Response of nitrogen pollution in surface water to land use and social-economic factors in the Weihe River watershed, northwest China

Water pollution is affecting China's water resources and environmental sustainability. Specifically, nitrogen pollution has become highly problematic in water management. To identify nitrogen pollution and its sources in the Weihe River, northwest China, concentrations of different forms of nitrogen, and delta N-15 and delta O-18 compositions were analyzed. The mean nitrate nitrogen (NO3--N), ammonium nitrogen (NH4+-N), and nitrite nitrogen (NO2--N) concentrations were 4.70, 0.34 and 0.09 mg L-1, respectively, indicating that nitrate was the dominant form of nitrogen pollution. The middle and lower reaches of the Weihe River suffered more severe nitrogen pollution problems than the upper reaches. The stable isotope and Bayesian model revealed that manure, domestic sewage and industrial wastewater discharge constituted the main sources of NO3--N pollution, with the contribution of 71.3% of the three sources. The redundancy analysis (RDA) showed a positive correlation between population, industrial wastewater, domestic sewage, livestock and nitrogen pollution concentrations. Forestlands and grasslands had negative effects on surface water nitrogen concentrations, with a total reduction effect of 55.8%. To improve the surface water quality in the Weihe River, it is recommended to increase sewage treatment, improve livestock manure utilization and refine the land-use structure of the watershed.