# Evolution of the Residual Pollution in Soils after Bioremediation Treatments
## 3 Results and Discussion  
### Evolution in Soil Properties over Time  
Soils affected by the spill were related to the fluvial regime, involving the main soil groups usually found in this environment: Fluvisols and Regosols according to International Union of Soil Sciences (IUSS) Working Group World Reference Base for soil resources (WRB) [20] or Entisols according to SSS [21]. These soils are usually characterized by low development profiles and high fertility, producing areas with high anthropic influence were crops and grazing are dominants [22; 23]. According to the main properties, the GGC was divided into two sectors: (1) Sector 1 (S1), involving soils with acidic pH (5-6), mean CaCO\({}_{3}\) content <2% and organic carbon <0.9% in the surface horizon; and (2) Sector 2 (S2), involving soils with neutral or slightly alkaline pH (7-8), mean CaCO\({}_{3}\) content around 10% and organic carbon around 2% in the surface horizon [16]. The initial impact of the spill produced acidification and increase of electrical conductivity (EC) in the uppermost 10 cm of the soils. This was related to the oxidation of the sulphides generating a release of complex sulphates and the solubilization of the main pollutants (Pb, As, Zn, and Cu) [11]. The initial remediation actions started with the removal of the tailings and the uppermost part of the soils strongly polluted, followed by the application of different organic and inorganic amendments to promote the improvement of the soil properties [24]. In this period, phytoextraction techniques wereapplied in experimental plots into the affected area; the main species used was _Brassica juncea_, but the results indicated a low total uptake of heavy metals in the plants and pointed out the problems related to the use of phytoextraction in the clean-up of multi-element contaminated sites [25]. Other studies developed in the GGC also indicated the low success of the phytoextraction technique applied to this area, so the restoration actions were focused in phytostabilization or phytoimmobilization [25, 26]. Twenty years after the accident and the implementation of the recovery and bioremediation measures, soils significantly changed in the most important parameters, with differences in relation to the initial soil properties and the sector involved (Table 1). In this way, the acidic sector (S1) increased the pH in the uppermost 10 cm of the soils, without significant differences in depth. The increase in pH is related to the limiting treatments applied during the first two years [16]; however, this rise in pH came at the expense of the dissolution of the CaCO\({}_{3}\) after a process related to the tailing oxidation of sulphides remained in the soil matrix, so the differences in the CaCO\({}_{3}\) content was not evident over time. The OC content was significantly increased in the uppermost 10 cm of the soils. This rise in OC is directly related to the success of the phytostabilization in the whole area, although twenty years later, the increase in OC is only significant in the surface depth and no differences were observed in depth. One of the parameters more sensitive to this type of pollution is the EC, related to the oxidation of sulphides to sulphates [11]; in this case, the reduction in the EC over time was significant after the accident, indicating a positive evolution of the soils, although high values of EC were still detected in some plots of the S1 sector, indicating the presence of residual pollution [14]. In the neutral-alkaline sector (S2), the changes in soil properties twenty years after the accident were similar to those described for the S1 sector. The initial values of pH increased by the addition of calcium carbonate, but only significantly affected to the uppermost 10 cm of the soils in this sector. The OC content also increased in the top part of the soils by the inputs of organic matter coming after the implementation of the vegetation (Figure 2).