# Metal Mobility in Afforested Sites of an Abandoned Zn-Pb Ore Mining Area
## 4 Discussion
Moreover, under relatively comparable pH conditions, the main soil properties influencing metal migration can be total organic carbon and cation exchange capacity [38]. The chemical forms of metals (Figure 6) are related to the properties of the particular element rather than to the position of the profile within each heap. The average acid values of the extractable fraction indicate that Zn and Pb follow the highest potential mobility of Cd, reflecting a well-known regularity for these elements [39; 40; 41; 3]. The mobility of the elements is also generally weakly affected by depth. This is particularly evident for Zn and Cd. The mobility of cadmium is generally higher in the heap material than in the former soil, except for the Paryz III profile, where the total content of Cd is low. Lead behaves differently in terms of mobility. Considering the sum of the acid of the extractable and reducible fractions, it is higher in the former soil than in the heap material. The general lack of a clear speciation pattern suggests that the considered elements are mobilised at rather short depths, where parts of the mobile forms are mixed/occur in parallel with parent sulphide or carbonate minerals (e.g., smithsonite) or are easily bounded with Fe, which is usually present in concentrations over 1%. This may be partly due to the relatively small number of samples tested and only lithological criteria of sample selection for speciation. Similar results were obtained for heaps in the neighbouring non-forested areas [27]. The phenomenon, according to the authors, resulted from the high variability of natural soil features. The data obtained are not directly comparable with earlier metal mobility studies in soil profiles, because we used a different estimation method for metal migration than coring to the depths until traces of metals are detected [42; 43]. The observed rate of metal migration seems to be more rapid than in sandstone or clay material [42] and it is more comparable to subsurface levels of loess than alluvial sandy soils [43]. Nevertheless, the factors controlling metal migration are the same as those indicated in other studies, with some variations related to specific local conditions. The applied estimation method of metal enrichment in the former soil horizons suggests, that the mobility of metals in the investigated soil profiles is controlled mainly by pH. It is elevated over neutral, particularly in central parts of both heaps, where the percolation of water through the gravel-size dolomite material minimises metal migration despite its high contamination. The presence of organic matter seems to be the more important factor controlling pH at the heap peripheries, most of all in coniferous forest areas, confirming the earlier findings of higher metal solubility in acid forest soils [16; 19]. Finally, the role of grain size in facilitating metal migration appears to be less important than in the studies by [PERSON] et al. [43] because all the former soils are composed of comparable proportions of sand. Generally, the analysis revealed the slow metal migration rate below the heaps and it does not seem to be an important source of groundwater pollution, only being significant at the heap peripheries. Many studies have shown that afforestation, as well as changes in the forest species composition, may lead to modifications in soil properties, primarily pH and cation-exchange capacity but also leading to changes in the quantity and quality of soil organic matter [44; 45; 46]. These factors are important for the behaviour of metals. In strongly acidic forest soils, the risks of metal mobility and migration may occur in soluble or particulate forms and increases their bioavailability [47; 48]. It has been shown that when metals stored in the investigated heaps are mobilised, the calcareous waste material of the heaps has a relatively high buffer capacity which inhibits the leaching of metals.