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

Nitrate contamination in surface water has become an environmental problem widespread concern. In this study, environmental isotopes (δ(15)N-NO3 (-) and δ(18)O-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.