3.1. Soil properties
Soil granulometric analysis was compared between the soils sampled at the beginning of the accident (1998) and twenty years later (2018), to check whether the removal of the surface layer of the soils (Aguilar et al., 2004b) generated differences in the textural class. We found no significant textural differences between the soils sampled just after the accident and twenty years later (Table 2), thus, the soils on which T1, T2 and T3 treatments were applied have a more coarse texture (“Loam” component), while the soils where T4 treatment was applied have a finer particle size (“Silty-clay” component).
Twenty years after the accident and with the application of remediation treatments, the pH, electrical conductivity, calcium carbonate and organic carbon content significantly changed in relation to the initial soil values and depending on the applied treatment (Table 3). The soils where the treatments were applied, were divided in two different sectors (Aguilar et al., 2004a), one including moderately acidic soils, with loam to sandy loam textures and low CaCO3 content, located in the first third of the area affected by the spill (first 15 km downstream from the mine) and coinciding with the soils where T1 and T2 treatments were applied, and a second sector of neutral or slightly alkaline soils, with clay component textures and a high content of CaCO3, located in the following two thirds of the affected area (from the end of the moderately acidic soil sector to the end of the affected area) and coinciding with the soils where T3 and T4 treatments were applied.
The pH values of the soils did not significantly increase after the application of the treatments 6 years after the accident (2004), while these differences were significant at 20 years in the soils where the T2, T3 and T4 were applied. In all cases, the mean pH values are above 7, reaching values higher than 8 in the case of T3 and T4; anyway, soils with treatments T1 and T2 presented values significantly lower than soils with treatments T3 and T4 both at the beginning (1998) and twenty years after the accident.
Electrical conductivity (EC) is a parameter closely related to this type of soil contamination, since the oxidation of the sulphides in the residual tailings that were mixed with the soils after the initial cleaning actions, generated soluble salts (mainly sulphates) that increased this parameter in the soil (Simón et al., 2001). The different treatments applied were effective in reducing the EC in all cases, the differences being significant 6 years after the accident and remaining constant over time.
The application of organic amendments was also widespread throughout the area affected by the spill. In this sense, the OC content increased in the soils after the application of all treatments, although these increases were statistically significant only in the last sampling period (2018).