Soil geochemical follow-up in the Cilento World Heritage Park (Campania, Italy) through exploratory compositional data analysis and C-A fractal model

Campania Region (southern Italy) is characterized by several elemental soil anomalies both geogenic and anthropogenic. Parts of these anomalies occur into the World Heritage Territory known as “National Park of the Cilento and Vallo di Diano”, where this follow-up study has been carried out. In this paper, a methodology based on compositional data analysis (CoDA) and factor score C-A fractal model was applied on geochemical data from the above study in order to identify geochemical signatures associated anomalies. Eighty-one top soil samples were collected over an area of 98 km2, and analyzed by ICP-MS after aqua regia digestion. Frequency based method (edaplots, classical and robust compositional biplot) and frequency space-method (factor score maps) were applied to visualize the correlation between variables and their main features into the survey areas. The different geochemical patterns were distinguished by a multivariate analysis combined with Concentration-Area (C-A) fractal method. Results show that geochemical data should be transformed under a compositional perspective to avoid artefacts, prior to statistical computations. Indeed, ilr-transformed data show a distinct bimodal distribution for several elements. This type of distribution appears masked considering raw and lognormal data. A “robustification” of the variables dataset permits to found a more clear relationship between variables. Factor score maps based on ilr-transformed variables and C-A plot cut off threshold displayed different geochemical patterns. In our survey area soil alteration phenomena could mask the nature of parental rock. The factor scores maps highlight an antithetic behaviour of many elements. Elements characterized by elevated geochemical mobility, such as Ca and Mg, are negatively correlated with Chemical Alteration Index values. Instead, Sn, Th, Be, Al are characterized by elevated geochemical stability; for this reason they are found where high-weathered soils occurs. The presence of elements such as Co, Cu Fe, Ni, Cr, Zn, K, Mn is mainly controlled by terrigenous flysch deposition. As, Pb (exceeding the CSC) and Sb association mainly occur in correspondence of urban areas and where traffic jams are frequent.