The Po river water from the Alps to the Adriatic Sea (Italy): new insights from geochemical and isotopic (δ18O-δD) data

Although the Po river is the most important fluvial system of Northern Italy, the systematic geochemical and isotopic investigations of its water are rare and were never reported for the whole basin. The present contribution aims to fill this knowledge gap, reporting a comprehensive data-set including oxygen and hydrogen stable isotopes, as well as major and trace element concentration of dissolved species for 54 Po river water samples, mainly collected in different hydrological conditions (peak discharge in April, drought in August) at increasing distance from the source, i.e. from the upper part of the catchment to the terminal (deltaic) part of the river at the confluence with the Adriatic Sea. The isotopic compositions demonstrate that the predominant part of the runoff derives from the Alpine sector of the catchment through important tributaries such as Dora Baltea, Ticino, Adda and Tanaro rivers, whereas the contribution from the Apennines tributaries is less important. The geochemical and isotopic compositions show that the Po river water attains a homogeneous composition at ca. 100 km from the source. The average composition is characterized by delta18O -9.8‰, deltaD – 66.2‰, TDS (Total Dissolved Solids) 268 mg/L, chloride 17 mg/L and by a general Ca–HCO3 hydrochemical facies, which is maintained for most of the river stream, only varying in the terminal part where the river is diverted in a complex deltaic system affected by more significant evaporation and mixing with saline water evidenced by higher TDS and chloride content (up to 8,198 mg/L and 4,197 mg/L, respectively). Geochemical and isotopic maps have been drawn to visualize spatial gradients, which reflect the evolution of the river water composition at progressive distance from the source; more detailed maps were focused on the deltaic part in order to visualize the processes occurring in the transitional zone toward the Adriatic Sea. The data also highlight anthropogenic contributions, mainly represented by significant concentrations of nitrate (average 8 mg/L) and possibly arsenic (average 12 μg/L). These data allow the calculation of geochemical fluxes transferred from the river to the sea, and generally they contribute to the definition of a “hydro-archive” which is useful to highlight ongoing variations in the related ecosystems.