Nitrate pollution source apportionment, uncertainty and sensitivity analysis across a rural-urban river network based on δ15N/δ18O-NO3- isotopes and SIAR modeling

Nitrate pollution is of considerable global concern as a threat to human health and aquatic ecosystems. Nowadays, delta N-15/delta O-18-NO3- combined with a Bayesian-based SIAR model are widely used to identify riverine nitrate sources. However, little is known regarding the effect of variations in pollution source isotopic composition on nitrate source contributions. Herein, we used delta N-15/delta O-18-NO3-, SIAR modeling, probability statistical analysis and a perturbing method to quantify the contributions and uncertainties of riverine nitrate sources in the Wen-Rui Tang River of China and to further investigate the model sensitivity of each nitrate source. The SIAR model confirmed municipal sewage (MS) as the major nitrate source (58.5-75.7%). Nitrogen fertilizer (NF, 8.6-20.9%) and soil nitrogen (SN, 7.8-20.1%) were also identified as secondary nitrate sources, while atmospheric depo-sition (AD, <0.1-7.9%) was a minor source. Uncertainties associated with NF (UI90 = 0.32) and SN (UI90 = 0.30) were high, whereas those associated with MS (UI90 = 0.14) were moderate and AD low (UI90 = 0.0087). A sensitivity analysis was performed for the SIAR modeling and indicated that the isotopic composition of the predominant source (i.e., MS in this study) had the strongest effect on the overall riverine nitrate source apportionment results.

Automated summary

This study examined the sources and contributions of riverine nitrate in the Wen-Rui Tang River in China using various methods. The SIAR model confirmed municipal sewage as the major source of nitrate, with nitrogen fertilizer and soil nitrogen also identified as secondary sources. The study found that the uncertainties associated with different sources varied, and the isotopic composition of the predominant source had the strongest effect on the overall results of nitrate source apportionment.