Potentially harmful elements in soils and holm-oak trees (Quercus ilex L.) growing in mining sites at the Valle de Alcudia Pb-Zn district (Spain)-Some clues on plant metal uptake

This work is aimed to the establishment of some clues concerning metal uptake by the holm oak (Quercus ilex L.) in abandoned base metals (Pb-Zn) mining sites from central Spain. The study is based on total contents of potentially harmful elements (PHEs), including Pb, Zn, Cu, As, Sb, Cd and Hg, in leaves from holm-oak trees as well as in the corresponding soils, in three different sites included in the Alcudia Valley Mining District. Besides, we present other analytical data for the corresponding soils: pH; electrical conductivity; organic matter; and selective extractions: i) Modified Geological Survey Field Leach Test (using deionized water); extraction with ammonium acetate solution at two different pH values; EPA Method 1312 (using a H2SO4:HNO3 solution); and extraction with an EDTA solution. This is aimed to search for relationships between PHE concentrations in the leaves and the different parameters measured in the corresponding soil, in order to determine bioavailability of PHE in this type of environments. The results show areas impacted by different degrees of PHE; in the most polluted area Pb concentrations reach 300 times the regional baseline and background values indicated by World Health Organization. Also, detected concentrations of the elements in leaves show that the holm-oak tree is a species with a very low bioaccumulation capacity, as shown by a selective uptake of PHE, favouring Zn, Cd and Cu with respect to Pb and As, and with Hg as a peculiar case, influenced by foliar uptake. However, metal concentrations in leaves reach levels above published toxicity levels thresholds, in particular for Pb and Zn. After this study, the soil to plant transfer capacity depends on different parameters for each element, being extremely difficult to generalise a common scenario favouring or preventing this process. Instead, we have found a complex case, conditioned by the soil proprieties (reactivity, salt contents and presence and concentration of organic matter) and to lesser extent, by PHE speciation in the soils.