Studying the spatial structure of potentially toxic trace elements (PTEs) using multivariate geostatistics in an olive orchard of southern Italy

Agricultural soil geochemistry is an important research issue because soil pollution by potentially toxic trace elements (PTEs) is of concern worldwide. High concentrations of PTEs in soil pose a threat for human health because they can be taken up into the trophic chain via assimilation by plants. Mapping the PTEs in soil is essential to delineate contamination. Geostatistical methods provide us a valuable tool to study spatial structure of the PTEs. They take into account spatial autocorrelation of data to create mathematical models of spatial correlation structures commonly expressed by variograms. The aims of this study were to quantify the spatial structure and the relationships of some PTEs (Ag, Be, Bi, Cd, Co, Cr, Cu, Ni, Pb, Sn, Tl, Zn): To do that a multivariate geostatistical approach was used. The experimental area (100 m x 100 m) was an olive orchard located in southern Italy (Calabria) where at 100 locations topsoil (0-0.20 m) samples were collected. Soil samples were ground, dried, weighed, digested in aqua regia and analyzed in laboratory for Ag, Be, Bi, Cd, Co, Cr, Cu, Ni, Pb, Sn, Tl, and Zn by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Four elements exceeded the regulatory threshold value (Limit A) in analysed soils: Be, Sn, Zn, and V, while Cr, Pb, Cu, Tl, Co, Ni and Cd were within the tolerability limits. A variographic analysis was carried out to quantify the spatial structure of PTEs and no significant difference as a function of direction was found. Mapping the spatial distribution of PTEs allowed defining their spatial relationships and delineating the potentially risky areas. Finally, stochastic images generated by the sequential Gaussian simulation were jointly combined to calculate the probability of exceeding the regulatory threshold values that might cause concern for human health.