Quantification of nitrate sources and fates in rivers in an irrigated agricultural area using environmental isotopes and a Bayesian isotope mixing model.

Nitrate (NO) pollution in rivers caused by intensive human activities is becoming a serious problem in irrigated agricultural areas. To identify NO sources and reveal the impact of irrigation projects on NO pollution in rivers, the hydrochemistry and isotopes of irrigation water from the Yellow River (IW) and river water (RW), and potential source samples were analyzed. The mean NO concentrations in the IW and RW were 24.4 mg/L and 49.9 mg/L, respectively. Approximately 45.2% of RW samples (n = 31) exceeded the Chinese drinking water standard for NO (45 mg/L). The δN and δO values, combined with the Cl/Na, SO/Ca ratio distributions, indicate that the NO in the RW mainly originated from chemical fertilizers, manure and sewage. A Bayesian model showed that manure and sewage contributed the most to the overall NO levels of the IW. In the RW, chemical fertilizers and IW contributed the most to the overall NO levels. The mean nitrate contribution to the RW from the combination of chemical fertilizers and IW is estimated to be 51.6%. Nitrogen from manure and sewage, soil N and precipitation also contributed. The NO pollution in rivers was largely influenced by the irrigation regime, with a large amount of nitrogen in chemical fertilizer lost because of low utilization efficiency and subsequent transfer, via irrigation runoff, into the rivers. This study suggests that with a detailed assessment of the sources and fate of NO, effective reduction strategies and better management practices can be implemented to control NO pollution in rivers.