Isotope and isotopomer ratios of dissolved N2O as indicators of nitrogen apportionment and transformation in shallow groundwater in Dongguan, southern China

Abstract

Nitrogen source identification is critical to effectively govern nitrogen contamination, as excessive nitrogen content in groundwater has become a crucial global environmental problem. The traditional stable isotope approach (δ15N-NO3–, δ18O-NO3–, δ15N-NH4+) has been widely used to identify nitrogen sources by comparing the isotopic signals between groundwater samples and the possible nitrogen source. However, nitrogen source apportionment based only on that method would be inaccurate because isotopic signals overlap between the nitrogen sources and nitrogen that has undergone biochemical reactions. To solve the shortcomings of the traditional stable isotope approach, the isotope ratios of N2O, the SP-δ18O model and the traditional stable isotope approach were combined to depict the nitrogen transformation processes and reconstruct the “initial” isotopic compositions of nitrogen sources. A total of 58 groundwater samples were collected in the Dongguan Basin and were analysed during June-July 2018. NO3–-N was the dominant species of dissolved inorganic nitrogen in most of the groundwater samples. In the groundwater flow direction, NO3– contents decreased, while the δ15N-NO3– and δ18O-NO3– values increased. The positive relationship between δ15N-NO3– and δ18O-NO3– and the calculation results of the SP-δ18O model indicated that both denitrification and nitrification processes occurred in the groundwater, and denitrification acted as the dominant nitrogen transformation process. The initial isotope ratio of nitrogen obtained from the combined use of the calculation results of the SP-δ18O model and δ15N-NO3– (or δ15N-NH4+) value suggested that manure and septic wastes were the primary sources of NO3– and NH4+ in the groundwater. The redox-sensitive components and the calculation results of the SP-δ18O model depicted the reductive level of NO3– in the groundwater flow direction: NO3– reduction did not occur in the upstream area, NO3– reduction was strong in the intermediate transition zone, and NO3– contents were completely consumed and CH4 occurred in the downstream region. This study demonstrated that the combination of the traditional stable isotope approach and the isotope ratios of N2O are useful tools for revealing the source and transformation of nitrogen in groundwater. The method used in this study provides new insight for nitrogen source identification.