• compositional data
• Tiber river basin
• water chemistry

This thesis illustrates the results of a three years Ph.D. research concerning the Tiber River basin (TRB), the largest catchment in central Italy, draining an area of 17,156 km2. The basin is mainly featured by terrigenous deposits in its upper part, the carbonatic Apennine ridge towards south-east and potassic and ultra-potassic volcanic complexes in the south-western area. This heterogeneous geological environment coupled with a significant presence of anthropic activities has made the TRB, a catchment that has driven very little attention so far, the ideal case of study to evaluate the nature of the variability for geochemical landscapes. Dissolved major and trace species for 222 water samples from the Tiber River and its tributaries define the river geochemistry under different flow conditions. Additionally, the chemical composition of 57 stream sediments assesses chemical-physical weathering processes, whereas analysis made on their magnetic susceptibility monitor possible pollution sources. The gathered data were processed by means of advanced compositional methods and graphical-numerical elaborations based on the analysis of data variability, pairwise robust compositional distances and frequency distribution shapes. These methods were developed in the framework of the Compositional Data Analysis (CoDA) approach, exploring the collective behaviour of river chemistry and its interlinks to possible environmental drivers. As a result of the analysis, geochemical landscapes of the TRB were determined, revealing a high level of structural complexity characterised by a multi-scale variability linked to water-rock interaction processes and to the influence of diffuse human activities. The TRB resilience to compositional changes both in space and time was defined and proved to be a useful tool to increase our knowledge on the response of the river system to potential contamination events or climate-induced modifications. Moreover, this research has highlighted the remarkable influence of landscape properties of the basin, such as slope and elevation patterns, on the chemical composition of surface waters, opening up new interesting perspectives for the study of complex river systems from a multidisciplinary point of view.