Vol.:(0123456789) 1 3 Environmental Earth Sciences (2018) 77:229 https://doi.org/10.1007/s12665-018-7395-5 ORIGINAL ARTICLE Nitrogen and oxygen isotopes as indicators of pollution sources in the Faxinal Dam watershed, Southern Brazil Tiago De Vargas 1,2  · Ari Roisenberg 1 Received: 9 May 2017 / Accepted: 28 February 2018 / Published online: 15 March 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Isotopic signatures of δD-H 2 O, δ 18 O-H 2 O, δ 15 N-NO 3 − , δ 18 O-NO 3 − and δ 15 N Fertilizer isotopes were used in order to evalu- ate the impact of agricultural inputs (fertilizers and calcium nitrate) and sewage efuents on the water of tributaries of the Faxinal Dam, which supply the city of Caxias do Sul in Southern Brazil. δ 2 H and δ 18 O were identifed by spectroscopy laser absorption tunable diode-type cavity ring-down spectroscopy, while the isotopic ratios of 15 N and 18 O in nitrate were determined by isotopic ratio mass spectrometry, applying the chemical denitrifcation method. The results showed that most of the analyzed samples are compatible with the Global Meteoric Water Line, Local Meteoric Water Line and Carlos Bar- bosa Wells and Fountains Tendency Line, except for the Fx-07 sample that presented greater similarity to δ 18 O values for groundwater. The isotopic signatures of δ 15 N-NO 3 − versus δ 18 O-NO 3 − , in the great majority of the samples, pointed out the infuence and the contribution of the fertilizers. In a monitored afuent, the results show the impact of the direct discharge of domestic sewage, while in the sewage treatment plant there is clear evidence of denitrifcation. This study demonstrated that isotope application of δ 15 N-NO 3 − and δ 18 O-NO 3 − is efcient tools for identifcation of N-NO 3 − of synthetic fertilizers and domestic sewage. Keywords Nitrate · Stable isotopes · Nitrate source · Fertilizers · Domestic sewage Introduction The watersheds used for public water supply usually do not receive adequate planning, control and management to pre- serve the quality of water resources. Nitrate is a common contaminant in water sources and in excess can contribute to the eutrophication of water accumulated in the reservoirs, and the intake of this water can cause diseases like methe- moglobinemia and stomach cancer (Gehle 2015). In surface water used for human supply, nitrate ion concentration val- ues should not exceed 50 mg L −1 to avoid harmful efects on human health (WHO 2017), and the Brazil Health Ministry Ordinance No. 2914 of 2011 indicates a concentration limit of 10 mg L −1 for N-NO 3 − , in drinking water. Nitrate may originate from point or difuse sources, such as domestic sewage and fertilizers, and may be present at various concentrations in the environment. The identifcation of the origin of NO 3 − in small watersheds can be accom- plished by measuring the variation in 15 N and 18 O isotopes. In general, the Cartesian relation between δ 15 N-NO 3 − and δ 18 O-NO 3 − leads to typical intervals of the main sources of nitrate as well as nitrifcation and denitrifcation processes. Several studies conducted to determine the characteris- tic intervals of δ 15 N-NO 3 − and δ 18 O-NO 3 − were cited by Kendall et al. (2007) to demonstrate the variation range of the main sources of nitrate. The variation in the inorganic fertilizers is represented by δ 15 N-4 to + 4‰ (σ: − 4 and + 3‰) and δ 18 O of + 17 to + 25‰, whereas organic ferti- lizers present greater intervals, with an amplitude of + 2 to + 30‰ of δ 15 N. Microbiological action responsible for the nitrifcation process of fertilizer (NH 4 + ) refects a range for the δ 18 O of − 5 to + 15‰. Human and animal wastes typically have a * Tiago De Vargas vargas.geo@gmail.com Ari Roisenberg ari.roisenberg@ufrgs.br 1 Institute of Geosciences of the Federal University of Rio Grande do Sul (UFRGS), Av. Bento Gonçalves, 9500, P.O. Box 15001, Porto Alegre 91501-970, Brazil 2 Service Autonomous Municipal of Water and Sewage (SAMAE), Nestor Moreira street, n° 719, neighborhood Lourdes, Caxias do Sul, RS 95052-500, Brazil