Riverine nitrate source identification combining δ¹⁵N/δ¹⁸O-NO₃^- with Δ¹⁷O-NO₃^- and a nitrification ¹⁵N-enrichment...

Dual nitrate isotopes (δ¹⁵N/δ¹⁸O-NO₃^-) are an effective tool for tracing nitrate sources in freshwater systems worldwide. However, the initial δ¹⁵N/δ¹⁸O values of different nitrate sources might be altered by isotopic fractionation during nitrification, thereby limiting the efficiency of source apportionment results. This study integrated hydrochemical parameters, site-specific isotopic compositions of potential nitrate sources, multiple stable isotopes (δD/δ¹⁸O-H₂O, δ¹⁵N/δ¹⁸O-NO₃^- and Δ¹⁷O-NO₃^-), soil incubation experiments assessing the nitrification ¹⁵N-enrichment factor (ε_N), and a Bayesian mixing model (MixSIAR) to reduce/eliminate the influence of ¹⁵N/¹⁸O-fractionations on nitrate source apportionment. Surface water samples from a typical drinking water source region were collected quarterly (June 2021 to March 2022). Nitrate concentrations ranged from 0.35 to 3.06 mg/L (mean = 0.78 ± 0.46 mg/L), constituting ∼70 % of total nitrogen. A MixSIAR model was developed based on δ¹⁵N/δ¹⁸O-NO₃^- values of surface waters and the incorporation of a nitrification ε_N (-6.9 ± 1.8 ‰). Model source apportionment followed: manure/sewage (46.2 ± 10.7 %) > soil organic nitrogen (32.3 ± 18.5 %) > nitrogen fertilizer (19.7 ± 13.1 %) > atmospheric deposition (1.8 ± 1.6 %). An additional MixSIAR model coupling δ¹⁵N/δ¹⁸O-NO₃^- with Δ¹⁷O-NO₃^- and ε_N was constructed to estimate the potential nitrate source contributions for the June 2021 water samples. Results revealed similar nitrate source contributions (manure/sewage = 43.4 ± 14.1 %, soil organic nitrogen = 29.3 ± 19.4 %, nitrogen fertilizer = 19.8 ± 13.8 %, atmospheric deposition = 7.5 ± 1.6 %) to the original MixSIAR model based on ε_N and δ¹⁵N/δ¹⁸O-NO₃^-. Finally, an uncertainty analysis indicated the MixSIAR model coupling δ¹⁵N/δ¹⁸O-NO₃^- with Δ¹⁷O-NO₃^- and ε_N performed better as it generated lower uncertainties with uncertainty index (UI₉₀) of 0.435 compared with the MixSIAR model based on δ¹⁵N/δ¹⁸O-NO₃^- (UI₉₀ = 0.522) and the MixSIAR model based on δ¹⁵N/δ¹⁸O-NO₃^- and ε_N (UI₉₀ = 0.442).