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.