Statistical and geostatistical analysis of heavy metals distribution in the mine waste dump of Coren del Cucì (Gromo, Val Seriana, BG)

Since Roman Age and for long time in the past the Coren del Cucì mining area, located close to Gromo village in Upper Val Seriana (BG), has been exploited for Fe, Cu, Pb, Zn and Ag (Jervis, 1881). Mineralization consists of pyrite + chalcopyrite + arsenopyrite + tetrahedrite ± sphalerite ± galena (De Capitani et al., 2003) hosted into a quartz breccia at the boundary between the Crystalline Basement and the late Paleozoic Collio Formation (Servida et al., 2006). Differently from other small waste disposals in the area, the occurrence of the main dump is highlighted by a colour of the covering vegetation (Calluna Vulgaris) which is different from the other vegetation in the area. In the first stage a judgemental sampling survey (EPA, 2002) has been planned; using FOREGS sampling methods (Salminen et al., 1998) about 60 soil samples and 15 vegetation samples have been collected. All the samples have been analyzed with ICP-AES for the determination of major elements (Ca, Fe, Mg) and heavy metals (Ag, Cd, Co, Cu, Ni, Pb, Zn) concentration. Moreover, to get more information about the possible geochemical mobility of elements, pH values of soils have been determined. Starting from obtained data and by means of the calculation of statistical descriptive parameters to remove outlier values of concentration, a statistical analysis has been performed which allowed to subdivide data in populations: in particular, the outstanding features are the presence of a single population matching with the local background for Mg and Fe and the occurrence of two populations matching with local and dump background for Ca. Also for Co, Cu, Ni, Zn two populations of data have been found, while for Ag, Cd and Pb a third population emerged matching with the ore itself. The geostatistical analysis of the element distribution in the area has been performed by the elaboration of contour maps. The overall of collected data and their elaboration led to draw a geochemical profile of the dump and to establish relations between the geochemical behaviour of several elements and the geological-environmental variables. In particular Ag, Co, Cu, Ni and Zn show higher concentrations inside the dump and several anomalies located close to the main tunnel entrances, while for Cd emerges a higher concentration only in vegetation outside the dump. Possible zones in danger of heavy metals pollution have been highlighted through the elaboration of comparison maps between the concentration of heavy metals and the threshold values of soils pollution according to the law in force (D.M. 471/99, D.Lgs. 152/2006), allowing thus to estimate the hazard related to the site. Finally, a strong relation has been observed between mine waste disposal and Calluna Vulgaris (De Capitani et al., 2003), the only vegetation growing on this terrain. The analysis performed on Calluna Vulgaris samples allowed to determine Biological Absorption Coefficient (BAC) for different metals; analytical results confirmed the capability of this species of growing on acid substrates with occurrence of toxic elements and poor of macro-nutrients, but without bioaccumulation.