3.3. Solubility and availability of contaminants in soils
In studies involving contamination of inorganic elements, besides to analyse the total concentration, it is essential to evaluate the solubility and bioavailability of pollutants in soils. These forms are directly related to the risk of dispersal of pollutants within the soil and throughout the rest of the ecosystem, as well as the risk of toxicity when absorbed by organisms, both animals and plants (Kim et al., 2015). In our case, we have analysed the concentrations of elements soluble in water, as the fraction with the highest risk of dispersion in the shortest term, and the concentrations of elements extractable with EDTA, as the potentially bioavailable fraction in the medium-long term (Quevauviller et al., 1998). EDTA tends to extract soluble, exchangeable and weakly adsorbed elements from the soil (Schramel et al., 2000, Álvarez et al., 2006), and at pH 7.0 it can also dissolve elements that are bound to humic substances in the soil. Due to the high stability of the complexes, EDTA could also dissolve elements that are present in the soil, such as water-insoluble carbonates (Yotova et al., 2018).
The heterogeneity of the contamination in our study area, described in the previous section, makes that the total concentration of the pollutants shows great variability; therefore, to evaluate the evolution of the mobility of the pollutants after the application of the different treatments in a comparative way, we have calculated the relative percentages of soluble (S) and bioavailable (E) fractions in relation to the total concentrations (Table 5).
The water soluble and potentially bioavailable forms present differences depending on the element considered and the treatment applied. In the case of Pb, after the application of treatments (2004), the solubility and bioavailability significantly decrease in all cases, and it remains low over time, however, 20 years after the accident, the soils where T3 and T4 treatments were applied have higher relative concentrations of soluble and bioavailable Pb than where T1 and T2 treatments were applied. However, although the total concentration of Pb is lower in T3 and T4 (Table 4), the soluble and bioavailable concentrations are not lower than in T1 and T2.
Arsenic shows a different behavior, since its relative solubility in water does not decrease over time for treatments T1 and T2, while an increase in treatments T3 and T4 is observed twenty years after the accident, furthermore, as with Pb, T3 and T4 treatments show a higher relative solubility of As in relation to T1 and T2 treatments. The bioavailability of As significantly decreases in all cases after the applied treatments and remains low over time; however, T3 and T4 treatments show a higher bioavailability of As in relation to T1 and T2 treatments in relative terms, considering that the total As concentration in T3 and T4 is significantly lower than in T1 and T2.
The solubility and relative bioavailability of Zn are significantly reduced after the application of treatments and this reduction continues until 2018. In 2004 it is observed that the relative solubility of Zn in soils where T1 and T2 treatments were applied was higher than where T3 and T4 were applied, although the opposite occurs with the total concentrations (Table 4). Therefore, the remediation measures were more effective in the case of Zn in the most distant sector from the mine, where although liming was less intense, the carbonated character of the soils influenced in the reduction of its solubility.
Cu does not significantly reduce its relative solubility over time, and an increase in T4 treatment is observed 20 years after the accident. There are no significant differences in the relative solubility of Cu between the different treatments, which is related to the low mobility of this element in soils. Cu does not reduce its relative bioavailability after the application of treatments (2004), although it does significantly decrease in the long term (2018), it is also observed that the relative bioavailability of Cu is higher in soils where T3 and T4 treatments were applied than where T1 and T2 were applied.
The percentages of the soluble and bioavailable concentrations of Pb, As, Zn and Cu have been represented in the form of radial graphs (Figure 2), to estimate the effects of each treatment on the studied elements, the smaller the polygon, the lower the risk of potential toxicity (Xiong et al., 2015).
From the data in Figure 2, the soil remediation index (RA) was calculated according to Xiong et al. (2015), who define it as the relationship between the surface of each polygon and the surface of the initial polygon (1998), therefore considering RA equal to 1 for the year 1998. In this sense, the lower the value of RA, the greater it will be the reduction of the soluble or bioavailable forms and, therefore, greater efficiency has been in the recovery treatments. The results of the calculation of RA are shown in Table 6