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

Nitrate (NO3−) contamination in water is an environmental problem of widespread concern. In this study, we combined the stable isotopes of NO3− (δ15N and δ18O) and water (δ2H and δ18O) with a Bayesian mixing model (SIAR) to identify the sources and transformation of NO3− in groundwater and rivers in the Ye River basin of North China. The results showed that the mean NO3− 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 NO3− in the Ye River basin (p < 0.05). However, the NO3− 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 NO3−. The sources of NO3− mainly originated from sewage and manure, soil nitrogen, and NH4+ 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.

Using Multiple Isotopes to Identify Sources and Transports of Nitrate in Urban Residential Stormwater Runoff

Increased nitrogen (N) from urban stormwater runoff aggravates the deterioration of aquatic ecosystems as urbanisation develops. In this study, the sources and transport of nitrate (NO3−) in urban stormwater runoff were investigated by analysing different forms of N, water isotopes (δD-H2O and δ18O-H2O), and NO3− isotopes (δ15N-NO3− and δ18O-NO3−) in urban stormwater runoff in a residential area in Hangzhou, China. The results showed that the concentrations of total N and nitrate N in road runoff were higher than those in roof runoff. Moreover, high concentrations of dissolved organic N and particulate N in road runoff led to significantly different TN concentrations in road runoff (mean: 3.76 mg/L) and roof runoff (mean: 1.23 mg/L). The high δ18O-NO3− values (mean: 60 ± 13.1‰) indicated that atmospheric deposition was the predominant NO3− source in roof runoff, as confirmed by the Bayesian isotope mixing model (SIAR model), contributing 83.6–97.8% to NO3−. The SIAR model results demonstrated that atmospheric deposition (34.2–91.9%) and chemical fertilisers (6.27–54.3%) were the main NO3− sources for the road runoff. The proportional contributions from soil and organic N were smaller than other sources in both the road runoff and roof runoff. For the initial period, the NO3− contributions from atmospheric deposition and chemical fertilisers were higher and lower, respectively, than those in the middle and late periods in road runoff during storm events 3 and 4, while an opposite trend of road runoff in storm event 7 highlighted the influence of short antecedent dry weather period. It was suggested that reducing impervious areas and more effective management of fertiliser application in urban green land areas were essential to minimize the presence of N in urban aquatic ecosystems.