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