4.3. Solubility and availability of contaminants in soils
The behavior observed in Pb could be related to the higher vegetation development where T3 and T4 treatments were applied and the affinity of the organic matter to form organic complexes with Pb easily extracted with EDTA (Sierra et al., 2019). In the case of As, it does not form stable complexes with EDTA (Přibil, 1982), which could explain the low values of bioavailability extracted. Arsenic is a low mobility element and its availability depends on soil properties such as Fe oxides, pH, calcium carbonate, clay content, cation exchange capacity and organic matter content (Martín et al., 2012, Romero- Freire et al., 2014).
The influence of carbonates and pH on the solubility of Zn is widely reported in the literature (Kraus and Wiegand, 2006, Madejón et al., 2006, Clemente et al., 2008, Romero-Freire et al., 2016b), where the decrease in the solubility of Zn is directly related to the increase in pH. Regarding the decrease in the bioavailability of Zn, the effect of liming on the soils of the area is recognized as a very effective measure according to Simón et al. (2005b), and, according to our data, remains over time.
Cu mobility is strongly related to pH, increasing its solubility in acidic soil conditions (García et al., 2009). Rocco et al. (2018), reported that the absorption by plants was positively related to the bioavailability of potentially harmful elements in the soil, presenting Zn greater mobility with respect to Cu. Moreover, Cu is strongly retained by the soil, is less mobile and less sensitive to changes in soil pH compared to Zn, with relative adsorption at pH values below 5. For this metal, the content of organic matter and iron oxides are the most significant parameters that govern their adsorption processes in soils (García-Carmona et al., 2019).
The influence of climate in our study area is also a key factor to be considered; in this sense, Liu et al. (2020) indicate that high temperatures accelerate the transformation of soluble fractions of potentially harmful elements, contributing to the increase in mobility of heavy metals. In acidic soils, solubility increases because the degradation of particulate organic carbon and the dissolution of Fe-Mn particles are accelerated (Zhang et al., 2014).
The results of the calculation of RA (Table 6), show a positive evolution in all treatments, since the values are below 1 in all cases. In the case of the solubility of the studied elements, the reduction is very strong after the application of the remediation treatments (2004), although this reduction is maintained over time in the case of T1 and T2 treatments, but increases in T3 and T4 treatments. This increase in soluble forms were mainly observed for Pb and As, which in these soils seem to be related to the competing effects between these elements and the soil organic matter content (Sierra et al., 2019). In the case of bioavailability, the reduction is greater for all treatments applied and this reduction strongly increases over time, indicating a general reduction in the availability of potentially harmful elements throughout the study area regardless of the treatments applied. However, the presence of residual contamination and the specific evolution in some sectors (Pastor-Jáuregui et al., 2020), makes it necessary to monitor the whole area and to apply remediation measures in those sectors where a potential risk for toxicity is detected.