4.1. Soil properties
The CaCO3 content is closely related to the increase of pH in soil and it is associated with the generalized application of carbonate amendments throughout the area affected by the spill (Aguilar et al., 2004a). However, the variations in the CaCO3 content are not very significant, both between the different treatments applied and between the years considered. In treatments T1 and T2, higher doses of carbonate material were applied in relation to treatments T3 and T4, but in the case of the former, the soils on which they were applied had a lower initial content of carbonates and a higher degree of residual contamination, which caused a partial dissolution of CaCO3 that buffered the pH of the soils (Simón et al., 2005a). In the case of the T3 and T4 treatments, there are also no significant differences in the carbonate content of the soils over time, since liming was carried out at lower doses and only during the first year, since these soils were originally carbonated and had less residual contamination, anyway, no significant increase was observed because a partial dissolution of calcium carbonates occurred (Simón et al., 2008).
The reduction of the EC has been considered as a general indicator of the degree of recovery in the area (García-Carmona et al., 2017). However, the soils in which T1 and T2 treatments were applied have higher mean EC values than in the other treatments (Table 3), which is related to the presence of residual contamination in these areas (Martín et al., 2015).
The application of organic amendments during the restoration actions recovered the soil OC values before the accident, so the differences between 1998 (before the removal of the contaminated soils and the upper part of the soils) and 2004 samplings (after completion of remediation measures) are not significant. The evolution over time has favored the growth of vegetation on all the GGC soils (Pastor-Jáuregui et al., 2020), which has promoted the contribution of organic matter to the soils and has produced a significant increase in the OC content observed twenty years later.