3.2. Total concentration of pollutants in soils
Soil contamination was initially very high, due to the entry of tailings and acidic waters into the soil matrix (Simón et al., 1999). In 1998 there were no official regulatory levels to declare a soil as contaminated in Spain, so the evaluation of potential contamination in the area was made based on the current proposal of the Regional Government of Andalucía (CMA, 1999). In this proposal, the intervention levels for agricultural soils were set at: Pb: 350 mg kg-1 (soils with pH<7) and 500 mg kg-1 (soils with pH>7), As: 50 mg kg-1, Zn: 600 mg kg-1 (soils with pH<7) and 1000 mg kg-1 (soils with pH>7), and Cu: 300 mg kg-1 (soils with pH<7) and 500 mg kg-1 (soils with pH>7). In this sense, just after the accident, the mean values of the different pollutants in the soils revealed an intense initial contamination (Table 4), since the intervention levels for As were exceeded in all the affected sectors, while that for Pb and Zn were exceeded in soils where T1 and T2 treatments were applied. The mean values for Cu did not exceed the intervention levels in any sector, although they did occasionally exceed in certain soils on which the T1 treatment was applied.
The subsequent removal of tailings and soils significantly reduced the total concentration of potentially contaminating elements in the soils, although subsequent studies indicated that this cleaning left a heterogeneous contamination pattern distributed throughout the affected area (Aguilar et al., 2004a). If we compare the initial concentrations with those analyzed 20 years after the accident, we observe that for Pb, As, and Cu there has been a significant decrease in the soils of treatments T3 and T4, but not in soils where T1 and T2 treatments were applied. Zn has been the element whose mean values have decreased in all sectors, these differences being significant 6 years after the accident. Our data confirm the decrease in mean concentrations in most of the restored area (soils with T3 and T4 treatments), although potentially dangerous concentrations of Pb and As were detected in some soils located in the first third of the area affected by the spill (soils with T1 and T2 treatments). Our results are in agreement with those reported by Martín et al. (2015), in areas randomly distributed close to the mine.
Seventeen years after the accident, the Regional Government of Andalucía published the Generic Reference Levels (GRL) to declare a soil as potentially contaminated (BOJA, 2015). In the case of the studied contaminants, the values established for the protection of human health were as follows: 275 mg kg-1 for Pb, 36 mg kg-1 for As, 10000 mg kg-1 for Zn and 595 mg kg-1for Cu. According to these values, the total mean concentrations of Zn and Cu throughout the GGC twenty years after the accident do not exceed the GRL in any of the sectors. The total concentrations of Pb exceed the GRL in 13.1% of all analysed soils, with strong differences between treatments: 35.7% of the areas where T1 was applied, 40% where T2 was applied, 5.4% of soils with T4 and was not exceeded in any case within the area where T3 was applied. As is considered the element with the highest risk of potential contamination, since 69% of the soils analysed exceed the GRL, with differences in percentages between treatments: 86% of the soils where T1 was applied, 90% where T2 was applied, 61% where T3 was applied and 62% where T4 was applied.