Nitrogen and oxygen isotopes in nitrate and nitrite in the polluted surface waters from the Arno River Basin (Central Italy)

The Arno River Basin (Central Italy) is affected by a considerable anthropogenic pressure due to the presence of large cities and widespread industrial and agricultural practices. In this work, 26 water samples from the Arno River and its main tributaries were analyzed to assess the water pollution status. The geochemical composition of the Arno River changes from the source (dominated by a Ca-HCO₃ facies) to the mouth (where a Na-Cl(SO₄) chemistry prevails) with an increasing quality deterioration, as suggested by the Chemical Water Quality Index, due to anthropogenic contributions and seawater intrusion before flowing into the Ligurian Sea. The Ombrone and Usciana tributaries introduce anthropogenic pollutants into the Arno River, whilst Elsa tributary supplies significant contents of geogenic sulfate. The concentrations of dissolved nitrate and nitrite (up to 63 and 9 mg/L, respectively) and the respective isotopic values of δ¹⁵N and δ¹⁸O were also determined to understand origin and fate of the N-species in the Arno River Basin surface waters. The combined application of δ¹⁵N-NO₃ and δ¹⁸O-NO₃ and N-source apportionment modelling allowed the identification of soil organic nitrogen and sewage and domestic wastes as primary sources for dissolved NO₃^-. The δ¹⁵N-NO₂ and δ¹⁸O-NO₂ values suggest that the nitrification process affects the ARB waters, thus controlling the abundances and proportion of the N-species. Our work indicates that additional efforts are needed to improve management strategies to reduce the release of nitrogenated species to the surface waters of the Arno River Basin, since little progress has been made from the early 2000s.