## 4 Discussion
Recently, the areas around the refinery have been monitored to define the pollution matrix levels [53] and in some cases, As, Hg, Pb concentration in soil were above the Italian law threshold, such as in the area occupied by the Site 1. By the soil characterization carried out in this study, Site 1 was contaminated only with Pb on the soil surface, probably due both to the presence of the refinery nearby and by the geochemical background values [53]. The high amount of contaminant metals such as Cd, Pb and Zn, observed in the sites 2 and 3, was due to mine metal deposition [47; 49; 50; 54; 55]. Site 2 and Site 3 differ from each other despite being close and from the same geological matrix. Furthermore, Site 2 was expected to be more contaminated because it was closer to the source of the flood, but instead Site 3 was significantly more contaminated. Plants collected in Site 3 showed a presence of toxic metals (Cd, Pb, Zn) strictly related to the high contents observed in soils. The levels of toxic metals detected in soils of Sites 4 and 5 were much lower than the Italian law thresholds. For Site 5, on the other hand, where contamination in the soil was not expected, this was confirmed by the soil chemical analysis but, when the plants were analysed, the presence of contaminants were revealed. It can be hypothesized that in the soil, the metals are diluted, while the concentration in the plants can be high due to atmospheric deposition. Metal dust originated from industries or by metalliferous mining with inadequate closure management, could be transported to other areas, by water and aeolian dispersion [28; 33; 56]. In southwest Sardinia, pollutants by industrial activities are correlated with environment and biota [57] and metal pollution affects people living near industrial sites [58]. In the Sulcis-Iglesiente area, included in San Giovanni Suergiu municipality (where Site 5 was located), remarkable levels of metal deposition were detected between the years 2008 and 2014 as reported by the Regional Environmental Protection Agency of Sardinia (ARPAS) in their technical reports [59]. Describing the chemical behaviour of HMs in soil, it can be observed that Zn is very similar to Cd. The mobilization of Zn is favoured by conditions of low pH and oxidizing environment, which make this element rather mobile in relation to other metals. Although Zn easily forms complexes in soils with organic matter and carbonates that can lead to an accumulation of the element in the surface horizons, its association with Fe and Al hydroxides and clays seems to be predominant. There is an inverse proportionality between the solubility and bioavailability of Zn and the presence in soils of Ca and P compounds, given the chalcophilia of this element. In general, it can be said that acidic, sandy soils poor in organic matter are those that mostly favour the mobilization of Zn [60]. The applications of Zn are many, but its dispersion into the environment originates mainly from the metallurgical industry and agriculture, which can lead to significant accumulations in the surface horizons, sometimes creating significant environmental problems [9; 61; 62]. Additionally, Pb has a marked chalcophilia, and as a result of weathering processes it is able to bind to carbonates, clays, organic matter and oxides of Fe and Mn present in the soils. Its affinity with calcium and generally with group 2 elements such as K, Ba, Sr, also allows it to replace these elements both in minerals and within the soil [60; 61]. The solubility of Pb is inversely proportional to the pH values, as at high values it precipitates as carbonate, phosphate, and hydroxide, while at low pH value is weakly solubilized. All these characteristics make Pb one of the least mobile metals of all and therefore has very low natural concentrations. The typical presence of Pb in soils is concentrated in the surface horizons [56], favoured by the accumulation of organic substances, which also represents the most important \"sink\" of this element in polluted soils [63].