Geochemical and Isotopic Investigation on the Po river waters from Monviso spurces to its Delta: natural and anthropogenic components
The Po river cross east-west the whole Northern Italy and flows within a densely populated area characterized by intense agriculture and industrial activities. In spite of its importance, systematic geochemical and isotopic investigations of its water are rare and never reported for the whole basin scale.
The aim of this study was to fill this knowledge gap investigating the Po river water through a chemical and multi-isotopic approach that includes oxygen, hydrogen, carbon, nitrogen sulphur and strontium isotope data.
The Po river flows within the Padanian plain, which is a sedimentary basin bound to the N by the Alpine mountains, to the SW by the Apennine hills and to the E by the Adriatic Sea and cover an area of more than 71,000 km2 (a quarter of the national territory). From the geological point of view the basin was characterized by a marine sedimentation in the Pliocene to Early Pleistocene, followed by fluvial sedimentation that enhanced during the glaciation periods. Within the plain the most important fluvial system is represented by the Po river which is progressively fed by many tributaries (and groundwater flows) from both the Alps and the Apennines. The Alpine streams are mainly fed by snow melting and show a seasonal peak flow during the spring-early summer period, while the Apennines streams have a rainy, and therefore intermittent, influence showing a minimum seasonal flow (drought) during the summer. These tributaries account for the annual regime of the Po river which is characterized by two low-level hydrometric periods (winter and summer) and two flood periods (late fall and spring). The first flood period reflects the intense late fall rains, while the second is due to snow melt from the higher sectors of the. The average discharge rate is 1,500 m3/s and maximum peak flows of 10,300 m3/s at section of Occhiobello, which is located in the lower reach of the river (close to the city of Ferrara), right upstream of the deltaic system.
The isotopic compositions (δ18O - δD ) 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. Geochemical and isotopic data show that the Po river water attains a homogeneous composition at ca. 100 km from the spring. The average composition measured at Occhiobello is characterized by δ18O 9.6‰, δD 64.0‰, TDS (Total Dissolved Solids) 260 mg/L, chloride 15 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 8,000 mg/L and 4,000 mg/L, respectively). The comparison of the data presented in this thesis with historical chemical analyses of Po river (available for the past fifty years) indicates that the major components (i.e. Ca2+, Mg2+, Na+, K+, HCO3, Cl, SO4) of the runoff water remained constant over the last decades suggesting that they reflect natural compositions unaffected by anthropogenic activities; in contrast, nitrate drastically increases from less than 1 mg/L to an average value of 9 mg/L as result of diffuse contamination. Coherently, δ13C (between -11.4‰ and -4.4‰) and δ34S (between 4.2‰ and 8.0‰) and also the 87Sr/86Sr (between 0.70896 and 0.70974) are compatible with the weathering and dissolution processes that involve the lithologies outcropping in the basin, while extremely variable δ15N (between -4.1‰ and 18.0‰) indicates contribution from pollutants of urban origin as well as components released by the agricultural and zootechnical activities. These observations confirm that although the origin of the main constituents of the Po river water is geogenic, anthropogenic contributions are also effective. The data also highlight an evolution of the dissolved nitrogen species that appear extremely reactive, especially in the deltaic part of the river, which is characterized by denitrification processes. 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.
It has to be noted that the presented data represent a snapshot of the nowadays river condition, and that future monitoring will be useful to highlight a) progressive involvement of further anthropogenic components and b) on-going environmental (climatic) changes. For these reasons this research contribute to integrate the GLObal River Chemistry (GLORICH) database (Hartmann et al. 2014, Proc Earth Plan Sci ) which is a useful tool to monitor earth surface processes at extensive scales and high resolution, and also to implement existing isotopic hydro-archives that provide additional information that cannot routinely be recovered from elementary chemistry alone (Bowen et al. 2009, Earth Plan Sc).