Identification of Sources and Transformations of Nitrate in the Intense Human Activity Region of North China Using a Multi-Isotope and Bayesian Model

Nitrate (NO(3)(−)) contamination in water is an environmental problem of widespread concern. In this study, we combined the stable isotopes of NO(3)(−) (δ(15)N and δ(18)O) and water (δ(2)H and δ(18)O) with a Bayesian mixing model (SIAR) to identify the sources and transformation of NO(3)(−) in groundwater and rivers in the Ye River basin of North China. The results showed that the mean NO(3)(−) concentrations in groundwater were 133.5 and 111.7 mg/L in the dry and flood seasons, respectively, which exceeded the required Chinese drinking water standards for groundwater (88.6 mg/L) (GB14848-2017). This suggests that groundwater quality has been severely impacted by human activity. Land use significantly affected the concentration of NO(3)(−) in the Ye River basin (p < 0.05). However, the NO(3)(−) concentrations in groundwater and river water had no obvious temporal variation (p > 0.05). The principal mode of nitrogen transformation for both groundwater and river water was nitrification, whereas denitrification did not significantly affect the isotopic compositions of NO(3)(−). The sources of NO(3)(−) mainly originated from sewage and manure, soil nitrogen, and NH(4)(+) in fertilizer for groundwater and from sewage and manure for the river water. According to the SIAR model, the primary sources of nitrate found in groundwater and river were sewage and manure in the Ye River basin. The proportional contributions of sewage and manure to nitrate contamination of groundwater and river were 58% and 48% in the dry season and 49% and 54% in the flood season, respectively. Based on these results, we suggest that the local government should enhance the sewage treatment infrastructure, construct an effective waste storage system to collect manure, and pursue a scientific fertilization strategy (such as soil formula fertilization) to increase the utilization rate of nitrogen fertilizer and prevent nitrate levels from increasing further.