussion
### Pollutant Concentration in Soils
The total concentration of PHEs in the area can be presented as cumulative frequency-distribution curves (Figure 3) to evaluate the degree of soil recovery, and to indicate the presence of potentially polluted soils in the GGC by calculating the percentage of soils that exceed the values to declare a soil as polluted. This potential risk was previously calculated by [PERSON] et al. [31], and according to the thresholds recommended by the Regional Government of Andalusia in 1999 [22] (fixed in 1000 mg kg\({}^{-1}\) for Pb, 100 mg kg\({}^{-1}\) for As, 1000 mg kg\({}^{-1}\) for Zn and 500 mg kg\({}^{-1}\) for Cu [32]), the 30% of the soils for As and the 20% for Zn exceeded the proposed values. In the following years, these thresholds were revised according to the risk assessment methodology proposed by the EU [33], considering the sources, receptors, exposure risk and pathways, and the regional government approved the current regulatory values in 2015 [34], which were strongly different from the previous ones: 275 mg kg\({}^{-1}\) for Pb, 36 mg kg\({}^{-1}\) for As, 10,000 mg kg\({}^{-1}\) for Zn, and 595 mg kg\({}^{-1}\) for Cu. According to these regulatory values and the mean concentrations measured in the soils, no potential risk related to Zn and Cu was detected. In the case of Pb, around 13% of the studied soils exceeded the regulatory value at 0-10 and 10-30 cm depth, and around 11% considering the 30-50 cm depth. The As is the element that presented the greatest risk of potential contamination in all depths, with around 70% of the studied soils exceeding the regulatory value at 0-10 and 30-50 cm depth and around 80% of at 10-30 cm depth. These results indicate a potential risk of pollution in the area, but the assessment made using total concentrations is only indicative and should be monitored in relation to the vegetation evolution in the area and according to toxicity bioassays [35]. Anyway, the use of the area is restricted since the accident occurred, so the agriculture, hunting and grazing is forbidden, and only recreational activities are permitted into the GGC after the bioremediation treatments were completed. The monitoring of the area has continued to assess the risk of dispersion of the residual pollution into the environment. Further studies indicated that the leaching of the most mobile elements was minimized over time as the concentration of PHEs in soils at 100 cm were close or below (depending on the element) to those of unaffected soils, so the risk of groundwater contamination was avoided [36]. Moreover, the risk related to the grazing horses (the only cattle that are allowed to enter the area) was assessed by analyzing PHEs in horse hair and dung, indicating no significant differences between animals from residual polluted areas and control pastures [37]. \begin{table}
\begin{tabular}{c c c c} \hline  & **UCS**[30] & **S1** & **S2** \\ \hline
**Element** & **(mg kg\({}^{-1}\))** & & **(Times above UCS)** & \\ \hline Pb & 41.8 & 9.4 & 2.4 \\ As & 18.1 & 10.4 & 2.8 \\ Zn & 230.8 & 1.6 & 1.7 \\ Cu & 40.1 & 3.0 & 2.2 \\ \hline \end{tabular}
\end{table} 
Table 3: Concentration of PHEs in the uncontaminated soils in the area (UCS) according to [PERSON] et al., and times above these values in the acidic (S1) and neutral-alkaline (S2) sectors. The main plant species introduced in the area promoted two main functions: on one hand, they covered the bare soils, promoting the soil coverage, reducing the soil erosion and the risk of dispersion of pollutants in the solid phase of runoff waters [38, 39]; on the other hand, the vegetation establishment changed the soil properties and reduced the mobility and availability of PHEs into the affected soil [40, 41]. The main tree species introduced in the phytostabilization of the area were: _Populus alba, Celtis australis, Frazinus angustifolia, Quercus _ilex, Olea europaea, Ceratonia siliqua,_ and _Pinus pinea_), being _C.