Origin and evolution of Po river water (Northern Italy): insights from geochemical and isotopic (delta18O-deltaD) data

The Po river is the most important fluvial system of Northern Italy and the related basin, the Padanian Plain, is strongly influenced by urban settlements, as well as by industrial, agricultural and zootechnical activities. In spite of its importance, systematic geochemical and isotopic investigations of its water are rare, difficultly accessible and never reported for the whole basin scale. 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 collected in different seasons at increasing distance from the spring, 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 of 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 spring. The average composition is characterized by delta18O -9.8‰, delta D – 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). A series of geochemical and isotopic maps have been carried out 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 concentration of nitrate (average 8 mg/L) and possibly arsenic (average 12 μg/L), which have to be monitored to minimize environmental risks. On the whole the data allow the calculation of geochemical fluxes transferred by the river to the sea, and more in general contribute to the definition of a “hydro-archive” which is useful to highlight ongoing variation in the related ecosystems.