Nitrogen and oxygen isotopes as indicators of pollution sources in the Faxinal Dam watershed, Southern Brazil
2018
De Vargas, Tiago | Roisenberg, Ari
Isotopic signatures of δD-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻ and δ¹⁵NFₑᵣₜᵢₗᵢzₑᵣ isotopes were used in order to evaluate the impact of agricultural inputs (fertilizers and calcium nitrate) and sewage effluents on the water of tributaries of the Faxinal Dam, which supply the city of Caxias do Sul in Southern Brazil. δ²H and δ¹⁸O were identified by spectroscopy laser absorption tunable diode-type cavity ring-down spectroscopy, while the isotopic ratios of ¹⁵N and ¹⁸O in nitrate were determined by isotopic ratio mass spectrometry, applying the chemical denitrification method. The results showed that most of the analyzed samples are compatible with the Global Meteoric Water Line, Local Meteoric Water Line and Carlos Barbosa Wells and Fountains Tendency Line, except for the Fx-07 sample that presented greater similarity to δ¹⁸O values for groundwater. The isotopic signatures of δ¹⁵N-NO₃⁻ versus δ¹⁸O-NO₃⁻, in the great majority of the samples, pointed out the influence and the contribution of the fertilizers. In a monitored affluent, the results show the impact of the direct discharge of domestic sewage, while in the sewage treatment plant there is clear evidence of denitrification. This study demonstrated that isotope application of δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ is efficient tools for identification of N-NO₃⁻ of synthetic fertilizers and domestic sewage.
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