The sources and fate of nitrogen in groundwater under different land use types: Stable isotope combined with a hydrochemical approach

Identifying the sources and fate of nitrogen in groundwater is challenging in areas with diverse land use types and multiple potential inputs. The Abujiaohe river basin in China's Sanjiang Plain, which has mixed land use types, including forestland, upland, paddy fields, and residential areas, was selected as the study area. Multiple environmental tracers, including hydrochemical and stable isotopes of nitrate (δ15N-NO3 - and δ18O-NO3 -) from groundwater, were examined. A biplot approach (δ15N-NO3 - and δ18O-NO3 -) combined with multivariate statistical analysis of hydrochemical data was used to investigate the sources and fate of nitrogen in groundwater. The results showed that severe nitrogen pollution-the highest concentration found in this study exceeded the national limit by four times-was present in both towns and agricultural areas. Land use was not only the dominant determining factor of nitrogen concentration and groundwater chemistry type, but also had a key impact on the regional nitrogen cycle and nitrogen sources. In the forestland, dominant nitrate was mainly derived from organic nitrogen nitrification in the soil. In residential areas, excessive NO3 - with high Cl- concentrations derived from manure and septic waste, which was controlled by the nitrification process. In paddy fields, low NO3 -, combined with high NH4 + and stable isotopes of nitrate concentrations, was observed. The linear relationship between δ15N-NO3 - and δ18O-NO3 - indicated that the occurrence of significant denitrification led to the limited presence of isotopic tracer. The uplands, affected by multiple factors such as rainfall and denitrification, had a more complex N cyclic process. Ultimately, multivariate statistical analysis results showed that the reduction of nitrogen fertilizer input was the main reason for nitrogen pollution. Therefore, the combined approach of using nitrogen isotopic ratios and conventional multivariate statistical analysis allowed for the more reliable separation of nitrogen sources in groundwater. This approach can help us to initiate more targeted remediation strategies. © 2019, Science Press. All right reserved.