stable isotope approach and its application for identifying nitrate source and transformation process in water

Nitrate contamination of water is a worldwide environmental problem. Recent studies have demonstrated that the nitrogen (N) and oxygen (O) isotopes of nitrate (NO₃ ⁻) can be used to trace nitrogen dynamics including identifying nitrate sources and nitrogen transformation processes. This paper analyzes the current state of identifying nitrate sources and nitrogen transformation processes using N and O isotopes of nitrate. With regard to nitrate sources, δ¹⁵N-NO₃ ⁻ and δ¹⁸O-NO₃ ⁻ values typically vary between sources, allowing the sources to be isotopically fingerprinted. δ¹⁵N-NO₃ ⁻ is often effective at tracing NO⁻ ₃ sources from areas with different land use. δ¹⁸O-NO₃ ⁻ is more useful to identify NO₃ ⁻ from atmospheric sources. Isotopic data can be combined with statistical mixing models to quantify the relative contributions of NO₃ ⁻ from multiple delineated sources. With regard to N transformation processes, N and O isotopes of nitrate can be used to decipher the degree of nitrogen transformation by such processes as nitrification, assimilation, and denitrification. In some cases, however, isotopic fractionation may alter the isotopic fingerprint associated with the delineated NO₃ ⁻ source(s). This problem may be addressed by combining the N and O isotopic data with other types of, including the concentration of selected conservative elements, e.g., chloride (Cl⁻), boron isotope (δ¹¹B), and sulfur isotope (δ³⁵S) data. Future studies should focus on improving stable isotope mixing models and furthering our understanding of isotopic fractionation by conducting laboratory and field experiments in different environments.