CNS stable isotopes in the Adige river waters: insights on natural and anthropogenic components

Previous investigation by Natali et al. (2016) indicated that the Adige river waters display a common Ca-HCO3 hydrochemical facies, reflecting a natural (geogenic) signature related to the weathering processes of the rocks outcropping in the basin; on the other hand nitrate increased in the last decades reflecting the growing of anthropogenic inputs. To crosscheck these statements 18 water samples have been collected along the riverine profile in May 2015 in order to analyze the isotopic compositions of Carbon, Nitrogen and Sulphur. These isotope analyses were performed in the laboratories of the Helmholtz-Zentrum für Umweltforschung (UFZ) Leipzig/Halle (Germany). Isotope analyses of carbon and associated oxygen were carried out on dissolved inorganic component (DIC) using the method described by Atekwana and Krishnamurthy (1998). Isotope analyses of sulphur and associated oxygen were carried out on sulphate recovered using the method described by Knöller et al (2005). Isotope analyses of nitrogen and associated oxygen were carried out on dissolved inorganic nitrogen (DIN) using the bacteria denitrification method described by Sigman et al (2001) and Casciotti et al (2002). This method involved the use of bacteria to transform NO3 in N2O. Isotope measurements were performed using a Delta V plus mass spectrometer (Electron with a Gasbench II, Thermo Electron GmbH). The δ13CDIC isotopic composition varies regularly along the riverine profile between -4.5‰ and -9.5‰. Analogously, the δ34S SO4 isotopic composition varies regularly along the riverine profile between +4.4‰ and +11.4‰. On the other hand δ15N NO3 isotopic composition varies between +3.9‰ and +10.5‰ with a more irregular pattern along the riverine profile. Isotopic data have been compared with another important riverine system in the same season, the Po river, described in Marchina et al. 2016 to highlight potential analogies or difference of the two Alpine rivers. Coherently with what observed in the Po River, carbon and Sulphur isotopic compositions reflect a geogenic signature, whereas the nitrogen isotopic composition are influenced by local effects related to anthropogenic contributions.