Sources of nitrate in the Arno River waters: Constraints from d¹⁵N and d¹⁸O

Running waters in anthropogenically affected areas are susceptible to nitrate contamination. Source identification is a fundamental step for the development of effective remediation. Previous studies pointed to pollution by nitrogen-bearing contaminants in the Arno Basin. In this paper, eleven surface water samples have been analysed for main and trace components and ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios, with the aim of identifying for the first time the origin of nitrate in the Arno River Basin so that further investigations can appropriately be designed. d¹⁸O(NO₃)and d¹⁵N(NO₃) values have allowed to hypothesise the main sources of nitrate, as follows: i) mineralized fertilizer, ii) soil-organic nitrogen, iii) manure and septic waste. The anomalously high d¹⁵N and d¹⁸O values in the Chiana (d¹⁵N=24.9‰ and d¹⁸O=15.5‰) and Usciana tributaries (d¹⁵N=30.1‰ and d¹⁸O=7.2‰) show a low probability of belonging to the same population as that of the other samples and can be related to denitrification process of nitrate from animal waste/sewage and/or an industrial process (e.g. tanneries).