Assessment of temporal and spatial differences of source apportionment of nitrate in an urban river in China, using delta N-15 and delta O-18 values and an isotope mixing model

摘要

Nitrate contamination in surface water has become an environmental problem widespread concern. In this study, environmental isotopes (delta N-15-NO3- and delta O-18-NO3-) and the chemical compositions of water samples from an urban river in Chongqing, China, were analyzed to evaluate the primary sources of nitrate pollution. A Bayesian isotope mixing model was applied to estimate the relative contributions of five potential NO3- sources to river pollution (sewage/manure, soil N, NH4+ in fertilizer and precipitation, NO3- fertilizer, and NO3- in precipitation). The results show that the urban river was affected by NO3- pollution from multiple sources. The major sources of NO3- pollution in the dry season were sewage/manure (38-50 %) and soil N (22-26 %); in the wet season, the major sources of NO3- pollution were sewage/manure (30-37 %), soil N (16-25 %), and precipitation (14-24 %). The higher contribution of N to the river water by precipitation indicates that atmospheric N deposition has become an important source of pollution in surface water in China. We conclude that domestic sewage is still the main contributor to NO3- pollution in urban rivers in China. The discharge of domestic sewage into rivers should be prohibited as a priority measure to prevent NO3- contamination.