Background levels of potentially toxic elements in ultramafic soils from the Voltri Unit: a mineralogical and geochemical approach

Ultramafic soils are characterized by the presence of several potentially toxic elements (PTE's; such as Cr, Co, and Ni) that commonly exceed residential and industrial concentration limits according to Italian laws (D.M. 471/1999; D.Lgs 152/2006). Therefore, the determination of the background levels and baseline values of PTE's within serpentinitic and peridotitic soils is of paramount importance in order to evaluate the potential risk of contamination for ecosystems as well as to distinguish between lithogenic and anthropogenic inputs. With this work we investigated the mineralogy, the mineral chemistry, and the bulk chemistry of undisturbed soils and associated parent rocks from four selected sites occurring in the "Bric del Dente Serpentinite" and "Monte Tobbio Peridotite" Formations (Voltri Unit, Ligurian Alps). Selected metal and semimetal concentrations (V, Cr, Co, Ni, Cu, Zn, As, Cd, Sb, Pb) were assessed by ICP-OES at the ARPA-Liguria laboratories according to the EPA-3050B and EPA-6010 methods. The mineralogy and the mineral chemistry of soils and rocks have been determined by means of optical microscopy (transmitted- and reflected light) and electron microprobe analyses (WDS). The bulk chemistry analyses evidenced very high concentrations of Cr (up to 3020 ppm in rocks and 1269 ppm in soils), Ni (up to 1855 ppm in rocks and 1041 ppm in soils), Co (up to 85 ppm in rocks and 64 ppm in soils), and V (up to 199 ppm in rocks and 39 ppm in soils). The mineralogical results showed that most of the Cr is contained within primary Fe-oxides and spinels (mainly magnetite and chromite). Olivine and serpentine group minerals are the main Ni-bearing minerals though non-trascurable Ni concentration have been also detected in Fe-oxides and Ni-sulphides. Among the other identified primary minerals only chlorites are characterized by significant, though very variable, amounts of Co, V, Ni. In general, most of the detected metal-bearing mineral species resulted completely unaltered either in rocks and soils evidencing their resistance to weathering and their tendency to remain as stable residual minerals within soils. Secondary authigenic minerals in soils are mainly represented by Fe-oxides and -oxyhydroxides and clay minerals. In general they contain relative small amounts of PTE's if compared to primary minerals and cannot be considered as the main source for the bulk metal content of soils. These preliminary results evidenced that all the studied rocks and soils have a baseline values of Cr, Ni, Co, and V well above (up to one order of magnitude) concentration limits of the Italian laws for industrial and residential sites. These baseline values should be considered as the natural background levels of the studied soils since most of the metals are related to the residual mineral species derived from the parent rocks. Further studies are in progress, through leaching experiments, to determine the relative mobility and bioavailability of the studied metals.