Isotopic compositions reveal the driving forces of high nitrate level in an urban river: Implications for pollution control

Abstract

High levels of nitrogen (N) in urban rivers are a global concern, but key evidence is lacking about their driving forces, which has limited the efficiency of water quality management and the sustainability of water resources. In this study, multiple stable isotopes (δ15N, δ13C, δ18O, and δD) and an isotopic mixing model were used to reveal the causes of high nitrate (NO3−) levels in a typical urban basin and to identify appropriate methods to control N pollution in urban rivers. The NO3− concentrations and their correlations with isotopes (δ15N–NO3– and δ18O–NO3–) indicated weak NO3− removal mechanisms (such as assimilation and denitrification). In-stream nitrification was typically prevailing, suggesting that NO3− in urban rivers could be extensively generated from in-stream process in addition to direct inputs from external sources. More than 60% of the annual NO3− flux of the river was from urban sources. A waste water treatment plant was an important yet underestimated NO3− source, which contributed 26% of the NO3− flux. Insufficient source control and prevailing in-stream nitrification, along with a lack of significant NO3− removal mechanisms, could be the major driving forces of the high NO3− levels. To reduce the levels, special attention should be paid to the waste water treatment plant and non-point sources aside from sewage and wastewater. Focus should also be placed on internal transformation processes that regulate NO3− levels. This study provides new implications for the restoration of urban rivers in China and other countries that are in a similar situation of tackling N pollution.