Identification of Rare Earth Elements (REEs) distribution patterns in the soils of Campania region (Italy) using compositional and multivariate data analysis

A big monitoring programme was carried out in Italy from 2015 to 2018 with the aim to assess the environmental conditions of the Campania region territory. Among the different environmental media, 7300 samples of topsoil (10-15 cm) were collected and analysed for 52 elements through an analytical methodology combining ICP-MS and ICP-ES. Specifically, this paper focuses on the geochemical distribution of La, Ce, Sc and Y with implications for the whole REEs group. Machine learning techniques, Compositional Data Analysis (CoDA) and multivariate statistics were used in order to understand the behaviour of these elements in different geogenic and anthropogenic conditions and their interaction with other elements. Lasso regression (LR) was performed to select the variables most related with REEs. Subsequently, robust Principal Component Analysis (rPCA) was performed, using 20 elements selected through the LR to identify the main geochemical associations. Finally, using rPCA scores and additive log-ratio (alr) coordinates, distribution maps were generated to assess the patterns of the REEs sources identified. The results show that median concentration values of the investigated elements (Ce = 80.3, La = 41.3, Y = 15.4 and Sc = 3.3 mg/kg) are significantly higher than those of European and Italian soils. The type of lithologic substrate and the degree of soil alteration are the main factors responsible of the REEs concentrations. Cerium and La have the same behaviour and show higher concentrations in volcanic soils, especially the more altered ones, which are associated with high values of low-mobile elements like Th, Fe, Hf, Zr, Mn, or where the pyroclastic cover lies on limestone rocks. Otherwise, Sc is enriched in soils formed on siliciclastic and calcareous rocks while Y shows an intermediate behaviour between Sc and, La and Ce. This study has shown that Th, Fe and Mn are the elements most closely related to REEs in soils of Campania region, therefore they can be used to estimate their natural concentration in soil.