Geochemical baseline values of chalcophile and siderophile elements in soils around the former mining area of Abbadia San Salvatore (Mt. Amiata, Southern Tuscany, Italy).

Abstract

Determining the background values of chemical components in environmental matrices is a difficult task. This is particularly true in regions where the human impact due to industrial, mining, agricultural and urban activities coexists with a geological (geogenic) anomaly, which influences the concentration of certain elements in soils, waters and air. In these cases, the term geochemical baseline (GB) is preferable, since it considers the actual content of that element in the superficial environment at a given point in time, including both geogenic and anthropogenic contribution. In this study, a total of 102 top- and sub-soil (collected at 10-50 cm and 50-154 cm depth, respectively) samples and seven rocks, onto which the soils developed, were collected for the determination of GBs for selected chalcophile (As, Cu, Hg and Sb) and siderophile (Co, Cr, Ni, and V) elements in 25.6 km2 around the former mining area of Abbadia San Salvatore (Mt. Amiata, Southern Tuscany, Italy). For about one century, cinnabar (HgS) ore deposits have been exploited to produce liquid mercury from the Mt. Amiata volcanic system and its surroundings, which represents a world-class mercury district. The < 2 mm (as required by the national regulamentation) fraction of the samples was pulverized and analysed by ICP-MS (As, Hg and Sb) and ICP-AES (Co, Cr, Ni, and V) after aqua regia digestion. The compositional data analysis of multivariate compositional vectors, based on the log-ratio approach was used to assess the nature of the geochemical . According to our findings, the centred log-ratio (clr) opposed to that of raw/log transformation, enhances the spatial mapping. This also allowed to obtain better-separated variables in the robust Principal Component Analysis (rPCA). Log-ratio geographical maps evidenced that the underlying bedrock geology (parent lithologies), rather than anthropogenic causes, controls the distribution of the  great majority of the elements in the top- and sub-soils. The resulting clr-PCA approach, associated with the geological features, indicates that the geochemical pattern of Hg-As is to be related to the volcanic rocks and ore-deposits, although an anthropogenic influence due to the past mining activity in the topsoils cannot be ruled out. Sb, Co, Cr, Ni, and V distribution patterns are in most cases attributed to calcareous and clay lithologies. The anomalous content of Sb found within the volcanic rocks was likely due to the presence of previously undetected old mining dump. The two data populations (volcanic and calcareus-clay lithologis) were separated into two different databases and the outliers were removed when necessary. By processing the two datasets, the US-EPA’s ProUCL software was used for calculating the GBs for the selected suite of elements. The obtained values are paramount for establishing specific guidelines and quality standards in environmental legislation and policy-making to be applied by the Municipality of Abbadia San Salvatore